publicationDate stringlengths 10 10 | title stringlengths 17 233 | abstract stringlengths 20 3.22k | id stringlengths 9 12 |
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2008-02-20 | In-plane current-voltage characteristics and oscillatory Josephson-vortex flow resistance in La-free Bi$_{2+x}$Sr$_{2-x}$CuO$_{6+δ}$ single crystals in high magnetic fields | We have investigated the in-plane $I(V)$ characteristics and the Josephson
vortex flow resistance in high-quality La-free
Bi$_{2+x}$Sr$_{2-x}$CuO$_{6+\delta}$ (Bi2201) single crystals in parallel and
tilted magnetic fields at temperatures down to 40 mK. For parallel magnetic
fields below the resistive upper critical field $H^{*}_{c2}$, the $I(V)$
characteristic obey a power-law with a smooth change with increasing
magnetic-field of the exponent from above 5 down to 1. In contrast to the
double-layer cuprate Bi2212, the observed smooth change suggests that there is
no change in the mechanism of dissipation (no Kosterlitz-Thouless transition)
over the range of temperatures investigated. At small angles between the
applied field and the $ab$-plane, prominent current steps in the $I(V)$
characteristics and periodic oscillations of Josephson-vortex flow resistance
are observed. While the current steps are periodic in the voltage at constant
fields, the voltage position of the steps, together with the flux-flow voltage,
increases nonlinearly with magnetic field. The $ab$-flow resistance oscillates
as a function of field with a constant period over a wide range of magnetic
fields and temperatures. The current steps in the $I(V)$ characteristics and
the flow resistance oscillations can be linked to the motion of Josephson
vortices across layers. | 0802.2791v1 |
2009-06-01 | Phase study of oscillatory resistances in microwave-irradiated- and dark- GaAs/AlGaAs devices: Indications of a new class of integral quantum Hall effect | We report the experimental results from a dark study and a photo-excited
study of the high mobility GaAs/AlGaAs system at large filling factors, $\nu$.
At large-$\nu$, the dark study indicates several distinct phase relations
("Type-1", "Type-2", and "Type-3") between the oscillatory diagonal- and Hall-
resistances, as the canonical Integral Quantum Hall Effect (IQHE) is manifested
in the "Type-1" case of approximately orthogonal diagonal- and Hall resistance-
oscillations. Surprisingly, the investigation indicates quantum Hall plateaus
also in the "Type-3" case characterized by approximately "anti-phase" Hall- and
diagonal- resistance oscillations, suggesting a new class of IQHE.
Transport studies under microwave photo-excitation exhibit radiation-induced
magneto-resistance oscillations in both the diagonal, $R_{xx}$, and
off-diagonal, $R_{xy}$, resistances. Further, when the radiation-induced
magneto-resistance oscillations extend into the quantum Hall regime, there
occurs a radiation-induced non-monotonic variation in the amplitude of
Shubnikov-de Haas (SdH) oscillations in $R_{xx}$ \textit{vs}. B, and a
non-monotonic variation in the width of the quantum Hall plateaus in $R_{xy}$.
The latter effect leads into the vanishing of IQHE at the minima of the
radiation-induced $R_{xx}$ oscillations with increased photo-excitation. We
reason that the mechanism which is responsible for producing the non-monotonic
variation in the amplitude of SdH oscillations in $R_{xx}$ under
photo-excitation is also responsible for eliminating, under photo-excitation,
the novel "Type-3" IQHE in the high mobility specimen. | 0906.0032v1 |
2016-10-19 | Pressure induced superconductivity in Bi single crystals | Measurements on resistivity and magnetic susceptibility have been carried out
for Bi single crystals under pressures up to 10.5GPa. The temperature dependent
resistivity shows a semimetallic behavior at ambient and low pressures (below
about 1.6GPa). This is followed by an upturn of resistivity in low temperature
region when the pressure is increased, which is explained as a semiconductor
behavior. This feature gradually gets enhanced up to a pressure of about
2.52GPa. Then a non-monotonic temperature dependent resistivity appears upon
further increasing pressure, which is accompanied by a strong suppression to
the low temperature resistivity upturn. Simultaneously, a superconducting
transition occurs at about 3.92K under a pressure of about 2.63GPa. With
further increasing pressure, a second superconducting transition emerges at
about 7K under about 2.8GPa. For these two superconducting states, the
superconductivity induced magnetic screening volumes are quite large. As the
pressure further increased to 8.1GPa, we observe the third superconducting
transition at about 8.2K. The resistivity measurements under magnetic field
allow us to determine the upper critical fields $\mu_0 H_{c2}$ of the
superconducting phases. The upper critical field for the phase with $T_c=3.92$K
is extremely low. Based on the Werthamer-Helfand-Hohenberg (WHH) theory, the
estimated value of $\mu_0 H_{c2}$ for this phase is about 0.103T. While the
upper critical field for the phase with $T_c$=7K is very high with a value of
about 4.56T. Finally, we present a pressure dependent phase diagram of Bi
single crystals. Our results reveal the interesting and rich physics in bismuth
single crystals under high pressure. | 1610.06062v1 |
2021-01-14 | Effects of resistivity on standing shocks in low angular momentum flows around black holes | We study two dimensional low angular momentum flow around the black hole
using the resistive magnetohydrodynamic module of PLUTO code. Simulations have
been performed for the flows with parameters of specific angular momentum,
specific energy, and magnetic field which may be expected for the flow around
Sgr A*. For flows with lower resistivity $\eta=10^{-6}$ and $0.01$, the
luminosity and the shock location on the equator vary quasi-periodically. The
power density spectra of luminosity variation show the peak frequencies which
correspond to the periods of $5 \times 10^5$, $1.4 \times 10^5$, and $5 \times
10^4$ seconds, respectively. These quasi-periodic oscillations (QPOs) occur due
to the interaction between the outer oscillating standing shock and the inner
weak shocks occurring at the innermost hot blob. While for cases with higher
resistivity $\eta=0.1$ and 1.0, the high resistivity considerably suppresses
the magnetic activity such as the MHD turbulence and the flows tend to be
steady and symmetric to the equator. The steady standing shock is formed more
outward compared with the hydrodynamical flow. The low angular momentum flow
model with the above flow parameters and with low resistivity has a possibility
for the explanation of the long-term flares with $\sim$ one per day and $\sim 5
- 10$ days of Sgr A* in the latest observations by Chandra, Swift, and
XMM-Newton monitoring of Sgr A*. | 2101.05474v2 |
2014-06-19 | AC loss and coupling currents in YBCO coated conductors with varying number of filaments | Striation of HTS coated conductors (CCs) as a way to reduce their
magnetization AC losses has been the subject of intense research in the past
years by several groups. While the principle of this approach is well
understood, its practical application on commercial material to be used in
power application is still far to be implemented due to manufacturing and
technological constraints. Recent advances in tape quality and striation
technology are now enabling systematic investigations of the influence of the
number of filaments on AC loss reduction with a consistency that was not
available in the past. In this work we demonstrate the technological
feasibility of reducing the magnetization losses of commercially available CC
by striating them into a high number of filaments (up to 120). The loss
reduction exceeds one order of magnitude and does not come at the expense of
current-carrying capability: samples with 10 and 20 filaments are unaffected by
the striation process, while samples with 80 and 120 filaments still retain 80
and 70% of the current-carrying potential, respectively. We also investigate
the transverse resistivity in order to understand the paths followed by the
coupling currents: we found that the coupling current prevalently flows in the
metallic substrate, rather than in and out of the filaments. Finally, we use
oxidation as a method to reduce the coupling currents and losses. The
contribution of this work is three-fold: 1) It describes the know-how to
produce a large number of high quality striations in commercially available
CCs, greatly reducing their losses without extensively degrading their
transport properties; 2) It provides a comprehensive characterization of said
samples (e.g. measurements in a wide frequency range, transverse resistance
profiles, influence of oxidation on DC and AC behavior); 3) It provides new
insight on the patterns of the coupling currents. | 1406.4990v2 |
2017-09-21 | Electrical properties of single crystal Yttrium Iron Garnet ultra-thin films at high temperatures | We report a study on the electrical properties of 19 nm thick Yttrium Iron
Garnet (YIG) films grown by liquid phase epitaxy. The electrical conductivity
and Hall coefficient are measured in the high temperature range [300,400]~K
using a Van der Pauw four-point probe technique. We find that the electrical
resistivity decreases exponentially with increasing temperature following an
activated behavior corresponding to a band-gap of $E_g\approx 2$ eV, indicating
that epitaxial YIG ultra-thin films behave as large gap semiconductor, and not
as electrical insulator. The resistivity drops to about $5\times 10^3$~$\Omega
\cdot \text{cm}$ at $T=400$ K. We also infer the Hall mobility, which is found
to be positive ($p$-type) at 5 cm$^2$/(V$\cdot$sec) and about independent of
temperature. We discuss the consequence for non-local transport experiments
performed on YIG at room temperature. These electrical properties are
responsible for an offset voltage (independent of the in-plane field direction)
whose amplitude, odd in current, grows exponentially with current due to Joule
heating. These electrical properties also induce a sensitivity to the
perpendicular component of the magnetic field through the Hall effect. In our
lateral device, a thermoelectric offset voltage is produced by a temperature
gradient along the wire direction proportional to the perpendicular component
of the magnetic field (Righi-Leduc effects). | 1709.07207v1 |
2020-04-01 | Perovskite-type cobalt oxide at the multiferroic Co/Pb Zr$_{0.2}$Ti$_{0.8}$O$_{3}$ interface | Magnetic Tunnel Junctions whose basic element consists of two ferromagnetic
electrodes separated by an insulating non-magnetic barrier have become
intensely studied and used in non-volatile spintronic devices. Since ballistic
tunnel of spin-polarized electrons sensitively depends on the chemical
composition and the atomic geometry of the lead/barrier interfaces their proper
design is a key issue for achieving the required functionality of the devices
such as e.g. a high tunnel magneto resistance. An important leap in the
development of novel spintronic devices is to replace the insulating barrier by
a ferroelectric which adds new additional functionality induced by the
polarization direction in the barrier giving rise to the tunnel electro
resistance (TER). The multiferroic tunnel junction
Co/PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$/La$_{2/3}$Sr$_{1/3}$MnO$_3$ (Co/PZT/LSMO)
represents an archetype system for which - despite intense studies - no
consensus exists for the interface geometry and their effect on transport
properties. Here we provide the first analysis of the Co/PZT interface at the
atomic scale using complementary techniques, namely x-ray diffraction and
extended x-ray absorption fine structure in combination with x-ray magnetic
circular dichroism and ab-initio calculations. The Co/PZT interface consists of
one perovskite-type cobalt oxide unit cell [CoO$_{2}$/CoO/Ti(Zr)O$_{2}$] on
which a locally ordered cobalt film grows. Magnetic moments (m) of cobalt lie
in the range between m=2.3 and m=2.7$\mu_{B}$, while for the interfacial
titanium atoms they are small (m=+0.005 $\mu_{B}$) and parallel to cobalt which
is attributed to the presence of the cobalt-oxide interface layers. These
insights into the atomistic relation between interface and magnetic properties
is expected to pave the way for future high TER devices. | 2004.00489v1 |
2020-05-26 | Influence of Different Surface Morphologies on the Performance of High Voltage, Low Resistance Diamond Schottky Diodes | Vertical diamond Schottky diodes with blocking voltages $V_{\text{BD}} > 2.4
\text{ kV}$ and on-resistances $R_{\text{On}} < 400 \text{ m}\Omega
\text{cm}^{2}$ were fabricated on homoepitaxially grown diamond layers with
different surface morphologies. The morphology (smooth as-grown, hillock-rich,
polished) influences the Schottky barrier, the carrier transport properties,
and consequently the device performance. The smooth as-grown sample exhibited a
low reverse current density $J_{\text{Rev}} < 10^{-4} \text{ A}/\text{cm}^{2}$
for reverse voltages up to $2.2 \text{ kV}$. The hillock-rich sample blocked
similar voltages with a slight increase in the reverse current density
($J_{\text{Rev}} < 10^{-3} \text{ A}/\text{cm}^{2}$). The calculated
1D-breakdown field, however, was reduced by $30 \text{ } \%$, indicating a
field enhancement induced by the inhomogeneous surface. The polished sample
demonstrated a similar breakdown voltage and reverse current density as the
smooth as-grown sample, suggesting that a polished surface can be suitable for
device fabrication. However, a statistical analysis of several diodes of each
sample showed the importance of the substrate quality: A high density of
defects both reduces the feasible device area and increases the reverse current
density. In forward direction, the hillock-rich sample exhibited a secondary
Schottky barrier, which could be fitted with a modified thermionic emission
model employing the Lambert W-function. Both polished and smooth sample showed
nearly ideal thermionic emission with ideality factors $1.08$ and $1.03$,
respectively. Compared with literature, all three diodes exhibit an improved
Baliga Figure of Merit for diamond Schottky diodes with $V_{\text{BD}} > 2
\text{ kV}$. | 2005.12591v1 |
2023-11-21 | Evidence of filamentary superconductivity in pressurized La3Ni2O7 | Recently, the signatures of superconductivity near 80 K have been discovered
in the single crystal of La3Ni2O7 under pressure, which makes it a new
candidate of the high-temperature superconductors dominated by 3d transition
elements after the cuprate and iron-pnictide superconductors, and thus has
attracted significant attention. However, the superconductivity, characterized
by the zero resistance and the Meissner effect, is anomalously irreproducible
in the compressed La3Ni2O7. In this study, we report the experimental results
obtained through highly sensitive modulated ac susceptibility measurements,
which reveal that the maximum superconducting volume fraction in the La3Ni2O7
sample is only approximately 1% (employing superconducting element vanadium as
a reference). In tandem with our observation of the zero-resistance state only
in some of the samples, we suggest that the superconductivity in this nickelate
is filamentary-like. In combination of our scanning transmission electron
microscopy (STEM) investigations, we propose that the filamentary
superconductivity most likely emerges at the interface between the La3Ni2O7 and
La4Ni3O10 phases. Further, the connection of the oxygen vacancy in La3Ni2O7
with the presence of superconductivity and superconducting transition
temperature (Tc) has been established, through which the upper and lower bounds
of the oxygen content for the presence of superconductivity were determined to
be 6.89 and 7.35, respectively. Our results not only provide new insights into
understanding the puzzling issues in this material, but also highlight a new
route for exploring new high-Tc nickelate superconductors with a higher
superconducting volume fraction, as well as pose a new challenge in
comprehending the complex nature of this filamentary superconductivity. | 2311.12361v2 |
2024-03-01 | Electrochemical Evaluation of Mg and a Mg-Al 5%Zn Metal Rich Primers for Protection of Al-Zn-Mg-Cu Alloy in NaCl | High purity magnesium and a Mg-Al 5wt% Zn metal rich primer (MRP) were
compared for their ability to suppress intergranular corrosion (IGC) and
intergranular stress corrosion cracking (IG-SCC) in peak aged AA 7075-T651 by
sacrificial anode-based cathodic prevention. Tests were conducted in 0.6 M NaCl
solution under full immersion. These evaluations considered the ability of the
primer to attain an intermediate negative open circuit potential (OCP) such
that the galvanic couple potential with bare aluminum alloy (AA) 7075-T651
resided below a range of potentials where IGC is prevalent. The ability of the
primer to achieve an OCP negative enough that the AA 7075-T651 could be
protected by sacrificial anode-based cathodic prevention and the ability to
sustain this function over time were evaluated as a first step by utilizing a
NaCl solution. The primers consisted of epoxy resins embedded with either (1)
Mg flake pigments (MgRP) or (2) Mg flake pigments and spherical Al-5 wt.% Zn
together as a composite (MgAlRP). MgRP was an effective coating for cathodic
protection but dispensed less anodic charge than the composite MgAlRP.
Cross-sectional analysis demonstrated that some Mg flakes dissolved while
uniform surface oxidation occurred on the remaining Mg flakes which led to
impaired activation. The composite MgAlRP maintained a suitably negative OCP
over time, remained activated, dispensed high anodic charge, and remained an
anode in zero resistance ammeter testing. Chemical stability modeling and zero
resistance ammeter testing suggest that Mg corrosion elevates the pH which
dissolved aluminum oxides and hydroxide thereby activates the Al-5wt.% Zn
pigments, thereby providing a primary (i.e. Mg corrosion) and secondary process
to enable superior (activation of Al-5wt%Zn) sacrificial anode-based cathodic
protection. | 2403.00610v1 |
2007-09-14 | High pressure study on the strong-coupling superconductivity in non-centrosymmetric compound CeIrSi_3 | We have carried out high pressure experiment on the pressure-induced
superconductor CeIrSi$_3$ without inversion center. The electrical resistivity
and ac heat capacity were measured in the same run for the same sample. The
critical pressure of the antiferromagnetic state was determined to be $P_{\rm
c}$ = 2.25 GPa. The heat capacity $C_{\rm ac}$ shows both antiferromagnetic and
superconducting transitions at pressures close to $P_{\rm c}$. The
superconducting transition temperature $T_{\rm sc}$ shows a maximum value of
1.6 K around $2.5-2.7$ GPa. At 2.58 GPa, a large heat capacity anomaly was
observed at $T_{\rm sc}$ = 1.59 K. The jump of the heat capacity in the form of
${\Delta}{C_{\rm ac}}/C_{\rm ac}(T_{\rm sc})$ is 5.7 $\pm$ 0.1. This is the
largest value observed among all superconductors studied previously, suggesting
the strong-coupling superconductivity in CeIrSi$_3$. The large magnitude and
anisotropy of the upper critical field $B_{\rm c2}$ at 2.65 GPa is discussed
from view points of the strong-coupling superconductivity and the reduced
paramagnetic effect in the non-centrosymmetric superconductor. Above $P_{\rm
c}$, the electrical resistivity shows the anomalous $T$-linear dependence in
the wide temperature region from $T_{\rm sc}$ to 30 K, which is different from
the Fermi liquid theory. Meanwhile, the heat capacity $C_{\rm ac}/T$ shows a
simple temperature dependence in the normal state above $T_{\rm sc}$. These
features do not seem to be explained simply by the spin fluctuation theory. The
electronic specific heat coefficient at $T_{\rm sc}$ is approximately unchanged
as a function of pressure, even at $P_{\rm c}$. | 0709.2199v1 |
2016-06-09 | Ternary borides Nb$_7$Fe$_3$B$_8$ and Ta$_7$Fe$_3$B$_8$ with Kagome-type iron framework | Two new ternary borides $TM$$_7$Fe$_3$B$_8$ ($TM$ = Nb, Ta) were synthesized
by high-temperature thermal treatment of samples obtained by arc-melting. This
new type of structure with space group $P$6/$mmm$, comprises $TM$ slabs
containing isolated planar hexagonal [B$_6$] rings and iron centered $TM$
columns in a Kagome type of arrangement. Chemical bonding analysis in
Nb$_7$Fe$_3$B$_8$ by means of the electron localizability approach reveals
two-center interactions forming the Kagome net of Fe and embedded B, while
weaker multicenter bonding present between this net and Nb atoms. Magnetic
susceptibility measurements reveal antiferromagnetic order below $T_N$ = 240 K
for Nb$_7$Fe$_3$B$_8$ and $T_N$ =265 K for Ta$_7$Fe$_3$B$_8$. Small remnant
magnetization below 0.01 $\mu_B$/f.u. is observed in the antiferromagnetic
state. The bulk nature of the magnetic transitions was confirmed by the
hyperfine splitting of the M\"o{\ss}bauer spectra, the sizable anomalies in the
specific heat capacity, and the kinks in the resistivity curves. The high-field
paramagnetic susceptibilities fitted by the Curie-Weiss law show effective
paramagnetic moments $\mu_{eff}$ about 3.1 $\mu_B$/Fe in both compounds. The
temperature dependence of the electrical resistivity also reveals metallic
character of both compounds. Density functional calculations corroborate the
metallic behaviour of both compounds and demonstrate the formation of a sizable
local magnetic moment on the Fe-sites. They indicate the presence of both
antiferro- and ferromagnetic interactions. | 1606.03123v1 |
2020-01-24 | Quantum transport in topological surface states of Bi$_2$Te$_3$ nanoribbons | Quasi-1D nanowires of topological insulators are emerging candidate
structures in superconductor hybrid architectures for the realization of
Majorana fermion based quantum computation schemes. It is however technically
difficult to both fabricate as well as identify the 1D limit of topological
insulator nanowires. Here, we investigated selectively-grown Bi$_2$Te$_3$
topological insulator nanoribbons and nano Hall bars at cryogenic temperatures
for their topological properties. The Hall bars are defined in deep-etched
Si$_3$N$_4$/SiO$_2$ nano-trenches on a silicon (111) substrate followed by a
selective area growth process via molecular beam epitaxy. The selective area
growth is beneficial to the device quality, as no subsequent fabrication needs
to be performed to shape the nanoribbons. Transmission line measurements are
performed to evaluate contact resistances of Ti/Au contacts applied as well as
the specific resistance of the Bi$_2$Te$_3$ binary topological insulator. In
the diffusive transport regime of these unintentionally $n$-doped Bi$_2$Te$_3$
topological insulator nano Hall bars, we identify distinguishable electron
trajectories by analyzing angle-dependent universal conductance fluctuation
spectra. When the sample is tilted from a perpendicular to a parallel magnetic
field orientation, these high frequent universal conductance fluctuations merge
with low frequent Aharonov-Bohm type oscillations originating from the
topologically protected surface states encircling the nanoribbon cross section.
For 500 nm wide Hall bars we also identify low frequent Shubnikov-de Haas
oscillations in the perpendicular field orientation, that reveal a topological
high-mobility 2D transport channel, partially decoupled from the bulk of the
material. | 2001.09028v1 |
2020-06-01 | PrVO$_4$ under High Pressure: Effects on Structural, Optical and Electrical Properties | In pursue of a systematic characterization of rare-earth vanadates under
compression, in this work we present a multifaceted study of the phase behavior
of zircon-type orthovanadate PrVO$_4$ under high pressure conditions, up until
24 GPa. We have found that PrVO$_4$ undergoes a zircon to monazite transition
at around 6 GPa, confirming previous results found by Raman experiments. A
second transition takes place above 14 GPa, to a BaWO$_4$-I--type structure.
The zircon to monazite structural sequence is an irreversible first-order
transition, accompanied by a volume collapse of about 9.6%. Monazite phase is
thus a metastable polymorph of PrVO$_4$. The monazite-BaWO$_4$-II transition is
found to be reversible instead and occurs with a similar volume change. Here we
report and discuss the axial and bulk compressibility of all phases. We also
compare our results with those for other rare-earth orthovanadates. Finally, by
means of optical-absorption experiments and resistivity measurements we
determined the effect of pressure on the electronic properties of PrVO$_4$. We
found that the zircon-monazite transition produces a collapse of the band gap
and an abrupt decrease of the resistivity. The physical reasons for this
behavior are discussed. Density-functional-theory simulations support our
conclusions. | 2006.01299v2 |
2024-04-04 | Electronic transport, metal-insulator transition, and Wigner crystallization in transition metal dichalcogenide monolayers | Two recent electronic transport experiments from Columbia University and
Harvard University have reported record high mobility and low channel densities
in transition metal dichalcogenide (TMD) WSe$_2$ monolayers [J. Pack, et al.,
arXiv:2310.19782; A. Y. Joe, et al., Phys. Rev. Lett. 132, 056303 (2024)]. A
two-dimensional (2D) metal-insulator transition (MIT) is demonstrated in the
Columbia sample at low densities, a regime where the formation of a Wigner
crystal (WC) is theoretically anticipated in the absence of disorder. We employ
the finite-temperature Boltzmann theory to understand the low-temperature
transport properties of monolayer TMDs, taking into account realistic disorder
scattering. We analyze the experimental results, focusing on the 2D MIT
behavior and the influence of temperature and density on mobility and
resistivity in the metallic phase. We provide a discussion of the nontrivial
carrier density dependence of our transport results. Our analysis elucidates
the linear-in-$T$ resistivity in the metallic phase, attributing it to Friedel
oscillations associated with screened charged impurities. Furthermore, we
explore whether Coulomb disorder could lead to the MIT through either a quantum
Anderson localization transition or a classical percolation transition. Our
theoretical estimates of the disorder-induced MIT critical densities, although
smaller, are within a factor of ~2 of the experimental critical density. We
examine the exceptionally high melting temperature ~10 K of WCs observed
experimentally in the MoSe$_2$ systems at low density, an order of magnitude
larger than the pristine melting temperature. This suggests that the observed
2D low-density MIT behavior is likely a result of the complex interplay between
disorder effects and interaction-driven WC physics, offering a comprehensive
understanding of the low-temperature transport phenomena in TMD monolayers. | 2404.03488v1 |
2005-06-08 | X-ray Diagnostics of Grain Depletion in Matter Accreting onto T Tauri Stars | Recent analysis of high resolution Chandra X-ray spectra has shown that the
Ne/O abundance ratio is remarkably constant in stellar coronae. Based on this
result, we point out the utility of the Ne/O ratio as a discriminant for
accretion-related X-rays from T Tauri stars, and for probing the measure of
grain-depletion of the accreting material in the inner disk. We apply the Ne/O
diagnostic to the classical T Tauri stars BP Tau and TW Hya--the two stars
found to date whose X-ray emission appears to originate, at least in part, from
accretion activity. We show that TW Hya appears to be accreting material which
is significantly depleted in O relative to Ne. In constrast, BP Tau has an Ne/O
abundance ratio consistent with that observed for post-T Tauri stars. We
interpret this result in terms of the different ages and evolutionary states of
the circumstellar disks of these stars. In the young BP Tau disk (age 0.6 Myr)
dust is still present near the disk corotation radius and can be ionized and
accreted, re-releasing elements depleted onto grains. In the more evolved TW
Hya disk (age 10 Myr), evidence points to ongoing coagulation of grains into
much larger bodies, and possibly planets, that can resist the drag of
inward-migrating gas, and accreting gas is consequently depleted of
grain-forming elements. | 0506185v1 |
2003-02-13 | Electronic phase separation in the rare earth manganates, (La1-xLnx)0.7Ca0.3MnO3 (Ln = Nd, Gd and Y) | All the three series of manganates showsaturation magnetization
characteristic of ferromagnetism, with the ferromagnetic Tc decreasing with
increasing in x up to a critical value of x, xc (xc = 0.6, 0.3, 0.2
respectively for Nd, Gd, Y). For x > xc, the magnetic moments are considerably
smaller showing a small increase around TM, the value of TM decreasing slightly
with increase in x or decrease in < rA >. The ferromagnetic compositions (x xc)
show insulator-metal (IM) transitions, while the compositions with x > xc are
insulating. The magnetic and electrical resistivity behavior of these
manganates is consistent with the occurrence of phase separation in the
compositions around xc, corresponding to a critical average radius of the
A-site cation, < rAc >, of 1.18 A. Both Tc and TIM increase linearly when < rA
> > < rAc > or x xc as expected of a homogenous ferromagnetic phase. Both Tc
and TM decrease linearly with the A-site cation size disorder at the A-site as
measured by the variance s2. Thus, an increase in s2 favors the insulating AFM
state. Percolative conduction is observed in the compositions with < rA > > <
rAc >. Electron transport properties in the insulating regime for x > xc
conforms to the variable range hopping mechanism. More interestingly, when x >
xc, the real part of dielectric constant (e') reaches a high value (104-106) at
ordinary temperatures dropping to a very small (~500) value below a certain
temperature, the value of which decreases with decreasing frequency. | 0302268v1 |
2003-03-21 | Independent Electronic and Magnetic Doping in (Ga,Mn)As Based Digital Ferromagnetic Heterostructures | Ferromagnetic semiconductors promise the extension of metal-based spintronics
into a material system that combines widely tunable electronic, optical, and
magnetic properties. Here, we take steps towards realizing that promise by
achieving independent control of electronic doping in the ferromagnetic
semiconductor (Ga,Mn)As. Samples are comprised of superlattices of 0.5
monolayer (ML) MnAs alternating with 20 ML GaAs and are grown by low
temperature (230 C) atomic layer epitaxy (ALE). This allows for the reduction
of excess As incorporation and hence the number of charge-compensating
As-related defects. We grow a series of samples with either Be or Si doping in
the GaAs spacers (p- and n-type dopants, respectively), and verify their
structural quality by in situ reflection high-energy electron diffraction
(RHEED) and ex situ x-ray diffraction. Magnetization measurements reveal
ferromagnetic behavior over the entire doping range, and show no sign of MnAs
precipitates. Finally, magneto-transport shows the giant planar Hall effect and
strong (20%) resistance fluctuations that may be related to domain wall motion. | 0303461v1 |
2006-02-26 | Physical properties and magnetic structure of TbRhIn5 intermetallic compound | In this work we report the physical properties of the new intermetallic
compound TbRhIn5 investigated by means of temperature dependent magnetic
susceptibility, electrical resistivity, heat-capacity and resonant x-ray
magnetic diffraction experiments. TbRhIn5 is an intermetallic compound that
orders antiferromagnetically at TN = 45.5 K, the highest ordering temperature
among the existing RRhIn5 (1-1-5, R = rare earth) materials. This result is in
contrast to what is expected from a de Gennes scaling along the RRhIn5 series.
The X-ray resonant diffraction data below TN reveal a commensurate
antiferromagnetic (AFM) structure with a propagation vector (1/2 0 1/2) and the
Tb moments oriented along the c-axis. Strong (over two order of magnitude)
dipolar enhancements of the magnetic Bragg peaks were observed at both Tb
absorption edges LII and LIII, indicating a fairly high polarization of the Tb
5d levels. Using a mean field model including an isotropic first-neighbors
exchange interaction J(R-R) and the tetragonal crystalline electrical field
(CEF), we were able to fit our experimental data and to explain the direction
of the ordered Tb-moments and the enhanced TN of this compound. The evolution
of the magnetic properties along the RRhIn5 series and its relation to CEF
effects for a given rare-earth is discussed. | 0602612v1 |
2006-06-28 | Dielectric anomaly at the orbital order-disorder transition in LaMnO_(3+delta) | We report a novel dielectric anomaly around the Jahn-Teller orbital
order-disorder transition temperature T_JT in LaMnO_(3+delta). The transition
has been characterized by resistivity (rho)versus temperature (T), calorimetry,
and temperature-dependent X-ray diffraction studies. Measurements of complex
dielectric permittivity epsilon* (= epsilon'-i.epsilon'') over a low-frequency
range (1 Hz - 10 MHz)across T_JT reveal a distinct anomaly. This observation,
and the reported relatively high static dielectric constant at T = 0 (epsilon0
\~18-20), possibly indicate that the orbital order gives rise to intrinsic
polarization that undergoes transition at T_JT. The frequency dispersion of the
dielectric response at any given temperature, however, reveals that the
dielectric response consists of Maxwell-Wagner component, due to interfaces,
within such a low frequency range. The T_JT and the nature of the anomaly in
epsilon'(omega,T), epsilon''(omega,T) at T_JT, of course, vary - from a sharp
upward feature to a smeared plateau and then a downward trend - depending on
the Mn^4+ concentration of the sample. The observation of an intrinsic
dielectric response due to long-range orbital order in LaMnO_3 - where no
ferroelectric order is possible due to the absence of off-centre distortion in
MnO_6 octahedra - may throw a new light onto these classes of materials
vis-a-vis multiferroic materials. | 0606731v1 |
2003-10-07 | Role of Oxygen and Carbon Impurities in the Radiation Resistance of Silicon Detectors | The influence of oxygen and carbon impurities on the concentrations of
defects in silicon for detector uses, in complex fields of radiation (proton
cosmic field at low orbits around the Earth, at Large Hadron Collider and at
the next generation of accelerators as Super-LHC) is investigated in the frame
of the quantitative model developed previously by the authors. The generation
rate of primary defects is calculated starting from the projectile - silicon
interaction and from recoil energy redistribution in the lattice. The
mechanisms of formation of complex defects are explicitly analysed.
Vacancy-interstitial annihilation, interstitial and vacancy migration to sinks,
divacancy, vacancy and interstitial impurity complex formation and
decomposition are considered. Oxygen and carbon impurities present in silicon
could monitor the concentration of all stable defects, due to their interaction
with vacancies and interstitials. Their role in the mechanisms of formation and
decomposition of the following stable defects: VP, VO, V_2, V_2O, C_i, C_iO_i
and C_iC_s is studied. The model predictions could be a useful clue in
obtaining harder materials for detectors at the new generation of accelerators,
for space missions or for industrial applications. | 0310032v1 |
2009-01-22 | Signal height in silicon pixel detectors irradiated with pions and protons | Pixel detectors are used in the innermost part of multi purpose experiments
at the Large Hadron Collider (LHC) and are therefore exposed to the highest
fluences of ionising radiation, which in this part of the detectors consists
mainly of charged pions. The radiation hardness of the detectors has thoroughly
been tested up to the fluences expected at the LHC. In case of an LHC upgrade
the fluence will be much higher and it is not yet clear up to which radii the
present pixel technology can be used. In order to establish such a limit, pixel
sensors of the size of one CMS pixel readout chip (PSI46V2.1) have been bump
bonded and irradiated with positive pions up to 6E14 Neq/cm^2 at PSI and with
protons up to 5E15 Neq/cm^2. The sensors were taken from production wafers of
the CMS barrel pixel detector. They use n-type DOFZ material with a resistance
of about 3.7kOhm cm and an n-side read out. As the performance of silicon
sensors is limited by trapping, the response to a Sr-90 source was
investigated. The highly energetic beta-particles represent a good
approximation to minimum ionising particles. The bias dependence of the signal
for a wide range of fluences will be presented. | 0901.3422v1 |
2012-04-17 | Beta-Ag2Te: A topological insulator with strong anisotropy | We present evidence of topological surface states in beta-Ag2Te through
first-principles calculations and periodic quantum interference effect in
single crystalline nanoribbon. Our first-principles calculations show that
beta-Ag2Te is a topological insulator with a gapless Dirac cone with strong
anisotropy. To experimentally probe the topological surface state, we
synthesized high quality beta-Ag2Te nanoribbons and performed electron
transport measurements. The coexistence of pronounced Aharonov-Bohm
oscillations and weak Altshuler-Aronov-Spivak oscillations clearly demonstrates
coherent electron transport around the perimeter of beta-Ag2Te nanoribbon and
therefore the existence of metallic surface states, which is further supported
by the temperature dependence of resistivity for beta-Ag2Te nanoribbons with
different cross section areas. Highly anisotropic topological surface state of
beta-Ag2Te suggests that the material is a promising material for fundamental
study and future spintronic devices. | 1204.3816v6 |
2012-09-16 | Glass-like recovery of antiferromagnetic spin ordering and dimensional crossover in a photo-excited manganite Pr$_{0.7}$Ca$_{0.3}$MnO$_3$ | Electronic orderings of charges, orbitals and spins are observed in many
strongly correlated electron materials, and revealing their dynamics is a
critical step toward understanding the underlying physics of important emergent
phenomena. Here we use time-resolved resonant soft x-ray scattering
spectroscopy to probe the dynamics of antiferromagnetic spin ordering in the
manganite Pr$_{0.7}$Ca$_{0.3}$MnO$_3$ following ultrafast photo-exitation. Our
studies reveal a glass-like recovery of the spin ordering and a crossover in
the dimensionality of the restoring interaction from quasi-1D at low pump
fluence to 3D at high pump fluence. This behavior arises from the metastable
state created by photo-excitation, a state characterized by spin disordered
metallic droplets within the larger charge- and spin-ordered insulating
domains. Comparison with time-resolved resistivity measurements suggests that
the collapse of spin ordering is correlated with the insulator-to-metal
transition, but the recovery of the insulating phase does not depend on the
re-establishment of the spin ordering. | 1209.3452v2 |
2013-05-30 | Kelvin Probe Microscopy and Electronic Transport Measurements in Reduced Graphene Oxide Chemical Sensors | Reduced Graphene Oxide (RGO) is an electronically hybrid material that
displays remarkable chemical sensing properties. Here, we present a
quantitative analysis of the chemical gating effects in RGO-based chemical
sensors. The gas sensing devices are patterned in a field-effect transistor
geometry, by dielectrophoretic assembly of RGO platelets between gold
electrodes deposited on SiO2/Si substrates. We show that these sensors display
highly selective and reversible responses to the measured analytes, as well as
fast response and recovery times (tens of seconds). We use combined electronic
transport/Kelvin Probe Microscopy measurements to quantify the amount of charge
transferred to RGO due to chemical doping when the device is exposed to
electron-acceptor (acetone) and electron-donor (ammonia) analytes. We
demonstrate that this method allows us to obtain high-resolution maps of the
surface potential and local charge distribution both before and after chemical
doping, to identify local gate-susceptible areas on the RGO surface, and to
directly extract the contact resistance between the RGO and the metallic
electrodes. The method presented is general, suggesting that these results have
important implications for building graphene and other nanomaterial-based
chemical sensors. | 1305.7222v1 |
2014-04-02 | Direct observation of the leakage current in epitaxial diamond Schottky barrier devices by conductive-probe atomic force microscopy and Raman imaging | The origin of the high leakage current measured in several vertical-type
diamond Schottky devices is conjointly investigated by conducting probe atomic
force microscopy (CP-AFM) and confocal micro-Raman/Photoluminescence (PL)
imaging analysis. Local areas characterized by a strong decrease of the local
resistance (5-6 orders of magnitude drop) with respect to their close
surrounding have been identified in several different regions of the sample
surface. The same local areas, also referenced as electrical hot-spots, reveal
a slightly constrained diamond lattice and three dominant Raman bands in the
low-wavenumber region (590, 914 and 1040 cm-1). These latter bands are usually
assigned to the vibrational modes involving boron impurities and its possible
complexes that can electrically act as traps for charge carriers. Local
current-voltage measurements performed at the hot-spots point out a
trap-filled-limited (TFL) current as the main conduction mechanism favoring the
leakage current in the Schottky devices. | 1404.0472v2 |
2014-09-20 | Crystallization characteristics and chemical bonding properties of nickel carbide thin film nanocomposites | The crystal structure and chemical bonding of magnetron-sputtering deposited
nickel carbide Ni$_{1-x}$C$_{x}$ (0.05$\leq$x$\leq$0.62) thin films have been
investigated by high-resolution X-ray diffraction, transmission electron
microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, and soft
X-ray absorption spectroscopy. By using X-ray as well as electron diffraction,
we found carbon-containing hcp-Ni (hcp-NiC$_{y}$ phase), instead of the
expected rhombohedral-Ni$_{3}$C. At low carbon content (4.9 at\%) the thin film
consists of hcp-NiC$_{y}$ nanocrystallites mixed with a smaller amount of
fcc-NiC$_{x}$. The average grain size is about 10-20 nm. With the increase of
carbon content to 16.3 at\%, the film contains single-phase hcp-NiC$_{y}$
nanocrystallites with expanded lattice parameters. With further increase of
carbon content to 38 at\%, and 62 at\%, the films transform to X-ray amorphous
materials with hcp-NiC$_{y}$ and fcc-NiC$_{x }$ nanodomain structures in an
amorphous carbon-rich matrix. Raman spectra of carbon indicate dominant
$sp^{2}$ hybridization, consistent with photoelectron spectra that show a
decreasing amount of C-Ni phase with increasing carbon content. The Ni $3d$ - C
$2p$ hybridization in the hexagonal structure gives rise to the salient
double-peak structure in Ni $2p$ soft X-ray absorption spectra at 16.3 at\%
that changes with carbon content. We also show that the resistivity is not only
governed by the amount of carbon, but increases by more than a factor of two
when the samples transform from crystalline to amorphous. | 1409.5912v1 |
2015-03-04 | Signature of strong spin-orbital coupling in the large non-saturating magnetoresistance material WTe2 | We report the detailed electronic structure of WTe$_2$ by high resolution
angle-resolved photoemission spectroscopy. Unlike the simple one electron plus
one hole pocket type of Fermi surface topology reported before, we resolved a
rather complicated Fermi surface of WTe$_2$. Specifically, there are totally
nine Fermi pockets, including one hole pocket at the Brillouin zone center
$\Gamma$, and two hole pockets and two electron pockets on each side of
$\Gamma$ along the $\Gamma$-$X$ direction. Remarkably, we have observed
circular dichroism in our photoemission spectra, which suggests that the
orbital angular momentum exhibits a rich texture at various sections of the
Fermi surface. As reported previously for topological insulators and Rashiba
systems, such a circular dichroism is a signature for spin-orbital coupling
(SOC). This is further confirmed by our density functional theory calculations,
where the spin texture is qualitatively reproduced as the conjugate consequence
of SOC. Since the backscattering processes are directly involved with the
resistivity, our data suggest that the SOC and the related spin and orbital
angular momentum textures may be considered in the understanding of the
anomalous magnetoresistance of WTe$_2$. | 1503.01422v1 |
2015-11-03 | Two-dimensional epitaxial superconductor-semiconductor heterostructures: A platform for topological superconducting networks | Progress in the emergent field of topological superconductivity relies on
synthesis of new material combinations, combining superconductivity, low
density, and spin-orbit coupling (SOC). For example, theory [1-4] indicates
that the interface between a one-dimensional (1D) semiconductor (Sm) with
strong SOC and a superconductor (S) hosts Majorana modes with nontrivial
topological properties [5-8]. Recently, epitaxial growth of Al on InAs
nanowires was shown to yield a high quality S-Sm system with uniformly
transparent interfaces [9] and a hard induced gap, indicted by strongly
suppressed sub gap tunneling conductance [10]. Here we report the realization
of a two-dimensional (2D) InAs/InGaAs heterostructure with epitaxial Al,
yielding a planar S-Sm system with structural and transport characteristics as
good as the epitaxial wires. The realization of 2D epitaxial S-Sm systems
represent a significant advance over wires, allowing extended networks via
top-down processing. Among numerous potential applications, this new material
system can serve as a platform for complex networks of topological
superconductors with gate-controlled Majorana zero modes [1-4]. We demonstrate
gateable Josephson junctions and a highly transparent 2D S-Sm interface based
on the product of excess current and normal state resistance. | 1511.01127v2 |
2015-11-08 | Real-time Stress Measurements in Germanium Thin Film Electrodes during Electrochemical Lithiation/delithiation Cycling | An in situ study of stress evolution and mechanical behavior of germanium as
a lithium-ion battery electrode material is presented. Thin films of germanium
are cycled in a half-cell configuration with lithium metal foil as
counter/reference electrode, with 1M LiPF6 in ethylene carbonate, diethyl
carbonate, dimethyl carbonate solution (1:1:1, wt. %) as electrolyte. Real-time
stress evolution in the germanium thin-film electrodes during electrochemical
lithiation/delithiation is measured by monitoring the substrate curvature using
the multi-beam optical sensing method. Upon lithiation a-Ge undergoes extensive
plastic deformation, with a peak compressive stress reaching as high as -0.76
+/- 0.05 GPa (mean +/- standard deviation). The compressive stress decreases
with lithium concentration reaching a value of approximately -0.3 GPa at the
end of lithiation. Upon delithiation the stress quickly became tensile and
follows a trend that mirrors the behavior on compressive side; the average peak
tensile stress of the lithiated Ge samples was approximately 0.83 GPa. The peak
tensile stress data along with the SEM analysis was used to estimate a lower
bound fracture resistance of lithiated Ge, which is approximately 5.3 J/m^2. It
was also observed that the lithiated Ge is rate sensitive, i.e., stress depends
on how fast or slow the charging is carried out. | 1511.02442v1 |
2016-02-03 | Full range of proximity effect probed with Superconductor/Graphene/Superconductor junctions | The high tunability of the density of states of graphene makes it an ideal
probe of quantum transport in different regimes. In particular, the
supercurrent that can flow through a non-superconducting (N) material connected
to two superconducting electrodes, crucially depends on the lenghth of the N
relative to the superconducting coherence length. Using graphene as the N
material we have investigated the full range of the superconducting proximity
effect, from short to long diffusive junctions. By combining several
S/graphene/S samples with different contacts and lengths, and measuring their
gate-dependent critical currents ($ I_c $) and normal state resistance $ R_N $,
we compare the product $eR_NI_c$ to the relevant energies, the Thouless energy
in long junctions and the superconducting gap of the contacts in short
junctions, over three orders of magnitude of Thouless energy. The experimental
variations strikingly follow a universal law, close to the predictions of the
proximity effect both in the long and short junction regime, as well as in the
crossover region, thereby revealing the interplay of the different energy
scales. Differences in the numerical coefficients reveal the crucial role
played by the interfacial barrier between graphene and the superconducting
electrodes, which reduces the supercurrent in both short and long junctions.
Surprisingly the reduction of supercurrent is independent of the gate voltage
and of the nature of the electrodes. A reduced induced gap and Thouless energy
are extracted, revealing the role played by the dwell time in the barrier in
the short junction, and an effective increased diffusion time in the long
junction. We compare our results to the theoretical predictions of Usadel
equations and numerical simulations which better reproduce experiments with
imperfect NS interfaces. | 1602.01489v1 |
2016-02-16 | An Integrated Tantalum Sulfide - Boron Nitride - Graphene Oscillator: A Charge-Density-Wave Device Operating at Room Temperature | The charge-density-wave (CDW) phase is a macroscopic quantum state consisting
of a periodic modulation of the electronic charge density accompanied by a
periodic distortion of the atomic lattice in quasi-1D or layered 2D metallic
crystals. Several layered transition metal dichalcogenides, such as 1T-TaSe2,
1T-TaS2 and 1T-TiSe2, exhibit unusually high transition temperatures to
different CDW symmetry-reducing phases. These transitions can be affected by
environmental conditions, film thickness and applied electric bias. However,
device applications of these intriguing systems at room temperature or their
integration with other 2D materials have not been explored. Here we show that
in 2D CDW 1T-TaS2, the abrupt change in the electrical conductivity and
hysteresis at the transition point between nearly-commensurate and
incommensurate charge-density-wave phases can be used for constructing an
oscillator that operates at room temperature. The hexagonal boron nitride was
capped on 1T-TaS2 thin film to provide protection from oxidation, and an
integrated graphene transistor provides a voltage tunable, matched,
low-resistance load enabling precise voltage control of the oscillator
frequency. The integration of these three disparate two-dimensional materials,
in a way that exploits the unique properties of each, yields a simple,
miniaturized, voltage-controlled oscillator device. Theoretical considerations
suggest that the upper limit of oscillation frequency to be in the THz regime. | 1602.05147v1 |
2016-05-05 | Structural, optical and complex impedance spectroscopy study of multiferroic Bi2Fe4O9 ceramic | Multiferroic bismuth ferrite Bi_2Fe_4O_9 (BFO) ceramic was synthesized by
conventional solid state reaction route. X-ray diffraction and Rietveld
refinement show formation of single phase ceramic with orthorhombic crystal
structure (space group Pbam). The morphological study depicted a well-defined
grain of size $\simeq$2{\mu}m. The optical studies were carried out by using
UV-Vis spectrophotometer which shows a band gap of 1.53 eV and a green emission
spectrum at 537 is observed in the Photoluminescence study. The frequency
dependent dielectric study at various temperature revealed that the dielectric
constant decreases with increase in frequency. A noticeable peak shift towards
higher frequency with increasing temperature is observed in the frequency
dependent dielectric loss plot. The impedance spectroscopy shows a substantial
shift in imaginary impedance (Z") peaks toward the high frequency side
described that the conduction in material favoring the long range motion of
mobile charge carriers. The presence of non-Debye type multiple relaxations has
been confirmed by complex modulus analysis. The frequency dependent ac
conductivity at different temperatures indicates that the conduction process is
thermally activated. The variation of dc conductivity exhibited a negative
temperature coefficient of resistance behavior. The activation energy
calculated from impedance, modulus and conductivity data confirmed that the
oxygen vacancies play a vital role in the conduction mechanism. | 1605.01574v1 |
2016-08-24 | Engineering quantum spin Hall insulators by strained-layer heterostructures | Quantum spin Hall insulators (QSHIs), also known as two-dimensional
topological insulators, have emerged as an unconventional class of quantum
states with insulating bulk and conducting edges originating from nontrivial
inverted band structures, and have been proposed as a platform for exploring
spintronics applications and exotic quasiparticles related to the spin-helical
edge modes. Despite theoretical proposals for various materials, however,
experimental demonstrations of QSHIs have so far been limited to two
systems--HgTe/CdTe and InAs/GaSb--both of which are lattice-matched
semiconductor heterostructures. Here we report transport measurements in yet
another realization of a band-inverted heterostructure as a QSHI
candidate--InAs/In$_{x}$Ga$_{1-x}$Sb with lattice mismatch. We show that the
compressive strain in the In$_{x}$Ga$_{1-x}$Sb layer enhances the band overlap
and energy gap. Consequently, high bulk resistivity, two orders of magnitude
higher than for InAs/GaSb, is obtained deep in the band-inverted regime. The
strain also enhances bulk Rashba spin-orbit splitting, leading to an unusual
situation where the Fermi level crosses only one spin branch for electronlike
and holelike bands over a wide density range. These properties make this system
a promising platform for robust QSHIs with unique spin properties and
demonstrate strain to be an important ingredient for tuning spin-orbit
interaction. | 1608.06751v2 |
2016-11-29 | Magnetoelectric effect in antiferromagnetic multiferroic Pb(Fe1/2Nb1/2)O3 and its solid solutions with PbTiO3 | Antiferromagnets (AFMs) are presently considered as promising materials for
applications in spintronics and random access memories due to the robustness of
information stored in AFM state against perturbing magnetic fields (P. Wadley
et al., Science 351, 587 (2016)). In this respect, AFM multiferroics maybe
attractive alternatives for conventional AFMs as the coupling of magnetism with
ferroelectricity (magnetoelectric effect) offers an elegant possibility of
electric field control and switching of AFM domains. Here we report the results
of comprehensive experimental and theoretical investigations of the quadratic
magnetoelectric (ME) effect in single crystals and high-resistive ceramics of
Pb(Fe1/2Nb1/2)O3 (PFN) and (1- x)Pb(Fe1/2Nb1/2)O3xPbTiO3 (PFNxPT). We are
interested primarily in the temperature range of multiferroic phase, T < 150 K,
where the ME coupling coefficient is extremely large (as compared to well-known
multiferroic BiFeO3) and shows sign reversal at
paramagnetic-to-antiferromagnetic phase transition. Moreover, we observe strong
ME response nonlinearity in the AFM phase in the magnetic fields of only few
kOe. To describe the temperature and magnetic field dependencies of the above
unusual features of ME effect in PFN and PFN-xPT, we use simple
phenomenological Landau approach which explains experimental data surprisingly
well. Our ME measurements demonstrate that the electric field of only 20-25
kV/cm is able to switch the AFM domains and align them with ferroelectric ones
even in PFN ceramic samples. | 1611.09899v1 |
2017-11-21 | Nanofeatures Induced by Severe Shot Peening (SSP) on Magnesium Alloy AZ31 | Considering the sensitivity of both fatigue strength and corrosion rate to
the surface characteristics, apposite surface treatments could address the
related challenges for biodegradable magnesium-based materials. Herein, we
treated the surface of a biocompatible magnesium alloy by a low cost and
versatile severe plastic deformation technique, severe shot peening, to
evaluate the potential of surface grain refinement to enhance functionality in
biological environment. The evolution of surface grain structure and surface
morphology were investigated using optical as well as scanning and transmission
electron microscopy. Surface roughness, wettability and chemical composition,
as well as in depth-microhardness and residual stress distribution, and
corrosion resistance were investigated. Successive light surface grinding was
used after severe shot peening to eliminate the effect of surface roughness and
separately investigate the influence of grain refinement alone.
Cytocompatibility tests with osteoblasts (or bone forming cells) were performed
using sample extracts. Results revealed for the first time that severe shot
peening can significantly enhance mechanical properties without causing adverse
effects to the growth of surrounding osteoblasts. The corrosion behavior, on
the other hand, was not improved by severe shot peening; nevertheless, slight
grinding of the rough surface layer with a high density of crystallographic
lattice defects, without removing the entire nanocrystallized layer, provided a
good potential for improving corrosion characteristics after severe shot
peening and thus, this method should be studied for a wide range of orthopedic
applications in which biodegradable magnesium is used. | 1712.02834v1 |
2017-12-13 | Quantum effects on dislocation motion from Ring-Polymer Molecular Dynamics | Quantum motion of atoms known as zero-point vibrations is recognized to be
important at low temperatures in condensed matter systems comprised of light
atoms or ions, affecting such properties and behaviors as proton-transfer
reactions, vibrational spectra of water and ice, and mechanical properties of
low temperature helium. Recently, quantum motion of atoms was proposed to
explain a long-standing discrepancy between theoretically computed and
experimentally measured low-temperature resistance (Peierls stress) to
dislocation motion in iron and possibly other metals with high atomic masses.
Here we report the first direct simulations of quantum motion of screw
dislocations in iron within the exact formalism of Ring-Polymer Molecular
Dynamics (RPMD) that rigorously accounts for quantum effects on the statistics
of condensed-phase systems. Our quantum RPMD simulations predict only a modest
($\approx\!13\%$) reduction in the Peierls stress in iron compared to its fully
classical prediction. Our simulations confirm that reduction in the Peierls
stress solely due to the zero-point energy is close to $50\%$ predicted
earlier, but its effect is substantially offset by an increase in the effective
atom size with decreasing temperature, an effect known as quantum dispersion.
Thus, quantum motion of atoms does not resolve the notorious discrepancy
between theoretical and experimental values of the Peierls stress in iron. | 1712.04629v2 |
2018-02-06 | Quasiparticle dynamics in granular aluminum close to the superconductor to insulator transition | Superconducting high kinetic inductance elements constitute a valuable
resource for quantum circuit design and millimeter-wave detection. Granular
aluminum (GrAl) in the superconducting regime is a particularly interesting
material since it has already shown a kinetic inductance in the range of
nH$/\Box$ and its deposition is compatible with conventional Al/AlOx/Al
Josephson junction fabrication. We characterize microwave resonators fabricated
from GrAl with a room temperature resistivity of $4 \times
10^3\,\mu\Omega\cdot$cm, which is a factor of 3 below the superconductor to
insulator transition, showing a kinetic inductance fraction close to unity. The
measured internal quality factors are on the order of $Q_{\mathrm{i}} = 10^5$
in the single photon regime, and we demonstrate that non-equilibrium
quasiparticles (QP) constitute the dominant loss mechanism. We extract QP
relaxation times in the range of 1 s and we observe QP bursts every $\sim 20$
s. The current level of coherence of GrAl resonators makes them attractive for
integration in quantum devices, while it also evidences the need to reduce the
density of non-equilibrium QPs. | 1802.01858v2 |
2018-01-06 | Apically Dominant Mechanism for Improving Catalytic Activities of N-Doped Carbon Nanotube Arrays in Rechargeable Zinc-Air Battery | The oxygen reduction (ORR) and oxygen evolution reactions (OER) in Zn-air
batteries (ZABs) require highly efficient, cost-effective and stable
electrocatalysts as replacements to traditionally high cost, inconsistently
stable and low poison resistant Platinum group metals (PGM) catalysts.
Although, nitrogen-doped carbon nanotube (NCNT) arrays have been developed over
recent decades through various advanced technologies are now capable of
catalyzing ORR efficiently, their underdeveloped bifunctional property,
hydrophobic surface, and detrimental preparation strategy are found to limit
practical large-scale commercialization for effective rechargeable ZABs. Here,
we have demonstrated fabrication of a three-dimensional (3D) nickel foam
supported NCNT arrays with CoNi nanoparticles (NPs) encapsulated within the
apical domain (denoted as CoNi@NCNT/NF) that exhibits excellent bifunctional
catalytic performance toward both ORR (onset potential of 0.97 V vs. RHE) and
OER (overpotential of 1.54 V vs. RHE at 10 mA cm$^{-2}$). We further examined
the practicability of this CoNi@NCNT/NF material being used as an air electrode
for rechargeable ZAB coin cell and pouch cell systems. The ZAB coin cell showed
a peak power density of 108 mW cm$^{-2}$ with an energy density of 845 Wh
kg$_{Zn}^{-1}$ and robust rechargeability over 28h under ambient conditions,
which exceeds the performance of PGM catalysts and leading non-PGM
electrocatalysts. In addition, density functional theory (DFT) calculations
revealed that the ORR and OER catalytic performance of the CoNi@NCNT/NF
electrode are mainly derived from the d-orbitals from the CoNi NPs encapsulated
within the apical dominant end of the NCNTs. | 1802.02063v1 |
2018-03-06 | Magic-angle graphene superlattices: a new platform for unconventional superconductivity | The understanding of strongly-correlated materials, and in particular
unconventional superconductors, has puzzled physicists for decades. Such
difficulties have stimulated new research paradigms, such as ultra-cold atom
lattices for simulating quantum materials. Here we report on the realization of
intrinsic unconventional superconductivity in a 2D superlattice created by
stacking two graphene sheets with a small twist angle. For angles near
$1.1^\circ$, the first `magic' angle, twisted bilayer graphene (TBG) exhibits
ultra-flat bands near charge neutrality, which lead to correlated insulating
states at half-filling. Upon electrostatic doping away from these correlated
insulating states, we observe tunable zero-resistance states with a critical
temperature $T_c$ up to 1.7 K. The temperature-density phase diagram shows
similarities with that of the cuprates, including superconducting domes.
Moreover, quantum oscillations indicate small Fermi surfaces near the
correlated insulating phase, in analogy with under-doped cuprates. The relative
high $T_c$, given such small Fermi surface (corresponding to a record-low 2D
carrier density of $10^{11} \textrm{cm}^{-2}$ , renders TBG among the strongest
coupling superconductors, in a regime close to the BCS-BEC crossover. These
novel results establish TBG as the first purely carbon-based 2D superconductor
and as a highly tunable platform to investigate strongly-correlated phenomena,
which could lead to insights into the physics of high-$T_c$ superconductors and
quantum spin liquids. | 1803.02342v2 |
2018-03-31 | The Role of Crystal Orientation in the Dissolution of UO$_2$ Thin Films | Epitaxial thin films have been utilised to investigate the radiolytic
dissolution of uranium dioxide interfaces. Thin films of UO$_2$ deposited on
single crystal yttria stabilised zirconia substrates have been exposed to water
in the presence of a high flux, monochromatic, synchrotron x-ray source. In
particular, this technique was applied to induce dissolution of three UO$_2$
thin films, grown along the principle UO$_2$ crystallographic orientations:
(001), (110) and (111). Dissolution of each film was induced for 9 accumulative
corrosion periods, totalling 270s, after which XRR spectra were recorded to
observe the change in morphology of the films as a function of exposure time.
While the (001) and (110) oriented films were found to corrode almost linearly
and at comparable rates, the (111) film was found to be significantly more
corrosion resistant, with no loss of UO$_2$ material being observed after the
initial 90s corrosion period. These results distinctly show the effect of
crystallographic orientation on the rate of x-ray induced UO$_2$ dissolution.
This result may have important consequences for theoretical dissolution models,
as it is evident that orientation dependence must be taken into consideration
to obtain accurate predictions of the dissolution behaviour of UO$_2$. | 1804.00201v1 |
2018-05-24 | Impact of thermal fluctuations on transport in antiferromagnetic semimetals | Recent demonstrations on manipulating antiferromagnetic (AF) order have
triggered a growing interest in antiferromagnetic metal (AFM), and potential
high-density spintronic applications demand further improvements in the
anisotropic magnetoresistance (AMR). The antiferromagnetic semimetals (AFS) are
newly discovered materials that possess massless Dirac fermions that are
protected by the crystalline symmetries. In this material, a reorientation of
the AF order may break the underlying symmetries and induce a finite energy
gap. As such, the possible phase transition from the semimetallic to insulating
phase gives us a choice for a wide range of resistance ensuring a large AMR. To
further understand the robustness of the phase transition, we study thermal
fluctuations of the AF order in AFS at a finite temperature. For macroscopic
samples, we find that the thermal fluctuations effectively decrease the
magnitude of the AF order by renormalizing the effective Hamiltonian. Our
finding suggests that the insulating phase exhibits a gap narrowing at elevated
temperatures, which leads to a substantial decrease in AMR. We also examine
spatially correlated thermal fluctuations for microscopic samples by solving
the microscopic Landau-Lifshitz-Gilbert equation finding a qualitative
difference of the gap narrowing in the insulating phase. For both cases, the
semimetallic phase shows a minimal change in its transmission spectrum
illustrating the robustness of the symmetry protected states in AFS. Our
finding may serve as a guideline for estimating and maximizing AMR of the AFS
samples at elevated temperatures. | 1805.09826v1 |
2018-09-03 | Imparting icephobicity with substrate flexibility | Ice accumulation hinders the performance of, and poses safety threats for
infrastructure both on the ground and in the air. Previously, rationally
designed superhydrophobic surfaces have demonstrated some potential as a
passive means to mitigate ice accretion; however, further studies on material
solutions that reduce impalement and contact time for impacting supercooled
droplets and can also repel droplets that freeze during surface contact are
urgently needed. Here we demonstrate the collaborative effect of substrate
flexibility and surface nanotexture on enhancing both icephobicity and the
repellency of viscous droplets. We first investigate the influence of increased
viscosity on impalement resistance and droplet-substrate contact time after
impact. Then we examine the effect of droplet partial solidification on recoil
and simulate more challenging icing conditions by impacting supercooled water
droplets onto flexible and rigid surfaces containing ice nucleation promoters.
We demonstrate a passive mechanism for shedding partially solidified droplets
under conditions where partial solidification occurs much faster than the
natural droplet oscillation which does not rely on converting droplet surface
energy into kinetic energy. Using an energy-based model, we identify a
previously unexplored mechanism whereby the substrate oscillation and velocity
govern the rebound process, with low-areal density and moderately stiff
substrates acting to efficiently absorb the incoming droplet kinetic energy and
rectify it back, allowing droplets to overcome adhesion and gravitational
forces, and recoil. This mechanism applies for a range of droplet viscosities,
spanning from low to high viscosity fluids and even ice slurries, which do not
rebound from rigid superhydrophobic substrates. | 1809.00490v1 |
2018-11-12 | Antiferromagnet-based spintronic functionality by controlling isospin domains in a layered perovskite iridate | The novel electronic state of the canted antiferromagnetic (AFM) insulator,
strontium iridate (Sr2IrO4) has been well described by the spin-orbit-entangled
isospin Jeff = 1/2, but the role of isospin in transport phenomena remains
poorly understood. In this study, antiferromagnet-based spintronic
functionality is demonstrated by combining unique characteristics of the
isospin state in Sr2IrO4. Based on magnetic and transport measurements, large
and highly anisotropic magnetoresistance (AMR) is obtained by manipulating the
antiferromagnetic isospin domains. First-principles calculations suggest that
electrons whose isospin directions are strongly coupled to in-plane net
magnetic moment encounter the isospin mismatch when moving across
antiferromagnetic domain boundaries, which generates a high resistance state.
By rotating a magnetic field that aligns in-plane net moments and removes
domain boundaries, the macroscopically-ordered isospins govern dynamic
transport through the system, which leads to the extremely angle-sensitive AMR.
As with this work that establishes a link between isospins and magnetotransport
in strongly spin-orbit-coupled AFM Sr2IrO4, the peculiar AMR effect provides a
beneficial foundation for fundamental and applied research on AFM spintronics. | 1811.04562v1 |
2018-11-30 | Spin Gapless Semiconducting Nature in Co-rich Co1+xFe1-xCrGa: Insight and Advancements | In this report, we present structural, electronic, magnetic and transport
properties of Co-rich spin gapless semiconductor CoFeCrGa using both
theoretical and experimental techniques. The key advantage of Co-rich samples
$\mathrm{Co_{1+x}Fe_{1-x}CrGa}$ is the high Curie temperature (T$\mathrm{_C}$)
and magnetization, without compromising the SGS nature (up to x = 0.4), and
hence our choice. The quaternary Heusler alloys $\mathrm{Co_{1+x}Fe_{1-x}CrGa}$
(x = 0.1 to 0.5) are found to crystallize in LiMgPdSn-type structure having
space group $F\bar{4}3m$ (\# 216). The measured Curie temperature increases
from 690 K (x = 0) to 870 K (x = 0.5). Observed magnetization values follow the
Slater-Pauling rule. Measured electrical resistivity, in the temperature range
of 5-350 K, suggests that the alloys retain the SGS behavior up to x = 0.4,
beyond which it reflects metallic character. Unlike conventional
semiconductors, the conductivity value ($\mathrm{\sigma_{xx}}$) at 300 K lies
in the range of 2289 S $\mathrm{cm^{-1}}$ to 3294 S $\mathrm{cm^{-1}}$, which
is close to that of other reported SGS materials. The anomalous Hall effect is
comparatively low. The intrinsic contribution to the anomalous Hall
conductivity increase with x, which can be correlated with the enhancement in
chemical order. The anomalous Hall coefficient is found to increase from 38
S/cm for x = 0.1 to 43 S/cm for 0.3. Seebeck coefficients turn out to be
vanishingly small below 300 K, another signature for being SGS. All the alloys
(for different x) are found to be both chemically and thermally stable.
Simulated magnetization agrees fairly with the experiment. As such Co-rich
CoFeCrGa is a promising candidate for room temperature spintronic applications,
with enhanced T$\mathrm{_C}$, magnetic properties and SGS nature. | 1811.12684v1 |
2019-09-24 | Spin-transfer dynamics in MgO-based magnetic tunnel junctions with an out-of-plane magnetized free layer and an in-plane polarizer | Here, we present an analytical and numerical model describing the
magnetization dynamics in MgO-based spin-torque nano-oscillators with an
in-plane magnetized polarizer and an out-of-plane free layer. We introduce the
spin-transfer torque asymmetry by considering the cosine angular dependence of
the resistance between the two magnetic layers in the stack. For the analytical
solution, dynamics are determined by assuming a circular precession trajectory
around the direction perpendicular to the plane, as set by the effective field,
and calculating the energy integral over a single precession period. In a more
realistic approach, we include the bias dependence of the tunnel
magnetoresistance, which is assumed empirically to be a piecewise linear
function of the applied voltage. The dynamical states are found by solving the
stability condition for the Jacobian matrix for out-of-plane static states. We
find that the bias dependence of the tunnel magnetoresistance, which is an
inseparable effect in every tunnel junction, exhibits drastic impact on the
spin-torque nano-oscillator phase diagram, mainly by increasing the critical
current for dynamics and quenching the oscillations at high currents. The
results are in good agreement with our experimental data published elsewhere. | 1909.10983v1 |
2020-03-16 | Unconventional Hall response in the quantum limit of HfTe5 | Interacting electrons confined to their lowest Landau level in a high
magnetic field can form a variety of correlated states, some of which manifest
themselves in a Hall effect. Although such states have been predicted to occur
in three dimensional semimetals, a corresponding Hall response has not yet been
experimentally observed. Here, we report the observation of an unconventional
Hall response in the quantum limit of the bulk semimetal HfTe5, adjacent to the
three-dimensional quantum Hall effect of a single electron band at low magnetic
fields. The additional plateau-like feature in the Hall conductivity of the
lowest Landau level is accompanied by a Shubnikov-de Haas minimum in the
longitudinal electrical resistivity and its magnitude relates as 3/5 to the
height of the last plateau of the three-dimensional quantum Hall effect. Our
findings are consistent with strong electron-electron interactions, stabilizing
an unconventional variant of the Hall effect in a three-dimensional material in
the quantum limit. | 2003.07213v3 |
2017-03-07 | Fast multicolor photodetectors based on graphene-contacted p-GaSe/n-InSe van der Waals heterostructures | The integration of different two-dimensional materials within a multilayer
van der Waals (vdW) heterostructure offers a promising technology for realizing
high performance opto-electronic devices such as photodetectors and light
sources1-3. Transition metal dichalcogenides, e.g. MoS2 and WSe2, have been
employed as the optically-active layer in recently developed heterojunctions.
However, MoS2 and WSe2 become direct band gap semiconductors only in mono- or
bilayer form4,5. In contrast, the metal monochalcogenides InSe and GaSe retain
a direct bandgap over a wide range of layer thicknesses from bulk crystals down
to exfoliated flakes only a few atomic monolayers thick6,7. Here we report on
vdW heterojunction diodes based on InSe and GaSe: the type II band alignment
between the two materials and their distinctive spectral response, combined
with the low electrical resistance of transparent graphene electrodes, enable
effective separation and extraction of photoexcited carriers from the
heterostructure even when no external voltage is applied. Our devices are fast
(< 10 {\mu}s), self-driven photodetectors with multicolor photoresponse ranging
from the ultraviolet to the near-infrared and have the potential to accelerate
the exploitation of two-dimensional vdW crystals by creating new routes to
miniaturized optoelectronics beyond present technologies. | 1703.02534v1 |
2017-03-28 | Co-appearance of superconductivity and ferromagnetism in a Ca$_2$RuO$_4$ nanofilm crystal | By tuning the physical and chemical pressures of layered perovskite materials
we can realize the quantum states of both superconductors and insulators. By
reducing the thickness of a layered crystal to a nanometer level, a nanofilm
crystal can provide novel quantum states that have not previously been found in
bulk crystals. Here we report the realization of high-temperature
superconductivity in Ca$_2$RuO$_4$ nanofilm single crystals. Ca$_2$RuO$_4$ thin
film with the highest transition temperature $T_c$ (midpoint) of 64~K exhibits
zero resistance in electric transport measurements. The superconducting
critical current exhibited a logarithmic dependence on temperature and was
enhanced by an external magnetic field. Magnetic measurements revealed a
ferromagnetic transition at 180~K and diamagnetic magnetization due to
superconductivity. Our results suggest the co-appearance of superconductivity
and ferromagnetism in Ca$_2$RuO$_4$ nanofilm crystals. We also found that the
induced bias current and the tuned film thickness caused a
superconductor-insulator transition. The fabrication of micro-nanocrystals made
of layered material enables us to discuss rich superconducting phenomena in
ruthenates. | 1703.09459v2 |
2018-10-16 | The superconductor-superinsulator transition: S-duality and QCD on the desktop | We show that the nature of quantum phases around the superconductor-insulator
transition (SIT) is controlled by charge-vortex topological interactions and
does not depend on the details of material parameters and disorder. We find
three distinct phases, superconductor, superinsulator and bosonic topological
insulator. The superinsulator is a state of matter with infinite resistance in
a finite temperature range, which is the S-dual of the superconductor and in
which charge transport is prevented by electric strings binding charges of
opposite sign. The electric strings ensuring linear confinement of charges are
generated by instantons and are dual to superconducting Abrikosov vortices.
Material parameters and disorder enter the London penetration depth of the
superconductor, the string tension of the superinsulator and the quantum
fluctuation parameter driving the transition between them. They are entirely
encoded in four phenomenological parameters of a topological gauge theory of
the SIT. Finally, we point out that, in the context of strong coupling gauge
theories, the many-body localization phenomenon that is often referred to as an
underlying mechanism for superinsulation is a mere transcription of the
well-known phenomenon of confinement into solid state physics language and is
entirely driven by endogenous disorder embodied by instantons with no need of
exogenous disorder. | 1810.06862v1 |
2020-04-14 | Intrinsically Activated SrTiO3: Photocatalytic H2 Evolution from Neutral Aqueous Methanol Solution in the Absence of Any Noble Metal Cocatalyst | Noble metal cocatalysts are conventionally a crucial factor in
oxide-semiconductor-based photocatalytic hydrogen generation. In the present
work, we show that optimized high-temperature hydrogenation of commercially
available strontium titanate (SrTiO3) powder can be used to engineer an
intrinsic cocatalytic shell around nanoparticles that can create a
photocatalyst that is highly effective without the use of any additional
cocatalyst for hydrogen generation from neutral aqueous methanol solutions.
This intrinsic activation effect can also be observed for SrTiO3[100] single
crystal as well as Nb-doped SrTiO3 (100) single crystal. For all types of
SrTiO3 samples (nanopowders and either of the single crystals), hydrogenation
under optimum conditions leads to a surface-hydroxylated layer together with
lattice defects visible by transmission electron microscopy, electron
paramagnetic resonance (EPR), and photoluminescence (PL). Active samples
provide states in a defective matrix -- this is in contrast to the inactive
defects formed in other reductive atmospheres. In aqueous media, active SrTiO3
samples show a significant negative shift of the flatband potential (in
photoelectrochemical as well as in capacitance data) and a lower
charge-transfer resistance for photoexcited electrons. We therefore ascribe the
remarkable cocatalyst-free activation of the material to a synergy between
thermodynamics (altered interface energetics induced by hydroxylation) and
kinetics (charge transfer mediation by suitable Ti3+ states). | 2004.06573v1 |
2022-02-10 | Dynamics of Multi-Domains in Ferroelectric Tunnel Junction | The Discovery of giant tunnel electroresistance (TER) in Ferroelectric Tunnel
Junction (FTJ) paves a futuristic possibility of utilizing the FTJ as a
bistable resistive device with an enormously high ON/OFF ratio. In the last 20
years, numerous studies have reported that the formation of multidomain in
ferroelectric material is an inevitable process to minimize the total system
energy. Recent studies based on phase-field simulations have demonstrated that
domain nucleation/motion substantially alters the electrostatics of a
ferroelectric material. However, the impact of domain dynamics on quantum
transport in FTJ remains elusive. This paper presents a comprehensive study of
multidomain dynamics in a ferroelectric tunnel junction. Analysis of this
article is twofold; firstly, we study the impact of domain dynamics on
electrostatics in an FTJ. Subsequently, the obtained electrostatics is used to
study the variations in tunneling current, and TER originated from multidomain
dynamics. We show that ON/OFF current density and TER vary locally in the
ferroelectric region. Furthermore, the device's electrostatics and quantum
transport exhibit an oscillatory nature due to periodic domain texture. ON/OFF
current density shows a sine/cosine distribution in ferroelectric, and
approximately one-decade local variation in current density is observed. These
local fluctuations in current density cause oscillations in the device's ON/OFF
ratio. Optimization techniques to achieve a uniform and maximum TER are also
discussed. A 2D analytical and explicit model is derived by solving coupled 2D
Poisson's equation and Landau-Ginzburg equation. The model incorporates the
switching and nucleation of domains by minimizing net ferroelectric energy
(depolarization+free+gradient energy density). Furthermore, the impact of the
bottom insulator layer on ferroelectric's gradient energy is also studied. | 2202.04926v1 |
2022-02-15 | A Ta-TaS2 monolithic catalyst with robust and metallic interface for superior hydrogen evolution | The use of highly active and robust catalysts is crucial for producing green
hydrogen by water electrolysis as we strive to achieve global carbon
neutrality. Noble metals like platinum are currently used in industry for the
hydrogen evolution reaction (HER), but suffer from scarcity, high price and
unsatisfied performance and stability at large current density, restricting
their large scale implementations. Here we report the synthesis of a new type
of monolithic catalyst (MC) consisting of a metal disulfide (e.g., TaS2)
catalyst vertically bonded to a conductive substrate of the same metal by
strong covalent bonds. These features give the MC a mechanically robust and
electrically near zero resistance interface, leading to an outstanding HER
performance including rapid charge transfer and excellent durability, together
with a low overpotential of 398 mV to achieve a current density of 2,000 mA
cm-2 as required by industry. The Ta TaS2 MC has a negligible performance decay
after 200 h operation at large current densities. In light of its unique
interface and the various choice of metal elements giving the same structure,
such monolithic materials may have broad uses besides catalysis. | 2202.07339v1 |
2017-04-19 | Potential Fluctuations at Low Temperatures in Mesoscopic-Scale SmTiO$_{3}$/SrTiO$_{3}$/SmTiO$_{3}$ Quantum Well Structures | Heterointerfaces of SrTiO$_{3}$ with other transition metal oxides make up an
intriguing family of systems with a bounty of coexisting and competing physical
orders. Some examples, such as LaAlO$_{3}$/SrTiO$_{3}$, support a high carrier
density electron gas at the interface whose electronic properties are
determined by a combination of lattice distortions, spin-orbit coupling,
defects, and various regimes of magnetic and charge ordering. Here, we study
electronic transport in mesoscale devices made with heterostructures of
SrTiO$_{3}$ sandwiched between layers of SmTiO$_{3}$, in which the transport
properties can be tuned from a regime of Fermi-liquid like resistivity ($\rho
\sim T^{2}$) to a non-Fermi liquid ($\rho \sim T^{5/3}$) by controlling the
SrTiO$_{3}$ thickness. In mesoscale devices at low temperatures, we find
unexpected voltage fluctuations that grow in magnitude as $T$ is decreased
below 20 K, are suppressed with increasing contact electrode size, and are
independent of the drive current and contact spacing distance.
Magnetoresistance fluctuations are also observed, which are reminiscent of
universal conductance fluctuations but not entirely consistent with their
conventional properties. Candidate explanations are considered, and a mechanism
is suggested based on mesoscopic temporal fluctuations of the Seebeck
coefficient. An improved understanding of charge transport in these model
systems, especially their quantum coherent properties, may lead to insights
into the nature of transport in strongly correlated materials that deviate from
Fermi liquid theory. | 1704.05744v1 |
2017-08-06 | Electrical transport and optical band gap of NiFe$_\textrm{2}$O$_\textrm{x}$ thin films | We fabricated NiFe$_\textrm{2}$O$_\textrm{x}$ thin films on
MgAl$_2$O$_4$(001) substrates by reactive dc magnetron co-sputtering varying
the oxygen partial pressure during deposition. The fabrication of a variable
material with oxygen deficiency leads to controllable electrical and optical
properties which would be beneficial for the investigations of the transport
phenomena and would, therefore, promote the use of such materials in spintronic
and spin caloritronic applications. We used several characterization techniques
in order to investigate the film properties, focusing on their structural,
magnetic, electrical, and optical properties. From the electrical resistivity
measurements we obtained the conduction mechanisms that govern the systems in
high and low temperature regimes, extracting low thermal activation energies
which unveil extrinsic transport mechanisms. The thermal activation energy
decreases in the less oxidized samples revealing the pronounced contribution of
a large amount of electronic states localized in the band gap to the electrical
conductivity. Hall effect measurements showed the mixed-type semiconducting
character of our films. The optical band gaps were determined via
ultraviolet-visible spectroscopy. They follow a similar trend as the thermal
activation energy, with lower band gap values in the less oxidized samples. | 1708.01937v1 |
2019-03-05 | All-optical cryogenic thermometry based on NV centers in nanodiamonds | The nitrogen-vacancy (NV) center in diamond has been recognized as a
high-sensitivity nanometer-scale metrology platform. Thermometry has been a
recent focus, with attention largely confined to room temperature applications.
Thermometry has been a recent focus, with attention largely confined to room
temperature applications. Temperature sensing at low temperatures, however,
remains challenging as the sensitivity decreases for many commonly used
techniques which rely on a temperature dependent frequency shift of the NV
centers spin resonance and its control with microwaves. Here we use an
alternative approach that does not require microwaves, ratiometric all-optical
thermometry, and demonstrate that it may be utilized to liquid nitrogen
temperatures without deterioration of the sensitivity. The use of an array of
nanodiamonds embedded within a portably polydimethylsiloxane (PDMS) sheet
provides a versatile temperature sensing platform that can probe a wide variety
of systems without the configurational restrictions needed for applying
microwaves. With this device, we observe a temperature gradient over tens of
microns in a ferromagnetic-insulator substrate (YIG) under local heating by a
resistive heater. This thermometry technique provides a cryogenically
compatible, microwave-free, minimally invasive approach capable of probing
local temperatures with few restrictions on the substrate materials. | 1903.01605v1 |
2021-01-03 | 3D Fluorescent Mapping of Invisible Molecular Damage after Cavi-tation in Hydrogen Exposed Elastomers | Elastomers saturated with gas at high pressure suffer from cavity nucleation,
inflation, and deflation upon rapid or explosive de-compression. Although this
process often results in undetectable changes in appearance, it causes internal
damage, hampers func-tionality (e.g., permeability), and shortens lifetime.
Here, we tag a model poly(ethyl acrylate) elastomer with {\pi}-extended
anthracene-maleimide adducts that fluoresce upon polymer chain scission, and
map in 3D the internal damage present after a cycle of gas satu-ration and
rapid decompression. Interestingly, we observe that each cavity observable
during the decompression results in a dam-aged region, the shape of which
reveals a fracture locus of randomly oriented penny-shape cracks (i.e., with a
flower-like morpholo-gy) that contain crack arrest lines. Thus, cavity growth
likely proceeds discontinuously (i.e., non-steadily) through the stable and
unstable fracture of numerous 2D crack planes. This non-destructive methodology
to visualize in 3D molecular damage in polymer networks is novel and serves to
understand how fracture occurs under complex 3D loads, predict mechanical aging
of pristine look-ing elastomers, and holds potential to optimize
cavitation-resistant materials. | 2101.00709v2 |
2021-01-28 | Macroscopic yarns of FeCl$_{3}$-intercalated collapsed carbon nano-tubes with high doping and stability | Macroscopic arrays of highly crystalline nanocarbons offer the possibility of
modifying the electronic structure of their low dimensional constituents, for
example through doping, and studying the resulting collective bulk behaviour.
Insertion of electron donors or acceptors between graphitic layers is an
attractive method to reversibly increase charge carrier concentra-tion without
disruption of the sp$2$-conjugated system. This work demonstrates FeCl$_{3}$
intercalation into fibres made up of collapsed (flattened) carbon nanotubes.
The bundles of collapsed CNTs, similar to crystallites of graphitic
nanoribbons, host elongated layered FeCl$_{3}$ crystals of hundreds of $nm$
long, much longer than previous reports on graphitic materials and directly
observable by transmission electron microscopy and X-ray diffraction.
Intercalated CNT fibres remain stable after months of exposure to ambient
conditions, partly due to the spontaneous formation of passivating monolayers
of FeClO at the crystal edge, preventing both desorption of intercalant and
further hydrolysis. Raman spectroscopy shows substantial electron transfer from
the CNTs to FeCl$_{3}$, a well-known acceptor, as observed by G band upshifts
as large as $25 cm^{-1}$. After resolving Raman features for the inner and
outer layers of the collapsed CNTs, strain and dynamic effect contributions of
charge transfer to the Raman upshift could be decoupled, giving a Fermi level
downshift of $- 0.72 eV$ and a large average free carrier concentration of
$5.3X10^{13}$ $cm^{-2}$ ($0.014$ electrons per carbon atom) in the intercalated
system. Four-probe resistivity measurements show an increase in conductivity by
a factor of six upon intercalation | 2101.12077v1 |
2021-01-29 | Doping isolated one-dimensional antiferro-magnetic semiconductor Vanadium tetrasulfide ($VS_4$) nanowires with carriers induces half-metallicity | Quasi one-dimensional (1D) vanadium tetrasulfide ($VS_4$) nanowires (NWs) are
synthetic semiconductors which combine with each other through Van der Waals
interactions to form bulk phases. However, the properties of these individual
nanowires remain unknown. Nevertheless, our calculations of their stability
indicate that $VS_4$) NWs can be separated from their bulk structures.
Accordingly, we theoretically investigated the geometrical, electronic, and
magnetic properties of bulk phase and isolated $VS_4$ NWs. Our results indicate
that both bulk phase and isolated $VS_4$ NWs are semiconductors with band gaps
of 2.24 and 2.64 eV, respectively, and that they prefer the antiferromagnetic
(AFM) ground state based on DFT calculations. These calculations also suggested
that isolated $VS_4$ NWs show half-metallic antiferromagnetism upon electron
and hole doping because carrier doping splits the spin degeneracy to induce
local spin polarisation. As a result, spin polarisation currents in isolated
$VS_4$ NWs can be manipulated with locally applied gate voltage. Therefore,
these 1D AFM materials have a high potential for advancing both fundamental
research and spintronic applications because they are more resistant to
magnetic perturbation than their 1D ferromagnetic counterparts. | 2101.12658v1 |
2016-09-29 | Quantum imaging of current flow in graphene | Since its first isolation in 2004, graphene has been found to host a plethora
of unusual electronic transport phenomena, making it a fascinating system for
fundamental studies in condensed-matter physics as well as offering tremendous
opportunities for future electronic and sensing devices. However, to fully
realise these goals a major challenge is the ability to non-invasively image
charge currents in monolayer graphene structures and devices. Typically,
electronic transport in graphene has been investigated via resistivity
measurements, however, such measurements are generally blind to spatial
information critical to observing and studying landmark transport phenomena
such as electron guiding and focusing, topological currents and viscous
electron backflow in real space, and in realistic imperfect devices. Here we
bring quantum imaging to bear on the problem and demonstrate high-resolution
imaging of current flow in graphene structures. Our method utilises an
engineered array of near-surface, atomic-sized quantum sensors in diamond, to
map the vector magnetic field and reconstruct the vector current density over
graphene geometries of varying complexity, from mono-ribbons to junctions, with
spatial resolution at the diffraction limit and a projected sensitivity to
currents as small as 1 {\mu}A. The measured current maps reveal strong spatial
variations corresponding to physical defects at the sub-{\mu}m scale. The
demonstrated method opens up an important new avenue to investigate fundamental
electronic and spin transport in graphene structures and devices, and more
generally in emerging two-dimensional materials and thin film systems. | 1609.09208v1 |
2019-01-08 | Quadratic to linear magnetoresistance tuning in TmB4 | The change of a material's electrical resistance (R) in response to an
external magnetic field (B) provides subtle information for the
characterization of its electronic properties and has found applications in
sensor and storage related technologies. In good metals, Boltzmann's theory
predicts a quadratic growth in magnetoresistance (MR) at low B, and saturation
at high fields. On the other hand, a number of nonmagnetic materials with weak
electronic correlation and low carrier concentration for metallicity, such as
inhomogeneous conductors, semimetals, narrow gap semiconductors and topological
insulators, two-dimensional electron gas (2DEG) show positive, non-saturating
linear magnetoresistance (LMR). However, observation of LMR in single crystals
of a good metal is rare. Here we present low-temperature, angle dependent
magnetotransport in single crystals of the antiferromagnetic metal, TmB4. We
observe large, positive and anisotropic MR(B), which can be tuned from
quadratic to linear by changing the direction of the applied field. In view of
the fact that isotropic, single crystalline metals with large Fermi surface
(FS) are not expected to exhibit LMR, we attribute our observations to the
anisotropic FS topology of TmB4. Furthermore, the linear MR is found to be
temperature-independent, suggestive of quantum mechanical origin. | 1901.02165v1 |
2019-01-30 | Cathodoluminescence of CdZnSSe crystals synthesized in 19th century bead glass | The article presents an experimental investigation of band-edge
cathodoluminescence of CdZnSSe crystals that nucleated and grew in silicate
glass melt during its production. We have studied Zn-rich red glass made for
manufacture of seed beads in the 19th century and found it to contain CdZnSSe
crystals. Due to colloidal staining using the CdZnSSe crystallites embedded in
glass, glass makers were enabled to produce lustrous red glass that, as we
presently know, manifests bright luminescence. The CdZnSSe crystallites exhibit
intense band-edge cathodoluminescence both at room temperature and at liquid
nitrogen temperature. We have found the band-edge cathodoluminescence of these
crystals to peak in the range from 2.00 to 2.03 eV at 300 K and from 2.02 to
2.06 eV at 80 K. We have also estimated the value of the band gap derivative
d$E_g$/d$T$ in the interval from 80 to 300 K and found it to lie in the range
from $-2.5 \times 10^{-4}$ to 0 eV/K. CdZnSSe crystals in glass demonstrate
high temperature and temporal stability. The glass-crystal composite on their
basis is resistant to the electron-beam irradiation and long-term weathering.
Possible applications of this composite in modern technologies, and processes
and components that might be used for making glass stained with CdZnSSe in the
past are also discussed. | 1901.10764v3 |
2019-10-21 | Revisiting Effects of Nitrogen Incorporation and Graphitization on Conductivity of Ultra-nano-crystalline Diamond Films | Detailed structural and electrical properties of ultra-nano-crystalline
diamond (UNCD) films grown in H$_\text{2}$/CH$_\text{4}$/N$_\text{2}$ plasma
were systematically studied as a function of deposition temperature ($T_d$) and
nitrogen content ($\%$ N$_2$) to thoroughly evaluate their effects on
conductivity. $T_d$ was scanned from 1000 to 1300 K for N$_2$ fixed at 0, 5, 10
and 20 $\%$. It was found that even the films grown in the synthetic gas
mixture with no nitrogen could be made as conductive as 1$-$10$^{-2}$ $\Omega$
cm with overall resistivity of all the films tuned over 4 orders of magnitude
through varying growth parameters. On a set of 27 samples, Raman spectroscopy
and scanning electron microscopy show a progressive and highly reproducible
film material phase transformation, from ultra-nano-crystalline diamond to
nano-crystalline graphite as deposition temperature increases. The rate of this
transformation is heavily dependent on N$_2$ content. Addition of nitrogen
greatly increases the amount of $sp^2$ bonded carbon in the films thus
enhancing the physical connectivity in the GB network that have high electronic
density of states. However, addition of nitrogen greatly slows down
crystallization of $sp^2$ phase in the GBs. Therefore, proper balance between
GB connectivity and crystallinity is the key in conductivity engineering of
(N)UNCD. | 1910.09595v1 |
2020-05-28 | Powering Electronic Devices from Salt Gradients in AA Battery-Sized Stacks of Hydrogel-Infused Paper | Strongly electric fish use gradients of ions within their bodies to generate
stunning external electrical discharges; the most powerful of these organisms,
the Atlantic torpedo ray, can produce pulses of over 1 kW from its electric
organs. Despite extensive study of this phenomenon in nature, the development
of artificial power generation schemes based on ion gradients for portable,
wearable, or implantable human use has remained out of reach. Previously,
inspired by the electric eel, we developed an artificial electric organ that
generated electricity from ion gradients within stacked hydrogels and, like the
eel, was optimized to deliver large voltages that exceeded 100 V. Due to its
high internal resistance, the current of this power source was, however, too
low to power standard electronics. Here we introduce an artificial electric
organ that takes inspiration from the unique morphologies of torpedo rays for
maximal current output. This power source uses a hybrid material of
hydrogel-infused paper to create, organize, and reconfigure stacks of thin,
arbitrarily large gel films both in series and in parallel. The resulting
increase in electrical power by almost two orders of magnitude compared to the
original eel-inspired design makes it possible to power electronic devices and
establishes that biology's mechanism of generating significant electrical power
can now be realized from benign and soft materials in a portable size. | 2005.13775v1 |
2020-06-27 | Controlling local resistance via electric-field induced dislocations | Dislocations are one-dimensional (1D) topological line defects where the
lattice deviates from the perfect crystal structure. The presence of
dislocations transcends condensed matter research and gives rise to a diverse
range of emergent phenomena [1-6], ranging from geological effects [7] to light
emission from diodes [8]. Despite their ubiquity, to date, the controlled
formation of dislocations is usually achieved via strain fields, applied either
during growth [9,10] or retrospectively via deformation, e.g., (nano
[11-14])-indentation [15]. Here we show how partial dislocations can be induced
using local electric fields, altering the structure and electronic response of
the material where the field is applied. By combining high-resolution imaging
techniques and density functional theory calculations, we directly image these
dislocations in the ferroelectric hexagonal manganite Er(Ti,Mn)O3 and study
their impact on the local electric transport behaviour. The use of an electric
field to induce partial dislocations is a conceptually new approach to the
burgeoning field of emergent defect-driven phenomena and enables local property
control without the need of external macroscopic strain fields. This control is
an important step towards integrating and functionalising dislocations in
practical devices for future oxide electronics. | 2006.15252v1 |
2020-07-13 | Approaching the Practical Conductivity Limits of Aerosol Jet Printed Silver | Previous efforts to directly write conductive metals have been narrowly
focused on nanoparticle ink suspensions that require aggressive sintering (>200
{\deg}C) and result in low-density, small-grained agglomerates with electrical
conductivities <25% of bulk metal. Here, we demonstrate aerosol jet printing of
a reactive ink solution and characterize high-density (93%) printed silver
traces having near-bulk conductivity and grain sizes greater than the electron
mean free path, while only requiring a low-temperature (80 {\deg}C) treatment.
We have developed a predictive electronic transport model which correlates the
microstructure to the measured conductivity and identifies a strategy to
approach the practical conductivity limit for printed metals. Our analysis of
how grain boundaries and tortuosity contribute to electrical resistivity
provides insight into the basic materials science that governs how an ink
formulator or process developer might approach improving the conductivity.
Transmission line measurements validate that electrical properties are
preserved up to 20 GHz, which demonstrates the utility of this technique for
printed RF components. This work reveals a new method of producing robust
printed electronics that retain the advantages of rapid prototyping and
three-dimensional fabrication while achieving the performance necessary for
success within the aerospace and communications industries. | 2007.06645v1 |
2020-07-14 | A phase field model for elastic-gradient-plastic solids undergoing hydrogen embrittlement | We present a gradient-based theoretical framework for predicting hydrogen
assisted fracture in elastic-plastic solids. The novelty of the model lies in
the combination of: (i) stress-assisted diffusion of solute species, (ii)
strain gradient plasticity, and (iii) a hydrogen-sensitive phase field fracture
formulation, inspired by first principles calculations. The theoretical model
is numerically implemented using a mixed finite element formulation and several
boundary value problems are addressed to gain physical insight and showcase
model predictions. The results reveal the critical role of plastic strain
gradients in rationalising decohesion-based arguments and capturing the
transition to brittle fracture observed in hydrogen-rich environments. Large
crack tip stresses are predicted, which in turn raise the hydrogen
concentration and reduce the fracture energy. The computation of the steady
state fracture toughness as a function of the cohesive strength shows that
cleavage fracture can be predicted in otherwise ductile metals using sensible
values for the material parameters and the hydrogen concentration. In addition,
we compute crack growth resistance curves in a wide variety of scenarios and
demonstrate that the model can appropriately capture the sensitivity to: the
plastic length scales, the fracture length scale, the loading rate and the
hydrogen concentration. Model predictions are also compared with fracture
experiments on a modern ultra-high strength steel, AerMet100. A promising
agreement is observed with experimental measurements of threshold stress
intensity factor $K_{th}$ over a wide range of applied potentials. | 2007.07093v1 |
2020-07-16 | Magnetotransport properties of the topological nodal-line semimetal CaCdSn | Topological nodal-line semimetals support protected band crossings which form
nodal lines or nodal loops between the valence and conduction bands and exhibit
novel transport phenomena. Here we address the topological state of the
nodal-line semimetal candidate material, CaCdSn, and report magnetotransport
properties of its single crystals grown by the self-flux method. Our
first-principles calculations show that the electronic structure of CaCdSn
harbors a single nodal loop around the $\Gamma$ point in the absence of
spin-orbit coupling (SOC) effects. The nodal crossings in CaCdSn are found to
lie above the Fermi level and yield a Fermi surface that consists of both
electron and hole pockets. CaCdSn exhibits high mobility ($\mu \approx
3.44\times 10^4$ cm$^2$V$^{-1}$s$^{-1}$) and displays a field-induced
metal-semiconductor like crossover with a plateau in resistivity at low
temperature. We observe an extremely large and quasilinear non-saturating
transverse as well as longitudinal magnetoresistance (MR) at low temperatures
($\approx 7.44\times 10^3 \%$ and $\approx 1.71\times 10^3\%$, respectively, at
4K). We also briefly discuss possible reasons behind such a large quasilinear
magnetoresistance and its connection with the nontrivial band structure of
CaCdSn. | 2007.08156v1 |
2020-07-27 | Thermodynamic and corrosion study of Sm$_{1-x}$Mg$_x$Ni$_y$ (y = 3.5 or 3.8) compounds forming reversible hydrides | AB5 compounds (A = rare earth, B = transition metal) have been widely studied
as anodes for Ni-MH applications. However, they have reached their technical
limitations and the search for new promising materials with high capacity is
foreseen. ABy compounds (2 < y < 5) are good candidates. They are made by
stacking [AB5] and [A2B4] units along the c crystallographic axis. The latter
unit allows a large increase in capacity, while the [AB5] unit provides good
cycling stability. Consequently, the AB3.8 composition (i.e. A5B19 with three
[AB5] for one [A2B4]) is expected to exhibit better cycling stability than the
AB3.5 (i.e. A2B7 with two [AB5] for one [A2B4]). Furthermore, substitution of
rare earth by light magnesium improves both the capacity and cycling stability.
In this paper, we compare the hydrogenation and corrosion properties of two
binary compounds SmNi$_{3.5}$ and SmNi$_{3.8}$ and two pseudo-binary ones
(Sm,Mg)Ni$_{3.5}$ and (Sm,Mg)Ni$_{3.8}$. A better solid-gas cycling stability
is highlighted for the binary SmNi$_{3.8}$. The pseudo-binary compounds also
exhibit higher cycling stability than the binary ones. Furthermore, their
resistance to corrosion was investigated. | 2007.13456v1 |
2020-08-12 | Investigation of the lead-free double perovskites Cs2AgSbX6 (X= Cl, Br, I) for optoelectronic and thermoelectric applications | Perovskite compounds have the potential to harvest solar energy as well as
exploit the thermoelectric potential of a number of available materials. Here,
we present the electronic, structural, thermoelectric, and optical properties
of Cs2AgSbX6 (X = Cl, Br, I) perovskite with the help of the density functional
theory (DFT). The WC-GGA approximation was used to calculate the structural
parameters. All these compounds crystalize in a cubic unit cell with lattice
constant increasing from 10.65 {\AA} (Cl) to 11.14 {\AA} (Br) to 11.86 {\AA}
(I). The mBJ-functional shows a semiconducting nature for these compounds with
an indirect band gap lying at the L-X symmetry points. The optical conductivity
and absorption coefficient show their peaks in the ultraviolet region, moving
towards a lower energy range by inserting large size anion. The band gap of
these compounds (2.08, 1.37, 0.64 eV) indicates their potential in single and
multijunction solar cells. The value of refractive index at zero energy was
evaluated to be 3.1, 2.2, and 1.97 for Cs2AgSbCl6, Cs2AgSbBr6 and Cs2AgSbI6.
Effective mass of electrons is smaller than those of holes resulting in higher
carrier mobility for electrons. The Seebeck coefficient, power factor, and the
figure of merit were computed using the BoltzTrap code. The negative
temperature coefficient of resistivity also supports the semiconductor nature
of these compounds. The high electrical, small thermal conductivity, positive
Seebeck coefficient, and the optimum figure of merit make these compounds
suitable for thermoelectric applications. | 2008.06384v3 |
2020-08-27 | Direct Imaging and Electronic Structure Modulation of Moiré Superlattices at the 2D/3D Interface | The atomic structure at the interface between two-dimensional (2D) and
three-dimensional (3D) materials influences properties such as contact
resistance, photo-response, and high-frequency electrical performance. Moir\'e
engineering is yet to be utilized for tailoring this 2D/3D interface, despite
its success in enabling correlated physics at 2D/2D interfaces. Using
epitaxially aligned MoS2/Au{111} as a model system, we demonstrate the use of
advanced scanning transmission electron microscopy (STEM) combined with a
geometric convolution technique in imaging the crystallographic 32 A moir\'e
pattern at the 2D/3D interface. This moir\'e period is often hidden in
conventional electron microscopy, where the Au structure is seen in projection.
We show, via ab initio electronic structure calculations, that charge density
is modulated according to the moir\'e period, illustrating the potential for
(opto-)electronic moir\'e engineering at the 2D/3D interface. Our work presents
a general pathway to directly image periodic modulation at interfaces using
this combination of emerging microscopy techniques. | 2008.12215v2 |
2020-10-08 | Microstructure, mechanical properties, corrosion resistance and cytocompatibility of WE43 Mg alloy scaffolds fabricated by laser powder bed fusion for biomedical applications | Open-porous scaffolds of WE43 Mg alloy with a body-center cubic cell pattern
were manufactured by laser powder bed fusion with different strut diameters.
The geometry of the unit cells was adequately reproduced during additive
manufacturing and the porosity within the struts was minimized. The
microstructure of the scaffolds was modified by means of thermal solution and
ageing heat treatments and was analysed in detail by means of X-ray
microtomography, optical, scanning and transmission electron microscopy.
Moreover, the corrosion rates and the mechanical properties of the scaffolds
were measured as a function of the strut diameter and metallurgical condition.
The microstructure of the as-printed scaffolds contained a mixture of Y-rich
oxide particles and Rare Earth-rich intermetallic precipitates. The latter
could be modified by heat treatments. The lowest corrosion rates of 2-3 mm/year
were found in the as-printed and solution treated scaffolds and they could be
reduced to ~0.1 mm/year by surface treatments using plasma electrolytic
oxidation. The mechanical properties of the scaffolds improved with the strut
diameter: the yield strength increased from 8 to 40 MPa and the elastic modulus
improved from 0.2 to 0.8 GPa when the strut diameter increased from 275 \mu m
to 800 \mu m. Nevertheless, the strength of the scaffolds without plasma
electrolytic oxidation treatment decreased rapidly when immersed in simulated
body fluid. In vitro biocompatibility tests showed surface treatments by plasma
electrolytic oxidation were necessary to ensure cell proliferation in scaffolds
with high surface-to-volume ratio. | 2010.03812v1 |
2020-10-11 | Capping layer influence and isotropic in-plane upper critical field of the superconductivity at the FeSe/SrTiO3 interface | Understanding the superconductivity at the interface of FeSe/SrTiO3 is a
problem of great contemporary interest due to the significant increase in
critical temperature (Tc) compared to that of bulk FeSe, as well as the
possibility of an unconventional pairing mechanism and topological
superconductivity. We report a study of the influence of a capping layer on
superconductivity in thin films of FeSe grown on SrTiO3 using molecular beam
epitaxy. We used in vacuo four-probe electrical resistance measurements and ex
situ magneto-transport measurements to examine the effect of three capping
layers that provide distinctly different charge transfer into FeSe: compound
FeTe, non-metallic Te, and metallic Zr. Our results show that FeTe provides an
optimal cap that barely influences the inherent Tc found in pristine
FeSe/SrTiO3, while the transfer of holes from a non-metallic Te cap completely
suppresses superconductivity and leads to insulating behavior. Finally, we used
ex situ magnetoresistance measurements in FeTe-capped FeSe films to extract the
angular dependence of the in-plane upper critical magnetic field. Our
observations reveal an almost isotropic in-plane upper critical field,
providing insight into the symmetry and pairing mechanism of high temperature
superconductivity in FeSe. | 2010.05308v1 |
2020-10-16 | Synthesis of narrow SnTe nanowires using alloy nanoparticles | Topological crystalline insulator tin telluride (SnTe) provides a rich
playground to examine interactions of correlated electronic states, such as
ferroelectricity, topological surface states, and superconductivity. Making
SnTe into nanowires further induces novel electronic states due to
one-dimensional (1D) confinement effects. Thus, for transport measurements,
SnTe nanowires must be made narrow in their diameters to ensure the 1D
confinement and phase coherence of the topological surface electrons. This
study reports a facile growth method to produce narrow SnTe nanowires with a
high yield using alloy nanoparticles as growth catalysts. The average diameter
of the SnTe nanowires grown using the alloy nanoparticles is 85 nm, nearly a
factor of three reduction from the previous average diameter of 240 nm using
gold nanoparticles as growth catalysts. Transport measurements reveal the
effect of the nanowire diameter on the residual resistance ratio and
magnetoresistance. Particularly, the ferroelectric transition temperature for
SnTe is observed to change systematically with the nanowire diameter. In situ
cryogenic cooling of narrow SnTe nanowires in a transmission electron
microscope directly reveals the cubic to rhombohedral structural transition,
which is associated with the ferroelectric transition. Thus, these narrow SnTe
nanowires represent a model system to study electronic states arising from the
1D confinement, such as 1D topological superconductivity as well as a potential
multi-band superconductivity. | 2010.08078v1 |
2021-02-12 | Universal size-dependent nonlinear charge transport in single crystals of the Mott insulator Ca$_2$RuO$_4$ | The surprisingly low current density required for inducing the insulator to
metal transition has made Ca$_2$RuO$_4$ an attractive candidate material for
developing Mott-based electronics devices. The mechanism driving the resistive
switching, however, remains a controversial topic in the field of strongly
correlated electron systems. Here we probe an uncovered region of phase space
by studying high-purity Ca$_2$RuO$_4$ single crystals, using the sample size as
principal tuning parameter. Upon reducing the crystal size, we find a four
orders of magnitude increase in the current density required for driving
Ca$_2$RuO$_4$ out of the insulating state into a non-equilibrium (also called
metastable) phase which is the precursor to the fully metallic phase. By
integrating a microscopic platinum thermometer and performing thermal
simulations, we gain insight into the local temperature during simultaneous
application of current and establish that the size dependence is not a result
of Joule heating. The findings suggest an inhomogeneous current distribution in
the nominally homogeneous crystal. Our study calls for a reexamination of the
interplay between sample size, charge current, and temperature in driving
Ca$_2$RuO$_4$ towards the Mott insulator to metal transition. | 2102.06556v3 |
2021-02-23 | Huge permittivity and premature metallicity in Bi$_2$O$_2$Se single crystals | Bi$_2$O$_2$Se is a promising material for next-generation semiconducting
electronics. It exhibits premature metallicity on the introduction of a tiny
amount of electrons, the physics behind which remains elusive. Here we report
on transport and dielectric measurements in Bi$_2$O$_2$Se single crystals at
various carrier densities. The temperature-dependent resistivity ($\rho$)
indicates a smooth evolution from the semiconducting to the metallic state. The
critical concentration for the metal-insulator transition (MIT) to occur is
extraordinarily low ($n_\textrm{c}\sim10^{16}$ cm$^{-3}$). The relative
permittivity of the insulating sample is huge
($\epsilon_\textrm{r}\approx155(10)$) and varies slowly with temperature.
Combined with the light effective mass, a long effective Bohr radius
($a_\textrm{B}^*\approx36(2)$ $\textrm{nm}$) is derived, which provides a
reasonable interpretation of the metallic prematurity according to Mott's
criterion for MITs. The high electron mobility ($\mu$) at low temperatures may
result from the screening of ionized scattering centers due to the huge
$\epsilon_\textrm{r}$. Our findings shed light on the electron dynamics in two
dimensional (2D) Bi$_2$O$_2$Se devices. | 2102.11451v1 |
2021-04-30 | Local inhomogeneities resolved by scanning probe techniques and their impact on local 2DEG formation in oxide heterostructures | Lateral inhomogeneities in the formation of 2-dimensional electron gases
(2DEG) directly influence their electronic properties. Understanding their
origin is an important factor for fundamental interpretations, as well as high
quality devices. Here, we studied the local formation of the buried 2DEG at
LaAlO3/SrTiO3 (LAO/STO) interfaces grown on STO (100) single crystals with
partial TiO2 termination, utilizing in-situ local conductivity atomic force
microscopy (LC-AFM) and scattering-type scanning near-field optical microscopy
(s-SNOM). Using substrates with different degrees of chemical surface
termination, we can link the resulting interface chemistry to an inhomogeneous
2DEG formation. In conductivity maps recorded by LC-AFM, a significant lack of
conductivity is observed at topographic features, indicative of a local
SrO/AlO2 interface stacking order, while significant local conductivity can be
probed in regions showing TiO2/LaO interface stacking order. These results
could be corroborated by s SNOM, showing a similar contrast distribution in the
optical signal which can be linked to the local electronic properties of the
material. The results are further complimented by low-temperature conductivity
measurements, which show an increasing residual resistance at 5 K with
increasing portion of insulating SrO terminated areas. Therefore, we can
correlate the macroscopic electrical behavior of our samples to its nanoscopic
structure. Using proper parameters, 2DEG mapping can be carried out without any
visible alteration of sample properties, proving LC AFM and s SNOM to be viable
and destruction-free techniques for the identification of local 2DEG formation.
Furthermore, applying LC AFM and s SNOM in this manner opens the exciting
prospect to link macroscopic low temperature transport to its nanoscopic
origin. | 2104.14838v1 |
2021-05-04 | Thermal transport in two-dimensional C3N/C2N superlattices: A molecular dynamics approach | Nanostructured superlattices have been the focus of many researchers due to
their physical and manipulatable properties. They aim to find promising
materials for new electronic and thermoelectric devices. In the present study,
we investigate the thermal conductivity of two-dimensional (2D) C3N/ C2N
superlattices using non-equilibrium molecular dynamics. We analyze the
dependence of thermal conductivity on the total length, temperature, and the
temperature difference between thermal baths for the superlattices. The minimum
thermal conductivity and the phonon mean free path at a superlattice period of
5.2 nm are 23.2W/m.K and 24.7 nm, respectively. Our results show that at a
specific total length, as the period increases, the number of interfaces
decreases, thus the total thermal resistance decreases, and the effective
thermal conductivity of the system increases. We found that at long lengths
(L_x >80 nm), the high-frequency and low-wavelength phonons are scattered
throughout the interfaces, while at short lengths, there is a wave interference
that reduces the thermal conductivity. The combination of these two effects,
i.e., the wave interference and the interface scattering, is the reason for the
existence of a minimum thermal conductivity in superlattices. | 2105.01378v2 |
2021-07-09 | Lithium-Metal Batteries Using Sustainable Electrolyte Media and Various Cathode Chemistries | Lithium-metal batteries employing concentrated glyme-based electrolytes and
different cathode chemistries are herein evaluated in view of a safe use of the
highly energetic alkali-metal anode. Indeed, diethylene-glycol dimethyl-ether
(DEGDME) and triethylene-glycol dimethyl-ether (TREGDME) dissolving lithium
bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium nitrate (LiNO3) in
concentration approaching the solvents saturation limit are used in lithium
batteries employing either a conversion sulfur-tin composite (S:Sn 80:20 w/w)
or a Li+ (de-)insertion LiFePO4 cathode. Cyclic voltammetry (CV) and
electrochemical impedance spectroscopy (EIS) clearly show the suitability of
the concentrated electrolytes in terms of process reversibility and low
interphase resistance, particularly upon a favorable activation. Galvanostatic
measurements performed in the lithium-sulfur (Li/S) batteries reveal promising
capacities at room temperature (25 {\deg}C) and a value as high as 1300 mAh
gS-1 for DEGDME-based electrolyte at 35 {\deg}C. On the other hand, the
lithium-LiFePO4 (Li/LFP) cells exhibit satisfactory cycling behavior, in
particular when employing an additional reduction step at low voltage cutoff
(i.e., 1.2 V) during the first discharge to consolidate the solid electrolyte
interphase (SEI). This procedure allows a coulombic efficiency near 100 %, a
capacity approaching 160 mAh g-1 and relevant retention particularly for the
cell using TREGDME-based electrolyte. Therefore, this work suggests the use of
concentrated glyme-based electrolytes, the fine tuning of the operative
conditions, and the careful selection of active materials chemistry as
significant steps to achieve practical and safe lithium-metal batteries. | 2107.04446v1 |
2021-08-30 | Realization of A Non-Markov Chain in A Single 2D Crystal RRAM | The non-Markov processes widely exist in thermodymanic processes, while it
usually requires packing of many transistors and memories with great system
complexity in traditional device architecture to minic such functions.
Two-dimensional (2D) material-based resistive random access memory (RRAM)
devices show potential for next-generation computing systems with much-reduced
complexity. Here, we achieve the non-Markov chain in an individual RRAM device
based on 2D mica with a vertical metal/mica/metal structure. We find that the
internal potassium ions (K+) in 2D mica gradually move along the direction of
the applied electric field, making the initially insulating mica conductive.
The accumulation of K+ is tuned by electrical field, and the 2D-mica RRAM
possesses both unipolar and bipolar memory windows, high on/off ratio, decent
stability and repeatability.Importantly, the non-Markov chain algorithm is
established for the first time in a single RRAM, in which the movement of K+ is
dependent on the stimulated voltage as well as their past states. This work not
only uncovers the inner ionic conductivity of 2D mica, but also opens the door
for such novel RRAM devices with numerous functions and applications. | 2108.13244v1 |
2021-09-23 | Creating superconductivity in WB2 through pressure-induced metastable planar defects | High-pressure electrical resistivity measurements reveal that the mechanical
deformation of ultra-hard WB2 during compression induces superconductivity
above 50 GPa with a maximum superconducting critical temperature, Tc of 17 K at
90 GPa. Upon further compression up to 190 GPa, the Tc gradually decreases.
Theoretical calculations show that electron-phonon mediated superconductivity
originates from the formation of metastable stacking faults and twin boundaries
that exhibit a local structure resembling MgB2} (hP3, space group 191,
prototype AlB2). Synchrotron x-ray diffraction measurements up to 145 GPa} show
that the ambient pressure hP12 structure (space group 194, prototype WB2)
continues to persist to this pressure, consistent with the formation of the
planar defects above 50 GPa. The abrupt appearance of superconductivity under
pressure does not coincide with a structural transition but instead with the
formation and percolation of mechanically-induced stacking faults and twin
boundaries. The results identify an alternate route for designing
superconducting materials. | 2109.11521v2 |
2021-12-08 | Precise Damage Shaping in Self-Sensing Composites Using Electrical Impedance Tomography and Genetic Algorithms | Fiber-reinforced composites with nanofiller-modified polymer matrices have
immense potential to improve the safety of high-risk engineering structures.
These materials are intrinsically self-sensing because their electrical
conductivity is affected by deformations and damage. This property, known as
piezoresistivity, has been extensively leveraged for conductivity-based damage
detection via electrical resistance change methods and tomographic imaging
techniques such as electrical impedance tomography (EIT). Although these
techniques are very effective at detecting the presence of damage, they suffer
from an inability to provide precise information about damage shape, size, or
mechanism. This is particularly detrimental for laminated composites which can
suffer from complex failure modes, such as delaminations, that are difficult to
detect. To that end, we herein propose a new technique for precisely
determining damage shape and size in self-sensing composites. Our technique
makes use of a genetic algorithm (GA) integrated with realistic physics-based
damage models to recover precise damage shape from conductivity changes imaged
via EIT. We experimentally validate this technique on carbon nanofiber
(CNF)-modified glass fiber-reinforced polymer (GFRP) laminates by considering
two specific damage mechanisms: through-holes and delaminations. Our results
show that this novel technique can accurately reconstruct multiple
through-holes with radii as small as 1.19 mm and delaminations caused by low
velocity impacts. These findings illustrate that coupling piezoresistivity with
conductivity-based spatial imaging techniques and physics-based inversion
strategies can enable damage shaping capabilities in self-sensing composite
structures. | 2112.04419v2 |
2022-01-10 | $B^2$ to $B$-linear magnetoresistance due to impeded orbital motion | Strange metals exhibit a variety of anomalous magnetotransport properties,
the most striking of which is a resistivity that increases linearly with
magnetic field $B$ over a broad temperature and field range. The ubiquity of
this behavior across a spectrum of correlated metals - both single- and
multi-band, with either dominant spin and/or charge fluctuations, of varying
levels of disorder or inhomogeneity and in proximity to a quantum critical
point or phase - obligates the search for a fundamental underlying principle
that is independent of the specifics of any material. Strongly anisotropic
(momentum-dependent) scattering can generate $B$-linear magnetoresistance but
only at intermediate field strengths. At high enough fields, the
magnetoresistance must eventually saturate. Here, we consider the ultimate
limit of such anisotropy, a region or regions on the Fermi surface that impede
all orbital (cyclotron) motion through them, but whose imposition can be
modelled nonetheless through a modified Boltzmann theoretical treatment.
Application of the proposed theorem suggests that the realization of
quadratic-to-linear magnetoresistance requires the presence of a bounded sector
on the Fermi surface possibly separating two distinct types of carriers. While
this bounded sector may have different origins or manifestations, we expect its
existence to account for the anomalous magnetotransport found in a wide range
of correlated materials. | 2201.03292v1 |
2022-01-12 | Controlled chemical functionalization toward 3D-2D carbon nanohorn-MoS2 heterostructures with enhanced electrocatalytic activity for protons reduction | The realization of novel heterostructures arising from the combination of
nanomaterials is an effective way to modify their physicochemical and
electrocatalytic properties, giving them enhanced characteristics stemming from
their individual constituents. Interfacing carbon nanohorns (CNHs) possessing
high porosity, large specific surface area and good electrical conductivity,
with MoS2 owning multiple electrocatalytic active sites but lacking significant
conductivity, robust interactions and effective structure, can be a strategy to
boost the electrocatalytic reduction of protons to molecular hydrogen. Herein,
we covalently introduce, in a stepwise approach, complementary functional
groups at the conical tips and sidewalls of CNHs, along with the basal plane of
MoS2, en route the construction of 3D-2D CNH-MoS2 heterostructures. The
increased MoS2 loading onto CNHs, improving and facilitating charge
delocalization and transfer in neighboring CNHs, along with the plethora of
active sites, results in excellent electrocatalytic activity for protons
reduction same to that of commercial Pt/C. We have registered minute
overpotential, low Tafel slope and small charge-transfer resistance for
electrocatalyzing the evolution of hydrogen from the newly prepared
heterostructure of 0.029 V, 71 mV/dec and 34.5 {\Omega}, respectively.
Furthermore, the stability of the 3D-2D CNH-MoS2 heterostructure was validated
after performing 10,000 ongoing electrocatalytic cycles. | 2201.05450v1 |
2022-01-26 | AI-Aided Mapping of the Structure-Composition-Conductivity Relationships of Glass-Ceramic Lithium Thiophosphate Electrolytes | Lithium thiophosphates (LPS) with the composition
(Li$_2$S)$_x$(P$_2$S$_5$)$_{1-x}$ are among the most promising prospective
electrolyte materials for solid-state batteries (SSBs), owing to their
superionic conductivity at room temperature ($>10^{-3}$ S cm$^{-1}$), soft
mechanical properties, and low grain boundary resistance. Several glass-ceramic
(gc) LPS with different compositions and good Li conductivity have been
previously reported, but the relationship between composition, atomic
structure, stability, and Li conductivity remains unclear due to the challenges
in characterizing non-crystalline phases in experiments or simulations. Here,
we mapped the LPS phase diagram by combining first principles and artificial
intelligence (AI) methods, integrating density functional theory, artificial
neural network potentials, genetic-algorithm sampling, and ab initio molecular
dynamics simulations. By means of an unsupervised structure-similarity
analysis, the glassy/ceramic phases were correlated with the local structural
motifs in the known LPS crystal structures, showing that the energetically most
favorable Li environment varies with the composition. Based on the discovered
trends in the LPS phase diagram, we propose a candidate solid-state electrolyte
composition, (Li$_{2}$S)$_{x}$(P$_{2}$S$_{5}$)$_{1-x}$ ($x\sim{}0.725$), that
exhibits high ionic conductivity ($>10^{-2}$ S cm$^{-1}$) in our simulations,
thereby demonstrating a general design strategy for amorphous or glassy/ceramic
solid electrolytes with enhanced conductivity and stability. | 2201.11203v1 |
2022-03-02 | Emergence of insulating ferrimagnetism and perpendicular magnetic anisotropy in 3d-5d perovskite oxide composite films for insulator spintronic | Magnetic insulators with strong perpendicular magnetic anisotropy (PMA) play
a key role in exploring pure spin current phenomena and developing
ultralow-dissipation spintronic devices, thereby it is highly desirable to
develop new material platforms. Here we report epitaxial growth of
La2/3Sr1/3MnO3 (LSMO)-SrIrO3 (SIO) composite oxide films (LSMIO) with different
crystalline orientations fabricated by sequential two-target ablation process
using pulsed laser deposition. The LSMIO films exhibit high crystalline quality
with homogeneous mixture of LSMO and SIO at atomic level. Ferrimagnetic and
insulating transport characteristics are observed, with the
temperature-dependent electric resistivity well fitted by Mott
variable-range-hopping model. Moreover, the LSMIO films show strong PMA.
Through further constructing all perovskite oxide heterostructures of the
ferrimagnetic insulator LSMIO and a strong spin-orbital coupled SIO layer,
pronounced spin Hall magnetoresistance (SMR) and spin Hall-like anomalous Hall
effect (SH-AHE) were observed. These results illustrate the potential
application of the ferrimagnetic insulator LSMIO in developing all-oxide
ultralow-dissipation spintronic devices. | 2203.00818v1 |
2022-04-07 | Spin sensitive transport in a spin liquid material: revealing a robustness of spin anisotropy | Alpha-phase (a-) RuCl_3 has emerged as a prime candidate for a quantum spin
liquid (QSL) that promises exotic quasiparticles relevant for fault-tolerant
quantum computation. Here, we report spin sensitive transport measurements to
probe spin correlation in a-RuCl_3 using a proximal spin Hall metal platinum
(Pt). Both transverse and longitudinal resistivities exhibit oscillations as
function of the angle between an in-plane magnetic field and the current, akin
to previously measured spin Hall magnetoresistance (SMR) in antiferromagnet/Pt
heterostructures. The oscillations are observed from 1.5 T to 18 T, both within
and beyond the magnetic field range where the antiferromagnetic order and QSL
state are reported in a-RuCl_3. The SMR oscillations show that spins in a-RuCl3
are largely locked to an in-plane quantization axis transverse to the magnetic
field, constituting a continuous-symmetry-broken state that does not
necessarily represent a long-range order. This robust anisotropy of spin axis
uncovers critical energy scales connected with reported QSL signatures in
a-RuCl_3. Simulations suggest a predominantly antiferromagnetic correlation to
moderately high magnetic-fields, that may support the SMR oscillations. The
coupling of the spin states within a-RuCl_3 and Pt demonstrated in our
experiment opens a transport route to exploring exotic spin phases and device
functionalities of QSL materials. | 2204.03158v1 |
2022-06-14 | Probing of large interfacial contribution to spin orbit coupling in CoFeB/Ta heterostructure by ultrafast THz emission spectroscopy | Ultrafast THz radiation generation from ferromagnetic/nonmagnetic bilayer
heterostructure-based spintronic emitters generally exploits the conversion
from spin- to charge-current within the nonmagnetic layer and its interface
with the ferromagnetic layer. Various possible sub-contributions to the
underlying mechanism of inverse spin Hall effect for the THz emission from such
structures, need to be exploited for not only investigating the intricacies at
the fundamental level in the material properties themselves but also for
improving their performance for broadband and high-power THz emission. Here, we
report ultrafast THz emission from CoFeB/Ta bilayer at varying sample
temperatures in a large range to unravel the role of intrinsic and extrinsic
spin to charge conversion processes. In addition to an enhancement in the THz
emission, its temperature dependence shows a THz signal polarity reversal if
the CoFeB/Ta sample is annealed at an elevated temperature. We extract the
behaviour of the spin Hall resistivity, determine the intrinsic spin Hall
conductivity contribution in it and compare those with the standard Fe/Pt
system. Our results clearly demonstrate a giant interfacial contribution to the
overall spin Hall angle arising from the modified interface in the annealed
CoFeB/Ta, where a sign reversal in the corresponding spin Hall angle is
manifested from the THz amplitude variation with the temperature. | 2206.06718v2 |
2022-08-04 | Studies on tuning surface electronic properties of hydrogenated diamond by oxygen functionalization | Ultra-wide bandgap and the absence of shallow dopants are the major
challenges in realizing diamond based electronics. However, the surface
functionalization offers an excellent alternative to tune electronic structure
of diamonds. Herein, we report on tuning the surface electronic properties of
hydrogenated polycrystalline diamond films through oxygen functionalization.
The hydrogenated diamond (HD) surface transforms from hydrophobic to
hydrophilic nature and the sheet resistance increases from ~ 8 kohms/sq. to
over 10 Gohms/sq. with progressive ozonation. The conductive atomic force
microscopic (c-AFM) studies reveal preferential higher current conduction on
selective grain interiors (GIs) than that of grain boundaries confirming the
surface charge transfer doping on these HDs. In addition, the local current
conduction is also found to be much higher on (111) planes as compared to (100)
planes on pristine and marginally O-terminated HD. However, there is no current
flow on the fully O-terminated diamond (OD) surface. Further, X-ray
photoelectron spectroscopic (XPS) studies reveal a redshift in binding energy
(BE) of C1s on pristine and marginally O-terminated HD surfaces indicating
surface band bending whilst the BE shifts to higher energy for OD. Moreover,
XPS analysis also corroborate c-AFM study for the possible charge transfer
doping mechanism on the diamond films which results in high current conduction
on GIs of pristine and partially O-terminated HDs. | 2208.02465v1 |
2022-09-01 | Four-point bending piezoelectric energy harvester with uniform surface strain toward better energy conversion performance and material usage | Improving the energy conversion efficiency of piezoelectric energy harvesters
is of great importance, and one approach is to make more uniform use of the
working material by ensuring a uniform strain state. To achieve better
performance, this paper presents a four-point bending piezoelectric energy
harvester with extensive investigation and modeling to identify the influential
parameters. An electromechanical analytical model is presented and verified by
experimental data. The frequency-domain method extracts the solutions for a
general time-variable force and impact. Four-point bending is compared with the
standard cantilever harvesters regarding voltage generation, mechanical strain,
and figure of merit. Strain contours are analyzed and interpreted for this
innovative approach, and the power generation by the optimal resistance load is
studied. Dimensionless parameters are introduced and investigated to find the
optimal operating conditions for the four-point bending harvester. Finally, the
four-point bending performance and the best figure of merit are discussed with
a view to the long-term fatigue life of the harvester. The results show that in
the best four-point bending energy conversion conditions; the energy conversion
coefficient is more than three times higher than that of typical cantilever
energy harvesters. The results also illustrate that the axial strain
experienced in a standard cantilever harvester is more than three times higher
than that of the four-point bending harvester, suggesting the latter device may
have favorable fatigue performance. Overall, the presented piezoelectric
harvester has improved energy conversion efficiency and experiences a reduced
and uniform surface strain, making it appropriate for high-efficiency energy
harvesting systems. | 2209.00252v1 |
2022-09-08 | Observation of strange metal in hole-doped valley-spin insulator | Temperature-linear resistance at low temperatures in strange metals is an
exotic characteristic of strong correlation systems, as observed in high-TC
superconducting cuprates, heavy fermions, Fe-based superconductors, ruthenates,
and twisted bilayer graphene. Here, we introduce a hole-doped valley-spin
insulator, V-doped WSe2, with hole pockets in the valence band. The strange
metal characteristic was observed in VxW1-xSe2 at a critical carrier
concentration of 9.5 x 10^20 cm-3 from 150 K to 1.8 K. The unsaturated
magnetoresistance is almost linearly proportional to the magnetic field. Using
the ansatz R(H,T) - R(0,0) ~ [(alpha.k.T)^2+(gamma.mu.B)^2]^1/2, the
gamma/alpha ratio is estimated approximately to 4, distinct from that for the
quasiparticles of LSCO, BaFe2(As1-xPx)2 (gamma/alpha=1) and bosons of YBCO
(gamma/alpha=2). Our observation opens up the possible routes that induce
strong correlation and superconductivity in two-dimensional materials with
strong spin-orbit coupling. | 2209.03672v1 |
2022-09-26 | Study of pnictides for photovoltaic applications | For the transition into a sustainable mode of energy usage, it is important
to develop photovoltaic materials that exhibit better solar-to-electricity
conversion efficiencies, a direct optimal band gap, and made of non-toxic,
earth abundant elements compared to the state-of-the-art silicon photovoltaics.
Here, we explore the non-redox-active pnictide chemical space, including binary
A$_3$B$_2$, ternary AA'$_2$B$_2$, and quaternary AA'A"B$_2$ compounds (A, A',
A" = Ca, Sr, or Zn; B = N or P), as candidate beyond-Si photovoltaics using
density functional theory calculations. Specifically, we evaluate the ground
state configurations, band gaps, and 0 K thermodynamic stability for all 20
pnictide compositions considered, besides computing the formation energy of
cation vacancies, anion vacancies, and cation anti-sites in a subset of
candidate compounds. Importantly, we identify SrZn$_2$N$_2$, SrZn$_2$P$_2$, and
CaZn$_2$P$_2$ to be promising candidates, exhibiting optimal (1.1-1.5 eV)
hybrid-functional-calculated band gaps, stability at 0 K, and high resistance
to point defects (formation energies $>$1 eV), while other possible candidates
include ZnCa$_2$N$_2$ and ZnSr$_2$N$_2$, which may be susceptible to N-vacancy
formation. We hope that our study will contribute to the practical development
of pnictide semiconductors as beyond-silicon light absorbers. | 2209.12458v1 |
2022-09-28 | Dissolution and Recrystallization Behavior of Li3PS4 in Different Organic Solvents | Solid state batteries can be built based on thiophosphate electrolytes such
as beta-Li3PS4. For the preparation of these electrolytes, various
solvent-based routes have been reported. For recycling of end-of-life solid
state batteries based on such thiophosphates, we consider the development of
dissolution and recrystallization strategies for the recovery of the model
compound beta-Li3PS4. We show that recrystallization can only be performed in
polar, slightly protic solvents such as N-methylformamide (NMF). The
recrystallization is comprehensively studied, showing that it proceeds via an
intermediate phase with composition Li3PS4*2NMF, which is structurally
characterized. This phase has a high resistivity for the transport of lithium
ions and must be removed in order to obtain a recrystallized product with a
conductivity similar to the pristine material. Moreover, the recrystallization
from solution results in an increase of the amorphous phase fraction next to
crystalline beta-Li3PS4, which results in a decrease of the activation energy
to 0.2 eV compared to 0.38 eV for the pristine phase. | 2209.13955v1 |
2022-11-02 | One-Step Formation of Plasmonic Cu Nanodomains in p-Type Cu$_2$O Matrix Films for Enhanced Photoconversion of n-ZnO/p-Cu$_2$O Heterojunctions | Plasmonic Cu nanoparticles were in-situ grown into a Cu$_2$O semiconductor
matrix by using reactive magnetron sputtering and adjusting the amount of
oxygen available during the synthesis in order to prevent the oxidation of part
of copper atoms landed on the film surface. Varying only the oxygen flowrate
(OFR) and using a single Cu target it was possible to observe the evolution in
the simultaneous formation of metallic Cu and Cu$_2$O phases for oxygen-poor
conditions. Suchformation is accompanied by the development of the surface
plasmon band (SPB) corresponding to Cu, as evidenced by UV-Vis
spectrophotometry and spectroscopic ellipsometry. The bandgap values of the
elaborated composites containing embedded Cu plasmonic nanodomains were lower
than the bandgap of single-phased Cu$_2$O films, likely due to the higher
defect density associated to the nanocrystalline nature of films, promoted by
the presence of metallic Cu. The resistivity of the thin films increased with
more oxidative deposition conditions and was associated to an increase in
Cu$_2$O/Cu ratio and smaller and more isolated Cu particles, as evidenced by
high resolution transmission electron microscopy and X-ray diffraction.
Photoconversion devices based on the studied nanocomposites were characterized
by I-V and spectral photocurrent measurements, showing an increase in the
photocurrent density under light illumination as consequence of the plasmonic
particles excitation leading to hot carrier's injection in the nearby ZnO and
Cu$_2$O semiconductors. | 2211.01010v1 |
2023-02-25 | Strain-adjustable reflectivity of polyurethane nanofiber membrane for thermal management applications | Passive radiative cooling technologies are highly attractive in pursuing
sustainable development. However, current cooling materials are often static,
which makes it difficult to cope with the varying needs of all-weather thermal
comfort management. Herein, a strategy is designed to obtain flexible
thermoplastic polyurethane nanofiber (Es-TPU) membranes via electrospinning,
realizing reversible in-situ solvent-free switching between radiative cooling
and solar heating through changes in its optical reflectivity by stretching. In
its radiative cooling state (0% strain), the Es-TPU membrane shows a high and
angular-independent reflectance of 95.6% in the 0.25-2.5 {\mu}m wavelength
range and an infrared emissivity of 93.3% in the atmospheric transparency
window (8-13 {\mu}m), reaching a temperature drop of 10 {\deg}C at midday, with
a corresponding cooling power of 118.25 W/m2. The excellent mechanical
properties of the Es-TPU membrane allows the continuous adjustment of
reflectivity by reversibly stretching it, reaching a reflectivity of 61.1%
({\Delta}R=34.5%) under an elongation strain of 80%, leading to a net
temperature increase of 9.5 {\deg}C above ambient of an absorbing substrate and
an equivalent power of 220.34 W/m2 in this solar heating mode. The strong haze,
hydrophobicity and outstanding aging resistance exhibited by this scalable
membrane hold promise for achieving uniform illumination with tunable strength
and efficient thermal management in practical applications. | 2302.13043v2 |
2023-03-26 | Molecular lighting goes less powering | The present era has seen tremendous demands for low-cost electrochromic
materials for visible-region multicolor display technology, paper-based,
flexible, and wearable electronic devices, smart windows, and optoelectronic
applications. Towards this goal, we report large-scale polyelectrochromic
devices fabricated on rigid to flexible ITO substrates comprising novel
anthracene containing viologen,
(1,1'-bis(anthracen-9-ylmethyl)-[4,4'-bipyridine]-1,1'-diium bromide,
abbreviated as AnV2+), and polythiophene (P3HT). Inter-estingly, the devices
show three states of reversible visible color in response to the applied bias,
sub-second to second switching time (0.7 s/1.6 s), and high coloration
efficiency (484 cm2/C), longer cycling stability up to 3000 s (103 switching
cycles). Thanks to the anthracenes moieties introduced to viologen that inhibit
formation of undesired dimer of cation radicals formed in response to the
applied bias, other-wise it would hamper the devices reconfiguration. The
devices are fully characterized, and electrochromic performances are ensured by
bias-dependent UV-Vis, and Raman spectroscopy. The fabricated electro-chromic
devices are tested with the commercially available low-cost cells to perform,
which is highly de-sired for practical applications. The computational study
facilitates the understanding of experimental re-sults. The alternating current
(AC)-based electrical impedance spectroscopy reveals that P3HT facilitates
reducing charge transfer resistance of the devices. Our work shows CMOS
compatibility and one of the best-performing devices that could pave the way
for developing cost-effective flexible, and wearable electrochromic devices. | 2303.14648v2 |
2023-04-12 | Potential Major Improvement in Superconductors for High-Field Magnets | Fusion reactors are limited by the magnetic field available to confine their
plasma. The commercial fusion industry uses the larger magnetic field and
higher operating temperature of the cuprate superconductor
$\mathbf{YBa_{2}Cu_{3}O_{7-\delta}}$ (YBCO) in order to confine their plasma
into a dense volume. A superconductor is a macroscopic quantum state that is
protected from the metallic (resistive) state by an energy gap. Unfortunately,
YBCO has an anisotropic gap, known as D-wave because it has the shape of a
$\mathbf{d_{x^2-y^2}}$ chemical orbital. This D-wave gap means that
poly-crystalline wire cannot be made because a few degree misalignment between
grains in the wire leads to a drastic loss in its supercurrent carrying
ability, and thereby its magnetic field limit. The superconductor industry has
responded by growing nearly-single-crystal superconducting YBCO films on
carefully prepared substrate tapes kilometers in length. Heroic development
programs have made such tapes commercially available, but they are very
expensive and delicate. MRI magnet superconductors, such as $\mathbf{NbTi}$ and
$\mathbf{Nb_{3}Sn}$, are formed into poly-crystalline wires because they have
an isotropic gap in the shape of an s chemical orbital (called S-wave) that
makes them insensitive to grain misalignment. However, these materials are
limited to lower magnetic fields and liquid-He temperatures. Here, we modified
YBCO by doping the Y site with Ca and Ce atoms to form
$\mathbf{(Y_{1-x-y}Ca_{x}Ce_{y})Ba_{2}Cu_{3}O_{7-\delta}}$, and show evidence
that it changes to an S-wave gap. Its superconducting transition temperature,
$\mathbf{T_c}$, of $\mathbf{\sim 70K}$, while lower than that of D-wave YBCO at
$\mathbf{\sim 90K}$, is easily maintained using common, economic cryogenic
equipment. | 2304.06171v1 |
2023-05-11 | Highly tunable lateral homojunction formed in 2D layered CuInP2S6 via in-plane ionic migration | As basic building blocks for next-generation information technologies
devices, high-quality p-n junctions based on van der Waals (vdW) materials have
attracted widespread interest.Compared to traditional two dimensional (2D)
heterojunction diodes, the emerging homojunctions are more attractive owing to
their intrinsic advantages, such as continuous band alignments and smaller
carrier trapping. Here, utilizing the long-range migration of Cu + ions under
in-plane electric field, a novel lateral p-n homojunction was constructed in
the 2D layered copper indium thiophosphate (CIPS). The symmetric Au/CIPS/Au
devices demonstrate an electric-field-driven resistance switching (RS)
accompanying by a rectification behavior without any gate control. Moreover,
such rectification behavior can be continuously modulated by poling voltage. We
deduce that the reversable rectifying RS behavior is governed by the effective
lateral build-in potential and the change of the interfacial barrier during the
poling process. Furthermore, the CIPS p-n homojuction is evidenced by the
photovoltaic effect, with the spectral response extending up to visible region
due to the better photogenerated carrier separation efficiency. Our study
provides a facile route to fabricate homojuctions through electric-field-driven
ionic migration and paves the way towards the use of this method in other vdW
materials. | 2305.06607v1 |
2023-06-26 | Spin-flop quasi metamagnetic, anisotropic magnetic, and electrical transport behavior of Ho substituted kagome magnet ErMn$_6$Sn$_6$ | We report on the magnetic and electrical properties of a (Mn$_3$Sn)$_2$
triangular network kagome structured high quality Ho substituted ErMn$_6$Sn$_6$
single-crystal sample by magneto-transport measurements.
Er$_{0.5}$Ho$_{0.5}$Mn$_6$Sn$_6$ orders antiferromagnetically at N\'{e}el
temperature $T_\mathrm{N} \sim$ 350 K followed by a ferrimagnetic (FiM)
transition at $T_\mathrm{C} \sim$ 114 K and spin-orientation transition at
$T_\mathrm{t} \sim$ 20 K. The field-manifestations of these magnetic phases in
the \textit{ab}-basal plane and along the \textit{c}-axis are illustrated
through temperature-field \textit{T-H} phase diagrams. In
\textit{H}$\parallel$\textit{c}, narrow hysteresis between spin reorientation
and field-induced FiM phases below $T_\mathrm{t}$, enhanced/strengthened FiM
phase below $T_\mathrm{C}$ and stemming of FiM phase out of strongly coexisting
AFM and FiM phases below $T_\mathrm{N}$ through a non-meta-magnetic transition
are confirmed to arise from strong R-Mn sublattices interaction. In contrast,
\textit{H}$\parallel$\textit{ab}-plane, between $T_\mathrm{N}$ and
$T_\mathrm{C}$, individually contributing R-Mn sublattices with weak
antiferromagnetic interactions undergo a field-induced spin-flop
quasi-metamagnetic transition to FiM state. The temperature dependent
electrical resistivity suggests metallic nature with Fermi liquid behavior at
low temperatures. Essentially, the current study stimulates interest to
investigate the magnetic and electrical properties of mixed rare-earth layered
kagome magnetic metals for possible novel and exotic behavior. | 2306.14417v1 |
2023-07-28 | Pressure-induced Superconductivity in Zintl Topological Insulator SrIn2As2 | The Zintl compound AIn2X2 (A = Ca, Sr, and X = P, As), as a theoretically
predicted new non-magnetic topological insulator, requires experiments to
understand their electronic structure and topological characteristics. In this
paper, we systematically investigate the crystal structures and electronic
properties of the Zintl compound SrIn2As2 under both ambient and high-pressure
conditions. Based on systematic angle-resolved photoemission spectroscopy
(ARPES) measurements, we observed the topological surface states on its (001)
surface as predicted by calculations, indicating that SrIn2As2 is a strong
topological insulator. Interestingly, application of pressure effectively tuned
the crystal structure and electronic properties of SrIn2As2. Superconductivity
is observed in SrIn2As2 for pressure where the temperature dependence of the
resistivity changes from a semiconducting-like behavior to that of a metal. The
observation of nontrivial topological states and pressure-induced
superconductivity in SrIn2As2 provides crucial insights into the relationship
between topology and superconductivity, as well as stimulates further studies
of superconductivity in topological materials. | 2307.15629v1 |
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