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2016-07-26 | Experimental Observation of Topological Superconductivity and Majorana Zero Modes on beta-Bi2Pd Thin Films | Using a cryogenic scanning tunneling microscopy, we report the observation of
topologically nontrivial superconductivity on a single material of \beta-Bi2Pd
films grown by molecular beam epitaxy. The superconducting gap associated with
spinless odd-parity pairing opens on the surface and appears much larger than
the bulk one due to the Dirac-fermion enhanced parity mixing of surface pair
potential. Majorana zero modes (MZMs), supported by such superconducting
states, are identified at magnetic vortices. The superconductivity and MZMs
exhibit resistance to nonmagnetic defects, characteristic of
time-reversal-invariant topological superconductors. Our results demonstrate a
simple platform to generate, manipulate and braid MZMs for quantum computation. | 1607.07551v4 |
2016-07-28 | Imaging current-induced switching of antiferromagnetic domains in CuMnAs | The magnetic order in antiferromagnetic (AF) materials is hard to control
with external magnetic fields. However, recent advances in detecting and
manipulating AF order electrically have opened up new prospects for these
materials in basic and applied spintronics research. Using x-ray magnetic
linear dichroism microscopy, we show here that staggered effective fields
generated by electrical current can induce reproducible and reversible
modification of the antiferromagnetic domain structure in microdevices
fabricated from a tetragonal CuMnAs thin film. The current-induced domain
switching is inhomogeneous at the submicron level. A clear correlation between
the average domain orientation and the anisotropy of the electrical resistance
is demonstrated. | 1607.08478v2 |
2016-08-11 | Evolution of Insulator-Metal Phase Transitions in Epitaxial Tungsten Oxide Films during Electrolyte-Gating | An interface between an oxide and an electrolyte gives rise to various
processes as exemplified by electrostatic charge accumulation/depletion and
electrochemical reactions such as intercalation/decalation under electric
field. Here we directly compare typical device operations of those in electric
double layer transistor geometry by adopting ${A}$-site vacant perovskite
WO$_3$ epitaxial thin films as a channel material and two different
electrolytes as gating agent. $\textit{In situ}$ measurements of x-ray
diffraction and channel resistance performed during the gating revealed that in
both the cases WO$_3$ thin film reaches a new metallic state through multiple
phase transitions, accompanied by the change in out-of-plane lattice constant.
Electrons are electrostatically accumulated from the interface side with an
ionic liquid, while alkaline metal ions are more uniformly intercalated into
the film with a polymer electrolyte. We systematically demonstrate this
difference in the electrostatic and electrochemical processes, by comparing
doped carrier density, lattice deformation behavior, and time constant of the
phase transitions. | 1608.03357v1 |
2016-10-15 | Dynamical Conductivity of Dirac Materials | For graphene (a Dirac material) it has been theoretically predicted and
experimentally observed that DC resistivity is proportional to $ T^4$ when the
temperature is much less than Bloch- Gr\"{u}neisen ($\Theta_{BG}$) temperature
and T linear in opposite case ($T>>\Theta_{BG}$). Going beyond the DC case, we
investigate the dynamical conductivity in graphene using the powerful method of
memory function formalism. In the DC (zero frequency regime) limit, we obtained
the above mention behavior which was previously obtained using the
Bloch-Boltzmann kinetic equation. In the finite frequency regime, we obtained
several new results: (1) the generalized Drude scattering rate, in the zero
temperature limit, shows $\omega^4 $ behavior at low frequencies ($\omega <<
k_B \Theta_{BG}/ \hbar$) and saturates at higher frequencies. We also observed
the Holstein Mechanism, however, with different power laws from that in the
case of metals; (2) At higher frequencies, $\omega>>k_B \Theta_{BG}/ \hbar$,
and higher temperatures $T>>\Theta_{BG}$, we observed that the generalized
Drude scattering rate is linear in temperature. In addition, several other
results are also obtained. With the experimental advancement of this field,
these results should be experimentally tested. | 1610.04697v1 |
2016-10-18 | Magnetic properties of low-moment ferrimagnetic Heusler Cr2CoGa thin films grown by molecular beam epitaxy | Recently, theorists have predicted many materials with a low magnetic moment
and large spin-polarization for spintronic applications. These compounds are
predicted to form in the inverse Heusler structure, however, many of these
compounds have been found to phase segregate. In this study, ordered Cr2CoGa
thin films were synthesized without phase segregation using molecular beam
epitaxy. The present as-grown films exhibit a low magnetic moment from
antiferromagnetically coupled Cr and Co atoms as measured with SQUID
magnetometry and soft X-ray magnetic circular dichroism. Electrical
measurements demonstrated a thermally-activated semiconductor-like resistivity
with an activation energy of 87 meV. These results confirm spin gapless
semiconducting behavior, which makes these thin films well positioned for
future devices. | 1610.05808v1 |
2016-10-21 | On the impact of strain on the electronic properties of InAs/GaSb quantum well systems | Electron-hole hybridization in InAs/GaSb double quantum well structures leads
to the formation of a mini band gap. We experimentally and theoretically
studied the impact of strain on the transport properties of this material
system. Thinned samples were mounted to piezo electric elements to exert strain
along the [011] and [001] crystal directions. When the Fermi energy is tuned
through the mini gap, a dramatic impact on the resistivity at the charge
neutrality point is found which depends on the amount of applied external
strain. In the electron and hole regimes, strain influences the Landau level
structure. By analyzing the intrinsic strain from the epitaxial growth, the
external strain from the piezo elements and combining our experimental results
with numerical simulations of strained and unstrained quantum wells, we
compellingly illustrate why the InAs/GaSb material system is regularly found to
be semimetallic. | 1610.06776v1 |
2016-11-09 | Parallel and series conduction model in Topological Insulators | In the past few years there has been a surge in the material science
engineering in order to synthesize bulk insulating and surface metallic
Topological Insulating (TI) materials. This quest is not only theoretically
important but also promising from the novel application perspective. The
dependence of temperature on resistance (R-T) of a particular sample reveals a
plethora of information about the electronic properties especially in a unique
sample like TI where there are two components comprising of an insulating bulk
and metallic surface states. Depending on the amount of intrinsic doping during
the sample formation, the bulk can either couple or remain decoupled with the
surface. The former leads to a metallic R-T profile whereas the latter is
captured by an insulating R-T behavior. These two behaviors can be represented
by series and parallel resistor models respectively. In this work we study the
R-T behavior in the framework of resistor models capturing the essential
features of our sample. | 1611.02856v1 |
2016-12-11 | Superconductivity in Se-doped new materials EuSr2Bi2S4F4 and Eu2SrBi2S4F4 | From our powder x ray diffraction pattern, electrical transport and magnetic
studies we report the effect of isovalent Se substitution at S sites in the
newly discovered systems EuSr2Bi2S4F4 and Eu2SrBi2S4F4. We have synthesized two
new variants of 3244 type superconductor with Eu replaced by Sr which is
reported elsewhere [Z. Haque et. al.]. We observe superconductivity at Tc 2.9 K
(resistivity) and 2.3 K (susceptibility) in EuSr2Bi2S4-xSexF4 series for x = 2.
In the other series Eu2SrBi2S4-xSexF4, two materials (x= 1.5; Tc = 2.6 K and x
= 2; Tc = 2.75 K) exhibit superconductivity. | 1612.03415v1 |
2017-02-16 | Millimeter-scale layered MoSe2 grown on sapphire and evidence for negative magnetoresistance | Molecular beam epitaxy technique has been used to deposit a single layer and
a bilayer of MoSe 2 on sapphire. Extensive characterizations including in-situ
and ex-situ measurements show that the layered MoSe 2 grows in a scalable
manner on the substrate and reveals characteristics of a stoichiometric
2H-phase. The layered MoSe 2 exhibits polycrystalline features with domains
separated by defects and boundaries. Temperature and magnetic field dependent
resistivity measurements unveil a carrier hopping character described within
two-dimensional variable range hopping mechanism. Moreover, a negative
magnetoresistance was observed, stressing a fascinating feature of the charge
transport under the application of a magnetic field in the layered MoSe 2
system. This negative magnetoresistance observed at millimeter-scale is similar
to that observed recently at room temperature inWS2 flakes at a micrometer
scale [Zhang et al., Appl. Phys. Lett. 108, 153114 (2016)]. This scalability
highlights the fact that the underlying physical mechanism is intrinsic to
these two-dimensional materials and occurs at very short scale. | 1702.05121v1 |
2017-06-16 | Gate-tunable large magnetoresistance in an all-semiconductor spin-transistor-like device | A large spin-dependent and electric field-tunable magnetoresistance of a
two-dimensional electron system (2DES) is a key ingredient for the realization
of many novel concepts for spin-based electronic devices. The low
magnetoresistance observed during the last decades in devices with lateral
semiconducting (SC) transport channels between ferromagnetic (FM) source (S)
and drain (D) contacts has been the main obstacle for realizing spin field
effect transistor proposals. Here, we show both, a large two terminal
magnetoresistance in lateral 2DES-based spin valve geometry, with up to 80%
resistance change, and tunability of the magnetoresistance by an electric gate.
The large magnetoresistance is due to finite electric field effects at the
FM/SC interface, which boost spin-to-charge conversion. The gating scheme we
use is based on switching between uni- and bi-directional spin diffusion,
without resorting to the spin-orbit coupling. | 1706.05239v1 |
2017-06-19 | Electrical signature of individual magnetic skyrmions in multilayered systems | Magnetic skyrmions are topologically protected whirling spin textures that
can be stabilized in magnetic materials in which a chiral interaction is
present. Their limited size together with their robustness against the external
perturbations promote them as the ultimate magnetic storage bit in a novel
generation of memory and logic devices. Despite many examples of the signature
of magnetic skyrmions in the electrical signal, only low temperature
measurements, mainly in magnetic materials with B20 crystal structure, have
demonstrated the skyrmions contribution to the electrical transport properties.
Using the combination of Magnetic Force Microscopy (MFM) and Hall resistivity
measurements, we demonstrate the electrical detection of sub-100 nm skyrmions
in multilayered thin film at room temperature (RT). We furthermore analyse the
room temperature Hall signal of a single skyrmion which contribution is mainly
dominated by anomalous Hall effect. | 1706.05809v1 |
2017-06-26 | The Sobering Reality of Perovskite/Si Tandem Solar Cells under Realistic Operating Conditions | Perovskite/Si tandem solar cells have the potential to considerably
out-perform conventional solar cells. Under standard test conditions,
perovskite/Si tandem solar cells already outperform the Si single junction.
Under realistic conditions, however, as we show, those tandem solar cells are
hardly more efficient than the Si cell alone. We model the performance of
realistic perovskite/Si tandem solar cells under real-world climate conditions,
by incorporating parasitic cell resistances, non-radiative recombination, and
optical losses into the detailed-balance limit. We show quantitatively that
optimizing these parameters in the perovskite top cell, perovskite/Si tandem
solar cells reach an efficiency advantage of up to 14% absolute, even while
leaving the Si cell untouched. Despite the rapid efficiency increase of
perovskite solar cells, our results emphasize the need for further material
development, careful device design, and light management strategies, all
necessary for highly efficient perovskite/Si tandem solar cells. | 1706.08610v1 |
2017-09-11 | Signature of growth-deposition technique on the properties of PECVD and thermal SiO2 | In this article, we report the process induced variation in the
characteristics of PECVD deposited and thermally grown silicon dioxide (SiO2)
thin film. We find key differences in the porosity, arrangement of the
nano-pores, surface roughness, refractive index and electrical resistivity of
the SiO2 thin films obtained by the two methods. While the occurrence of the
nanoporous structure is an inherent property of the material and independent of
the process of film growth or deposition, the arrangements of these nano-pores
in the oxide film is process dependent. The distinct arrangements of the
nano-pores are signatures of the deposition/growth processes. Morphological
analysis has been carried out to demonstrate the difference between oxides
either grown by thermal oxidation or through PECVD deposition. The tunable
conductive behavior of the metal filled nano-porous oxides is also
demonstrated, which has potential to be used as conductive oxides in various
applications. | 1709.03257v1 |
2017-10-10 | Evidence of a topological Hall effect in Eu$_{1-x}$Sm$_x$TiO$_3$ | We report on the observation of a possible topological Hall effect in thin
films of the itinerant ferromagnet Eu1-xSmxTiO3. EuTiO3 and Eu0.955Sm0.045TiO3
films were grown by molecular beam epitaxy. The EuTiO3 film is insulating. The
Hall resistivity of the Eu0.955Sm0.045TiO3 films exhibits the anomalous Hall
effect below the Curie temperature of ~ 5 K as well as additional features that
appear at 2 K. It is shown that these features are magnetic in origin and
consistent with the topological Hall effect seen in materials systems with
topologically nontrivial spin textures, such as skyrmions. The results open up
interesting possibilities for epitaxial hybrid heterostructures that combine
topological magnetic states, tunable carrier densities, and other phenomena. | 1710.03729v1 |
2017-10-12 | Thermal conductivity reduction in rough silicon nanomembranes | Nanostructured silicon is a promising material for thermoelectric conversion,
because the thermal conductivity in silicon nanostructures can be strongly
reduced with respect to that of bulk materials. We present thermal conductivity
measurements, performed with the 3$\omega$ technique, of suspended
monocrystalline silicon thin films (nanomembranes or nanoribbons) with smooth
and rough surfaces. We find evidence for a significant effect of surface
roughness on phonon propagation: the measured thermal conductivity for the
rough structures is well below that predicted by theoretical models which take
into account diffusive scattering on the nanostructure walls. Conversely, the
electrical conductivity appears to be substantially unaffected by surface
roughness: the measured resistance of smooth and rough nanostructures are
comparable, if we take into account the geometrical factors. Nanomembranes are
more easily integrable in large area devices with respect to nanowires and are
mechanically stronger and able to handle much larger electrical currents (thus
enabling the fabrication of thermoelectric devices that can supply higher power
levels with respect to current existing solutions). | 1710.04403v1 |
2017-10-17 | XPS study of the chemical stability of DyBa2Cu3O6+δ superconductor | The chemical stability of the powder DyBa2Cu3O6+{\delta} has been studied by
X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and thermal
analysis at ambient conditions. The powder was subjected to mechanical
processing in a ball mill-activator to accelerate chemical degradation. The
kinetic regularities of hydrolytic decomposition of DyBa2Cu3O6+{\delta} under
the influence of air moisture have been determined. The resistive properties of
DyBa2Cu3O6+{\delta} to water have been found to be better, but not much
different from analogous properties of YBa2Cu3O6+{\delta} which is unstable in
a wet environment. Chemical degradation of the material is triggered by crucial
concentrating of water particles near the free surface of the solid reactant
(due to their low diffusibility in the bulk) leading to rapid chemical
decomposition of the respective regions. | 1710.06271v1 |
2017-10-27 | Percolative nature of the dc paraconductivity in the cuprate superconductors | We present an investigation of the planar direct-current (dc)
paraconductivity of the model cuprate material HgBa$_2$CuO$_{4+\delta}$ in the
underdoped part of the phase diagram. The simple quadratic
temperature-dependence of the Fermi-liquid normal-state resistivity enables us
to extract the paraconductivity above the macroscopic $T_c$ with great
accuracy. The paraconductivity exhibits unusual exponential temperature
dependence, with a characteristic temperature scale that is distinct from
$T_c$. In the entire temperature range where it is discernable, the
paraconductivity is quantitatively explained by a simple superconducting
percolation model, which implies that underlying gap disorder dominates the
emergence of superconductivity. | 1710.10220v1 |
2017-10-31 | Suppression of vacancies boosts thermoelectric performance in type-I clathrates | Intermetallic type-I clathrates continue to attract attention as promising
thermoelectric materials. Here we present structural and thermoelectric
properties of single crystalline Ba8(Cu,Ga,Ge,v)46, where v denotes a vacancy.
By single crystal X-ray diffraction on crystals without Ga we find clear
evidence for the presence of vacancies at the 6c site in the structure. With
increasing Ga content, vacancies are successively filled. This increases the
charge carrier mobility strongly, even within a small range of Ga substitution,
leading to reduced electrical resistivity and enhanced thermoelectric
performance. The largest figure of merit ZT =0.9 at 900 K is found for a single
crystal of approximate composition Ba8Cu4.6Ga1.0Ge40.4. This value, that may
further increase at higher temperatures, is one of the largest to date found in
transition metal element-based clathrates. | 1710.11536v1 |
2018-01-02 | Edge-Grafted Molecular Junctions between Graphene Nanoplatelets: Applied Chemistry to Enhance Heat Transfer in Nanomaterials | The edge-functionalization of graphene nanoplatelets (GnP) was carried out
exploiting diazonium chemistry, aiming at the synthesis of edge decorated
nanoparticles to be used as building blocks in the preparation of engineered
nanostructured materials for enhanced heat transfer. Indeed, both phenol
functionalized and dianiline-bridged GnP (GnP-OH and E-GnP, respectively) were
assembled in nanopapers exploiting the formation of non-covalent and covalent
molecular junctions, respectively. Molecular dynamics allowed to estimate the
thermal conductance for the two different types of molecular junction,
suggesting a factor 6 between conductance of covalent vs. non-covalent
junctions. Furthermore, the chemical functionalization was observed to drive
the self-organization of the nanoflakes into the nanopapers, leading to a 20%
enhancement of the thermal conductivity for GnP-OH and E-GnP while the cross
plane thermal conductivity was boosted by 150% in the case of E-GnP. The
application of chemical functionalization to the engineering of contact
resistance in nanoparticles network was therefore validated as a fascinating
route for the enhancement of heat exchange efficiency on nanoparticle networks,
with great potential impact in low-temperature heat exchange and recovery
applications | 1801.00859v1 |
2018-01-03 | Ion-exchange synthesis and superconductivity at 8.6 K of Na2Cr3As3 with quasi-one-dimensional crystal structure | A new Cr-based quasi-one-dimensional superconductor Na2Cr3As3 was synthesized
by an ion-exchange method in sodium naphthalenide solution. The crystals are
thread-like and the structure was analyzed by X-ray diffraction with a
noncentrosymmetric hexagonal space group P-6m2 (No. 187), in which the
(Cr3As3)2- linear chains are separated by Na+ ions, and the refined lattice
parameters are a = 9.239(2) {\AA} and c = 4.209(6) {\AA}. The measurements for
electrical resistivity, magnetic susceptibility, and heat capacity reveal a
superconducting transition with unconventional characteristic at the Tc of 8.6
K, which exceeds that of all previously reported Cr-based superconductors. | 1801.01010v1 |
2018-01-04 | Crystal growth and magnetic anisotropy in the spin-chain ruthenate Na$_2$RuO$_4$ | We report single crystal growth, electrical resistivity $\rho$, anisotropic
magnetic susceptibiltiy $\chi$, and heat capacity $C_p$ measurements on the
one-dimensional spin-chain ruthenate Na$_2$RuO$_4$. We observe variable range
hopping (VRH) behaviour in $\rho(T)$. The magnetic susceptibility with magnetic
field perpendicular ($\chi_\perp$) and parallel ($\chi_\parallel$) to the
spin-chains is reported. The magnetic properties are anisotropic with
$\chi_\perp > \chi_\parallel$ in the temperature range of measurements T
$\approx 2$ to $305$ K with $\chi_\perp / \chi_\parallel$ $\approx 1.4$ at
$305$ K. Analysis of the $\chi(T)$ data reveals an anisotropy in the $g$-factor
and Van-Vleck paramagnetic contribution. An anomaly in $\chi(T)$ and a
corresponding lambda-like anomaly in $C_p$ at $T_N = 37$ K confirms long-range
antiferromagnetic ordering. This temperature is an order of magnitude smaller
than the Weiss temperature $\theta \sim -250$ K and points to suppression of
long range magnetic order due to low dimensionality. However, we were unable to
get a satisfactory fit of the experimental $\chi(T)$ by an isolated
one-dimensional spin-chain model, suggesting the importance of inter-chain
interactions in Na$_2$RuO$_4$. | 1801.01524v1 |
2018-01-17 | The simultaneous discharge of liquid and grains from a silo | The flow rate of water through an orifice at the bottom of a container
depends on the hydrostatic pressure whereas for a dry granular material is
nearly constant. But what happens during the simultaneous discharge of grains
and liquid from a silo? By measuring the flow rate as a function of time, we
found that: (i) different regimes appear, going from constant flow rate
dominated by the effective fluid viscosity to a hydrostatic-like discharge
depending on the aperture and grain size, (ii) the mixed material is always
discharged faster than dry grains but slower than liquid, (iii) for the
mixture, the liquid level drops faster than the grains level but they are
always linearly proportional to one another, and (iv) a sudden growth in the
flow rate happens during the transition from a biphasic to a single phase
discharge. These results are associated to the competition between the decrease
of hydrostatic pressure above the granular bed and the hydrodynamic resistance.
A model combining the Kozeny-Carman, Bernoulli and mass conservation equations
is proposed and the numerical results are in good agreement with experiments. | 1801.05747v1 |
2018-02-16 | Levitated electromechanics: all-electrical cooling of charged nano- and micro-particles | We show how charged levitated nano- and micro-particles can be cooled by
interfacing them with an $RLC$ circuit. All-electrical levitation and cooling
is applicable to a wide range of particle sizes and materials, and will enable
state-of-the-art force sensing within an electrically networked system.
Exploring the cooling limits in the presence of realistic noise we find that
the quantum regime of particle motion can be reached in cryogenic environments
both for passive resistive cooling and for an active feedback scheme, paving
the way to levitated quantum electromechanics. | 1802.05928v3 |
2018-02-21 | Design of Chern Insulating Phases in Honeycomb Lattices | The search for robust examples of the magnetic version of topological
insulators, referred to as quantum anomalous Hall insulators or simply Chern
insulators, so far lacks success. Our groups have explored two distinct
possibilities based on multiorbital 3d oxide honeycomb lattices. Each has a
Chern insulating phase near the ground state, but materials parameters were not
appropriate to produce a viable Chern insulator. Further exploration of one of
these classes, by substituting open shell 3d with 4d and 5d counterparts, has
led to realistic prediction of Chern insulating ground states. Here we recount
the design process, discussing the many energy scales that are active in
participating (or resisting) the desired Chern insulator phase. | 1802.07411v1 |
2018-02-21 | Meta-screening and permanence of polar distortion in metallized ferroelectrics | Ferroelectric materials are characterized by a spontaneous polar distortion.
The behavior of such distortions in the presence of free charge is the key to
the physics of metallized ferroelectrics in particular, and of
structurally-polar metals more generally. Using first-principles simulations,
here we show that a polar distortion resists metallization and the attendant
suppression of long-range dipolar interactions in the vast majority of a sample
of 11 representative ferroelectrics. We identify a meta-screening effect,
occurring in the doped compounds as a consequence of the charge rearrangements
associated to electrostatic screening, as the main factor determining the
survival of a non-centrosymmetric phase. Our findings advance greatly our
understanding of the essentials of structurally-polar metals, and offer
guidelines on the behavior of ferroelectrics upon field-effect charge injection
or proximity to conductive device elements. | 1802.07464v1 |
2018-04-09 | Canted antiferromagnetism in phase-pure CuMnSb | We report the low-temperature properties of phase-pure single crystals of the
half-Heusler compound CuMnSb grown by means of optical float-zoning. The
magnetization, specific heat, electrical resistivity, and Hall effect of our
single crystals exhibit an antiferromagnetic transition at $T_{\mathrm{N}} =
55~\mathrm{K}$ and a second anomaly at a temperature $T^{*} \approx
34~\mathrm{K}$. Powder and single-crystal neutron diffraction establish an
ordered magnetic moment of $(3.9\pm0.1)~\mu_{\mathrm{B}}/\mathrm{f.u.}$,
consistent with the effective moment inferred from the Curie-Weiss dependence
of the susceptibility. Below $T_{\mathrm{N}}$, the Mn sublattice displays
commensurate type-II antiferromagnetic order with propagation vectors and
magnetic moments along $\langle111\rangle$ (magnetic space group $R[I]3c$).
Surprisingly, below $T^{*}$, the moments tilt away from $\langle111\rangle$ by
a finite angle $\delta \approx 11^{\circ}$, forming a canted antiferromagnetic
structure without uniform magnetization consistent with magnetic space group
$C[B]c$. Our results establish that type-II antiferromagnetism is not the
zero-temperature magnetic ground state of CuMnSb as may be expected of the
face-centered cubic Mn sublattice. | 1804.03223v1 |
2018-04-24 | Electrical and thermal transport in coplanar polycrystalline graphene-hBN heterostructures | We present a theoretical study of electronic and thermal transport in
polycrystalline heterostructures combining graphene (G) and hexagonal boron
nitride (hBN) grains of varying size and distribution. By increasing the hBN
grain density from a few percents to $100\%$, the system evolves from a good
conductor to an insulator, with the mobility dropping by orders of magnitude
and the sheet resistance reaching the M$\Omega$ regime. The Seebeck coefficient
is suppressed above $40\%$ mixing, while the thermal conductivity of
polycrystalline hBN is found to be on the order of $30-120\,{\rm W}{\rm
m}^{-1}{\rm K}^{-1}$. These results, agreeing with available experimental data,
provide guidelines for tuning G-hBN properties in the context of
two-dimensional materials engineering. In particular, while we proved that both
electrical and thermal properties are largely affected by morphological
features (like e.g. by the grain size and composition), we find in all cases
that nm-sized polycrystalline G-hBN heterostructures are not good
thermoelectric materials. | 1804.09272v2 |
2018-05-08 | Relativistic Gurzhi effect in channels of Dirac materials | Charge transport in channel-shaped 2D Dirac systems is studied employing the
Boltzmann equation. The dependence of the resistivity on temperature and
chemical potential is investigated. An accurate understanding of the influence
of electron-electron interaction and material disorder allows us to identify a
parameter regime, where the system reveals hydrodynamic transport behavior. We
point out the conditions for three Dirac fermion specific features: heat flow
hydrodynamics, pseudo\-diffusive transport, and the electron-hole scattering
dominated regime. It is demonstrated that for clean samples the relativistic
Gurzhi effect, a definite indicator of hydrodynamic transport, can be observed. | 1805.02987v2 |
2018-05-28 | Phonon Fingerprints of CsPb2Br5 Single Crystals | CsPb2Br5 is a stable, water-resistant, material derived from CsPbBr3
perovskite and featuring two-dimensional Pb-Br framework separated by Cs
layers. Both compounds can coexist at nano- length scale, which often produces
conflicting optical spectroscopy results. We present a complete set of
polarized Raman spectra of nonluminescent CsPb2Br5 single crystals that reveals
the symmetry and frequency of nondegenerate Raman active phonons accessible
from the basal (001) plane. The experimental results are in good agreement with
density functional perturbation theory simulations, which suggests that the
calculated frequencies of yet unobserved double degenerate Raman and infrared
phonons are also reliable. Unlike CsPbBr3, the lattice dynamics of CsPb2Br5 is
stable as evidenced by the calculated phonon dispersion. The sharp Raman lines
and lack of a dynamic- disorder-induced central peak in the spectra at room
temperature indicate that the coupling of Cs anharmonic motion to Br atoms,
known to cause the dynamic disorder in CsPbBr3, is absent in CsPb2Br5. | 1805.11181v1 |
2018-06-01 | Flat bands and the physics of strongly correlated Fermi systems | Some materials can have the dispersionless parts in their electronic spectra.
These parts are usually called flat bands and generate the corps of unusual
physical properties of such materials. These flat bands are induced by the
condensation of fermionic quasiparticles, being very similar to the Bose
condensation. The difference is that fermions to condense, the Fermi surface
should change its topology, leading to violation of time-reversal (T) and
particle-hole (C) symmetries. Thus, the famous Landau theory of Fermi liquids
does not work for the systems with fermion condensate (FC) so that several
experimentally observable anomalies have not been explained so far. Here we use
FC approach to explain recent observations of the asymmetric tunneling
conductivity in heavy-fermion compounds and graphene and its restoration in
magnetic fields, as well as the violation of Leggett theorem, recently observed
experimentally in overdoped cuprates, and recent observation of the challenging
universal scaling connecting linear-$T$-dependent resistivity to the
superconducting superfluid density. | 1806.00438v1 |
2018-06-19 | Influence of Interfaces on the Transport Properties of Graphite revealed by Nanometer Thickness Reduction | We investigated the influence of thickness reduction on the transport
properties of graphite microflakes. Using oxygen plasma etching we decreased
the thickness of highly oriented pyrolytic graphite (HOPG) microflakes from
$\sim 100$~nm to $\sim 20$~nm systematically. Keeping current and voltage
electrodes intact, the electrical resistance $R(T)$, the magnetoresistance (MR)
and Raman spectra were measured in every individual sample and after each
etching step of a few nm. The results show that $R(T)$ and MR can increase or
decrease with the sample thickness in a non-systematic way. The results
indicate that HOPG samples are inhomogeneous materials, in agreement with
scanning transmission electron microscopy images and X-ray diffraction data.
Our results further indicate that the quantum oscillations in the MR are not an
intrinsic property of the ideal graphite structure but their origin is related
to internal conducting interfaces. | 1806.07417v3 |
2018-06-20 | Concurrent transitions in wear rate and surface microstructure in nanocrystalline Ni-W | Nanocrystalline metals are promising materials for wear-resistant
applications due to their superior strength and hardness, but prior work has
shown that cyclic loading can lead to coarsening. In this study, scratch wear
tests were carried out on nanocrystalline Ni-19 at.% W films with an
as-deposited grain size of 3 nm, with systematic characterization performed
after different wear cycles. A new gradient nanograined microstructure is
observed and a direct connection between wear rate and subsurface
microstructure is discovered. A second Ni-W specimen with the same composition
and a 45 nm average grain size is produced by annealing the original specimen.
Subsequent wear testing shows that an identical subsurface microstructure is
produced in this sample, emphasizing the importance of the cross-over in
deformation mechanisms for determining the steady-state grain size during wear. | 1806.07919v2 |
2018-06-27 | Spin splitting induced in a superconductor by an antiferromagnetic insulator | Inspired by recent feats in exchange coupling antiferromagnets to an adjacent
material, we demonstrate the possibility of employing them for inducing spin
splitting in a superconductor, thereby avoiding the detrimental, parasitic
effects of ferromagnets employed to this end. We derive the Gor'kov equation
for the matrix Green's function in the superconducting layer, considering a
microscopic model for its disordered interface with a two-sublattice magnetic
insulator. We find that an antiferromagnetic insulator with effectively
uncompensated interface induces a large, disorder-resistant spin splitting in
the adjacent superconductor. In addition, we find contributions to the
self-energy stemming from the interfacial disorder. Within our model, these
mimic impurity and spin-flip scattering, while another breaks the symmetries in
particle-hole and spin spaces. The latter contribution, however, drops out in
the quasi-classical approximation and thus, does not significantly affect the
superconducting state. | 1806.10356v2 |
2019-08-30 | Efficient Pourbaix diagrams of many-element compounds | Pourbaix diagrams have long been an essential tool for determining the phase
stability of solids and their associated ionic species under electrochemical
conditions. In recent years, Pourbaix diagrams have been used for applications
ranging from corrosion-resistance alloy design to electrocatalysis, and the
data from which they are generated has been enhanced by the availability of
materials data in various online databases. However, generation of
multi-element Pourbaix diagrams has a critical bottleneck which makes 3-element
systems difficult to analyze quickly, and 4 and 5 element systems intractable.
In this work, we present a method for constructing Pourbaix diagrams which uses
a pre-processing step to circumvent the most egregious computational bottleneck
and make many-element Pourbaix diagrams computationally efficient. | 1909.00035v1 |
2019-09-02 | Nematic properties of FeSe$_{1-x}$Te$_{x}$ crystals with a low Te content | We report on the synthesis and physical properties of FeSe$_{1-x}$Te$_x$
single crystals with a low Te content (x = 0.17, 0.21, 0.25), where the
replacement of Se with Te partially suppresses superconductivity. Resistivity
and Hall effect measurements indicate weak anomalies at elevated temperatures
ascribed to nematic transitions. A quasi-classical analysis of transport data,
including in a pulsed magnetic field of up to 25 T, confirms the inversion of
majority carriers type from holes in FeSe to electrons in FeSe$_{1-x}$Te$_x$ at
x $>$ 0.17. The temperature-dependent term in the elastoresistance of the
studied compositions has a negative sign, which means that for substituted FeSe
compositions, the elastoresistance is positive for hole-doped materials and
negative for electron-doped materials just like in semiconductors such as
silicon and germanium. | 1909.00711v1 |
2019-09-10 | Ferroelectric enhancement of superconductivity in compressively strained SrTiO$_3$ films | SrTiO$_3$ is an incipient ferroelectric on the verge of a polar instability,
which is avoided at low temperatures by quantum fluctuations. Within this
unusual quantum paraelectric phase, superconductivity persists despite
extremely dilute carrier densities. Ferroelectric fluctuations have been
suspected to play a role in the origin of superconductivity by contributing to
electron pairing. To investigate this possibility, we used optical second
harmonic generation to measure the doping and temperature dependence of the
ferroelectric order parameter in compressively strained SrTiO$_3$ thin films.
At low temperatures, we uncover a spontaneous out-of-plane ferroelectric
polarization with an onset that correlates perfectly with normal-state
electrical resistivity anomalies. These anomalies have previously been
associated with an enhancement of the superconducting critical temperature in
doped SrTiO$_3$ films, directly linking the ferroelectric and superconducting
phases. We develop a long-range mean-field Ising model of the ferroelectric
phase transition to interpret the data and extract the relevant energy scales
in the system. Our results support a long-suspected connection between
ferroelectricity and superconductivity in SrTiO$_3$, but call into question the
role played by ferroelectric fluctuations. | 1909.04782v1 |
2014-08-07 | Magneto-transport properties of oriented Mn2CoAl films sputtered on thermally oxidized Si substrates | Spin gapless semiconductors are interesting novel class of materials by
embracing both magnetism and semiconducting. Its potential application in
future spintronics requires realization in thin film form. In this letter, we
report a successful growth of spin gapless Mn2CoAl films on thermally oxidized
Si substrates by magnetron sputtering deposition. The films deposited at 673K
are well oriented to (001) direction and display a uniform-crystalline surface.
Magnetotransport measurements on the oriented films reveal a
semiconducting-like resistivity, small anomalous Hall conductivity and linear
magnetoresistance (MR) representative of the transport signatures of spin
gapless semiconductors. The magnetic properties of the films have also been
investigated and compared to that of bulk Mn2CoAl, with small discrepancy
induced by the composition deviation. | 1408.1547v1 |
2017-03-06 | Pressure-Induced Insulator-to-Metal Transition Provides Evidence for Negative-$U$ Centers in Large-Gap Disordered Insulators | Attractive negative-$U$ interactions between electrons facilitated by strong
electron-phonon interaction are common in highly polarizable and disordered
materials such as amorphous chalcogenides, but there is no direct evidence for
them in large-band-gap insulators. Here we report how such negative-$U$ centers
are responsible for widespread insulator-to-metal transitions in amorphous
HfO$_2$ and Al$_2$O$_3$ thin films with a 10$^9$-fold resistance drop.
Triggered by a static hydraulic pressure or a 0.1 ps impulse of magnetic
pressure, the transition can proceed at such low pressure that there is very
little overall deformation (strain~10$^{-5}$). Absent a significant energy
change overall, the transition is attributed to the reversal of localized
electron-phonon interaction: By reversing the sign of $U$, trapped electrons
are destabilized and released, thus clearing conduction paths previously
blocked by charged traps. The results also suggest that Mott insulators when
disordered may become Anderson insulators with strong electron-phonon
interactions regulating incipient conduction paths, a novel finding of
technological significance for electronic devices. | 1703.02003v1 |
2017-03-20 | Optimized spin-injection efficiency and spin MOSFET operation based on low-barrier ferromagnet/insulator/n-Si tunnel contact | We theoretically investigate the spin injection in different FM/I/n-Si tunnel
contacts by using the lattice NEGF method. We find that the tunnel contacts
with low barrier materials such as TiO$_2$ and Ta$_{2}$O$_{5}$, have much lower
resistances than the conventional barrier materials, resulting in a wider and
attainable optimum parameters window for improving the spin injection
efficiency and MR ratio of a vertical spin MOSFET. Additionally, we find the
spin asymmetry coefficient of TiO$_2$ tunnel contact has a negative value,
while that of Ta$_{2}$O$_{5}$ contact can be tuned between positive and
negative values, by changing the parameters. | 1703.06835v3 |
2013-12-24 | All-Metallic Electrically-Gated Tantalum Diselenide Switches and Logic Circuits | We report the fabrication and performance of all-metallic three-terminal
devices with tantalum diselenide thin-film conducting channels. For this
proof-of-concept demonstration, the layers of 2H-TaSe2 were exfoliated
mechanically from single crystals grown by the chemical vapor transport method.
Devices with nanometer-scale thicknesses exhibit strongly non-linear
current-voltage characteristics, unusual optical response, and electrical
gating at room temperature. We have found that the drain-source current in
thin-film 2H-TaSe2-Ti/Au devices reproducibly shows an abrupt transition from a
highly resistive to a conductive state, with the threshold tunable via the gate
voltage. Such current-voltage characteristics can be used in principle for
implementing radiation-hard all-metallic logic circuits. These results may open
new application space for thin films of van der Waals materials. | 1312.6863v1 |
2014-10-27 | Giant supercurrent states in a superconductor-InAs/GaSb-superconductor junction | Superconductivity in topological materials has attracted a great deal of
interest in both electron physics and material sciences since the theoretical
predictions that Majorana fermions can be realized in topological
superconductors [1-4]. Topological superconductivity could be realized in a
type II, band-inverted, InAs/GaSb quantum well if it is in proximity to a
conventional superconductor. Here we report observations of the proximity
effect induced giant supercurrent states in an InAs/GaSb bilayer system that is
sandwiched between two superconducting tantalum electrodes to form a
superconductor-InAs/GaSb-superconductor junction. Electron transport results
show that the supercurrent states can be preserved in a surprisingly large
temperature-magnetic field (T-H) parameter space. In addition, the evolution of
differential resistance in T and H reveals an interesting superconducting gap
structure. | 1410.7342v3 |
2014-10-30 | Kondo effect and non-Fermi liquid behavior in Dirac and Weyl semimetals | We study the Kondo effect in three-dimensional (3D) Dirac materials and Weyl
semimetals. We find the scaling of the Kondo temperature with respect to the
doping $n$ and the coupling $J$ between the moment of the magnetic impurity and
the carriers of the semimetal. We find that when the temperature is much
smaller than the Kondo temperature the resistivity due to the Kondo effect
scales as the $n^{-4/3}$.We also study the effect of the interplay of
long-range scalar disorder and Kondo effect. In the presence of
disorder-induced long-range carrier density inhomogeneities the Kondo effect is
not characterized by a Kondo temperature but by a distribution of Kondo
temperatures. We obtain the expression of such distribution and show that its
features cause the appearance of strong non-Fermi liquid behavior. Finally we
compare the properties of the Kondo effect in 3D Dirac materials and 2D Dirac
systems like graphene and topological insulators. | 1410.8532v2 |
2014-11-07 | Temperature evolution of magnetic and transport behavior in 5\textit{d} Mott insulator Sr$_2$IrO$_4$: Significance of magneto-structural coupling | We have investigated the temperature evolution of magnetism and its
interrelation with structural parameters in perovskite-based layered compound
Sr$_2$IrO$_4$, which is believed to be a $J_{eff}$ = 1/2 Mott insulator. The
structural distortion plays an important role in this material which induces a
weak ferromagnetism in otherwise antiferromagnetically ordered magnetic state
with transition temperature around 240 K. Interestingly, at low temperature
below around 100 K, a change in magnetic moment has been observed. Temperature
dependent x-ray diffraction measurements show sudden changes in structural
parameters around 100 K are responsible for this. Resistivity measurements show
insulating behavior throughout the temperature range across the magnetic phase
transition. The electronic transport can be described with Mott's
two-dimensional variable range hopping (VRH) mechanism, however, three
different temperature ranges are found for VRH, which is a result of varying
localization length with temperature. A negative magnetoresistance (MR) has
been observed at all temperatures in contrast to positive behavior generally
observed in strongly spin-orbit coupled materials. The quadratic field
dependence of MR implies a relevance of a quantum interference effect. | 1411.1946v1 |
2014-11-19 | Electronic structure and transport properties of Cu-deficient kuramite Cu3-xSnS4 | Electrical and thermal transport properties of Cu-deficient kuramite
Cu3-xSnS4 (CTS) was examined as a possible earth-abundant thermoelectric
material. Crystallographic structure of CTS was characterized by partial
disorder between Cu and Sn. In contrast to semiconducting electrical transport
of related compounds, such as Cu2ZnSnS4 and Cu3SbS4, metallic conduction with
an electrical resistivity of 0.4 mohmcm and a carrier concentration of 3 \times
10^21 cm-3 was observed at 300 K. Lattice thermal conductivity was calculated
at 2.6 Wm^-1K^-1, which was probably reduced by Cu-deficiency and or partial
cation disorder. Density functional theory calculation indicates valence band
was composed of hybridization between Cu 3d orbitals and S 3p orbitals. | 1411.5122v2 |
2015-05-20 | III-V/Si Wafer Bonding Using Transparent, Conductive Oxide Interlayers | We present a method for low temperature plasma-activated direct wafer bonding
of III-V materials to Si using a transparent, conductive indium zinc oxide
interlayer. The transparent, conductive oxide (TCO) layer provides excellent
optical transmission as well as electrical conduction, suggesting suitability
for Si/III-V hybrid devices including Si-based tandem solar cells. For bonding
temperatures ranging from 100$^{\circ}$C to 350$^{\circ}$C, Ohmic behavior is
observed in the sample stacks, with specific contact resistivity below 1
$\Omega$cm$^2$ for samples bonded at 200$^{\circ}$C. Optical absorption
measurements show minimal parasitic light absorption, which is limited by the
III-V interlayers necessary for Ohmic contact formation to TCOs. These results
are promising for Ga$_{0.5}$In$_{0.5}$P/Si tandem solar cells operating at one
sun or low concentration conditions. | 1505.05453v2 |
2016-06-10 | Pressure sensing using vertically aligned carbon nanotubes on a flexible substrate | Sensing technologies have been under research and development for their
varied applications from microelectronics to space exploration. With the end of
Moores law in sight, there is growing demand for shrinking materials and
improving sensitivity and range of sensing of sensors. Carbon nanotubes (CNTs)
offer an excellent combination of small size (in the order of nanometers in two
dimensions and micrometers in the third dimension), varied current conductivity
(from insulating to metallic), flexibility, mechanical strength and feasibility
of mass production. Here we used CNTs to fabricate pressure sensors to sense
static loads of pressure and studied the characteristics of different methods
of building the sensors. We offer an adhesive-absorption technique of
fabrication of pressure sensors that tackles the issue of endurance of the
sensors to repeated operation. We demonstrate a significant change in
resistance of a vertically aligned forest of nanotubes upon application of
static loads. This study will enable building of better pressure sensors for
several applications. | 1606.03190v1 |
2016-06-18 | Failure mechanisms of single-crystal silicon electrodes in lithium-ion batteries | Long-term durability is a major obstacle limiting the widespread use of
lithium ion batteries (LIBs) in heavy-duty applications and others demanding
extended lifetime. As one of the root causes of degradation and failure of
battery performance, the electrode failure mechanisms are still unknown. Here,
we reveal the fundamental fracture mechanisms of single-crystal silicon
electrodes over extended lithiation/delithiation cycles, using electrochemical
testing, microstructure characterization, fracture mechanics, and finite
element analysis. Anisotropic lithium invasion causes crack initiation
perpendicular to the electrode surface, followed by growth through the
electrode thickness. The low fracture energy of the lithiated/unlithiated
silicon interface provides a weak microstructural path for crack deflection,
accounting for the crack patterns and delamination observed after repeated
cycling. Based on this physical understanding, we demonstrate how electrolyte
additives can heal electrode cracks and provide strategies to enhance the
fracture resistance in future LIBs from surface chemical, electrochemical, and
material science perspectives. | 1606.06283v1 |
2016-06-26 | (CaO)(FeSe): A Layered Wide Gap Oxychalcogenide Semiconductor | A new iron-oxychalcogenide (CaO)(FeSe) was obtained which crystallizes in the
orthorhombic space group Pnma (No. 62) with a = 5.9180(12) {\AA}, b = 3.8802(8)
{\AA}, c = 13.193(3) {\AA}. The unique structure of (CaO)(FeSe) is built up of
a quasi-two-dimensional network of corrugated infinite layers of corner-shared
FeSe2O2 tetrahedra that extend in the ab-plane. The corrugated layers composed
of corner-shared FeSe2O2 tetrahedra stack along the c-axis with Ca2+ cations
sandwiched between the layers. Optical spectroscopy and resistivity
measurements reveal semiconducting behavior with an indirect optical band gap
of around 1.8 eV and an activation energy of 0.19(1) eV. Electronic band
structure calculations at the density function level predict a magnetic
configuration as ground state and confirm the presence of an indirect wide gap
in (CaO)(FeSe). | 1606.08000v1 |
2017-01-17 | Strange metal state near a heavy-fermion quantum critical point | Recent experiments on quantum criticality in the Ge-substituted
heavy-electron material YbRh2Si2 under magnetic field have revealed a possible
non-Fermi liquid (NFL) strange metal (SM) state over a finite range of fields
at low temperatures, which still remains a puzzle. In the SM region, the
zero-field antiferromagnetism is suppressed. Above a critical field, it gives
way to a heavy Fermi liquid with Kondo correlation. The T (temperature)-linear
resistivity and the T-logarithmic followed by a power-law singularity in the
specific heat coefficient at low T, salient NFL behaviours in the SM region,
are un-explained. We offer a mechanism to address these open issues
theoretically based on the competition between a quasi-2d fluctuating
short-ranged resonant- valence-bonds (RVB) spin-liquid and the Kondo
correlation near criticality. Via a field-theoretical renormalization group
analysis on an effective field theory beyond a large-N approach to an anti-
ferromagnetic Kondo-Heisenberg model, we identify the critical point, and
explain remarkably well both the crossovers and the SM behaviour. | 1701.04599v1 |
2017-01-17 | Lifshitz transition mediated electronic transport anomaly in bulk ZrTe5 | Zirconium pentatelluride ZrTe$_5$, a fascinating topological material
platform, hosts exotic chiral fermions in its highly anisotropic
three-dimensional Dirac band and holds great promise advancing the
next-generation information technology. However, the origin underlying its
anomalous resistivity peak has been under debate for decades. Here we provide
transport evidence substantiating the anomaly to be a direct manifestation of a
Lifshitz transition in the Dirac band with an ultrahigh carrier mobility
exceeding 3$\times$10$^5$ cm$^2$ V$^{-1}$ s$^{-1}$. We demonstrate that the
Lifshitz transition is readily controllable by means of carrier doping, which
sets the anomaly peak temperature $T_p$. $T_p$ is found to scale approximately
as $n_H^{0.27}$, where the Hall carrier concentration $n_H$ is linked with the
Fermi level by $\epsilon_F$ $\propto$ $n_H^{1/3}$ in a linearly dispersed Dirac
band. This relation indicates $T_p$ monotonically increases with $\epsilon_F$,
which serves as an effective knob for fine tuning transport properties in
pentatelluride-based Dirac semimetals. | 1701.04737v2 |
2017-01-26 | Spin mediated magneto-electro-thermal transport behavior in Ni80Fe20/MgO/p-Si thin films | In Si, spin-phonon interaction is the primary spin relaxation mechanism. At
low temperatures, the absence of spin-phonon relaxation will lead to enhanced
spin accumulation. Spin accumulation may change the electro-thermal transport
within the material, and thus may serve as an investigative tool for
characterizing spin-mediated behavior. Here we present the first experimental
proof of spin accumulation induced electro-thermal transport behavior in a Pd
(1 nm)/Ni80Fe20 (25 nm)/MgO (1 nm)/p-Si (2 um) specimen. The spin accumulation
originates from the spin-Hall effect. The spin accumulation changes the
phononic thermal transport in p-Si causing the observed magneto-electro-thermal
transport behavior. We also observe the inverted switching behavior in
magnetoresistance measurement at low temperatures in contrast to magnetic
characterization, which is attributed to the canted spin states in p-Si due to
spin accumulation. The spin accumulation is elucidated by current dependent
anomalous Hall resistance measurement, which shows a decrease as the electric
current is increased. This result may open a new paradigm in the field of
spin-mediated transport behavior in semiconductor and semiconductor
spintronics. | 1701.07854v2 |
2017-11-27 | Extreme magnetoresistance induced by Zeeman effect-driven electron-hole compensation and topological protection in MoSi$_2$ | The magnetoresistance is the magnetic field induced change of electrical
resistivity of a material. Recent studies have revealed extremely large
magnetoresistance in several non-magnetic semimetals, which has been explained
on the basis of either electron-hole compensation or the Fermi surface
topology, or the combination of both. Here, we present a single crystal study
on MoSi$_2$, which exhibits extremely large magnetoresistance, approaching
almost 10$^7$ % at 2 K and 14 T magnetic field. It is found that the
electron-hole compensation level in MoSi$_2$ evolves with magnetic field, which
is resulted from strong Zeeman effect, and found beneficial in boosting the
large non-saturating magnetoresistance. The non-trivial Berry phase in the de
Haas-van Alphen oscillations and the moderate suppression of backward
scattering of the charge carriers lend support for the topological nature of
this semimetal. The ultra-large carrier mobility of the topologically protected
charge carriers reinforces the magnetoresistance of MoSi2 to an unprecedented
large value. | 1711.09646v1 |
2017-11-30 | Experimental observation of node-line-like surface states in LaBi | In a Dirac nodal line semimetal, the bulk conduction and valence bands touch
at extended lines in the Brillouin zone. To date, most of the theoretically
predicted and experimentally discovered nodal lines derive from the bulk bands
of two- and three-dimensional materials. Here, based on combined angle-resolved
photoemission spectroscopy measurements and first-principles calculations, we
report the discovery of node-line-like surface states on the (001) surface of
LaBi. These bands derive from the topological surface states of LaBi and bridge
the band gap opened by spin-orbit coupling and band inversion. Our
first-principles calculations reveal that these "nodal lines" have a tiny gap,
which is beyond typical experimental resolution. These results may provide
important information to understand the extraordinary physical properties of
LaBi, such as the extremely large magnetoresistance and resistivity plateau. | 1711.11174v2 |
2017-12-19 | Synthesis, Crystal Structure, and Physical Properties of New Layered Oxychalcogenide La2O2Bi3AgS6 | We have synthesized a new layered oxychalcogenide La2O2Bi3AgS6. From
synchrotron X-ray diffraction and Rietveld refinement, the crystal structure of
La2O2Bi3AgS6 was refined using a model of the P4/nmm space group with a =
4.0644(1) {\AA} and c = 19.412(1) {\AA}, which is similar to the related
compound LaOBiPbS3, while the interlayer bonds (M2-S1 bonds) are apparently
shorter in La2O2Bi3AgS6. The tunneling electron microscopy (TEM) image
confirmed the lattice constant derived from Rietveld refinement (c ~ 20 {\AA}).
The electrical resistivity and Seebeck coefficient suggested that the
electronic states of La2O2Bi3AgS6 are more metallic than those of LaOBiS2 and
LaOBiPbS3. The insertion of a rock-salt-type chalcogenide into the van der
Waals gap of BiS2-based layered compounds, such as LaOBiS2, will be a useful
strategy for designing new layered functional materials in the layered
chalcogenide family. | 1712.06726v1 |
2017-12-05 | A Holistic Approach to Evaluate EMI Shielding Characteristics of Carbon Nanotube-based Polymer Composites | The work presents a comprehensive methodology to determine the Shielding
Effectiveness (SE) of single-walled carbon nanotubes (SWCNTs)/polymer
nanocomposites. Here, an algorithm based on Ant Colony Optimization (ACO) was
employed to determine the electrical conductivity ({\sigma}) of these
nanocomposites as a function of SWCNT concentration. Then, these {\sigma}
values were used to compute the SE values as a function of frequency and
concentration. Specifically, a pseudo three-dimensional (3D) percolation model
was developed to study the effects of random connectivity of SWCNTs to one
another on the {\sigma} values of the nanocomposites. Both intrinsic and
tunneling resistances were taken into account. The consequences of the presence
of both well-exfoliated and aggregated SWCNTs with varying lengths distributed
inhomogeneously on {\sigma} and SE values were investigated. | 1712.08188v1 |
2018-10-13 | Tuning the electronic structures and transport properties of zigzag blue phosphorene nanoribbons | In recent years, single element two-dimensional atom crystal materials have
aroused extensive interest in many applications. Blue phosphorus, successfully
synthesized on Au substrate by molecular beam epitaxy not long ago, shows
unusual geometrical and electronic structures. We investigate the electronic
structures and transport properties of zigzag blue phosphorene nanoribbons by
using a first-principles method, which can be obviously tuned via different
groups passivation on the both edges. The ZBPNRs-H and ZBPNRs-OH present a wide
gap semiconductor property. While the ZBPNRs-O are metallic. Interestingly, the
current-voltage curves of ZBPNRs-O show a negative differential resistive
effect, which is independent on the ribbon width. The electric current through
the ZBPNRs-O is mainly flowing along the both outside zigzag phosphorus chains
via the way of P-P bond current. Through modifying the both edges with various
functional groups, the ZBPNRs can display some important functional
characteristics and become a candidate of NDR devices. | 1810.05792v1 |
2018-10-18 | Coating of Aluminum Alloys by Micro Arc Oxidation in Nitrate Salt | Plasma electrolytic oxidation (PEO) is a process for obtaining oxide coatings
on valve metals. Mostly PEO is done in an aqueous solution electrolyte which
limits the size of treated parts due to the system heating up. In presented
work an alternative method of PEO processing applied in aluminum 1050 alloy in
nitrate molten salt was investigated. The morphology, phase and chemical
compositions, micro-hardness, and corrosion resistance were examined using. The
obtained results showed that formed coating contains from two sub-layers, outer
soft layer with the thickness of 4 micrometer and inner, denser layer with the
thickness of 5 micrometer. The formed coating consists of corundum, {\gamma} -
Al2O3, {\theta} - Al2O3 and is free of any contaminants originated from the
electrolyte. | 1810.08018v1 |
2018-11-26 | Synthesis of Oxidation-Resistant Electrochemical-Active Copper Nanowires Using Phenylenediamine Isomers | Phenylenediamine (PDA) was chosen as a coordinating, reducing, and capping
agent to effectively direct growth and protect against oxidation of Cu
nanowires (Cu NWs) in an aqueous solution. PDA was found to reduce Cu (II) to
Cu (I) at room temperature, and stabilize the resulting Cu (I) by forming a
coordination complex. The presence of a stable Cu (I) complex is the key step
in the synthesis of Cu NWs under mild conditions. Different PDA isomers lead to
different growth paths of forming Cu NWs. Both pPDA and mPDA-synthesized Cu NWs
were covered with a thin layer of polyphenylenediamine and show excellent
anti-oxidation properties, even in the presence of water. The usefulness of the
present and electrochemical active Cu NWs for a variety of nanotechnology
applications is discussed. | 1811.10510v1 |
2019-01-31 | Unconventional Bloch-Grüneisen scattering in hybrid Bose-Fermi systems | We report on the novel mechanism of electron scattering in hybrid Bose-Fermi
systems consisting of a two-dimensional electron gas in the vicinity of an
exciton condensate: We show that a pair-of-bogolons--mediated scattering proves
to be dominating over the conventional acoustic phonon channel and over the
single-bogolon scattering, even if the screening is taken into account. We
develop a microscopic theory of this effect, focusing on GaAs and MoS$_2$
materials, and find the principal temperature dependence of resistivity,
distinct from the conventional phonon--mediated processes. Further, we
scrutinize parameters and suggest a way to design composite samples with
predefined electron mobilities and propose a mechanism of electron pairing for
superconductivity. | 1902.01214v1 |
2019-02-14 | Charge transport in oxygen-deficient EuTiO$_3$: the emerging picture of dilute metallicity in quantum-paraelectric perovskite oxides | We report on a study of charge transport in EuTiO$_{3-\delta}$ single
crystals with carrier density tuned across several orders of magnitude.
Comparing this system with other quasi-cubic perovskites, in particular
strontium titanate, we draw a comprehensive picture of metal-insulator
transition and dilute metallicity in this $AB$O$_3$ family. Because of a lower
electric permittivity, the metal-insulator transition in EuTiO$_{3-\delta}$
occurs at higher carrier densities compared to SrTiO$_3$. At low temperature, a
distinct $T^2$ resistivity is visible. Its prefactor $A$ smoothly decreases
with increasing carrier concentration in a similar manner in three different
perovskites. Our results draw a comprehensive picture of charge transport in
doped quantum paraelectrics. | 1902.05512v2 |
2019-02-27 | Stokes flow analogous to viscous electron current in graphene | Electron transport in two-dimensional conducting materials such as graphene,
with dominant electron-electron interaction, exhibits unusual vortex flow that
leads to a nonlocal current-field relation (negative resistance), distinct from
the classical Ohm's law. The transport behavior of these materials is best
described by low Reynolds number hydrodynamics, where the constitutive
pressure-speed relation is Stoke's law. Here we report evidence of such
vortices observed in a viscous flow of Newtonian fluid in a microfluidic device
consisting of a rectangular cavity$-$analogous to the electronic system. We
extend our experimental observations to elliptic cavities of different
eccentricities, and validate them by numerically solving bi-harmonic equation
obtained for the viscous flow with no-slip boundary conditions. We verify the
existence of a predicted threshold at which vortices appear. Strikingly, we
find that a two-dimensional theoretical model captures the essential features
of three-dimensional Stokes flow in experiments. | 1902.10383v1 |
2019-06-30 | Dissipative-regime measurements as a tool for confirming and characterizing near-room-temperature superconductivity | The search for new superconducting materials approaching room temperature
benefits from having a variety of testing methodologies to confirm and
characterize the presence of superconductivity. Often the first signatures of
new superconducting species occur incompletely and in very small volume
fractions. These trace amounts may be too weak to produce an observable
Meissner effect and the resistance may not go completely to zero if the
percolation threshold is not met. Under these conditions, secondary
behavior--such as transitions or cross overs in the temperature dependence of
magnetoresistance, magnetic irreversibility, or thermopower--are often used as
indications for the presence of superconductivity. Our group has developed a
rather unique set of fast-timescale and dissipative transport measurements that
can provide another tool set for confirming and characterizing suspected
superconductivity. Here we provide some background for these methods and
elucidate their collaborative value in the search for new superconducting
materials. Keywords: pairbreaking, pair-breaking, vortex, vortices, theory,
tutorial, RTS, room-temperature superconductivity, superconductor, detection,
characterization | 1907.00425v1 |
2019-07-08 | Theory of Spin Injection in Two-dimensional Metals with Proximity-Induced Spin-Orbit Coupling | Spin injection is a powerful experimental probe into a wealth of
nonequilibrium spin-dependent phenomena displayed by materials with spin-orbit
coupling (SOC). Here, we develop a theory of coupled spin-charge diffusive
transport in two-dimensional spin-valve devices. The theory describes a
realistic proximity-induced SOC with both spatially uniform and random
components of the SOC due to adatoms and imperfections, and applies to the two
dimensional electron gases found in two-dimensional materials and van der Walls
heterostructures. The various charge-to-spin conversion mechanisms known to be
present in diffusive metals, including the spin Hall effect and several
mechanisms contributing current-induced spin polarization are accounted for.
Our analysis shows that the dominant conversion mechanisms can be discerned by
analyzing the nonlocal resistance of the spin-valve for different polarizations
of the injected spins and as a function of the applied in-plane magnetic field. | 1907.03727v1 |
2019-07-09 | Low-Temperature Dielectric Anomalies at the Mott Insulator-Metal Transition | The correlation-driven Mott transition is commonly characterized by a drop in
resistivity across the insulator-metal phase boundary; yet, the complex
permittivity provides a deeper insight into the microscopic nature. We
investigate the frequency- and temperature-dependent dielectric response of the
Mott insulator $\kappa$-(BEDT-TTF)$_{2}$-Cu$_2$(CN)$_3$ when tuning from a
quantum spin liquid into the Fermi-liquid state by applying external pressure
and chemical substitution of the donor molecules. At low temperatures the
coexistence region at the first-order transition leads to a strong enhancement
of the quasi-static dielectric constant $\epsilon_1$ when the effective
correlations are tuned through the critical value. Several dynamical regimes
are identified around the Mott point and vividly mapped through pronounced
permittivity crossovers. All experimental trends are captured by dynamical
mean-field theory of the single-band Hubbard model supplemented by percolation
theory. | 1907.04437v2 |
2019-07-20 | Generating Optimal Grasps Under A Stress-Minimizing Metric | We present stress-minimizing (SM) metric, a new metric of grasp qualities.
Unlike previous metrics that ignore the material of target objects, we assume
that target objects are made of homogeneous isotopic materials. SM metric
measures the maximal resistible external wrenches without causing fracture in
the target objects. Therefore, SM metric is useful for robot grasping valuable
and fragile objects. In this paper, we analyze the properties of this new
metric, propose grasp planning algorithms to generate globally optimal grasps
maximizing the SM metric, and compare the performance of the SM metric and a
conventional metric. Our experiments show that SM metric is aware of the
geometries of target objects while the conventional metric are not. We also
show that the computational cost of the SM metric is on par with that of the
conventional metric. | 1907.08749v1 |
2019-07-23 | Electronic transport in thin films of BaPbO$_3$: Unraveling two-dimensional quantum effects | Recently, perovskite related BaPbO$_3$ has attracted attention due to its
hidden topological properties and, moreover, has been used as a thin layer in
heterostructures to induce two-dimensional superconductivity. Here we
investigate the normal state electronic transport properties of thin films of
BaPbO$_3$. Temperature and magnetic field dependent sheet resistances are
strongly affected by two-dimensional quantum effects. Our analysis decodes the
interplay of spin--orbit coupling, disorder, and electron--electron interaction
in this compound. Similar to recently discussed topological materials, we find
that weak antilocalization is the dominant protagonist in magnetotransport,
whereas electron--electron interactions play a pronounced role in the
temperature dependence. A systematic understanding of these quantum effects is
essential to allow for an accurate control of properties not only of thin films
of BaPbO$_3$, but also of topological heterostructures. | 1907.09839v2 |
2019-07-28 | Magneto-ionic control of spin polarization in magnetic tunnel junctions | Magnetic tunnel junctions (MTJs) with Hf0.5Zr0.5O2 barriers are reported to
show both tunneling magnetoresistance effect (TMR) and tunneling
electroresistance effect (TER), displaying four resistance states by magnetic
and electric field switching. Here we show that, under electric field cycling
of large enough magnitude, the TER can reach values as large as 10^6%.
Moreover, concomitant with this TER enhancement, the devices develop electrical
control of spin polarization, with sign reversal of the TMR effect. Currently,
this intermediate state exists for a limited number of cycles and understanding
the origin of these phenomena is key to improve its stability. The experiments
presented here point to the magneto-ionic effect as the origin of the large TER
and strong magneto-electric coupling, showing that ferroelectric polarization
switching of the tunnel barrier is not the main contribution. | 1907.12111v1 |
2020-02-11 | On the Mechanical and Thermal Stability of Free-standing Monolayer Amorphous Carbon | Recently (C.-T. Toh et al., Nature 577, 199 (2020)), the first synthesis of
free-standing monolayer amorphous carbon (MAC) was achieved. MAC is a pure
carbon structure composed of five, six, seven and eight atom rings randomly
distributed. MAC proved to be surprisingly stable and highly fracture
resistant. Its electronic properties are similar to boron nitride. In this
work, we have investigated the mechanical properties and thermal stability of
MAC models using fully-atomistic reactive molecular dynamics simulations. For
comparison purposes, the results are contrasted against pristine graphene (PG)
models of similar dimensions. Our results show that MAC and PG exhibit distinct
mechanical behavior and fracture dynamics patterns. While PG after a critical
strain threshold goes directly from elastic to brittle regimes, MAC shows
different elastic stages between these two regimes. Remarkably, MAC is
thermally stable up to 3600 K, which is close to the PG melting point. These
exceptional physical properties make MAC-based materials promising candidates
for new technologies, such as flexible electronics. | 2002.04682v1 |
2020-02-28 | Evidence for metastable photo-induced superconductivity in K$_3$C$_{60}$ | Far and mid infrared optical pulses have been shown to induce non-equilibrium
unconventional orders in complex materials, including photo-induced
ferroelectricity in quantum paraelectrics, magnetic polarization in
antiferromagnets and transient superconducting correlations in the normal state
of cuprates and organic conductors. In the case of non-equilibrium
superconductivity, femtosecond drives have generally resulted in electronic
properties that disappear immediately after excitation, evidencing a state that
lacks intrinsic rigidity. Here, we make use of a new optical device to drive
metallic K$_3$C$_{60}$ with mid-infrared pulses of tunable duration, ranging
between one picosecond and one nanosecond. The same superconducting-like
optical properties observed over short time windows for femtosecond excitation
are shown here to become metastable under sustained optical driving, with
lifetimes in excess of ten nanoseconds. Direct electrical probing becomes
possible at these timescales, yielding a vanishingly small resistance. Such a
colossal positive photo-conductivity is highly unusual for a metal and, when
taken together with the transient optical conductivities, it is rather
suggestive of metastable light-induced superconductivity. | 2002.12835v1 |
2020-03-20 | Quantum Fluctuations in the Non-Fermi Liquid System CeCo$_{2}$Ga$_{8}$ Investigated Using $μ$SR | Reduced dimensionality offers a crucial information in deciding the type of
the quantum ground state in heavy fermion materials. Here we have examined
stoichiometric CeCo$_{2}$Ga$_{8}$ compound, which crystallizes in a
quasi-one-dimensional crystal structure with Ga-Ce-Co chains along the
$c$-axis. The low-temperature behavior of magnetic susceptibility
($\chi\sim-\ln T$), heat capacity ($C_p/T\sim-\ln T$), and resistivity
($\rho\sim T^{n}$) firmly confirm the non-Fermi liquid ground state of
CeCo$_{2}$Ga$_{8}$. We studied the low-energy spin dynamics of
CeCo$_{2}$Ga$_{8}$ compound utilizing zero field (ZF-) and longitudinal field
(LF-) muon spin relaxation ($\mu$SR) measurements. ZF-$\mu$SR measurement
reveals the absence of long-range magnetic ordering down to 70 mK, and
interestingly below 1 K, the electronic relaxation rate sharply rises,
intimating the appearance of low energy quantum spin fluctuations in
CeCo$_{2}$Ga$_{8}$. | 2003.09104v1 |
2020-03-28 | Superconductivity in the nonsymmorphic line-nodal compound CaSb$_2$ | We found superconductivity in CaSb$_2$ with the transition temperature of 1.7
K by means of electrical-resistivity, magnetic-susceptibility, and
specific-heat measurements. This material crystallizes in a nonsymmorphic
structure and is predicted to have multiple Dirac nodal lines in the bulk
electronic band structure protected by symmetry even in the presence of
spin-orbit coupling. We discuss a possible topological superconductivity for
the quasi-2-dimensional band originating mainly from one of the antimony sites. | 2003.12800v1 |
2020-02-25 | Advanced protection against environmental degradation of silver mirror stacks for space application | Protection of silver mirror stacks from environmental degradation before
launching is crucial for space applications. Hereby, we report a comparative
study of the advanced protection of silver mirror stacks for space telescopes
provided by SiO2 and Al2O3 coatings in conditions of accelerated aging by
sulfidation. The model silver stack samples were deposited by cathodic
magnetron sputtering on a reference silica substrate for optical applications
and a surface-pretreated SiC substrate. Accelerated aging was performed in dry
and more severe wet conditions. Optical micrographic observations, surface and
interface analysis by Time-of Flight Secondary Ion Mass Spectrometry (ToF-SIMS)
and reflectivity measurements were combined to comparatively study the effects
of degradation. The results show a lower kinetics of degradation by accelerated
aging of the stacks protected by the alumina coating in comparable test
conditions. | 2004.06590v1 |
2020-04-20 | Lindemann unjamming of emulsions | We study the bulk and shear elastic properties of barely-compressed,
"athermal" emulsions and find that the rigidity of the jammed solid fails at
remarkably large critical osmotic pressures. The minuscule yield strain and
similarly small Brownian particle displacement of solid emulsions close to this
transition suggests that this catastrophic failure corresponds to a
plastic-entropic instability: the solid becomes too soft and weak to resist the
thermal agitation of the droplets that compose it and fails. We propose a
modified Lindemann stability criterion to describe this transition and derive a
scaling law for the critical osmotic pressure that agrees quantitatively with
experimental observations. | 2004.09594v2 |
2020-09-07 | Quantum Sensing of Insulator-to-Metal Transitions in a Mott Insulator | Nitrogen vacancy (NV) centers, optically-active atomic defects in diamond,
have attracted tremendous interest for quantum sensing, network, and computing
applications due to their excellent quantum coherence and remarkable
versatility in a real, ambient environment. Taking advantage of these
strengths, we report on NV-based local sensing of the electrically driven
insulator-to-metal transition (IMT) in a proximal Mott insulator. We studied
the resistive switching properties of both pristine and ion-irradiated VO2 thin
film devices by performing optically detected NV electron spin resonance
measurements. These measurements probe the local temperature and magnetic field
in electrically biased VO2 devices, which are in agreement with the global
transport measurement results. In pristine devices, the electrically-driven IMT
proceeds through Joule heating up to the transition temperature while in
ion-irradiated devices, the transition occurs non-thermally, well below the
transition temperature. Our results provide the first direct evidence for
non-thermal electrically induced IMT in a Mott insulator, highlighting the
significant opportunities offered by NV quantum sensors in exploring nanoscale
thermal and electrical behaviors in Mott materials. | 2009.02886v1 |
2020-09-09 | Population Dynamics Model and Analysis for Bacteria Transformation and Conjugation | We present a two-species population model in a well-mixed environment where
the dynamics involves, in addition to birth and death, changes due to
environmental factors and inter-species interactions. The novel dynamical
components are motivated by two common mechanisms for developing antibiotic
resistance in bacteria: plasmid {\it transformation}, where external genetic
material in the form of a plasmid is transferred inside a host cell; and {\it
conjugation} by which one cell transfers genetic material to another by direct
cell-to-cell contact. Through analytical and numerical methods, we identify the
effects of transformation and conjugation individually. With transformation
only, the two-species system will evolve towards one species' extinction, or a
stable co-existence in the long-time limit. With conjugation only, we discover
interesting oscillations for the system. Further, we quantify the combined
effects of transformation and conjugation, and chart the regimes of stable
co-existence, a result with ecological implications. | 2009.04276v1 |
2020-09-28 | Two-dimensional Janus van der Waals heterojunctions: a review of recent research progresses | Two-dimensional Janus van der Waals (vdW) heterojunctions, referring to the
junction containing at least one Janus material, are found to exhibit tuneable
electronic structures, wide light adsorption spectra, controllable contact
resistance, and sufficient redox potential due to the intrinsic polarization
and unique interlayer coupling. These novel structures and properties are
promising for the potential applications in electronics and energy conversion
devices. To provide a comprehensive picture about the research progress and
guide the following investigations, here we summarize their fundamental
properties of different types of two-dimensional Janus vdW heterostructures
including electronic structure, interface contact and optical properties, and
discuss the potential applications in electronics and energy conversion
devices. The further challenges and possible research directions of the novel
heterojunctions are discussed at the end of this review. | 2009.12985v1 |
2022-02-11 | Unusual electrical and magnetic properties in layered EuZn2As2 | Eu-based compounds often exhibit unusual magnetism, which is critical for
nontrivial topological properties seen in materials such as EuCd2As2. We
investigate the structure and physical properties of EuZn2As2 through
measurements of the electrical resistivity, Hall effect, magnetization, and
neutron diffraction. Our data show that EuZn2As2 orders antiferromagnetically
with an A-type spin configuration below TN = 19 K. Surprisingly, there is
strong evidence for dominant ferromagnetic fluctuations above TN, as reflected
by positive Curie-Weiss temperature and extremely large negative
magnetoresistance (MR) between TN and Tfl {\guillemotright} 200 K. Furthermore,
the angle dependence of the MRab indicates field-induced spin reorientation
from the ab-plane to a direction approximately 45{\deg} from the ab plane.
Compared to EuCd2As2, the doubled TN and Tfl make EuZn2As2 a better platform
for exploring topological properties in both magnetic fluctuation (TN < T <
Tfl) and ordered (T < TN) regimes. | 2202.05884v1 |
2022-02-16 | Emergence of Intergranular Tunneling Dominated Negative Magnetoresistance in Helimagnetic Manganese Phosphide Nanorod Thin Films | Helical magnets are emerging as a novel class of materials for spintronics
and sensor applications; however, research on their charge and spin transport
properties in a thin film form is less explored. Herein, we report the
temperature and magnetic field dependent charge transport properties of a
highly crystalline MnP nanorod thin film over a wide temperature range (2-350
K). The MnP nanorod films of 100 nm thickness were grown on Si substrates at
500 oC using molecular beam epitaxy. The temperature dependent resistivity data
exhibits a metallic behavior over the entire measured temperature range.
However, large negative magnetoresistance of up to 12% is observed below 50 K
at which the system enters a stable helical (screw) magnetic state. In this
temperature regime, the MR(H,T) dependence seems to show a magnetic field
manipulated phase coexistence. The observed magnetoresistance is dominantly
governed by the intergranular spin dependent tunneling mechanism. These
findings pinpoint a correlation between the transport and magnetism in this
helimagnetic system. | 2202.07915v1 |
2014-01-30 | Infrared luminescence in Bi-doped Ge-S and As-Ge-S chalcogenide glasses and fibers | Experimental and theoretical studies of spectral properties of chalcogenide
Ge-S and As-Ge-S glasses and fibers are performed. A broad infrared (IR)
luminescence band which covers the 1.2-2.3~$\mu$m range with a lifetime about
6~$\mu$s is discovered. Similar luminescence is also present in optical fibers
drawn from these glasses. Arsenic addition to Ge-S glass significantly enhances
both its resistance to crystallization and the intensity of the luminescence.
Computer modeling of Bi-related centers shows that interstitial Bi$^+$ ions
adjacent to negatively charged S vacancies are most likely responsible for the
IR luminescence. | 1401.7815v1 |
2016-03-02 | Energy filtering enhancement of thermoelectric performance of nanocrystalline Cr-Si composites | We report on thermoelectric properties of nanocrystalline Cr$_{\rm
1-x}$Si$_{\rm x}$ composite films. As-deposited amorphous films were
transformed into a nanocrystalline state with average grain size of 10--20~nm
by annealing during in-situ thermopower and electrical resistivity
measurements. The partially crystallized films, i.e. the films consisting of
crystalline grains dispersed in the amorphous matrix, are a new type of the
heterogeneous material where the nanocrystalline phase plays the role of
scattering centers giving rise to a large contribution to the thermopower. We
show that the thermopower enhancement is related to the energy dependent
scattering (energy filtering) of the charge carriers on the nanograin
interfaces. | 1603.00626v4 |
2016-03-07 | Tuning of thermoelectric properties with changing Se content in Sb2Te3 | Polycrystalline Sb 2 Te 3-x Se x (0.0 < x < 1.0) samples were synthesized by
the solid state reaction method. The structural analysis showed that up to the
maximal concentration of Se, the samples possess the Rhombohedral crystal
symmetry (space group R 3 m ). Increase of Se content increases the resistivity
of the samples. Variation of phonon frequencies, observed from Raman
spectroscopic study, depict anomalous behaviour around x = 0.2. The sample Sb 2
Te 2.8 Se 0.2 also shows maximum Seebeck coefficient, carrier concentration and
thermoelectric power factor. Nature of scattering mechanism controlling the
thermopower data has been explored. The thermoelectric properties of the
synthesized materials have been analyzed theoretically in the frame of
Boltzmann equation approach. | 1603.01998v1 |
2016-03-28 | Mechanism for the Large Conductance Modulation in Electrolyte-gated Thin Gold Films | Electrolyte gating using ionic liquid electrolytes has recently generated
considerable interest as a method to achieve large carrier density modulations
in a variety of materials. In noble metal thin films, electrolyte gating
results in large changes in sheet resistance. The widely accepted mechanism for
these changes is the formation of an electric double layer with a charged layer
of ions in the liquid and accumulation or depletion of carriers in the thin
film. We report here a different mechanism. In particular, we show using x-ray
absorption near edge structure (XANES) that the previously reported large
conductance modulation in gold films is due to reversible oxidation and
reduction of the surface rather than the charging of an electric double layer.
We show that the double layer capacitance accounts for less than 10\% of the
observed change in transport properties. These results represent a significant
step towards understanding the mechanisms involved in electrolyte gating. | 1603.08286v1 |
2017-04-05 | Superconductivity at 33 - 37 K in $ALn_2$Fe$_4$As$_4$O$_2$ ($A$ = K and Cs; $Ln$ = Lanthanides) | We have synthesized 10 new iron oxyarsenides, K$Ln_2$Fe$_4$As$_4$O$_2$ ($Ln$
= Gd, Tb, Dy, and Ho) and Cs$Ln_2$Fe$_4$As$_4$O$_2$ ($Ln$ = Nd, Sm, Gd, Tb, Dy,
and Ho), with the aid of lattice-match [between $A$Fe$_2$As$_2$ ($A$ = K and
Cs) and $Ln$FeAsO] approach. The resultant compounds possess hole-doped
conducting double FeAs layers, [$A$Fe$_4$As$_4$]$^{2-}$, that are separated by
the insulating [$Ln_2$O$_2$]$^{2+}$ slabs. Measurements of electrical
resistivity and dc magnetic susceptibility demonstrate bulk superconductivity
at $T_\mathrm{c}$ = 33 - 37 K. We find that $T_\mathrm{c}$ correlates with the
axis ratio $c/a$ for all 12442-type superconductors discovered. Also,
$T_\mathrm{c}$ tends to increase with the lattice mismatch, implying a role of
lattice instability for the enhancement of superconductivity. | 1704.01488v1 |
2017-04-09 | Properties of In-Plane Graphene/MoS2 Heterojunctions | The graphene/MoS2 heterojunction formed by joining the two components
laterally in a single plane promises to exhibit a low-resistance contact
according to the Schottky-Mott rule. Here we provide an atomic-scale
description of the structural, electronic, and magnetic properties of this type
of junction. We first identify the energetically favorable structures in which
the preference of forming C-S or C-Mo bonds at the boundary depends on the
chemical conditions. We find that significant charge transfer between graphene
and MoS2 is localized at the boundary. We show that the abundant 1D boundary
states substantially pin the Fermi level in the lateral contact between
graphene and MoS2, in close analogy to the effect of 2D interfacial states in
the contacts between 3D materials. Furthermore, we propose specific ways in
which these effects can be exploited to achieve spin-polarized currents. | 1704.02669v1 |
2017-04-11 | Neutron powder diffraction study on the iron-based nitride superconductor ThFeAsN | We report neutron diffraction and transport results on the newly discovered
superconducting nitride ThFeAsN with $T_c=$ 30 K. No magnetic transition, but a
weak structural distortion around 160 K, is observed cooling from 300 K to 6 K.
Analysis on the resistivity, Hall transport and crystal structure suggests this
material behaves as an electron optimally doped pnictide superconductors due to
extra electrons from nitrogen deficiency or oxygen occupancy at the nitrogen
site, which together with the low arsenic height may enhance the electron
itinerancy and reduce the electron correlations, thus suppress the static
magnetic order. | 1704.03119v1 |
2017-04-29 | Infrared-transmittance tunable metal-insulator conversion device with thin-film-transistor-type structure on a glass substrate | Infrared (IR) transmittance tunable metal-insulator conversion was
demonstrated on glass substrate by using thermochromic vanadium dioxide (VO2)
as the active layer in three-terminal thin-film-transistor-type device with
water-infiltrated glass as the gate insulator. Alternative positive/negative
gate-voltage applications induce the reversible protonation/deprotonation of
VO2 channel, and two-orders of magnitude modulation of sheet-resistance and 49%
modulation of IR-transmittance were simultaneously demonstrated at room
temperature by the metal-insulator phase conversion of VO2 in a non-volatile
manner. The present device is operable by the room-temperature protonation in
all-solid-state structure, and thus it will provide a new gateway to future
energy-saving technology as advanced smart window. | 1705.00130v1 |
2017-05-02 | Nodeless superconductivity and the peak effect in the quasi-skutterudites $\mathrm{Lu}_3\mathrm{Os}_4\mathrm{Ge}_{13}$ and $\mathrm{Y}_3\mathrm{Ru}_4\mathrm{Ge}_{13}$ | We report an investigation of the superconducting states of
$\mathrm{Lu}_3\mathrm{Os}_4\mathrm{Ge}_{13}$ and
$\mathrm{Y}_3\mathrm{Ru}_4\mathrm{Ge}_{13}$ single crystals by measurements of
the electrical resistivity, ac susceptibility and London penetration depth. The
analysis of the penetration depth and the derived superfluid density indicates
the presence of nodeless superconductivity and suggest that there are multiple
superconducting gaps in both materials. Furthermore, ac susceptibility
measurements of both compounds display the peak effect in the low temperature
region of the $H-T$ phase diagram. This anomalous increase of the critical
current with field gives an indication of a change of the arrangement of flux
lines in the mixed state, as found in some of the isostructural stannide
materials. | 1705.00818v1 |
2017-05-15 | Novel metal-insulator-transition at the SrTiO3/SmTiO3 interface | We report on a metal-insulator transition (MIT) that is observed in an
electron system at the SmTiO3/SrTiO3 interface. This MIT is characterized by an
abrupt transition at a critical temperature, below which the resistance changes
by more than an order of magnitude. The temperature of the transition
systematically depends on the carrier density, which is tuned from ~ 1x10^14
cm^-2 to 3x10^14 cm^-2 by changing the SmTiO3 thickness. Analysis of the
transport properties shows non-Fermi liquid behavior and mass enhancement as
the carrier density is lowered. We compare the MIT characteristics with those
of known MITs in other materials systems and show that they are distinctly
different in several aspects. We tentatively conclude that both long range
Coulomb interactions and the fixed charge at the polar interface are likely to
play a role in this MIT. The strong dependence on the carrier density makes
this MIT of interest for field-tunable devices. | 1705.05447v1 |
2017-07-11 | Strain-dependent solid surface stress and the stiffness of soft contacts | Surface stresses have recently emerged as a key player in the mechanics of
highly compliant solids. The classic theories of contact mechanics describe
adhesion with a compliant substrate as a competition between surface energies
driving deformation to establish contact and bulk elasticity resisting this.
However, it has recently been shown that surface stresses provide an additional
restoring force that can compete with and even dominate over elasticity in
highly compliant materials, especially when length scales are small compared to
the ratio of the surface stress to the elastic modulus, $\Upsilon/E$. Here, we
investigate experimentally the contribution of surface stresses to the force of
adhesion. We find that the elastic and capillary contributions to the adhesive
force are of similar magnitude, and that both are required to account for
measured adhesive forces between rigid silica spheres and compliant, silicone
gels. Notably, the strain-dependence of the solid surface stress contributes
significantly to the stiffness of soft solid contacts. | 1707.03089v1 |
2017-08-02 | Review on Modeling of Mechanical and Thermal Properties of Nano- and Micro-Composites | This article deals with the prediction of thermomechanical properties of
fiber reinforced composites using several micromechanics models. These include
strength of material approach, Halpin-Tsai equations, multi-phase mechanics of
materials approaches, multi-phase Mori-Tanaka models, composite cylindrical
assemblage model, Voigt-Reuss models, modified mixture rule, Cox model,
effective medium approach and method of cells. Several composite systems
reinforced with short and long, aligned, random and wavy reinforcements were
considered. In addition, different aspects such as fiber-matrix interphase,
fiber-matrix interfacial thermal resistance, fiber geometry, and multiple types
of reinforcements were considered to model the composites systems. The current
study also presents some important preliminary concepts and application of
developed micromechanics models to advanced nanocomposites such as carbon
nanotube reinforced composite. Main contribution of the current work is the
investigation of several analytical micromechanical models, while most of the
existing studies on the subject deal with only one or two approaches
considering few aspects. | 1708.00764v1 |
2017-08-10 | Observation of two-dimensional Fermi surface and Dirac dispersion in YbMnSb$_2$ | We present the crystal structure, electronic structure, and transport
properties of the material YbMnSb$_2$, a candidate system for the investigation
of Dirac physics in the presence of magnetic order. Our measurements reveal
that this system is a low-carrier-density semimetal with a 2D Fermi surface
arising from a Dirac dispersion, consistent with the predictions of density
functional theory calculations of the antiferromagnetic system. The low
temperature resistivity is very large, suggesting scattering in this system is
highly efficient at dissipating momentum despite its Dirac-like nature. | 1708.03308v1 |
2018-08-06 | Short Note on Superconductivity at Ambient Temperature and Pressure in Silver Embedded Gold Nano-particles: A Goldsmith job ahead | Very recent observation of superconductivity in both transport (resistance
versus temperature) and magnetization (Zero field cooled) at around 235 K has
obviously raised many eyebrows. The experimental fraternity in particular is
more thrilled. Although the results presented in the paper are clean and both
transport and magnetization measurements do approve the observation of
superconductivity to some extent (the shielding fraction is low), the problem
is to synthesize or fabricate the material. This I name a goldsmith job,
primarily because one has to deal with Gold and Silver nano particles
combination/amalgamation/alloying or even who knows the inter-facial new phase,
which could be superconducting. This short note is written in view to let the
spark continues and the interesting work of ) authors is reproduced
independently. | 1808.01797v2 |
2018-08-12 | A Facile Approach to Prepare Self-Assembled, Nacre-Inspired Clay/Polymer Nano-Composites | Nature provides many paradigms for the design and fabrication of artificial
composite materials. Inspired by the relationship between the well-ordered
architecture and biopolymers found in natural nacre, we present a facile
strategy to construct large-scale organic/inorganic nacre-mimetics with
hierarchical structure via a water-evaporation self-assembly process. Through
hydrogen bonding, we connect Laponite-nanoclay platelets with each other using
naturally abundant cellulose creating thin, flexible films with a local
brick-and-mortar architecture. While the aqueous solution displays liquid
crystalline textures, the dried films show a pronounced Young's modulus (9.09
GPa) with a maximum strength of 298.02 MPa and toughness of 16.63 MJm-3. In
terms of functionalities, we report excellent glass-like transparency along
with exceptional shape-persistent flame shielding. We also demonstrate that
through metal ion-coordination we can further strengthen the interactions
between the polymers and the nanoclays. These ion-treated hybrid films exhibit
further enhanced mechanical, and thermal properties as well as resistance
against swelling and dissolution in aqueous environments. We believe that our
simple pathway to fabricate such versatile polymer/clay nanocomposites can open
avenues for inexpensive production of environmentally friendly, biomimetic
materials in aerospace, wearable electrical devices, artificial muscle, and
food packaging industry. | 1808.03972v1 |
2019-03-16 | Fundamental limits to radiative heat transfer: the limited role of nanostructuring in the near field | In a complementary article, we exploited algebraic properties of Maxwell's
equations and fundamental principles such as electromagnetic reciprocity and
passivity, to derive fundamental limits to radiative heat transfer applicable
in near- through far-field regimes. The limits depend on the choice of material
susceptibilities and bounding surfaces enclosing arbitrarily shaped objects. In
this article, we apply these bounds to two different geometric configurations
of interest, namely dipolar particles or extended structures of infinite area
in the near field of one another, and compare these predictions to prior
limits. We find that while near-field radiative heat transfer between dipolar
particles can saturate purely geometric "Landauer" limits, bounds on extended
structures cannot, instead growing much more slowly with respect to a material
response figure of merit, an "inverse resistivity" for metals, due to the
deleterious effects of multiple scattering; nanostructuring is unable to
overcome these limits, which can be practically reached by planar media at the
surface polariton condition. | 1903.07968v3 |
2019-04-17 | Low temperature saturation of phase coherence length in topological insulators | Implementing topological insulators as elementary units in quantum
technologies requires a comprehensive understanding of the dephasing mechanisms
governing the surface carriers in these materials, which impose a practical
limit to the applicability of these materials in such technologies requiring
phase coherent transport. To investigate this, we have performed
magneto-resistance (MR) and conductance fluctuations\ (CF) measurements in both
exfoliated and molecular beam epitaxy grown samples. The phase breaking length
($l_{\phi}$) obtained from MR shows a saturation below sample dependent
characteristic temperatures, consistent with that obtained from CF
measurements. We have systematically eliminated several factors that may lead
to such behavior of $l_{\phi}$ in the context of TIs, such as finite size
effect, thermalization, spin-orbit coupling length, spin-flip scattering, and
surface-bulk coupling. Our work indicates the need to identify an alternative
source of dephasing that dominates at low $T$ in topological insulators,
causing saturation in the phase breaking length and time. | 1904.08517v1 |
2019-08-28 | A Cantilever Torque Magnetometry Method for the Measurement of Hall Conductivity of Highly Resistive Samples | We present the first measurements of Hall conductivity utilizing a new torque
magnetometry method designed for insulators. A Corbino disk exhibits a magnetic
dipole moment proportional to Hall conductivity when voltage is applied across
a test material. This magnetic dipole moment can be measured through torque
magnetometry. The symmetry of this contactless technique allows for the
measurement of Hall conductivity in previously inaccessible materials. Finally,
a low-temperature noise bound, the lack of systematic errors on dummy devices,
and a measurement of the Hall conductivity of sputtered indium tin oxide
demonstrate the efficacy of the technique. | 1908.10857v3 |
2011-04-18 | Manipulation of heat current by the interface between graphene and white graphene | We investigate the heat current flowing across the interface between graphene
and hexagonal boron nitride (so-called white graphene) using both molecular
dynamics simulation and nonequilibrium Green's function approaches. These two
distinct methods discover the same phenomena that the heat current is reduced
linearly with increasing interface length, and the zigzag interface causes
stronger reduction of heat current than the armchair interface. These phenomena
are interpreted by both the lattice dynamics analysis and the transmission
function explanation, which both reveal that the localized phonon modes at
interfaces are responsible for the heat management. The room temperature
interface thermal resistance is about $7\times10^{-10}$m$^{2}$K/W in zigzag
interface and $3.5\times10^{-10}$m$^{2}$K/W in armchair interface, which
directly results in stronger heat reduction in zigzag interface. Our
theoretical results provide a specific route for experimentalists to control
the heat transport in the graphene and hexagonal boron nitride compound through
shaping the interface between these two materials. | 1104.3371v2 |
2012-06-17 | The efficiency and power of the martensite rotor heat engine. I | The physical aspects - mechanics and thermodynamics - of operation of
martensite rotor heat engine (MRHE) on the basis of martensite-austenite
structural phase transition with the transition temperature in the region of
low-potential water temperatures have been studied. The engine converts the
thermal energy of low-potential water into the elastic energy of working body
(spring, ribbon or wire) made of the material with shape memory effect. At some
simplifying assumptions, the analytical expressions are obtained for the
thermal efficiency and the power of MRHE of different type. The registration of
head hydraulic resistance and heat conductivity of working body material is
made and the maximum value of power produced by the engine at the given
mechanical and heat conditions is calculated.
The recommendations are given on the optimal choice of engine parameters. On
the basis of numerical estimations for nitinol, the possibility of application
of MRHE is shown for efficient and ecologically pure production of electric
energy both on local (geothermal waters, waste water of industrial enterprises,
etc.) and global (warm ocean stream) scales. | 1206.3733v1 |
2012-06-29 | Synthesis of a new alkali metal-organic solvent intercalated iron selenide superconductor with Tc{\approx}45K | We report on a new iron selenide superconductor with a TC onset of 45K and
the nominal composition Lix(C5H5N)yFe2-zSe2, synthesized via intercalation of
dissolved alkaline metal in anhydrous pyridine at room temperature. This
superconductor exhibits a broad transition, reaching zero resistance at 10K.
Magnetization measurements reveal a superconducting shielding fraction of
approximately 30%. Analogous phases intercalated with Na, K and Rb were also
synthesized and characterized. The superconducting transition temperature of
Lix(C5H5N)yFe2-zSe2 is clearly enhanced in comparison to the known
superconductors FeSe0.98 (Tc ~ 8K) and AxFe2-ySe2 (TC ~ 27-32K) and is in close
agreement with critical temperatures recently reported for Lix(NH3)yFe2-zSe2.
Post-annealing of intercalated material (Lix(C5H5N)yFe2-zSe2) at elevated
temperatures drastically enlarges the c-parameter of the unit cell (~44%) and
increases the amount of superconducting shielding fraction to nearly 100%. Our
findings indicate a new synthesis road leading to possibly even higher critical
temperatures in this class of materials by intercalation of organic compounds
between Fe-Se layers. | 1206.7022v1 |
2015-06-04 | Thermal conductivity of bulk and nanoscaled Si/Ge alloys from the Kinetic Collective Model | Several hitherto unexplained features of thermal conductivity in group IV
materials, such as the change in the slope as a function of sample size for
pure vs. alloyed samples and the fast decay in thermal conductivity for low
impurity concentration, are described in terms of a transition from a
collective to kinetic regime in phonon transport. We show that thermal
transport in pure bulk silicon samples is mainly collective, and that
impurity/alloy and boundary scattering are responsible for the destruction of
this regime with an associated strong reduction in thermal conductivity,
leaving kinetic transport as the only one allowed when those resistive
scattering mechanisms are dominant. | 1506.01522v2 |
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