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2022-04-01 | Magnetic Kagome Superconductor CeRu$_2$ | Materials with a kagome lattice provide a platform for searching for new
electronic phases and investigating the interplay between correlation and
topology. Various probes have recently shown that the kagome lattice can host
diverse quantum phases with intertwined orders, including charge density wave
states, bond density wave states, chiral charge order, and, rarely,
superconductivity. However, reports of the coexistence of superconductivity and
magnetic order in kagome materials remain elusive. Here we revisit a magnetic
superconductor CeRu$_2$ with a kagome network formed by Ru atoms. Our
first-principles calculations revealed a kagome flat band near the Fermi
surface, indicative of flat-band magnetism. At ambient pressure, CeRu$_2$
exhibits a superconducting transition temperature ($T_{\text{c}}$) up to ~ 6 K
and a magnetic order at ~ 40 K. Notably, superconductivity and related behavior
can be tuned by adjusting the amount of Ru. We conducted a systematic
investigation of the superconductivity and magnetic order in CeRu$_2$ via
magnetic, resistivity, and structural measurements under pressure up to ~ 168
GPa. An unusual phase diagram that suggests an intriguing interplay between the
compound's superconducting order parameters has been constructed. A
$T_{\text{c}}$ resurgence was observed above pressure of ~ 28 GPa, accompanied
by the sudden appearance of a secondary superconducting transition. Our
experiments have identified tantalizing phase transitions driven by high
pressure and suggest that the superconductivity and magnetism in CeRu$_2$ are
strongly intertwined. | 2204.00553v2 |
2004-12-06 | Resistivity and Thermoelectric power of NaxCoO2 (x =1.0, 0.7 and 0.6) system | Results of thermo-electric power (S) and electrical resistivity (r)
measurements are reported on NaxCoO2 compounds with x = 1.0, 0.7 and 0.6. These
are single-phase compounds crystallizing in the hexagonal structure (space
group P63/mmc) at room temperature. Thermo-electric power values at 300K
(S300K) are, 80mV/K, 39mV/K and 37mV/K for x = 1.0, 0.7 and 0.6 samples,
respectively. The samples with x=0.7 and 1.0 are metallic down to 5 K, while
the x = 0.6 sample is semiconducting. The value of r300K for x = 1.0 sample is
\~0.895 mW-cm and the power factor (S2/r) is = 7.04 x 10-3 W/mK2 which
qualifies it as a good thermo-electric material. In x =1.0 sample, S(T) is
positive throughout 300-5K temperature range and decreases monotonically to
zero as temperature T= 0. In contrast, S(T) of x = 0.7 and 0.6 samples changes
sign and shows negative values between 90 K and 16 K before approaching zero as
T = 0. Anomalous S(T) behavior of x = 0.6 and 0.7 samples, which are
coincidentally the precursor materials to the reported superconductivity in
this class of materials, indicates a dramatic change in the electronic
structure of these compounds on lowering the Na content. | 0412122v1 |
2011-11-21 | Anisotropic Magnetoresistance Effects in Fe, Co, Ni, Fe_4N, and Half-Metallic Ferromagnet: A Systematic Analysis | We theoretically analyze the anisotropic magnetoresistance (AMR) effects of
bcc Fe (+), fcc Co (+), fcc Ni (+), Fe$_4$N (-), and a half-metallic
ferromagnet (-). The sign in each ( ) represents the sign of the AMR ratio
observed experimentally. We here use the two-current model for a system
consisting of a spin-polarized conduction state and localized d states with
spin--orbit interaction. From the model, we first derive a general expression
of the AMR ratio. The expression consists of a resistivity of the conduction
state of the $\sigma$ spin ($\sigma=\uparrow$ or $\downarrow$), $\rho_{s
\sigma}$, and resistivities due to s--d scattering processes from the
conduction state to the localized d states. On the basis of this expression, we
next find a relation between the sign of the AMR ratio and the s--d scattering
process. In addition, we obtain expressions of the AMR ratios appropriate to
the respective materials. Using the expressions, we evaluate their AMR ratios,
where the expressions take into account the values of $\rho_{s
\downarrow}/\rho_{s \uparrow}$ of the respective materials. The evaluated AMR
ratios correspond well to the experimental results. | 1111.4864v3 |
2013-09-25 | Piezoresistance in Silicon and its nanostructures | Piezoresistance is the change in the electrical resistance, or more
specifically the resistivity, of a solid induced by an applied mechanical
stress. The origin of this effect in bulk, crystalline materials like Silicon,
is principally a change in the electronic structure which leads to a
modification of the charge carriers effective mass. The last few years have
seen a rising interest in the piezoresistive properties of semiconductor
nanostructures, motivated in large part by claims of a giant piezoresistance
effect in Silicon nanowires that is more than two orders of magnitude bigger
than the known bulk effect. This review aims to present the controversy
surrounding claims and counter-claims of giant piezoresistance in Silicon
nanostructures by presenting a summary of the major works carried out over the
last 10 years. The main conclusions that can be drawn from the literature are
that i) reproducible evidence for a giant piezoresistance effect in un-gated
Silicon nanowires is limited, ii) in gated nanowires a giant effect has been
reproduced by several authors, iii) the giant effect is fundamentally different
from either the bulk Silicon piezoresistance or that due to quantum confinement
in accumulation layers and heterostructures, the evidence pointing to an
electrostatic origin for the piezoresistance, iv) released nanowires tend to
have slightly larger piezoresistance coefficients than un-released nanowires,
and v) insufficient work has been performed on bottom-up grown nanowires to be
able to rule out a fundamental difference in their properties when compared
with top-down nanowires. On the basis of this, future possible research
directions are suggested. | 1309.6445v2 |
2014-09-25 | Superconductivity in 122-type antimonide BaPt$_2$Sb$_2$ | The crystal structure, superconducting properties, and electronic structure
of a novel superconducting 122-type antimonide, BaPt$_2$Sb$_2$, have been
investigated by measurements of powder X-ray diffraction patterns, electrical
resistivity, ac magnetic susceptibility, specific heat as well as ab-initio
calculations. This material crystallizes in a new-type of monoclinic variant of
the CaBe$_2$Ge$_2$-type structure, in which Pt$_2$Sb$_2$ layers consisting of
PtSb$_4$ tetrahedra and Sb$_2$Pt$_2$ layers consisting of SbPt$_4$ tetrahedra
are stacked alternatively and Ba atoms are located between the layers.
Measurements of electrical resistivity, ac magnetic susceptibility and specific
heat revealed that BaPt$_2$Sb$_2$ is a superconducting material with a $T_{\rm
c}$ of 1.8 K. The electronic heat capacity coefficient $\gamma_{\rm n}$ and
Debye temperature $\theta_{\rm D}$ were 8.6(2) mJ/mol K$^2$ and 146(4) K, where
the figures in parentheses represent the standard deviation. The upper critical
field $\mu_{\rm 0}H_{\rm c2}(0)$ and the Ginzburg-Landau coherent length
$\xi(0)$ were determined to be 0.27 T and 35 nm. Calculations showed that it
has two three-dimensional Fermi surfaces (FSs) and two two-dimensional FSs,
leading to anisotropic transport properties. The d-states of the Pt atoms in
the Pt2Sb2 layers mainly contribute to $N(E_{\rm F})$. A comparison between
experimental and calculated results indicates that BaPt$_2$Sb$_2$ is a
superconducting material with moderate coupling. | 1409.7147v1 |
2014-11-11 | Tunable Cobalt Vacancies and Related Properties in LaCoxAs2 | The origin of transition metal vacancies and their effects on the properties
of ThCr2Si2-type compounds have been less studied and poorly understood. Here
we carefully investigate the structure, physical properties, and electronic
structure for a series of lanthanum cobalt arsenides with nominal composition
of LaCoxAs2 (1.6 < = x < = 2.1). It is revealed that the occupancy of Co can be
tuned between 1.98(1) and 1.61(1). The structural analyses based on X-ray and
neutron diffractions show the existence of Co vacancies results from charge
balance due to the formation of bond between As-As. These Co vacancies affect
the magnetic and electrical properties greatly, adjusting the Curie temperature
from 205 to 47 K and increasing the resistivity by more than 100%. First
principles calculations indicate that the Co vacancies weaken the spin
polarization and reduce the density of states at the Fermi level, resulting in
decreased Curie temperature and increased resistivity, respectively. Our
results address the importance of transition metal vacancies in ThCr2Si2-type
materials and offer a reliable route to tune the magnetism of ThCr2Si2-type
structure. | 1411.2788v1 |
2017-10-02 | Realization of the Axion Insulator State in Quantum Anomalous Hall Sandwich Heterostructures | The 'magnetoelectric effect' arises from the coupling between magnetic and
electric properties in materials. The Z2 invariant of topological insulators
(TIs) leads to a quantized version of this phenomenon, known as the topological
magnetoelectric (TME) effect. This effect can be realized in a new topological
phase called an 'axion insulator' whose surface states are all gapped but the
interior still obeys time reversal symmetry. We demonstrate such a phase using
electrical transport measurements in a quantum anomalous Hall (QAH) sandwich
heterostructure, in which two compositionally different magnetic TI layers are
separated by an undoped TI layer. Magnetic force microscopy images of the same
sample reveal sequential magnetization reversals of the top and bottom layers
at different coercive fields, a consequence of the weak interlayer exchange
coupling due to the spacer. When the magnetization is antiparallel, both the
Hall resistance and Hall conductance show zero plateaus, accompanied by a large
longitudinal resistance and vanishing longitudinal conductance, indicating the
realization of an axion insulator state. Our findings thus show evidences for a
phase of matter distinct from the established QAH state and provide a promising
platform for the realization of the TME effect. | 1710.00471v2 |
2018-04-20 | Pressure induced superconductivity bordering a charge-density-wave state in NbTe4 with strong spinorbit coupling | Transition-metal chalcogenides host various phases of matter, such as
charge-density wave (CDW), superconductors, and topological insulators or
semimetals. Superconductivity and its competition with CDW in low-dimensional
compounds have attracted much interest and stimulated considerable research.
Here we report pressure induced superconductivity in a strong spin-orbit (SO)
coupled quasi-one-dimensional (1D) transition-metal chalcogenide NbTe$_4$,
which is a CDW material under ambient pressure. With increasing pressure, the
CDW transition temperature is gradually suppressed, and superconducting
transition, which is fingerprinted by a steep resistivity drop, emerges at
pressures above 12.4 GPa. Under pressure $p$ = 69 GPa, zero resistance is
detected with a transition temperature $T_c$ = 2.2 K and an upper critical
field $H_{c2}$= 2 T. We also find large magnetoresistance (MR) up to 102\% at
low temperatures, which is a distinct feature differentiating NbTe$_4$ from
other conventional CDW materials. | 1804.07448v1 |
2018-11-12 | Laser writable high-K dielectric for van der Waals nano-electronics | Like silicon-based semiconductor devices, van der Waals heterostructures will
require integration with high-K oxides. This is needed to achieve suitable
voltage scaling, improved performance as well as allowing for added
functionalities. Unfortunately, commonly used high-k oxide deposition methods
are not directly compatible with 2D materials. Here we demonstrate a method to
embed a multi-functional few nm thick high-k oxide within van der Waals devices
without degrading the properties of the neighbouring 2D materials. This is
achieved by in-situ laser oxidation of embedded few layer HfS2 crystals. The
resultant oxide is found to be in the amorphous phase with a dielectric
constant of k~15 and break-down electric fields in the range of 0.5-0.6 V/nm.
This transformation allows for the creation of a variety of fundamental
nano-electronic and opto-electronic devices including, flexible Schottky
barrier field effect transistors, dual gated graphene transistors as well as
vertical light emitting and detecting tunnelling transistors. Furthermore, upon
dielectric break-down, electrically conductive filaments are formed. This
filamentation process can be used to electrically contact encapsulated
conductive materials. Careful control of the filamentation process also allows
for reversible switching between two resistance states. This allows for the
creation of resistive switching random access memories (ReRAMs). We believe
that this method of embedding a high-k oxide within complex van der Waals
heterostructures could play an important role in future flexible
multi-functional van der Waals devices. | 1811.04829v1 |
2018-10-09 | Magnetic behavior, Griffiths phase and magneto-transport study in 3$d$ based nano-crystalline double perovskite Pr$_2$CoMnO$_6$ | Double perovskite (DP) oxide material receive extensive research interest due
to exciting physical properties with potential technological application. 3$d$
based DP oxides are promising for exciting physics like magnetodielectric,
ferroelectric, Griffith phase etc., specially Co/Mn DPs are gaining much
research interest. In this paper we present the study of magnetic phase and
transport properties in nano-crystalline Pr$_2$CoMnO$_6$ a 3$d$ based double
perovskite compound. This material shows a paramagnetic (PM) to ferromagnetic
(FM) phase transition below 173 K marked by a rapid increase in magnetic moment
due to spin ordering. We found divergence in inverse magnetic susceptibility
($\chi$$^{-1}$) from Curie weiss behavior around 206 K which indicates the
evolution of Griffiths phase before actual PM-FM transition. We found that the
Griffiths phase suppressed with increasing applied magnetic filed. For the
understanding of charge transport in this material we have measured temperature
dependent electrical resistivity. Pr$_2$CoMnO$_6$ is a strong insulator where
resistivity increase abruptly below magnetic phase transition. To understand
the effect of magnetic field on transport behavior we have also measured the
magnetoresistance (MR) at different temperatures. Sample shows the negative MR
with maximum value $\sim$22 $\%$ under applied magnetic field of 50 kOe at 125
K. MR follows quadratic field dependency above $T_c$ however below $T_c$ the MR
shows deviation from this field dependency at low field. | 1810.03895v1 |
2020-02-11 | A Multiscale Constitutive Model for Metal Forming of Dual Phase Titanium Alloys by Incorporating Inherent Deformation and Failure Mechanisms | Ductile metals undergo a considerable amount of plastic deformation before
failure. Void nucleation, growth and coalescence is the mechanism of failure in
such metals. {\alpha}/{\beta} titanium alloys are ductile in nature and are
widely used for their unique set of properties like specific strength, fracture
toughness, corrosion resistance and resistance to fatigue failures. Voids in
these alloys were reported to nucleate on the phase boundaries between {\alpha}
and {\beta} phase. Based on the findings of crystal plasticity finite element
method (CPFEM) based investigation of the void growth at the interface of
{\alpha} and {\beta} phases [1], [2], a void nucleation, growth, and
coalescence model has been formulated. An existing single-phase crystal
plasticity theory is extended to incorporate underlying physical mechanisms of
deformation and failure in dual phase titanium alloys. Effects of various
factors (stress triaxiality, Lode parameter, deformation state (equivalent
strain), and phase boundary inclination) on void nucleation, growth and
coalescence are used to formulate the constitutive model while their
interaction with a conventional crystal plasticity theory is established. An
extensive parametric assessment of the model is carried out to quantify and
understand the effects of the material parameters on the overall material
response. Performance of the proposed model is then assessed and verified by
comparing the results of the proposed model with the RVE study results.
Application of the constitutive model for utilisation in the design and
optimisation of the forming process of {\alpha}/{\beta} titanium alloy
components is also demonstrated using experimental data. | 2002.04459v1 |
2020-04-23 | Hybrid Graphene/Carbon Nanofiber Wax Emulsion for Paper-based Electronics and Thermal Management | Materials for electronics that function as electrical and/or thermal
conductors are often rigid, expensive, difficult to be sourced and sometimes
toxic. An electrically and thermally conductive nanocomposite that is
lightweight, flexible and eco-friendly could improve the environmental
friendliness of the electronics sector and enable new applications. Considering
this, we have fabricated electrically and thermally conductive flexible
materials by functionalizing paper with nanocarbon conductive inks. Carnauba
wax is emulsified in isopropyl alcohol and mixed with graphene nanoplatelets
(GNPs) or with hybrids of GNPs and carbon nanofibers (CNFs). The percolation
threshold of the hybrid samples is lowered compared with the pure GNPs
composites, due to their increased filler aspect ratio. The hybrid samples also
exhibit superior bending and folding stability. Densification of the coating to
decrease their sheet resistance enables them to achieve as low as ~ 50 {\Omega}
sq-1 for the GNP-based paper. The densification procedure improves the bending
stability, the abrasion resistance, and the electromagnetic interference
shielding of the paper-based conductors. Finally, the compressed samples show
an impressive enhancement of their thermal diffusivity. The flexible and
multifunctional nanocarbon coated paper is a promising electronic conductor and
thermally dissipative material and, at the same time, can increase the
environmental sustainability of the electronics sector. | 2004.11476v1 |
2020-09-21 | Photocurrent Imaging of Multi-Memristive Charge Density Wave Switching in Two-Dimensional 1T-TaS2 | Transport studies of atomically thin 1T-TaS2 have demonstrated the presence
of intermediate resistance states across the nearly commensurate (NC) to
commensurate (C) charge density wave (CDW) transition, which can be further
switched electrically. While this presents exciting opportunities for the
material in memristor applications, the switching mechanism has remained
elusive and could be potentially attributed to the formation of inhomogeneous C
and NC domains across the 1T-TaS2 flake. Here, we present simultaneous
electrical driving and scanning photocurrent imaging of CDWs in ultrathin
1T-TaS2 using a vertical heterostructure geometry. While micron-sized CDW
domains form upon changing temperature, electrically driven transitions result
in largely uniform changes, indicating that states of intermediate resistance
for the latter likely correspond to true metastable CDW states in between the
NC and C phases, which we then explain by a free energy analysis. Additionally,
we are able to perform repeatable and bidirectional switching across the
multiple CDW states without changing sample temperature, demonstrating that
atomically thin 1T-TaS2 can be further used as a robust and reversible
multi-memristor material. | 2009.10179v1 |
2018-12-21 | Transfer of mass and momentum at rough and porous surfaces | The surface texture of materials plays a critical role in wettability,
turbulence and transport phenomena. In order to design surfaces for these
applications, it is desirable to characterise non-smooth and porous materials
by their ability to exchange mass and momentum with flowing fluids. While the
underlying physics of the tangential (slip) velocity at a fluid-solid interface
is well understood, the importance and treatment of normal (transpiration)
velocity and normal stress is unclear. We show that, when slip velocity varies
at an interface above the texture, a non-zero transpiration velocity arises
from mass conservation. The ability of a given surface texture to accommodate
for a normal velocity of this kind is quantified by a transpiration length. We
further demonstrate that normal momentum transfer gives rise to a pressure
jump. For a porous material, the pressure jump can be characterised by so
called resistance coefficients. By solving five Stokes problems, the introduced
measures of slip, transpiration and resistance can be determined for any
anisotropic non-smooth surface consisting of regularly repeating geometric
patterns. The proposed conditions are a subset of effective boundary conditions
derived from formal multi-scale expansion. We validate and demonstrate the
physical significance of the effective conditions on two canonical problems --
a lid-driven cavity and a turbulent channel flow, both with non-smooth bottom
surfaces. | 1812.09401v2 |
2019-06-25 | Titanium Contacts to MoS2 with Interfacial Oxide: Interface Chemistry and Thermal Transport | The deposition of a thin oxide layer at metal/semiconductor interfaces has
been previously reported as a means of reducing contact resistance in 2D
electronics. Using X-ray photoelectron spectroscopy with in-situ Ti deposition,
we fabricate Au/Ti/TiOx/MoS2 samples as well as Au/Ti/MoS2 and Au/TiOx/MoS2 for
comparison. Elemental titanium reacts strongly with MoS2 whereas no interface
reactions are observed in the two types of samples containing TiOx/MoS2
interfaces. Using time domain thermoreflectance for the measurement of thermal
boundary conductance, we find that samples contacted with Ti and a thin TiOx
layer at the interface (less than or equal to 1.5 nm) exhibit the same behavior
as samples contacted solely with pure Ti. The Au/TiOx/MoS2 samples exhibit
approximately 20% lower thermal boundary conductance, despite having the same
MoS2 interface chemistry as the samples with thin oxide at the Ti/MoS2
interface. We identify the mechanism for this phenomenon, attributing it to the
different interfaces with the top Au contact. Our work demonstrates that the
use of thin interfacial oxide layers to reduce electrical contact resistance
does not compromise heat flow in 2D electronic devices. We note that the
thicknesses of the Ti and TiOx layers must be considered for optimal thermal
transport. | 1906.10727v1 |
2019-06-27 | The Essential Work of Fracture Parameters for 3D printed polymer sheets | Additive manufacturing is becoming increasingly popular in academia and
industry. Accordingly, there has been a growing interest in characterizing 3D
printed samples to determine their structural integrity behaviour. We employ
the Essential Work of Fracture (EWF) to investigate the mechanical response of
polymer sheets obtained through additive manufacturing. Our goal is twofold;
first, we aim at gaining insight into the role of fibre reinforcement on the
fracture resistance of additively manufactured polymer sheets. Deeply
double-edge notched tensile (DDEN-T) tests are conducted on four different
polymers: Onyx, a crystalline, nylon-reinforced polymer, and three standard
polymers used in additive manufacturing - PLA, PP and ABS. Results show that
fibre-reinforcement translates into a notable increase in fracture resistance,
with the fracture energy of Onyx being an order of magnitude higher than that
reported for non-reinforced polymers. On the other hand, we propose the use of
a miniature test specimen, the deeply double-edge notched small punch specimens
(DDEN-SP), to characterize the mechanical response using a limited amount of
material. The results obtained exhibit good alignment with the DDEN-T data,
suggesting the suitability of the DDEN-SP test for measuring fracture
properties of additively manufactured polymers in a cost-effective manner. | 1906.11512v1 |
2019-10-09 | The roles of adhesion, internal heat generation and elevated temperatures in normally loaded, sliding rough surfaces | The thermal effects of plastic and frictional heat generation and elevated
temperature were examined along with the role of adhesion in the context of
galling wear, using a representative crystal plasticity, normally loaded,
sliding surface model. Galling frequency behaviour was predicted for 316L
steel. Deformation of the surfaces was dominated by the surface geometry, with
no significant effect due to variations in frictional models. Plastic and
frictional heating were found to have a minimal effect on the deformation of
the surface, with the rapid conduction of heat preventing any highly localised
heating. There was no corresponding effect on the predicted galling frequency
response.
Isothermal, elevated temperature conditions caused a decrease in galling
resistance, driven by the temperature sensitivity of the critical resolved
shear stress. The extent of deformation, as quantified by the area of
plastically deformed material and plastic reach, increased with temperature.
Comparisons were made with literature results for several surface amplitude and
wavelength conditions. Model results compared favourably with those in the
literature. However, the reduction in predicted galling resistance with
elevated temperature for a fixed surface was not as severe as observations in
the literature, suggesting other mechanisms (e.g. phase transformations,
surface coatings and oxides) are likely important. | 1910.03830v1 |
2020-01-06 | Two-dimensional antiferroelectric tunnel junction | Ferroelectric tunnel junctions (FTJs), which consist of two metal electrodes
separated by a thin ferroelectric barrier, have recently aroused significant
interest for technological applications as nanoscale resistive switching
devices. So far, most of existing FTJs have been based on perovskite-oxide
barrier layers. The recent discovery of the two-dimensional (2D) van der Waals
ferroelectric materials opens a new route to realize tunnel junctions with new
functionalities and nm-scale dimensions. Due to the weak coupling between the
atomic layers in these materials, the relative dipole alignment between them
can be controlled by applied voltage. This allows transitions between
ferroelectric and antiferroelectric orderings, resulting in significant changes
of the electronic structure. Here, we propose to realize 2D antiferroelectric
tunnel junctions (AFTJs), which exploit this new functionality, based on
bilayer In$_2$X$_3$ (X = S, Se, Te) barriers and different 2D electrodes. Using
first-principles density functional theory calculations, we demonstrate that
the In$_2$X$_3$ bilayers exhibit stable ferroelectric and antiferroelectric
states separated by sizable energy barriers, thus supporting a non-volatile
switching between these states. Using quantum-mechanical modeling of the
electronic transport, we explore in-plane and out-of-plane tunneling across the
In$_2$S$_3$ van der Waals bilayers, and predict giant tunneling
electroresistance (TER) effects and multiple non-volatile resistance states
driven by ferroelectric-antiferroelectric order transitions. Our proposal opens
a new route to realize nanoscale memory devices with ultrahigh storage density
using 2D AFTJs. | 2001.01639v3 |
2020-01-28 | Interlayer band-to-band tunneling and negative differential resistance in van der Waals BP/InSe field effect transistors | Atomically thin layers of van der Waals (vdW) crystals offer an ideal
material platform to realize tunnel field effect transistors (TFETs) that
exploit the tunneling of charge carriers across the forbidden gap of a vdW
heterojunction. This type of device requires a precise energy band alignment of
the different layers of the junction to optimize the tunnel current. Amongst
two-dimensional (2D) vdW materials, black phosphorus (BP) and indium selenide
(InSe) have a Brillouin zone-centered conduction and valence bands, and a type
II band offset, both ideally suited for band-to-band tunneling. Here, we
demonstrate TFETs based on BP/InSe heterojunctions with diverse electrical
transport characteristics: forward rectifying, Zener-tunneling and backward
rectifying characteristics are realized in BP/InSe junctions with different
thickness of the BP layer or by electrostatic gating of the junction.
Electrostatic gating yields a large on/off current ratio of up to 108 and
negative differential resistance at low applied voltages (V ~ 0.2V). These
findings illustrate versatile functionalities of TFETs based on BP and InSe,
offering opportunities for applications of these 2D materials beyond the device
architectures reported in the current literature. | 2001.10273v1 |
2020-08-31 | Creep deformation of WC hardmetals with iron-based binders | Iron is a candidate to replace cobalt in WC hardmetals, due to its lower cost
and toxicity. A WC-FeCr hardmetal was compression tested at 900-1200 {\deg}C.
Particular attention is paid to the steady-state creep rates and
stress-exponents (n) during isostress treatments. Three regimes of stress
dependence are observed. Two of these were previously reported for WC-Co: power
law creep (n~3) at medium stresses; and grain boundary sliding (n~1) at higher
stresses, generally >100MPa. A previously unreported low stress (<10MPa) regime
with an exponent of n~2 is also observed. By combining electron microscopy with
X-ray diffraction texture measurements, the low stress regime is attributed to
viscous flow of the binder, which is accommodated by diffusional creep in the
WC skeleton. The mechanism may be applicable to other hardmetals. Compared to
analogous WC-Co materials, WC-FeCr shows improved creep resistance below 1000
{\deg}C, which can be explained by its lower self-diffusivity, and a lower
solubility for WC than Co. However, at temperatures corresponding to liquid
eutectic formation (~1140 {\deg}C), its creep resistance becomes inferior.
These results indicate FeCr may be a suitable replacement for Co provided the
eutectic temperature is not exceeded. | 2008.13565v1 |
2020-10-06 | Role of f-d exchange interaction and Kondo scattering in Nd doped pyrochlore Iridate (Eu1-xNdx)2Ir2O7 | We report study of magnetization, resistivity, magnetoresistance and specific
heat of the pyrochlore Iridate (Eu1-xNdx)2Ir2O7 with x=0.0, 0.5 and 1.0, where
spin orbit coupling, electronic correlation, magnetic frustration and Kondo
scattering coexists. Metal insulator transition temperature (T_MI) decrease
with increase in Nd content but always coincides with magnetic irreversibility
temperature (field induced moment). Resistivity below T_MI do not fit with
either activated (gap) or to any power law (gapless) dependence. The Curie
constant show surprising result, that Nd induces singlet correlation (reduction
of para-moment) in Ir sublattice. Magnetoresistance is negative at low
temperatures below 10 K and increases strongly with increase in x and vary
quadratically with field switching over to linear dependence above 50 kOe. Low
temperature specific heat shows Schottky peak, coming from Nd moments, showing
existence of doublet split in Nd energy level, arising from f-d exchange
interaction. All materials show presence of a linear specific heat in the
insulating region. The coefficient of linear specific heat for x= 0.0 does not
vary with external magnetic field but varies superlinearly for x = 1.0
materials. We argue that linear specific heat probably rules out weakly
correlated phases like Weyl fermions. We propose that with the introduction of
Nd at Eu site the system evolves from chiral spin liquid with gapless spinon
excitations with a very small charge gap to Kondo type interaction superposed
on chiral spin liquid coexisting with long range antiferromagnetic ordering.
Huge increase of magnetoresistance with increase in Nd concentrations shows
importance of Kondo scattering in the chiral spin liquid material by rare earth
moments. | 2010.02685v1 |
2020-10-15 | Multipurpose and Reusable Ultrathin Electronic Tattoos Based on PtSe2 and PtTe2 | Wearable bioelectronics with emphasis on the research and development of
advanced person-oriented biomedical devices have attracted immense interest in
the last decade. Scientists and clinicians find it essential to utilize
skin-worn smart tattoos for on-demand and ambulatory monitoring of an
individual's vital signs. Here we report on the development of novel ultrathin
platinum-based two-dimensional dichalcogenide (Pt-TMDs) based electronic
tattoos as advanced building blocks of future wearable bioelectronics. We made
these ultrathin electronic tattoos out of large-scale synthesized platinum
diselenide (PtSe2) and platinum ditelluride (PtTe2) layered materials and used
them for monitoring human physiological vital signs, such as the electrical
activity of the heart and the brain, muscle contractions, eye movements, and
temperature. We show that both materials can be used for these applications;
yet, PtTe2 was found to be the most suitable choice due to its metallic
structure. In terms of sheet resistance, skin-contact, and electrochemical
impedance, PtTe2 outperforms state-of-the-art gold and graphene electronic
tattoos and performs on par with medical-grade Ag/AgCl gel electrodes. The
PtTe2 tattoos show four times lower impedance and almost 100 times lower sheet
resistance compared to monolayer graphene tattoos. One of the possible prompt
implications of the work is perhaps in the development of advanced
human-machine interfaces. To display the application, we built a multi-tattoo
system that can easily distinguish eye movement and identify the direction of
an individual's sight. | 2010.07534v1 |
2020-10-22 | Development, Processing and Applications of a UV-Curable Polymer with Surface Active Thiol Groups | We present here a novel resist formulation with active thiol groups at the
surface. The material is UV curable, and can be patterned at the micro- and
nanoscale by UV nanoimprint lithography. The resist formulation development,
its processing, patterning and surface characterization are presented here. In
addition, a possible application, including its use to modify the electrical
properties of graphene devices is shown. The cured material is highly
transparent, intrinsically hydrophilic and can be made more hydrophilic
following a UV-ozone or an O2 plasma activation. We evaluated the
hydrophilicity of the polymer for different polymer formulations and curing
conditions. In addition, a protocol for patterning of the polymer in the micro
and nanoscale by nanoimprinting is given and preliminary etching rates together
with the polymer selectivity are measured. The main characteristic and unique
advantage of the polymer is that it has thiol functional groups at the surface
and in the bulk after curing. These groups allow for direct surface
modifications with thiol-based chemistry e.g., thiol-ene reactions. We prove
the presence of the thiol groups by Raman spectroscopy and perform a thiol-ene
reaction to show the potential of the easy click chemistry. This opens the way
for very straightforward surface chemistry on nanoimprinted polymer samples.
Furthermore, we show how the polymer improves the electrical properties of a
graphene field effect transistor, allowing for optimal performance at ambient
conditions. | 2010.11878v1 |
2021-04-02 | Crystal, local atomic and electronic structures of YbFe$_2$Zn$_{20-x}$Cd$_x$ ($0 \leq x \leq 1.4$): a multi-band system with possible coexistence of light and heavy fermions | The partial (up to 7 %) substitution of Cd for Zn in the Yb-based
heavy-fermion material YbFe$_2$Zn$_{20}$ is known to induce a slight ($\sim 20$
%) reduction of the Sommerfeld specific heat coefficient $\gamma$ and a huge
(up to two orders of magnitude) reduction of the $T^2$ resistivity coefficient
$A$, corresponding to a drastic and unexpected reduction of the Kadowaki-Woods
ratio $A/\gamma ^2$. Here, Yb $L_{3}$-edge X-ray absorption spectroscopy shows
that the Yb valence state is close to $3+$ for all $x$, whereas X-ray
diffraction reveals that Cd replace the Zn ions only at the $16c$ site of the
$Fd\bar{3}m$ cubic structure, leaving the $48f$ and $96g$ sites with full Zn
occupation. Ab-initio electronic structure calculations in pure and Cd-doped
materials, carried out without considering correlations, show multiple
conduction bands with only minor modifications of the band dispersions near the
Fermi level and therefore do not explain the resistivity drop introduced by Cd
substitution. We propose that the site-selective Cd substitution introduces
light conduction bands with substantial contribution of Cd($16c$) $5p$ levels
that have weak coupling to the Yb$^{3+}$ $4f$ moments. These light fermions
coexist with heavy fermions originated from other conduction bands with larger
participation of Zn($48f$ and $96g$) $4p$ levels that remain strongly coupled
with the Yb$^{3+}$ local moments. | 2104.01050v1 |
2021-08-18 | Robust narrow-gap semiconducting behavior in square-net La$_{3}$Cd$_{2}$As$_{6}$ | ABSTRACT: Narrow-gap semiconductors are sought-after materials due to their
potential for long-wavelength detectors, thermoelectrics, and more recently
non-trivial topology. Here we report the synthesis and characterization of a
new family of narrow-gap semiconductors, $R$$_{3}$Cd$_{2}$As$_{6}$ ($R=$ La,
Ce). Single crystal x-ray diffraction at room temperature reveals that the As
square nets distort and Cd vacancies order in a monoclinic superstructure. A
putative charge-density ordered state sets in at 279~K in
La$_{3}$Cd$_{2}$As$_{6}$ and at 136~K in Ce$_{3}$Cd$_{2}$As$_{6}$ and is
accompanied by a substantial increase in the electrical resistivity in both
compounds. The resistivity of the La member increases by thirteen orders of
magnitude on cooling, which points to a remarkably clean semiconducting ground
state. Our results suggest that light square net materials within a $I4/mmm$
parent structure are promising clean narrow-gap semiconductors. | 2108.08006v1 |
2021-10-01 | Effects of nonmagnetic impurities and subgap states on the kinetic inductance, complex conductivity, quality factor and depairing current density | We investigate how a combination of a nonmagnetic-impurity scattering rate
$\gamma$ and finite subgap states parametrized by Dynes $\Gamma$ affects
various physical quantities relevant to to superconducting devices: kinetic
inductance $L_k$, complex conductivity $\sigma$, surface resistance $R_s$,
quality factor $Q$, and depairing current density $J_d$. All the calculations
are based on the Eilenberger formalism of the BCS theory. We assume the device
materials are extreme type-II $s$-wave superconductors. It is well known that
the optimum impurity concentration ($\gamma/\Delta_0 \sim 1$) minimizes $R_s$.
Here, $\Delta_0$ is the pair potential for the idealized ($\Gamma\to 0$)
superconductor for the temperature $T\to 0$. We find the optimum $\Gamma$ can
also reduce $R_s$ by one order of magnitude for a clean superconductor
($\gamma/\Delta_0 < 1$) and a few tens $\%$ for a dirty superconductor
($\gamma/\Delta_0 > 1$). Also, we find a nearly-ideal ($\Gamma/\Delta_0 \ll 1$)
clean-limit superconductor exhibits a frequency-independent $R_s$ for a broad
range of frequency $\omega$, which can significantly improve $Q$ of a very
compact cavity with a few tens of GHz frequency. As $\Gamma$ or $\gamma$
increases, the plateau disappears, and $R_s$ obeys the $\omega^2$ dependence.
The subgap-state-induced residual surface resistance $R_{\rm res}$ is also
studied, which can be detected by an SRF-grade high-$Q$ 3D resonator. We
calculate $L_k(\gamma, \Gamma,T)$ and $J_d(\gamma, \Gamma,T)$, which are
monotonic increasing and decreasing functions of $(\gamma, \Gamma,T)$,
respectively. Measurements of $(\gamma, \Gamma)$ of device materials can give
helpful information on engineering $(\gamma, \Gamma)$ via materials processing,
by which it would be possible to improve $Q$, engineer $L_k$, and ameliorate
$J_d$. | 2110.00573v1 |
2021-10-21 | Angular harmonic Hall voltage and magnetoresistance measurements of Pt/FeCoB and Pt-Ti/FeCoB bilayers for spin Hall conductivity determination | Materials with significant spin-orbit coupling enable efficient
spin-to-charge interconversion, which can be utilized in novel spin electronic
devices. A number of elements, mainly heavy-metals (HM) have been identified to
produce a sizable spin current ($j_\mathrm{s}$), while supplied with a charge
current ($j$), detected mainly in the neighbouring ferromagnetic (FM) layer.
Apart from the spin Hall angle $\theta_\mathrm{SH}$ = $j_\mathrm{s}$/$j$, spin
Hall conductivity ($\sigma_\mathrm{SH}$) is an important parameter, which takes
also the resistivity of the material into account. In this work, we present a
measurement protocol of the HM/FM bilayers, which enables for a precise
$\sigma_\mathrm{SH}$ determination. Static transport measurements, including
resistivity and magnetization measurements are accompanied by the angular
harmonic Hall voltage analysis in a dedicated low-noise rotating probe station.
Dynamic characterization includes effective magnetization and magnetization
damping measurement, which enable HM/FM interface absorption calculation. We
validate the measurement protocol in Pt and Pt-Ti underlayers in contact with
FeCoB and present the $\sigma_\mathrm{SH}$ of up to 3.3$\times$10$^5$ S/m,
which exceeds the values typically measured in other HM, such as W or Ta. | 2110.11483v1 |
2021-12-23 | Hydrogen induced electronic transition within correlated perovskite nickelates with heavy rare-earth composition | Although discovery in hydrogen induced electronic transition within
perovskite family of rare-earth nickelate (ReNiO3) opens up a new paradigm in
exploring both the new materials functionality and device applications, the
existing research stays at ReNiO3 with light rare-earth compositions. To
further extend the cognition towards heavier rare-earth, herein we demonstrate
the hydrogen induced electronic transitions for quasi-single crystalline
ReNiO3/LaAlO3 (001) heterostructures, covering a large variety of the
rare-earth composition from Nd to Er. The hydrogen induced elevations in the
resistivity of ReNiO3 (RH/R0) show an unexpected non-monotonic tendency with
the atomic number of the rare-earth composition, e.g., firstly increase from Nd
to Dy and afterwards decreases from Dy to Er. Although ReNiO3 with heavy
rare-earth composition (e.g. DyNiO3) exhibits large RH/R0 up to 107, their
hydrogen induced electronic transition is not reversible. Further probing the
electronic structures via near edge X-ray absorption fine structure analysis
clearly demonstrates the respective transition in electronic structures of
ReNiO3 from Ni3+ based electron itinerant orbital configurations towards the
Ni2+ based electron localized state. Balancing the hydrogen induced transition
reversibility with the abruption in the variations of material resistivity, we
emphasize that the ReNiO3 with middle rare-earth compositions (e.g. Sm) to be
most suitable that caters for the potential applications in correlated
electronic devices. | 2112.12357v1 |
2022-12-01 | Extrinsic to intrinsic mechanism crossover of anomalous Hall effect in the Ir-doped MnPtSn Heusler system | Recent findings of large anomalous Hall signal in nonferromagnetic and
nonferrimagnetic materials suggest that the magnetization of the system is not
a critical component for the realization of the anomalous Hall effect (AHE).
Here, we present a combined theoretical and experimental study demonstrating
the evolution of different mechanisms of AHE in a cubic Heusler system
MnPt$_{1-x}$Ir$_x$Sn. With the help of magnetization and neutron diffraction
studies, we show that the substitution of nonmagnetic Ir in place of Pt
significantly reduces the net magnetic moment from 4.17 $ \mu _B$/f.u. in
MnPtSn to 2.78 $ \mu _B$/f.u. for MnPt$_{0.5}$Ir$_{0.5}$Sn. In contrast, the
anomalous Hall resistivity is enhanced by nearly three times from 1.6 $ \mu
\Omega $ cm in MnPtSn to about 5 $ \mu \Omega $ cm for
MnPt$_{0.5}$Ir$_{0.5}$Sn. The power law analysis of the Hall resistivity data
suggests that the extrinsic contribution of AHE that dominates in the case of
the parent MnPtSn almost vanishes for MnPt$_{0.5}$Ir$_{0.5}$Sn, where the
intrinsic mechanism plays the major role. The experimental results are well
supported by our theoretical study, which shows a considerable enhancement of
the spin-orbit coupling when Ir is introduced into the system. Our finding of a
crossover of the anomalous Hall effect with chemical engineering is a major
contribution toward the recent interest in controlling the band topology of
topological materials, both in bulk and thin-film forms. | 2212.00360v1 |
2023-06-26 | Large electro-opto-mechanical coupling in VO2 neuristors | Biological neurons are electro-mechanical systems, where the generation and
propagation of an action potential is coupled to generation and transmission of
an acoustic wave. Neuristors, such as VO2, characterized by insulator-metal
transition (IMT) and negative differential resistance, can be engineered as
self-oscillators, which are good approximations of biological neurons in the
domain of electrical signals. In this study, we show that these
self-oscillators are coupled electro-opto-mechanical systems, with better
energy conversion coefficients than the conventional electromechanical or
electrooptical materials. This is due to the significant contrast in the
material's resistance, optical refractive index and density across the induced
temperature range in a Joule heating driven IMT. We carried out laser
interferometry to measure the opto-mechanical response while simultaneously
driving the devices electrically into self-oscillations of different kinds. We
analyzed films of various thicknesses, engineered device geometry and performed
analytical modelling to decouple the effects of refractive index change
vis-a-vis mechanical strain in the interferometry signal. We show that the
effective piezoelectric coefficient (d13*) for our neuristor devices is 660
pm/V, making them viable alternatives to Pb-based piezoelectrics for MEMS
applications. Furthermore, we show that the effective electro-optic coefficient
(r13*) is ~22 nm/V, which is much larger than that in thin-film and bulk
Pockels materials. | 2306.14367v1 |
2023-08-28 | Crystal-Chemical Origins of the Ultrahigh Conductivity of Metallic Delafossites | Despite their highly anisotropic complex-oxidic nature, certain delafossite
compounds (e.g., PdCoO2, PtCoO2) are the most conductive oxides known, for
reasons that remain poorly understood. Their room-temperature conductivity can
exceed that of Au, while their low-temperature electronic mean-free-paths reach
an astonishing 20 microns. It is widely accepted that these materials must be
ultrapure to achieve this, although the methods for their growth (which produce
only small crystals) are not typically capable of such. Here, we first report a
new approach to PdCoO2 crystal growth, using chemical vapor transport methods
to achieve order-of-magnitude gains in size, the highest structural qualities
yet reported, and record residual resistivity ratios (>440). Nevertheless, the
first detailed mass spectrometry measurements on these materials reveal that
they are not ultrapure, typically harboring 100s-of-parts-per-million impurity
levels. Through quantitative crystal-chemical analyses, we resolve this
apparent dichotomy, showing that the vast majority of impurities are forced to
reside in the Co-O octahedral layers, leaving the conductive Pd sheets highly
pure (~1 ppm impurity concentrations). These purities are shown to be in
quantitative agreement with measured residual resistivities. We thus conclude
that a previously unconsidered "sublattice purification" mechanism is essential
to the ultrahigh low-temperature conductivity and mean-free-path of metallic
delafossites. | 2308.14257v2 |
2023-10-26 | Slow and Non-Equilibrium Dynamics due to Electronic Ferroelectricity in a Strongly-Correlated Molecular Conductor | Using a combination of resistance fluctuation (noise) and dielectric
spectroscopy we investigate the nature of relaxor-type electronic
ferroelectricity in the organic conductor $\kappa$-(BETS)$_2$Mn[N(CN)$_2$]$_3$,
a system representative for a wider class of materials, where strong
correlations of electrons on a lattice of dimerized molecules results in an
insulating ground state. The two complementary spectroscopies reveal a distinct
low-frequency dynamics. By dielectric spectroscopy we detect an intrinsic
relaxation that is typical for relaxor ferroelectrics below the
metal-to-insulator transition at $T_{\rm{MI}}\sim 25\,$K. Resistance noise
spectroscopy reveals fluctuating two-level processes above $T_{\rm MI}$ which
strongly couple to the applied electric field, a signature of fluctuating polar
nanoregions (PNR), i.e. clusters of quantum electric dipoles fluctuating
collectively. The PNR preform above the metal insulator transition. Upon
cooling through $T_{\rm MI}$, a drastic increase of the low-frequency
$1/f$-type fluctuations and slowing down of the charge carrier dynamics is
accompanied by the onset of strong non-equilibrium dynamics indicating a glassy
transition of interacting dipolar clusters, the scaling properties of which are
consistent with a droplet model. The freezing of nano-scale polar clusters and
non-equilibrium dynamics is suggested to be a common feature of organic
relaxor-type electronic ferroelectrics and needs to be considered in
theoretical models describing these materials. | 2310.17242v2 |
2024-01-01 | Nonlinear charge transport induced by gate voltage oscillation in few-layer MnBi2Te4 | Nonlinear charge transport, including nonreciprocal longitudinal resistance
and nonlinear Hall effect, has garnered significant attention due to its
ability to explore inherent symmetries and topological properties of novel
materials. An exciting recent progress along this direction is the discovery of
significant nonreciprocal longitudinal resistance and nonlinear Hall effect in
the intrinsic magnetic topological insulator MnBi2Te4 induced by the quantum
metric dipole. Given the importance of this finding, the inconsistent response
with charge density, and conflicting requirement of C3z symmetry, it is
imperative to elucidate every detail that may impact the nonlinear transport
measurement. In this study, we reveal an intriguing experimental factor that
inevitably gives rise to sizable nonlinear transport signal in MnBi2Te4. We
demonstrate that this effect stems from the gate voltage oscillation caused by
the application of a large alternating current to the sample. Furthermore, we
propose a methodology to significantly suppress this effect by individually
grounding the voltage electrodes during the second-harmonic measurements. Our
investigation emphasizes the critical importance of thoroughly assessing the
impact of gate voltage oscillation before determining the intrinsic nature of
nonlinear transport in all 2D material devices with an electrically connected
operative gate electrode. | 2401.00679v1 |
1997-12-15 | Spin Tunneling in Conducting Oxides | Direct tunneling in ferromagnetic junctions is compared with
impurity-assisted, surface state assisted, and inelastic contributions to a
tunneling magnetoresistance (TMR). Theoretically calculated direct tunneling in
iron group systems leads to about a 30% change in resistance, which is close to
experimentally observed values. It is shown that the larger observed values of
the TMR might be a result of tunneling involving surface polarized states. We
find that tunneling via resonant defect states in the barrier radically
decreases the TMR (down to 4% with Fe-based electrodes), and a resonant tunnel
diode structure would give a TMR of about 8%. With regards to inelastic
tunneling, magnons and phonons exhibit opposite effects: one-magnon emission
generally results in spin mixing and, consequently, reduces the TMR, whereas
phonons are shown to enhance the TMR. The inclusion of both magnons and phonons
reasonably explains an unusual bias dependence of the TMR.
The model presented here is applied qualitatively to half-metallics with 100%
spin polarization, where one-magnon processes are suppressed and the change in
resistance in the absence of spin-mixing on impurities may be arbitrarily
large. Even in the case of imperfect magnetic configurations, the resistance
change can be a few 1000 percent. Examples of half-metallic systems are
CrO$_2$/TiO$_2$ and CrO$_2$/RuO$_2$, and an account of their peculiar band
structures is presented. The implications and relation of these systems to CMR
materials which are nearly half-metallic, are discussed. | 9712170v2 |
2000-09-14 | Experimental study of negative photoconductivity in n-PbTe(Ga) epitaxial films | We report on low-temperature photoconductivity (PC) in n-PbTe(Ga) epitaxial
films prepared by the hot-wall technique on <111>-BaF_2 substrates. Variation
of the substrate temperature allowed us to change the resistivity of the films
from 10^8 down to 10_{-2} Ohm x cm at 4.2 K. The resistivity reduction is
associated with a slight excess of Ga concentration, disturbing the Fermi level
pinning within the energy gap of n-PbTe(Ga). PC has been measured under
continuous and pulse illumination in the temperature range 4.2-300 K. For films
of low resistivity, the photoresponse is composed of negative and positive
parts. Recombination processes for both effects are characterized by
nonexponential kinetics depending on the illumination pulse duration and
intensity. Analysis of the PC transient proves that the negative
photoconductivity cannot be explained in terms of nonequilibrium charge
carriers spatial separation of due to band modulation. Experimental results are
interpreted assuming the mixed valence of Ga in lead telluride and the
formation of centers with a negative correlation energy. Specifics of the PC
process is determined by the energy levels attributed to donor Ga III, acceptor
Ga I, and neutral Ga II states with respect to the crystal surrounding. The
energy level corresponding to the metastable state Ga II is supposed to occur
above the conduction band bottom, providing fast recombination rates for the
negative PC. The superposition of negative and positive PC is considered to be
dependent on the ratio of the densities of states corresponding to the donor
and acceptor impurity centers. | 0009209v1 |
2002-04-20 | Possible Magnetic separation in Ru doped La0.67Ca0.33MnO3 | X-ray diffraction, resistivity, ac susceptibility and magnetization studies
on La0.67Ca0.33Mn1-xRuxO3 (0 x < 0.1) were carried out. A significant increase
in the lattice parameters indicated the presence of mixed valance state of Ru:
Ru3+ and Ru4+. The resistivity of the doped compounds exhibited two features: a
broad maximum and a relatively sharp peak. While a para to ferromagnetic
transition could be observed for the latter peak, no magnetic signal either in
ac susceptibility or in magnetization measurements could be observed for the
broad maximum. The magnetic moment decreases non linearly from 3.55 to 3 mB
over the Ru composition from 0 to 8.5 at.%. Based on the results of the present
studies and on existing literature on the Mn-site substituted systems, we argue
that a magnetic phase separation occurs in the Ru doped system. While the sharp
peak in the resistivity corresponds to Ru4+ enriched region with a
ferromagnetic coupling with neighboring Mn ions, the broad peak corresponds to
a Ru3+ rich regions, with an antiferromagnetic coupling with neighboring Mn
ions. | 0204441v1 |
2003-02-03 | Metallicity and its low temperature behavior in dilute 2D carrier systems | We theoretically consider the temperature and density dependent transport
properties of semiconductor-based 2D carrier systems within the RPA-Boltzmann
transport theory, taking into account realistic screened charged impurity
scattering in the semiconductor. We derive a leading behavior in the transport
property, which is exact in the strict 2D approximation and provides a zeroth
order explanation for the strength of metallicity in various 2D carrier
systems. By carefully comparing the calculated full nonlinear temperature
dependence of electronic resistivity at low temperatures with the corresponding
asymptotic analytic form obtained in the $T/T_F \to 0$ limit, both within the
RPA screened charged impurity scattering theory, we critically discuss the
applicability of the linear temperature dependent correction to the low
temperature resistivity in 2D semiconductor structures. We find quite generally
that for charged ionized impurity scattering screened by the electronic
dielectric function (within RPA or its suitable generalizations including local
field corrections), the resistivity obeys the asymptotic linear form only in
the extreme low temperature limit of $T/T_F \le 0.05$. We point out the
experimental implications of our findings and discuss in the context of the
screening theory the relative strengths of metallicity in different 2D systems. | 0302047v3 |
2003-04-21 | Chemical, Structural, and Transport Properties of Na1-xCoO2 | We report measurement of room-temperature compressibility, thermal expansion,
thermoelectric power a(T) at various pressures P < 20 kbar, basal-plane
resistivity rab (T), magnetic susceptibility and thermal conductivity k(T)
taken on single-crystal or cold-pressed Na0.57CoO2. An enhancement of a large
thermopower with a change of slope occurs on heating near 100 K, but this
enhancement is progressively suppressed by pressure. The c-axis thermal
expansion is large in the interval 150 K <T < 250 K where the c-axis
resistivity exhibits a smooth transition from a metallic to a non-metallic
temperature dependence; but the basal-plane thermal expansion remains
negligible for all temperatures T < 300 K. On the other hand, the basal-plane
room-temperature compressibility is large in the interval 0 < P < 22 kbar,
becoming negligible in the range 22 < P < 45 kbar, whereas the c-axis
room-temperature compressibility is anomalously large in the pressure range 22
< P < 35 kbar. The basal plane resistivity is prop. to T^(3/2) below 175 K
where there is 3D metallic conduction; it rises less rapidly with temperature
where the metallic conduction is confined to 2D. The phonon contribution to the
thermal conductivity of a cold-pressed ceramic sample is not suppressed, as
previously reported. These findings are rationalized with the aid of the virial
theorem, recognition of a pinning of the nominal Co(IV)/Co(III) redox couple at
the top of the O2-:2p6 bands, and a schematic location of the a1T and eT
antibonding bands of this couple with respect to the Fermi energy. | 0304455v1 |
2004-11-04 | Electron transport, penetration depth and upper critical magnetic field of ZrB12 and MgB2 | We report on the synthesis and measurements of the temperature dependence of
resistivity, R(T), the penetration depth, l(T), and upper critical magnetic
field, Hc2(T), for polycrystalline samples of dodecaboride ZrB12 and diboride
MgB2. We conclude that ZrB12 as well as MgB2 behave like simple metals in the
normal state with usual Bloch-Gruneisen temperature dependence of resistivity
and with rather low resistive Debye temperature, TR=280 K, for ZrB12 (as
compared to MgB2 with TR=900 K). The R(T) and l(T) dependencies of ZrB12 reveal
a superconducting transition at Tc=6.0 K. Although a clear exponential
l(T)dependence in MgB2 thin films and ceramic pellets was observed at low
temperatures, this dependence was almost linear for ZrB12 below Tc/2. These
features indicate s-wave pairing state in MgB2, whereas a d-wave pairing state
is possible in ZrB12. A fit to the data gives a reduced energy gap
2D(0)/kTc=1.6 for MgB2 films and pellets, in good agreement with published data
for 3D \pi - sheets of the Fermi surface. Contrary to conventional theories we
found a linear temperature dependence of Hc2(T) for ZrB12 (Hc2(0)=0.15 T). | 0411116v1 |
2008-03-03 | Phase separation and the effect of quenched disorder in $Pr_{0.5}Sr_{0.5}MnO_3$ | The nature of phase separation in $Pr_{0.5}Sr_{0.5}MnO_3$ has been probed by
linear as well as nonlinear magnetic susceptibilities and resistivity
measurements across the 2nd order paramagnetic to ferromagnetic transition
($T_C$) and 1st order ferromagnetic to antiferromagnetic transition ($T_N$). We
found that the ferromagnetic (metallic) clusters, which form with the onset of
long-range order in the system at $T_C$, continuously decrease their size with
the decrease in temperature and coexist with non-ferromagnetic (insulating)
clusters. These non-ferromagnetic clusters are identified to be
antiferromagnetic. Significantly, it is shown that they do not arise because of
the superheating effect of the lower temperature 1st order transition. Thus
reveals unique phase coexistence in a manganite around half-doping encompassing
two long-range order transitions. Both the ferromagnetic and antiferromagnetic
clusters form at $T_C$ and persist much below $T_N$. Substitution of quenched
disorder (Ga) at Mn-site promotes antiferromagnetism at the cost of
ferromagnetism without adding any magnetic interaction or introducing any
significant lattice distortion. Moreover, increase in disorder decreases the
ferromagnetic cluster size and with 7.5% Ga substitution clusters size reduces
to the single domain limit. Yet, all the samples show significant short-range
ferromagnetic interaction much above $T_C$. Resistivity measurements also
reveal the novel phase coexistence identified from the magnetic measurements.
It is significant that, increase in disorder up to 7.5% increases the
resistivity of the low temperature antiferromagnetic phase by about four
orders. | 0803.0085v1 |
2008-04-16 | Properties of the hole and electron doped perovskites LnCoO3 | Two extreme members of the cobaltite series, LaCoO3 and DyCoO3, were
investigated by the electrical resistivity and thermopower measurements up to
800-1000 K. Special attention was given to effects of extra holes or electrons,
introduced by light doping of Co sites by Mg2+ or Ti4+ ions. The experiments on
the La based compounds were complemented with magnetic measurements. The study
shows that both kinds of charge carriers induce magnetic states on surrounding
CoIII sites and form thus thermally stable polarons of large total spin. Their
itinerancy is characterized by low temperature resistivity, which is of
Arrhenius type r~exp(EA/kT) for the hole (CoIV) doped samples, while an unusual
dependence r~1/Tn (n=8-10) is observed for the electron (CoII) doped samples.
At higher temperatures, additional hole carriers are massively populated in the
CoIII background, leading to a resistivity drop. This transition become evident
at ~300 K and 450 K and culminates at TI-M=540 and 780 K for the La and Dy
based samples, respectively.
The electronic behaviours of the cobaltites are explained considering two
excitation processes in parent compounds. The first one is related to a local
excitation from the diamagnetic LS CoIII to close-lying paramagnetic HS CoIII
state. Secondarily, a metallic phase of the IS CoIII character is formed
through a charge transfer mechanism between LS/HS pairs. The magnetic polarons
associated with doped carriers are interpreted as droplets of such IS phase. | 0804.2685v3 |
2008-08-22 | Single crystal growth and physical properties of the layered arsenide BaRh_2As_2 | Single crystals of BaRh_2As_2 have been synthesized from a Pb flux. We
present the room temperature crystal structure, single crystal x-ray
diffraction measurements as a function of temperature T, anisotropic magnetic
susceptibility \chi versus T, electrical resistivity in the ab-plane \rho
versus T, Hall coefficient versus T and magnetic field H, and heat capacity C
versus T measurements on the crystals. The single crystal structure
determination confirms that BaRh_2As_2 forms in the tetragonal ThCr_2Si_2 type
structure (space group I4/mmm) with lattice parameters a = b = 4.0564(6)\AA and
c = 12.797(4) \AA. Band structure calculations show that BaRh_2As_2 should be
metallic with a small density of states at the Fermi energy N(E_ F) = 3.49
states/eV f.u. (where f.u. \equiv formula unit) for both spin directions.
\rho(T) data in the ab-plane confirm that the material is indeed metallic with
a residual resistivity \rho(2K) = 29 \mu \Omega cm, and with a residual
resistivity ratio \rho(310K)/\rho(2K) = 5.3. The observed \chi(T) is small
(\sim 10^{-5} cm^3/mol) and weakly anisotropic with \chi_{ab}/\chi_ c \approx
2. The C(T) data indicate a small density of states at the Fermi energy with
the low temperature Sommerfeld coefficient \gamma = 4.7(9) mJ/mol K^2. There
are no indications of superconductivity, spin density wave, or structural
transitions between 2K and 300K. We compare the calculated density of states
versus energy of BaRh_2As_2 with that of BaFe_2As_2. | 0808.3116v1 |
2009-04-24 | Anisotropic magnetoresistance of spin-orbit coupled carriers scattered from polarized magnetic impurities | Anisotropic magnetoresistance (AMR) is a relativistic magnetotransport
phenomenon arising from combined effects of spin-orbit coupling and broken
symmetry of a ferromagnetically ordered state of the system. In this work we
focus on one realization of the AMR in which spin-orbit coupling enters via
specific spin-textures on the carrier Fermi surfaces and ferromagnetism via
elastic scattering of carriers from polarized magnetic impurities. We report
detailed heuristic examination, using model spin-orbit coupled systems, of the
emergence of positive AMR (maximum resistivity for magnetization along
current), negative AMR (minimum resistivity for magnetization along current),
and of the crystalline AMR (resistivity depends on the absolute orientation of
the magnetization and current vectors with respect to the crystal axes)
components. We emphasize potential qualitative differences between pure
magnetic and combined electro-magnetic impurity potentials, between short-range
and long-range impurities, and between spin-1/2 and higher spin-state carriers.
Conclusions based on our heuristic analysis are supported by exact solutions to
the integral form of the Boltzmann transport equation in archetypical
two-dimensional electron systems with Rashba and Dresselhaus spin-orbit
interactions and in the three-dimensional spherical Kohn-Littinger model. We
include comments on the relation of our microscopic calculations to standard
phenomenology of the full angular dependence of the AMR, and on the relevance
of our study to realistic, two-dimensional conduction-band carrier systems and
to anisotropic transport in the valence band of diluted magnetic
semiconductors. | 0904.3785v2 |
2011-01-28 | Cooper pair insulator in amorphous films induced by nanometer-scale thickness variations | Unusual transport properties of superconducting (SC) materials, such as the
under doped cuprates, low dimensional superconductors in strong magnetic
fields, and insulating films near the Insulator Superconductor Transition
(IST), have been attributed to the formation of inhomogeneous phases.
Difficulty correlating the behaviors with observations of the inhomogeneities
make these connections uncertain. Of primary interest here are proposals that
insulating films near the IST, which show an activated resistance and giant
positive magnetoresistance, contain islands of Cooper Pairs (CPs). Here we
present evidence that these types of inhomogeneities are essential to such an
insulating phase in amorphous Bi (a-Bi) films deposited on substrates patterned
with nanometer-sized holes. The patterning induces film thickness variations,
and corresponding coupling constant variations, that transform the composition
of the insulator from localized electrons to CPs. Analyses near the
thickness-tuned ISTs of films on nine different substrates show that weak links
between SC islands dominate the transport. In particular, the ISTs all occur
when the link resistance approaches the resistance quantum for pairs. These
observations lead to a detailed picture of CPs localized by spatial variations
of the superconducting coupling constant. | 1101.5642v1 |
2011-09-15 | Surface impedance of superconductors with magnetic impurities | Motivated by the problem of the residual surface resistance of the
superconducting radio-frequency (SRF) cavities, we develop a microscopic theory
of the surface impedance of s-wave superconductors with magnetic impurities. We
analytically calculate the current response function and surface impedance for
a sample with spatially uniform distribution of impurities, treating magnetic
impurities in the framework of the Shiba theory. The obtained general
expressions hold in a wide range of parameter values, such as temperature,
frequency, mean free path, and exchange coupling strength. This generality, on
the one hand, allows for direct numerical implementation of our results to
describe experimental systems (SRF cavities, superconducting qubits) under
various practically relevant conditions. On the other hand, explicit analytical
expressions can be obtained in a number of limiting cases, which makes possible
further theoretical investigation of certain regimes. As a feature of key
relevance to SRF cavities, we show that in the regime of "gapless
superconductivity" the surface resistance exhibits saturation at zero
temperature. Our theory thus explicitly demonstrates that magnetic impurities,
presumably contained in the oxide surface layer of the SRF cavities, provide a
microscopic mechanism for the residual resistance. | 1109.3395v2 |
2012-09-25 | Sb concentration dependent structural and resistive properties of polycrystalline Bi-Sb alloys | Polycrystalline Bi-Sb alloys have been synthesized over a wide range of
antimony concentration (8 at% to 20 at%) by solid state reaction method. In
depth structural analysis using X-Ray diffraction (XRD) and temperature
dependent resistivity measurement of synthesized samples have been performed.
XRD data confirmed single phase nature of polycrystalline samples and revealed
that complete solid solution is formed between bismuth and antimony. Rietveld
refinement technique, utilizing MAUD software, has been used to perform detail
structural analysis of the samples and lattice parameters of synthesized Bi-Sb
alloys have been estimated. Lattice parameter and unit cell volume decreases
monotonically with increasing antimony content. The variation of lattice
parameters with antimony concentration depicts a distinct slope change beyond
12 at% Sb content sample. Band gap has been estimated from the thermal
variation of resistivity data, with the 12% Sb content sample showing maximum
value. It has been observed that, with increasing antimony concentration the
transition from direct to indirect gap semiconductor is intimately related to
the variation of the estimated lattice parameters. Band diagram for the
polycrystalline Bi-Sb alloy system has also been proposed. | 1209.5506v1 |
2012-12-18 | Growth and physical property study of single nanowire (diameter ~ 45nm) of half doped Manganite | We report here the growth and characterization of functional oxide nanowire
of hole doped manganite of La0.5Sr0.5MnO3 (LSMO). We also report four probe
electrical resistance measurement of single nanowire of LSMO (diameter ~ 45nm)
using FIB fabricated electrodes. The wires were fabricated by hydrothermal
method using autoclave at a temperature of 270 oC. The elemental analysis and
physical property like electrical resistivity were studied at individual
nanowire level. The quantitative determination of Mn valency and elemental
mapping of constituent elements was done by using Electron Energy Loss
Spectroscopy (EELS) in the Scanning Transmission Electron Microscopy (STEM)
mode. We addressed the important issue of whether as a result of size reduction
the nanowires can retain the desired composition, structure and physical
properties. The nanowires used were found to have a ferromagnetic transition
(TC) at around 325 K which is very close to the bulk value of around 330 K
found in single crystal of the same composition confirming that the functional
behavior is likely to be retained even after size reduction of the nanowires to
a diameter of 45 nm. The electrical resistivity shows insulating behavior
within the temperature range measured, which is very much similar to the bulk
system. | 1212.4374v2 |
2013-10-16 | Spin-Flipping in Pt and at Co/Pt Interfaces | There has been recent controversy about the magnitude of spin-flipping in the
heavy metal Pt, characterized by the spin-diffusion length, lsf(Pt) We propose
a resolution of this controversy, and also present evidence for the importance
of a phenomenon neglected in prior studies of transport across sputtered
Ferromagnetic/Pt (F/Pt) interfaces, spin-flipping at the interface. The latter
is characterized by an interface spin-flipping parameter, delta(Co/Pt) that
specifies the probability P = [1 - exp(-delta)] of a conduction electron
flipping its spin direction as it traverses a Co/Pt interface. From studies of
the Current-Perpendicular-to-Plane (CPP) Resistances and Magnetoresistances of
sputtered ferromagnetically coupled Co/Pt multilayers by themselves, and
embedded within Py-based Double Exchange-biased Spin-Valves, we derive values
at 4.2K of delta(Co/Pt) = 0.9 (+0.5/-0.2), the interface specific resistance,
AR*(Co/Pt) = 0.74 +/- 0.15 fohm-m(2). and the interface spin-scattering
asymmetry, gamma(Co/Pt) = 0.58 +/- 0.12. This value of delta(Co/Pt) is much
larger than ones previously found for interfaces involving Co but not Pt. To
derive delta requires knowledge of the spin-diffusion length, lsf(Pt), for our
sputtered Pt. We derive lsf(Pt) from separate measurements. Combining our
results with those from others, we find that lsf(Pt) for Pt is approximately
proportional to the inverse resistivity, 1/rho(Pt). | 1310.4364v2 |
2013-11-02 | Experimental evidence for direct insulator-quantum Hall transition in multi-layer graphene | We have performed magnetotransport measurements on a multi-layer graphene
flake. At the crossing magnetic field Bc, an approximately
temperature-independent point in the measured longitudinal resistivity, which
is ascribed to the direct insulator-quantum Hall (I-QH) transition, is
observed. By analyzing the amplitudes of the magnetoresistivity oscillations,
we are able to measure the quantum mobility of our device. It is found that at
the direct I-QH transition, the product of the quantum mobility and is about
0.37 which is considerably smaller than 1. In contrast, at Bc, the longitudinal
resistivity is close to the Hall resistivity, i.e., the product of the
classical mobility and the crossing field is about 1. Therefore our results
suggest that different mobilities need to be introduced for the direct I-QH
transition observed in multi-layered graphene. Combined with existing
experimental results obtained in various material systems, our data obtained on
graphene suggest that the direct I-QH transition is a universal effect in 2D. | 1311.0353v1 |
2013-12-16 | Which Memristor Theory is Best for Relating Devices Properties to Memristive Function? | There are three theoretical models which purport to relate
experimentally-measurable or fabrication-controllable device properties to the
memristor's operation: 1. Strukov et al's phenomenological model; 2. Georgiou
et al's Bernoulli rewrite of that phenomenological model; 3. Gale's
memory-conservation model. They differ in their prediction of the effect on
memristance of changing the electrode size and factors that affect the
hysteresis. Using a batch of TiO$_2$ sol-gel memristors fabricated with
different top electrode widths we test and compare these three theories. It was
found that, contrary to model 2's prediction, the `dimensionless lumped
parameter', $\beta$, did not correlate to any measure of the hysteresis.
Contrary to model 1, memristance was found to be dependent on the three spatial
dimensions of the TiO$_2$ layer, as was predicted by model 3. Model 3 was found
to fit the change in resistance value with electrode size. Simulations using
model 3 and experimentally derived values for contact resistance gave
hysteresis values that were linearly related to (and only one order of
magnitude out) from the experimentally-measured values. Memristor hysteresis
was found to be related to the ON state resistance and thus the electrode size
(as those two are related). These results offer a verification of the
memory-conservation theory of memristance and its association of the vacancy
magnetic flux with the missing magnetic flux in memristor theory. This is the
first paper to experimentally test various theories pertaining to the operation
of memristor devices. | 1312.4422v1 |
2015-11-13 | Studies on proximity effect in Mo/Bi1.95Sb0.05Se3 hybrid structure | Proximity effect in a mechanically exfoliated Bi1.95Sb0.05Se3 topological
insulator (TI) single crystal partially covered with disordered superconducting
(SC) Mo thin film is reported. Magnetotransport measurement was performed
simultaneously across three different regions of the sample viz. SC, TI and
SC/TI junction. Resistance measured across SC shows a TC at 4.3 K concomitantly
the resistance measurement on TI showed a metallic trend with a steep upturn at
TC. Magneto-resistance (MR) measurement on TI exhibit a positive MR with
Shubnikov-de Haas (SdH) oscillations, whereas on SC a positive MR superimposed
with steep cusp close to TC is observed. Across SC/TI junction both SdH
oscillation and the cusp were observed. The frequency of SdH oscillation on
SC/TI junction is found to be lesser (~ 125 T) as compared to a reference
Bi1.95Sb0.05Se3sample (~ 174 T). Upper critical field HC2 deduced from WHH fit
was found to be 17.14 T for a reference Mo film whereas Mo film deposited on TI
showed a decreased HC2 of 4.05 T. The coherence length for the former was found
to be 4.38 nm and for the latter 9.01 nm. The interaction between the spin-less
Cooper pairs in SC with the spin-momentum locked carriers on the surface of TI
is believed to cause such changes in transport properties. | 1511.04213v1 |
2016-06-28 | Enhanced superconductivity, Kondo behavior and negative-curvature resistivity of oxygen-irradiated thin films of aluminium | We followed the evolution of the normal and superconducting properties of Al
thin films after each session of various successive oxygen irradiations at
ambient temperature. Such irradiated films, similar to the granular ones,
exhibit enhanced superconductivity, Kondo behavior and negative-curvature
resistivity. Two distinct roles of oxygen are identified: as a damage-causing
projectile and as an implanted oxidizing agent. The former gives rise to the
processes involved in the conventional recovery stages. The latter, considered
within the context of the Cabrera-Mott model, gives rise to a multistep process
which involves charges transfer and creation of stabilized vacancies and
charged defects. Based on the outcome of this multistep process, we consider
(i) the negative curvature resistivity as a manifestation of a
thermally-assisted liberation of trapped electric charges, (ii) the Kondo
contribution as a spin-flip scattering from paramagnetic, color-center-type
defects, and (iii) the enhancement of T_{c} as being due to a lattice softening
facilitated by the stabilized defects and vacancies. The similarity in the
phase diagrams of granular and irradiated films as well as the aging effects
are discussed along the same line of reasoning. | 1606.08918v2 |
2016-06-30 | Quadrupole-Driven Non-Fermi Liquid and Magnetic-Field Induced Heavy Fermion States in a Non-Kramers Doublet System | Orbital degrees of freedom in condensed matters could play important roles in
forming a variety of exotic electronic states by interacting with conduction
electrons. In 4f electron systems, because of strong intra-atomic spin-orbit
coupling, an orbitally degenerate state inherently carries quadrupolar degrees
of freedom. The present work has focussed on a purely quadrupole-active system
PrIr2Zn20 showing superconductivity in the presence of an antiferroquadrupole
order at TQ = 0.11 K. We observed non-Fermi liquid (NFL) behaviors emerging in
the electrical resistivity and the 4f contribution to the specific heat, C_4f,
in the paramagnetic state at T > TQ. Moreover, in magnetic fields below 6 T,
all data set of the electrical resistivity and C_4f(T) are well scaled with
characteristic temperatures T0's. This is the first observation of the NFL
state in the nonmagnetic quadrupole-active system, whose origin is
intrinsically different from that observed in the vicinity of the conventional
quantum critical point. It implies possible formation of a quadrupole Kondo
lattice resulting from hybridization between the quadrupoles and the conduction
electrons. Below 0.13 K, the electrical resistivity and C_4f(T) exhibit
anomalies as B approaches 5 T. This is the manifestation of a field-induced
crossover toward a Fermi-liquid ground state in the quadrupole Kondo lattice. | 1606.09571v1 |
2016-10-27 | Magnetoresistance and robust resistivity plateau in MoAs2 | We have grown the MoAs$_2$ single crystal which crystallizes in a monoclinic
structure with C2/m space group. Transport measurements show that MoAs$_2$
displays a metallic behavior at zero field and undergoes a
metal-to-semiconductor crossover at low temperatures when the applied magnetic
field is over 5 T. A robust resistivity plateau appears below 18 K and persists
for the field up to 9 T. A large positive magnetoresistance (MR), reaching
about 2600\% at 2 K and 9 T, is observed when the field is perpendicular to the
current.The MR becomes negative below 40 K when the field is rotated to be
parallel to the current. The Hall resistivity shows the non-linear
field-dependence below 70 K. The analysis using two-band model indicates a
compensated electron-hole carrier density at low temperatures. A combination of
the breakdown of Kohler's rule, the abnormal drop and the cross point in Hall
data implies that a possible Lifshitz transition has occurred between 30 K and
60 K, likely driving the compensated electron-hole density, the large MR as
well as the metal-semiconductor transition in MoAs$_2$. Our results indicate
that the family of centrosymmetric transition-metal dipnictides has rich
transport behavior which can in general exhibit variable metallic and
topological features. | 1610.08594v2 |
2017-10-24 | Absence of Metallic Behavior in Epitaxial NiCo2O4 Thin Films: Role of Microstructural Disorder | Despite the low resistivity (~ 1 mohm cm), the metallic electrical transport
has not been commonly observed in the inverse spinel NiCo2O4, except in certain
epitaxial thin films. Previous studies have stressed the effect of valence
mixing and degree of spinel inversion on the electric conduction of NiCo2O4
films. In this work, we have studied the effect of microstructure by comparing
the NiCo2O4 epitaxial films grown on MgAl2O4 (111) and on Al2O3 (0001)
substrates. Although the optimal growth condition and the magnetic properties
are similar for the NiCo2O4/MgAl2O4 and the NiCo2O4/Al2O3, they show metallic
and semiconducting electrical transport respectively. Despite similar
temperature and field dependence of magnetization, the NiCo2O4/Al2O3 show much
larger magnetoresistance at low temperature. Post-growth annealing decreases
the resistivity of NiCo2O4/Al2O3, but the annealed films are still
semiconducting. The correlation between the structural correlation length and
the resistivity suggests that the microstructural disorder, generated by the
dramatic mismatch between the NiCo2O4 and Al2O3 crystal structures, may be the
origin of the absence of the metallic electrical transport in NiCo2O4. These
results reveal microstructural disorder as another key factor in controlling
the electrical transport of NiCo2O4, with potentially large magnetoresistance
for spintronics application. | 1710.08608v2 |
2017-12-05 | Peculiarities of the electronic transport in half-metallic Co-based Heusler alloys | Electrical, magnetic and galvanomagnetic properties of half-metallic Heusler
alloys of Co$_2$YZ (Y = Ti, V, Cr, Mn, Fe, Ni, and Z = Al, Si, Ga, Ge, In, Sn,
Sb) were studied in the temperature range 4.2--900 K and in magnetic fields of
up to 100 kOe. It was found that varying Y in affects strongly the electric
resistivity and its temperature dependence $\rho(T)$, while this effect is not
observed upon changing Z. When Y is varied, extrema (maximum or minimum) are
observed in $\rho(T)$ near the Curie temperature $T_C$. At $T < T_C$, the
$\rho(T)$ behavior can be ascribed to a change in electronic energy spectrum
near the Fermi level. The coefficients of the normal and anomalous Hall effect
were determined. It was shown that the latter coefficient, $R_S$, is related to
the residual resistivity $\rho_0$ by a power law $R_S \sim \rho_0^k/M_S$ with
$M_S$ the spontaneous magnetization. The exponent $k$ was found to be 1.8 for
Co$_2$FeZ alloys, which is typical for asymmetric scattering mechanisms, and
2.9 for Co$_2$YAl alloys, which indicates an additional contribution to the
anomalous Hall effect. The temperature dependence of resistivity at low
temperatures is analyzed and discussed in the framework of the two-magnon
scattering theory. | 1712.01584v1 |
2017-12-25 | Effects of nuclear spins on the transport properties of the edge of two-dimensional topological insulators | The electrons in the edge channels of two-dimensional topological insulators
can be described as a helical Tomonaga-Luttinger liquid. They couple to nuclear
spins embedded in the host materials through the hyperfine interaction, and are
therefore subject to elastic spin-flip backscattering on the nuclear spins. We
investigate the nuclear-spin-induced edge resistance due to such backscattering
by performing a renormalization-group analysis. Remarkably, the effect of this
backscattering mechanism is stronger in a helical edge than in nonhelical
channels, which are believed to be present in the trivial regime of InAs/GaSb
quantum wells. In a system with sufficiently long edges, the disordered nuclear
spins lead to an edge resistance which grows exponentially upon lowering the
temperature. On the other hand, electrons from the edge states mediate an
anisotropic Ruderman-Kittel-Kasuya-Yosida nuclear spin-spin interaction, which
induces a spiral nuclear spin order below the transition temperature. We
discuss the features of the spiral order, as well as its experimental
signatures. In the ordered phase, we identify two backscattering mechanisms,
due to charge impurities and magnons. The backscattering on charge impurities
is allowed by the internally generated magnetic field, and leads to an
Anderson-type localization of the edge states. The magnon-mediated
backscattering results in a power-law resistance, which is suppressed at zero
temperature. Overall, we find that in a sufficiently long edge the nuclear
spins, whether ordered or not, suppress the edge conductance to zero as the
temperature approaches zero. | 1712.09040v2 |
2018-01-26 | Local Magnetic Measurements of Trapped Flux Through a Permanent Current Path in Graphite | Temperature and field dependent measurements of the electrical resistance of
different natural graphite samples, suggest the existence of superconductivity
at room temperature in some regions of the samples. To verify whether
dissipationless electrical currents are responsible for the trapped magnetic
flux inferred from electrical resistance measurements, we localized them using
magnetic force microscopy on a natural graphite sample in remanent state after
applying a magnetic field. The obtained evidence indicates that at room
temperature a permanent current flows at the border of the trapped flux region.
The current path vanishes at the same transition temperature $T_c\approx370$~K
as the one obtained from electrical resistance measurements on the same sample.
This sudden decrease of the phase is different from what is expected for a
ferromagnetic material. Time dependent measurements of the signal show the
typical behavior of flux creep of a permanent current flowing in a
superconductor. The overall results support the existence of room-temperature
superconductivity at certain regions in the graphite structure and indicate
that magnetic force microscopy is suitable to localize them. Magnetic coupling
is excluded as origin of the observed phase signal. | 1801.08836v1 |
2018-01-29 | Observation of the Meissner effect at room temperature in single-layer graphene brought into contact with alkanes | There are claims of synthesis of a room temperature superconductor. However,
these claims have not been officially accepted by scientific communities.
Currently, the highest transition temperature (Tc) recognized in scientific
articles is 135 K at 1 atm of Hg-Ba-Ca-Cu-O system which is a copper oxide
superconductor. We packed graphite flakes into a ring-shaped
polytetrafluoroethylene (PTFE) tube and further injected heptane or octane.
Then we generated circulating current in this ring tube by electromagnetic
induction and showed that this circulating current continues to flow
continuously at room temperature for 50 days. This experiment suggests that
bringing alkane into contact with graphite may result in a material with zero
resistance at room temperature. In addition, we showed by means of AC
resistance measurements using the two-terminal method that the resistances of
graphite fibers brought into contact with various alkanes suddenly change at
specific critical temperatures between 363 and 504 K. In this study, we show
that after a magnetic field is applied to a single-layer graphene at room
temperature, alkane is brought into contact with the single-layer graphene,
then the graphene excludes the magnetic field immediately. This phenomenon
demonstrates that the alkane-wetted single-layer graphene shows Meissner effect
at room temperature. Furthermore, we applied a magnetic field perpendicularly
to the annular single-layer graphene brought into contact with n-hexane and
immediately removed the magnetic field. After that we observed that a constant
magnetic field generates from this annular graphene for some time. In
conclusion, the single-layer graphene brought into contact with alkane shows
Meissner effect at room temperature, which provides definitive evidence for
room temperature superconductivity. | 1801.09376v1 |
2019-05-10 | Magnetic-Field-Induced Phenomena in the Paramagnetic Superconductor UTe$_{2}$ | We present magnetoresistivity measurements on the heavy-fermion
superconductor UTe$_{2}$ in pulsed magnetic fields $\mu_0H$ up to 68~T and
temperatures $T$ from 1.4 to 80~K. Magnetic fields applied along the three
crystallographic directions $\mathbf{a}$ (easy magnetic axis), $\mathbf{b}$,
and $\mathbf{c}$ (hard magnetic axes), are found to induce different phenomena
- depending on the field direction - beyond the low-field suppression of the
superconducting state. For $\mathbf{H}\parallel\mathbf{a}$, a broad anomaly in
the resistivity is observed at $\mu_0H^*\simeq10$~T and $T = 1.4$~K. For
$\mathbf{H}\parallel\mathbf{c}$, no magnetic transition nor crossover are
observed. For $\mathbf{H}\parallel\mathbf{b}$, a sharp first-order-like step in
the resistivity indicates a metamagnetic transition at the field $\mu_0H_m
\simeq 35$~T. When the temperature is raised signature of first-order
metamagnetism is observed up to a critical endpoint at $T_{CEP}\simeq7$~K. At
higher temperatures a crossover persists up to 28~K, i.e., below the
temperature $T_\chi^{max} = 35$~K where the magnetic susceptibility is maximal.
A sharp maximum in the Fermi-liquid quadratic coefficient $A$ of the
low-temperature resistivity is found at $H_m$. It indicates an enhanced
effective mass associated with critical magnetic fluctuations, possibly coupled
with a Fermi surface instability. Similarly to the URhGe case, we show that
UTe$_{2}$ is a candidate for field-induced reentrant superconductivity in the
proximity of $H_m$. | 1905.03990v1 |
2019-05-13 | Extending Policy from One-Shot Learning through Coaching | Humans generally teach their fellow collaborators to perform tasks through a
small number of demonstrations. The learnt task is corrected or extended to
meet specific task goals by means of coaching. Adopting a similar framework for
teaching robots through demonstrations and coaching makes teaching tasks highly
intuitive. Unlike traditional Learning from Demonstration (LfD) approaches
which require multiple demonstrations, we present a one-shot learning from
demonstration approach to learn tasks. The learnt task is corrected and
generalized using two layers of evaluation/modification. First, the robot
self-evaluates its performance and corrects the performance to be closer to the
demonstrated task. Then, coaching is used as a means to extend the policy
learnt to be adaptable to varying task goals. Both the self-evaluation and
coaching are implemented using reinforcement learning (RL) methods. Coaching is
achieved through human feedback on desired goal and action modification to
generalize to specified task goals. The proposed approach is evaluated with a
scooping task, by presenting a single demonstration. The self-evaluation
framework aims to reduce the resistance to scooping in the media. To reduce the
search space for RL, we bootstrap the search using least resistance path
obtained using resistive force theory. Coaching is used to generalize the
learnt task policy to transfer the desired quantity of material. Thus, the
proposed method provides a framework for learning tasks from one demonstration
and generalizing it using human feedback through coaching. | 1905.04841v1 |
2019-09-18 | Blockade of vortex flow by thermal fluctuations in atomically thin clean-limit superconductors | Resistance in superconductors arises from the motion of vortices driven by
flowing supercurrents or external electromagnetic fields and may be strongly
affected by thermal or quantum fluctuations. The common expectation borne out
in previous experiments is that as the temperature is lowered, vortex motion is
suppressed, leading to a decreased resistance. A new generation of materials
provides access to the previously inaccessible regime of clean-limit
superconductivity in atomically thin superconducting layers. We show
experimentally that for few-layer 2H-NbSe$_2$ the resistance below the
superconducting transition temperature may be non-monotonic, passing through a
minimum and then increasing again as temperature is decreased further. The
effects exists over a wide range of current and magnetic fields, but is most
pronounced in monolayer devices at intermediate currents. Analytical and
numerical calculations confirm that the findings can be understood in a
two-fluid vortex model, in which a fraction of vortices flow in channels while
the rest are pinned but thermally fluctuating in position. We show
theoretically that the pinned, fluctuating vortices effectively control the
mobility of the free vortices. The findings provide a new perspective on
fundamental questions of vortex mobility and dissipation in superconductors. | 1909.08469v1 |
2019-09-19 | Thermal rectification and interface thermal resistance in hybrid pillared-graphene and graphene: A molecular dynamics approach | In this study, we investigate the thermal rectification and thermal
resistance in the hybrid pillared-graphene and graphene (PGG) system. This is
done through the classical molecular dynamics simulation (MD) and also with a
continuum model. At first, the thermal conductivity of both pillared-graphene
and graphene is calculated employing MD simulation and Fourier low. Our results
show that the thermal conductivity of the pillared-graphene is much smaller
than the graphene by an order of magnitude. Next, by applying positive and
negative temperature gradients along the longitudinal direction of PGG, the
thermal rectification is examined. The MD results indicate that for the lengths
in the range of 36 to 86nm, the thermal rectification remains almost constant
(~3-5%). We have also studied the phonon density of states (DOS) on both sides
of the interface of PGG. The DOS curves show that there is phonon scattering at
low frequencies (acoustic mode) that depends on the imposed temperature
gradient direction in the system. Therefore, we can introduce the PGG as a
promising thermal rectifier at room temperature. Furthermore, in the following
of this work, we also explore the temperature distribution over the PGG by
using the continuum model. The results that obtained from the continuum model
predict the MD results such as the temperature distribution in the upper half
layer and lower full layer graphene, the temperature gap and also the thermal
resistance at the interface. | 1909.08971v1 |
2014-08-19 | Device Perspective for Black Phosphorus Field-Effect Transistors: Contact Resistance, Ambipolar and Scaling | Although monolayer black phosphorus (BP) or phosphorene has been successfully
exfoliated and its optical properties have been explored, most of electrical
performance of the devices is demonstrated on few-layer phosphorene and
ultra-thin BP films. In this paper, we study the channel length scaling of
ultra-thin BP field-effect transistors (FETs), and discuss a scheme for using
various contact metals to change transistor characteristics. Through studying
transistor behaviors with various channel lengths, the contact resistance can
be extracted from the transfer length method (TLM). With different contact
metals, we find out that the metal/BP interface has different Schottky barrier
heights, leading to a significant difference in contact resistance, which is
quite different from previous studies of transition metal dichalcogenides
(TMDs) such as MoS2 where Fermi-level is strongly pinned near conduction band
edge at metal/MoS2 interface. The nature of BP transistors are Schottky barrier
FETs, where the on and off states are controlled by tuning the Schottky
barriers at the two contacts. We also observe the ambipolar characteristics of
BP transistors with enhanced n-type drain current and demonstrate that the
p-type carriers can be easily shifted to n-type or vice versus by controlling
the gate bias and drain bias, showing the potential to realize BP CMOS logic
circuits. | 1408.4206v2 |
2014-08-19 | Exploring quantum phase transition in Pd_{1-x}Ni_x nanoalloys | Pd$_{1-x}$Ni$_x$ alloy system is an established ideal transition metal system
possessing a composition induced paramagnetic to ferromagnetic quantum phase
transition (QPT) at the critical concentration $x_c \sim$ 0.026 in bulk. A
low-temperature non-Fermi liquid (NFL) behaviour around $x_c$ usually indicates
the presence of quantum criticality (QC) in this system. In this work, we
explore the existence of such a QPT in nanoparticles of this alloy system. We
synthesized single-phase, polydispersed and 40-50 nm mean diameter crystalline
nanoparticles of Pd$_{1-x}$Ni$_x$ alloys, with $x$ near $x_c$ and beyond, by a
chemical reflux method. In addition to the determination of the size,
composition, phase and crystallinity of the alloys by microscopic and
spectroscopic techniques, the existence of a possible QPT was explored by
resistivity and DC magnetization measurements. A dip in the value of the
exponent $n$ near $x_c$, and a concomitant peak in the constant $A$, of the
$AT^n$ dependence of the low temperature ($T$) resistivity indicate the
presence of a quantum-like phase transition in the system. The minimum value of
$n$, however, remains within the Fermi liquid regime ($n >$ 2). The DC
magnetization results suggest an anticipatory presence of a superparamagnetic
to ferromagnetic QPT in the mean-sized nanoparticles. The observation of a
possible quantum critical NFL behaviour ($n <$ 2) through resistivity is argued
to be inhibited by the electron-magnon scatterings present in the smaller
nanoparticles. | 1408.4316v1 |
2017-01-03 | Conduction Channel Formation and Dissolution Due to Oxygen Thermophoresis/Diffusion in Hafnium Oxide Memristors | Transition metal oxide memristors, or resistive random-access memory (RRAM)
switches, are under intense development for storage-class memory because of
their favorable operating power, endurance, speed, and density. Their
commercial deployment critically depends on predictive compact models based on
understanding nanoscale physico-chemical forces, which remains elusive and
controversial owing to the difficulties in directly observing atomic motions
during resistive switching, Here, using scanning transmission synchrotron x-ray
spectromicroscopy to study in-situ switching of hafnium oxide memristors, we
directly observed the formation of a localized oxygen-deficiency-derived
conductive channel surrounded by a low-conductivity ring of excess oxygen.
Subsequent thermal annealing homogenized the segregated oxygen, resetting the
cells towards their as-grown resistance state. We show that the formation and
dissolution of the conduction channel are successfully modeled by radial
thermophoresis and Fick diffusion of oxygen atoms driven by Joule heating. This
confirmation and quantification of two opposing nanoscale radial forces that
affect bipolar memristor switching are important components for any future
physics-based compact model for the electronic switching of these devices. | 1701.00864v1 |
2018-10-05 | Effect of impurities on morphology and growth mode of (111) and (001) epitaxial-like ScN films | ScN material is an emerging semiconductor with an indirect bandgap. It has
attracted attention for its thermoelectric properties, use as seed layers, and
for alloys for piezoelectric application. ScN or other transition metal nitride
semiconductors used for their interesting electrical properties are sensitive
to contaminants, such as oxygen or fluorine. In this present article, the
influence of depositions conditions on the amount of oxygen contaminants
incorporated in ScN films were investigated and their effects on the electrical
properties (electrical resistivity and Seebeck coefficient) were studied. The
epitaxial-like films of thickness 125 +-5 nm to 155 +-5 nm were deposited by
D.C.-magnetron sputtering on c-plane Al2O3, MgO(111) and r-plane Al2O3 at a
substrate temperature ranging from 700 to 950 degree C. The amount of oxygen
contaminants presents in the film, dissolved into ScN or as an oxide, was
related to the adatom mobility during growth, which is affected by the
deposition temperature and the presence of twin domain growth. The lowest
values of electrical resistivity of 50 micro-ohm cm were obtained on
ScN(111)/MgO(111) and on ScN(001)/r-plane Al2O3 grown at 950 degree C with no
twin domains and the lowest amount of oxygen contaminant. At the best, the
films exhibited an electrical resistivity of 50 micro-ohm cm with Seebeck
coefficient values maintained at -40 microV K-1, thus a power factor estimated
at 3.2 10-3 W m-1 K-2 (at room temperature). | 1810.02593v1 |
2019-07-05 | A post mortem analysis of the strain-induced crystallization effects on fatigue of elastomers | Natural rubber (NR) is the most commonly used elastomer in the automotive
industry thanks to its outstanding fatigue resistance. Strain-induced
crystallization (SIC) is found to play a role of paramount importance in the
great crack growth resistance of NR [1]. Typically, NR exhibits a lifetime
reinforcement for non-relaxing loadings [2-3]. At the microscopic scale,
fatigue striations were observed on the fracture surface of Diabolo samples
tested in fatigue. They are the signature of SIC [2,4,5]. In order to provide
additional information on the role of SIC in the fatigue crack growth
resistance of NR, striations are investigated through post-mortem analysis
after fatigue experiments using loading ranging from-0.25 to 0.25. No striation
was observed in the case of tests performed at 90{\textdegree}C. This confirms
that the formation of striation requires a certain crystallinity level in the
material. At 23{\textdegree}C, two striation regimes were identified: small
striation patches with different orientations (Regime 1) and zones with large
and well-formed striations (Regime 2). Since fatigue striations are observed
for all the loading ratios applied, they are therefore not the signature of the
reinforcement. Nevertheless, increasing the minimum value of the strain
amplified the striation phenomenon and the occurrence of Regime 2. | 1907.02688v1 |
2022-02-12 | Expansion of Graphene-Based Device Technology for Resistance Metrology | The field of Quantum Hall metrology had a strong start with the implemntation
of GaAs-based devices, given that 2D materials systems provided access to
interesting quantum phenomena, including the infrastructure associated with
making relevant measurements. With the technology laid out, further
improvements in both infrastructure and standards were achieved in the previous
two decades as EG-based quantized Hall resistance (QHR) devices became
established as national standards. Since the metrology community has reached
some understanding that a comparison against GaAs-based QHR devices had been
accomplished, the next steps became clearer as far as how the EG-based QHR with
a single Hall bar could be further developed. Since the early 90s, it has been
of modest interest that QHR devices have a means of interconnecting several
single Hall bar elements and has since been a subject of research. NMIs are now
presently at a juncture where consideration must be granted beyond just
simplicity of operation. A natural direction for resistance standards would be
to increase the total accessible parameter space. This means using EG-based QHR
devices to output more than the single value at the $\nu = 2$ plateau (about
12.9 k$\Omega$). A first natural question is whether one may use the $\nu = 6$
plateau or $\nu = 10$ plateau, and though some work has been done with these
Landau levels in graphene, they simply do not offer the same level of precision
as the $\nu = 2$ plateau. | 2202.05954v1 |
2016-03-21 | Influence of rhombohedral stacking order in the electrical resistance of bulk and mesoscopic graphite | The electrical, in-plane resistance as a function of temperature $R(T)$ of
bulk and mesoscopic thin graphite flakes obtained from the same batch was
investigated. Samples thicker than $\sim 30$ nm show metalliclike contribution
in a temperature range that increases with the sample thickness, whereas a
semiconductinglike behavior was observed for thinner samples. The temperature
dependence of the in-plane resistance of all measured samples and several
others from literature can be very well explained between 2 K and 1100 K
assuming three contributions in parallel: a metalliclike conducting path at the
interfaces between crystalline regions, composed of two semiconducting phases,
i.e. Bernal and rhombohedral stacking. From the fits of $R(T)$ we obtain a
semiconducting energy gap of $110 \pm 20$meV for the rhombohedral and $38\pm 8
$meV for the Bernal phase. The presence of these crystalline phases was
confirmed by x-ray diffraction measurements. We review similar experimental
data from literature of the last 33 years and two more theoretical models used
to fit $R(T)$. | 1603.06365v3 |
2017-04-03 | Plasmonic heating in Au nanowires at low Temperatures: The role of thermal boundary resistance | Inelastic electron tunneling and surface-enhanced optical spectroscopies at
the molecular scale require cryogenic local temperatures even under
illumination - conditions that are challenging to achieve with plasmonically
resonant metallic nanostructures. We report a detailed study of the laser
heating of plasmonically active nanowires at substrate temperatures from 5 to
60 K. The increase of the local temperature of the nanowire is quantified by a
bolometric approach and could be as large as 100 K for a substrate temperature
of 5 K and typical values of laser intensity. We also demonstrate that a $\sim
3\times$ reduction of the local temperature increase is possible by switching
to a sapphire or quartz substrate. Finite element modeling of the heat
dissipation reveals that the local temperature increase of the nanowire at
temperatures below $\sim$50 K is determined largely by the thermal boundary
resistance of the metal-substrate interface. The model reproduces the striking
experimental trend that in this regime the temperature of the nanowire varies
nonlinearly with the incident optical power. The thermal boundary resistance is
demonstrated to be a major constraint on reaching low temperatures necessary to
perform simultaneous inelastic electron tunneling and surface enhanced Raman
spectroscopies. | 1704.00771v1 |
2017-05-18 | Effects of the Functional Group on the Lithium Ions Across the Port of Carbon Nanotube | The mean axial velocity of lithium irons across the entrance of carbon
nanotube VLi is an important factor for the charge-discharge performances of
rechargeable Lithium battery. The molecular dynamics simulation method is
adopted to evaluate the factors and their effects on VLi which include the
diameter of carbon nanotube, functional group type on the port and the number
of a given type of functional group. The statistical analysis of the
calculation results shows that: In the selected carbon nanotubes of four
different diameters, VLi will gradually rise with the increase of CNT diameter
due to lithium irons migration resistance decreasing; as the port of CNT is
successively modified to hydrogen (-H), hydroxyl (-OH), amino (-NH2) and
carboxyl (-COOH), the corresponding migration resistance of lithium ions is
enhanced resulting in the dropping of VLi; in comparison to the effect strength
of four types of functional groups on VLi, -COOH shows strongest, -NH2 and -OH
perform relatively weaker, and the effect difference between -NH2 and -OH is
very small, -H displays weakest; When the number of a given non-hydrogen
functional group on the port sequentially increases, it also shows a trend that
lithium ion migration resistance gradually increases which makes VLi decreases
in turn. The more influential the functional group, the greater the impact of
functional group number changes on VLi. The results of this paper have some
significance on the precise production of lithium-ion battery electrode
materials, enhancing the overall battery cycle efficiency and charging speed. | 1705.06650v1 |
2017-08-24 | Memory matrix theory of the dc resistivity of a disordered antiferromagnetic metal with an effective composite operator | We perform the calculation of the dc resistivity as a function of temperature
of the "strange-metal" state that emerges in the vicinity of a
spin-density-wave phase transition in the presence of weak disorder. This
scenario is relevant to the phenomenology of many important correlated
materials, such as, e.g., the pnictides, the heavy-fermion compounds and the
cuprates. To accomplish this task, we implement the memory-matrix approach that
allows the calculation of the transport coefficients of the model beyond the
quasiparticle paradigm. Our computation is also inspired by the $\epsilon=3-d$
expansion in a hot-spot model embedded in $d$-space dimensions recently put
forth by Sur and Lee [Phys. Rev. B 91, 125136 (2015)], in which they find a new
low-energy non-Fermi liquid fixed point that is perturbatively accessible near
three dimensions. As a consequence, we are able to establish here the
temperature and doping dependence of the electrical resistivity at intermediate
temperatures of a two-dimensional disordered antiferromagnetic metallic model
with a composite operator that couples the order-parameter fluctuations to the
entire Fermi surface. We argue that our present theory provides a good basis in
order to unify the experimental transport data, e.g., in the cuprates and the
pnictide superconductors, within a wide range of doping regimes. | 1708.07537v1 |
2019-04-03 | Patterning of diamond like carbon films for sensor applications using silicon containing thermoplastic resist (SiPol) as a hard mask | Patterning of diamond-like carbon (DLC) and DLC:metal nanocomposites is of
interest for an increasing number of applications. We demonstrate a nanoimprint
lithography process based on silicon containing thermoplastic resist combined
with plasma etching for straightforward patterning of such films. A variety of
different structures with few hundred nanometer feature size and moderate
aspect ratios were successfully realized. The quality of produced patterns was
directly investigated by the means of optical and scanning electron microscopy
(SEM). Such structures were further assessed by employing them in the
development of gratings for guided mode resonance (GMR) effect. Optical
characterization of such leaky waveguide was compared with numerical
simulations based on rigorous coupled wave analysis method with good agreement.
The use of such structures as refractive index variation sensors is
demonstrated with sensitivity up to 319 nm/RIU, achieving an improvement close
to 450% in sensitivity compared to previously reported similar sensors. This
pronounced GMR signal fully validates the employed DLC material, the technology
to pattern it and the possibility to develop DLC based gratings as corrosion
and wear resistant refractometry sensors that are able to operate under harsh
conditions providing great value and versatility. | 1904.01880v1 |
2019-04-02 | Filament mechanics in a half-space via regularised Stokeslet segments | We present a generalisation of efficient numerical frameworks for modelling
fluid-filament interactions via the discretisation of a recently-developed,
non-local integral equation formulation to incorporate regularised Stokeslets
with half-space boundary conditions, as motivated by the importance of
confining geometries in many applications. We proceed to utilise this framework
to examine the drag on slender inextensible filaments moving near a boundary,
firstly with a relatively-simple example, evaluating the accuracy of resistive
force theories near boundaries using regularised Stokeslet segments. This
highlights that resistive force theories do not accurately quantify filament
dynamics in a range of circumstances, even with analytical corrections for the
boundary. However, there is the notable and important exception of movement in
a plane parallel to the boundary, where accuracy is maintained. In particular,
this justifies the judicious use of resistive force theories in examining the
mechanics of filaments and monoflagellate microswimmers with planar flagellar
patterns moving parallel to boundaries. We proceed to apply the numerical
framework developed here to consider how filament elastohydrodynamics can
impact drag near a boundary, analysing in detail the complex responses of a
passive cantilevered filament to an oscillatory flow. In particular, we
document the emergence of an asymmetric periodic beating in passive filaments
in particular parameter regimes, which are remarkably similar to the power and
reverse strokes exhibited by motile 9+2 cilia. Furthermore, these changes in
the morphology of the filament beating, arising from the fluid-structure
interactions, also induce a significant increase in the hydrodynamic drag of
the filament. | 1904.02543v2 |
2015-07-23 | Spatially resolved TiOx phases in RRAM conductive nanofilaments using soft X-ray spectromicroscopy | Reduction in metal-oxide thin films has been suggested as the key mechanism
responsible for forming conductive nanofilaments within solid-state memory
devices, enabling their resistive switching capacity. The quantitative spatial
identification of such filaments is a daunting task, particularly for
metal-oxides capable of exhibiting multiple phases as in the case of TiOx.
Here, we spatially resolve and chemically characterize distinct TiOx phases in
localized regions of a TiOx-based memristive device by combining full-field
transmission X-ray microscopy with soft X-ray spectroscopic analysis that is
performed on lamella samples. We particularly show that electrically
pre-switched devices in low-resistive states comprise reduced disordered phases
with O/Ti ratios close to Ti2O3 stoichiometry that aggregate in a ~ 100 nm
filamentary region electrically conducting the top and bottom electrodes of the
devices. We have also identified crystalline rutile and orthorhombic-like TiO2
phases in the region adjacent to the filament, suggesting that the temperature
increases locally up to 1000 K, validating the role of Joule heating in
resistive switching. Contrary to previous studies, our approach enables to
simultaneously investigate morphological and chemical changes in a quantitative
manner without incurring difficulties imposed by interpretation of electron
diffraction patterns acquired via conventional electron microscopy techniques. | 1507.06588v1 |
2019-01-16 | Quantum oscillations in strongly correlated topological Kondo insulators | The observation of quantum oscillations in topological Kondo insulators SmB6
and YbB12 is a recent puzzling experimental discovery. Quantum oscillations
observed in the resistivity and the magnetization are usually explained by the
existence of the Fermi surface. However, Kondo insulators do not have a Fermi
surface and thus should not show quantum oscillations. By performing dynamical
mean-field calculations for topologically nontrivial Kondo insulators in a
magnetic field, we analyze the effect of correlations on the emergence of
quantum oscillations in narrow-gap topological Kondo insulators and demonstrate
that the interplay between correlations and nonlocal hybridization,
ubiquitously occurring in topological Kondo insulators, can lead to observable
quantum oscillations without the necessity of a Fermi surface. Particularly, we
show that correlations make it easier to observe quantum oscillations in the
magnetization and the resistivity of the bulk material. The fundamental
mechanism for these quantum oscillations is a combination of correlation
effects and Landau levels coming very close to the Fermi energy. We furthermore
demonstrate that quantum oscillations in a three-dimensional system can be
understood by analyzing the physics on the two-dimensional planes in the
momentum space for which the hybridization in direction of the magnetic field
vanishes. We believe that this scenario is relevant to understanding the
observation of quantum oscillations in the magnetic torque for SmB6 as well as
oscillations in the resistivity and the magnetic torque of YbB12. | 1901.05099v2 |
2019-10-18 | Probing quantum spin liquids in equilibrium using the inverse spin Hall effect | We propose an experimental method utilizing a strongly spin-orbit coupled
metal to quantum magnet bilayer that will probe quantum magnets lacking long
range magnetic order, e.g., quantum spin liquids, via examination of the
voltage noise spectrum in the metal layer. The bilayer is held in thermal and
chemical equilibrium, and spin fluctuations arising across the single interface
are converted into voltage fluctuations in the metal as a result of the inverse
spin Hall effect. We elucidate the theoretical workings of the proposed bilayer
system, and provide precise predictions for the frequency characteristics of
the enhancement to the ac electrical resistance measured in the metal layer for
three candidate quantum spin liquid models. Application to the Heisenberg
spin-$1/2$ kagom{\'e} lattice model should allow for the extraction of any
spinon gap present. A quantum spin liquid consisting of fermionic spinons
coupled to a $U(1)$ gauge field should cause subdominant $\W^{4/3}$ scaling of
the resistance of the coupled metal. Finally, if the magnet is well-captured by
the Kitaev model in the gapless spin liquid phase, then the proposed bilayer
can extract the two-flux gap which arises in spite of the gapless spectrum of
the fermions. We therefore show that spectral analysis of the ac resistance in
the metal in a single interface, equilibrium bilayer can test the relevance of
a quantum spin liquid model to a given candidate material. | 1910.08610v2 |
2020-05-23 | Mixed ground state in Fe-Ni Invar alloys | We investigate the ground state properties of Invar alloys via detailed study
of the electronic structure of Fe$_{1-x}$Ni$_x$ alloys ($x$ = 0.2, 0.3, 0.4,
0.5, 0.6, 0.7, 0.9) employing $x$-ray photoelectron spectroscopy (XPS). While
all the alloys exhibit soft ferromagnetic behavior with Curie temperature much
higher than the room temperature, the results for invar alloy,
Fe$_{0.6}$Ni$_{0.4}$ exhibit anomalous behavior. Moreover, the
magneto-resistance of the Invar alloy becomes highly negative while the end
members possess positive magneto-resistance. The core level spectra of the
Invar alloy exhibit emergence of a distinct new feature below 20~K while all
other Fe-Ni alloys exhibit no temperature dependence down to 10~K.
Interestingly, the shallow core level spectra (3$s$, 3$p$) of Fe and Ni of the
Invar alloy reveal stronger deviation at low temperatures compared to the deep
core levels (2$s$, 2$p$) indicating crystal field effect. It appears that there
is a large precipitation of antiferromagnetic $\gamma^\prime$ phase below 20 K
possessing low magnetic moment (0.5$\mu_B$) on Fe within the $\alpha$ phase.
The discovery of negative magneto-resistance, anomalous magnetization at low
temperature and the emergence of unusual new features in the core levels at low
temperature provide an evidence of mixed phase in the ground state of Invar
alloys. | 2005.11493v1 |
2020-07-01 | Impact of V substitution on the physical properties of Ni-Zn-Co ferrites: structural, magnetic, dielectric and electrical properties | We have investigated the Vanadium- (V) substituted Ni-Zn-Co ferrites where
the samples were prepared using solid-state reaction technique. The impact of
V5+ substitution on the structural, magnetic, dielectric and electrical
properties of Ni-Zn-Co ferrites has been studied. XRD analysis confirmed the
formation of a single-phase cubic spinel structure. The lattice constants have
been calculated both theoretically and experimentally along with other
structural parameters such as bulk density, X-ray density and porosity. The
FESEM images are taken to study the surface morphology. FTIR measurement is
also performed which confirms spinel structure formation. The saturation
magnetization (Ms), coercive field (Hc) and Bohr magneton (B) were calculated
from the obtained M-H loops. The temperature dependent permeability is studied
to obtain the Curie temperature. Frequency and composition dependence of
permeability was also analyzed. Dielectric behavior and ac resistivity are also
subjected to investigate the frequency dependency. An inverse relationship was
observed between the composition dependence of dielectric constant and ac
resistivity. The obtained results such as the electrical resistivity,
dielectric constants and magnetic properties suggest the appropriateness of the
studied ferrites in microwave device applications. | 2007.00602v1 |
2020-07-20 | Capping and gate control of anomalous Hall effect and hump structure in ultra-thin SrRuO$_3$ films | Ferromagnetism and exotic topological structures in SrRuO$_3$ (SRO) induce
sign-changing anomalous Hall effect (AHE). Recently, hump structures have been
reported in the Hall resistivity of SRO thin films, especially in the
ultra-thin regime. We investigate the AHE and hump structure in the Hall
resistivity of SRO ultra-thin films with an SrTiO$_3$ (STO) capping layer and
ionic liquid gating. STO capping results in sign changes in the AHE and
modulation of the hump structure. In particular, the hump structure in the Hall
resistivity is strongly modulated and even vanishes in STO-capped 4 unit cell
(uc) films. In addition, the conductivity of STO-capped SRO ultra-thin films is
greatly enhanced with restored ferromagnetism. We also performed ionic liquid
gating to modulate the electric field at SRO/STO interface. Drastic changes in
the AHE and hump structure are observed with different gate voltages. Our study
shows that the hump structure as well as the AHE can be controlled by tuning
inversion symmetry and the electric field at the interface. | 2007.09872v2 |
2020-07-23 | Origin of gap-like behaviors in URu$_2$Si$_2$: Combined study via quasiparticle scattering spectroscopy and resistivity measurements | We address two long-standing questions regarding the hidden order in URu2Si2:
Is it associated with the hybridization process, and what are the distinct
roles played by the localized and itinerant electrons? Our quasiparticle
scattering spectroscopy reveals a hybridization gap ubiquitous in the entire
phase space spanned by P and Fe substitutions in URu2Si2, including the
no-order and antiferromagnetic regions, with minimal change upon crossing the
phase boundary. This indicates its opening isn't associated with the ordering,
and thus localized electrons must be the major player. Towards a consistent
understanding of all the other gap-like behaviors observed only below
transition temperatures, we analyze the electrical resistivity using a model in
which gapped bosonic excitations are the dominant scattering source. With their
stiffness set to follow an unusual temperature dependence (decreasing with
decreasing temperature), this model fits all of our resistivity data well
including the jump at the transition. Remarkably, the extracted gap increases
slowly with increasing Fe content, similarly to the gap detected by inelastic
neutron scattering at Q1 = (1.4, 0, 0), suggesting a common origin. Such a
model can also naturally explain the Hall effect temperature dependence without
invoking Fermi surface gapping. | 2007.12235v2 |
2020-10-11 | Hydrothermal synthesis and complete phase diagram of FeSe$_{1-x}$S$_{x}$ $(0 \leq x \leq 1)$ single crystals | We report the successful synthesis of FeSe$_{1-x}$S$_{x}$ single crystals
with $x$ ranging from 0 to 1 via a hydrothermal method. A complete phase
diagram of FeSe$_{1-x}$S$_{x}$ has been obtained based on resistivity and
magnetization measurements. The nematicity is suppressed with increasing $x$,
and a small superconducting dome appears within the nematic phase. Outside the
nematic phase, the superconductivity is continuously suppressed and reaches a
minimum $T_c$ at $x$ = 0.45; beyond this point, $T_c$ slowly increases until
$x$ = 1. Intriguingly, an anomalous resistivity upturn with a characteristic
temperature $T^*$ in the intermediate region of $0.31 \leq x \leq 0.71$ is
observed. $T^{*}$ shows a dome-like behavior with a maximum value at $x$ =
0.45, which is opposite the evolution of $T_c$, indicating competition between
$T^*$ and superconductivity. The origin of $T^*$ is discussed in detail.
Furthermore, the normal state resistivity evolves from non-Fermi-liquid to
Fermi-liquid behavior with S doping at low temperatures, accompanied by a
reduction in electronic correlations. Our study addresses the lack of single
crystals in the high-S doping region and provides a complete phase diagram,
which will promote the study of relations among nematicity, superconductivity,
and magnetism. | 2010.05191v3 |
2021-03-12 | Area-selective deposition and B $δ$-doping of Si(100) with BCl$_{3}$ | B-doped $\delta$-layers were fabricated in Si(100) using BCl$_{3}$ as a
dopant precursor in ultrahigh vacuum. BCl$_{3}$ adsorbed readily at room
temperature, as revealed by scanning tunneling microscopy (STM) imaging.
Annealing at elevated temperatures facilitated B incorporation into the Si
substrate. Secondary ion mass spectrometry (SIMS) depth profiling demonstrated
a peak B concentration $>$ 1.2(1) $\times$ 10$^{21}$ cm$^{-3}$ with a total
areal dose of 1.85(1) $\times$ 10$^{14}$ cm$^{-2}$ resulting from a 30 L
BCl$_{3}$ dose at 150 $^{\circ}$C. Hall bar measurements of a similar sample
were performed at 3.0 K revealing a sheet resistance of $R_{\mathrm{s}}$ = 1.91
k$\Omega\square^{-1}$, a hole concentration of $n$ = 1.90 $\times$ 10$^{14}$
cm$^{-2}$ and a hole mobility of $\mu$ = 38.0 cm$^{2}$V$^{-1}$s$^{-1}$ without
performing an incorporation anneal. Further, the conductivity of several
B-doped $\delta$-layers showed a log dependence on temperature suggestive of a
two-dimensional system. Selective-area deposition of BCl$_{3}$ was also
demonstrated using both H- and Cl-based monatomic resists. In comparison to a
dosed area on bare Si, adsorption selectivity ratios for H and Cl resists were
determined by SIMS to be 310(10):1 and 1529(5):1, respectively, further
validating the use of BCl$_{3}$ as a dopant precursor for atomic precision
fabrication of acceptor-doped devices in Si. | 2103.07529v1 |
2021-05-21 | Convection, Heat Generation and Particle Deposition in Direct Laser Writing of Metallic Microstructures | Three-dimensional metallic microstructures find applications as stents in
medicine, as ultrabroadband antennas in communications, in micromechanical
parts or as structures of more fundamental interest in photonics like
metamaterials. Direct metal printing of such structures using three-dimensional
laser lithography is a promising approach, which is not extensively applied
yet, as fabrication speed, surface quality, and stability of the resulting
structures are limited so far. In order to identify the limiting factors, we
investigate the influence of light-particle interactions and varying scan speed
on heat generation and particle deposition in direct laser writing of silver.
We introduce a theoretical model which captures diffusion of particles and heat
as well as the fluid dynamics of the photo-resist. Chemical reactions are
excluded from the model but particle production is calibrated using
experimental data. We find that optical forces generally surmount those due to
convection of the photo-resist. Simulations predict overheating of the
photo-resist at laser powers similar to those found in experiments. The thermal
sensitivity of the system is essentially determined by the largest particles
present in the laser focus. Our results suggest that to improve particle
deposition and to achieve higher writing speeds in metal direct laser writing,
strong optical trapping of the emerging particles is desirable. Furthermore,
precise control of the particle size reduces the risk of spontaneous
overheating. | 2105.10243v2 |
2021-10-29 | Optimal asymmetry of transistor-based terahertz detectors | Detectors of terahertz radiation based on field-effect transistors (FETs) are
among most promising candidates for low-noise passive signal rectification both
in imaging systems and wireless communications. However, it was not realised so
far that geometric asymmetry of common FET with respect to source-drain
interchange is a strong objective to photovoltage harvesting. Here, we break
the traditional scheme and reveal the optimally-asymmetric FET structure
providing the maximization of THz responsivity. We fabricate a series of
graphene transistors with variable top gate position with respect to
mid-channel, and compare their sub-THz responsivities in a wide range of
carrier densities. We show that responsivity is maximized for input gate
electrode shifted toward the source contact. Theoretical simulations show that
for large channel resistance, exceeding the gate impedance, such recipe for
responsivity maximisation is universal, and holds for both resistive
self-mixing and photo-thermoelectric detection pathways. In the limiting case
of small channel resistance, the thermoelectric and self-mixing voltages react
differently upon changing the asymmetry, which may serve to disentangle the
origin of nonlinearities in novel materials. | 2110.15810v1 |
2021-12-06 | Dynamical Mean Field Theory of Moiré Bilayer Transition Metal Dichalcogenides: Phase Diagram, Resistivity, and Quantum Criticality | We present a comprehensive dynamical mean field study of the triangular
lattice moir\'e Hubbard model, which is believed to represent the physics of
moir\'e bilayer transition metal dichalcogenides. In these materials, important
aspects of the band structure including the bandwidth and the order and
location of van Hove singularities can be tuned by varying the interlayer
potential. We present a magnetic and metal-insulator phase diagram and a
detailed study of the dependence of the resistivity on temperature, band
filling and interlayer potential. We find that transport displays Fermi liquid,
strange metal and quantum critical behaviors in distinct regions of the phase
diagram. Specifically, we find that the cube-root van Hove singularity
($\rho(\epsilon) \sim|\epsilon|^{-1 / 3}$) gives a strange metal behavior with
a $T$-linear scattering rate and $\omega/T$ scaling. We show how magnetic order
affects the resistivity. Our results elucidate the physics of the correlated
states and the metal-insulator continuous transition recently observed in
twisted homobilayer WSe$_2$ and heterobilayer MoTe$_2$/WSe$_2$ experiments. | 2112.03080v3 |
2022-03-15 | Challenges and opportunities of srf theory for next generation particle accelerators | We suggest a program to establish theoretical performance limits of srf
cavities using modern theories of nonequilibrium superconductivity under a
strong electromagnetic field. These theories will be used to calculate the main
parameter of merit of srf cavities: the quality factor Q and its dependencies
on the field amplitude, temperature and frequency, which would allow us to
understand how far the srf cavity performance could be pushed from the current
state of the art. Given that the quality factor is determined by multiple
mechanisms operating on very different length scales, we will address the
interconnected problems of a nonlinear surface resistance, rf losses of
vortices trapped in the cavity, the effect of materials defects and surface
topography, and the opportunities to boost the srf performance by surface
nano-structuring, impurity management and multilayers. We suggest the following
directions of theoretical srf research to address the goals of boosting the
performance of the next generation particle accelerators: 1. Establishing the Q
limit, mechanisms of nonlinear surface resistance and the residual resistance
in a nonequilibrium superconductor under a strong RF field. 2. Establishing the
srf breakdown field limit, dynamic superheating field and its dependencies on
frequency, temperature and concentration of impurities. 3. Losses due to
trapped vortices and extreme dynamics of ultrafast vortices driven by strong rf
Meissner currents in srf cavities. 4. Optimization of srf performance due to
surface nanostructuring of the cavity surface, multilayers and impurity
management. | 2203.08315v1 |
2022-05-19 | Electrical Circuit Modelling of Nanofluidic Systems | Nanofluidic systems exhibit transport characteristics that have made
technological marvels such as desalination, energy harvesting, and highly
sensitive biomolecule sensing possible by virtue of their ability to influence
small currents due to the selective transport of ions. Traditionally many of
these applications have relied on the use of nanoporous membranes. The immense
complexities of membrane geometry often impede a comprehensive understanding of
the underlying physics. To bypass the associated difficulties, here we consider
the much simpler nanochannel array comprised of numerous nanochannels and
elucidate the effects of interchannel interactions on the Ohmic response of the
array. We demonstrate that a nanochannel array is equivalent to an array of
mutually independent but identical unit-cells whereby the array can be
represented by an equivalent electrical circuit of unit-cell resistances
connected in a parallel configuration. We show that the total resistance of the
system scales inversely to the number of channels. We further deconstruct the
unit-cell to be a combination of multiple contributing resistances connected in
series. We validate the theoretical model underlying these electrical
abstractions using numerical simulations and experiments. Our approach to
modeling realistic nanofluidic systems by their equivalent electrical circuit
provides an invaluable tool for analyzing and interpreting experimental
measurements, characterization of surface charge properties of newly developed
materials, and a method for the design and development of function-specific
nanofluidic devices. | 2205.09437v3 |
2022-06-08 | Phonon-limited resistivity of multilayer graphene systems | We calculate the theoretical contribution to the doping and temperature ($T$)
dependence of electrical resistivity due to scattering by acoustic phonons in
Bernal bilayer graphene (BBG) and rhombohedral trilayer graphene (RTG). We
focus on the role of nontrivial geometric features of the detailed, anisotropic
$k\cdot p$ band structures of these systems - e.g. Van Hove singularities,
Lifshitz transitions, Fermi surface anisotropy, and band curvature near the gap
- whose effects on transport have not yet been systematically studied. We find
that these geometric features strongly influence the temperature and doping
dependencies of the resistivity. In particular, the band geometry leads to a
nonlinear $T$-dependence in the high-$T$ equipartition regime, complicating the
usual $T^4$ to $T$ Bloch-Gr\"{u}neisen crossover. Our focus on BBG and RTG is
motivated by recent experiments in these systems that have discovered several
exotic low-$T$ superconductivity proximate to complicated hierarchies of
isospin-polarized phases. These interaction-driven phases are intimately
related to the geometric features of the band structures, highlighting the
importance of understanding the influence of band geometry on transport. While
resolving the effects of the anisotropic band geometry on the scattering times
requires nontrivial numerical solution, our approach is rooted in intuitive
Boltzmann theory. We compare our results with recent experiment and discuss how
our predictions can be used to elucidate the relative importance of various
scattering mechanisms in these systems. | 2206.04080v2 |
2022-07-18 | Confinement-Induced Chiral Edge Channel Interaction in Quantum Anomalous Hall Insulators | In quantum anomalous Hall (QAH) insulators, the interior is insulating but
electrons can travel with zero resistance along one-dimensional conducting
paths known as chiral edge channels (CECs). These CECs have been predicted to
be confined to the one-dimensional (1D) edges and exponentially decay in the
two-dimensional (2D) bulk. In this work, we present the results of a systematic
study of QAH devices fashioned in a Hall bar geometry of different widths. At
the charge neutral point, the QAH effect persists in a Hall bar device with a
width of only ~72 nm, implying the intrinsic decaying length of CECs is less
than ~36 nm. In the electron-doped regime, we find that the Hall resistance
deviates quickly from the quantized value when the sample width is less than 1
um. Our theoretical calculations suggest that the deviation from the quantized
Hall resistance in narrow QAH samples originates from the interaction between
two opposite CECs mediated by disorder-induced bulk states in QAH insulators,
consistent with our experimental observations. | 2207.08371v1 |
2022-09-12 | Robust design optimization for enhancing delamination resistance of composites | Recent developments in the field of computational modeling of fracture have
opened up possibilities for designing structures against failure. A special
case, called interfacial fracture or delamination, can occur in loaded
composite structures where two or more materials are bonded together at
comparatively weak interfaces. Due to the potential crack growth along these
interfaces, the structural problem suffers from snap-back/snap-through
instabilities and bifurcations with respect to the model parameters, leading to
noisy and discontinuous responses. For such a case, the design optimization
problem for a selected quantity of interest is ill-posed, since small
variations in the design parameters can lead to large jumps in the structural
response. To this end, this paper presents a stochastic optimization approach
to maximize delamination resistance that is less sensitive to small
perturbations of the design and thereby leads to a robust solution. To overcome
the intractability of Monte Carlo methods for estimating the expected value of
the expensive-to-evaluate response function, a global, piecewise-constant
surrogate is constructed based on nearest-neighbor interpolation that is
iteratively refined during the optimization run. We found that by taking a
large stochastic region at the beginning of the optimization and gradually
reducing it to the desired one can help overcome poor local optima. Our results
demonstrate the effectiveness of the proposed framework using an example of
shape optimization of hard inclusions embedded in a double-cantilever beam,
which significantly enhances delamination resistance. | 2209.05241v1 |
2022-10-25 | Enhanced Thermoelectric Performance of Nanostructured Nickel Doped Ag2Te | We report on the thermoelectric properties of nickel doped Ag2-xNixTe (x = 0,
0.015, 0.025 & 0.055, 0.115, 0.155) nanostructures in the temperature (T) range
of 5 K to 575 K. The electrical resistivity of Ag2Te nanostructure shows
metallic behaviour in 5 K to 300 K initially that evolves into two metal to
insulator transitions (MITs) at low and mid-temperature regimes with increasing
x due to Mott-variable range hopping (VRH) and Arrhenius transports,
respectively. Their Seebeck coefficient varies nearly in a linear fashion in
this temperature range, showing metallic or doped-degenerate semiconducting
behaviour. Notably, this behaviour of the Seebeck coefficient is in contrast to
Mott VRH conduction as observed in resistivity. The steady increase in
resistivity and S with the sharp decrease in thermal conductivity between 410 K
to 425 K associated with the structural phase transition accomplishes a maximum
thermoelectric figure of merit (ZT) of 0.86 near 480 K in x = 0.155. This is
about 83 % more compared to that of bulk Ag2Te, and shows a significant
improvement over the best value reported for Ag2Te nanostructures thus far.
This study, therefore, shows that simultaneous nanocomposite formation, doping
and nanostructuring could be an effective strategy for tuning the electron and
phonon transports to improve the thermoelectric properties of a material. | 2210.13903v1 |
2022-12-28 | An Atomistic Model of Field-Induced Resistive Switching in Valence Change Memory | In Valence Change Memory (VCM) cells, the conductance of an insulating
switching layer is reversibly modulated by creating and redistributing point
defects under an external field. Accurate simulations of the switching dynamics
of these devices can be difficult due to their typically disordered atomic
structures and inhomogeneous arrangements of defects. To address this, we
introduce an atomistic framework for modelling VCM cells. It combines a
stochastic Kinetic Monte Carlo approach for atomic rearrangement with a quantum
transport scheme, both parameterized at the ab-initio level by using inputs
from Density Functional Theory (DFT). Each of these steps operates directly on
the underlying atomic structure. The model thus directly relates the energy
landscape and electronic structure of the device to its switching
characteristics. We apply this model to simulate non-volatile switching between
high- and low-resistance states in an TiN/HfO2/Ti/TiN stack, and analyze both
the kinetics and stochasticity of the conductance transitions. We also resolve
the atomic nature of current flow resulting from the valence change mechanism,
finding that conductive paths are formed between the undercoordinated Hf atoms
neighboring oxygen vacancies. The model developed here can be applied to
different material systems to evaluate their resistive switching potential,
both for use as conventional memory cells and as neuromorphic computing
primitives. | 2212.14090v1 |
2023-05-02 | Direct observations of spin fluctuations in spin-hedgehog-anti-hedgehog lattice states in MnSi$_{1-x}$Ge$_x$ ($x=0.6$ and $0.8$) at zero magnetic field | The helimagnetic compounds MnSi$_{1-x}$Ge$_{x}$ show the three-dimensional
multiple-$q$ order as referred to as spin-hedgehog-anti-hedgehog (SHAH)
lattice. Two representative forms of SHAH are cubic-3$q$ lattice with $q \|
\langle100\rangle$ and tetrahedral-4$q$ lattice with $q \| \langle111\rangle$,
which show up typically for $x=1.0-~0.8$ and for $x=0.6$, respectively. Here,
we have investigated the spin fluctuations in the MnSi$_{1-x}$Ge$_{x}$
polycrystalline samples with $x=0.6$ and $0.8$ by using the time-of-flight
(TOF) neutron inelastic scattering and MIEZE-type neutron spin echo techniques
to elucidate the microscopic origin of the unconventional Hall effect in the
SHAH lattice states. This research is motivated by the observation of a sign
change in the unconventional Hall resistivity as a function of temperature [Y.
Fujishiro et al., Nat. Comm. $\textbf{10}$, 1059 (2019)]. The present results
reveal the correspondences between the temperature ranges where the positive
Hall resistivity and spin fluctuations are observed. These results agree well
with the theoretical model of the conduction electrons scattered by the
fluctuating spin clusters with a non-zero average of sign-biased scalar spin
chirality as a mechanism of the positive Hall resistivity [H. Ishizuka and N.
Nagaosa, Sci. Adv. $\textbf{4}$, eaap9962 (2018)]. | 2305.01172v1 |
2023-08-30 | A Deep Dive into the Design Space of a Dynamically Reconfigurable Cryogenic Spiking Neuron | Spiking neural network offers the most bio-realistic approach to mimic the
parallelism and compactness of the human brain. A spiking neuron is the central
component of an SNN which generates information-encoded spikes. We present a
comprehensive design space analysis of the superconducting memristor (SM)-based
electrically reconfigurable cryogenic neuron. A superconducting nanowire (SNW)
connected in parallel with an SM function as a dual-frequency oscillator and
two of these oscillators can be coupled to design a dynamically tunable spiking
neuron. The same neuron topology was previously proposed where a fixed
resistance was used in parallel with the SNW. Replacing the fixed resistance
with the SM provides an additional tuning knob with four distinct combinations
of SM resistances, which improves the reconfigurability by up to ~70%.
Utilizing an external bias current (Ibias), the spike frequency can be
modulated up to ~3.5 times. Two distinct spike amplitudes (~1V and ~1.8 V) are
also achieved. Here, we perform a systematic sensitivity analysis and show that
the reconfigurability can be further tuned by choosing a higher input current
strength. By performing a 500-point Monte Carlo variation analysis, we find
that the spike amplitude is more variation robust than spike frequency and the
variation robustness can be further improved by choosing a higher Ibias. Our
study provides valuable insights for further exploration of materials and
circuit level modification of the neuron that will be useful for system-level
incorporation of the neuron circuit | 2308.15754v1 |
2023-12-03 | Experimental apparatus for non-contact resistivity measurements of the rock core plug based on magnetic induction | A new apparatus has been developed to measure the conductivity of rock
samples. The probe, which consists of multi-coil transmitters and receivers
doesn't require physical contact with the samples. The measurement is based on
the induction principle. The measurement system is validated by using saline
solutions and water-saturated sands of known conductivity. This work presents
details of the development of a system of magnetic resistivity measurements by
magnetic induction for petrophysical applications. The first application
consists of measuring the resistivity of the core plug which is 0.038 m in
diameter. Currently the system is operating properly at a frequency of 50 kHz
with a current of up to 500 mA at 20 {\deg}C. During the study two types of
samples were investigated: aqueous solutions with conductivities between 1 to
100 mS/cm and rocks. Several tests were carried out with the objective of
investigating the performance of the instrument, such as the experiment to
obtain sensitivity for the measurement system as a function of the current
applied to the transmitter coil. | 2312.01375v1 |
2023-12-08 | The Kernel Method for Electrical Resistance Tomography | In this paper we consider the inverse problem of electrical conductivity
retrieval starting from boundary measurements, in the framework of Electrical
Resistance Tomography (ERT). In particular, the focus is on non-iterative
reconstruction algorithms, compatible with real-time applications. In this work
a new non-iterative reconstruction method for Electrical Resistance Tomography,
termed Kernel Method, is presented. The imaging algorithm deals with the
problem of retrieving the shape of one or more anomalies embedded in a known
background. The foundation of the proposed method is given by the idea that if
there exists a current flux at the boundary (Neumann data) able to produce the
same voltage measurements on two different configurations, with and without the
anomaly, respectively, then the corresponding electric current density for the
problem involving only the background material vanishes in the region occupied
by the anomaly. Coherently with this observation, the Kernel Method consists in
(i) evaluating a proper current flux at the boundary $g$, (ii) solving one
direct problem on a configuration without anomaly and driven by $g$, (iii)
reconstructing the anomaly from the spatial plot of the power density as the
region in which the power density vanishes. This new tomographic method has a
very simple numerical implementation at a very low computational cost. Beside
theoretical results and justifications of our method, we present a large number
of numerical examples to show the potential of this new algorithm. | 2312.05059v1 |
2024-01-08 | Microwave-assisted synthesis of LaMnO3+d: Tuning physical properties with microwave power | Synthesis of transition metal oxides by microwave irradiation is a faster and
energy-saving method compared to conventional heating in an electrical furnace
because microwave energy is directly converted into heat within precursors.
However, not much is known about how the physical properties are modified by
the power of microwaves. We synthesized LaMnO3+d by irradiating oxide
precursors with microwaves and studied the impact of microwave power (P = 1000
W, 1200 W, 1400 W and 1600 W) on magnetism, resistivity, magnetoresistance,
thermopower, magnetic entropy change, magnetostriction, and electron spin
resonance. It is found that paramagnetic to ferromagnetic transition becomes
sharper, saturation magnetization increases, and electrical resistivity at low
temperatures dramatically decreases as P increases. While the resistivity of
samples irradiated with MW power of P less than or equal to 1400 W show
insulating-like behavior down to 50 K, an insulator-metal transition occurs in
the sample exposed to P = 1600 W and this sample also shows a maximum
magnetoresistance (= -55%), magneto-thermopower (=-87%), magnetostriction (-180
x10-6) for H = 50 kOe and magnetic entropy change of 4.78 J/kg. K for H = 30
kOe around the Curie temperature. The intensity of electron spin resonance
spectra at 300 K increases with P. We postulate that the much enhanced physical
properties observed for the P = 1600 W sample arise from the creation of higher
hole density, chemical homogeneity, and increased grain size. Our study shows
that microwave power can be used as a knob to tune magnetism and other physical
properties to our advantage. | 2401.04087v1 |
2024-01-30 | Two-Dimensional Phase-Fluctuating Superconductivity in Bulk-Crystalline NdO$_{0.5}$F$_{0.5}$BiS$_2$ | We present a combined growth and transport study of superconducting
single-crystalline NdO$_{0.5}$F$_{0.5}$BiS$_2$. Evidence of two-dimensional
superconductivity with significant phase fluctuations of preformed Cooper pairs
preceding the superconducting transition is reported. This result is based on
three key observations. (1) The resistive superconducting transition
temperature $T_c$ (defined by resistivity $\rho \rightarrow 0$) increases with
increasing disorder. (2) As $T\rightarrow T_c$, the conductivity diverges
significantly faster than what is expected from Gaussian fluctuations in two
and three dimensions. (3) Non-Ohmic resistance behavior is observed in the
superconducting state. Altogether, our observations are consistent with a
temperature regime of phase-fluctuating superconductivity. The crystal
structure with magnetic ordering tendencies in the NdO$_{0.5}$F$_{0.5}$ layers
and (super)conductivity in the BiS$_2$ layers is likely responsible for the
two-dimensional phase fluctuations. As such, NdO$_{0.5}$F$_{0.5}$BiS$_2$ falls
into the class of unconventional ``laminar" bulk superconductors that include
cuprate materials and 4Hb-TaS$_2$. | 2401.16980v2 |
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