publicationDate stringlengths 10 10 | title stringlengths 17 233 | abstract stringlengths 20 3.22k | id stringlengths 9 12 |
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2008-06-11 | Anisotropic thermodynamic and transport properties of single crystalline (Ba$_{1-x}$K$_x$)Fe$_2$As$_2$ (x = 0 and 0.45) | Single crystals of BaFe$_2$As$_2$ and (Ba$_{0.55}$K$_{0.45}$)Fe$_2$As$_2$
have been grown out of excess Sn with 1% or less incorporation of solvent. The
crystals are exceptionally micaceous, are easily exfoliated and can have
dimensions as large as 3 x 3 x 0.2 mm$^3$. The BaFe$_2$As$_2$ single crystals
manifest a structural phase transition from a high temperature tetragonal phase
to a low temperature orthorhombic phase near 85 K and do not show any sign of
superconductivity down to 1.8 K. This transition can be detected in the
electrical resistivity, Hall resistivity, specific heat and the anisotropic
magnetic susceptibility. In the (Ba$_{0.55}$K$_{0.45}$)Fe$_2$As$_2$ single
crystals this transition is suppressed and instead superconductivity occurs
with a transition temperature near 30 K. Whereas the superconducting transition
is easily detected in resistivity and magnetization measurements, the change in
specific heat near $T_c$ is small, but resolvable, giving $\Delta C_p/\gamma
T_c \approx 1$. The application of a 140 kOe magnetic field suppresses $T_c$ by
only $\sim 4$ K when applied along the c-axis and by $\sim 2$ K when applied
perpendicular to the c-axis. The ratio of the anisotropic upper critical
fields, $\gamma = H_{c2}^{\perp c} / H_{c2}^{\| c}$, varies between 2.5 and 3.5
for temperatures down to $\sim 2$ K below $T_c$. | 0806.1874v2 |
2008-06-16 | Transport and anisotropy in single-crystalline SrFe$_2$As$_2$ and $A_{0.6}$K$_{0.4}$Fe$_2$As$_2$ ($A$ = Sr, Ba) superconductors | We have successfully grown high quality single crystals of SrFe$_2$As$_2$ and
A$_{0.6}$K$_{0.4}$Fe$_2$As$_2$(A=Sr, Ba) using flux method. The resistivity,
specific heat and Hall coefficient have been measured. For parent compound
SrFe$_2$As$_2$, an anisotropic resistivity with $\rho_c$ / $\rho_{ab}$ as large
as 130 is obtained at low temperatures. A sharp drop in both in-plane and
out-plane resistivity due to the SDW instability is observed below 200 K. The
angular dependence of in-plane magnetoresistance shows 2-fold symmetry with
field rotating within ab plane below SDW transition temperature. This is
consistent with a stripe-type spin ordering in SDW state. In K doped
A$_{0.6}$K$_{0.4}$Fe$_2$As$_2$(A=Sr. Ba), the SDW instability is suppressed and
the superconductivity appears with T$_c$ above 35 K. The rather low anisotropy
in upper critical field between H$\parallel$ab and H$\parallel$c indicates
inter-plane coupling play an important role in hole doped Fe-based
superconductors. | 0806.2648v3 |
2008-07-03 | Pressure induced superconductivity in CaFe$_2$As$_2$ | CaFe$_2$As$_2$ has been found to be exceptionally sensitive to the
application of hydrostatic pressure and superconductivity has been found to
exist in a narrow pressure region that appears to be at the interface between
two different phase transitions. The pressure - temperature ($P - T$) phase
diagram of CaFe$_2$As$_2$ reveals that this stoichiometric, highly ordered,
compound can be easily tuned to reveal all the salient features associated with
FeAs-based superconductivity without introducing any disorder. Whereas at
ambient pressure CaFe$_2$As$_2$ does not superconduct for $T > 1.8$ K and
manifests a first order structural phase transition near $T \approx 170$ K, the
application of $\sim 5$ kbar hydrostatic pressure fully suppresses the
resistive signature of the structural phase transition and instead
superconductivity is detected for $T < 12$ K. For $P \ge 5.5$ kbar a different
transition is detected, one associated with a clear reduction in resistivity
and for $P > 8.6$ kbar superconductivity is no longer detected. This higher
pressure transition temperature increases rapidly with increasing pressure,
exceeding 300 K by $P \sim 17$ kbar. The low temperature, superconducting dome
is centered around 5 kbar, extending down to 2.3 kbar and up to 8.6 kbar. This
superconducting phase appears to exist when the low pressure transition is
suppressed sufficiently, but before the high pressure transition has reduced
the resistivity, and possibly the associated fluctuations, too dramatically. | 0807.0616v2 |
2008-08-06 | Divergent resistance at the Dirac point in graphene: Evidence for a transition in a high magnetic field | We have investigated the behavior of the resistance of graphene at the $n=0$
Landau Level in an intense magnetic field $H$. Employing a low-dissipation
technique (with power $P<$3 fW), we find that, at low temperature $T$, the
resistance at the Dirac point $R_0(H)$ undergoes a 1000-fold increase from
$\sim$10 k$\Omega$ to 40 M$\Omega$ within a narrow interval of field. The
abruptness of the increase suggests that a transition to an insulating, ordered
state occurs at the critical field $H_c$. Results from 5 samples show that
$H_c$ depends systematically on the disorder, as measured by the offset gate
voltage $V_0$. Samples with small $V_0$ display a smaller critical field $H_c$.
Empirically, the steep increase in $R_0$ fits acccurately a
Kosterlitz-Thouless-type correlation length over 3 decades. The curves of $R_0$
vs. $T$ at fixed $H$ approach the thermal-activation form with a gap
$\Delta\sim$15 K as $H\to H_c^{-}$, consistent with a field-induced insulating
state. | 0808.0906v3 |
2009-09-22 | Experimental study on the cyclic resistance of a natural loess from Northern France | In order to analyze the instability phenomenon observed along the Northern
High Speed Line of R\'eseau Ferr\'e de France (RFF), soil blocks were taken at
a site near the railway, at four different depths (1.2, 2.2, 3.5 and 4.9 m).
Cyclic triaxial tests were carried out on saturated and unsaturated soil
specimens. The results from tests on initially saturated specimens showed that
the soil taken at 2.2 m depth has the lowest resistance to cyclic loading, in
relation to its highest porosity and lowest clay fraction. This soil was then
studied at unsaturated state with various initial water contents. Unsaturated
soil specimens were first subjected to cyclic loadings to decrease their
volume. These cyclic loadings was stopped when the volume decrease was
approximately equal to the initial pore air volume, or when the pores filled by
air were eliminated and the soil was considered to become saturated.
Afterwards, the back-pressure tubing was saturated with de-aired water and
cycles were applied under undrained condition. Significant effect of initial
water content was evidenced: the lower the initial water content, the higher
the cyclic resistance. This can be explained by the densification of the soil
during the initial cyclic loadings. | 0909.3932v1 |
2011-03-23 | On the origin of non-monotonic doping dependence of the in-plane resistivity anisotropy in Ba(Fe$_{1-x}T_x$)$_2$As$_2$, $T$ = Co, Ni and Cu | The in-plane resistivity anisotropy has been measured for detwinned single
crystals of Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$ and Ba(Fe$_{1-x}$Cu$_x$)$_2$As$_2$.
The data reveal a non-monotonic doping dependence, similar to previous
observations for Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$. Magnetotransport measurements
of the parent compound reveal a non-linear Hall coefficient and a strong linear
term in the transverse magnetoresistance. Both effects are rapidly suppressed
with chemical substitution over a similar compositional range as the onset of
the large in-plane resistivity anisotropy. It is suggested that the relatively
small in-plane anisotropy of the parent compound in the spin density wave state
is due to the presence of an isotropic, high mobility pocket of reconstructed
Fermi surface. Progressive suppression of the contribution to the conductivity
arising from this isotropic pocket with chemical substitution eventually
reveals the underlying in-plane anisotropy associated with the remaining FS
pockets. | 1103.4535v2 |
2011-11-21 | Coulomb drag in monolayer and bilayer graphene | We theoretically calculate the interaction-induced frictional Coulomb drag
resistivity between two graphene monolayers as well as between two graphene
bilayers, which are spatially separated by a distance "$d$". We show that the
drag resistivity between graphene monolayers can be significantly affected by
the intralayer momentum-relaxation mechanism. For energy independent intralayer
scattering, the frictional drag induced by inter-layer electron-electron
interaction goes asymptotically as $\rho_D \sim T^2/n^4d^6$ and $\rho_D \sim
T^2/n^2d^2$ in the high-density ($k_F d \gg 1$) and low-density ($k_F d \ll 1$)
limits, respectively. When long-range charge impurity scattering dominates
within the layer, the monolayer drag resistivity behaves as $\rho_D \sim
T^2/n^3d^4$ and $T^2 \ln (\sqrt{n} d) /n$ for $k_F d \gg 1$ and $k_F d \ll 1$,
respectively. The density dependence of the bilayer drag is calculated to be
$\rho_D \propto T^2/n^{3}$ both in the large and small layer separation limit.
In the large layer separation limit, the bilayer drag has a strong $1/d^4$
dependence on layer separation, whereas this goes to a weak logarithmic
dependence in the strong inter-layer correlation limit of small layer
separation. In addition to obtaining the asymptotic analytical formula for
Coulomb drag in graphene, we provide numerical results for arbitrary values of
density and layer separation interpolating smoothly between our asymptotic
theoretical results. | 1111.5022v2 |
2011-12-12 | Self-similar solutions of viscous and resistive ADAFs with thermal conduction | We have studied the effects of thermal conduction on the structure of viscous
and resistive advection-dominated accretion flows (ADAFs). The importance of
thermal conduction on hot accretion flow is confirmed by observations of hot
gas that surrounds Sgr A$^*$ and a few other nearby galactic nuclei. In this
research, thermal conduction is studied by a saturated form of it, as is
appropriated for weakly-collisional systems. It is assumed the viscosity and
the magnetic diffusivity are due to turbulence and dissipation in the flow. The
viscosity also is due to angular momentum transport. Here, the magnetic
diffusivity and the kinematic viscosity are not constant and vary by position
and $\alpha$-prescription is used for them. The govern equations on system have
been solved by the steady self-similar method. The solutions show the radial
velocity is highly subsonic and the rotational velocity behaves sub-Keplerian.
The rotational velocity for a specific value of the thermal conduction
coefficient becomes zero. This amount of conductivity strongly depends on
magnetic pressure fraction, magnetic Prandtl number, and viscosity parameter.
Comparison of energy transport by thermal conduction with the other energy
mechanisms implies that thermal conduction can be a significant energy
mechanism in resistive and magnetized ADAFs. This property is confirmed by
non-ideal magnetohydrodynamics (MHD) simulations. | 1112.2678v1 |
2012-02-14 | Viscous and resistive accretion flows with radially self-similar and outflows | The existence of outflow in accretion flows is confirmed by observations and
magnetohydrodynamics (MHD) simulations. In this paper, we study outflows of
accretion flows in the presence of resistivity and toroidal magnetic field. The
mechanism of energy dissipation in the flow is assumed to be the viscosity and
the magnetic diffusivity due to turbulence in the accretion flow. It is also
assumed that the magnetic diffusivity and the kinematic viscosity are not
constant and vary by position and $\alpha$-prescription is used for them. The
influence of outflow emanating from accretion disc is considered as a sink for
mass, angular momentum and energy. The self-similar method is used to solve the
integrated equations that govern the behavior of the accretion flow in the
presence of outflow. The solutions represent the disc which rotates faster and
becomes cooler for stronger outflows. Moreover, by adding the magnetic
diffusivity, the surface density and rotational velocity decrease, while the
radial velocity and temperature increase. The study of present model with the
magnitude of magnetic field implies that the disc rotates and accretes faster
and becomes hotter, while the surface density decreases. The disc thickness
increases by adding the magnetic field or resistivity, while it becomes thinner
for more losses of mass and energy due to the outflows. | 1202.3095v1 |
2012-05-17 | Viscous-Resistive ADAF with a general Large-Scale Magnetic Field | We have studied the structure of hot accretion flow bathed in a general
large-scale magnetic field. We have considered magnetic parameters $
\beta_{r,\varphi,z}[=c^2_{r,\varphi,z}/(2c^2_{s})] $, where $ c^2_{r, \varphi,
z} $ are the Alfv\'{e}n sound speeds in three direction of cylindrical
coordinate $ (r,\varphi,z) $. The dominant mechanism of energy dissipation is
assumed to be the magnetic diffusivity due to turbulence and viscosity in the
accretion flow. Also, we adopt a more realistic model for kinematic viscosity $
(\nu=\alpha c_{s} H) $, with both $ c_{s} $ and $ H $ as a function of magnetic
field. As a result in our model, the kinematic viscosity and magnetic
diffusivity $ (\eta=\eta_{0}c_{s} H) $ are not constant. In order to solve the
integrated equations that govern the behavior of the accretion flow, a
self-similar method is used. It is found that the existence of magnetic
resistivity will increase the radial infall velocity as well as sound speed and
vertical thickness of the disk. However the rotational velocity of the disk
decreases by the increase of magnetic resistivity. Moreover, we study the
effect of three components of global magnetic field on the structure of the
disk. We found out that the radial velocity and sound speed are Sub-Keplerian
for all values of magnetic field parameters, but the rotational velocity can be
Super-Keplerian by the increase of toroidal magnetic field. Also, Our numerical
results show that all components of magnetic field can be important and have a
considerable effect on velocities and vertical thickness of the disk. | 1205.3888v1 |
2012-10-29 | Numerical Study of Charge Transport of Overdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ within Semiclassical Boltzmann Transport Theory | The in-plane resistivity of the high-temperature oxide superconductor
La$_{2-x}$Sr$_{x}$CuO$_{4}$ [LSCO] shows a strong growth of a contribution
linear in temperature as the doping is reduced in the overdoped region toward
optimal. This linear term is a signature of non-Fermi liquid behavior. We find
that the appearance of a linear term in the resistivity can arise in a
semiclassical Boltzmann transport theory which uses renormalized quasiparticle
scattering rates and an empirical band structure fitted to ARPES data on LSCO.
The linearized Boltzmann equation is solved numerically by discretizing the
Brillouin zone in a way that fits best to the Fermi surface geometry. The main
trends in the development of the anomalous temperature dependence are well
reproduced. There is a substantial underestimation of the magnitude of the
resistivity which is expected in view of the moderate to weak values we chose
for the onsite repulsion to stay within the one-loop renormalization group
approximation. The analysis was extended to the Seebeck coefficient with
similar agreement with the main trends in the data. | 1210.7668v2 |
2012-11-07 | c-axis resistivity, pseudogap, superconductivity and Widom line in doped Mott insulators | Layered doped Mott insulators, such as the cuprates, show unusual temperature
dependence of the resistivity. Intriguingly, the resistivity perpendicular to
the CuO$_2$ planes, $\rho_c(T)$, shows both metallic ($d\rho_c/dT > 0$) and
semi-conducting ($d\rho_c/dT<0$) behavior. We shed light on this puzzle by
calculating $\rho_c$ for the two-dimensional Hubbard model within plaquette
cellular dynamical mean-field theory and strong-coupling continuous-time
quantum Monte Carlo as the impurity solver. The temperature, $T$, and doping,
$\delta$, dependencies of $\rho_c$ are controlled by the first-order transition
between pseudogap and correlated metal phases from which superconductivity can
emerge. On the large doping side of the transition $\rho_c(T)$ is metallic,
while on the low-doping side $\rho_c(T)$ changes from metallic to
semi-conducting behavior with decreasing $T$. As a function of doping, the jump
in $\rho_c$ across the first-order transition evolves into a sharp crossover at
higher temperatures. This crossover coincides with the pseudogap temperature
$T^*$ in the single-particle density of states, the spin susceptibility and
other observables. Such coincidence in crossovers is expected along the
continuation of the first-order transition into the super-critical regime,
called the Widom line. This implies that not only the dynamic and the
thermodynamic properties but also the DC transport in the normal state are
governed by the hidden first-order transition. $\rho_c(T)$ has a
high-temperature quasi-linear regime where it can exceed the Mott-Ioffe-Regel
limit and when it has a minimum it is nearly parallel to the Widom line. | 1211.1702v1 |
2013-04-24 | Anisotropic transport and magnetic properties, and magnetic-field tuned states of CeZn11 single crystals | We present detailed temperature and field dependent data obtained from
magnetization, resistivity, heat capacity, Hall resistivity and thermoelectric
power measurements performed on single crystals of CeZn11. The compounds orders
antiferromagnetically at $\sim$ 2 K. The zero-field resistivity and TEP data
show features characteristic of a Ce-based intermetallic with crystal electric
field splitting and possible correlated, Kondo lattice effects. We constructed
the T-H phase diagram for the magnetic field applied along the easy, [110],
direction which shows that the magnetic field required to suppress T_N below
0.4 K is in the range of 45-47.5 kOe. A linear behavior of the rho(T) data,
H||[110], was observed only for H=45 kOe for 0.46 K<T<1.96 K followed by the
Landau-Fermi-liquid regime for a limited range of fields, 47.5 kOe< H<60 kOe.
From the analysis of our data, it appears that CeZn11 is a weakly to moderately
correlated local moment compound with rather small Kondo temperature. The
thermoelectric and transport properties of CeZn11 are mostly governed by the
CEF effects. Given the very high quality of our single crystals, quantum
oscillations are found for both CeZn11 and its non-magnetic analogue, LaZn11. | 1304.6752v1 |
2013-07-12 | Optical properties and electronic structure of the nonmetallic metal FeCrAs | The complex optical properties of a single crystal of hexagonal FeCrAs ($T_N
\simeq 125$ K) have been determined above and below $T_N$ over a wide frequency
range in the planes (along the $b$ axis), and along the perpendicular ($c$
axis) direction. At room temperature, the optical conductivity
$\sigma_1(\omega)$ has an anisotropic metallic character. The electronic band
structure reveals two bands crossing the Fermi level, allowing the optical
properties to be described by two free-carrier (Drude) contributions consisting
of a strong, broad component and a weak, narrow term that describes the
increase in $\sigma_1(\omega)$ below $\simeq 15$ meV. The dc-resistivity of
FeCrAs is ``non-metallic'', meaning that it rises in power-law fashion with
decreasing temperature, without any signature of a transport gap. In the
analysis of the optical conductivity, the scattering rates for both Drude
contributions track the dc-resistivity quite well, leading us to conclude that
the non-metallic resistivity of FeCrAs is primarily due to a scattering rate
that increases with decreasing temperature, rather than the loss of free
carriers. The power law $\sigma_1(\omega) \propto \omega^{-0.6}$ is observed in
the near-infrared region and as $T\rightarrow T_N$ spectral weight is
transferred from low to high energy ($\gtrsim 0.6$ eV); these effects may be
explained by either the two-Drude model or Hund's coupling. We also find that a
low-frequency in-plane phonon mode decreases in frequency for $T < T_N$,
suggesting the possibility of spin-phonon coupling. | 1307.3528v2 |
2013-09-26 | Anomalous Fermi liquid phase in metallic Skyrmion crystals | In non-centrosymmetric crystals such as MnSi, magnetic order can take the
form of a skyrmion crystal (SkX) . In this phase, conduction electrons coupled
to the local magnetic moments acquire a Berry's phase, leading to an emergent
electromagnetism. Motivated by experimental reports of a non-Fermi liquid phase
in MnSi, in which resistivity is observed to scale as $\Delta \rho \sim
T^{3/2}$, here we examine the effect of coupling phonons of an incommensurate
SkX to electrons. Despite the formal similarity to a system consisting of a
Fermi surface coupled to an electromagnetic field, the Berry phase fluctuations
do not lead to non-Fermi liquid behavior. Instead, we propose a different
mechanism in which electrons scatter off columnar fluctuation in a
three-dimensional SkX. When the effects of lattice induced anisotropy are
neglected, these fluctuations are ultra-soft and induce an `anomalous Fermi
liquid' in which Landau quasiparticles survive but with an anomalous $\Delta
\rho(T)\sim T^{7/4}$ resistivity perpendicular to the columns, and a Fermi
liquid resistivity along them. | 1309.7047v3 |
2014-07-11 | Self-consistent stationary MHD shear flows in the solar atmosphere as electric field generators | Magnetic fields and flows in coronal structures, for example, in gradual
phases in flares, can be described by 2D and 3D magnetohydrostatic (MHS) and
steady magnetohydrodynamic (MHD) equilibria. Within a physically simplified,
but exact mathematical model, we study the electric currents and corresponding
electric fields generated by shear flows. Starting from exact and analytically
calculated magnetic potential fields, we solveid the nonlinear MHD equations
self-consistently. By applying a magnetic shear flow and assuming a nonideal
MHD environment, we calculated an electric field via Faraday's law. The formal
solution for the electromagnetic field allowed us to compute an expression of
an effective resistivity similar to the collisionless Speiser resistivity. We
find that the electric field can be highly spatially structured, or in other
words, filamented. The electric field component parallel to the magnetic field
is the dominant component and is high where the resistivity has a maximum. The
electric field is a potential field, therefore, the highest energy gain of the
particles can be directly derived from the corresponding voltage. In our
example of a coronal post-flare scenario we obtain electron energies of tens of
keV, which are on the same order of magnitude as found observationally. This
energy serves as a source for heating and acceleration of particles. | 1407.3227v1 |
2014-07-22 | Rapid Change of Field Line Connectivity and Reconnection in Stochastic Magnetic Fields | Magnetic fields without a direction of continuous symmetry have the generic
feature that neighboring field lines exponentiate away from each other and
become stochastic, hence the ideal constraint of preserving magnetic field line
connectivity becomes exponentially sensitive to small deviations from ideal
Ohm's law. The idea of breaking field line connectivity by stochasticity as a
mechanism for fast reconnection is tested with numerical simulations based on
reduced magnetohydrodynamics equations with a strong guide field line-tied to
two perfectly conducting end plates. Starting from an ideally stable force-free
equilibrium, the system is allowed to undergo resistive relaxation. Two
distinct phases are found in the process of resistive relaxation. During the
quasi-static phase, rapid change of field line connectivity and strong induced
flow are found in regions of high field line exponentiation. However, although
the field line connectivity of individual field lines can change rapidly, the
overall pattern of field line mapping appears to deform gradually. From this
perspective, field line exponentiation appears to cause enhanced diffusion
rather than reconnection. In some cases, resistive quasi-static evolution can
cause the ideally stable initial equilibrium to cross a stability threshold,
leading to formation of intense current filaments and rapid change of field
line mapping into a qualitatively different pattern. It is in this onset phase
that the change of field line connectivity is more appropriately designated as
magnetic reconnection. Our results show that rapid change of field line
connectivity appears to be a necessary, but not a sufficient condition for fast
reconnection. | 1407.6069v1 |
2014-12-17 | Contacts for organic switches with carbon-nanotube leads | Molecular devices, as future electronics, seek low-resistivity contacts for
the energy saving. At the same time, the contacts should intensify desired
properties of tailored electronic elements. In this work, we focus our
attention on two classes of organic switches connected to carbon-nanotube leads
and operating due to photo- or field-induced proton transfer (PT) process. By
means of the first-principles atomistic simulations of the ballistic
conductance, we search for atomic contacts which strengthen diversity of the
two swapped I-V characteristics between two tautomers of a given molecular
system. We emphasize, that the low-resistive character of the contacts is not
necessarily in accordance with the switching properties. Very often, the
higher-current flow makes it more difficult to distinguish between the logic
states of the molecular device. Instead, the resistive contacts multiply a
current gear at the tautomeric transition to a larger extent. The low- and
high-bias work regimes set additional conditions, which are fulfilled by
different contacts. In some cases, the peroxide contacts or the direct
connection to the tube perform better than the popular sulfur contact.
Additionally, we find that the switching-bias value is not an inherent property
of the conducting molecule, but it strongly depends on the chosen contacts. | 1412.5667v3 |
2015-09-07 | Finite Size Effect in Amorphous Indium oxide | We study the low temperature magneto-transport properties of several highly
disordered amorphous Indium oxide(a:InO) samples. Simultaneously fabricated
devices comprising a 2-dimensional (2D) film and 10 $\mu$m long wires of
different widths were measured to investigate the effect of size as we approach
the 1D limit, which is around 4 times the correlation length, and happens to be
around 100 nm for a:InO. The film and the wires showed magnetic field ({\it B})
induced superconductor to insulator transition (SIT). In the superconducting
side, the resistance increased with decrease in wire width, whereas, an
opposite trend is observed in the insulating side. We find that this effect can
be explained in light of charge-vortex duality picture of the SIT. Resistance
of the 2D film follows an activated behavior over the temperature ($T$),
whereas, the wires show a crossover from the high-$T$ activated to a
$T$-independent behavior. At high temperature regime the wires' resistance
follow the film's until they deviate and became independent of $T$. We find
that temperature at which this deviation occurs evolve with magnetic field and
the width of the wire, which show the effect of finite size on the transport. | 1509.02033v2 |
2015-09-18 | Effect of Strain on Ferroelectric Field Effect in Strongly Correlated Oxide Sm$_{0.5}$Nd$_{0.5}$NiO$_3$ | We report the effect of epitaxial strain on the magnitude and retention of
the ferroelectric field effect in high quality PbZr$_{0.3}$Ti$_{0.7}$O$_3$
(PZT)/3.8-4.3 nm Sm$_{0.5}$Nd$_{0.5}$NiO$_3$ (SNNO) heterostructures grown on
(001) LaAlO$_3$ (LAO) and SrTiO$_3$ (STO) substrates. For SNNO on LAO, which
exhibits a first-order metal-insulator transition (MIT), switching the
polarization of PZT induces a 10 K shift in the transition temperature
$T_{MI}$, with a maximum resistance change between the on and off states of
$\Delta$$R$/$R_{on}$ ~75%. In sharp contrast, only up to 5% resistance change
has been induced in SNNO on STO, where the MIT is second-order, with the
modulation of $T_{MI}$ negligibly small. We also observe thermally activated
retention of the off state resistance $R_{off}$ in both systems, with the
activation energy of 22 meV (28 meV) for devices on LAO (STO). The time
dynamics and thermal response of the field effect instability points to
phonon-assisted interfacical trapping of charged mobile defects, which are
attributed to strain induced oxygen vacancies. At room temperature, $R_{off}$
stabilizes at ~55% and ~19% of the initial switching levels for SNNO on LAO and
STO, respectively, reflecting the significantly different oxygen vacancy
densities in these two systems. Our results reveal the critical role of strain
in engineering and modeling the complex oxide composite structures for
nanoelectronic and spintronic applications. | 1509.05493v2 |
2015-09-19 | Collapse of the low temperature insulating state in Cr-doped V$_2$O$_3$ thin films | We have grown epitaxial Cr-doped V$_2$O$_3$ thin films with Cr concentrations
between $0$ and $20\%$ on $(0001)$-Al$_2$O$_3$ by oxygen-assisted molecular
beam epitaxy. For the highly doped samples (> $3\%$), a regular and monotonous
increase of the resistance with decreasing temperature is measured. Strikingly,
in the low doping samples (between $1\%$ and $3\%$), a collapse of the
insulating state is observed with a reduction of the low temperature
resistivity by up to 5 orders of magnitude. A vacuum annealing at high
temperature of the films recovers the low temperature insulating state for
doping levels below $3\%$ and increases the room temperature resistivity
towards the values of Cr-doped V$_2$O$_3$ single crystals. It is well-know that
oxygen excess stabilizes a metallic state in V$_2$O$_3$ single crystals. Hence,
we propose that Cr doping promotes oxygen excess in our films during deposition
leading to the collapse of the low temperature insulating state at low Cr
concentrations. These results suggest that slightly Cr-doped V$_2$O$_3$ films
can be interesting candidates for field effect devices. | 1509.05866v1 |
2015-10-23 | Identifying lineage effects when controlling for population structure improves power in bacterial association studies | Bacteria pose unique challenges for genome-wide association studies (GWAS)
because of strong structuring into distinct strains and substantial linkage
disequilibrium across the genome. While methods developed for human studies can
correct for strain structure, this risks considerable loss- of-power because
genetic differences between strains often contribute substantial phenotypic
variability. Here we propose a new method that captures lineage-level
associations even when locus-specific associations cannot be fine-mapped. We
demonstrate its ability to detect genes and genetic variants underlying
resistance to 17 antimicrobials in 3144 isolates from four taxonomically
diverse clonal and recombining bacteria: Mycobacterium tuberculosis,
Staphylococcus aureus, Escherichia coli and Klebsiella pneumoniae. Strong
selection, recombination and penetrance confer high power to recover known
antimicrobial resistance mechanisms, and reveal a candidate association between
the outer membrane porin nmpC and cefazolin resistance in E. coli. Hence our
method pinpoints locus-specific effects where possible, and boosts power by
detecting lineage-level differences when fine-mapping is intractable. | 1510.06863v3 |
2016-07-26 | Resistive Tearing Instability in Electron-MHD: Application to Neutron Star Crusts | We study a resistive tearing instability developing in a system evolving
through the combined effect of Hall drift in the Electron-MHD limit and Ohmic
dissipation. We explore first the exponential growth of the instability in the
linear case and we find the fastest growing mode, the corresponding eigenvalues
and dispersion relation. The instability growth rate scales as $\gamma \propto
B^{2/3} \sigma^{-1/3}$ where $B$ is the magnetic field and $\sigma$ the
electrical conductivity. We confirm the development of the tearing resistive
instability in the fully non-linear case, in a plane parallel configuration
where the magnetic field polarity reverses, through simulations of systems
initiating in Hall equilibrium with some superimposed perturbation. Following a
transient phase, during which there is some minor rearrangement of the magnetic
field, the perturbation grows exponentially. Once the instability is fully
developed the magnetic field forms the characteristic islands and X-type
reconnection points, where Ohmic decay is enhanced. We discuss the implications
of this instability for the local magnetic field evolution in neutron stars'
crusts, proposing that it can contribute to heating near the surface of the
star, as suggested by models of magnetar post-burst cooling. In particular, we
find that a current sheet a few meters thick, covering as little as $1\%$ of
the total surface can provide $10^{42}~$erg in thermal energy within a few
days. We briefly discuss applications of this instability in other systems
where the Hall effect operates such as protoplanetary discs and space plasmas. | 1607.07874v2 |
2016-07-29 | Using a terrestrial laser scanner to characterize vegetation-induced flow resistance in a controlled channel | Vegetation characteristics providing spatial heterogeneity at the channel
reach scale can produce complex flow patterns and the relationship between
plant patterns morphology and flow resistance is still an open question (Nepf
2012). Unlike experiments in laboratory, measuring the vegetation
characteristics related to flow resistance on open channel in situ is
difficult. Thanks to its high resolution and light weight, scanner lasers allow
now to collect in situ 3D vegetation characteristics. In this study we used a
1064 nm usual Terrestrial Laser Scanner (TLS) located 5 meters at nadir above a
8 meters long equipped channel in order to both i) characterize the vegetation
structure heterogeneity within the channel form a single scan (blockage factor,
canopy height) and ii) to measure the 2D water level all over the channel
during steady flow within a few seconds scan. This latter measuring system was
possible thanks to an additive dispersive product sprinkled at the water
surface. Vegetation characteristics and water surfaces during steady flows from
6 different plant spatial design on channel bottom for 4 plant species were
thus measured. Vegetation blockage factors at channel scale were estimated from
TLS points clouds and analyzed. | 1608.02483v1 |
2016-10-09 | Chaos-based Wireless Communication Resisting Multipath Effects | In additive white gaussian noise (AWGN) channel, chaos has been proved to be
the optimal coherent communication waveform in the sense of using very simple
matched filter to maximize the signal-to-noise ratio (SNR). Recently, Lyapunov
exponent spectrum of the chaotic signals after being transmitted through a
wireless channel has been shown to be unaltered, paving the way for wireless
communication using chaos. In wireless communication systems, inter-symbol
interference (ISI) caused by multipath propagation is one of the main obstacles
to achieve high bit transmission rate and low bit error rate (BER). How to
resist multipath effect is a fundamental problem in a chaos-based wireless
communication system (CWCS). In this paper, implementation of a CWCS is
presented. It is built to transmit chaotic signals generated by a hybrid
dynamical system and then to filter the received signals by using the
corresponding matched filter to decrease the noise effect and to detect the
binary information. We find that the multipath effect can be effectively
resisted by regrouping the return map of the received signal and by setting the
corresponding threshold based on the available information. We show that the
optimal threshold is a function of the channel parameters and of the
transmitted information symbols. Practically, the channel parameters are
time-variant, and the future information symbols are unavailable. In this case,
a suboptimal threshold (SOT) is proposed, and the BER using the SOT is derived
analytically. Simulation results show that the CWCS achieves a remarkable
competitive performance even under inaccurate channel parameters. | 1611.02325v3 |
2016-11-22 | Sampling Random Spanning Trees Faster than Matrix Multiplication | We present an algorithm that, with high probability, generates a random
spanning tree from an edge-weighted undirected graph in
$\tilde{O}(n^{4/3}m^{1/2}+n^{2})$ time (The $\tilde{O}(\cdot)$ notation hides
$\operatorname{polylog}(n)$ factors). The tree is sampled from a distribution
where the probability of each tree is proportional to the product of its edge
weights. This improves upon the previous best algorithm due to Colbourn et al.
that runs in matrix multiplication time, $O(n^\omega)$. For the special case of
unweighted graphs, this improves upon the best previously known running time of
$\tilde{O}(\min\{n^{\omega},m\sqrt{n},m^{4/3}\})$ for $m \gg n^{5/3}$ (Colbourn
et al. '96, Kelner-Madry '09, Madry et al. '15).
The effective resistance metric is essential to our algorithm, as in the work
of Madry et al., but we eschew determinant-based and random walk-based
techniques used by previous algorithms. Instead, our algorithm is based on
Gaussian elimination, and the fact that effective resistance is preserved in
the graph resulting from eliminating a subset of vertices (called a Schur
complement). As part of our algorithm, we show how to compute
$\epsilon$-approximate effective resistances for a set $S$ of vertex pairs via
approximate Schur complements in $\tilde{O}(m+(n + |S|)\epsilon^{-2})$ time,
without using the Johnson-Lindenstrauss lemma which requires $\tilde{O}(
\min\{(m + |S|)\epsilon^{-2}, m+n\epsilon^{-4} +|S|\epsilon^{-2}\})$ time. We
combine this approximation procedure with an error correction procedure for
handing edges where our estimate isn't sufficiently accurate. | 1611.07451v2 |
2016-11-30 | Reconnection and particle acceleration in interacting flux ropes I. Magnetohydrodynamics and test particles in 2.5D | Magnetic reconnection and non-thermal particle distributions associated with
current-driven instabilities are investigated by means of resistive
magnetohydrodynamics (MHD) simulations combined with relativistic test particle
methods. We propose a system with two parallel, repelling current channels in
an initially force-free equilibrium, as a simplified representation of flux
ropes in a stellar magnetosphere. The current channels undergo a rotation and
separation on Alfv\'enic timescales, forming secondary islands and (up to
tearing unstable) current sheets in which non-thermal energy distributions are
expected to develop. Using the recently developed particle module of our
open-source grid-adaptive MPI-AMRVAC software, we simulate MHD evolution
combined with test particle treatments in MHD snapshots. We explore under which
plasma-$\beta$ conditions the fastest reconnection occurs in two-and-a-half
dimensional (2.5D) scenarios and in these settings test particles are evolved.
We quantify energy distributions, acceleration mechanisms, relativistic
corrections to the particle equations of motion and effects of resistivity in
magnetically dominated proton-electron plasmas. Due to large resistive electric
fields and indefinite acceleration of particles in the infinitely long current
channels, hard energy spectra are found in 2.5D configurations. Solutions to
these numerical artifacts are proposed for both 2.5D setups and future 3D work.
We discuss the magnetohydrodynamics of an additional kink instability in 3D
setups and the expected effects on energy distributions. The obtained results
hold as a proof-of-principle for test particle approaches in MHD simulations,
relevant to explore less idealised scenarios like solar flares and more exotic
astrophysical phenomena, like black hole flares, magnetar magnetospheres and
pulsar wind nebulae. | 1611.09966v2 |
2016-12-06 | Pressure-induced magnetic transition exceeding 30 K in the Yb-based heavy fermion superconductor $β$-YbAlB$_4$ | Measurements of the electric resistivity $\rho(T)$ under pressure up to 8 GPa
were performed on high-quality single-crystals of the Yb-based heavy fermion
system $\beta$-YbAlB$_4$ in the temperature range $2<T<$ 300 K. In the
resistivity data, we observed pressure-induced magnetic ordering above the
critical pressure $P_{\rm c} \sim$ 2 GPa. Clear difference in the phase diagram
under pressure using two types of pressure mediums indicates that the
transition temperature may be further enhanced under application of uniaxial
pressure. With pressure, this phase transition temperature $T_{\rm M}$ is
enhanced reaching 32 K at 8 GPa, which is the highest transition temperature so
far recorded for the Yb-based heavy fermion compounds. The power-law exponent
$\alpha$ in $\rho=\rho_0+ AT^{\alpha}$ below $T_{\rm M}$ gradually changes from
3/2 to 5/2 with increasing pressure from 2 to 8 GPa. In contrast, the
resistivity exhibits a $T$-linear behavior in the temperature range 2 $\le T
\le$ 20 K and is insensitive to pressure below $P_{\rm c}$. In this pressure
regime, the magnetization is also nearly independent of pressure and shows no
anomaly above 2 K. Our results indicate that a quantum critical point for
$\beta$-YbAlB$_4$ is also located near $P_{\rm c}$ in addition to the strange
metal region near the ambient pressure. | 1612.01800v2 |
2017-10-07 | Intrinsically shunted Josephson junctions for electronics applications | Conventional Josephson metal-insulator-metal devices are inherently
underdamped and exhibit hysteretic current-voltage response due to a very high
subgap resistance compared to that in the normal state. At the same time,
overdamped junctions with single-valued characteristics are needed for most
superconducting digital applications. The usual way to overcome the hysteretic
behavior is to place an external low-resistance normal-metal shunt in parallel
with each junction. Unfortunately, such solution results in a considerable
complication of the circuitry design and introduces parasitic inductance
through the junction. This paper provides a concise overview of some generic
approaches that have been proposed in order to realize internal shunting in
Josephson heterostructures with a barrier that itself contains the desired
resistive component. The main attention is paid to self-shunted devices with
local weak-link transmission probabilities so strongly disordered in the
interface plane that transmission probabilities are tiny for the main part of
the transition region between two superconducting electrodes, while a small
part of the interface is well transparent. We consider the possibility of
realizing a universal bimodal distribution function and emphasize advantages of
such junctions that can be considered as a new class of self-shunted Josephson
devices promising for practical applications in superconducting electronics
operating at 4.2 K. | 1710.02707v1 |
2018-03-13 | Graph Ranking and the Cost of Sybil Defense | Ranking functions such as PageRank assign numeric values (ranks) to nodes of
graphs, most notably the web graph. Node rankings are an integral part of
Internet search algorithms, since they can be used to order the results of
queries. However, these ranking functions are famously subject to attacks by
spammers, who modify the web graph in order to give their own pages more rank.
We characterize the interplay between rankers and spammers as a game. We define
the two critical features of this game, spam resistance and distortion, based
on how spammers spam and how rankers protect against spam. We observe that all
the ranking functions that are well-studied in the literature, including the
original formulation of PageRank, have poor spam resistance, poor distortion,
or both. Finally, we study Min-PPR, the form of PageRank used at Google itself,
but which has received no (theoretical or empirical) treatment in the
literature. We prove that Min-PPR has low distortion and high spam resistance.
A secondary benefit is that Min-PPR comes with an explicit cost function on
nodes that shows how important they are to the spammer; thus a ranker can focus
their spam-detection capacity on these vulnerable nodes. Both Min-PPR and its
associated cost function are straightforward to compute. | 1803.05001v5 |
2018-04-25 | Jet launching in resistive GR-MHD black hole - accretion disk systems | We investigate the launching mechanism of relativistic jets from black hole
sources, in particular the strong winds from the surrounding accretion disk.
Numerical investigations of the disk wind launching - the simulation of the
accretion-ejection transition - have so far almost only been done for
non-relativistic systems. From these simulations we know that resistivity, or
magnetic diffusivity, plays an important role for the launching process. Here,
we extend this treatment to general relativistic magnetohydrodynamics (GR-MHD)
applying the resistive GR-MHD code rHARM. Our model setup considers a thin
accretion disk threaded by a large-scale open magnetic field. We run a series
of simulations with different Kerr parameter, field strength and diffusivity
level. Indeed we find strong disk winds with, however, mildly relativistic
speed, the latter most probably due to our limited computational domain.
Further, we find that magnetic diffusivity lowers the efficiency of accretion
and ejection, as it weakens the efficiency of the magnetic lever arm of the
disk wind. As major driving force of the disk wind we disentangle the toroidal
magnetic field pressure gradient, however,magneto-centrifugal driving may also
contribute. Black hole rotation in our simulations suppresses the accretion
rate due to an enhanced toroidal magnetic field pressure that seems to be
induced by frame-dragging. Comparing the energy fluxes from the
Blandford-Znajek-driven central spine and the surrounding disk wind, we find
that the total electromagnetic energy flux is dominated by the total matter
energy flux of the disk wind (by a factor 20). The kinetic energy flux of the
matter outflow is comparatively small and comparable to the Blandford-Znajek
electromagnetic energy flux. | 1804.09652v1 |
2018-05-18 | Radio frequency performance projection and stability trade-off of h-BN encapsulated graphene field-effect transistors | Hexagonal boron nitride (h-BN) encapsulation significantly improves carrier
transport in graphene. This work investigates the benefit of implementing the
encapsulation technique in graphene field-effect transistors (GFET) in terms of
their radio frequency (RF) performance. For such a purpose, a drift-diffusion
self-consistent simulator is prepared to get the GFET electrical
characteristics. Both the mobility and saturation velocity information are
obtained by means of an ensemble Monte Carlo simulator upon considering the
relevant scattering mechanisms that affect carrier transport. RF figures of
merit are simulated using an accurate small-signal model that includes
non-reciprocal capacitances. Results reveal that the cutoff frequency could
scale up to the physical limit given by the inverse of the transit time.
Projected maximum oscillation frequencies, in the order of few THz, are
expected to exceed the values demonstrated by InP and Si based RF transistors.
The existing trade-off between power gain and stability and the role played by
the gate resistance are also studied. High power gain and stability are
feasible even if the device is operated far away from current saturation.
Finally, the benefits of device unilateralization and the exploitation of the
negative differential resistance region to get negative-resistance gain are
discussed. | 1805.07138v4 |
2018-06-21 | Development and Characterization of 6-gap Bakelite Multi-gap Resistive Plate Chamber | The Multi-gap Resistive Plate Chamber (MRPC) is an advanced form of Resistive
Plate Chamber (RPC) detector where the gas gap is divided into sub-gaps. MRPCs
are known for their good time resolution and detection efficiency for charged
particles. They have found suitable applications in several high energy physics
experiments like ALICE in LHC, CERN, Geneva, Switzerland, and STAR in RHIC,
BNL, USA. As they have very good time resolution and are of low cost, they can
be a suitable replacement for very expensive scintillators used in Positron
Emission Tomography Imaging. The MRPCs that are being used nowadays are
developed with glass electrodes. We have made an attempt to develop a 6-gap
MRPC using bakelite electrodes. The outer electrodes are of dimensions 15 cm
$\times$ 15 cm $\times$ 0.3 cm and the inner electrodes are of dimension 14 cm
$\times$ 14 cm $\times$ 0.05 cm. The glossy finished electrode surfaces have
not been treated with any oil like linseed, silicone for smoothness. The
performance of the detector has been studied measuring the efficiency, noise
rate and time resolution with cosmic rays. This effort is towards the
development of a prototype for Positron Emission Tomography with the
Time-Of-Flight technique using MRPCs. Details of the development procedure and
performance studies have been presented here. | 1806.08265v1 |
2018-06-23 | Bilayer h-BN Barriers for Tunneling Contacts in Fully-Encapsulated Monolayer $\mathbf{MoSe_2}$ Field-Effect Transistors | The performance of electronic and spintronic devices based on two-dimensional
semiconductors (2D SC) is largely dependent on the quality and resistance of
the metal/SC electrical contacts, as well as preservation of the intrinsic
properties of the SC channel. Direct Metal/SC interaction results in highly
resistive contacts due to formation of large Schottky barriers and considerably
affects the properties of the 2D SC. In this work, we address these two
important issues in monolayer $\mathrm{MoSe_2}$ Field-Effect transistors
(FETs). We encapsulate the $\mathrm{MoSe_2}$ channel with hexagonal Boron
Nitride (h-BN), using bilayer h-BN at the metal/SC interface. The bilayer h-BN
eliminates the metal/$\mathrm{MoSe_2}$ chemical interactions, preserves the
electrical properties of $\mathrm{MoSe_2}$ and reduces the contact resistances
by prevention of Fermi-level pinning. We investigate electrical transport in
the monolayer $\mathrm{MoSe_2}$ FETs that yields close to intrinsic electron
mobilities ($\approx 26\ \mathrm{cm^2 V^{-1} s^{-1}}$) even at room
temperature. Moreover, we experimentally study the charge transport through
Metal/h-BN/$\mathrm{MoSe_2}$ tunnel contacts and we explicitly show that the
dielectric bilayer of h-BN provides highly efficient gating (tuning the Fermi
energy) of the $\mathrm{MoSe_2}$ channel at the contact regions even with small
biases. Also we provide a theoretical model that allows to understand and
reproduce the experimental $I-V$ characteristics of the contacts. These
observations give an insight into the electrical behavior of the metal/h-BN/2D
SC heterostructure and introduce bilayer h-BN as a suitable choice for high
quality tunneling contacts that allows for low energy charge and spin
transport. | 1806.08954v1 |
2018-09-25 | Neural Networks with Structural Resistance to Adversarial Attacks | In adversarial attacks to machine-learning classifiers, small perturbations
are added to input that is correctly classified. The perturbations yield
adversarial examples, which are virtually indistinguishable from the
unperturbed input, and yet are misclassified. In standard neural networks used
for deep learning, attackers can craft adversarial examples from most input to
cause a misclassification of their choice.
We introduce a new type of network units, called RBFI units, whose non-linear
structure makes them inherently resistant to adversarial attacks. On
permutation-invariant MNIST, in absence of adversarial attacks, networks using
RBFI units match the performance of networks using sigmoid units, and are
slightly below the accuracy of networks with ReLU units. When subjected to
adversarial attacks, networks with RBFI units retain accuracies above 90% for
attacks that degrade the accuracy of networks with ReLU or sigmoid units to
below 2%. RBFI networks trained with regular input are superior in their
resistance to adversarial attacks even to ReLU and sigmoid networks trained
with the help of adversarial examples.
The non-linear structure of RBFI units makes them difficult to train using
standard gradient descent. We show that networks of RBFI units can be
efficiently trained to high accuracies using pseudogradients, computed using
functions especially crafted to facilitate learning instead of their true
derivatives. We show that the use of pseudogradients makes training deep RBFI
networks practical, and we compare several structural alternatives of RBFI
networks for their accuracy. | 1809.09262v1 |
2019-09-22 | Ship resistance when operating in floating ice floes: a combined CFD&DEM approach | Whilst climate change is transforming the Arctic into a navigable ocean where
small ice floes are floating on the sea surface, the effect of such ice
conditions on ship performance has yet to be understood. The present work
combines a set of numerical methods to simulate the ship-wave-ice interaction
in such ice conditions. Particularly, Computational Fluid Dynamics is applied
to provide fluid solutions for the floes and it is incorporated with the
Discrete Element Method to govern ice motions and account for ship-ice/ice-ice
collisions, by which, the proposed approach innovatively includes wave effects
in the interaction. In addition, this work introduces two algorithms that can
implement computational models with natural ice-floe fields, which takes
randomness into consideration thus achieving high-fidelity modelling of the
problem. Following validation against experiments, the model is shown accurate
in predicting the ice-floe resistance of a ship, and then a series of
simulations are performed to investigate how the resistance is influenced by
ship speed, ice concentration, ice thickness and floe diameter. This paper
presents a useful approach that can provide power estimates for Arctic shipping
and has the potential to facilitate other polar engineering purposes. | 1909.10018v1 |
2014-08-26 | Mean-field dynamics of tumor growth and control using low-impact chemoprevention | Cancer poses danger because of its unregulated growth, development of
resistant subclones, and metastatic spread to vital organs. Although the major
transitions in cancer development are increasingly well understood, we lack
quantitative theory for how chemoprevention is predicted to affect survival. We
employ master equations and probability generating functions, the latter well
known in statistical physics, to derive the dynamics of tumor growth as a
mean-field approximation. We also study numerically the associated stochastic
birth-death process. Our findings predict exponential tumor growth when a
cancer is in its early stages of development and hyper-exponential growth
thereafter. Numerical simulations are in general agreement with our analytical
approach. We evaluate how constant, low impact treatments affect both
neoplastic growth and the frequency of chemoresistant clones. We show that
therapeutic outcomes are highly predictable for treatments starting either
sufficiently early or late in terms of initial tumor size and the initial
number of chemoresistant cells, whereas stochastic dynamics dominate therapies
starting at intermediate neoplasm sizes, with high outcome sensitivity both in
terms of tumor control and the emergence of resistant subclones. The outcome of
chemoprevention can be understood in terms of both minimal physiological
impacts resulting in long-term control and either preventing or slowing the
emergence of resistant subclones. We argue that our model and results can also
be applied to the management of early, clinically detected cancers after tumor
excision. | 1408.6052v1 |
2011-07-22 | Ferromagnetic Quantum Critical Endpoint in UCoAl | Resistivity and magnetostriction measurements were performed at high magnetic
fields and under pressure on UCoAl. At ambient pressure, the 1st order
metamagnetic transition at H_m ~ 0.7 T from the paramagnetic ground state to
the field-induced ferromagnetic state changes to a crossover at finite
temperature T_0 ~11 K. With increasing pressure, H_m linearly increases, while
T_0 decreases and is suppressed at the quantum critical endpoint (QCEP, P_QCEP
~ 1.5 GPa, H_m ~ 7 T). At higher pressure, the value of H_m identified as a
crossover continuously increases, while a new anomaly appears above P_QCEP at
higher field H* in resistivity measurements. The field dependence of the
effective mass (m*) obtained by resistivity and specific heat measurements
exhibits a step-like drop at H_m at ambient pressure. With increasing pressure,
it gradually changes into a peak structure and a sharp enhancement of m* is
observed near the QCEP. Above P_QCEP, the enhancement of m* is reduced, and a
broad plateau is found between H_m and H*. We compare our results on UCoAl with
those of the ferromagnetic superconductor UGe2 and the itinerant metamagnetic
ruthenate Sr3Ru2O7. | 1107.4590v1 |
2013-12-02 | Revisiting heat capacity of bulk polycrystalline YBa2Cu3O7-δ | In this letter, we present the superconducting property characterization of a
phase pure reasonably good quality YBa2Cu3O7-{\delta} sample. Studied compound
is crystallized in orthorhombic Pmmm space group with lattice parameters a, b,
and c are 3.829(2) {\AA}, 3.887(1) {\AA} and 11.666(3) {\AA} respectively. Bulk
superconductivity is observed below 90K as evidenced by resistivity and dc/ac
magnetization measurements. The resistivity under magnetic field ({\rho}TH)
measurements showed clearly both the intra-grain and inter-grain transitions,
which are supplemented by detailed (varying frequency and amplitude) ac
susceptibility studies as well. The upper critical field at 0K i.e., Hc2(0)
being determined from {\rho}TH measurements with 50% criteria of resistivity
drope is ~ 70 Tesla. Studied polycrystalline YBa2Cu3O7-{\delta} is subjected to
detailed heat capacity (CP) studies. Cp exhibited well defined anomaly at below
90 K, which decreases with applied field. Though the Cp anomaly/peak at Tc
reduces with applied field, the same is not completely suppressed in high
applied fields of up to 12 Tesla. The Sommerfeld constant ({\gamma}) and Debye
temperature ({\Theta}D) as determined from low temperature fitting of CP(T)
data to Sommerfeld-Debye model, are 10.65 mJ/mole-K2 and 312.3 K respectively.
The results are compared with existing literature on bulk polycrystalline
superconducting YBa2Cu3O7-{\delta} sample | 1312.0417v2 |
2013-12-22 | Crystal growth, transport phenomena and two gap superconductivity in the mixed alkali metal $(K_{1-z}Na_z)_{x}Fe_{2-y}Se_2$ iron selenide | Using the self-flux technique we grew superconducting
$(K_{1-z}Na_z)_{x}Fe_{2-y}Se_2$ (z = 0.3) single crystals. The EDX mapping
revealed the uniform elements distribution on the crystal surface while the XRD
measurements indicate that the crystals are compositionally inhomogenous on
nanoscale. The physical properties of the as-prepared sample are characterized
by electrical resistivity, magnetization and specific heat measurements.
Resistivity measurements show the onset of the superconducting transition at 33
K and zero resistivity at 31.7 K. The large upper critical field $H_{c2}$(0)
was estimated as high as about of 140 T for the in-plane field and 38 T for the
out-of-plane field. The anisotropy of $H_{c2}^{ab}(0)/H_{c2}^{c}(0)$ and
coherence lengths $\xi^{ab}(0)/\xi^{c}$(0) was found to be around 3.7. The
pioneer studies by multiple Andreev reflections effect spectroscopy
("break-junction" technique) revealed the presence of two anisotropic
superconducting gaps $\Delta_L\,=\,(9.3\pm1.5)\,meV$,
$\Delta_S\,=\,(1.9\pm0.4)\,meV$, and provided measuring of the $\Delta_L$(T)
temperature dependence. The BCS-ratio for the large gap
$2\Delta_L/k_BT_c^{bulk}\,\approx\,6.3$ points to a strong electron-boson
coupling in the "driving" condensate characterized by $\Delta_L$ order
parameter. | 1312.6441v2 |
2014-11-27 | Electron-phonon interaction in a spin-orbit coupled quantum wire with a gap | Interaction between electron and acoustic phonon in an in-plane magnetic
field induced gapped quantum wire with Rashba spin-orbit interaction is
studied. We calculate acoustic phonon limited resistivity ($\rho$) and
phonon-drag thermopower ($S_g$) due to two well known mechanisms of
electron-phonon interaction namely, deformation potential (DP) and
piezoelectric (PE) scattering. In the so called Bloch-Gruneisen temperature
limit both $\rho$ and $S_g$ depend on temperature ($T$) in a power law fashion
i.e. $\rho$ or $S_g\sim T^{\nu_T}$. For resistivity, $\nu_T$ takes the value
$5$ and $3$ due to DP and PE scattering respectively. On the other hand,
$\nu_T$ is $4$ and $2$ due to DP and PE scattering, respectively for
phonon-drag thermopower. Additionally, we find numerically that $\nu_T$ depends
on Rashba parameter ($\alpha$) and electron density ($n$). The dependence of
$\nu_T$ on $\alpha$ becomes more prominent at lower density. We also study the
variations of $\rho$ and $S_g$ with carrier density in the Bloch-Gruneisen
regime. Through a numerical analysis a similar power law dependence $\rho$ or
$S_g\sim n^{-\nu_n}$ is established in which the effective exponent $\nu_n$
undergoes a smooth transition from a low density behavior to a high density
behavior. At a higher density regime, $\nu_n$ matches excellently with the
value obtained from theoretical arguments. Approximate analytical expressions
for both resistivity and phonon-drag thermopower in the Bloch-Gruneisen regime
are given. | 1411.7620v1 |
2019-02-01 | Observation of Two Collapsed Phases in CaRbFe4As4 | We report the observation of the pressure-induced, fully-collapsed tetragonal
phase of CaRbFe4As4 for P~ 22 GPa via high-pressure x-ray diffraction and
magnetotransport measurements. The x-ray measurements, along with resistivity
measurements, show that there is an initial half-collapsed tetragonal phase for
6 < P < 22 GPa, in which superconductivity is continuously suppressed from Tc=
35K at P= 3.1 GPa to Tc <2K for P >17.2 GPa, as well as signs of the
fully-collapsed tetragonal phase near P=22 GPa. Density functional calculations
suggest that both of these transitions are driven by increased As-As bonding,
first across the Ca layer, and then at the second transition, across the Rb
layer. Although electrical resistivity measurements in the fully-collapsed
tetragonal phase do not show superconductivity, there is a change in the slope
of both the Hall coefficient and the longitudinal resistance near 22 GPa,
suggesting a strong correlation between the electronic and lattice degrees of
freedom in this new iron-based superconductor. | 1902.00472v3 |
2019-02-04 | Gauge phonon dominated resistivity in twisted bilayer graphene near magic angle | Recent experiments on twisted bilayer graphene (tBG) close to magic angle
show that a small relative rotation in a van der Waals heterostructure greatly
alters its electronic properties. We consider various scattering mechanisms and
show that the carrier transport in tBG is dominated by a combination of charged
impurities and acoustic gauge phonons. Charged impurities still dominate at low
temperature and densities because of the inability of Dirac fermions to screen
long-range Coulomb potentials at charge neutrality; however, the gauge phonons
dominate for most of the experimental regime because although they couple to
current, they do not induce charge and are therefore unscreened by the large
density of states close to magic angle. We show that the resistivity has a
strong monotonically decreasing carrier density dependence at low temperature
due to charged impurity scattering, and weak density dependence at high
temperature due to gauge phonons. Away from charge neutrality, the resistivity
increases with temperature, while it does the opposite close to the Dirac
point. A non-monotonic temperature dependence observed only at low temperature
and carrier density is a signature of our theory that can be tested in
experimentally available samples. | 1902.01405v1 |
2019-05-06 | A Differential Game Approach to Decentralized Virus-Resistant Weight Adaptation Policy over Complex Networks | Increasing connectivity of communication networks enables large-scale
distributed processing over networks and improves the efficiency for
information exchange. However, malware and virus can take advantage of the high
connectivity to spread over the network and take control of devices and servers
for illicit purposes. In this paper, we use an SIS epidemic model to capture
the virus spreading process and develop a virus-resistant weight adaptation
scheme to mitigate the spreading over the network. We propose a differential
game framework to provide a theoretic underpinning for decentralized mitigation
in which nodes of the network cannot fully coordinate, and each node determines
its own control policy based on local interactions with neighboring nodes. We
characterize and examine the structure of the Nash equilibrium, and discuss the
inefficiency of the Nash equilibrium in terms of minimizing the total cost of
the whole network. A mechanism design through a penalty scheme is proposed to
reduce the inefficiency of the Nash equilibrium and allow the decentralized
policy to achieve social welfare for the whole network. We corroborate our
results using numerical experiments and show that virus-resistance can be
achieved by a distributed weight adaptation scheme. | 1905.02237v2 |
2020-03-29 | Deficiency of the scaling collapse as an indicator of a superconductor-insulator quantum phase transition | Finite-size scaling analysis is a well-accepted method for identification and
characterization of quantum phase transitions (QPTs) in superconducting,
magnetic and insulating systems. We formally apply this analysis in the form
suitable for QPTs in 2-dimensional superconducting films to magnetic-field
driven superconductor-metal transition in 1-dimensional MoGe nanowires. Despite
being obviously inapplicable to nanowires, the 2d scaling equation leads to a
high-quality scaling collapse of the nanowire resistance in the temperature and
resistance ranges comparable or better to what is accepted in the analysis of
the films. Our results suggest that the appearance and the quality of the
scaling collapse by itself is not a reliable indicator of a QPT. We have also
observed a sign-change of the zero-bias anomaly (ZBA) in the non-linear
resistance, occurring exactly at the critical field of the accidental QPT. This
behavior is often taken as an additional confirmation of the transition. We
argue that in nanowires, the non-linearity is caused by electron heating and
has no relation to the critical fluctuations. Our observation suggests that
similar to the scaling collapse, the sign-change of ZBA can be a misleading
indicator of QPT. | 2003.13157v2 |
2016-03-05 | Stationary Axisymmetric Configuration of the Resistive Thick Accretion Tori around a Schwarzschild Black Hole | We examine a thick accretion disc in the presence of external gravity and
intrinsic dipolar magnetic field due to a non-rotating central object. In this
paper, we generalize the Newtonian theory of stationary axisymmetric resistive
tori of Tripathy, Prasanna $\&$ Das (1990) by including the fully general
relativistic features. If we are to obtain the steady state configuration, we
have to take into account the finite resistivity for the magnetofluid in order
to avoid the piling up of the field lines anywhere in the accretion discs. The
efficient value of conductivity must be much smaller than the classical
conductivity to be astrophysically interesting. The accreting plasma in the
presence of an external dipole magnetic field gives rise to a current in the
azimuthal direction. The azimuthal current produced due to the motion of the
magnetofluid modifies the magnetic field structure inside the disc and
generates a poloidal magnetic field for the disc. The solutions we have found
show that the radial inflow, pressure and density distributions are strongly
modified by the electrical conductivity both in relativistic and Newtonian
regimes. However, the range of conductivity coefficient is different for both
regimes, as well as that of the angular momentum parameter and the radius of
the innermost stable circular orbit. Furthermore, it is shown that the
azimuthal velocity of the disc which is not dependent on conductivity is
sub-Keplerian in all radial distances for both regimes. Owing to the presence
of pressure gradient and magnetic forces. This work may also be important for
the general relativistic computational magnetohydrodynamics that suffers from
the lack of exact analytic solutions that are needed to test computer codes. | 1603.01712v1 |
2016-03-15 | The Resistive-Plate WELL with Argon mixtures - a robust gaseous radiation detector | A thin single-element THGEM-based, Resistive-Plate WELL (RPWELL) detector was
operated with 150 GeV/c muon and pion beams in Ne/(5%CH$_4$), Ar/(5%CH$_4$) and
Ar/(7%CO$_2$); signals were recorded with 1 cm$^2$ square pads and SRS/APV25
electronics. Detection efficiency values greater than 98% were reached in all
the gas mixtures, at average pad multiplicity of 1.2. The use of the
10$^9${\Omega}cm resistive plate resulted in a completely discharge-free
operation also in intense pion beams. The efficiency remained essentially
constant at 98-99% up to fluxes of $\sim$10$^4$Hz/cm$^2$, dropping by a few %
when approaching 10$^5$ Hz/cm$^2$. These results pave the way towards
cost-effective, robust, efficient, large-scale detectors for a variety of
applications in future particle, astro-particle and applied fields. A potential
target application is digital hadron calorimetry. | 1603.04820v1 |
2017-07-27 | Reconnection and particle acceleration in interacting flux ropes -- II. 3D effects on test particles in magnetically dominated plasmas | We analyze particle acceleration in explosive reconnection events in
magnetically dominated proton-electron plasmas. Reconnection is driven by
large-scale magnetic stresses in interacting current-carrying flux tubes. Our
model relies on development of current-driven instabilities on macroscopic
scales. These tilt-kink instabilities develop in an initially force-free
equilibrium of repelling current channels. Using MHD methods we study a 3D
model of repelling and interacting flux tubes in which we simultaneously evolve
test particles, guided by electromagnetic fields obtained from MHD. We identify
two stages of particle acceleration; Initially particles accelerate in the
current channels, after which the flux ropes start tilting and kinking and
particles accelerate due to reconnection processes in the plasma. The explosive
stage of reconnection produces non-thermal energy distributions with slopes
that depend on plasma resistivity and the initial particle velocity. We also
discuss the influence of the length of the flux ropes on particle acceleration
and energy distributions. This study extends previous 2.5D results to 3D
setups, providing all ingredients needed to model realistic scenarios like
solar flares, black hole flares and particle acceleration in pulsar wind
nebulae: formation of strong resistive electric fields, explosive reconnection
and non-thermal particle distributions. By assuming initial energy
equipartition between electrons and protons, applying low resistivity in
accordance with solar corona conditions and limiting the flux rope length to a
fraction of a solar radius we obtain realistic energy distributions for solar
flares with non-thermal power law tails and maximum electron energies up to 11
MeV and maximum proton energies up to 1 GeV. | 1707.08920v2 |
2016-09-23 | Parallelization of JOREK-STARWALL for non-linear MHD simulations including resistive walls (Report of the EUROfusion High Level Support Team Projects JORSTAR/JORSTAR2) | Large scale plasma instabilities inside a tokamak can be influenced by the
currents flowing in the conducting vessel wall. This involves non linear plasma
dynamics and its interaction with the wall current. In order to study this
problem the code that solves the magneto-hydrodynamic (MHD) equations, called
JOREK [Huysmans G.T.A. and Czarny O. NF 47, 659 (2007); Czarny O. and Huysmans
G. JCP 227, 7423 (2008)], was coupled [Hoelzl M., et al. Journal of Physics:
Conference Series, 401, 012010 (2012)] with the model for the vacuum region and
the resistive conducting structure named STARWALL [Merkel P., Strumberger E.,
arXiv:150804911 (2015)]. The JOREK-STARWALL model has been already applied to
perform simulations of Vertical Displacement Events (VDEs), Resistive Wall
Modes (RWMs), Quiescent H-Mode, and vertical kick ELM triggering. At the
beginning of the project it was not possible to resolve the realistic wall
structure with a large number of finite element triangles due to the huge
consumption of memory and wall clock time by STARWALL and the corresponding
coupling routine in JOREK. Moreover, both the STARWALL code and the JOREK
coupling routine were only partially parallelized via OpenMP. The aim of this
project is to implement an MPI parallelization to reduce memory consumption and
execution time such that simulations with large resolutions become possible. | 1609.07441v2 |
2018-07-03 | Weak anti-localization of two-dimensional holes in germanium beyond the diffusive regime | Gate-controllable spin-orbit coupling is often one requisite for spintronic
devices. For practical spin field-effect transistors, another essential
requirement is ballistic spin transport, where the spin precession length is
shorter than the mean free path such that the gate-controlled spin precession
is not randomized by disorder. In this letter, we report the observation of a
gate-induced crossover from weak localization to weak anti-localization in the
magneto-resistance of a high-mobility two-dimensional hole gas in a strained
germanium quantum well. From the magneto-resistance, we extract the
phase-coherence time, spin-orbit precession time, spin-orbit energy splitting,
and cubic Rashba coefficient over a wide density range. The mobility and the
mean free path increase with increasing hole density, while the spin precession
length decreases due to increasingly stronger spin-orbit coupling. As the
density becomes larger than $\sim6\times 10^{11}$cm$^{-2}$, the spin precession
length becomes shorter than the mean free path, and the system enters the
ballistic spin transport regime. We also report here the numerical methods and
code developed for calculating the magneto-resistance in the ballistic regime,
where the commonly used HLN and ILP models for analyzing weak localization and
anti-localization are not valid. These results pave the way toward
silicon-compatible spintronic devices. | 1807.01400v1 |
2019-01-21 | Charge, lattice and magnetism across the valence crossover in EuIr$_2$Si$_2$ single crystals | We present a detailed study of the temperature evolution of the crystal
structure, specific heat, magnetic susceptibility and resistivity of single
crystals of the paradigmatic valence-fluctuating compound EuIr$_2$Si$_2$. A
comparison to stable-valent isostructural compounds EuCo$_2$Si$_2$ (with
Eu$^{3+}$), and EuRh$_2$Si$_2$, (with Eu$^{2+}$) reveals an anomalously large
thermal expansion indicative of the lattice softening associated to valence
fluctuations. A marked broad peak at temperatures around 65-75 K is observed in
specific heat, susceptibility and the derivative of resistivity, as thermal
energy becomes large enough to excite Eu into a divalent state, which localizes
one f electron and increases scattering of conduction electrons. In addition,
the intermediate valence at low temperatures manifests in a moderately
renormalized electron mass, with enhanced values of the Sommerfeld coefficient
in the specific heat and a Fermi-liquid-like dependence of resistivity at low
temperatures. The high residual magnetic susceptibility is mainly ascribed to a
Van Vleck contribution. Although the intermediate/fluctuating valence duality
is to some extent represented in the interconfiguration fluctuation model
commonly used to analyze data on valence-fluctuating systems, we show that this
model cannot describe the different physical properties of EuIr$_2$Si$_2$ with
a single set of parameters. | 1901.06826v1 |
2019-06-12 | Influence of 3D plasmoid dynamics on the transition from collisional to kinetic reconnection | Within the resistive magnetohydrodynamic model, high-Lundquist number
reconnection layers are unstable to the plasmoid instability, leading to a
turbulent evolution where the reconnection rate can be independent of the
underlying resistivity. However, the physical relevance of these results
remains questionable for many applications. First, the reconnection electric
field is often well above the runaway limit, implying that collisional
resistivity is invalid. Furthermore, both theory and simulations suggest that
plasmoid formation may rapidly induce a transition to kinetic scales, due to
the formation of thin current sheets. Here, this problem is studied for the
first time using a first-principles kinetic simulation with a Fokker-Planck
collision operator in 3D. The low-$\beta$ reconnecting current layer thins
rapidly due to Joule heating before onset of the oblique plasmoid instability.
Linear growth rates for standard ($k_y = 0$) tearing modes agree with
semi-collisional boundary layer theory, but the angular spectrum of oblique
($|k_y|>0$) modes is significantly narrower than predicted. In the non-linear
regime, flux-ropes formed by the instability undergo complex interactions as
they are advected and rotated by the reconnection outflow jets, leading to a
turbulent state with stochastic magnetic field. In a manner similar to previous
2D results, super-Dreicer fields induce a transition to kinetic reconnection in
thin current layers that form between flux-ropes. These results may be testable
within new laboratory experiments. | 1906.04867v1 |
2019-06-17 | Non-equilibrium Green's Function and First Principle Approach to Modeling of Multiferroic Tunnel Junctions | Recently, multiferroic tunnel junctions (MFTJs) have gained significant
spotlight in the literature due to its high tunneling electro-resistance
together with its non-volatility. In order to analyze such devices and to have
insightful understanding of its characteristics, there is a need for developing
a multi-physics modeling and simulation framework. The simulation framework
discussed in this paper is motivated by the scarcity of such multi-physics
studies in the literature. In this study, a theoretical analysis of MFTJs is
demonstrated using self-consistent analysis of spin-based non-equilibrium
Green's function (NEGF) method to estimate the tunneling current,
Landau-Khalatnikov (LK) equation to model the ferroelectric polarization
dynamics, together with landau-Lifshitz-Gilbert's (LLG) equations to capture
the magnetization dynamics. The spin-based NEGF method is equipped with a
magnetization dependent Hamiltonian that eases the modeling of the tunneling
electro-resistance (TER), tunneling magneto-resistance (TMR), and the
magnetoelectric effect (ME) in MFTJs. Moreover, we apply the first principle
calculations to estimate the screening lengths of the MFTJ electrodes that are
necessary for estimation of tunneling current. The simulation results of the
proposed framework are in good agreement with the experimental results.
Finally, a comprehensive analysis of TER and TMR of MFTJs and their dependence
on various device parameters is illustrated. | 1906.06986v1 |
2020-05-02 | All-electrical monitoring of bacterial antibiotic susceptibility in a microfluidic device | The lack of rapid antibiotic susceptibility tests adversely affects the
treatment of bacterial infections and contributes to increased prevalence of
multidrug resistant bacteria. Here, we describe an all-electrical approach that
allows for ultra-sensitive measurement of growth signals from only tens of
bacteria in a microfluidic device. Our device is essentially a set of
microfluidic channels, each with a nano-constriction at one end and
cross-sectional dimensions close to that of a single bacterium. Flowing a
liquid bacteria sample (e.g., urine) through the microchannels rapidly traps
the bacteria in the device, allowing for subsequent incubation in drugs. We
measure the electrical resistance of the microchannels, which increases (or
decreases) in proportion to the number of bacteria in the microchannels. The
method and device allow for rapid antibiotic susceptibility tests in about two
hours. Further, the short-time fluctuations in the electrical resistance during
an antibiotic susceptibility test are correlated with the morphological changes
of bacteria caused by the antibiotic. In contrast to other electrical
approaches, the underlying geometric blockage effect provides a robust and
sensitive signal, which is straightforward to interpret without electrical
models. The approach also obviates the need for a high-resolution microscope
and other complex equipment, making it potentially usable in resource-limited
settings. | 2005.00846v1 |
2020-05-04 | Environment-friendly gas mixtures for Resistive Plate Chambers: an experimental and simulation study | Resistive Plate Chambers (RPC) have shown stable operation at the Large
Hadron Collider and satisfactory efficiency for the entire Run 1 (2010-2013)
and Run 2 (2015-2018) with C$_{2}$H$_{2}$F$_{4}$-based gas mixtures and the
addition of SF$_{6}$ and i-C$_{4}$H$_{10}$. Since its global warming potential
(GWP) is high, C$_{2}$H$_{2}$F$_{4}$ is phasing out of production due to recent
European Union regulations and as a result its cost is progressively
increasing. Therefore, finding a new RPC gas mixture with a low GWP has become
extremely important. This contribution describes the simulation of the RPC
efficiency with tetrafluoropropene C$_{3}$H$_{2}$F$_{4}$ (HFO1234ze), a
hydrofluoroolefin with very low GWP. Simulation results are systematically
compared with measurements of RPC efficiency in C$_{3}$H$_{2}$F$_{4}$-based gas
mixtures with the addition of different combinations of Ar, He, CO$_{2}$,
O$_{2}$ and i-C$_{4}$H$_{10}$ in various concentrations. This simulation allows
the study of the interplay between C$_{3}$H$_{2}$F$_{4}$ and the other gas
components in the mixture as well as may allow the identification of the most
promising environment-friendly gas mixtures with C$_{3}$H$_{2}$F$_{4}$ for
RPCs. | 2005.01476v1 |
2020-05-04 | Evidence for a pressure-induced antiferromagnetic quantum critical point in intermediate valence UTe2 | UTe$_2$ is a recently discovered unconventional superconductor that has
attracted much interest due to its many intriguing properties - a large
residual density-of-states in the superconducting state, re-entrant
superconductivity in high magnetic fields, and potentially spin-triplet
topological superconductivity. Our ac calorimetry, electrical resistivity, and
x-ray absorption study of UTe$_2$ under applied pressure reveals key new
insights on the superconducting and magnetic states surrounding
pressure-induced quantum criticality at P$_{c1}$ = 1.3 GPa. First, our specific
heat data at low pressures, combined with a phenomenological model, show that
pressure alters the balance between two closely competing superconducting
orders. Second, near 1.5 GPa we detect two bulk transitions that trigger
changes in the resistivity which are consistent with antiferromagnetic order,
rather than ferromagnetism. The presence of both bulk magnetism and
superconductivity at pressures above P$_{c2}$ = 1.4 GPa results in a
significant temperature difference between resistively and thermodynamically
determined transitions into the superconducting state, which indicates a
suppression of the superconducting volume fraction by magnetic order. Third,
the emergence of magnetism is accompanied by an increase in valence towards a
U$^{4+}$ (5f2) state, which indicates that UTe$_2$ exhibits intermediate
valence at ambient pressure. Our results suggest that antiferromagnetic
fluctuations may play a more significant role on the superconducting state of
UTe$_2$ than previously thought. | 2005.01659v2 |
2020-05-22 | Improved-RPC for the CMS muon system upgrade for the HL-LHC | During Phase-2 of the LHC, known as the High Luminosity LHC (HL-LHC), the
accelerator will increase its instantaneous luminosity to 5 $\times$ 10$^{34}$
cm$^{-2}$ s$^{-1}$, delivering an integrated luminosity of 3000 fb$^{-1}$ over
10 years of operation starting from 2027. In view of the HL-LHC, the CMS muon
system will be upgraded to sustain efficient muon triggering and reconstruction
performance. Resistive Plate Chambers (RPCs) serve as dedicated detectors for
muon triggering due to their excellent timing resolution, and will extend
$|\eta|$ (pseudorapidity) up to a region of 2.4. Before Long Shutdown 3 (LS3),
the RE3/1 and RE4/1 stations of the endcap will be equipped with new improved
Resistive Plate Chambers (iRPCs) having different design and geometry than the
present RPC system. The iRPC geometry configuration improves the detector's
rate capability and its ability to survive the harsh background conditions of
the HL-LHC. Also, new electronics equipped with excellent timing precision
measurements with a resolution of less than 150 ps are developed to readout the
RPC detectors from both sides of the strips to allow for good spatial
resolution along them. The performance of the iRPC has been studied with gamma
radiation at the Gamma Irradiation Facility (GIF++) at CERN. Ongoing longevity
studies will help to certify the iRPCs for the HL-LHC running period. The main
detector parameters such as the currents, rate and resistivity are regularly
monitored as a function of the integrated charge. Preliminary results of the
detector performance will be presented. | 2005.11396v4 |
2020-06-08 | Flow through three-dimensional self-affine fractures | We investigate through numerical simulations of the Navier-Stokes equations
the influence of the surface roughness on the fluid flow through fracture
joints. Using the Hurst exponent $H$ to characterize the roughness of the
self-affine surfaces that constitute the fracture, our analysis reveal the
important interplay between geometry and inertia on the flow. Precisely, for
low values of Reynolds numbers Re, we use Darcy's law to quantify the hydraulic
resistance $G$ of the fracture and show that its dependence on $H$ can be
explained in terms of a simple geometrical model for the tortuosity $\tau$ of
the channel. At sufficiently high values of Re, when inertial effects become
relevant, our results reveal that nonlinear corrections up to third-order to
Darcy's law are aproximately proportional to $H$. These results imply that the
resistance $G$ to the flow follows a universal behavior by simply rescaling it
in terms of the fracture resistivity and using an effective Reynolds number,
namely, Re/$H$. Our results also reveal the presence of quasi-one-dimensional
channeling, even considering the absence of shear displacement between upper
and lower surfaces of the self-affine fracture. | 2006.04846v1 |
2020-06-22 | Effect of Landau quantization on linear magnetoresistance of periodically modulated two-dimensional electron gas | The linear response of two-dimensional electron gas in a perpendicular
magnetic field in the presence of a spatially dependent classically smooth
electrostatic potential is studied theoretically, by application of the Kubo
formula for nonlocal conductivity tensor. In the classical transport regime, a
general expression for the conductivity tensor through the correlation
functions of the homogeneous electron gas is derived. The quantum transport
regime, when Landau quantization is essential, is studied for the case of
unidirectional periodic potential modulation. Apart from the Shubnikov-de Haas
oscillations, the resistivity can demonstrate quantum oscillations with larger
periods and smaller amplitudes, which survive when temperature increases. These
oscillations exist when the modulation amplitude considerably exceeds the
cyclotron energy so the Landau subbands, formed out of the Landau levels by the
modulation potential, overlap in the energy domain. Both diagonal components of
the resistivity tensor demonstrate oscillations related to modification of the
density of states by the modulation. In addition, the resistivity component
perpendicular to the modulation axis, which is caused by the
scattering-assisted hopping transport, shows another kind of oscillations
related to enhancement of the hopping probability when the guiding center of
cyclotron orbit shifts by the doubled cyclotron radius. It is suggested that
such high-temperature oscillations can be detected under conditions when the
modulation period considerably exceeds the cyclotron radius. | 2006.12351v1 |
2020-11-06 | (Quasi-)Real-Time Inversion of Airborne Time-Domain Electromagnetic Data via Artificial Neural Network | The possibility to have results very quickly after, or even during, the
collection of electromagnetic data would be important, not only for quality
check purposes, but also for adjusting the location of the proposed flight
lines during an airborne time-domain acquisition. This kind of readiness could
have a large impact in terms of optimization of the Value of Information of the
measurements to be acquired. In addition, the importance of having fast tools
for retrieving resistivity models from airborne time-domain data is
demonstrated by the fact that Conductivity-Depth Imaging methodologies are
still the standard in mineral exploration. In fact, they are extremely
computationally efficient, and, at the same time, they preserve a very high
lateral resolution. For these reasons, they are often preferred to inversion
strategies even if the latter approaches are generally more accurate in terms
of proper reconstruction of the depth of the targets and of reliable retrieval
of true resistivity values of the subsurface. In this research, we discuss a
novel approach, based on neural network techniques, capable of retrieving
resistivity models with a quality comparable with the inversion strategy, but
in a fraction of the time. We demonstrate the advantages of the proposed novel
approach on synthetic and field datasets. | 2011.03522v1 |
2020-11-30 | Discovering novel cancer bio-markers in acquired lapatinib resistance using Bayesian methods | Genes/Proteins do not work alone within our body, rather as a group they
perform certain activities indicated as pathways. Signalling transduction
pathways (STPs) are some of the important pathways that transmit biological
signals from protein-to-protein controlling several cellular activities.
However, many diseases such as cancer target some of these signalling pathways
for their growth and malignance, but demystifying their underlying mechanisms
are a very complicated tasks. In this study, we use a fully Bayesian approach
to develop methodologies in discovering novel driver bio-markers in aberrant
STPs given two-conditional high-throughput gene expression data. This project,
namely PathTurbEr (Pathway Perturbation Driver), is applied on a global gene
expression dataset derived from the lapatinib (an EGFR/HER dual inhibitor)
sensitive and resistant samples from breast cancer cell lines (SKBR3).
Differential expression analysis revealed 512 differentially expressed genes
(DEGs) and their signalling pathway enrichment analysis revealed 22 singalling
pathways as aberrated including PI3K-AKT, Hippo, Chemokine, and TGF-beta
singalling pathway as highly dysregulated in lapatinib resistance. Next, we
model the aberrant activities in TGF-beta STP as a causal Bayesian network (BN)
from given observational datasets using three Markov Chain Monte Carlo (MCMC)
sampling methods, i.e. Neighbourhood sampler (NS) and Hit-and-Run (HAR)
sampler, which has already proven to have more robust inference with lower
chances of getting stuck at local optima and faster convergence compared to
other state-of-art methods. Next, we examined the structural features of the
optimal BN as a statistical process that generates the global structure using,
$p_1$-model, a special class of Exponential Random Graph Models (ERGMs) and
MCMC methods for their hyper-parameter sampling.... | 2012.00566v1 |
2021-02-02 | Long-term evolution of a merger-remnant neutron star in general relativistic magnetohydrodynamics I: Effect of magnetic winding | Long-term ideal and resistive magnetohydrodynamics (MHD) simulations in full
general relativity are performed for a massive neutron star formed as a remnant
of binary neutron star mergers. Neutrino radiation transport effects are taken
into account as in our previous papers. The simulation is performed in axial
symmetry and without considering dynamo effects as a first step. In the ideal
MHD, the differential rotation of the remnant neutron star amplifies the
magnetic-field strength by the winding in the presence of a seed poloidal field
until the electromagnetic energy reaches $\sim 10\%$ of the rotational kinetic
energy, $E_{\rm kin}$, of the neutron star. The timescale until the maximum
electromagnetic energy is reached depends on the initial magnetic-field
strength and it is $\sim 1$ s for the case that the initial maximum
magnetic-field strength is $\sim 10^{15}$ G. After a significant amplification
of the magnetic-field strength by the winding, the magnetic braking enforces
the initially differentially rotating state approximately to a rigidly rotating
state. In the presence of the resistivity, the amplification is continued only
for the resistive timescale, and if the maximum electromagnetic energy reached
is smaller than $\sim 3\%$ of $E_{\rm kin}$, the initial differential rotation
state is approximately preserved. In the present context, the post-merger mass
ejection is induced primarily by the neutrino irradiation/heating and the
magnetic winding effect plays only a minor role for the mass ejection. | 2102.01346v1 |
2021-03-15 | Electron Collision Cross Sections in Tetrafluoropropene HFO1234ze(E) for Gas Mixtures in Resistive Plate Chambers | In recent years, there has been growing interest in tetrafluoropropene
HFO1234ze(E) (C$_{3}$H$_{2}$F$_{4}$) for Resistive Plate Chambers (RPCs). This
novel gas is considered a promising alternative to the standard mixtures
currently used in RPCs, thanks to its low global warming potential. The
knowledge of electron collision cross sections in C$_{3}$H$_{2}$F$_{4}$ enables
reliable predictions of electron transport coefficients and reaction rates in
C$_{3}$H$_{2}$F$_{4}$-based gas mixtures. This allows for optimizing the
C$_{3}$H$_{2}$F$_{4}$-based gas mixtures to achieve the desired performance in
RPCs.
From measurements of electron transport coefficients and reaction rates, a
complete set of scattering cross sections for electrons in
C$_{3}$H$_{2}$F$_{4}$ has been derived. Validation of the electron collision
cross sections is achieved through systematic comparisons of electron swarm
parameters with experimental data in both pure C$_{3}$H$_{2}$F$_{4}$ and
C$_{3}$H$_{2}$F$_{4}$/CO$_{2}$ gas mixtures. Given the influence of electron
attachment in C$_{3}$H$_{2}$F$_{4}$ by the gas density, this work also includes
precise calculations of the critical electric field strength in such mixtures.
This set of cross sections has been further utilized to compute the effective
ionization Townsend coefficient in gas mixtures containing
C$_{3}$H$_{2}$F$_{4}$, potentially applicable for RPCs. | 2103.08643v2 |
2021-03-31 | Cascade-Forward Neural Network Based on Resilient Backpropagation for Simultaneous Parameters and State Space Estimations of Brushed DC Machines | A sensorless speed, average temperature and resistance estimation technique
based on Neural Network (NN) for brushed DC machines is proposed in this paper.
The literature on parameters and state spaces estimations of the Brushed DC
machines, shows a variety of approaches. However, these observers are sensitive
to a noise, on the model accuracy also are difficult to stabilize and to
converge. Furthermore, the majority of earlier works, estimate either the speed
or the temperature or the winding resistance. According to the literatures, the
Resilient backpropagation (RBP) as is the known as the faster BP algorithm,
Cascade-Forward Neural Network (CFNN), is known as the among accelerated
learning backpropagation algorithms, that's why where it is found in several
researches, also in several applications in these few years. The main objective
of this paper is to introduce an intelligent sensor based on resilient BP to
estimate simultaneously the speed, armature temperature and resistance of
brushed DC machines only from the measured current and voltage. A comparison
between the obtained results and the results of traditional estimator has been
made to prove the ability of the proposed method. This method can be embedded
in thermal monitoring systems, in high performance motor drives. | 2104.04348v1 |
2021-04-25 | Relativistic resistive dissipative magnetohydrodynamics from the relaxation time approximation | Here we derive the relativistic resistive dissipative second-order
magnetohydrodynamic evolution equations using the Boltzmann equation, thus
extending our work from the previous paper
\href{https://link.springer.com/article/10.1007/JHEP03(2021)216}{JHEP 03 (2021)
216} where we considered the non-resistive limit. We solve the Boltzmann
equation for a system of particles and antiparticles using the relaxation time
approximation and the Chapman-Enskog like gradient expansion for the
off-equilibrium distribution function, truncating beyond second-order. In the
first order, the bulk and shear stress are independent of the electromagnetic
field, however, the diffusion current, shows a dependence on the electric
field. In the first order, the transport coefficients~(shear and bulk stress)
are shown to be independent of the electromagnetic field. The diffusion
current, however, shows a dependence on the electric field. In the
second-order, the new transport coefficients that couple electromagnetic field
with the dissipative quantities appear, which are different from those obtained
in the 14-moment approximation~\cite{Denicol:2019iyh} in the presence of the
electromagnetic field. Also we found out the various components of conductivity
in this case. | 2104.12179v2 |
2021-05-17 | Strange Metals from Melting Correlated Insulators in Twisted Bilayer Graphene | Even as the understanding of the mechanism behind correlated insulating
states in magic-angle twisted bilayer graphene converges towards various kinds
of spontaneous symmetry breaking, the metallic "normal state" above the
insulating transition temperature remains mysterious, with its excessively high
entropy and linear-in-temperature resistivity. In this work, we focus on the
effects of fluctuations of the order-parameters describing correlated
insulating states at integer fillings of the low-energy flat bands on charge
transport. Motivated by the observation of heterogeneity in the order-parameter
landscape at zero magnetic field in certain samples, we conjecture the
existence of frustrating extended range interactions in an effective Ising
model of the order-parameters on a triangular lattice. The competition between
short-distance ferromagnetic interactions and frustrating extended range
antiferromagnetic interactions leads to an emergent length scale that forms
stripe-like mesoscale domains above the ordering transition. The gapless
fluctuations of these heterogeneous configurations are found to be responsible
for the linear-in-temperature resistivity as well as the enhanced low
temperature entropy. Our insights link experimentally observed
linear-in-temperature resistivity and enhanced entropy to the strength of
frustration, or equivalently, to the emergence of mesoscopic length scales
characterizing order-parameter domains. | 2105.08069v3 |
2021-07-21 | On the explosive phase of the tearing mode in double current sheet plasmas: effect of the equilibrium magnetic configuration on the onset threshold and growth rate | Magnetic reconnection associated with the tearing instability occurring in
double-current sheet systems is investigated within the framework of reduced
resistive magnetohydrodynamics (MHD) in a two-dimensional Cartesian geometry.
The explosive non linear phase is particularly explored using the adaptive
finite-element FINMHD code. The critical aspect ratio, that is defined as the
minimum $L/x_s$ ratio (with $L$ and $x_s$ being the periodic system length and
half-distance between the two current layers respectively) necessary for non
linear destabilization after the linear and early non linear saturation phases,
is obtained. The latter threshold is independent of the details of the chosen
initial equilibrium (double Harris-like magnetic profile) and of the
resistivity. Its value is shown to be $4.7$, that is close and slightly smaller
than the value of order $5$ deduced using a more particular equilibrium
configuration in previous studies. The time dependence of the kinetic energy
($E_K$) is shown to follow a double exponential law, $E_K \propto \exp \
[e^{(\gamma^* t)} ]$, with a pseudo-growth rate $\gamma^* \simeq 0.1 \ t_A^
{-1}$ ($t_A$ being the characteristic Alfv\'en time) that is again independent
of the configuration and resistivity. The mechanism offers a possible
explanation for the sudden onset of explosive magnetic energy release occurring
on the fast Alfv\'en time scale in disruptive events of astrophysical plasmas
with pre-existing double current sheets like in the solar corona. | 2107.10069v2 |
2021-11-22 | Magnetic attractive interaction induced superconductivity in metals | The BCS theory has described the conventional superconductors
successfully.However,it is still not clear when the superconductivity occurs
and why the resistance is zero.Also,there is no simple formula to calculate
Tc,and it is not well understood about how to improve Tc. Therefore,a theory of
superconductivity caused by magnetic attractive interactions is
presented.(1)The magnetic attractive interaction leads to
superconductivity.This interactions happen only when the electron velocity
reaches up to 106 m/s and the distance between two electrons is about 0.0529
nm. The number of superconducting electrons is about 10-5 of the number of free
electrons.(2)Resistance is redefined as the ratio of magnetic flux to electric
quantity. The resistance equals to zero, when the magnetic flux is 0. This is
why superconductors show Meissner effect.(3)Tc is determined by the magnetic
attractive energy. A simple formula to estimate Tc is derived and it is related
not only to the electron density n and Bc, but also to electronic degrees of
freedom i.(4)The Tc can be increased with increasing the ratio of the
superconducting electrons to total electrons. Mankowsky realized the transient
room temperature superconductivity in YBCO by illuminating in 2014. Bilayer
graphene superconductor was realized by Cao via applied a voltage in 2018. In
2020, roomtemperature superconductor was reported under high pressure for the
CSH system. Another way to improve Tc is to reduce electronic degrees of
freedom i. By forming Al"super atoms"to reduce i, Tc was further increased
experimentally to more than 100 K in 2015. We believe that room temperature
superconductors can be realized in Li by applied a voltage at atmospheric
pressure. | 2111.11048v1 |
2022-01-28 | Generalized statistics: applications to data inverse problems with outlier-resistance | The conventional approach to data-driven inversion framework is based on
Gaussian statistics that presents serious difficulties, especially in the
presence of outliers in the measurements. In this work, we present maximum
likelihood estimators associated with generalized Gaussian distributions in the
context of R\'enyi, Tsallis and Kaniadakis statistics. In this regard, we
analytically analyse the outlier-resistance of each proposal through the
so-called influence function. In this way, we formulate inverse problems by
constructing objective functions linked to the maximum likelihood estimators.
To demonstrate the robustness of the generalized methodologies, we consider an
important geophysical inverse problem with high noisy data with spikes. The
results reveal that the best data inversion performance occurs when the
entropic index from each generalized statistic is associated with objective
functions proportional to the inverse of the error amplitude. We argue that in
such a limit the three approaches are resistant to outliers and are also
equivalent, which suggests a lower computational cost for the inversion process
due to the reduction of numerical simulations to be performed and the fast
convergence of the optimization process. | 2201.12173v1 |
2022-04-02 | Mechanical and electrical properties of MWCNT/PP films and structural health monitoring of GF/PP joints | While welding of thermoplastic composites (TPCs) is a promising rivetless
method to reduce weight, higher confidence in joints' structural integrity and
failure prediction must be achieved for widespread use in industry. In this
work, we present an innovative study on damage detection for ultrasonically
welded TPC joints with multiwalled carbon nanotubes (MWCNTs) and embedded
buckypaper films. MWCNTs show promise for structural health monitoring (SHM) of
composite joints, assembled by adhesive bonding or fusion bonding, through
electrical resistance changes. This study focuses on investigating
multifunctional films and their suitability for ultrasonic welding (USW) of
TPCs, using two approaches: 1) MWCNT/filled polypropylene (PP) nanocomposites
prepared via solvent dispersion, and 2) high conductivity MWCNT buckypaper
embedded between PP films by hot pressing. Nanocomposite formulations
containing 5 wt and 10 wt MWCNTs were synthesized using solvent dispersion
method, followed by compression molding to manufacture films. The effect of
MWCNT concentration on electrical and dynamic mechanical behavior of
multifunctional films was examined with a Sourcemeter and Dynamic Mechanical
Analyzer, and a comparison was made between 5 to 20 wt MWCNT-PP films based on
previous research. Glass fiber/polypropylene (GF/PP) composite joints were
ultrasonically welded in a single lap shear configuration using buckypaper and
MWCNT/PP films. Furthermore, electrical resistance measurements were carried
out for joints under bending loads. It was observed that 15 wt and 20 wt
MWCNT/PP films had higher stability and sensitivity for resistance response
than embedded buckypaper and films with low MWCNT contents, demonstrating their
suitability for USW and potential for SHM. | 2204.00909v1 |
2022-05-09 | ResSFL: A Resistance Transfer Framework for Defending Model Inversion Attack in Split Federated Learning | This work aims to tackle Model Inversion (MI) attack on Split Federated
Learning (SFL). SFL is a recent distributed training scheme where multiple
clients send intermediate activations (i.e., feature map), instead of raw data,
to a central server. While such a scheme helps reduce the computational load at
the client end, it opens itself to reconstruction of raw data from intermediate
activation by the server. Existing works on protecting SFL only consider
inference and do not handle attacks during training. So we propose ResSFL, a
Split Federated Learning Framework that is designed to be MI-resistant during
training. It is based on deriving a resistant feature extractor via
attacker-aware training, and using this extractor to initialize the client-side
model prior to standard SFL training. Such a method helps in reducing the
computational complexity due to use of strong inversion model in client-side
adversarial training as well as vulnerability of attacks launched in early
training epochs. On CIFAR-100 dataset, our proposed framework successfully
mitigates MI attack on a VGG-11 model with a high reconstruction
Mean-Square-Error of 0.050 compared to 0.005 obtained by the baseline system.
The framework achieves 67.5% accuracy (only 1% accuracy drop) with very low
computation overhead. Code is released at:
https://github.com/zlijingtao/ResSFL. | 2205.04007v1 |
2022-06-18 | Quantifying the value of transient voltage sources | Some voltage sources are transient, lasting only for a moment of time, such
as the voltage generated by converting a human motion into electricity. Such
sources moreover tend to have a degree of randomness as well as internal
resistance. We investigate how to put a number to how valuable a given
transient source is. We derive several candidate measures via a systematic
approach. We establish an inter-convertibility hierarchy between such sources,
where inter-conversion means adding passive interface circuits to the sources.
Resistors at the ambient temperature are at the bottom of this hierarchy and
sources with low internal resistance and high internal voltages are at the top.
We provide three possible measures for a given source that assign a number to
the source respecting this hierarchy. One measure captures how much ``unitdc"
the source contains, meaning $1$ V dc with $1\Omega$ internal resistance for
$1$s. Another measure relates to the signal-to-noise ratio of the voltage
time-series whereas a third is based on the relative entropy between the
voltage probability distribution and a thermal noise resistor. We argue that
the unitdc measure is particularly useful by virtue of its operational
interpretation in terms of the number of unit dc sources that one needs to
combine to create the source or that can be distilled from the source. | 2206.09126v2 |
2022-09-14 | Accretion disc backflow in resistive MHD simulations | We investigate accretion onto a central star, with the size, rotation rate,
and magnetic dipole of a young stellar object, to study the flow pattern
(velocity and density) of the fluid within and outside of the disc. We perform
resistive MHD simulations of thin $\alpha$-discs, varying the parameters such
as the stellar rotation rate and magnetic field, and (anomalous) coefficients
of viscosity and resistivity in the disc. To provide a benchmark for the
results and to compare with known analytic results, we also perform purely
hydrodynamic simulations (HD) for the same problem. Although obtained for a
different situation with differing inner boundary condition, the disc structure
in the HD simulations closely follows the analytic solution of Klu\'zniak and
Kita (2000) -- in particular a region of "midplane" backflow exists in the
right range of radii, depending on the viscosity parameter. In the MHD
solutions, whenever the magnetic Prandtl number does not exceed a certain
critical value, the midplane backflow exists throughout the accretion disc,
extending all the way down to the inner transition zone where the disc
transitions to a magnetic funnel flow. For values of the magnetic Prandtl
number close to the critical value the backflow and the inner disc undergo a
quasiperiodic radial oscillation, otherwise the backflow is steady, as is the
disc solution. From our results, supplemented by our reading of the literature,
we conclude that midplane backflow is a real feature of at least some accretion
discs, whether HD $\alpha$-discs or MHD discs, including ones driven by MRI
turbulence. | 2209.06526v1 |
2022-11-02 | Weak antilocalization induced by Se substitution in layered BiCh$_2$-based (Ch = S, Se) superconductors LaO$_{1-x}$F$_x$BiS$_{2-y}$Se$_y$ | We report transport properties for layered BiCh2-based (Ch = S, Se)
superconductors LaO1-xFxBiS2-ySey (x = 0.2, 0.5, y = 0-1.05) and the
observation of weak antilocalization (WAL). Electrical resistivity and Hall
coefficients for the Se-poor samples increase with decreasing temperature. The
increase becomes less pronounced with increasing Se concentration indicating a
loss of insulating behavior. Interestingly, the moderately Se-substituted
samples exhibit metallic behavior in the high-temperature region and a weak
increase in the resistivity in the low-temperature regions, which indicates the
existence of carrier localization. The heavily Se-substituted compounds show
metallic behavior in the entire-temperature region. Sign changes of the Hall
coefficients are observed for the x = 0.2 samples, which possibly is related to
a charge-density wave (CDW). Magnetoresistance measurements indicate that WAL
is realized in the heavily Se-substituted systems. The WAL behavior is weakened
by the changes in F and Se concentrations. A crossover state of the WAL and WL
emerges around the moderately F-doped and Se-free LaO0.8F0.2BiS2. The change of
the resistivity behavior by the F and Se substitution clearly correlates to the
difference of the magnetoconductance. Moreover, the localization regions of the
WAL-WL crossover and weak WAL states are possibly associated with the CDW. We
propose that the BiCh2-based system is a good platform for studying
relationship between WAL, superconductivity, and electronic ordering because
those states are tunable by element substitutions with bulk single crystals. | 2211.00843v1 |
2022-11-17 | Charge diffusion in relativistic resistive second-order dissipative magnetohydrodynamics | We study charge diffusion in relativistic resistive second-order dissipative
magnetohydrodynamics. In this theory, charge diffusion is not simply given by
the standard Navier-Stokes form of Ohm's law, but by an evolution equation
which ensures causality and stability. This, in turn, leads to transient
effects in the charge diffusion current, the nature of which depends on the
particular values of the electrical conductivity and the charge-diffusion
relaxation time. The ensuing equations of motion are of so-called stiff
character, which requires special care when solving them numerically. To this
end, we specifically develop an implicit-explicit Runge-Kutta method for
solving relativistic resistive second-order dissipative magnetohydrodynamics
and subject it to various tests. We then study the system's evolution in a
simplified 1+1-dimensional scenario for a heavy-ion collision, where matter and
electromagnetic fields are assumed to be transversely homogeneous, and
investigate the cases of an initially non-expanding fluid and a fluid initially
expanding according to a Bjorken scaling flow. In the latter case, the scale
invariance is broken by the ensuing self-consistent dynamics of matter and
electromagnetic fields. However, the breaking becomes quantitatively important
only if the electromagnetic fields are sufficiently strong. The breaking of
scale invariance is larger for smaller values of the conductivity. Aspects of
entropy production from charge diffusion currents and stability are also
discussed. | 2211.09459v2 |
2023-08-21 | Neuromorphic Hebbian learning with magnetic tunnel junction synapses | Neuromorphic computing aims to mimic both the function and structure of
biological neural networks to provide artificial intelligence with extreme
efficiency. Conventional approaches store synaptic weights in non-volatile
memory devices with analog resistance states, permitting in-memory computation
of neural network operations while avoiding the costs associated with
transferring synaptic weights from a memory array. However, the use of analog
resistance states for storing weights in neuromorphic systems is impeded by
stochastic writing, weights drifting over time through stochastic processes,
and limited endurance that reduces the precision of synapse weights. Here we
propose and experimentally demonstrate neuromorphic networks that provide
high-accuracy inference thanks to the binary resistance states of magnetic
tunnel junctions (MTJs), while leveraging the analog nature of their stochastic
spin-transfer torque (STT) switching for unsupervised Hebbian learning. We
performed the first experimental demonstration of a neuromorphic network
directly implemented with MTJ synapses, for both inference and
spike-timing-dependent plasticity learning. We also demonstrated through
simulation that the proposed system for unsupervised Hebbian learning with
stochastic STT-MTJ synapses can achieve competitive accuracies for MNIST
handwritten digit recognition. By appropriately applying neuromorphic
principles through hardware-aware design, the proposed STT-MTJ neuromorphic
learning networks provide a pathway toward artificial intelligence hardware
that learns autonomously with extreme efficiency. | 2308.11011v1 |
2023-10-02 | Breaking Through the Plasma Wavelength Barrier to Extend the Transparency Range of Ultrathin Indium Tin Oxide Films into the Far Infrared | Indium tin oxide (ITO) film, which is the most commonly used transparent
conductive film (TCF), has traditionally been believed to be transparent in the
visible spectrum but to reflect infrared (IR) light beyond the plasma
wavelength (${\lambda}_p$). However, our theoretical analysis challenges this
notion by demonstrating that an ultrathin ITO TCF that is thinner than the
light's penetration depth, can overcome the transmission barrier at
${\lambda}_p$. To validate the theoretical modeling, we have successfully
fabricated ITO films that, despite having ${\lambda}_p \approx$ 1 ${\mu}$m,
remain transparent from 400 nm to 20 ${\mu}$m. This represents the broadest
transparency range ever reported for any In$_2$O$_3$-based TCF. The 10-nm-thick
ITO TCFs have high visible transmittance (91.0% at 550 nm), low resistivity (5
$\times$ 10$^{-4}$ ${\Omega}\cdot$ cm), and good IR transmittance (averaging
60% over 1.35 $\unicode{x2013}$ 18.35 ${\mu}$m). Their IR transparency
facilitates radiative cooling of the underlying circuitry. When an operational
resistor is enclosed by commercial ITO TCFs that are 140 nm thick, its
temperature increases. However, using 10-nm-thick ITO TCFs instead of the
commercial ones can completely avoid this temperature rise. Moreover, attaching
a silver grid to a 10-nm-thick ITO TCF can reduce the effective sheet
resistance to ~10 ${\Omega}/\square$ at the expense of only ~3% transmittance.
This development paves the way for large-scale applications that require low
sheet resistance and far-IR transparency. | 2310.00984v1 |
2024-01-31 | Large amplitude traveling waves for the non-resistive MHD system | We prove the existence of large amplitude bi-periodic traveling waves
(stationary in a moving frame) of the two-dimensional non-resistive
Magnetohydrodynamics (MHD) system with a traveling wave external force with
large velocity speed $\lambda (\omega_1, \omega_2)$ and of amplitude of order
$O(\lambda^{1^+})$ where $\lambda \gg 1$ is a large parameter. For most values
of $\omega = (\omega_1, \omega_2)$ and for $\lambda \gg 1$ large enough, we
construct bi-periodic traveling wave solutions of arbitrarily large amplitude
as $\lambda \to + \infty$. More precisely, we show that the velocity field is
of order $O(\lambda^{0^+})$, whereas the magnetic field is close to a constant
vector as $\lambda \to + \infty$. Due to the presence of small divisors, the
proof is based on a nonlinear Nash-Moser scheme adapted to construct nonlinear
waves of large amplitude. The main difficulty is that the linearized equation
at any approximate solution is an unbounded perturbation of large size of a
diagonal operator and hence the problem is not perturbative. The invertibility
of the linearized operator is then performed by using tools from micro-local
analysis and normal forms together with a sharp analysis of high and low
frequency regimes w.r. to the large parameter $\lambda \gg 1$. To the best of
our knowledge, this is the first result in which global in time, large
amplitude solutions are constructed for the 2D non-resistive MHD system with
periodic boundary conditions and also the first existence results of large
amplitude quasi-periodic solutions for a nonlinear PDE in higher space
dimension. | 2401.17943v1 |
2024-02-29 | Quantum transport properties of the topological Dirac Semimetal $α$-Sn | We report measurements of the electrical resistivity ($\rho$) and
thermoelectric power (S) in a thin film of strained single-crystalline
$\alpha$-Sn grown by molecular beam epitaxy on an insulating substrate. The
temperature (T) dependence of the resistivity of $\alpha$-Sn can be divided
into two regions:below T* $\approx$ 135 K $\rho$(T) shows a metallic-like
behaviour, while above this temperature an increasing contribution from
thermally excited holes to electrical transport is observed. However, it is
still dominated by highly mobile electrons, resulting in a negative sign of the
Seebeck coefficient above T = 47 K. In the low temperature limit, a small
positive S likely reflects the persistent contribution from low-mobility holes
or the positive phonon-drag thermopower. In the presence of the magnetic field
(B) applied along an electric field or thermal gradient, we note a negative
magnetoresistance or a negative slope of S(B), respectively. The theoretical
prediction for the former (calculated using density functional theory) agrees
well with the experiment. However, these characteristics quickly disappear when
the magnetic field is deviated from an orientation parallel to the electrical
field or the thermal gradient. We indicate that the behaviour of the electrical
resistivity and thermoelectric power can be explained in terms of the chiral
current arising from the topologically non-trivial electronic structure of
$\alpha$-Sn. Its decay at high temperature is a consequence of the decreasing
ratio between the intervalley Weyl relaxation time to the Drude scattering
time. | 2403.00083v1 |
2024-05-06 | Physical properties and electronic structure of the two-gap superconductor V$_{2}$Ga$_{5}$ | We present a thorough investigation of the physical properties and
superconductivity of the binary intermetallic V2Ga5. Electrical resistivity and
specific heat measurements show that V2Ga5 enters its superconducting state
below Tsc = 3.5 K, with a critical field of Hc2,perp c(Hc2,para c) = 6.5(4.1)
kOe. With H perp c, the peak effect was observed in resistivity measurements,
indicating the ultrahigh quality of the single crystal studied. The resistivity
measurements under high pressure reveal that the Tsc is suppressed linearly
with pressure and reaches absolute zero around 20 GPa. Specific heat and muon
spin relaxation measurements both indicate that the two-gap s-wave model best
describes the superconductivity of V2Ga5. The spectra obtained from
angle-resolved photoemission spectroscopy measurements suggest that two
superconducting gaps open at the Fermi surface around the Z and {\Gamma}
points. These results are verified by first-principles band structure
calculations. We therefore conclude that V2Ga5 is a phonon-mediated two-gap
s-wave superconductor | 2405.03499v1 |
2013-12-03 | Materials Design by Quantum-Chemical and other Theoretical/Computational Means: Applications to Energy Storage and Photoemissive Materials | The present paper discusses some recent developments in the field of rational
design for energy storage and photoemissive materials. Recent and new examples
of designer materials for Li-ion and Li-air type batteries with high capacity
and energy/power density as well as photoemissive materials with low
workfunctions and improved brightness are discussed as illustrative examples of
how quantum-chemical and other theoretical computational means can be used for
rational materials design. | 1312.0699v2 |
2015-10-05 | Low frequency sound attenuation in a flow duct using a thin slow sound material | We present a thin subwavelength material that can be flush mounted to a duct
and which gives a large wide band attenuation at remarkably low frequencies in
air flow channels. To decrease the material thickness, the sound is slowed in
the material using folded side branch tubes. The impedance of the material is
compared to the optimal value, which differs greatly from the characteristic
impedance. In particular, the viscous and thermal effects have to be very small
to have high transmission losses. Grazing flow on this material increases the
losses at the interface between the flow and the material. | 1606.01877v1 |
2005-06-06 | Fast high--voltage amplifiers for driving electro-optic modulators | We describe five high-voltage (60 to 550V peak to peak), high-speed (1-300ns
rise time; 1.3-300MHz bandwidth) linear amplifiers for driving capacitive or
resistive loads such as electro-optic modulators. The amplifiers use bipolar
transistors in various topologies. Two use electron tubes to overcome the speed
limitations of high-voltage semiconductors. All amplifiers have been built.
Measured performance data is given for each. | 0506050v1 |
2016-02-26 | Resistively detected high-order magnetoplasmons in a high-quality 2D electron gas | We report on high-order magnetoplasmon resonances detected in photoresistance
in high-mobility GaAs quantum wells. These resonances manifest themselves as a
series of photoresistance extrema in the regime of Shubnikov-de Haas
oscillations. Extending to orders above 20, the extrema exhibit alternating
strength, being less (more) pronounced at even (odd) order magnetoplasmon
modes. This experimental technique provides sensitive and elegant means to
detect and investigate multiple magnetoplasmon modes and could be applied to
other systems. | 1602.08407v1 |
2023-01-17 | High Speed Parallel Signal Crosstalk Cancellation Concept | High performance computing (HPC) systems make extensive use of high speed
electrical interconnects, in routing signals among processing elements, or
between processing elements and memory. Increasing bandwidth demands result in
high density, parallel I/O exposed to crosstalk due to tightly coupled
transmission lines. The crosstalk cancellation signaling concept discussed in
this paper utilizes the known, predictable theory of coupled transmission lines
to cancel crosstalk from neighboring traces with carefully chosen resistive
cross-terminations between them. Through simulation and analysis of practical
bus architectures, we explore the merits of crosstalk cancellation which could
be used in dense interconnect HPC (or other) applications. | 2301.10170v1 |
2017-10-04 | A Characterization of Effective Resistance Metrics | We produce a characterization of finite metric spaces which are given by the
effective resistance of a graph. This characterization is applied to the more
general context of resistance metrics defined by Kigami. A countably infinite
resistance metric gives rise to a sequence of finite, increasing graphs with
invariant effective resistance. We show that these graphs have a unique limit
graph in terms of the convergence of edge weights and that their associated
random walks converge weakly to the random walk on the limit graph. If the
limit graph is recurrent, its effective resistance is identified as the initial
resistance metric. | 1710.01587v2 |
2017-12-22 | Bacterial cooperation leads to heteroresistance | By challenging E. coli with sublethal norfloxacin for 10 days, Henry Lee and
James Collins suggests the bacterial altruism leads to the population-wide
resistance. By detailedly analyzing experiment data, we suggest that bacterial
cooperation leads to population-wide resistance under norfloxacin pressure and
simultaneously propose the bacteria shield is the possible feedback mechanism
of less resistant bacteria. The bacteria shield is that the less resistant
bacteria sacrifice the large number of themselves to consume norfloxacin and
then to relieve the norfloxacin burden from highly resistant bacteria. Thus,
due to highly resistant bacteria and less resistant bacteria extracted from the
same bacteria population, bacterial cooperation leads to heteroresistance. | 1712.08309v1 |
2005-08-31 | Spinel ferrites: old materials bring new opportunities for spintronics | Over the past few years, intensive studies of ultrathin epitaxial films of
perovskite oxides have often revealed exciting properties like giant
magnetoresistive tunnelling and electric field effects. Spinel oxides appear as
even more versatile due to their more complex structure and the resulting many
degrees of freedom. Here we show that the epitaxial growth of nanometric
NiFe2O4 films onto perovskite substrates allows the stabilization of novel
ferrite phases with properties dramatically differing from bulk ones. Indeed,
NiFe2O4 films few nanometres thick have a saturation magnetization at least
twice that of the bulk compound and their resistivity can be tuned by orders of
magnitude, depending on the growth conditions. By integrating such thin NiFe2O4
layers into spin-dependent tunnelling heterostructures, we demonstrate that
this versatile material can be useful for spintronics, either as a conductive
electrode in magnetic tunnel junctions or as a spin-filtering insulating
barrier in the little explored type of tunnel junction called spin-filter. Our
findings are thus opening the way for the realisation of monolithic spintronics
architectures integrating several layers of a single material, where the layers
are functionalised in a controlled manner. | 0508764v1 |
2007-05-10 | Influence of oxygen partial pressure on structural, transport and magnetic properties of Co doped TiO2 films | Thin films of Co-TiO2 are deposited on silicon and quartz substrates using
Pulse Laser Deposition (PLD) process at various oxygen partial pressures
ranging from 6.6 x 10-3 Pascals (Pa) to 53 Pa. Crystal structure, transport and
magnetic properties of reduced CoxTi(1-x)O2 (0 <x< 0.03) thin films are
investigated and are found to have a strong dependence on the oxygen partial
pressure. X-ray diffraction (XRD) data reveals the presence of mixed phase
material containing both anatase and rutile. However, these phases
intertransform with the change in the oxygen partial pressure in the chamber
during the growth of the films. X-ray Photoelectron Spectroscopy (XPS) shows no
Co or CoO related peaks for samples with Co concentration up to x=0.03.
However, the oxygen 1s peaks are asymmetric suggesting the presence of oxygen
vacancies. The transport and magnetic measurements show a clear dependence on
the concentration of oxygen vacancies. There is an enhancement in the
electrical conductivity and the magnetization as more vacancies are created in
the material. The resistivity as a function of temperature rho(T) follows the
polaronic behavior and the activation energies obtained, ~100 to 150meV, are
within the range that is typical for semiconducting materials. | 0705.1527v1 |
2008-11-25 | Structural and physical properties of $SrMn_{1-x}Ru_xO_3$ perovskites | We combine the results of magnetic and transport measurements with neutron
diffraction data to construct the structural and magnetic phase diagram of the
entire family of SrMn$_{1-x}$Ru$_{x}$O$_3$ ($0 \leqslant x \leqslant 1$)
perovskites. We have found antiferromagnetic ordering of the C type for lightly
Ru-substituted materials ($0.06 \leqslant x \leqslant 0.5$) in a similar manner
to $R_{y}$Sr$_{1-y}$MnO$_3$ ($R$=La, Pr), due to the generation of Mn$^{3+}$ in
both families of manganite perovskites by either $B$-site substitution of
Ru$^{5+}$ for Mn$^{4+}$ or $A$-site substitution of $R^{3+}$ for Sr$^{2+}$.
This similarity is driven by the same ratio of $d^4$ / $d^3$ ions in both
classes of materials for equivalent substitution level. In both cases, a
tetragonal lattice distortion is observed, which for some compositions ($0.06
\leqslant x \leqslant 0.2$) is coupled to a C-type AF transition and results in
a first order magnetic and resistive transition. Heavily substituted
SrMn$_{1-x}$Ru$_{x}$O$_3$ materials are ferromagnetic due to dominating
exchange interactions between the Ru$^{4+}$ ions. Intermediate substitution
($0.6 \leqslant x \leqslant 0.7$) leads to a spin-glass behavior instead of a
quantum critical point reported previously in single crystals, due to enhanced
disorder. | 0811.4181v1 |
2009-10-09 | Three-dimensional jamming and flows of soft glassy materials | Various disordered dense systems such as foams, gels, emulsions and colloidal
suspensions, exhibit a jamming transition from a liquid state (they flow) to a
solid state below a yield stress. Their structure, thoroughly studied with
powerful means of 3D characterization, exhibits some analogy with that of
glasses which led to call them soft glassy materials. However, despite its
importance for geophysical and industrial applications, their rheological
behavior, and its microscopic origin, is still poorly known, in particular
because of its nonlinear nature. Here we show from two original experiments
that a simple 3D continuum description of the behaviour of soft glassy
materials can be built. We first show that when a flow is imposed in some
direction there is no yield resistance to a secondary flow: these systems are
always unjammed simultaneously in all directions of space. The 3D jamming
criterion appears to be the plasticity criterion encountered in most solids. We
also find that they behave as simple liquids in the direction orthogonal to
that of the main flow; their viscosity is inversely proportional to the main
flow shear rate, as a signature of shear-induced structural relaxation, in
close similarity with the structural relaxations driven by temperature and
density in other glassy systems. | 0910.1821v2 |
2012-08-04 | Microscopic theory of the glassy dynamics of passive and active network materials | Signatures of glassy dynamics have been identified experimentally for a rich
variety of materials in which molecular networks provide rigidity. Here we
present a theoretical framework to study the glassy behavior of both passive
and active network materials. We construct a general microscopic network model
that incorporates nonlinear elasticity of individual filaments and steric
constraints due to crowding. Based on constructive analogies between structural
glass forming liquids and random field Ising magnets implemented using a
heterogeneous self-consistent phonon method, our scheme provides a microscopic
approach to determine the mismatch surface tension and the configurational
entropy, which compete in determining the barrier for structural rearrangements
within the random first order transition theory of escape from a local energy
minimum. The influence of crosslinking on the fragility of inorganic network
glass formers is recapitulated by the model. For active network materials, the
mapping, which correlates the glassy characteristics to the network
architecture and properties of nonequilibrium motor processes, is shown to
capture several key experimental observations on the cytoskeleton of living
cells: Highly connected tense networks behave as strong glass formers; intense
motor action promotes reconfiguration. The fact that our model assuming a
negative motor susceptibility predicts the latter suggests that on average the
motorized processes in living cells do resist the imposed mechanical load. Our
calculations also identify a spinodal point where simultaneously the mismatch
penalty vanishes and the mechanical stability of amorphous packing disappears. | 1208.0880v1 |
2014-09-24 | Field tunable spin density wave phases in Sr3Ru2O7 | The conduction electrons in a metal experience competing interactions with
each other and the atomic nuclei. This competition can lead to many types of
magnetic order in metals. For example, in chromium the electrons order to form
a spin-density-wave (SDW) antiferromagnetic state. A magnetic field may be used
to perturb or tune materials with delicately balanced electronic interactions.
Here we show that the application of a magnetic field can induce SDW magnetic
order in a metal, where none exists in the absence of the field. We use
magnetic neutron scattering to show that the application of a large (~8T)
magnetic field to the metamagnetic perovskite metal Sr3Ru2O7 can be used to
tune the material through two magnetically-ordered SDW states. The ordered
states exist over relatively small ranges in field (<0.4T) suggesting that
their origin is due to a new mechanism related to the electronic fine structure
near the Fermi energy, possibly combined with the stabilising effect of
magnetic fluctuations. The magnetic field direction is shown to control the SDW
domain populations which naturally explains the strong resistivity anisotropy
or electronic nematic behaviour observed in this material. | 1409.7054v2 |
2014-12-12 | Magnetoresistance from broken spin helicity | The propensity of some materials and multilayers to have a magnetic field
dependent resistance, called magnetoresistance, has found commercial
applications such as giant magnetoresistance harddisk read heads. But
magnetoresistance can also be a powerful probe of electronic and magnetic
interactions in matter. For example, magnetoresistance can be used to analyze
multiband conductivity, conduction inhomogeneity, localized magnetic moments,
and (fractional) Landau level structure. For materials with strong spin-orbit
interaction, magnetoresistance can be used as a probe for weak antilocalization
or a nontrivial Berry phase, such as in topological insulator surface states.
For the three dimensional topological insulators a large and linear
magnetoresistance is often used as indication for underlying non-trivial
topology, although the origin of this effect has not yet been established.
Here, we observe a large magnetoresistance in the conducting bulk state of
Bi$_2$Te$_3$. We show that this type of large magnetoresistance is due to the
competition between helical spin-momentum locking (i.e. spin rotates with
momentum direction) and the unidirectional spin alignment by an applied
magnetic field. Warping effects are found to provide the (quasi) linear
dependence on magnetic field. We provide a quantitative model for the helicity
breaking induced magnetoresistance that can be applied to a vast range of
materials, surfaces or interfaces with weak to strong spin-orbit interactions,
such as the contemporary oxide interfaces, bulk Rashba systems, and topological
insulator surface states. | 1412.4065v1 |
2015-09-16 | Photoinduced Kondo effect in CeZn$_{3}$P$_{3}$ | The Kondo effect, which originates from the screening of a localized magnetic
moment by a spin-spin interaction, is widely observed in non-artificial
magnetic materials, artificial quantum dots, and carbon nanotubes. In devices
based on quantum dots or carbon nanotubes that target quantum information
applications, the Kondo effect can be tuned by a gate voltage, a magnetic
field, or light. However, the manipulation of the Kondo effect in
non-artificial materials has not been thoroughly studied; in particular, the
artificial creation of the Kondo effect remains unexplored. Per this subject
study, however, a new route for the optical creation of the Kondo effect in the
non-artificial material $p$-type semiconductor CeZn$_{3}$P$_{3}$ is presented.
The Kondo effect emerges under visible-light illumination of the material by a
continuous-wave laser diode and is ultimately revealed by photoinduced
electrical resistivity, which clearly exhibits a logarithmic temperature
dependency. By contrast, a La-based compound (LaZn$_{3}$P$_{3}$) displays only
normal metallic behavior under similar illumination. The photoinduced Kondo
effect, which occurs at higher temperatures when compared with the Kondo effect
in artificial systems, provides a potential new range of operation for not only
quantum information/computation devices but also for operation of magneto-optic
devices thereby expanding the range of device applications based on the Kondo
effect. | 1509.04909v2 |
2015-11-06 | Finite size effects on crack front pinning at heterogeneous planar interfaces: Experimental, finite elements and perturbation approaches | Understanding the role played by the microstructure of materials on their
macroscopic failure properties is an important challenge in solid mechanics.
Indeed, when a crack propagates at a heterogeneous brittle interface, the front
is trapped by tougher regions and deforms. This pinning induces non-linearities
in the crack propagation problem, even within Linear Elastic Fracture Mechanics
theory, and modifies the overall failure properties of the material. For
example crack front pinning by tougher places could increase the fracture
resistance of multilayer structures, with interesting technological
applications. Analytical perturbation approaches, based on Bueckner-Rice
elastic line models, focus on the crack front perturbations, hence allow for a
description of these phenomena. Here, they are applied to experiments
investigating the propagation of a purely interfacial crack in a simple
toughness pattern: a single defect strip surrounded by homogeneous interface.
We show that by taking into account the finite size of the body, quantitative
agreement with experimental and finite elements results is achieved. In
particular this method allows to predict the toughness contrast, i.e. the
toughness difference between the single defect strip and its homogeneous
surrounding medium. This opens the way to a more accurate use of the
perturbation method to study more disordered heterogeneous materials, where the
finite elements method is less adequate. From our results, we also propose a
simple method to determine the adhesion energy of tough interfaces by measuring
the crack front deformation induced by known interface patterns. | 1511.02050v1 |
2016-05-26 | Measurement Techniques for Thermal Conductivity and Interfacial Thermal Conductance of Bulk and Thin Film Materials | Thermal conductivity and interfacial thermal conductance play crucial roles
in the design of engineering systems where temperature and thermal stress are
of concerns. To date, a variety of measurement techniques are available for
both bulk and thin film solid-state materials with a broad temperature range.
For thermal characterization of bulk material, the steady-state absolute
method, laser flash diffusivity method, and transient plane source method are
most used. For thin film measurement, the 3{\omega} method and transient
thermoreflectance technique including both frequency-domain and time-domain
analysis are employed widely. This work reviews several most commonly used
measurement techniques. In general, it is a very challenging task to determine
thermal conductivity and interface contact resistance with less than 5% error.
Selecting a specific measurement technique to characterize thermal properties
need to be based on: 1) knowledge on the sample whose thermophysical properties
is to be determined, including the sample geometry and size, and preparation
method; 2) understanding of fundamentals and procedures of the testing
technique and equipment, for example, some techniques are limited to samples
with specific geometrics and some are limited to specific range of
thermophysical properties; 3) understanding of the potential error sources
which might affect the final results, for example, the convection and radiation
heat losses. | 1605.08469v2 |
2016-04-02 | Modeling Mechanical Properties of Aluminum Composite Produced Using Stir Casting Method | ANN (Artificial Neural Networks) modeling methodology was adopted for
predicting mechanical properties of aluminum cast composite materials. For this
purpose aluminum alloy were developed using conventional foundry method. The
composite materials have complex nature which posses the nonlinear relationship
among heat treatment, processing parameters, and composition and affects their
mechanical properties. These nonlinear relation ships with properties can more
efficiently be modeled by ANNs. Neural networks modeling needs sufficient data
base consisting of mechanical properties, chemical composition and processing
parameters. Such data base is not available for modeling. Therefore, a large
range of experimental work was carried out for the development of aluminum
composite materials. Alloys containing Cu, Mg and Zn as matrix were reinforced
with 1- 15% Al2O3 particles using stir casting method. Alloys composites were
cast in a metal mold. More than eighty standard samples were prepared for
tensile tests. Sixty samples were given solution treatments at 580oC for half
an hour and tempered at 120oC for 24 hours. The samples were characterized to
investigate mechanical properties using Scanning Electron Microscope, X-Ray
Spectrometer, Optical Metallurgical Microscope, Vickers Hardness, Universal
Testing Machine and Abrasive Wear Testing Machine. A MLP (Multilayer
Perceptron) feedforward was developed and used for modeling purpose. Training,
testing and validation of the model were carried out using back propagation
learning algorithm. The modeling results show that an architecture of 14 inputs
with 9 hidden neurons and 4 outputs which includes the tensile strength,
elongation, hardness and abrasive wear resistance gives reasonably accurate
results with an error within the range of 2-7 % in training, testing and
validation. | 1605.09691v1 |
2016-07-26 | Superconductivity in CaBi$_{2}$ | Superconductivity is observed with critical temperature $T_{c}$ = 2.0 K in
self-flux-grown single crystals of $CaBi_{2}$. This material adopts the
$ZrSi_{2}$ structure type with lattice parameters a = 4.696(1) $\AA$, b =
7.081(2) $\AA$ and c = 4.611(1) $\AA$. The crystals of $CaBi_{2}$ were studied
by means of magnetic susceptibility, specific heat and electrical resistivity
measurements. The heat capacity jump at $T_{c}$ is $\Delta C/\gamma T_{c}$ =
1.41, confirming bulk superconductivity; the Sommerfeld coefficient $\gamma$ =
4.1 $mJ\: mol^{-1}\, K^{-2}$ and the Debye temperature $\Theta_{D}$ = 157 K.
The electron-phonon coupling strength is $\lambda_{el-ph}$ = 0.59, and the
thermodynamic critical field $H_{c}$ is low, between 111 and 124 Oe $CaBi_{2}$
is a moderate coupling type-I superconductor. Results of electronic structure
calculations are reported and charge densities, electronic bands, densities of
states and Fermi surfaces are discussed, focusing on the effects of
spin\textendash orbit coupling and electronic property anisotropy. We find a
mixed quasi-2D + 3D character in the electronic structure, which reflects the
layered crystal structure of the material. | 1607.07603v1 |
2016-08-21 | Unusual interlayer quantum transport behavior caused by the zeroth Landau level in YbMnBi2 | Relativistic fermions in topological quantum materials are characterized by
linear energy-momentum dispersion near band crossing points. Under magnetic
field, relativistic fermions acquire Berry phase of {\pi} in cyclotron motion,
leading to a zeroth Landau level (LL) at the crossing point. Such
field-independent zeroth LL, which distinguishes relativistic fermions from
conventional electron systems, is hardly probed in transport measurements since
the Fermi energy (EF) is usually not right at the band crossing points in most
topological materials. Here we report the observation of exotic quantum
transport behavior resulting from the zeroth LL in a multiband topological
semimetal YbMnBi2 which possesses linear band crossings both at and away from
the Fermi level (FL). We show that the Dirac bands with the crossing points
being above or below the FL leads to Shubnikov de-Haas oscillations in the
in-plane magnetoresistance, whereas the Dirac bands with the crossing points
being at the FL results in unusual angular dependences of the out-of-plane
magnetoresistance and in-plane Hall resistivity due to the dependence of the
zeroth LL's degeneracy on field orientation. Our results shed light on the
transport mechanism of the zeroth LL's relativistic fermions in layered
materials. | 1608.05956v1 |
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