publicationDate stringlengths 10 10 | title stringlengths 17 233 | abstract stringlengths 20 3.22k | id stringlengths 9 12 |
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2012-02-14 | Precision comparison of the quantum Hall effect in graphene and gallium arsenide | The half-integer quantum Hall effect in epitaxial graphene is compared with
high precision to the well known integer effect in a GaAs/AlGaAs
heterostructure. We find no difference between the quantised resistance values
within the relative standard uncertainty of our measurement of $8.7\times
10^{-11}$. The result places new tighter limits on any possible correction
terms to the simple relation $R_{\rm K}=h/e^2$, and also demonstrates that
epitaxial graphene samples are suitable for application as electrical
resistance standards of the highest metrological quality. We discuss the
characterisation of the graphene sample used in this experiment and present the
details of the cryogenic current comparator bridge and associated uncertainty
budget. | 1202.2985v1 |
2012-02-14 | Pressure suppression of unconventional charge-density-wave state in PrRu4P12 studied by optical conductivity | Optical conductivity s(w) of PrRu4P12 has been studied under high pressure to
14 GPa, at low temperatures to 8 K, and at photon energies 12 meV-1.1 eV. The
energy gap in s(w) at ambient pressure, caused by a metal-insulator transition
due to an unconventional charge-density-wave formation at 63 K, is gradually
filled in with increasing pressure to 10 GPa. At 14 GPa and below 30 K, s(w)
exhibits a pronounced Drude-type component due to free carriers. This indicates
that the initial insulating ground state at zero pressure has been turned into
a metallic one at 14 GPa. This is consistent with a previous resistivity study
under pressure, where the resistivity rapidly decreased with cooling below 30 K
at 14 GPa. The evolution of electronic structure with pressure is discussed in
terms of the hybridization between the 4f and conduction electrons. | 1202.3007v3 |
2012-02-16 | Approximate strange metallic behavior in AdS | We show for unit dynamical exponent, $z=1$, the appearance of the Fermi
liquid and non-Fermi liquid behavior as we tune the charge density and the
magnetic field in 3+1 dimensional field theory using the gauge-gravity duality.
There exists an upturn behavior of the resistivity only along the direction
perpendicular to the magnetic field. Also, there exists a universal behavior of
the resistivity, independent of the dimensionality of the spacetime, in a
specific corner of the parameter space, namely, in the large charge density and
small magnetic field limit: the longitudinal conductivity goes as $T^{-2/z}$,
whereas the Hall conductivity goes as $T^{-4/z}$. It means the Hall coefficient
goes as $T^{4/z}$. We compute the diffusion constant from the flow equation of
the conductivity. | 1202.3555v1 |
2012-04-23 | Cyclopentadienyl-Benzene Based Sandwich Molecular Wires Showing Efficient Spin Filtering, Negative Differential Resistance, and Pressure Induced Electronic Transitions | Using density functional theory, we investigate TM-cyclopentadienyl-benzene
sandwich molecular wires (SMWs) which are composites of TM-cyclopentadienyl and
TM-benzene wires (TM = transition metal (V, Fe)). All the SMWs are found to be
highly stable ferromagnetic half-metals, showing spin switching behavior.
Transport calculations show that finite size clusters display spin filter
property when coupled with Au electrodes on either side. I-V characteristics of
all systems confirm the spin filter property, with Au-BzVCpVBz-Au displaying
exceptionally high performance. In addition to spin filtering, the
Au-BzFeCpFeBz-Au system also shows negative differential resistance (NDR).
Compression causes an abrupt reduction in magnetic moment and a transition to a
metallic phase, while stretching causes an increase in magnetic moment.
Half-metallicity is preserved for modest amounts of stretching and compression. | 1204.5047v1 |
2012-05-17 | Microwave heating-induced DC magnetic flux penetration in YBa$_{2}$Cu$_{3}$O$_{7-δ}$ superconducting thin films | The magneto-optical imaging technique is used to visualize the penetration of
the magnetic induction in YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ thin films during
surface resistance measurements. The in-situ surface resistance measurements
were performed at 7 GHz using the dielectric resonator method. When only the
microwave magnetic field $H_{rf}$ is applied to the superconductor, no
$H_{rf}$-induced vortex penetration is observed, even at high rf power. In
contrast, in the presence of a constant magnetic field superimposed on $H_{rf}$
we observe a progression of the flux front as $H_{rf}$ is increased. A local
thermometry method based on the measurement of the resonant frequency of the
dielectric resonator placed on the YBa$_{2}$Cu$_{3}$O$_{7-\delta}$ thin film
shows that the $H_{rf}$--induced flux penetration is due to the increase of the
film temperature. | 1205.3962v1 |
2012-07-04 | Kondo-like behavior near the metal-to-insulator transition of nano-scale granular aluminum | We show that the normal state transport properties of nano-scale granular
Aluminum films, near the metal to insulator transition, present striking
similarities with those of Kondo systems. Those include a negative
magneto-resistance, a minimum of resistance R at a temperature Tm in metallic
films, a logarithmic rise at low temperatures and a negative curvature of R(T)
at high temperatures. These normal state properties are interpreted in terms of
spin-flip scattering of conduction electrons by local magnetic moments,
possibly located at the metal/oxide interfaces. Their co-existence with the
enhanced superconductivity seen in these films is discussed. | 1207.0970v3 |
2012-07-19 | Simulation Studies of Charge Transport on Resistive Structures in Gaseous Ionization Detectors | We developed a tool for the simulation of charge transport on a conducting
plate of finite dimensions. This tool is named Chani. Main motivation of
developing Chani was to provide a tool for the optimization of the dimensions
and resistivity of the anode electrodes in spark-protected Micropattern Gaseous
Detectors (MPGD). In this thesis, we start with the general description of the
LHC and the ATLAS Experiment. Then, we review the gaseous ionization detector
technologies and in particular, the micromegas technology. We then present the
working principles of Chani along with the example calculations. These examples
include comparisons with the analytically solvable problems which shows that
the simulation results are reasonable. | 1207.4585v1 |
2012-07-20 | Multi-Band Exotic Superconductivity in the New Superconductor Bi4O4S3 | Resistivity, Hall effect and magnetization have been investigated on the new
superconductor Bi4O4S3. A weak insulating behavior has been induced in the
normal state when the superconductivity is suppressed. Hall effect measurements
illustrate clearly a multiband feature dominated by electron charge carriers,
which is further supported by the magnetoresistance data. Interestingly, a kink
appears on the temperature dependence of resistivity at about 4 K at all high
magnetic fields when the bulk superconductivity is completely suppressed. This
kink can be well traced back to the upper critical field Hc2(T) in the low
field region, and is explained as the possible evidence of residual Cooper
pairs on the one dimensional chains. | 1207.4955v1 |
2012-08-14 | Superconductivity in Bismuth Oxysulfide Bi4O4S3 | Bismuth oxysulfide Bi4O4S3, which has recently been claimed to be an exotic
superconductor (Tc = 4.5 K), was investigated by magnetic susceptibility and
electrical resistivity measurements as well as by electron probe microanalysis.
Single-phase Bi4O4S3 was successfully prepared by a high-pressure method, and
its lattice parameters and normal-state resistivity, as well as the density of
states at the Fermi level, were found to be comparable to those determined
earlier. However, the observed superconductivity was most likely
impurity-driven, strictly contradictory to the observations in ongoing
experiments. The present results indicate that the superconductivity of Bi4O4S3
does not truly reflect the bulk nature of the BiS2 layered phase, regardless of
the manner in which Bi4O4S3 is synthesized. We discuss possible superconducting
impurities. | 1208.2818v3 |
2012-08-15 | Double negative differential thermal resistance induced by the nonlinear on-site potentials | We study heat conduction through one-dimensional homogeneous lattices in the
presence of the nonlinear on-site potentials containing the bounded and
unbounded parts, and the harmonic interaction potential. We observe the
occurrence of double negative differential thermal resistance (NDTR), namely,
there exist two regions of temperature difference, where the heat flux
decreases as the applied temperature difference increases. The nonlinearity of
the bounded part contributes to NDTR at low temperatures and NDTR at high
temperatures is induced by the nonlinearity of the unbounded part. The
nonlinearity of the on-site potentials is necessary to obtain NDTR for the
harmonic interaction homogeneous lattices. However, for the anharmonic
homogeneous lattices, NDTR even occurs in the absence of the on-site
potentials, for example the rotator model. | 1208.3008v1 |
2012-09-17 | Hair of astrophysical black holes | The "no hair" theorem is not applicable to black holes formed from collapse
of a rotating neutron star. Rotating neutron stars can self-produce particles
via vacuum breakdown forming a highly conducting plasma magnetosphere such that
magnetic field lines are effectively "frozen-in" the star both before and
during collapse. In the limit of no resistivity, this introduces a topological
constraint which prohibits the magnetic field from sliding off the newly-formed
event horizon. As a result, during collapse of a neutron star into a black
hole, the latter conserves the number of magnetic flux tubes N_B = e
\Phi_\infty /(\pi c \hbar), where \Phi_\infty is the initial magnetic flux
through the hemispheres of the progenitor and out to infinity. The black hole's
magnetosphere subsequently relaxes to the split monopole magnetic field
geometry with self-generated currents outside the event horizon. The
dissipation of the resulting equatorial current sheet leads to a slow loss of
the anchored flux tubes, a process that balds the black hole on long resistive
time scales rather than the short light-crossing time scales expected from the
vacuum "no-hair" theorem. | 1209.3785v1 |
2012-09-24 | Non-stationary Magnetic Microstructures in Stellar Thin Accretion Discs | We examine the morphology of magnetic structures in thin plasma accretion
discs, generalizing a stationary ideal MHD model to the time-dependent
visco-resistive case. Our analysis deals with small scale perturbations to a
central dipole-like magnetic field, which give rise -- as in the ideal case --
to the periodic modulation of magnetic flux surfaces along the radial
direction, corresponding to the formation of a toroidal current channels
sequence. These microstructures suffer an exponential damping in time because
of the non-zero resistivity coefficient, allowing us to define a configuration
lifetime which mainly depends on the midplane temperature and on the length
scale of the structure itself. By means of this lifetime we show that the
microstructures can exist within the inner region of stellar discs in a precise
range of temperatures, and that their duration is consistent with local
transient processes (minutes to hours). | 1209.5227v2 |
2012-09-29 | Contact induced spin relaxation in Hanle spin precession measurements | In the field of spintronics the "conductivity mismatch" problem remains an
important issue. Here the difference between the resistance of ferromagnetic
electrodes and a (high resistive) transport channel causes injected spins to be
backscattered into the leads and to lose their spin information. We study the
effect of the resulting contact induced spin relaxation on spin transport, in
particular on non-local Hanle precession measurements. As the Hanle line shape
is modified by the contact induced effects, the fits to Hanle curves can result
in incorrectly determined spin transport properties of the transport channel.
We quantify this effect that mimics a decrease of the spin relaxation time of
the channel reaching more than 4 orders of magnitude and a minor increase of
the diffusion coefficient by less than a factor of 2. Then we compare the
results to spin transport measurements on graphene from the literature. We
further point out guidelines for a Hanle precession fitting procedure that
allows to reliably extract spin transport properties from measurements. | 1210.0093v1 |
2012-10-02 | Magnetic structure of Coronal Mass Ejections | We present several models of the magnetic structure of solar coronal mass
ejections (CMEs). First, we model CMEs as expanding force-free magnetic
structures. While keeping the internal magnetic field structure of the
stationary solutions, expansion leads to complicated internal velocities and
rotation, while the field structures remain force-free.
Second, expansion of a CME can drive resistive dissipation within the CME
changing the ionization states of different ions. We fit in situ measurements
of ion charge states to the resistive spheromak solutions.
Finally, we consider magnetic field structures of fully confined stable
magnetic clouds containing both toroidal and poloidal magnetic fields and
having no surface current sheets. Expansion of such clouds may lead to sudden
onset of reconnection events. | 1210.0918v1 |
2012-10-25 | The RPC-based proposal for the ATLAS forward muon trigger upgrade in view of super-LHC | The innermost station of the present ATLAS forward muon detector needs to be
upgraded for the super-LHC. We present a proposal to replace it with a
sandwiched detector composed of several layers of small-radius Monitored Drift
Tube chambers (sMDT) for precision tracking measurement and two stations of
multi-gap Resistive Plate Chambers (mRPC) for triggering purpose. We describe
the layout of the upgraded detector and the trigger strategy. Several
modifications to the RPCs used in the ATLAS barrel region are needed to satisfy
the super-LHC requirements. Various studies with the proposed mRPC timing
resolution, spatial resolution and rate capability have been performed. | 1210.6728v1 |
2012-12-06 | The Reconstruction Algorithm Study of 2D Interpolating Resistive Readout Structure | Systematic investigations including both simulation and prototype tests have
been done about the interpolating resistive readout structure with GEM (Gaseous
Electron Multiplier) detector. From the simulation, we have a good knowledge of
the process of charges diffusion on the surface of the readout plane and
develop several reconstruction methods to determine the hit position. The total
signal duration time of a typical event with the readout structure is about
several hundred nanoseconds, which implied an ideal count rate up to 106Hz. A
stable worked prototype was designed and fabricated after the simulation. Using
55Fe 5.9keV X-Ray, the image performance of the prototype is examined with flat
field image and some special geometry shapes, meanwhile, an energy resolution
of about 17% is obtained. | 1212.1220v1 |
2012-12-12 | Metal-insulator transition in holography | We exhibit an interaction-driven metal-insulator quantum phase transition in
a holographic model. Use of a helical lattice enables us to break translation
invariance while preserving homogeneity. The metallic phase is characterized by
a sharp Drude peak and a d.c. resistivity that increases with temperature. In
the insulating phase the Drude spectral weight is transferred into a
`mid-infrared' peak and to energy scales of order the chemical potential. The
d.c. resistivity now decreases with temperature. In the metallic phase,
operators breaking translation invariance are irrelevant at low energy scales.
In the insulating phase, translation symmetry breaking effects are present at
low energies. We find the near horizon extremal geometry that captures the
insulating physics. | 1212.2998v2 |
2013-01-06 | Anomalous response to gate voltage application in mesoscopic LaAlO_3/SrTiO_3 devices | We report on resistivity and Hall measurements performed on a series of
narrow mesa devices fabricated from LaAlO_3/SrTiO_3 single interface
heterostructure with a bridge width range of 1.5-10 microns. Upon applying
back-gate voltage of the order of a few Volts, a strong increase in the sample
resistance (up to factor of 35) is observed, suggesting a relatively large
capacitance between the Hall-bar and the gate. The high value of this
capacitance is due to the device geometry, and can be explained within an
electrostatic model using the Thomas Fermi approximation. The Hall coefficient
is sometimes a non-monotonic function of the gate voltage. This behavior is
inconsistent with a single conduction band model. We show that a theoretical
two-band model is consistent with this transport behavior, and indicates a
metal to insulator transition in at least one of these bands. | 1301.1055v1 |
2013-04-22 | Stochastic Memristive Devices for Computing and Neuromorphic Applications | Nanoscale resistive switching devices (memristive devices or memristors) have
been studied for a number of applications ranging from non-volatile memory,
logic to neuromorphic systems. However a major challenge is to address the
potentially large variations in space and in time in these nanoscale devices.
Here we show that in metal-filament based memristive devices the switching can
be fully stochastic. While individual switching events are random, the
distribution and probability of switching can be well predicted and controlled.
Rather than trying to force high switching probabilities using excessive
voltage or time, the inherent stochastic nature of resistive switching allows
these binary devices to be used as building blocks for novel error-tolerant
computing schemes such as stochastic computing and provide a needed "analog"
feature in neuromorphic applications. To verify such potential, we demonstrated
memristor-based stochastic bitstreams in both time and space domains, and show
that an array of binary memristors can act as a multi-level "analog" device for
neuromorphic applications. | 1304.5993v1 |
2013-04-27 | Attacks and Countermeasures in Fingerprint Based Biometric Cryptosystems | We investigate implementations of biometric cryptosystems protecting
fingerprint templates (which are mostly based on the fuzzy vault scheme by
Juels and Sudan in 2002) with respect to the security they provide. We show
that attacks taking advantage of the system's false acceptance rate, i.e.
false-accept attacks, pose a very serious risk --- even if brute-force attacks
are impractical to perform. Our observations lead to the clear conclusion that
currently a single fingerprint is not sufficient to provide a secure biometric
cryptosystem. But there remain other problems that can not be resolved by
merely switching to multi-finger: Kholmatov and Yanikoglu in 2007 demonstrated
that it is possible to break two matching vault records at quite a high rate
via the correlation attack.
We propose an implementation of a minutiae fuzzy vault that is inherently
resistant against cross-matching and the correlation attack. Surprisingly,
achieving cross-matching resistance is not at the cost of authentication
performance. In particular, we propose to use a randomized decoding procedure
and find that it is possible to achieve a GAR=91% at which no false accepts are
observed on a database generally used. Our ideas can be adopted into an
implementation of a multibiometric cryptosystem. All experiments described in
this paper can fully be reproduced using software available for download. | 1304.7386v1 |
2013-06-19 | Nematode Locomotion in Unconfined and Confined Fluids | The millimeter-long soil-dwelling nematode {\it C. elegans} propels itself by
producing undulations that propagate along its body and turns by assuming
highly curved shapes. According to our recent study [PLoS ONE \textbf{7},
e40121 (2012)] all these postures can be accurately described by a
piecewise-harmonic-curvature (PHC) model. We combine this curvature-based
description with highly accurate hydrodynamic bead models to evaluate the
normalized velocity and turning angles for a worm swimming in an unconfined
fluid and in a parallel-wall cell. We find that the worm moves twice as fast
and navigates more effectively under a strong confinement, due to the large
transverse-to-longitudinal resistance-coefficient ratio resulting from the
wall-mediated far-field hydrodynamic coupling between body segments. We also
note that the optimal swimming gait is similar to the gait observed for
nematodes swimming in high-viscosity fluids. Our bead models allow us to
determine the effects of confinement and finite thickness of the body of the
nematode on its locomotion. These effects are not accounted for by the
classical resistive-force and slender-body theories. | 1306.4423v1 |
2013-07-08 | Effects of the non-uniform initial environment and the guide field on the plasmoid instability | Effects of non-uniform initial mass density and temperature on the plasmoid
instability are studied via 2.5-dimensional resistive magnetohydrodynamic(MHD)
simulations. Our results indicate that the development of the plasmoid
instability is apparently prevented when the initial plasma density at the
center of the current sheet is higher than that in the upstream region. As a
result, the higher the plasma density at the center and the lower the plasma
$\beta$ in the upstream region, the higher the critical Lundquist number needed
for triggering secondary instabilities. When $\beta =0.2$, the critical
Lundquist number is higher than $10^4$. For the same Lundquist number, the
magnetic reconnection rate is lower for the lower plasma $\beta$ case.
Oppositely, when the initial mass density is uniform and the Lundquist number
is low, the magnetic reconnection rate turns out to be higher for the lower
plasma $\beta$ case. For the high Lundquist number case ($>10^4$) with uniform
initial mass density, the magnetic reconnection is not affected by the initial
plasma $\beta$ and the temperature distribution. Our results indicate that the
guide field has a limited impact on the plasmoid instability in resistive MHD. | 1307.1963v1 |
2013-08-05 | Explosive reconnection of double tearing modes in relativistic plasmas: application to the Crab flares | Magnetic reconnection associated to the double tearing mode (DTM) is
investigated by means of resistive relativistic magnetohydrodynamic (RRMHD)
simulations. A linearly unstable double current sheet system in two dimensional
cartesian geometry is considered. For initial perturbations of large enough
longitudinal wavelengths, a fast reconnection event is triggered by a secondary
instability that is structurally driven by the nonlinear evolution of the
magnetic islands. The latter reconnection phase and time scale appear to weakly
depend on the plasma resistivity and magnetization parameter. We discuss the
possible role of such explosive reconnection dynamics to explain the MeV flares
observed in the Crab pulsar nebula. Indeed the time scale and the critical
minimum wavelength give constraints on the Lorentz factor of the striped wind
and on the location of the emission region respectively. | 1308.0906v1 |
2014-02-05 | Thermal-inertial effects on magnetic reconnection in relativistic pair plasmas | The magnetic reconnection process is studied in relativistic pair plasmas
when the thermal and inertial properties of the magnetohydrodynamical fluid are
included. We find that in both Sweet-Parker and Petschek relativistic scenarios
there is an increase of the reconnection rate owing to the thermal-inertial
effects, both satisfying causality. To characterize the new effects we define a
thermal-inertial number which is independent of the relativistic Lundquist
number, implying that reconnection can be achieved even for vanishing
resistivity as a result of only thermal-inertial effects. The current model has
fundamental importance for relativistic collisionless reconnection, as it
constitutes the simplest way to get reconnection rates faster than those
accessible with the sole resistivity. | 1402.1115v2 |
2014-02-14 | Unification of the Pressure and Composition Dependence of Superconductivity in Ru substituted BaFe2As2 | Temperature dependent high pressure electrical resistivity studies has been
carried out on Ba(Fe_{1-x}Ru_{x})_{2}As_{2} single crystals with x = 0.12, 0.26
and 0.35, which correspond to under doped, optimally doped and over doped
composition regimes respectively. The evolution of structural/magnetic
(T_{S-M}) and superconducting transition (T_(c)) temperatures, with pressure
for various compositions have been obtained. The normal state resistivity has
been analyzed in terms of a model that incorporates both spin fluctuations and
the opening of the gap in the spin density wave (SDW)phase. It is shown that Tc
scales with the strength of the spin fluctuation, B, and T_{S-M} scales with
the SDW gap parameter, \Delta. This provides a prescription for the unification
of the composition and pressure induced superconductivity in BaFe2As2. | 1402.3387v1 |
2014-03-24 | Physics of Fully Depleted CCDs | In this work we present simple, physics-based models for two effects that
have been noted in the fully depleted CCDs that are presently used in the Dark
Energy Survey Camera. The first effect is the observation that the point-spread
function increases slightly with the signal level. This is explained by
considering the effect on charge-carrier diffusion due to the reduction in the
magnitude of the channel potential as collected signal charge acts to partially
neutralize the fixed charge in the depleted channel. The resulting reduced
voltage drop across the carrier drift region decreases the vertical electric
field and increases the carrier transit time. The second effect is the
observation of low-level, concentric ring patterns seen in uniformly
illuminated images. This effect is shown to be most likely due to lateral
deflection of charge during the transit of the photogenerated carriers to the
potential wells as a result of lateral electric fields. The lateral fields are
a result of space charge in the fully depleted substrates arising from
resistivity variations inherent to the growth of the high-resistivity silicon
used to fabricate the CCDs. | 1403.6185v1 |
2014-05-25 | Negative differential resistance in graphene-based ballistic field effect transistor with oblique top gate | Negative differential resistance (NDR) with room temperature
peak-valley-ratio of 8 has been observed in a ballistic field-effect-transistor
(FET) based on graphene, having an oblique top gate. Graphene FETs with a top
gate inclination angle of 45 degrees and a drain-source distance of 400 nm were
fabricated on a chip cut from a 4 inch graphene wafer grown by CVD. From the 60
measured devices, NDR was observed only in the regions where the CVD graphene
displays the Raman signature of defectless monolayers. In other specific
positions on the wafer, where graphene quality was not high enough and the
Raman signature indicated the presence of defects, the ballistic character of
transport is lost and the graphene FETs display nonlinear drain-voltage
dependences tuned by the top and back gate voltage. | 1405.6407v1 |
2014-06-26 | The value of monitoring to control evolving populations | Populations can evolve in order to adapt to external changes. The capacity to
evolve and adapt makes successful treatment of infectious diseases and cancer
difficult. Indeed, therapy resistance has quickly become a key challenge for
global health. Therefore, ideas of how to control evolving populations in order
to overcome this threat are valuable. Here we use the mathematical concepts of
stochastic optimal control to study what is needed to control evolving
populations. Following established routes to calculate control strategies, we
first study how a polymorphism can be maintained in a finite population by
adaptively tuning selection. We then introduce a minimal model of drug
resistance in a stochastically evolving cancer cell population and compute
adaptive therapies, where decisions are based on monitoring the response of the
tumor, which can outperform established therapy paradigms. For both case
studies, we demonstrate the importance of high-resolution monitoring of the
target population in order to achieve a given control objective: to control one
must monitor. | 1406.6957v1 |
2014-07-28 | Critical point for the CAF-F phase transition at charge neutrality in bilayer graphene | We report on magneto-transport measurements up to 30 T performed on a bilayer
graphene Hall bar, enclosed by two thin hexagonal boron nitride flakes. Our
high mobility sample exhibits an insulating state at neutrality point which
evolves into a metallic phase when a strong in-plane field is applied, as
expected for a transition from a canted antiferromagnetic to a ferromagnetic
spin ordered phase. For the first time we individuate a temperature-independent
crossing in the four-terminal resistance as a function of the total magnetic
field, corresponding to the critical point of the transition. We show that the
critical field scales linearly with the perpendicular component of the field,
as expected from the underlying competition between the Zeeman energy and
interaction-induced anisotropies. A clear scaling of the resistance is also
found and an universal behavior is proposed in the vicinity of the transition. | 1407.7425v1 |
2014-11-28 | The tearing mode instability of thin current sheets: the transition to fast reconnection in the presence of viscosity | This paper studies the growth rate of reconnection instabilities in thin
current sheets in the presence of both resistivity and viscosity. In a previous
paper, Pucci and Velli (2014), it was argued that at sufficiently high
Lundquist number S it is impossible to form current sheets with aspect ratios
L/a which scale as $L/a\sim S^\alpha$ with $\alpha > 1/3$ because the growth
rate of the tearing mode would then diverge in the ideal limit
$S\rightarrow\infty$. Here we extend their analysis to include the effects of
viscosity, (always present in numerical simulations along with resistivity) and
which may play a role in the solar corona and other astrophysical environments.
A finite Prandtl number allows current sheets to reach larger aspect ratios
before becoming rapidly unstable in pile-up type regimes. Scalings with
Lundquist and Prandtl numbers are discussed as well as the transition to
kinetic reconnection | 1412.0047v2 |
2014-12-02 | Two-stage magnetic-field-tuned superconductor-insulator transition in underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ | In the underdoped pseudogap regime of cuprate superconductors, the normal
state is commonly probed by applying a magnetic field ($H$). However, the
nature of the $H$-induced resistive state has been the subject of a long-term
debate, and clear evidence for a zero-temperature ($T=0$) $H$-tuned
superconductor-insulator transition (SIT) has proved elusive. Here we report
magnetoresistance measurements in underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$,
providing striking evidence for quantum critical behavior of the resistivity --
the signature of a $H$-driven SIT. The transition is not direct: it is
accompanied by the emergence of an intermediate state, which is a
superconductor only at $T=0$. Our finding of a two-stage $H$-driven SIT goes
beyond the conventional scenario in which a single quantum critical point
separates the superconductor and the insulator in the presence of a
perpendicular $H$. Similar two-stage $H$-driven SIT, in which both disorder and
quantum phase fluctuations play an important role, may also be expected in
other copper-oxide high-temperature superconductors. | 1412.1134v1 |
2015-01-15 | Non-ergodic metallic and insulating phases of Josephson junction chains | Strictly speaking the laws of the conventional Statistical Physics, based on
the Equipartition Postulate and Ergodicity Hypothesis, apply only in the
presence of a heat bath. Until recently this restriction was not important for
real physical systems: a weak coupling with the bath was believed to be
sufficient. However, the progress in both quantum gases and solid state
coherent quantum devices demonstrates that the coupling to the bath can be
reduced dramatically. To describe such systems properly one should revisit the
very foundations of the Statistical Mechanics. We examine this general problem
for the case of the Josephson junction chain and show that it displays a novel
high temperature non-ergodic phase with finite resistance. With further
increase of the temperature the system undergoes a transition to the fully
localized state characterized by infinite resistance and exponentially long
relaxation. | 1501.03853v1 |
2015-02-17 | Variations of magnetic properties of UGa2 under pressure | Electrical resistivity \rho(T) of the 5f ferromagnet UGa2 was investigated
for single-crystal samples as a function of pressure and magnetic field. The
Curie temperature monotonously increases from T$_{C}$ = 124 K under
quasi-hydrostatic pressure up to 154 K at p = 14.2 GPa, after which it turns
down steeply and reaches T$_{C}$ = 147 K at p = 15.2 GPa. At 20 GPa the
compound is already non-magnetic. This dramatic variation is compatible with
exchange interactions mediated by the 5f hybridization with the non-f states.
The external pressure first enhances the exchange coupling of the 5f moments,
but eventually suppresses the order by washing out the 5f moments. Such a
two-band model is adequate for the weakly delocalized 5f states. The
spin-disorder resistivity, which produces very high \rho-values (300
\mu\Omega.cm) is gradually suppressed by the pressure. In the paramagnetic
state, this leads to a crossover from initial negative to positive d\rho/dT. | 1502.04948v1 |
2015-08-17 | Suppression of Nonmagnetic Insulating State by Application of Pressure in Mineral Tetrahedrite Cu$_{12}$Sb$_{4}$S$_{13}$ | The mineral tetrahedrite Cu$_{12}$Sb$_{4}$S$_{13}$ exhibits a first-order
metal--insulator transition (MIT) at $T_{\rm MI}$ = 85 K and ambient pressure.
We measured the $^{63}$Cu-NMR at ambient pressure and the resistivity and
magnetic susceptibility at high pressures. $^{63}$Cu-NMR results indicate a
nonmagnetic insulating ground state in this compound. The MIT is monotonically
suppressed by pressure and disappears at $\sim1.0$ GPa. Two other anomalies are
observed in the resistivity measurements, and the pressure -- temperature phase
diagram of Cu$_{12}$Sb$_{4}$S$_{13}$ is constructed. | 1508.03933v1 |
2015-09-25 | Turbulent Reconnection in Relativistic Plasmas And Effects of Compressibility | We report turbulence effects on magnetic reconnection in relativistic plasmas
using 3-dimensional relativistic resistive magnetohydrodynamics simulations. We
found reconnection rate became independent of the plasma resistivity due to
turbulence effects similarly to non-relativistic cases. We also found
compressible turbulence effects modified the turbulent reconnection rate
predicted in non-relativistic incompressible plasmas; The reconnection rate
saturates and even decays as the injected velocity approaches to the Alfv\'en
velocity. Our results indicate the compressibility cannot be neglected when
compressible component becomes about half of incompressible mode occurring when
the Alfv\'en Mach number reaches about $0.3$. The obtained maximum reconnection
rate is around $0.05$ to $0.1$, which will be able to reach around $0.1$ to
$0.2$ if injection scales are comparable to the sheet length. | 1509.07703v2 |
2015-09-26 | Destroying coherence in high temperature superconductors with current flow | The loss of single-particle coherence going from the superconducting state to
the normal state in underdoped cuprates is a dramatic effect that has yet to be
understood. Here, we address this issue by performing angle resolved
photoemission spectroscopy (ARPES) measurements in the presence of a transport
current. We find that the loss of coherence is associated with the development
of an onset in the resistance, in that well before the midpoint of the
transition is reached, the sharp peaks in the ARPES spectra are completely
suppressed. Since the resistance onset is a signature of phase fluctuations,
this implies that the loss of single-particle coherence is connected with the
loss of long-range phase coherence. | 1509.07937v3 |
2015-11-11 | The Effect of Large Scale Magnetic Field on Outflow in ADAFs: an Odd Symmetry Configuration | We construct self-similar inflow-outflow solutions for a hot
viscous-resistive accretion flow with large scale magnetic fields that have odd
symmetry with respect to the equatorial plane in $B_\theta$, and even symmetry
in $B_r$ and $B_\phi$. Following previous authors, we also assume that the
polar velocity $v_\theta$ is nonzero. We focus on four parameters:
$\beta_{r0}$, $\beta_{\phi0}$ (the plasma beta parameters for associated with
magnetic field components at the equatorial plane), the magnetic resistivity
$\eta_0$, and the density index $n=-d\ln\rho/d\ln r$. The resulting flow
solutions are divided into two parts consisting of an inflow region with a
negative radial velocity ($v_r<0$) and an outflow region with $v_r>0$. Our
results show that stronger outflows emerge for smaller $\beta_{r0}$
($\le10^{-2}$ for $n>1$) and larger values of $\beta_{\phi0}$, $\eta_0$ and
$n$. | 1511.04366v1 |
2015-12-08 | On hybrid circuits exploiting thermistive properties of slime mould | Slime mould Physarum polycephalum is a single cell visible by unaided eye.
Let the slime mould span two electrodes with a single protoplasmic tube: if the
tube is heated to approximately 40{\deg}C, the electrical resistance of the
protoplasmic tube increases from 3 M{\Omega} to approximatively 10'000
M{\Omega}. The organism's resistance is not proportional nor correlated to the
temperature of its environment. Slime mould can therefore not be considered as
a thermistor but rather as a thermic switch. We employ the P. polycephalum
thermic switch to prototype hybrid electrical analog summator, NAND gates, and
cascade the gates into Flip-Flop latch. Computing operations performed on this
bio-hybrid computing circuitry feature high repeatability, reproducibility and
comparably low propagation delays | 1512.02483v1 |
2015-12-11 | Dynamical Decentralized Voltage Control of Multi-Terminal HVDC Grids | High-voltage direct current (HVDC) is a commonly used technology for
long-distance electric power transmission, mainly due to its low resistive
losses. When connecting multiple HVDC lines into a multi-terminal HVDC (MTDC)
system, several challenges arise. To ensure safe and efficient operation of
MTDC systems, the voltage of all terminals need to be steered to within an
operational range. In this paper we study the commonly used decentralized
voltage droop controller, and show that it in general does not steer the
voltages to within the operational range. We propose a decentralized PI
controller with deadband, and show that it always steers the voltages to within
the operational range regardless of the loads. Additionally we show that the
proposed controller inherits the property of proportional power sharing from
the droop controller, provided that both the loads and the line resistances are
sufficiently low. The results are validated through simulation in MATLAB. | 1512.03838v2 |
2016-01-06 | Transfer Printing of CVD Graphene FETs on Patterned Substrates | We describe a simple and scalable method for the transfer of CVD graphene for
the fabrication of field effect transistors. This is a dry process that uses a
modified RCA cleaning step to improve the surface quality. In contrast to
conventional fabrication routes where lithographic steps are performed after
the transfer, here graphene is transferred to a pre-patterned substrate. The
resulting FET devices display nearly zero Dirac voltage, and the contact
resistance between the graphene and metal contacts is on the order of 910 +-
340 Ohm-micrometer. This approach enables formation of conducting graphene
channel lengths up to one millimeter. The resist-free transfer process provides
a clean graphene surface that is promising for use in high sensitivity graphene
FET biosensors. | 1601.01224v1 |
2016-01-25 | A Perceptually Motivated Filter Bank with Perfect Reconstruction for Audio Signal Processing | Many audio applications rely on filter banks (FBs) to analyze, process, and
re-synthesize sounds. To approximate the auditory frequency resolution in the
signal chain, some applications rely on perceptually motivated FBs, the
gammatone FB being a popular example. However, most perceptually motivated FBs
only allow partial signal reconstruction at high redundancies and/or do not
have good resistance to sub-channel processing. This paper introduces an
oversampled perceptually motivated FB enabling perfect reconstruction,
efficient FB design, and adaptable redundancy. The filters are directly
constructed in the frequency domain and linearly distributed on a perceptual
frequency scale (e.g. ERB, Bark, or Mel scale). The proposed design allows for
various filter shapes, uniform or non-uniform FB setting, and large
down-sampling factors. For redundancies $\geq$ 3 perfect reconstruction is
achieved by computing the canonical dual FB analytically. For lower
redundancies perfect reconstruction is achieved using an iterative method.
Experiments show performance improvements of the proposed approach when
compared to the gammatone FB in terms of reconstruction error and resistance to
sub-channel processing, especially at low redundancies. | 1601.06652v1 |
2016-04-06 | The Raspberry model for protein-like particles: ellipsoids and confinement in cylindrical pores | The study of protein mass transport via atomistic simulation requires time
and length scales beyond the computational capabilities of modern computer
systems. The raspberry model for colloidal particles in combination with the
mesoscopic hydrodynamic method of lattice Boltzmann facilitates coarse-grained
simulations that are on the order of microseconds and hundreds of nanometers
for the study of diffusive transport of protein-like colloid particles. The
raspberry model reproduces linearity in resistance to motion versus particle
size and correct enhanced drag within cylindrical pores at off-center
coordinates for spherical particles. Owing to the high aspect ratio of many
proteins, ellipsoidal raspberry colloid particles were constructed and
reproduced the geometric resistance factors of Perrin and of Happel and Brenner
in the laboratory-frame and in the moving body-frame. Accurate body-frame
rotations during diffusive motion have been captured for the first time using
projections of displacements. | 1604.01829v3 |
2016-04-29 | Superconductivity and abnormal pressure effect in Sr0.5La0.5FBiSe2 superconductor | Through the solid state reaction method, we synthesized a new BiSe2-based
superconductor Sr0:5La0:5FBiSe2 with superconducting transition temperature
Tc?3.8 K. A strong diamagnetic signal below Tc in susceptibility ?(T) is
observed indicating the bulk nature of superconductivity. Different to most
BiS2-based compounds where superconductivity develops from a
semiconducting-like normal state, the present compound exhibits a metallic
behavior down to Tc. Under weak magnetic field or pressure, however, a
remarkable crossover from metallic to insulating behaviors takes place around
Tmin where the resistivity picks up a local minimum. With increasing pressure,
Tc decreases monotonously and Tmin shifts to high temperatures, while the
absolute value of the normal state resistivity at low temperatures first
decreases and then increases with pressure up to 2.5 GPa. These results imply
that the electronic structure of Sr0:5La0:5FBiSe2 may be different to those in
the other BiS2-based systems. | 1604.08686v1 |
2016-05-04 | Eco-friendly gas mixtures for Resistive Plate Chambers based on Tetrafluoropropene and Helium | Due to the recent restrictions deriving from the application of the Kyoto
protocol, the main components of the gas mixtures presently used in the
Resistive Plate Chambers systems of the LHC experiments will be most probably
phased out of production in the coming years. Identifying possible replacements
with the adequate characteristics requires an intense R&D, which was recently
started, also in collaborations across the various experiments. Possible
candidates have been proposed and are thoroughly investigated. Some tests on
one of the most promising candidate - HFO-1234ze, an allotropic form of
tetrafluoropropane- have already been reported. Here an innovative approach,
based on the use of Helium, to solve the problems related to the too elevate
operating voltage of HFO-1234ze based gas mixtures, is discussed and the
relative first results are shown. | 1605.01691v1 |
2016-05-07 | Numerical study on the effect of design parameters and spacers on RPC signal and timing properties | Numerical calculations have been performed to understand the reason for the
observed non-uniform response of a Resistive Plate Chamber (RPC) in a few
critical regions such as near edge spacers and corners of the device. In this
context, the signal from a RPC due to the passage of muons through different
regions has been computed. Also, a simulation of RPC timing properties is
presented along with the effect of the applied field, gas mixture and
geometrical components. | 1605.02154v3 |
2016-05-26 | Preliminary results of Resistive Plate Chambers operated with eco-friendly gas mixtures for application in the CMS experiment | The operations of Resistive Plate Chambers in LHC experiments require
Fluorine based (F-based) gases for optimal performance. Recent European
regulations demand the use of environmentally unfriendly F-based gases to be
limited or banned. In view of the CMS experiment upgrade, several tests are
ongoing to measure the performance of the detector with these new ecological
gas mixtures, in terms of efficiency, streamer probability, induced charge and
time resolution. Prototype chambers with readout pads and with the standard CMS
electronic setup are under test. In this paper preliminary results on
performance of RPCs operated with a potential eco-friendly gas candidate
1,3,3,3-Tetrafluoropropene, commercially known as HFO-1234ze, with CO2 and CF3I
based gas mixtures are presented and discussed for the possible application in
the CMS experiment. | 1605.08172v1 |
2016-05-31 | First results of CMS RPC performance at 13 TeV | The muon spectrometer of the CMS (Compact Muon Solenoid) experiment at the
Large Hadron Collider (LHC) is equipped with a redundant system made of
Resistive Plate Chambers (RPCs) and Drift Tube (DT) chambers in the barrel, RPC
and Cathode Strip Chambers (CSCs) in the endcap region. In this paper, the
first results of the performance of the RPC system during 2015 with the LHC
running at 13 TeV is presented. The stability of the RPC performance, in terms
of efficiency, cluster size and noise, is reported. | 1605.09521v3 |
2016-07-21 | Magnetocapacitance oscillations and thermoelectric effect in two-dimensional electron gas irradiated by microwaves | To study the influence of microwave irradiation on two-dimensional electrons,
we apply a method based on capacitance measurements in GaAs quantum well
samples where the gate covers a central part of the layer. We find that the
capacitance oscillations at high magnetic fields, caused by the oscillations of
thermodynamic density of states, are not essentially modified by microwaves.
However, in the region of fields below 1 Tesla, we observe another set of
oscillation, with the period and the phase identical to those of microwave
induced resistance oscillations. The phenomenon of microwave induced
capacitance oscillations is explained in terms of violation of the Einstein
relation between conductivity and the diffusion coefficient in the presence of
microwaves, which leads to a dependence of the capacitor charging on the
anomalous conductivity. We also observe microwave-induced oscillations in the
capacitive response to periodic variations of external heating. These
oscillations appear due to the thermoelectric effect and are in antiphase with
microwave induced resistance oscillations because of the Corbino-like geometry
of our experimental setup. | 1607.06435v1 |
2016-08-01 | TorBricks: Blocking-Resistant Tor Bridge Distribution | Tor is currently the most popular network for anonymous Internet access. It
critically relies on volunteer nodes called bridges for relaying Internet
traffic when a user's ISP blocks connections to Tor. Unfortunately, current
methods for distributing bridges are vulnerable to malicious users who obtain
and block bridge addresses. In this paper, we propose TorBricks, a protocol for
distributing Tor bridges to n users, even when an unknown number t < n of these
users are controlled by a malicious adversary. TorBricks distributes O(tlog(n))
bridges and guarantees that all honest users can connect to Tor with high
probability after O(log(t)) rounds of communication with the distributor.
We also extend our algorithm to perform privacy-preserving bridge
distribution when run among multiple untrusted distributors. This not only
prevents the distributors from learning bridge addresses and bridge assignment
information, but also provides resistance against malicious attacks from a m/3
fraction of the distributors, where m is the number of distributors. | 1608.00509v1 |
2016-10-03 | Beyond ideal magnetohydrodynamics: Resistive, reactive and relativistic plasmas | We develop a new framework for the modelling of charged fluid dynamics in
general relativity. The model, which builds on a recently developed variational
multi-fluid model for dissipative fluids, accounts for relevant effects like
the inertia of both charge currents and heat and, for mature systems, the
decoupling of superfluid components. We discuss how the model compares to
standard relativistic magnetohydronamics and consider the connection between
the fluid dynamics, the microphysics and the underlying equation of state. As
illustrations of the formalism, we consider three distinct two-fluid models
describing i) an Ohm's law for resistive charged flows, ii) a relativistic heat
equation, and iii) an equation representing the momentum of a decoupled
superfluid component. As a more complex example, we also formulate a
three-fluid model which demonstrates the thermo-electric effect. This framework
allows us to model neutron stars (and related systems) at a hierarchy of
increasingly complex levels, and should enable us to make progress on a range
of exciting problems in astrophysics and cosmology. | 1610.00449v1 |
2016-10-13 | Straintronic magneto-tunneling-junction based ternary content addressable memory | Straintronic magneto-tunneling junction (s-MTJ) switches, whose resistances
are controlled with voltage-generated strain in the magnetostrictive free layer
of the MTJ, are extremely energy-efficient switches that would dissipate a few
aJ of energy during switching. Unfortunately, they are also relatively
error-prone and have low resistance on/off ratio. This suggests that as
computing elements, they are best suited for non-Boolean architectures. Here,
we propose and analyze a ternary content addressable memory implemented with
s-MTJs and some transistors. It overcomes challenges encountered by traditional
all-transistor implementations, resulting in exceptionally high cell density. | 1610.03902v2 |
2016-10-17 | Topological quantum phase transition and superconductivity induced by pressure in the bismuth tellurohalide BiTeI | A pressure-induced topological quantum phase transition has been
theoretically predicted for the semiconductor BiTeI with giant Rashba spin
splitting. In this work, the evolution of the electrical transport properties
in BiTeI and BiTeBr is investigated under high pressure. The pressure-dependent
resistivity in a wide temperature range passes through a minimum at around 3
GPa, indicating the predicted transition in BiTeI. Superconductivity is
observed in both BiTeI and BiTeBr while the resistivity at higher temperatures
still exhibits semiconducting behavior. Theoretical calculations suggest that
the superconductivity may develop from the multi-valley semiconductor phase.
The superconducting transition temperature Tc increases with applied pressure
and reaches a maximum value of 5.2 K at 23.5 GPa for BiTeI (4.8 K at 31.7 GPa
for BiTeBr), followed by a slow decrease. Our results demonstrate that BiTeX (X
= I, Br) compounds with non-trivial topology of electronic states display new
ground states upon compression. | 1610.05364v1 |
2016-11-29 | Sub 100nW volatile nano-metal-oxide memristor as synaptic-like encoder of neuronal spikes | Advanced neural interfaces mediate a bio-electronic link between the nervous
system and microelectronic devices, bearing great potential as innovative
therapy for various diseases. Spikes from a large number of neurons are
recorded leading to creation of big data that require on-line processing under
most stringent conditions, such as minimal power dissipation and on-chip space
occupancy. Here, we present a new concept where the inherent volatile
properties of a nano-scale memristive device are used to detect and compress
information on neural spikes as recorded by a multi-electrode array.
Simultaneously, and similarly to a biological synapse, information on spike
amplitude and frequency is transduced in metastable resistive state transitions
of the device, which is inherently capable of self-resetting and of continuous
encoding of spiking activity. Furthermore, operating the memristor in a very
high resistive state range reduces its average in-operando power dissipation to
less than 100 nW, demonstrating the potential to build highly scalable, yet
energy-efficient on-node processors for advanced neural interfaces. | 1611.09671v1 |
2016-12-21 | Magnetotransport studies of Superconducting Pr$_4$Fe$_2$As$_2$Te$_{1-x}$O$_4$ | We report a detailed study of the electrical transport properties of single
crystals of Pr$_4$Fe$_2$As$_2$Te$_{1-x}$O$_4$, a recently discovered iron-based
superconductor. Resistivity, Hall effect and magnetoresistance are measured in
a broad temperature range revealing the role of electrons as dominant charge
carriers. The significant temperature dependence of the Hall coefficient and
the violation of Kohler's law indicate multiband effects in this compound. The
upper critical field and the magnetic anisotropy are investigated in fields up
to 16 T, applied parallel and perpendicular to the crystallographic c-axis.
Hydrostatic pressure up to 2 GPa linearly increases the critical temperature
and the resistivity residual ratio. A simple two-band model is used to describe
the transport and magnetic properties of Pr$_4$Fe$_2$As$_2$Te$_{1-x}$O$_4$. The
model can successfully explain the strongly temperature dependent negative Hall
coefficient and the high magnetic anisotropy assuming that the mobility of
electrons is higher than that of holes. | 1612.07090v1 |
2016-12-23 | Strained graphene Hall bar | The effects of strain, induced by a Gaussian bump, on the magnetic field
dependent transport properties of a graphene Hall bar are investigated. The
numerical simulations are performed using both classical and quantum mechanical
transport theory and we found that both approaches exhibit similar
characteristic features. The effects of the Gaussian bump are manifested by a
decrease of the bend resistance, $R_B$, around zero-magnetic field and the
occurrence of side-peaks in $R_B$. These features are explained as a
consequence of bump-assisted scattering of electrons towards different
terminals of the Hall bar. Using these features we are able to give an estimate
of the size of the bump. Additional oscillations in $R_B$ are found in the
quantum description that are due to the population/depopulation of Landau
levels. The bump has a minor influence on the Hall resistance even for very
high values of the pseudo-magnetic field. When the bump is placed outside the
center of the Hall bar valley polarized electrons can be collected in the
leads. | 1612.07995v1 |
2017-02-01 | Superzone gap formation and low lying crystal electric field levels in PrPd$_2$Ge$_2$ single crystal | The magnetocrystalline anisotropy exhibited in PrPd$_2$Ge$_2$ single crystal
has been investigated by measuring the magnetization, magnetic susceptibility,
electrical resistivity and heat capacity. PrPd$_2$Ge$_2$ crystallizes in the
well known ThCr$_2$Si$_2$\--type tetragonal structure. The antiferromagnetic
ordering is confirmed as 5.1~K with the [001]-axis as the easy axis of
magnetization. A superzone gap formation is observed from the electrical
resistivity measurement when the current is passed along the [001] direction.
The crystal electric field (CEF) analysis on the magnetic susceptibility,
magnetization and the heat capacity measurements confirms a doublet ground
state with a relatively low over all CEF level splitting. The CEF level
spacings and the Zeeman splitting at high fields become comparable and lead to
metamagnetic transition at 34~T due to the CEF level crossing. | 1702.00220v1 |
2017-02-08 | Giant microwave-induced $B$-periodic magnetoresistance oscillations in a two-dimensional electron gas with a bridged-gate tunnel point contact | We have studied the magnetoresistance of the quantum point contact fabricated
on the high mobility two-dimensional electron gas (2DEG) exposed to microwave
irradiation. The resistance reveals giant $B$-periodic oscillations with the
relative amplitude $\Delta R/R$ of up to $700$\% resulting from the propagation
and interference of the edge magnetoplasmons (EMPs) in the sample. This giant
photoconductance is attributed to the considerably large local electron density
modulation in the vicinity of the point contact. We have also analyzed the
oscillation periods $\Delta B$ of the resistance oscillations and, comparing
the data with the EMP theory, extracted the EMP interference length $L$. We
have found that the length $L$ substantially exceeds the distance between the
contact leads but rather corresponds to the distance between metallic contact
pads measured along the edge of the 2DEG. This resolves existing controversy in
the literature and should help to properly design highly sensitive microwave
and terahertz spectrometers based on the discussed effect. | 1702.02309v1 |
2017-05-31 | An upper bound on transport | The linear growth of operators in local quantum systems leads to an effective
lightcone even if the system is non-relativistic. We show that consistency of
diffusive transport with this lightcone places an upper bound on the
diffusivity: $D \lesssim v^2 \tau_\text{eq}$. The operator growth velocity $v$
defines the lightcone and $\tau_\text{eq}$ is the local equilibration
timescale, beyond which the dynamics of conserved densities is diffusive. We
verify that the bound is obeyed in various weakly and strongly interacting
theories. In holographic models this bound establishes a relation between the
hydrodynamic and leading non-hydrodynamic quasinormal modes of planar black
holes. Our bound relates transport data --- including the electrical
resistivity and the shear viscosity --- to the local equilibration time, even
in the absence of a quasiparticle description. In this way, the bound sheds
light on the observed $T$-linear resistivity of many unconventional metals, the
shear viscosity of the quark-gluon plasma and the spin transport of unitary
fermions. | 1706.00019v2 |
2017-10-10 | Transport coefficients from QCD Kondo effect | We study the transport coefficients from the QCD Kondo effect in quark matter
which contains heavy quarks as impurity particles. We estimate the coupling
constant of the interaction between a light quark and a heavy quark at finite
density and temperature by using the renormalization group equation up to
two-loop order. We also estimate the coupling constant at zero temperature by
using the mean-field approximation as non-perturbative treatment. To calculate
the transport coefficients, we use the relativistic Boltzmann equation and
apply the relaxation time approximation. We calculate the electric resistivity
from the relativistic kinetic theory, and the viscosities from the relativistic
hydrodynamics. We find that the electric resistivity is enhanced and the shear
viscosity is suppressed due to the QCD Kondo effect at low temperature. | 1710.03434v2 |
2017-10-17 | Superconductivity of Cobalt in Thin Films | Due to competing long range ferromagnetic order, the transition metals Fe, Co
and Ni are not superconductors at ambient pressure. While superconductivity was
observed in a non-magnetic phase of Fe, stabilized under pressure, it is yet to
be discovered in Co and Ni under any experimental conditions. Here, we report
emergence of superconductivity in the recently discovered high-density
nonmagnetic face centered cubic phase in Co thin films below a transition
temperature (Tc) of ~5.4 K, as revealed in experiments based on point-contact
spectroscopy and resistance, and four-probe measurements of resistance at
ambient pressure. We confirm the non-magnetic nature of the dense fcc phase of
Co within first-principles density functional theory, and show that its
superconductivity below 5 K originates from anomalous softening of
zone-boundary phonons and their enhanced coupling with electrons upon biaxial
strain. | 1710.06114v1 |
2017-10-29 | Gauge Topological Nature of the Superconductor-Insulator Transition | It has long been believed that, at absolute zero, electrons can form only one
quantum coherent state, a superconductor. Yet, several two dimensional
superconducting systems were found to harbor the superinsulating state with
infinite resistance, a mirror image of superconductivity, and a metallic state
often referred to as Bose metal, characterized by finite longitudinal and
vanishing Hall resistances. The nature of these novel and mysterious quantum
coherent states is the subject of intense study.Here, we propose a topological
gauge description of the superconductor-insulator transition (SIT) that enables
us to identify the underlying mechanism of superinsulation as Polyakov's linear
confinement of Cooper pairs via instantons. We find a criterion defining
conditions for either a direct SIT or for the SIT via the intermediate Bose
metal and demonstrate that this Bose metal phase is a Mott topological
insulator in which the Cooper pair-vortex liquid is frozen by Aharonov-Bohm
interactions. | 1710.10575v2 |
2018-01-09 | Direct Detection of Ultralight Dark Matter via Astronomical Ephemeris | A novel idea of the direct detection to search for a ultralight dark matter
based on the interaction between the dark matter and a nucleon is proposed.
Solar system bodies feel the dark matter wind and it acts as a resistant force
opposing their motions. The astronomical ephemeris of solar system bodies is so
precise that it has a strong capability to detect a dark matter whose mass is
much lighter than O(1) eV. We have estimated the resistant force based on the
calculation of the elastic scattering cross section between the dark matter and
the bodies beyond the Born approximation, and show that the astronomical
ephemeris indeed put a very strong constraint on the interaction between the
dark matter and a nucleon, depending on how smoothly the ultralight dark matter
is distributed at the scale smaller than the celestial bodies in our solar
system. | 1801.02807v1 |
2018-01-09 | Flare particle acceleration in the interaction of twisted coronal flux ropes | The aim of this work is to investigate and characterise non-thermal particle
behaviour in a three-dimensional (3D) magnetohydrodynamical (MHD) model of
unstable multi-threaded flaring coronal loops. We have used a numerical scheme
which solves the relativistic guiding centre approximation to study the motion
of electrons and protons. The scheme uses snapshots from high resolution
numerical MHD simulations of coronal loops containing two threads, where a
single thread becomes unstable and (in one case) destabilises and merges with
an additional thread. The particle responses to the reconnection and
fragmentation in MHD simulations of two loop threads are examined in detail. We
illustrate the role played by uniform background resistivity and distinguish
this from the role of anomalous resistivity using orbits in an MHD simulation
where only one thread becomes unstable without destabilising further loop
threads. We examine the (scalable) orbit energy gains and final positions
recovered at different stages of a second MHD simulation wherein a secondary
loop thread is destabilised by (and merges with) the first thread. We compare
these results with other theoretical particle acceleration models in the
context of observed energetic particle populations during solar flares. | 1801.02907v1 |
2018-02-12 | First FBK Production of 50$μ$m Ultra-Fast Silicon Detectors | Fondazione Bruno Kessler (FBK, Trento, Italy) has recently delivered its
first 50 $\mu$m thick production of Ultra-Fast Silicon Detectors (UFSD), based
on the Low-Gain Avalanche Diode design. These sensors use high resistivity
Si-on-Si substrates, and have a variety of gain layer doping profiles and
designs based on Boron, Gallium, Carbonated Boron and Carbonated Gallium to
obtain a controlled multiplication mechanism. Such variety of gain layers will
allow identifying the most radiation hard technology to be employed in the
production of UFSD, to extend their radiation resistance beyond the current
limit of $\phi \sim$ 10$^{15}$ n$_{eq}$/cm$^2$. In this paper, we present the
characterisation, the timing performances, and the results on radiation damage
tolerance of this new FBK production. | 1802.03988v2 |
2018-02-12 | A candidate Theory for the "Strange Metal" phase at Finite Energy Window | We propose a lattice model for strongly interacting electrons with the
potential to explain the main phenomenology of the strange metal phase in the
cuprate high temperature superconductors. Our model is motivated by the
recently developed "tetrahedron" rank-3 tensor model that mimics much of the
physics of the better-known Sachdev-Ye-Kitaev (SYK) model. Our electron model
has the following advantageous properties: (1) it only needs one orbital per
site on the square lattice; (2) it does not require any quenched random
interaction; (3) it has local interactions and respects all the symmetries of
the system; (4) the soluble limit of this model has a longitudinal DC
resistivity that scales linearly with temperature within a finite temperature
window; (5) again the soluble limit of this model has a fermion pairing
instability in the infrared, which can lead to either superconductivity or a
"pseudogap" phase. The linear$-T$ longitudinal resistivity and the pairing
instability originate from the generic scaling feature of the SYK model and the
tetrahedron tensor model. | 1802.04293v2 |
2018-02-22 | Negative differential resistance in Van der Waals heterostructures due to moiré-induced spectral reconstruction | Formation of moir\'{e} superlattices is common in Van der Waals
heterostructures as a result of the mismatch between lattice constants and
misalignment of crystallographic directions of the constituent two-dimensional
crystals. We discuss theoretically electron transport in a Van der Waals
tunnelling transistor in which one of the electrodes is made of two crystals
forming a moir\'{e} superlattice at their interface. By investigating
structures containing either the aligned graphene/hexagonal boron nitride
heterostructure or twisted bilayer graphene, we show that negative differential
resistance is possible in such transistors as a consequence of the
superlattice-induced changes in the electronic density of states and without
the need of momentum conserving tunnelling present in high-quality exfoliated
devices. | 1802.08100v3 |
2018-04-10 | Development of Integration-Type Silicon-On-Insulator Monolithic Pixel Detectors by Using a Float Zone Silicon | In this paper, we describe the development of monolithic pixel detectors by
using a Silicon-on-Insulator (SOI) technology for X-ray and charged particle
applications. The detectors are based on a 0.2 {\mu}m CMOS fully depleted SOI
process (Lapis Semiconductor Co., Ltd). The SOI wafer consists of a thick
high-resistivity substrate for sensing and a thin low resistivity Si layer for
CMOS circuits.
We developed the integration-type SOI pixel detector, INTPIX4 mainly for
X-ray imaging; it is made of a Float Zone (FZ) or Czochralski (Cz) silicon
wafer. Since 2005, Cz SOI detectors were used initially. After 2011, FZ SOI
detectors were successfully fabricated. In this paper, we state recent
progresses and test results of the SOI monolithic pixel detector using a FZ
silicon and compare them with the results obtained using the Cz detector. | 1804.03338v1 |
2018-05-07 | Vacuum-dressed cavity magnetotransport of a 2D electron gas | We present a theory predicting how the linear magnetotransport of a
two-dimensional electron gas is modified by a passive electromagnetic cavity
resonator where no real photons are injected nor created. For a cavity photon
mode with in-plane linear polarization, the dc bulk magnetoresistivity of the
2D electron gas is anisotropic. In the regime of high filling factors of the
Landau levels, the envelope of the Shubnikov-de Haas oscillations is profoundly
modified and the resistivity can be increased or reduced depending on the
system parameters. In the limit of low magnetic fields, the resistivity along
the cavity-mode polarization direction is enhanced in the ultrastrong
light-matter coupling regime. Our work shows the crucial role of virtual
polariton excitations in controlling the dc charge transport properties of
cavity-embedded systems. | 1805.02623v2 |
2018-06-09 | Streamer studies in Resistive Plate Chambers | The present paper is meant as an update of the presentation given in a
previous Resistive Plate Chamber (RPC) workshop, aimed at finding an
eco-friendly gas mixture for streamer operation of RPCs. Indeed the streamer
working regime is still suitable for building large RPC systems dedicated to
low rate applications, such as cosmic ray and neutrino physics. In addition to
other studies about gas mixtures for streamer mode operation, in this paper the
replacement of R134a with CF4, a gas widely used in other gaseous detectors,
has been investigated. The effect of the gas gap thickness on the discharge
quenching has also been studied; this is an important check because thin gas
gaps of 1 mm, one half of the typical used value, have been introduced for high
rate applications. Finally preliminar results about the streamer formation
timing are also reported. | 1806.03443v2 |
2018-09-04 | Linear Wave Propagation for Resistive Relativistic Magnetohydrodynamics | We present a linear mode analysis of the relativistic MHD equations in the
presence of finite electrical conductivity. Starting from the fully
relativistic covariant formulation, we derive the dispersion relation in the
limit of small linear perturbations. It is found that the system supports ten
wave modes which can be easily identified in the limits of small or large
conductivities. In the resistive limit, matter and electromagnetic fields
decouple and solution modes approach pairs of light and acoustic waves as well
as a number of purely damped (non-propagating) modes. In the opposite (ideal)
limit, the frozen-in condition applies and the modes of propagation coincide
with a pair of fast magnetosonic, a pair of slow and Alfv\'en modes, as
expected. In addition, the contact mode is always present and it is unaffected
by the conductivity. For finite values of the conductivity, the dispersion
relation gives rise to either pairs of opposite complex conjugate roots or
purely imaginary (damped) modes. In all cases, the system is dissipative and
also dispersive as the phase velocity depends nonlineary on the wavenumber.
Occasionally, the group velocity may exceed the speed of light although this
does not lead to superluminal signal propagation. | 1809.01115v1 |
2018-09-16 | Temperature-dependent transport measurements with Arduino | The current performances of single-board microcontrollers render them
attractive not only for basic applications but also for more elaborate
projects, amongst which physics teaching or research. In this article, we show
how temperature-dependent transport measurements can be performed using an
Arduino microcontroller, from cryogenic temperatures up to room temperature or
above. We focus on two of the main issues for this type of experiments: the
determination of the sample temperature and the measurement of its resistance.
We also detail two student-led experiments: evidencing the magnetocaloric
effect in Gadolinium and measuring the resistive transition of a high critical
temperature superconductor. | 1809.08075v1 |
2018-09-27 | Impact of Integrated Circuit Packaging on Synaptic Dynamics of Memristive Devices | The memristor can be used as non volatile memory (NVM) and for emulating
neuron behavior. It has the ability to switch between low resistance $R_{on}$
and high resistance values $R_{off}$, and exhibit the synaptic dynamic
behaviour such as potentiation and depression. This paper presents a study on
potentiation and depression of memristors in Quad Flat Pack. A comparison is
drawn between the memristors with and without the impact of parasitics of
packaging, using measured data and equivalent circuit models. The parameters in
memristor and packaging models for the SPICE simulations were determined using
measured data to reflect the memristor parasitics in Quad Flat Packs. | 1809.10434v1 |
2019-09-30 | Numerical simulation of magnetic reconnection around a black hole | We performed numerical simulations of general relativistic
magnetohydrodynamics with uniform resistivity to investigatethe occurrence of
magnetic reconnection in a split-monopole magnetic field around a Schwarzschild
black hole. We found that magnetic reconnection happens near the black hole at
its equatorial plane. The magnetic reconnection has a point-like reconnection
region and slow shock waves, as in the Petschek reconnection model. The
magnetic reconnection rate decreases as the resistivity becomes smaller. When
the global magnetic Reynolds number is $10^4$ or larger, the magnetic
reconnection rate increases linearly with time from $2 \tau_{\rm S}$ to $\sim
10 \tau_{\rm S}$ ($\tau_{\rm S}=r_{\rm S}/c, r_{\rm S}$ is the Schwarzschild
radius and $c$ is the speed of light). The linear increase of the reconnection
rate agrees with the magnetic reconnection in the Rutherford regime of the
tearing mode instability. | 1909.13414v1 |
2014-08-01 | The linear tearing instability in three dimensional, toroidal gyrokinetic simulations | Linear gyro-kinetic simulations of the classical tearing mode in
three-dimensional toroidal geometry were performed using the global gyro
kinetic turbulence code, GKW . The results were benchmarked against a
cylindrical ideal MHD and analytical theory calculations. The stability, growth
rate and frequency of the mode were investigated by varying the current
profile, collisionality and the pressure gradients. Both collision-less and
semi-collisional tearing modes were found with a smooth transition between the
two. A residual, finite, rotation frequency of the mode even in the absense of
a pressure gradient is observed which is attributed to toroidal finite
Larmor-radius effects. When a pressure gradient is present at low
collisionality, the mode rotates at the expected electron diamagnetic
frequency. However the island rotation reverses direction at high
collisionality. The growth rate is found to follow a $\eta^{1/7}$ scaling with
collisional resistivity in the semi-collisional regime, closely following the
semi-collisional scaling found by Fitzpatrick. The stability of the mode
closely follows the stability using resistive MHD theory, however a
modification due to toroidal coupling and pressure effects is seen. | 1408.0112v1 |
2014-08-01 | Development of Multi-gap Resistive Plate Chamber (MRPC) for medical imaging | The low cost and high resolution Multi-gap Resistive Plate Chamber (MRPC)
opens up a new possibility to find an efficient alternative detector for the
Time of Flight (TOF) based Positron Emission Tomography, where the sensitivity
of the system depends largely on the time resolution of the detector. In a
layered structure, suitable converters can be used to increase the photon
detection efficiency. In this paper results of the cosmic ray test of a
four-gap bakelite-based prototype MRPC operated in streamer mode and six-gap
glass-based MRPC operated in avalanche mode are discussed. | 1408.0280v1 |
2014-08-17 | Quantum criticality in two dimensions and Marginal Fermi Liquid | Kinetic properties of a two dimensional model of fermions interacting with
antiferromagnetic spin excitations near the quantum critical point (QCP) are
considered. The temperature or doping are assumed to be sufficiently high, such
that the pseudogap does not appear. In contrast to standard spin-fermion
models, it is assumed that there are intrinsic inhomogeneities in the system
suppressing space correlations of the antiferromagnetic excitations. It is
argued that the inhomogeneities in the spin excitations in the "strange metal"
phase can be a consequence of existence of $\pi $-shifted domain walls in the
doped antiferromagnetic phase. Averaging over the inhomogeneities and
calculating physical quantities like resistivity and some others one can
explain unusual properties of cuprates unified under the name "Marginal Fermi
Liquid" (MFL). The dependence of the slope of the linear temperature dependence
of the resistivity on doping is compared with experimental data. | 1408.3865v3 |
2017-03-29 | Illumination effect by visible continuous-wave laser on bulk 40 K-superconductors | Visible CW (continuous wave) -laser heating effects on the bulk
superconductors CeFeAsO0.65F0.35 and MgB2 with 1.5 mm thickness have been
investigated by measuring the temperature dependence of electrical resistivity.
Each compound shows a critical-temperature Tc reduction with increasing fluence
rate. At the normal state, a parallel circuit model based on the Fourier's law
can well reproduce the temperature dependence of electrical resistivity of
illuminated sample. On the other hand, the predicted temperature-rise due to
the laser heating in the superconducting state is much smaller than the
observed Tc-reduction. A temperature gradient of a few K across the sample
thickness easily triggers the destruction of bulk superconductivity.
Furthermore we have found a slight Tc-enhancement in CeFeAsO0.65F0.35 after a
rather high fluence-rate irradiation. | 1703.10270v1 |
2004-06-12 | Thermally-activated current transport in MgB_2 films | Thermally-activated flux flow (TAFF) resistivity above the irreversibility
field is reported for two different c-axis textured MgB_2 superconducting
films. Transport measurements at different perpendicular magnetic fields 0 < B
< 9 T and temperatures from 5 to 40K reveal TAFF Ohmic resistivity described by
the Arrhenius law with the quadratic field dependence of the activation energy.
Our transport measurements on bulk MgB_2 ceramic samples also show the TAFF
behavior, but do not show the quadratic field dependence of U(T,B). We explain
our results in terms of thermally-activated drift of pre-existing quenched
dislocations in the vortex lattice. Our results indicate that thermal
fluctuations can be essential in determining the irreversibility field in MgB2
though to a much lesser extent than in high-temperature superconductors. | 0406294v1 |
2011-07-05 | Parametric oscillator based on non-linear vortex dynamics in low resistance magnetic tunnel junctions | Radiofrequency vortex spin-transfer oscillators based on magnetic tunnel
junctions with very low resistance area product were investigated. A high power
of excitations has been obtained characterized by a power spectral density
containing a very sharp peak at the fundamental frequency and a series of
harmonics. The observed behaviour is ascribed to the combined effect of spin
transfer torque and Oersted-Amp\`ere field generated by the large applied
dc-current. We furthermore show that the synchronization of a vortex
oscillation by applying a ac bias current is mostly efficient when the external
frequency is twice the oscillator fundamental frequency. This result is
interpreted in terms of a parametric oscillator. | 1107.0867v2 |
2013-12-02 | Robust superconductivity with large upper critical field in Nb2PdS5 | We report synthesis, structural details and complete superconducting
characterization of very recently discovered Nb2PdS5 new superconductor. The
synthesized compound is crystallized in mono-clinic structure. Bulk
superconductivity is seen in both magnetic susceptibility and electrical
resistivity measurements with superconducting transition temperature (Tc) at
6K. The upper critical field (Hc2), being estimated from high field
magneto-transport measure-ments is above 240kOe. The estimated Hc2(0) is
clearly above the Pauli paramagnetic limit. Heat capacity measurements show
clear transition with well defined peak at Tc, but with lower jump than as
expected for a BCS type superconductor. The Sommerfield constant and Debye
temperature as determined from low temperature fitting of heat capacity data
are 32mJ/moleK2 and 263K respectively. Hall coefficients and resistivity in
conjugation with electronic heat capacity indicates multiple gap
superconductivity signatures in Nb2PdS5. We also studied the impact of
hydrostatic pressure on superconductivity of Nb2PdS5 and found nearly no change
in Tc for the given pressure range. | 1312.0425v5 |
2013-12-12 | Pressure-induced superconductivity in EuFe2As2 without a quantum critical point: magnetotransport and upper critical field measurements under high pressure | Resistivity and Hall effect measurements of EuFe$_2$As$_2$ up to 3.2\,GPa
indicate no divergence of quasiparticle effective mass at the pressure
$P_\mathrm{c}$ where the magnetic and structural transition disappears. This is
corroborated by analysis of the temperature ($T$) dependence of the upper
critical field. $T$-linear resistivity is observed at pressures slightly above
$P_\mathrm{c}$. The scattering rates for both electrons and holes are shown to
be approximately $T$-linear. When a field is applied, a $T^2$ dependence is
recovered, indicating that the origin of the $T$-linear dependence is spin
fluctuations. | 1312.3380v1 |
2013-12-25 | Ambipolar Transport and Magneto-resistance Crossover in a Mott Insulator, Sr$_{2}$IrO$_{4}$ | Electric field effect (EFE) controlled magnetoelectric transport in thin
films of undoped and La-doped Sr$_{2}$IrO$_{4}$ (SIO) were investigated under
the action of ionic liquid gating. Despite large carrier density modulation,
the temperature dependent resistance measurements exhibit insulating behavior
in chemically and EFE doped samples with the band filling up to 10\%. The
ambipolar transport across the Mott gap is demonstrated by EFE tuning of the
activation energy. Further, we observe a crossover from a negative
magnetoresistance (MR) at high temperatures to positive MR at low temperatures.
The crossover temperature was around $\sim$80-90 K, irrespective of the
filling. This temperature and magnetic field dependent crossover is
qualitatively associated with a change in the conduction mechanism from Mott to
Coulomb gap mediated variable range hopping (VRH). This explains the origin of
robust insulating ground state of SIO in electrical transport studies and
highlights the importance of disorder and Coulombic interaction on electrical
properties of SIO. | 1312.7015v2 |
2014-11-18 | Tunneling transport in a few monolayer-thick WS2/graphene heterojunction | This paper demonstrates the high-quality tunnel barrier characteristics and
layer number controlled tunnel resistance of a transition metal dichalcogenide
(TMD) measuring just a few monolayers in thickness. Investigation of vertical
transport in WS2 and MoS2 TMDs in graphene/TMD/metal heterostructures revealed
that WS2 exhibits tunnel barrier characteristics when its thickness is between
2 to 5 monolayers, whereas MoS2 experiences a transition from tunneling to
thermionic emission transport with increasing thickness within the same range.
Tunnel resistance in a graphene/WS2/metal heterostructure therefore increases
exponentially with the number of WS2 layers, revealing the tunnel barrier
height of WS2 to be 0.37 eV. | 1411.4714v2 |
2016-06-01 | Variation of transition temperatures and residual resistivity ratio in vapor-grown FeSe | The study of the iron-based superconductor FeSe has blossomed with the
availability of high quality single crystals, obtained through
flux/vapor-transport growth techniques below the structural transformation
temperature of its tetragonal phase, T~450 C. Here, we report on the variation
of sample morphology and properties due to small modifications in the growth
conditions. A considerable variation of the superconducting transition
temperature Tc, from 8.8 K to 3 K, which cannot be correlated with the sample
composition, is observed. Instead, we point out a clear correlation between Tc
and disorder, as measured by the residual resistivity ratio. Notably, the
tetragonal-to-orthorhombic structural transition is also found to be quite
strongly disorder dependent (Ts =72 - 90 K), and linearly correlated with Tc. | 1606.00500v1 |
2016-06-14 | Effect of atomic disorder and Ce doping on superconductivity of Ca3Rh4Sn13: Electric transport properties under high pressure | We report the observation of a superconducting state below 8K coexistent with
a spin-glass state caused by atomic disorder in Ce substituted Ca3Rh4Sn13.
Measurements of specific heat, resistivity, and magnetism reveal the existence
of inhomogeneous superconductivity in samples doped with Ce with
superconducting critical temperatures Tc higher than those observed in the
parent compound. For Ca3Rh4Sn13, the negative value of the change in
resistivity with pressure P, correlates well with the calculated decrease in
the density of states at the Fermi energy with P. Based on band structure
calculations performed under pressure, we demonstrate how the change in DOS
would affect Tc of Ca3Rh4Sn13 under negative lattice pressure in samples that
are strongly defected by quenching. | 1606.04579v1 |
2017-01-19 | Spin-filtering in superconducting junction with the manganite interlayer | We report on the electronic transport and the impact of spin-filtering in
mesa-structures made of epitaxial thin films of cuprate superconductor
YBa2Cu3Ox(YBCO) and the manganite LaMnO3 (LMO) interlayer with the Au/Nb
counterelectrode. Ferromagnetic resonance measurements of heterostructure
Au/LMO/YBCO shows ferromagnetic state at temperatures below 150 K as in the
case of reference LMO film grown on the neodymium gallate substrate. The
heights of the tunneling barrier evaluated from resistive characteristics of
mesa-structures at different thickness of interlayer showed an exponential
decrease from 30 mV down to 5 mV with the increase of manganite interlayer
thickness. Temperature dependence of the conductivity of mesa-structures could
be described taking into account the d-wave superconductivity in YBCO and a
spin filtering of the electron transport. Spin filtering is supported also by
measurements of magneto-resistance and the high sensitivity of mesa-structure
conductivity to weak magnetic fields. | 1701.05364v1 |
2017-01-27 | Control of superconductivity with a single ferromagnetic layer in niobium/erbium bilayers | Superconducting spintronics in hybrid superconductor/ferromagnet (S-F)
heterostructures provides an exciting potential new class of device. The
prototypical super-spintronic device is the superconducting spin-valve, where
the critical temperature, $T_c$, of the S-layer can be controlled by the
relative orientation of two (or more) F-layers. Here, we show that such control
is also possible in a simple S/F bilayer. Using field history to set the
remanent magnetic state of a thin Er layer, we demonstrate for a Nb/Er bilayer
a high level of control of both $T_c$ and the shape of the resistive
transition, R(T), to zero resistance. We are able to model the origin of the
remanent magnetization, treating it as an increase in the effective exchange
field of the ferromagnet and link this, using conventional S-F theory, to the
suppression of $T_c$. We observe stepped features in the R(T) which we argue is
due to a fundamental interaction of superconductivity with inhomogeneous
ferromagnetism, a phenomena currently lacking theoretical description. | 1701.08065v2 |
2017-11-08 | Mechanical characterization of disordered and anisotropic cellular monolayers | We consider a cellular monolayer, described using a vertex-based model, for
which cells form a spatially disordered array of convex polygons that tile the
plane. Equilibrium cell configurations are assumed to minimize a global energy
defined in terms of cell areas and perimeters; energy is dissipated via dynamic
area and length changes, as well as cell neighbour exchanges. The model
captures our observations of an epithelium from a Xenopus embryo showing that
uniaxial stretching induces spatial ordering, with cells under net tension
(compression) tending to align with (against) the direction of stretch, but
with the stress remaining heterogeneous at the single-cell level. We use the
vertex model to derive the linearized relation between tissue-level stress,
strain and strain-rate about a deformed base state, which can be used to
characterize the tissue's anisotropic mechanical properties; expressions for
viscoelastic tissue moduli are given as direct sums over cells. When the base
state is isotropic, the model predicts that tissue properties can be tuned to a
regime with high elastic shear resistance but low resistance to area changes,
or vice versa. | 1711.02909v2 |
2017-11-21 | Strained graphene structures: from valleytronics to pressure sensing | Due to its strong bonds graphene can stretch up to 25% of its original size
without breaking. Furthermore, mechanical deformations lead to the generation
of pseudo-magnetic fields (PMF) that can exceed 300 T. The generated PMF has
opposite direction for electrons originating from different valleys. We show
that valley-polarized currents can be generated by local straining of
multi-terminal graphene devices. The pseudo-magnetic field created by a
Gaussian-like deformation allows electrons from only one valley to transmit and
a current of electrons from a single valley is generated at the opposite side
of the locally strained region. Furthermore, applying a pressure difference
between the two sides of a graphene membrane causes it to bend/bulge resulting
in a resistance change. We find that the resistance changes linearly with
pressure for bubbles of small radius while the response becomes non-linear for
bubbles that stretch almost to the edges of the sample. This is explained as
due to the strong interference of propagating electronic modes inside the
bubble. Our calculations show that high gauge factors can be obtained in this
way which makes graphene a good candidate for pressure sensing. | 1711.07904v1 |
2017-12-12 | Dynamical Negative Differential Resistance in Antiferromagnetically Coupled Few-Atom Spin-Chains | We present the appearance of negative differential resistance (NDR) in
spin-dependent electron transport through a few-atom spin-chain. A chain of
three antiferromagnetically coupled Fe atoms(Fe trimer) was positioned on a
Cu2N/Cu(100) surface and contacted with the spin-polarized tip of a scanning
tunneling microscope, thus coupling the Fe trimer to one non-magnetic and one
magnetic lead. Pronounced NDR appears at the low bias of 7 mV where inelastic
electron tunneling dynamically locks the atomic spin in a long-lived excited
state. This causes a rapid increase of the magnetoresistance between
spin-polarized tip and Fe trimer and quenches elastic tunneling. By varying the
coupling strength between tip and Fe trimer we find that in this transport
regime the dynamic locking of the Fe trimer competes with magnetic exchange
interaction, which statically forces the Fe trimer into the
high-magnetoresistance state and removes the NDR. | 1712.04213v1 |
2017-12-12 | Scaling Projections on Spin Transfer Torque Magnetic Tunnel Junctions | We investigate scaling of technologically relevant magnetic tunnel junction
devices in the trilayer and pentalayer configurations by varying the
cross-sectional area along the transverse direction using the non-equilibrium
Green's function spin transport formalism. We study the geometry dependence by
considering square and circular cross-sections. As the transverse dimension in
each case reduces, we demonstrate that the transverse mode energy profile plays
a major role in the resistance-area product. Both types of devices show
constant tunnel magnetoresistance at larger cross-sectional areas but achieve
ultra-high magnetoresistance at small cross-sectional areas, while maintaining
low resistance-area products. We notice that although the critical switching
voltage for switching the magnetization of the free layer nanomagnet in the
trilayer case remains constant at larger areas, it needs more energy to switch
at smaller areas. In the pentalayer case, we observe an oscillatory behavior at
smaller areas as a result of double barrier tunneling. We also describe how
switching characteristics of both kinds of devices are affected by the scaling. | 1712.04235v1 |
2017-12-14 | Conductivity of anisotropic inhomogeneous superconductors above critical temperature | We propose a model and derive analytical expressions for conductivity in
heterogeneous fully anisotropic conductors with ellipsoid superconducting
inclusions. This model and calculations are useful to analyze the observed
temperature dependence of conductivity anisotropy in various anisotropic
superconductors, where superconductivity onset happens inhomogeneously in the
form of isolated superconducting islands. The results are applied to explain
the experimental data on resistivity above the transition temperature $T_c$ in
the high-temperature superconductor $\mathrm{YBa_2Cu_4O_8}$ and in the organic
superconductor $\beta$-(BEDT-TTF)$_{2}$I$_{3}$. The comparison of resistivity
data and diamagnetic response in $\beta$-(BEDT-TTF)$_{2}$I$_{3}$ allows us to
estimate the size of superconducting inclusions as $d\sim 1\mu m$. | 1712.05347v2 |
2017-12-16 | Mitigating Asymmetric Nonlinear Weight Update Effects in Hardware Neural Network based on Analog Resistive Synapse | Asymmetric nonlinear weight update is considered as one of the major
obstacles for realizing hardware neural networks based on analog resistive
synapses because it significantly compromises the online training capability.
This paper provides new solutions to this critical issue through
co-optimization with the hardware-applicable deep-learning algorithms. New
insights on engineering activation functions and a threshold weight update
scheme effectively suppress the undesirable training noise induced by
inaccurate weight update. We successfully trained a two-layer perceptron
network online and improved the classification accuracy of MNIST handwritten
digit dataset to 87.8/94.8% by using 6-bit/8-bit analog synapses, respectively,
with extremely high asymmetric nonlinearity. | 1712.05895v1 |
2018-10-30 | Whole genome single nucleotide polymorphism genotyping of Staphylococcus aureus | Next-generation sequencing technology enables routine detection of bacterial
pathogens for clinical diagnostics and genetic research. Whole genome
sequencing has been of importance in the epidemiologic analysis of bacterial
pathogens. However, few whole genome sequencing-based genotyping pipelines are
available for practical applications. Here, we present the whole genome
sequencing-based single nucleotide polymorphism (SNP) genotyping method and
apply to the evolutionary analysis of methicillin-resistant Staphylococcus
aureus. The SNP genotyping method calls genome variants using next-generation
sequencing reads of whole genomes and calculates the pair-wise Jaccard
distances of the genome variants. The method may reveal the high-resolution
whole genome SNP profiles and the structural variants of different isolates of
methicillin-resistant S. aureus (MRSA) and methicillin-susceptible S. aureus
(MSSA) strains. The phylogenetic analysis of whole genomes and particular
regions may monitor and track the evolution and the transmission dynamic of
bacterial pathogens. The computer programs of the whole genome sequencing-based
SNP genotyping method are available to the public at
https://github.com/cyinbox/NGS. | 1810.13027v1 |
2018-11-02 | Ladder-like optical conductivity in the spin-fermion model | In the nested limit of the spin-fermion model for the cuprates,
one-dimensional physics in the form of half-filled two-leg ladders emerges. We
show that the renormalization group flow of the corresponding ladder is towards
the d-Mott phase, a gapped spin-liquid with short-ranged d-wave pairing
correlations, and reveals an intermediate SO(5)$\times$SO(3) symmetry. We use
the results of the renormalization group in combination with a memory-function
approach to calculate the optical conductivity of the spin-fermion model in the
high-frequency regime, where processes within the hot spot region dominate the
transport. We argue that umklapp processes play a major role. For finite
temperatures, we determine the resistivity in the zero-frequency (dc) limit.
Our results show an approximate linear temperature dependence of the
resistivity and a conductivity that follows a non-universal power law. A
comparison to experimental data supports our assumption that the conductivity
is dominated by the antinodal contribution above the pseudogap. | 1811.01103v2 |
2018-11-13 | Hydrodynamic and Solid Mechanics Analysis of Capillary Force-Induced Mold Deformation in Sub-10 nm UV Nanoimprint Lithography | A model has been developed to study the dynamic filling process and to
investigate the capillary force-induced deformation of nanostructures on the
imprint mold during ultraviolet nanoimprint lithography (UV-NIL) down to sub-10
nm resolution. The dynamic behavior of resist filling with varied physical
parameters was investigated by a hydrodynamic model. The capillary
force-induced deformation of mold structures was modeled using beam bending
mechanics for both wetting and non-wetting mold structures. Theoretically
calculated results were cross-validated with finite-element simulations using
two-phase flow and solid mechanics methods. Based on the theoretical analysis,
a general parameter of critical aspect ratio for design of imprint mold for
UV-NIL is developed. The investigation of capillary force-induced deformation
in UV-NIL helps to deepen the understanding of dynamic mechanism of resist
filling and structural deformation at sub-10 nm scale and enable optimization
for high-fidelity UV-NIL. | 1811.05103v2 |
2018-11-29 | Magneto-immutable turbulence in weakly collisional plasmas | We propose that pressure anisotropy causes weakly collisional turbulent
plasmas to self-organize so as to resist changes in magnetic-field strength. We
term this effect "magneto-immutability" by analogy with incompressibility
(resistance to changes in pressure). The effect is important when the pressure
anisotropy becomes comparable to the magnetic pressure, suggesting that in
collisionless, weakly magnetized (high-$\beta$) plasmas its dynamical relevance
is similar to that of incompressibility. Simulations of magnetized turbulence
using the weakly collisional Braginskii model show that magneto-immutable
turbulence is surprisingly similar, in most statistical measures, to critically
balanced MHD turbulence. However, in order to minimize magnetic-field
variation, the flow direction becomes more constrained than in MHD, and the
turbulence is more strongly dominated by magnetic energy (a nonzero "residual
energy"). These effects represent key differences between pressure-anisotropic
and fluid turbulence, and should be observable in the $\beta\gtrsim1$ turbulent
solar wind. | 1811.12421v2 |
2019-02-15 | Electrodynamics of granular aluminum from superconductor to insulator: observation of collective superconducting modes | We report on a detailed study of the optical response and $T_c-\rho$ phase
diagram ($T_c$ being the superconducting critical temperature and $\rho$ the
normal state resistivity of the film) of granular aluminum, combining transport
measurements and a high resolution optical spectroscopy technique. The
$T_c-\rho$ phase diagram is discussed as resulting from an interplay between
the phase stiffness, the Coulomb repulsion and the superconducting gap
$\Delta$. We provide a direct evidence for two different types of well resolved
sub-gap absorptions, at $\omega_1\simeq\Delta$ and at
$\Delta\lesssim\omega_2\lesssim2\Delta$ (decreasing with increasing
resistivity). | 1902.05843v2 |
2019-02-27 | Energy bandpass filtering in superlattice phase change memories | We propose energy bandpass filtering employed using the idea of
anti-reflection heterostructures as a means to reduce the energy requirements
of a superlattice phase change memory based on GeTe and Sb$_{2}$Te$_{3}$
heterostructures. Different configurations of GeTe/Sb$_{2}$Te$_{3}$
superlattices are studied using the non-equilibrium Green's function approach.
Our electronic transport simulations calculate the coupling parameter for the
high resistance covalent state, to $97 \%$ that of the stable low resistance
resonant state, maintaining the ON/OFF ratio of 100 for a reliable read
operation. By examining various configurations of the superlattice structures
we conclude that the inclusion of anti-reflection units on both sides of the
superlattice increases the overall ON/OFF ratio by an order of magnitude which
can further help in scaling down of the memory device. It is also observed that
the device with such anti-reflection units exhibits 32$\%$ lesser RESET voltage
than the most common PCM superlattice configurations and 27$\%$ in the presence
of elastic dephasing. Moreover, we also find that the ON/OFF ratios in these
devices are also resilient to the variations in the periodicity of the
superlattice. | 1902.10551v1 |
2019-05-15 | Automatic Long-Term Deception Detection in Group Interaction Videos | Most work on automated deception detection (ADD) in video has two
restrictions: (i) it focuses on a video of one person, and (ii) it focuses on a
single act of deception in a one or two minute video. In this paper, we propose
a new ADD framework which captures long term deception in a group setting. We
study deception in the well-known Resistance game (like Mafia and Werewolf)
which consists of 5-8 players of whom 2-3 are spies. Spies are deceptive
throughout the game (typically 30-65 minutes) to keep their identity hidden. We
develop an ensemble predictive model to identify spies in Resistance videos. We
show that features from low-level and high-level video analysis are
insufficient, but when combined with a new class of features that we call
LiarRank, produce the best results. We achieve AUCs of over 0.70 in a fully
automated setting. Our demo can be found at
http://home.cs.dartmouth.edu/~mbolonkin/scan/demo/ | 1905.08617v2 |
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