text
stringlengths 73
2.82k
| category
stringclasses 21
values |
|---|---|
Coexistence of Superconductivity and Antiferromagnetism in the Hubbard
model for cuprates: Antiferromagnetism and $d$-wave superconductivity are the most important
competing ground-state phases of cuprate superconductors. Using cellular
dynamical mean-field theory (CDMFT) for the Hubbard model, we revisit the
question of the coexistence and competition of these phases in the one-band
Hubbard model with realistic band parameters and interaction strengths. With an
exact diagonalization solver, we improve on previous works with a more complete
bath parametrization which is carefully chosen to grant the maximal possible
freedom to the hybridization function for a given number of bath orbitals.
Compared with previous incomplete parametrizations, this general bath
parametrization shows that the range of microscopic coexistence of
superconductivity and antiferromagnetism is reduced for band parameters for
NCCO, and confined to electron-doping with parameters relevant for YBCO. | cond-mat |
A new field-theoretical formulation for the motion of an electron in a
quenched disorder potential: Following a proposal by Aronov and Ioselevich, we express the Green functions
(GF) of a noninteracting disordered Fermi system as a functional integral on a
real time/frequency lattice. The normalizing denominator of this functional
integral is equal to unity, because of identities satisfied by the GF. The GF
can then be simply averaged with respect to the random disorder potential. We
describe the fermionic fields not belonging to the external frequency by means
of a bosonic auxiliary field g. The Hubbard-Stratonovich field Q is introduced
only with respect to the fermionic fields for the external frequency. | cond-mat |
Theoretical analysis of anisotropic upper critical field of
superconductivity in nodal-line semimetals: We study the properties of the upper critical field of superconductivity in
nodal-line semimetals in a continuous model, which has a nodal-line on the
$k_{z} = 0$ plane. Using the semiclassical Green's function method, we
calculate the upper critical field for the two limiting cases: the dirty limit
with many impurities and the clean limit with few impurities. The results show
the large anisotropy of the magnitude of the upper critical field and the
unusual temperature dependence. The obtained results are compared with recent
experimental data of PbTaSe$_{2}$. | cond-mat |
Molecular dynamics simulation of the fragile glass former
ortho-terphenyl: a flexible molecule model: We present a realistic model of the fragile glass former orthoterphenyl and
the results of extensive molecular dynamics simulations in which we
investigated its basic static and dynamic properties. In this model the
internal molecular interactions between the three rigid phenyl rings are
described by a set of force constants, including harmonic and anharmonic terms;
the interactions among different molecules are described by Lennard-Jones
site-site potentials. Self-diffusion properties are discussed in detail
together with the temperature and momentum dependencies of the
self-intermediate scattering function. The simulation data are compared with
existing experimental results and with the main predictions of the Mode
Coupling Theory. | cond-mat |
Bose-Einstein Condensation on Curved Manifolds: Here we describe a weakly interacting Bose gas on a curved manifold, which is
embedded in the three-dimensional Euclidean space.~To this end we start by
considering a harmonic trap in the normal direction of the manifold, which
confines the three-dimensional Bose gas in the vicinity of its
surface.~Following the notion of dimensional reduction as outlined in
[L.~Salasnich et al., Phys.~Rev.~A {\bf 65}, 043614 (2002)], we assume a large
enough trap frequency so that the normal degree of freedom of the condensate
wave function can be approximately integrated out. In this way we obtain an
effective condensate wave function on the quasi-two-dimensional surface of the
curved manifold, where the thickness of the cloud is determined
self-consistently. For the particular case when the manifold is a sphere, our
equilibrium results show how the chemical potential and the thickness of the
cloud increase with the interaction strength.~Furthermore, we determine within
a linear stability analysis the low-lying collective excitations together with
their eigenfrequencies, which turn out to reveal an instability for attractive
interactions. | cond-mat |
Fermi level dependent charge-to-spin current conversion by Dirac surface
state of topological insulators: The spin-momentum locking at the Dirac surface state of a topological
insulator (TI) offers a distinct possibility of a highly efficient
charge-to-spin current (C-S) conversion compared with spin Hall effects in
conventional paramagnetic metals. For the development of TI-based spin current
devices, it is essential to evaluate its conversion efficiency quantitatively
as a function of the Fermi level EF position. Here we exemplify a coefficient
of qICS to characterize the interface C-S conversion effect by using spin
torque ferromagnetic resonance (ST-FMR) for (Bi1-xSbx)2Te3 thin films whose EF
is tuned across the band gap. In bulk insulating conditions, interface C-S
conversion effect via Dirac surface state is evaluated as nearly constant large
values of qICS, reflecting that the qICS is inversely proportional to the Fermi
velocity vF that is almost constant. However, when EF traverses through the
Dirac point, the qICS is remarkably suppressed possibly due to the degeneracy
of surface spins or instability of helical spin structure. These results
demonstrate that the fine tuning of the EF in TI based heterostructures is
critical to maximizing the efficiency using the spin-momentum locking
mechanism. | cond-mat |
Pairing mechanism of high-temperature superconductivity: Experimental
constraints: Developing a theory of high-temperature superconductivity in copper oxides is
one of the outstanding problems in physics. It is a challenge that has defeated
theoretical physicists for more than twenty years. Attempts to understand this
problem are hindered by the subtle interplay among a few mechanisms and the
presence of several nearly degenerate and competing phases in these systems.
Here we present some crucial experiments that place essential constraints on
the pairing mechanism of high-temperature superconductivity. The observed
unconventional oxygenisotope effects in cuprates have clearly shown strong
electron-phonon interactions and the existence of polarons and/or bipolarons.
Angle-resolved photoemission and tunneling spectra have provided direct
evidence for strong coupling to multiple-phonon modes. In contrast, these
spectra do not show strong coupling features expected for magnetic resonance
modes. Angle-resolved photoemission spectra and the oxygen-isotope effect on
the antiferromagnetic exchange energy J in undoped parent compounds
consistently show that the polaron binding energy is about 2 eV, which is over
one order of magnitude larger than J = 0.14 eV. The normal-state
spin-susceptibility data of holedoped cuprates indicate that intersite
bipolarons are the dominant charge carriers in the underdoped region while the
component of Fermi-liquid-like polarons is dominant in the overdoped region.
All the experiments to test the gap or order-parameter symmetry consistently
demonstrate that the intrinsic gap (pairing) symmetry for the Fermi-liquid-like
component is anisotropic s-wave and the order-parameter symmetry of the
Bose-Einstein condensation of bipolarons is d-wave. | cond-mat |
Adiabatic quantum pumping of chiral Majorana fermions: We investigate adiabatic quantum pumping of chiral Majorana states in a
system composed of two Mach--Zehnder type interferometers coupled via a quantum
point contact. The pumped current is generated by periodic modulation of the
phases accumulated by traveling around each interferometer. Using scattering
matrix formalism we show that the pumped current reveals a definite signature
of the chiral nature of the Majorana states involved in transport in this
geometry. Furthermore, by tuning the coupling between the two interferometers
the pump can operate in a regime where finite pumped current and zero
two-terminal conductance is expected. | cond-mat |
Impact of charge distribution of soft layers on transient electroosmotic
flow of Maxwell fluids in soft nanochannels: We theoretically study transient electroosmotic flow of general Maxwell
fluids through polyelectrolyte grafted nanochannel with a layered distribution
of charges.
By applying the method of Laplace transform, we semi-analytically obtain
transient electroosmotic flow from Cauchy momentum equation and Maxwell
constitutive equation.
For nanochannels grafted with polyelectrolyte layers having different layered
distribution of charges, we study the influence of dimensionless relaxation
time, dimensionless polyelectrolyte layer thickness and dimensionless drag
coefficient on transient electroosmotic flow.
We present the results for some particular cases. Firstly, we unravel that
for the case of polyzwitterionic brush that the sum of positive and negative
structural charges is zero, total electroosmotic flow is non-zero. In
particular, depending on charge distribution within end part of polyelectrolyte
layers, the direction of electroosmotic flow can be reversed critically.
Secondly, in order to quantitatively evaluate a reversal of electroosmotic flow
for two polyelectrolyte layers of opposite signs, we introduce a critical
number ks as the ratio between layered charge densities of two polyelectrolyte
layers. Increasing ks allows electroosmotic flow to be reversed easily.
We verify that adjusting charge distributions of the layer can control
intentionally the direction of the flows as well as strength of electroosmotic
flow. | cond-mat |
Electron-phonon interaction in the dynamics of trap filling in quantum
dots: We analyze theoretically the effects of electron-phonon interaction in the
dynamics of an electron that can be trapped to a localized state and detrapped
to an extended band state of a small quantum dot (QD) using a simple model
system. In spite of its simplicity the time dependent model has no analytical
solution but a numerically exact one can be found producing a rich dynamics.
The electronic motion is quasi-periodic in time, with oscillations around a
mean value that are basic characteristics of the weak and strong coupling
regimes of electron-phonon interaction and set the time scales of the system.
Using values of the parameters appropriate for defects in semiconductor QDs, we
find these time scales to range typically from tenths of picoseconds to a few
picoseconds. The values of the time averaged trap occupancy strongly depend on
the the strength of the electron-phonon interaction and can be as large as
40$\%$ when the coupling is most efficient, independently of other parameters.
An interesting result of the present work is the formation of resonances at
specific values of the electron-phonon coupling parameter that only exist when
several levels are allowed to coherently cooperate in the filling of the trap.
They are characterized by a trap occupancy that is a periodic function of time
with large amplitude and period picturing an electron that is periodically
trapped and detrapped. We conclude that the formation of these resonances is a
robust consequence of electron-phonon interaction in small systems.
Electron-phonon interaction is an efficient mechanism that can provide ca.
50$\%$ filling of a deep trap state on a subpicoseconds to picoseconds time
scale, much faster than radiative decay occurring in time scales of tens of
picoseconds to nanoseconds, while the occupancy of this state will be smaller
than ca. 1$\%$ in the absence of electron-phonon coupling. | cond-mat |
Voltage-controlled transmission in a dielectric slab doped with the
quantum dot molecules: Transmission and reflection of an electromagnetic pulse through a dielectric
slab doped with the quantum dot molecules is investigated. It is shown that the
transmitted and reflected pulses depend on the inter-dot tunneling effect and
can be controlled by applying a gate voltage. | cond-mat |
Spontaneous Rotation of Ferrimagnetism Driven by Antiferromagnetic Spin
Canting: Spin-reorientation phase transitions that involve the rotation of a
crystal$'$s magnetization have been well characterized in distorted-perovskite
oxides such as the orthoferrites. In these systems spin reorientation occurs
due to competing rare-earth and transition metal anisotropies coupled via
$f$-$d$ exchange. Here, we demonstrate an alternative paradigm for spin
reorientation in distorted perovskites. We show that the
$R_2\mathrm{CuMnMn_4O_{12}}$ (R = Y or Dy) triple A-site columnar-ordered
quadruple perovskites have three ordered magnetic phases and up to two
spin-reorientation phase transitions. Unlike the spin-reorientation phenomena
in other distorted perovskites, these transitions are independent of rare-earth
magnetism, but are instead driven by an instability towards antiferromagnetic
spin canting likely originating in frustrated Heisenberg exchange interactions,
and the competition between Dzyaloshinskii-Moriya and single-ion anisotropies. | cond-mat |
Collective excitation of electric dipole on molecular dimer in organic
dimer-Mott insulator: The terahertz (THz) response in 10-100 cm^-1 was investigated in an organic
dimer-Mott (DM) insulator kappa-(ET)_2Cu_2(CN)_3 that exhibits a relaxor-like
dielectric anomaly. 30 cm^-1 band in the optical conductivity was attributable
to collective excitation of the intra-dimer electric dipoles which are formed
by an electron correlation. We succeeded in observing photoinduced enhancement
of this 30 cm^-1 band, reflecting the growth of the electric dipole cluster in
the DM phase. Such optical responses in kappa-(ET)_2Cu_2(CN)_3 reflect
instability near the boundary between the DM-ferroelectric charge ordered
phases. | cond-mat |
Effect of sodium pyrophosphate and understanding microstructure of
aqueous LAPONITE(R) dispersion using dissolution study: We investigate physical origin of ergodicity breaking in an aqueous colloidal
dispersion of synthetic hectorite clay, LAPONITE(R), by performing dissolution
and rheological experiments with monovalent salt and tetrasodium pyrophosphate
solution. We also study the effect of interface, nitrogen and paraffin oil on
the same. Dissolution experiments carried out for dispersions with both the
interfaces show similar results. However, for samples with nitrogen interface,
all the effects are observed to get expedited in time compared to paraffin oil
interface. When kept in contact with water, 1.5 wt. % and 2.8 wt. % colloidal
dispersion at pH 10 swell at small ages, while do not swell at large ages. The
solution of tetrasodium pyrophosphate, interestingly, dissolves the entire
colloidal dispersion samples with pH 10 irrespective of the clay concentration.
Experiments carried out on colloidal dispersions prepared in water having pH 13
demonstrate no effect of water as well as sodium pyrophosphate solution on the
same suggesting a possibility of the presence of negative charge on edge at
that pH. We believe that all the behaviors observed for samples at pH 10 can be
explained by an attractive gel microstructure formed by edge-to-face contact.
Furthermore, the absence of swelling in old colloidal dispersion at pH 10 and
dissolution of the same by sodium pyrophosphate solution cannot be explained by
merely repulsive interactions. This behavior suggests that attractive
interactions play an important role in causing ergodicity breaking in the
colloidal dispersions at pH 10 at all the ages irrespective of the clay
concentration. We substantiate the presence of fractal network structure formed
by interparticle edge-face association using rheological tools and cryo-TEM
imaging. We also conduct a comprehensive study of the effect of sodium
pyrophosphate in the sol-gel transition of LAPONITE(R) dispersion. | cond-mat |
Mode-Locking in Quantum-Hall-Effect Point Contacts: We study the effect of an ac drive on the current-voltage (I-V)
characteristics of a tunnel junction between two fractional Quantum Hall fluids
at filling $\nu ^{-1}$ an odd integer. Within the chiral Luttinger liquid model
of edge states, the point contact dynamics is described by a driven damped
quantum mechanical pendulum. In a semi-classical limit which ignores electron
tunnelling, this model exhibits mode-locking, which corresponds to current
plateaus in the I-V curve at integer multiples of $I= e\omega /2\pi$, with
$\omega$ the ac drive angular frequency. By analyzing the full quantum model at
non-zero $\nu$ using perturbative and exact methods, we study the effect of
quantum fluctuation on the mode-locked plateaus. For $\nu=1$ quantum
fluctuations smear completely the plateaus, leaving no trace of the ac drive.
For $\nu \ge 1/2$ smeared plateaus remain in the I-V curve, but are not
centered at the currents $I=n e \omega /2\pi$. For $\nu < 1/2$ rounded plateaus
centered around the quantized current values are found. The possibility of
using mode locking in FQHE point contacts as a current-to-frequency standard is
discussed. | cond-mat |
Liquid-gas phase behavior of polydisperse dipolar hard-sphere fluid:
Extended thermodynamic perturbation theory for central force associating
potential: The liquid-gas phase diagram for polydisperse dipolar hard-sphere fluid with
polydispersity in the hard-sphere size and dipolar moment is calculated using
extension of the recently proposed thermodynamic perturbation theory for
central force (TPT-CF) associating potential. To establish the connection with
the phase behavior of ferrocolloidal dispersions it is assumed that the dipole
moment is proportional to the cube of the hard-sphere diameter. We present and
discuss the full phase diagram, which includes cloud and shadow curves,
binodals and distribution functions of the coexisting daughter phases at
different degrees of the system polydispersity. In all cases studied
polydispersity increases the region of the phase instability and shifts the
critical point to the higher values of the temperature and density. The larger
size particles always fractionate to the liquid phase and the smaller size
particles tend to move to the gas phase. At relatively high values of the
system polydispersity three-phase coexistence is observed. | cond-mat |
Tungsten material properties at high temperature and high stress: Recently reported results on the long lifetime of the tungsten samples under
high temperature and high stress conditions expected in the Neutrino Factory
target have strengthened the case for a solid target option for the Neutrino
Factory. In order to study in more details the behaviour of basic material
properties of tungsten, a new method has been developed for measurement of
tungsten Young's modulus at high stress, high strain-rates (> 1000 s^-1) and
very high temperatures (up to 2650 C). The method is based on measurements of
the surface motion of tungsten wires, stressed by a pulsed current, using a
Laser Doppler Vibrometer. The measured characteristic frequencies of wire
expansion and contraction under the thermal loading have been used to directly
obtain the tungsten Young's modulus as a function of applied stress and
temperature. The experimental results have been compared with modelling results
and we have found that they agree very well. From the point of view of future
use of tungsten as a high power target material, the most important result of
this study is that Young's modulus of tungsten remains high at high
temperature, high stress and high strain-rates. | cond-mat |
Dynamics of point Josephson junctions in a microstrip line: We analyze a new long wave model describing the electrodynamics of an array
of point Josephson junctions in a superconducting cavity. It consists in a wave
equation with Dirac delta function sine nonlinearities. We introduce an adapted
spectral problem whose spectrum gives the resonances in the current-voltage
characteristic curve of any array. Using the associated inner product and
eigenmodes, we establish that at the resonances the solution is described by
two simple ordinary differential equations. | cond-mat |
Finite-frequency prethermalization in periodically driven ergodic
systems: We investigate the periodically driven dynamics of many-body systems, either
classical or quantum, finite-dimensional or mean-field, displaying an unbounded
phase-space. We find that the inclusion of a smooth periodic drive atop an
otherwise ergodic dynamics leads to a long-lived prethermalization, even at
moderate driving frequencies. In specific asymptotic limits, we compute the
corresponding prethermal Hamiltonian from an analytical perturbation scheme. | cond-mat |
Thermal Schwinger Effect: Defect Production in Compressed Filament
Bundles: We discuss the response of biopolymer filament bundles bound by transient
cross linkers to compressive loading. These systems admit a mechanical
instability at stresses typically below that of traditional Euler buckling. In
this instability, there is thermally-activated pair production of topological
defects that generate localized regions of bending -- kinks. These kinks
shorten the bundle's effective length thereby reducing the elastic energy of
the mechanically loaded structure. This effect is the thermal analog of the
Schwinger effect, in which a sufficiently large electric field causes
electron-positron pair production. We discuss this analogy and describe the
implications of this analysis for the mechanics of biopolymer filament bundles
of various types under compression. | cond-mat |
Modelling the Berezinskii-Kosterlitz-Thouless Transition in the
NiGa_2S_4: In the two-dimensional superfluidity, the proliferation of the vortices and
the anti-vortices results in a new class of phase transition,
Berezinskii-Kosterlitz-Thouless (BKT) transition. This class of the phase
transitions is also anticipated in the two-dimensional magnetic systems.
However, its existence in the real magnetic systems still remains mysterious.
Here we propose a phenomenological model to illustrate that the novel
spin-freezing transition recently uncovered in the NMR experiment on the
NiGa_2S_4 compound is the BKT-type. The novel spin-freezing state observed in
the NiGa_2S_4 possesses the power-law decayed spin correlation. | cond-mat |
A proposal of an orbital-dependent correlation energy functional for
energy-band calculations: An explicitly orbital-dependent correlation energy functional is proposed,
which is to be used in combination with the orbital-dependent exchange energy
functional in energy-band calculations. It bears a close resemblance to the
second-order direct and exchange perturbation terms calculated with Kohn-Sham
orbitals and Kohn-Sham energies except that one of the two Coulomb interactions
entering each term is replaced by an effective interaction which contains
information about long-, intermediate-, and short-range correlations beyond
second-order perturbation theory. Such an effective interaction can rigorously
be defined for the correlation energy of the uniform electron liquid and is
evaluated with high accuracy in order to apply to the orbital-dependent
correlation energy functional. The coupling-constant-averaged spin-parallel and
spin-antiparallel pair correlation functions are also evaluated with high
accuracy for the electron liquid. The present orbital-dependent correlation
energy functional with the effective interaction borrowed from the electron
liquid is valid for tightly-binding electrons as well as for nearly-free
electrons in marked contrast with the conventional local density approximation. | cond-mat |
Detection of qubit-oscillator entanglement in nanoelectromechanical
systems: Experiments over the past years have demonstrated that it is possible to
bring nanomechanical resonators and superconducting qubits close to the quantum
regime and to measure their properties with an accuracy close to the Heisenberg
uncertainty limit. Therefore, it is just a question of time before we will
routinely see true quantum effects in nanomechanical systems. One of the
hallmarks of quantum mechanics is the existence of entangled states. We propose
a realistic scenario making it possible to detect entanglement of a mechanical
resonator and a qubit in a nanoelectromechanical setup. The detection scheme
involves only standard current and noise measurements of an atomic point
contact coupled to an oscillator and a qubit. This setup could allow for the
first observation of entanglement between a continuous and a discrete quantum
system in the solid state. | cond-mat |
How crosslink numbers shape the large-scale physics of cytoskeletal
materials: Cytoskeletal networks are the main actuators of cellular mechanics, and a
foundational example for active matter physics. In cytoskeletal networks,
motion is generated on small scales by filaments that push and pull on each
other via molecular-scale motors. These local actuations give rise to large
scale stresses and motion. To understand how microscopic processes can give
rise to self-organized behavior on larger scales it is important to consider
what mechanisms mediate long-ranged mechanical interactions in the systems. Two
scenarios have been considered in the recent literature. The first are systems
which are relatively sparse, in which most of the large scale momentum transfer
is mediated by the solvent in which cytoskeletal filaments are suspended. The
second, are systems in which filaments are coupled via crosslink molecules
throughout. Here, we review the differences and commonalities between the
physics of these two regimes. We also survey the literature for the numbers
that allow us to place a material within either of these two classes. | cond-mat |
Magnetocaloric effect in the high-temperature antiferromagnet YbCoC2: The magnetic $H$-$T$ phase diagram and magnetocaloric effect in the recently
discovered high-temperature heavy-fermion compound YbCoC$_2$ have been studied.
With the increase in the external magnetic field YbCoC$_2$ experiences the
metamagnetic transition and then transition to the ferromagnetic state. The
dependencies of magnetic entropy change -$\Delta S_m (T)$ have segments with
positive and negative magnetocaloric effects for $\Delta H \leq 6$~T. For
$\Delta H = 9$~T magnetocaloric effect becomes positive with a maximum value of
-$\Delta S_m (T)$ is 4.1 J / kg K and a refrigerant capacity is 56.6 J / kg. | cond-mat |
Strong photon coupling to the quadrupole moment of an electron in solid
state: The implementation of circuit quantum electrodynamics allows coupling distant
qubits by microwave photons hosted in on-chip superconducting resonators.
Typically, the qubit-photon interaction is realized by coupling the photons to
the electric dipole moment of the qubit. A recent proposal suggests storing the
quantum information in the electric quadrupole moment of an electron in a
triple quantum dot. The qubit is expected to have improved coherence since it
is insensitive to dipolar noise produced by distant voltage fluctuators. Here
we experimentally realize a quadrupole qubit in a linear array of three quantum
dots in a GaAs/AlGaAs heterostructure. A high impedance microwave resonator
coupled to the middle dot interacts with the qubit quadrupole moment. We
demonstrate strong quadrupole qubit--photon coupling and observe improved
coherence properties when operating the qubit in the parameter space where the
dipole coupling vanishes. | cond-mat |
Quantum spin liquid in antiferromagnetic chain S=1/2 with Acoustic
Phonons: A spin and phonon excitations spectrum are studied using quantum Monte Carlo
method in antiferromagnetic chain with spins $S=1/2$ coupled nonadiabaticity
with acoustic phonons . It is found the critical coupling exists to open gap in
the triplet excitation spectrum for any phonon velocity. The phase boundaries
of delocalized phonons and propagated the bound states of magnon and a phonon
are calculated. It is shown that the spherical symmetry of the spin-spin
correlation functions is broken . The magnetic and optical properties $CuGeO_3$
are explained without using spin-Peierls transition. | cond-mat |
Quantum Hall effect in a p-type heterojunction with a lateral surface
quantum dot superlattice: The quantization of Hall conductance in a p-type heterojunction with lateral
surface quantum dot superlattice is investigated. The topological properties of
the four-component hole wavefunction are studied both in r- and k-spaces. New
method of calculation of the Hall conductance in a 2D hole gas described by the
Luttinger Hamiltonian and affected by lateral periodic potential is proposed,
based on the investigation of four-component wavefunction singularities in
k-space. The deviations from the quantization rules for Hofstadter "butterfly"
for electrons are found, and the explanation of this effect is proposed. For
the case of strong periodic potential the mixing of magnetic subbands is taken
into account, and the exchange of the Chern numbers between magnetic subands is
discussed. | cond-mat |
Photo-induced superconducting-like response in strongly correlated
systems: We propose a novel mechanism for the photo-induced superconducting-like
response recently reported in cuprates and other strongly correlated materials.
This mechanism relies on quantum-fluctuating bosons consisting of electron
pairs. With periodic drive, the electron pairs and vacancies of pairs form a
coherent non-equilibrium condensate, different from conventional
superconductors, yet showing superconducting-like response in some regimes even
with dissipation. Unlike the case of driven fermionic bands which results in
the familiar Floquet bands with hybridization gaps, for driven bosons the "gap"
opens up in the momentum direction, resulting in a resonant region in momentum
space where the eigenvalues are complex. We give a simple physical argument why
this picture leads to a "perfect conductor" which exhibits superconducting-like
frequency-dependent conductivity but no Meissner response. While our model is
quite general, in the case of cuprates, quantum-fluctuating pair density wave
in the pseudogap region may serve as the origin of the quantum-fluctuating
electron pairs. | cond-mat |
Flavors of Magnetic Noise in Quantum Materials: The complexity of electronic band structures in quantum materials offers new
charge-neutral degrees of freedom stable for transport, a promising example
being the valley (axial) degree of freedom in Weyl semimetals (WSMs). A
noninvasive probe of their transport properties is possible by exploiting the
frequency dependence of the magnetic noise generated in the vicinity of the
material. In this work, we investigate the magnetic noise generically
associated with diffusive transport using a systematic Langevin approach.
Taking a minimal model of magnetic WSMs for demonstration, we show that thermal
fluctuations of the charge current, the valley current, and the magnetic order
can give rise to magnetic noise with distinctively different spectral
characters, which provide a theoretical guidance to separate their
contributions. Our approach is extendable to the study of magnetic noise and
its spectral features arising from other transport degrees of freedom in
quantum materials. | cond-mat |
General solution to inhomogeneous dephasing and smooth pulse dynamical
decoupling: In order to achieve the high-fidelity quantum control needed for a broad
range of quantum information technologies, reducing the effects of noise and
system inhomogeneities is an essential task. It is well known that a system can
be decoupled from noise or made insensitive to inhomogeneous dephasing
dynamically by using carefully designed pulse sequences based on square or
delta-function waveforms such as Hahn spin echo or CPMG. However, such ideal
pulses are often challenging to implement experimentally with high fidelity.
Here, we uncover a new geometrical framework for visualizing all possible
driving fields, which enables one to generate an unlimited number of smooth,
experimentally feasible pulses that perform dynamical decoupling or dynamically
corrected gates to arbitrarily high order. We demonstrate that this scheme can
significantly enhance the fidelity of single-qubit operations in the presence
of noise and when realistic limitations on pulse rise times and amplitudes are
taken into account. | cond-mat |
Electric Circuit Simulation of Floquet Topological Insulators: We present a method for simulating any non-interacting and time-periodic
tight-binding Hamiltonian in Fourier space using electric circuits made of
inductors and capacitors. We first map the time-periodic Hamiltonian to a
Floquet Hamiltonian, which converts the time dimension into a Floquet
dimension. In electric circuits, this Floquet dimension is simulated as an
extra spatial dimension without any time dependency in the electrical elements.
The number of replicas needed in the Floquet Hamiltonian depends on the
frequency and strength of the drive. We also demonstrate that we can detect the
topological edge states (including the anomalous edge states in the dynamical
gap) in an electric circuit by measuring the two-point impedance between the
nodes. Our method paves a simple and promising way to explore and control
Floquet topological phases in electric circuits. | cond-mat |
Protein folding simulations with Interacting Growth Walk model: We demonstrate that the recently proposed interacting growth walk (IGW)
model, modified for generating self-avoiding heteropolymers, proves to be a
simpler alternative to the other Monte Carlo methods available in the
literature for obtaining minimum energy conformation of lattice proteins. In
fact, this simple growth algorithm seems to be capable of quickly leading to
low energy states for all the three dimensional bench mark HP-sequences
investigated. | cond-mat |
Intrinsic Response of Graphene Vapor Sensors: Graphene is a purely two-dimensional material that has extremely favorable
chemical sensor properties. It is known, however, that conventional
nanolithographic processing typically leaves a resist residue on the graphene
surface, whose impact on the sensor characteristics of the system has not yet
been determined. Here we show that the contamination layer both degrades the
electronic properties of the graphene and masks graphene s intrinsic sensor
responses. The contamination layer chemically dopes the graphene, enhances
carrier scattering, and acts as an absorbent layer that concentrates analyte
molecules at the graphene surface, thereby enhancing the sensor response. We
demonstrate a cleaning process that verifiably removes the contamination on the
device structure and allows the intrinsic chemical responses of graphene to be
measured. | cond-mat |
Simplistic Coulomb forces in molecular dynamics: Comparing the Wolf and
shifted-force approximations: This paper compares the Wolf method to the shifted forces (SF) method for
efficient computer simulation of isotropic systems interacting via Coulomb
forces, taking results from the Ewald summation method as representing the true
behavior. We find that for the Hansen-McDonald molten salt model the SF
approximation overall reproduces the structural and dynamical properties as
accurately as does the Wolf method. It is shown that the optimal Wolf damping
parameter depends on the property in focus, and that neither the potential
energy nor the radial distribution function are useful measures for the
convergence of theWolf method to the Ewald summation method. The SF
approximation is also tested for the SPC/Fw model of liquid water at room
temperature, showing good agreement with both the Wolf and the particle mesh
Ewald methods; this confirms previous findings [Fennell & Gezelter, J. Chem.
Phys. {\bf 124}, 234104 (2006)]. Beside its conceptualsimplicity the SF
approximation implies a speed-up of a factor 2 to 3 compared to the Wolf method
(which is in turn much faster than the Ewald method). | cond-mat |
Signatures of topological Josephson junctions: Quasiparticle poisoning and diabatic transitions may significantly narrow the
window for the experimental observation of the $4\pi$-periodic $dc$ Josephson
effect predicted for topological Josephson junctions. Here, we show that
switching current measurements provide accessible and robust signatures for
topological superconductivity which persist in the presence of quasiparticle
poisoning processes. Such measurements provide access to the phase-dependent
subgap spectrum and Josephson currents of the topological junction when
incorporating it into an asymmetric SQUID together with a conventional
Josephson junction with large critical current. We also argue that pump-probe
experiments with multiple current pulses can be used to measure the
quasiparticle poisoning rates of the topological junction. The proposed
signatures are particularly robust, even in the presence of Zeeman fields and
spin-orbit coupling, when focusing on short Josephson junctions. Finally, we
also consider microwave excitations of short topological Josephson junctions
which may complement switching current measurements. | cond-mat |
Weak chaos and fractional dynamics in an optically driven colloidal ring: Three colloidal spheres driven around a ring-like optical trap known as an
optical vortex have been predicted to undergo periodic collective motion due to
their hydrodynamic coupling. In fact, the quenched disorder in the
optically-implemented potential energy landscape drives a transition to
instability evolving into microscopic weak chaos with fractional dynamics. As a
result, the relation between the space-time selfsimilarity of the system's
collective transport properties and its microscopic weak chaos dynamics is
revealed. | cond-mat |
Optically-induced magnetization switching in NiCo2O4 thin films using
ultrafast lasers: Recently, all-optical magnetization control has been garnering considerable
attention in realizing next-generation ultrafast magnetic information devices.
Here, employing a magneto-optical Kerr effect (MOKE) microscope, we observed
the laser-induced magnetization switching of ferrimagnetic oxide NiCo2O4 (NCO)
epitaxial thin films with perpendicular magnetic anisotropy, where the sample
was pumped at 1030-nm laser pulses, and magnetic domain images were acquired
via the MOKE microscope with a white light emitting diode. Laser pulses
irradiated an NCO thin film at various temperatures from 300 K to 400 K while
altering the parameters of pulse interval, fluence, and the number of pulses
with the absence of the external magnetic field. We observed accumulative
all-optical switching at 380 K and above. Our observation of oxide NCO thin
films facilitates the realization of chemically stable magnetization switching
using ultrafast lasers, and without applying a magnetic field. | cond-mat |
Pressure-enhanced ferromagnetism in layered CrSiTe3 flakes: The research on van der Waals (vdW) layered ferromagnets have promoted the
development of nanoscale spintronics and applications. However, low-temperature
ferromagnetic properties of these materials greatly hinder their applications.
Here, we report pressure-enhanced ferromagnetic behaviours in layered CrSiTe3
flakes revealed by high-pressure magnetic circular dichroism (MCD) measurement.
At ambient pressure, CrSiTe3 undergoes a paramagnetic-to-ferromagnetic phase
transition at 32.8 K, with a negligible hysteresis loop, indicating a soft
ferromagnetic behaviour. Under 4.6 GPa pressure, the soft ferromagnet changes
into hard one, signalled by a rectangular hysteretic loop with remnant
magnetization at zero field. Interestingly, with further increasing pressure,
the coercive field (H_c) dramatically increases from 0.02 T at 4.6 GPa to 0.17
T at 7.8 GPa, and the Curie temperature (T_c^h: the temperature for closing the
hysteresis loop) also increases from ~36 K at 4.6 GPa to ~138 K at 7.8 GPa. The
influences of pressure on exchange interactions are further investigated by
density functional theory calculations, which reveal that the in-plane
nearest-neighbor exchange interaction and magneto-crystalline anisotropy
increase simultaneously as pressure increases, leading to increased H_c and
T_c^h in experiments. The effective interaction between magnetic couplings and
external pressure offers new opportunities for both searching room-temperature
layered ferromagnets and designing pressure-sensitive magnetic functional
devices. | cond-mat |
Method of Image and Transmission through Semi-infinite Nanowires: The method of functional integral bosonization is extended to examine the
transmission properties of semi-infinite nanowires. In particular, it is shown
that edge states will arise at the end point of the dimerized semi-infinite
spin-chain and by combining the method of image and the bosonization technique,
the system can be properly bosonized. Based on the bosonized action and a
renormalization group analysis, it is shown that unlike scattering due to
single bulk impurity in the nanowire, the scattering potential remains relevant
even for slightly attractive potential due to the interaction between the edge
state and its image. When the strength of potential goes beyond a critical
strength, the tip of the semi-infinite nanowire may become insulating. | cond-mat |
Tuning the confinement potential between spinons in the Ising chain
CoNb2O6 using longitudinal fields and quantitative determination of the
microscopic Hamiltonian: The Ising chain realizes the fundamental paradigm of spin fractionalization,
where locally flipping a spin creates two domain walls (spinons) that can
separate apart at no energy cost. In a quasi-one-dimensional system, the
mean-field effects of the weak three-dimensional couplings confine the spinons
into a Zeeman ladder of two-spinon bound states. Here, we experimentally tune
the confinement potential between spinons in the quasi-one-dimensional Ising
ferromagnet CoNb2O6 by means of an applied magnetic field with a large
component along the Ising direction. Using high-resolution single crystal
inelastic neutron scattering, we directly observe how the spectrum evolves from
the limit of very weak confinement at low field (with many closely-spaced bound
states with energies scaling as the field strength to the power 2/3) to very
strong confinement at high field (where it consists of a magnon and a
dispersive two-magnon bound state, with a linear field dependence). At
intermediate fields, we explore how the higher-order bound states disappear
from the spectrum as they move to higher energies and overlap with the
two-particle continuum. By performing a global fit to the observed spectrum in
zero field and high field applied along two orthogonal directions, combined
with a quantitative parameterization of the interchain couplings, we propose a
refined single chain and interchain Hamiltonian that quantitatively reproduces
all observed dispersions and their field dependence. | cond-mat |
Mode specific electronic friction in dissociative chemisorption on metal
surfaces: H$_2$ on Ag(111): Electronic friction and the ensuing nonadiabatic energy loss play an
important role in chemical reaction dynamics at metal surfaces. Using molecular
dynamics with electronic friction evaluated on-the-fly from Density Functional
Theory, we find strong mode dependence and a dominance of nonadiabatic energy
loss along the bond stretch coordinate for scattering and dissociative
chemisorption of H$_2$ on the Ag(111) surface. Exemplary trajectories with
varying initial conditions indicate that this mode-specificity translates into
modulated energy loss during a dissociative chemisorption event. Despite minor
nonadiabatic energy loss of about 5\%, the directionality of friction forces
induces dynamical steering that affects individual reaction outcomes,
specifically for low-incidence energies and vibrationally excited molecules.
Mode-specific friction induces enhanced loss of rovibrational rather than
translational energy and will be most visible in its effect on final energy
distributions in molecular scattering experiments. | cond-mat |
Deriving GENERIC from a generalized fluctuation symmetry: Much of the structure of macroscopic evolution equations for relaxation to
equilibrium can be derived from symmetries in the dynamical fluctuations around
the most typical trajectory. For example, detailed balance as expressed in
terms of the Lagrangian for the path-space action leads to gradient zero-cost
flow. We find a new such fluctuation symmetry that implies GENERIC, an
extension of gradient flow where a Hamiltonian part is added to the dissipative
term in such a way as to retain the free energy as Lyapunov function. | cond-mat |
A theory for the effect of patch / non-patch attractions on the
self-assembly of patchy colloids: In this paper, we develop a thermodynamic perturbation theory to describe the
self-assembly of patchy colloids which exhibit both patch-patch attractions as
well as patch / non-patch attractions. That is, patches attract other patches
as well as the no patch region. In general these attractions operate on
different energy scales, which allows for controlled self-assembly as well as
anomalous phase behavior. As an application we apply the model to the study of
liquid water. | cond-mat |
Visibility study of graphene multilayer structures: The visibility of graphene sheets on different types of substrates has been
investigated both theoretically and experimentally. Although single layer
graphene is observable on various types of dielectrics under an optical
microscope, it is invisible when it is placed directly on most of the
semiconductor and metallic substrates. We show that coating of a resist layer
with optimum thickness is an effective way to enhance the contrast of graphene
on various types of substrates and makes single layer graphene visible on most
semiconductor and metallic substrates. Experiments have been performed to
verify the results on quartz and NiFe-coated Si substrates. The results
obtained will be useful for fabricating graphene-based devices on various types
of substrates for electronics, spintronics and optoelectronics applications. | cond-mat |
Antiferromagnetic ordering and dipolar interactions of YbAlO$_3$: In this paper we report low-temperature magnetic properties of the rare-earth
perovskite material YbAlO$_3$. Results of elastic and inelastic neutron
scattering experiment, magnetization measurements along with the crystalline
electrical field (CEF) calculations suggest that the ground state of Yb moments
is a strongly anisotropic Kramers doublet, and the moments are confined in the
$ab$-plane, pointing at an angle of $\varphi = \pm 23.5^{\circ}$ to the
$a$-axis. With temperature decreasing below $T_{\rm N}=0.88$ K, Yb moments
order into the coplanar, but non-collinear antiferromagnetic (AFM) structure
$AxGy$, where the moments are pointed along their easy-axes. In addition, we
highlight the importance of the dipole-dipole interaction, which selects the
type of magnetic ordering and may be crucial for understanding magnetic
properties of other rare-earth orthorhombic perovskites. Further analysis of
the broad diffuse neutron scattering shows that one-dimensional interaction
along the $c$-axis is dominant, and suggests YbAlO$_3$ as a new member of one
dimensional quantum magnets. | cond-mat |
Superconducting microwave resonators with non-centrosymmetric
nonlinearity: We investigated both theoretically and experimentally open-ended coplanar
waveguide resonators with rf SQUIDs embedded in the central conductor at
different positions. These rf SQUIDs can be tuned by an external magnetic field
and thus may exhibit the non-centrosymmetric nonlinearity of $\chi^{(2)}$ type
with suppressed Kerr nonlinearity. We demonstrated that this nonlinearity
allows for efficient mixing of $\lambda/2$ and $\lambda$ modes in the cavity
and thus enables various parametric effects with three wave mixing. These
effects are the second harmonic generation, the half tone generation, the
parametric amplification in both degenerate and non-degenerate regimes and
deamplification in degenerate regime. | cond-mat |
Thermoelectric properties of Wigner crystal in two-dimensional periodic
potential: We study numerically transport and thermoelectric properties of electrons
placed in a two-dimensional (2D) periodic potential. Our results show that the
transition from sliding to pinned phase takes place at a certain critical
amplitude of lattice potential being similar to the Aubry transition for the
one-dimensional Frenkel-Kontorova model. We show that the 2D Aubry pinned phase
is characterized by high values of Seebeck coefficient S = 12. At the same time
we find that the value of Seebeck coefficient is significantly influenced by
the geometry of periodic potential. We discuss possibilities to test the
properties of 2D Aubry phase with electrons on a surface of liquid helium. | cond-mat |
Theoretical prediction of Curie temperature in two-dimensional
ferromagnetic monolayer: Theoretical prediction of Curie temperature (TC) is of vital importance for
designing the spintronic devices in two-dimensional (2D) ferromagnetic
materials. Herein, based on the extensive investigation of Monte Carlo
simulations, we summary and propose an improved method to estimate TC more
precisely, which includes the different contributions of multiple near-neighbor
interactions. Taking monolayer CrI3 as an example, the trends of TC with
biaxial strain are investigated via Monte Carlo simulations, mean-field
formulas and our method. Besides, our method is not only accurate and
convenient to predicting the TC in 2D ferromagnetic honeycomb lattice CrI3 but
it can be extended for predicting the TC of other 2D lattices. Our work paves
the way to accelerate the prediction and discovery of novel 2D ferromagnets for
spintronic applications. | cond-mat |
A microscopic theory of Curzon-Ahlborn heat engine: Abstract The Curzon-Ahlborn (CA) efficiency, as the efficiency at the maximum
power (EMP) of the endoreversible Carnot engine, has a significant impact on
finite-time thermodynamics. However, the CA engine model is based on many
assumptions. In the past few decades, although a lot of efforts have been made,
a microscopic theory of the CA engine is still lacking. By adopting the method
of the stochastic differential equation of energy, we formulate a microscopic
theory of the CA engine realized with an underdamped Brownian particle in a
class of non-harmonic potentials. This theory gives microscopic interpretation
of all assumptions made by Curzon and Ahlborn, and thus puts the results about
CA engine on a solid foundation. Also, based on this theory, we obtain
analytical expressions of the power and the efficiency statistics for the
Brownian CA engine. Our research brings new perspectives to experimental
studies of finite-time microscopic heat engines featured with fluctuations. | cond-mat |
Soft self-assembled nanoparticles with temperature-dependent properties: The fabrication of versatile building blocks that are reliably self-assemble
into desired ordered and disordered phases is amongst the hottest topics in
contemporary material science. To this end, microscopic units of varying
complexity, aimed at assembling the target phases, have been thought, designed,
investigated and built. Such a path usually requires laborious fabrication
techniques, especially when a specific funcionalisation of the building blocks
is required. Telechelic star polymers, i.e., star polymers made of a number $f$
of di-block copolymers consisting of solvophobic and solvophilic monomers
grafted on a central anchoring point, spontaneously self-assemble into soft
patchy particles featuring attractive spots (patches) on the surface. Here we
show that the tunability of such a system can be widely extended by controlling
the physical and chemical parameters of the solution. Indeed, at fixed external
conditions the self-assembly behaviour depends only on the number of arms
and/or on the ratio of solvophobic to solvophilic monomers. However, changes in
temperature and/or solvent quality makes it possible to reliably change the
number and size of the attractive patches. This allows to steer the mesoscopic
self-assembly behaviour without modifying the microscopic constituents.
Interestingly, we also demonstrate that diverse combinations of the parameters
can generate stars with the same number of patches but different radial and
angular stiffness. This mechanism could provide a neat way of further
fine-tuning the elastic properties of the supramolecular network without
changing its topology. | cond-mat |
Diffusion and localization for the Chirikov typical map: We consider the classical and quantum properties of the "Chirikov typical
map", proposed by Boris Chirikov in 1969. This map is obtained from the well
known Chirikov standard map by introducing a finite number $T$ of random phase
shift angles. These angles induce a random behavior for small time scales
($t<T$) and a $T$-periodic iterated map which is relevant for larger time
scales ($t>T$). We identify the classical chaos border $k_c\sim T^{-3/2} \ll 1$
for the kick parameter $k$ and two regimes with diffusive behavior on short and
long time scales. The quantum dynamics is characterized by the effect of
Chirikov localization (or dynamical localization). We find that the
localization length depends in a subtle way on the two classical diffusion
constants in the two time-scale regime. | cond-mat |
Bounds on current fluctuations in periodically driven systems: Small nonequilibrium systems in contact with a heat bath can be analyzed with
the framework of stochastic thermodynamics. In such systems, fluctuations,
which are not negligible, follow universal relations such as the fluctuation
theorem. More recently, it has been found that, for nonequilibrium stationary
states, the full spectrum of fluctuations of any thermodynamic current is
bounded by the average rate of entropy production and the average current.
However, this bound does not apply to periodically driven systems, such as heat
engines driven by periodic variation of the temperature and artificial
molecular pumps driven by an external protocol. We obtain a universal bound on
current fluctuations for periodically driven systems. This bound is a
generalization of the known bound for stationary states. In general, the
average rate that bounds fluctuations in periodically driven systems is
different from the rate of entropy production. We also obtain a local bound on
fluctuations that leads to a trade-off relation between speed and precision in
periodically driven systems, which constitutes a generalization to periodically
driven systems of the so called thermodynamic uncertainty relation. From a
technical perspective, our results are obtained with the use of a recently
developed theory for 2.5 large deviations for Markov jump processes with
time-periodic transition rates. | cond-mat |
Slow crack propagation through a disordered medium: Critical transition
and dissipation: We show that the intermittent and self-similar fluctuations displayed by a
slow crack during the propagation in a heterogeneous medium can be
quantitatively described by an extension of a classical statistical model for
fracture. The model yields the correct dynamical and morphological scaling, and
allows to demonstrate that the scale invariance originates from the presence of
a non-equilibrium, reversible, critical transition which in the presence of
dissipation gives rise to self organized critical behaviour. | cond-mat |
Observation of sub-kelvin superconductivity in Cd$_3$As$_2$ thin films: We report the first experimental observation of superconductivity in
Cd$_3$As$_2$ thin films without application of external pressure. Surface
studies suggest that the observed transport characteristics are related to the
polycrystalline continuous part of investigated films with homogeneous
distribution of elements and the Cd-to-As ratio close to stoichiometric
Cd$_3$As$_2$. The latter is also supported by Raman spectra of the studied
films, which are similar to those of Cd$_3$As$_2$ single crystals. The
formation of superconducting phase in films under study is confirmed by the
characteristic behavior of temperature and magnetic field dependence of samples
resistances, as well as by the presence of pronounced zero-resistance plateaux
in $dV/dI$ characteristics. The corresponding $H_c-T_c$ plots reveal a clearly
pronounced linear behavior within the intermediate temperature range, similar
to that observed for bulk Cd$_3$As$_2$ and Bi$_2$Se$_3$ films under pressure,
suggesting the possibility of nontrivial pairing in the films under
investigation. We discuss a possible role of sample inhomogeneities and crystal
strains in the observed phenomena. | cond-mat |
A curious relationship between Potts glass models: A Potts glass model proposed by Nishimori and Stephen[H. Nishimori and M. J.
Stephen, Phys. Rev. B 27, 5644 (1983)] is analyzed by means of the replica mean
field theory. This model is a discrete model, has a gauge symmetry, and is
called the Potts gauge glass model. By comparing the present results with the
results of the conventional Potts glass model, we find the coincidences and
differences between the models. We find a coincidence that the property for the
Potts glass phase in this model is coincident with that in the conventional
model at the mean field level. We find a difference that, unlike in the case of
the conventional $p$-state Potts glass model, this system for large $p$ does
not become ferromagnetic at low temperature under a concentration of
ferromagnetic interaction. The present results support the act of numerically
investigating the present model for study of the Potts glass phase in finite
dimensions. | cond-mat |
Tailoring population inversion in Landau-Zener-Stückelberg
interferometry of flux qubits: We distinguish different mechanisms for population inversion in flux qubits
driven by dc+ac magnetic fields. We show that for driving amplitudes such that
there are Landau-Zener-St\"uckelberg intereferences, it is possible to have
population inversion solely mediated by the environmental bath at long driving
times. We study the effect of the resonant frequency $\Omega_p$ of the
measuring circuit, finding different regimes for the asymptotic population of
the state of the flux qubit. By tailoring $\Omega_p$ the degree of population
inversion can be controlled. Our studies are based on realistic simulations of
the device for the Josephson flux qubit using the Floquet-Born-Markov
formalism. | cond-mat |
An analysis of localization transitions using non-parametric
unsupervised learning: We propose a new viewpoint on the study of localization transitions in
disordered quantum systems, showing how critical properties can be seen also as
a geometric transition in the data space generated by the classically encoded
configurations of the disordered quantum system. We showcase our approach to
the Anderson model on regular random graphs, known for displaying features of
interacting systems, despite being a single-particle problem. We estimate the
transition point and critical exponents in agreement with the best-known
results in the literature. We provide a simple and coherent explanation of our
findings, discussing the applicability of the method in real-world scenarios
with a modest number of measurements. | cond-mat |
Memristive Reservoirs Learn to Learn: Memristive reservoirs draw inspiration from a novel class of neuromorphic
hardware known as nanowire networks. These systems display emergent brain-like
dynamics, with optimal performance demonstrated at dynamical phase transitions.
In these networks, a limited number of electrodes are available to modulate
system dynamics, in contrast to the global controllability offered by
neuromorphic hardware through random access memories. We demonstrate that the
learn-to-learn framework can effectively address this challenge in the context
of optimization. Using the framework, we successfully identify the optimal
hyperparameters for the reservoir. This finding aligns with previous research,
which suggests that the optimal performance of a memristive reservoir occurs at
the `edge of formation' of a conductive pathway. Furthermore, our results show
that these systems can mimic membrane potential behavior observed in spiking
neurons, and may serve as an interface between spike-based and continuous
processes. | cond-mat |
Quasi-free-standing AA-stacked bilayer graphene induced by calcium
intercalation of the graphene-silicon carbide interface: We study quasi-freestanding bilayer graphene on silicon carbide intercalated
by calcium. The intercalation, and subsequent changes to the system, were
investigated by low-energy electron diffraction, angle-resolved photoemission
spectroscopy (ARPES) and density-functional theory (DFT). Calcium is found to
intercalate only at the graphene-SiC interface, completely displacing the
hydrogen terminating SiC. As a consequence, the system becomes highly n-doped.
Comparison to DFT calculations shows that the band dispersion, as determined by
ARPES, deviates from the band structure expected for Bernal-stacked bilayer
graphene. Instead, the electronic structure closely matches AA-stacked bilayer
graphene on Ca-terminated SiC, indicating a spontaneous transition from AB- to
AA-stacked bilayer graphene following calcium intercalation of the underlying
graphene-SiC interface. | cond-mat |
Multivalley effective mass theory simulation of donors in silicon: Last year, Salfi et al. made the first direct measurements of a donor wave
function and found extremely good theoretical agreement with atomistic
tight-binding [Salfi et al., Nat. Mater. 13, 605 (2014)]. Here, we show that
multi-valley effective mass theory, applied properly, does achieve close
agreement with tight-binding and hence gives reliable predictions. To
demonstrate this, we variationally solve the coupled six-valley Shindo-Nara
equations, including silicon's full Bloch functions. Surprisingly, we find that
including the full Bloch functions necessitates a tetrahedral, rather than
spherical, donor central cell correction to accurately reproduce the
experimental energy spectrum of a phosphorus impurity in silicon. We
cross-validate this method against atomistic tight-binding calculations,
showing that the two theories agree well for the calculation of donor-donor
tunnel coupling. Further, we benchmark our results by performing a statistical
uncertainty analysis, confirming that derived quantities such as the wave
function profile and tunnel couplings are robust with respect to variational
energy fluctuations. Finally, we apply this method to exhaustively enumerate
the tunnel coupling for all donor-donor configurations within a large search
volume, demonstrating conclusively that the tunnel coupling has no spatially
stable regions. Though this instability is problematic for reliably coupling
donor pairs for two-qubit operations, we identify specific target locations
where donor qubits can be placed with scanning tunneling microscopy technology
to achieve reliably large tunnel couplings. | cond-mat |
A tight-binding approach to uniaxial strain in graphene: We analyze the effect of tensional strain in the electronic structure of
graphene. In the absence of electron-electron interactions, within linear
elasticity theory, and a tight-binding approach, we observe that strain can
generate a bulk spectral gap. However this gap is critical, requiring threshold
deformations in excess of 20%, and only along preferred directions with respect
to the underlying lattice. The gapless Dirac spectrum is robust for small and
moderate deformations, and the gap appears as a consequence of the merging of
the two inequivalent Dirac points, only under considerable deformations of the
lattice. We discuss how strain-induced anisotropy and local deformations can be
used as a means to affect transport characteristics and pinch off current flow
in graphene devices. | cond-mat |
Colloidal Hard Spheres: Triumphs, Challenges and Mysteries: The simplicity of hard spheres as a model system is deceptive. Although the
particles interact solely through volume exclusion, that nevertheless suffices
for a wealth of static and dynamical phenomena to emerge, making the model an
important target for achieving a comprehensive understanding of matter. In
addition, while real colloidal suspensions are typically governed by complex
interactions, Pusey and Van Megen [Nature 320 340--342 (1986)] demonstrated
that suitably tuned suspensions result in hard-sphere like behavior, thus
bringing a valuable experimental complement to the celebrated theoretical
model. Colloidal hard spheres are thus both a material in their own right and a
platform upon which phenomena exhibited by simple materials can be explored in
great detail. These various purposes enable a particular synergy between
experiment, theory and computer simulation. Here we review the extensive body
of work on hard spheres, ranging from their equilibrium properties such as
phase behavior, interfaces and confinement to some of the non--equilibrium
phenomena they exhibit such as sedimentation, glass formation and nucleation. | cond-mat |
Minimum vertex cover problems on random hypergraphs: replica symmetric
solution and a leaf removal algorithm: We study minimum vertex cover problems on random \alpha-uniform hypergraphs
using two different approaches, a replica method in statistical mechanics of
random systems and a leaf removal algorithm. It is found that there exists a
phase transition at the critical average degree e/(\alpha-1). Below the
critical degree, a replica symmetric ansatz in the statistical-mechanical
method holdsand the algorithm estimates a solution of the problem which
coincide with that by the replica method. In contrast, above the critical
degree, the replica symmetric solution becomes unstable and these methods fail
to estimate the exact solution.These results strongly suggest a close relation
between the replica symmetry and the performance of approximation algorithm. | cond-mat |
Competition between heavy-fermion and Kondo interaction in isoelectronic
A-site ordered perovskites: With current research efforts shifting towards the 4$d$ and 5$d$ transition
metal oxides, understanding the evolution of the electronic and magnetic
structure as one moves away from 3$d$ materials is of critical importance. Here
we perform X-ray spectroscopy and electronic structure calculations on $A$-site
ordered perovskites with Cu in the $A$-site and the $B$-sites descending along
the 9th group of the periodic table to elucidate the emerging properties as
$d$-orbitals change from partially filled 3$d$, 4$d$, to 5$d$. The results show
that when descending from Co to Ir the charge transfers from the cuprate like
Zhang-Rice state on Cu to the t$_{2g}$ orbital of the B site. As the Cu
$d$-orbital occupation approaches the Cu$^{2+}$ limit, a mixed-valence state in
CaCu$_3$Rh$_4$O$_{12}$ and heavy fermion state in CaCu$_3$Ir$_4$O$_{12}$ are
obtained. The investigated d-electron compounds are mapped onto the Doniach
phase diagram of the competing RKKY and Kondo interactions developed for
f-electron systems. | cond-mat |
Study of implosion in an attractive Bose-Einstein condensate: By solving the Gross-Pitaevskii equation analytically and numerically, we
reexamine the implosion phenomena that occur beyond the critical value of the
number of atoms of an attractive Bose-Einstein condensate (BEC) with
cigar-shape trapping geometry. We theoretically calculate the critical number
of atoms in the condensate by using Ritz's variational optimization technique
and investigate the stability and collapse dynamics of the attractive BEC by
numerically solving the time dependent Gross-Pitavskii equation. | cond-mat |
Fermi surface truncation from thermal nematic fluctuations: We analyze how thermal fluctuations near a finite temperature nematic phase
transition affect the spectral function $A({\bf k},\omega)$ for single-electron
excitations in a two-dimensional metal. Perturbation theory yields a splitting
of the quasi-particle peak with a d-wave form factor, reminiscent of a
pseudogap. We present a resummation of contributions to all orders in the
Gaussian fluctuation regime. Instead of a splitting, the resulting spectral
function exhibits a pronounced broadening of the quasi-particle peak, which
varies strongly around the Fermi surface and vanishes upon approaching the
Brillouin zone diagonal. The Fermi surface obtained from a Brillouin zone plot
of $A({\bf k},0)$ seems truncated to Fermi arcs. | cond-mat |
Showcasing the necessity of the principle of relative motion in physical
statistics: Inconsistency of the `segmented Fermi surface': The hunt for exotic properties in flowing systems is a popular and active
field of study, and has recently gained renewed attention through claims such
as a ``segmented Fermi surface'' in a superconducting system that hosts steady
superflow of screening current driven by an external field. Apart from this
excitement and the promise of hosting Majorana zero modes, claims such as this
imply exotic gap-to-gapless quantum phase transitions merely through boost of
inertial frames of observation, and challenge the very concept behind the
principle of relative motion. Here, we first illustrate an obvious inescapable
physical inconsistency of such claims concerning the flow velocity. Taking into
account this basic principle from the beginning, we then demonstrate that a
proper employment of physical statistics naturally reproduces the experimental
observation without causing such a conceptual crisis. This example showcases
the importance of strict adherence to the basic principle of relative motion in
physical statistics, especially when pushing the frontiers of physics and
technology. | cond-mat |
Heat convection and radiation in flighted rotary kilns: A minimal model: We propose a minimal model aiming to describe heat transfer between particles
(i.e. grains) and gases in a model of flighted rotary kilns. It considers a
channel in which a convective gas interacts with a granular suspension and a
granular bed. Despite its simplicity it captures the main experimental findings
in the case of dilute suspension of heavy grains typical of what can be
observed in many industrial rotary kilns. Energy balance between each phase
takes into account the main heat transfer mechanisms between the transverse
granular motion and the convective gas. In the absence of radiation heat
transfer, the model predicts exponential variations of the temperatures
characterized by a length which depends on the granular and gas heat flow rates
as well as on the exchange areas. When radiation is taken into account, the
model can be solved numerically. For this case, the temperature variations can
be fitted by stretched exponentials whose parameters are found to be
independent of the studied phases. Finally, an efficiency criterion is proposed
to optimize the length of the system. | cond-mat |
Temperature evolution of spin accumulation detected electrically in a
nondegenerated silicon channel: We study temperature evolution of spin accumulation signals obtained by the
three-terminal Hanle effect measurements in a nondegenerated silicon channel
with a Schottky-tunnel-barrier contact. We find the clear difference in the
temperature-dependent spin signals between spin-extraction and spin-injection
conditions. In a spin-injection condition with a low bias current, the
magnitude of spin signals can be enhanced despite the rise of temperature. For
the interpretation of the temperature-dependent spin signals, it is important
to consider the sensitivity of the spin detection at the
Schottky-tunnel-barrier contact in addition to the spin diffusion in Si. | cond-mat |
Structure of BSCCO supermodulation from ab initio calculations: We present results of density functional theory (DFT) calculation of the
structural supermodulation in BSCCO-2212 structure, and show that the
supermodulation is indeed a spontaneous symmetry breaking of the nominal
crystal symmetry, rather than a phenomenon driven by interstitial O dopants.
The structure obtained is in excellent quantitative agreement with recent x-ray
studies, and reproduces several qualitative aspects of scanning tunnelling
microscopy (STM) experiments as well. The primary structural modulation
affecting the CuO_2 plane is found to be a buckling wave of tilted CuO_5
half-octahedra, with maximum tilt angle near the phase of the supermodulation
where recent STM experiments have discovered an enhancement of the
superconducting gap. We argue that the tilting of the half-octahedra and
concommitant planar buckling are directly modulating the superconducting pair
interaction. | cond-mat |
Giant Nernst effect in the crossover between Fermi liquid and strange
metal: The strange-metal state is a crucial problem in condensed matter physics
highlighted by its ubiquity in almost all major correlated systems[1-7]. Its
understanding could provide important insight into high-Tc superconductivity[2]
and quantum criticality[8]. However, with the Fermi liquid theory failing in
strange metals, understanding the highly unconventional behaviors has been a
long-standing challenge. Fundamental aspects of strange metals remain elusive,
including the nature of their charge carriers[1]. Here, we report the
observation of a giant Nernst response in the strange-metal state in a
two-dimensional superconductor 2M-WS2. A giant Nernst coefficient comparable to
the vortex Nernst signal in superconducting cuprates, and its high sensitivity
to carrier mobility, are found when the system enters the strange-metal state
from the Fermi liquid state. The temperature and magnetic field dependence of
the giant Nernst peak rule out the relevance of both Landau quasiparticles and
superconductivity. Instead, the giant Nernst peak at the crossover indicates a
dramatic change in carrier entropy when entering the strange-metal state. The
presence of such an anomalous Nernst response is further confirmed in other
iconic strange metals, suggesting its universality and places stringent
experimental constraints on the mechanism of strange metals. | cond-mat |
From Coherent Modes to Turbulence and Granulation of Trapped Gases: The process of exciting the gas of trapped bosons from an equilibrium initial
state to strongly nonequilibrium states is described as a procedure of symmetry
restoration caused by external perturbations. Initially, the trapped gas is
cooled down to such low temperatures, when practically all atoms are in
Bose-Einstein condensed state, which implies the broken global gauge symmetry.
Excitations are realized either by imposing external alternating fields,
modulating the trapping potential and shaking the cloud of trapped atoms, or it
can be done by varying atomic interactions by means of Feshbach resonance
techniques. Gradually increasing the amount of energy pumped into the system,
which is realized either by strengthening the modulation amplitude or by
increasing the excitation time, produces a series of nonequilibrium states,
with the growing fraction of atoms for which the gauge symmetry is restored. In
this way, the initial equilibrium system, with the broken gauge symmetry and
all atoms condensed, can be excited to the state, where all atoms are in the
normal state, with completely restored gauge symmetry. In this process, the
system, starting from the regular superfluid state, passes through the states
of vortex superfluid, turbulent superfluid, heterophase granular fluid, to the
state of normal chaotic fluid in turbulent regime. Both theoretical and
experimental studies are presented. | cond-mat |
Butterfly hysteresis loop at non-zero bias field in antiferromagnetic
molecular rings: cooling by adiabatic magnetization: At low temperatures, the magnetization of the molecular ferric wheel NaFe$_6$
exhibits a step at a critical field $B_c$ due to a field-induced
level-crossing. By means of high-field torque magnetometry we observed a
hysteretic behavior at the level-crossing with a characteristic butterfly shape
which is analyzed in terms of a dissipative two-level model. Several unusual
features were found. The non-zero bias field of the level-crossing suggests the
possibility of cooling by adiabatic magnetization. | cond-mat |
Microtubules: Montroll's kink and Morse vibrations: Using a version of Witten's supersymmetric quantum mechanics proposed by
Caticha, we relate Montroll's kink to a traveling, asymmetric Morse double-well
potential suggesting in this way a connection between kink modes and
vibrational degrees of freedom along microtubules | cond-mat |
Striped Ferronematic ground states in a spin-orbit coupled $S=1$ Bose
gas: We theoretically establish the mean-field phase diagram of a homogeneous
spin-$1$, spin-orbit coupled Bose gas as a function of the spin-dependent
interaction parameter, the Raman coupling strength and the quadratic Zeeman
shift. We find that the interplay between spin-orbit coupling and
spin-dependent interactions leads to the occurrence of ferromagnetic or
ferronematic phases which also break translational symmetry. For weak Raman
coupling, increasing attractive spin-dependent interactions (as in $^{87}$Rb or
$^7$Li) induces a transition from a uniform to a stripe XY ferromagnet (with no
nematic order). For repulsive spin-dependent interactions however (as in
$^{23}$Na), we find a transition from an $XY$ spin spiral phase ($<S_{z} >= 0$
and uniform total density) with uniaxial nematic order, to a biaxial
ferronematic, where the total density, spin vector and nematic director
oscillate in real space. We investigate the stability of these phases against
the quadratic Zeeman effect, which generally tends to favor uniform phases with
either ferromagnetic or nematic order but not both. We discuss the relevance of
our results to ongoing experiments on spin-orbit coupled, spinor Bose gases. | cond-mat |
Dirty Weyl fermions: rare region effects near 3D Dirac points: We study three-dimensional Dirac fermions with weak finite-range scalar
potential disorder. We show that even though disorder is perturbatively
irrelevant at 3D Dirac points, nonperturbative effects from rare regions give
rise to a nonzero density of states and a finite mean free path, with the
transport at the Dirac point being dominated by hopping between rare regions.
As one moves in chemical potential away from the Dirac point, there are
interesting intermediate-energy regimes where the rare regions produce
scattering resonances that determine the DC conductivity. We also discuss the
interplay of disorder with interactions at the Dirac point. Attractive
interactions drive a transition into a granular superconductor, with a critical
temperature that depends strongly on the disorder distribution. In the presence
of Coulomb repulsion and weak retarded attraction, the system can be a Bose
glass. Our results apply to all 3D systems with Dirac points, including Weyl
semimetals, and overturn a thirty year old consensus regarding the irrelevance
of weak disorder at 3D Dirac points. | cond-mat |
Orbital-Peierls State in NaTiSi2O6: Does the quasi one-dimensional titanium pyroxene NaTiSi2O6 exhibit the novel
{\it orbital-Peierls} state? We calculate its groundstate properties by three
methods: Monte Carlo simulations, a spin-orbital decoupling scheme and a
mapping onto a classical model. The results show univocally that for the spin
and orbital ordering to occur at the same temperature --an experimental
observation-- the crystal field needs to be small and the orbitals are active.
We also find that quantum fluctuations in the spin-orbital sector drive the
transition, explaining why canonical bandstructure methods fail to find it. The
conclusion that NaTiSi2O6 shows an orbital-Peierls transition is therefore
inevitable. | cond-mat |
From Floquet to Dicke: quantum spin-Hall insulator interacting with
quantum light: Time-periodic perturbations due to classical electromagnetic fields are
useful to engineer the topological properties of matter using the Floquet
theory. Here we investigate the effect of quantized electromagnetic fields by
focusing on the quantized light-matter interaction on the edge state of a
quantum spin-Hall insulator. A Dicke-type superradiant phase transition occurs
at arbitrary weak coupling, the electronic spectrum acquires a finite gap and
the resulting ground state manifold is topological with Chern number $\pm 1$.
When the total number of excitations is conserved, a photocurrent is generated
along the edge, being pseudo-quantized as $\omega\ln(1/\omega)$ in the low
frequency limit, and decaying as $1/\omega$ for high frequencies with $\omega$
the photon frequency. The photon spectral function exhibits a clean Goldstone
mode, a Higgs like collective mode at the optical gap and the polariton
continuum. | cond-mat |
Transport via classical percolation at quantum Hall plateau transitions: We consider transport properties of disordered two-dimensional electron gases
under high perpendicular magnetic field, focusing in particular on the peak
longitudinal conductivity $\sigma_{xx}^\mathrm{peak}$ at the quantum Hall
plateau transition. We use a local conductivity model, valid at temperatures
high enough such that quantum tunneling is suppressed, taking into account the
random drift motion of the electrons in the disordered potential landscape and
inelastic processes provided by electron-phonon scattering. A diagrammatic
solution of this problem is proposed, which leads to a rich interplay of
conduction mechanisms, where classical percolation effects play a prominent
role. The scaling function for $\sigma_{xx}^\mathrm{peak}$ is derived in the
high temperature limit, which can be used to extract universal critical
exponents of classical percolation from experimental data. | cond-mat |
Flat band induced non-Fermi liquid behavior of multicomponent fermions: We investigate multicomponent fermions in a flat band and predict
experimental signatures of non-Fermi liquid behavior. We use dynamical
mean-field theory to obtain the density, double occupancy and entropy in a Lieb
lattice for $\mathcal{N} = 2$ and $\mathcal{N} = 4$ components. We derive a
mean-field scaling relation between the results for different values of
$\mathcal{N}$, and study its breakdown due to beyond-mean field effects. The
predicted signatures occur at temperatures above the N\'eel temperature and
persist in presence of a harmonic trapping potential, thus they are observable
with current ultracold gas experiments. | cond-mat |
Liquid n-hexane condensed in silica nanochannels: A combined optical
birefringence and vapor sorption isotherm study: The optical birefringence of liquid n-hexane condensed in an array of
parallel silica channels of 7nm diameter and 400 micrometer length is studied
as a function of filling of the channels via the vapor phase. By an analysis
with the generalized Bruggeman effective medium equation we demonstrate that
such measurements are insensitive to the detailed geometrical (positional)
arrangement of the adsorbed liquid inside the channels. However, this technique
is particularly suitable to search for any optical anisotropies and thus
collective orientational order as a function of channel filling. Nevertheless,
no hints for such anisotropies are found in liquid n-hexane. The n-hexane
molecules in the silica nanochannels are totally orientationally disordered in
all condensation regimes, in particular in the film growth as well as in the
the capillary condensed regime. Thus, the peculiar molecular arrangement found
upon freezing of liquid n-hexane in nanochannel-confinement, where the
molecules are collectively aligned perpendicularly to the channels' long axes,
does not originate in any pre-alignment effects in the nanoconfined liquid due
to capillary nematization. | cond-mat |
Coulomb-interaction induced incomplete shell filling in the hole system
of InAs quantum dots: We have studied the hole charging spectra of self-assembled InAs quantum dots
in perpendicular magnetic fields by capacitance-voltage spectroscopy. From the
magnetic field dependence of the individual peaks we conclude that the s-like
ground state is completely filled with two holes but that the fourfold
degenerate p-shell is only half filled with two holes before the filling of the
d-shell starts. The resulting six-hole ground state is highly polarized. This
incomplete shell filling can be explained by the large influence of the Coulomb
interaction in this system. | cond-mat |
Callen-Welton fluctuation dissipation theorem and Nyquist theorem as a
consequence of detailed balance principle applied to an oscillator: We re-derive the Nyquist theorem and Callen-Welton fluctuation-dissipation
theorem (FDT) as a consequence of detailed balance principle applied to a
harmonic oscillator. The usage of electrical notions in the beginning makes the
consideration understandable for every physicists. Perhaps it is the simplest
derivation of these well-known theorems in statistical physics. The classical
limit is understandable as a consequence of Waterston-Herapath equipartition
theorem. | cond-mat |
Geometrical barriers and the growth of flux domes in thin ideal
superconducting disks: When an ideal (no bulk pinning) flat type-II superconducting disk is
subjected to a perpendicular magnetic field H_a, the first vortex nucleates at
the rim when H_a = H_0, the threshold field, and moves quickly to the center of
the disk. As H_a increases above H_0, additional vortices join the others, and
together they produce a domelike field distribution of radius b. In this paper
I present analytic solutions for the resulting magnetic-field and
sheet-current-density distributions. I show how these distributions vary as b
increases with H_a, and I calculate the corresponding field-increasing
magnetization. | cond-mat |
Anomalous Polymer Dynamics Is Non-Markovian: Memory Effects and The
Generalized Langevin Equation Formulation: Any first course on polymer physics teaches that the dynamics of a tagged
monomer of a polymer is anomalously subdiffusive, i.e., the mean-square
displacement of a tagged monomer increases as $t^\alpha$ for some $\alpha<1$
until the terminal relaxation time $\tau$ of the polymer. Beyond time $\tau$
the motion of the tagged monomer becomes diffusive. Classical examples of
anomalous dynamics in polymer physics are single polymeric systems, such as
phantom Rouse, self-avoiding Rouse, self-avoiding Zimm, reptation,
translocation through a narrow pore in a membrane, and many-polymeric systems
such as polymer melts. In this pedagogical paper I report that all these
instances of anomalous dynamics in polymeric systems are robustly characterized
by power-law memory kernels within a {\it unified} Generalized Langevin
Equation (GLE) scheme, and therefore, are non-Markovian. The exponents of the
power-law memory kernels are related to the relaxation response of the polymers
to local strains, and are derived from the equilibrium statistical physics of
polymers. The anomalous dynamics of a tagged monomer of a polymer in these
systems is then reproduced from the power-law memory kernels of the GLE via the
fluctuation-dissipation theorem (FDT). Using this GLE formulation I further
show that the characteristics of the drifts caused by a (weak) applied field on
these polymeric systems are also obtained from the corresponding memory
kernels. | cond-mat |
Repulsive van der Waals forces due to hydrogen exposure on bilayer
Graphene: We consider the effect of atomic hydrogen exposure to a system of two undoped
sheets of graphene grown near a silica surface (the first adsorbed to the
surface and the second freestanding near the surface). In the absence of atomic
hydrogen the van der Waals force between the sheets is attractive at all
separations causing the sheets to come closer together. However, with addition
of atomic hydrogen between the sheets the long range van der Waals interaction
turns repulsive at a critical concentration. The underlying triple layer
structure (SiO2 -Atomic Hydrogen Gas -Air) gives rise to a long range repulsion
that at large enough separations dominates over the more rapidly decaying
attraction between the two-dimensional undoped graphene sheets (and between the
outer graphene sheet and SiO2). This may be an avenue to tune the separation
between two graphene sheets with the gas concentration. Doping of the graphene
layers increases the attractive part of the interaction and hence reduces the
net repulsive interaction. | cond-mat |
Time-dependent quantum transport in a resonant tunnel junction coupled
to a nanomechanical oscillator: We present a theoretical study of time-dependent quantum transport in a
resonant tunnel junction coupled to a nanomechanical oscillator within the
non-equilibrium Green's function technique. An arbitrary voltage is applied to
the tunnel junction and electrons in the leads are considered to be at zero
temperature. The transient and the steady state behavior of the system is
considered here in order to explore the quantum dynamics of the oscillator as a
function of time. The properties of the phonon distribution of the
nanomechnical oscillator strongly coupled to the electrons on the dot are
investigated using a non-perturbative approach. We consider both the energy
transferred from the electrons to the oscillator and the Fano factor as a
function of time. We discuss the quantum dynamics of the nanomechanical
oscillator in terms of pure and mixed states. We have found a significant
difference between a quantum and a classical oscillator. In particular, the
energy of a classical oscillator will always be dissipated by the electrons
whereas the quantum oscillator remains in an excited state. This will provide
useful insight for the design of experiments aimed at studying the quantum
behavior of an oscillator. | cond-mat |
Field-induced transverse spin ordering in FeBr2: Weak first-order phase transitions from axial to oblique spin ordering in
FeBr2 are evidenced by SQUID magnetometry in axial fields H1(T) above the
multicritical point, Hm = 2.4 MA/m, Tm = 4.6 K, and below the
antiferro-to-paramagnetic phase line, Hc(T), in agreement with recent specific
heat data (Aruga Katori et al., 1996). The ordering of the in-plane moments is
probably due to non-diagonal coupling to the longitudinal ones, both of which
increase dis-continuously at H1(T) only under an additional symmetry-breaking
transverse field. | cond-mat |
Synthesis, Structural, and Transport Properties of Cr-doped BaTi_2As_2O: The interplay between unconventional superconductivity and the ordering of
charge/spin density wave is one of the most vital issues in both condensed
matter physics and material science. The Ti-based compound BaTi_2As_2O, which
can be seen as the parent phase of superconducting BaTi_2Sb_2O, has a layered
structure with a space group P4/mmm, similar to that of cuprate and iron-based
superconductors. This material exhibits a charge density wave (CDW) ordering
transition revealed by an anomaly at around 200 K in transport measurements.
Here, we report the synthesis and systematical study of the physical properties
in Cr-doped BaTi_{2-x}Cr_xAs_2O (x = 0 - 0.154), and demonstrate that the
transition temperature of the CDW ordering is suppressed gradually by the doped
Cr element. The magnetization measurements confirm the evolution of the CDW
ordering transition. These observed behaviors are similar to that observed in
iron-based superconductors, but no superconductivity emerges down to 2 K. In
addition, the first-principles calculations are also carried out for
well-understanding the nature of experimental observations. | cond-mat |
Broadband probing magnetization dynamics of the coupled vortex state
permalloy layers in nanopillars: Broadband magnetization response of coupled vortex state magnetic dots in
layered nanopillars was explored as a function of in-plane magnetic field and
interlayer separation. For dipolarly coupled circular Py(25 nm)/Cu(20 nm)/Py(25
nm) nanopillars of 600 nm diameter, a small in-plane field splits the
eigenfrequencies of azimuthal spin wave modes inducing an abrupt transition
between in-phase and out-of-phase kinds of the low-lying coupled spin wave
modes. The critical field for this splitting is determined by antiparallel
chiralities of the vortices in the layers. Qualitatively similar (although more
gradual) changes occur also in the exchange coupled Py(25 nm)/Cu(1 nm)/Py(25
nm) tri-layer nanopillars. These findings are in qualitative agreement with
micromagnetic dynamic simulations. | cond-mat |
X-ray imaging of the dynamic magnetic vortex core deformation: Magnetic platelets with a vortex configuration are attracting considerable
attention. The discovery that excitation with small in-plane magnetic fields or
spin polarised currents can switch the polarisation of the vortex core did not
only open the possibility of using such systems in magnetic memories, but also
initiated the fundamental investigation of the core switching mechanism itself.
Micromagnetic models predict that the switching is mediated by a
vortex-antivortex pair, nucleated in a dynamically induced vortex core
deformation. In the same theoretical framework, a critical core velocity is
predicted, above which switching occurs. Although these models are extensively
studied and generally accepted, experimental support has been lacking until
now. In this work, we have used high-resolution time-resolved X-ray microscopy
to study the detailed dynamics in vortex structures. We could reveal the
dynamic vortex core deformation preceding the core switching. Also, the
threshold velocity could be measured, giving quantitative comparison with
micromagnetic models. | cond-mat |
New Monte Carlo method for planar Poisson-Voronoi cells: By a new Monte Carlo algorithm we evaluate the sidedness probability p_n of a
planar Poisson-Voronoi cell in the range 3 \leq n \leq 1600. The algorithm is
developed on the basis of earlier theoretical work; it exploits, in particular,
the known asymptotic behavior of p_n as n\to\infty. Our p_n values all have
between four and six significant digits. Accurate n dependent averages, second
moments, and variances are obtained for the cell area and the cell perimeter.
The numerical large n behavior of these quantities is analyzed in terms of
asymptotic power series in 1/n. Snapshots are shown of typical occurrences of
extremely rare events implicating cells of up to n=1600 sides embedded in an
ordinary Poisson-Voronoi diagram. We reveal and discuss the characteristic
features of such many-sided cells and their immediate environment. Their
relevance for observable properties is stressed. | cond-mat |
Surfactant-Mediated Epitaxial Growth of Single-Layer Graphene in an
Unconventional Orientation on SiC: We report the use of a surfactant molecule during the epitaxy of graphene on
SiC(0001) that leads to the growth in an unconventional orientation, namely
$R0^\circ$ rotation with respect to the SiC lattice. It yields a very
high-quality single-layer graphene with a uniform orientation with respect to
the substrate, on the wafer scale. We find an increased quality and homogeneity
compared to the approach based on the use of a pre-oriented template to induce
the unconventional orientation. Using spot profile analysis low energy electron
diffraction, angle-resolved photoelectron spectroscopy, and the normal
incidence x-ray standing wave technique, we assess the crystalline quality and
coverage of the graphene layer. Combined with the presence of a
covalently-bound graphene layer in the conventional orientation underneath, our
surfactant-mediated growth offers an ideal platform to prepare epitaxial
twisted bilayer graphene via intercalation. | cond-mat |
In-plane Magnetization Induced Quantum Anomalous Hall Effect: In a two-dimensional electron gas, the quantized Hall conductance can be
induced by a strong magnetic field, known as the quantum Hall effect, and it
can also result from the strong exchange coupling of magnetic ions, dubbed as
the "quantum anomalous Hall effect". The quantum Hall effect requires the
out-of-plane magnetic field, and similarly, it is commonly believed that the
magnetization should be out-of-plane for the quantum anomalous Hall effect. In
the present work, we find this condition is not necessary and predict that the
quantum anomalous Hall effect can also be induced by the purely in-plane
magnetization in two realistic systems, including Bi$_2$Te$_3$ thin film with
magnetic doping and HgMnTe quantum wells with shear strains, when all the
reflection symmetries are broken. An experimental setup is proposed to confirm
this effect, the observation of which will pave the way to search for the
quantum anomalous Hall effect in a wider range of materials. | cond-mat |
Non-Linear Stochastic Equations with Calculable Steady States: We consider generalizations of the Kardar--Parisi--Zhang equation that
accomodate spatial anisotropies and the coupled evolution of several fields,
and focus on their symmetries and non-perturbative properties. In particular,
we derive generalized fluctuation--dissipation conditions on the form of the
(non-linear) equations for the realization of a Gaussian probability density of
the fields in the steady state. For the amorphous growth of a single height
field in one dimension we give a general class of equations with exactly
calculable (Gaussian and more complicated) steady states. In two dimensions, we
show that any anisotropic system evolves on long time and length scales either
to the usual isotropic strong coupling regime or to a linear-like fixed point
associated with a hidden symmetry. Similar results are derived for textural
growth equations that couple the height field with additional order parameters
which fluctuate on the growing surface. In this context, we propose
phenomenological equations for the growth of a crystalline material, where the
height field interacts with lattice distortions, and identify two special cases
that obtain Gaussian steady states. In the first case compression modes
influence growth and are advected by height fluctuations, while in the second
case it is the density of dislocations that couples with the height. | cond-mat |
Jamming of packings of frictionless particles with and without shear: By minimizing the enthalpy of packings of frictionless particles, we obtain
jammed solids at desired pressures and hence investigate the jamming transition
with and without shear. Typical scaling relations of the jamming transition are
recovered in both cases. In contrast to systems without shear, shear-driven
jamming transition occurs at a higher packing fraction and the jammed solids
are more rigid with an anisotropic force network. Furthermore, by introducing
the macro-friction coefficient, we propose an explanation of the packing
fraction gap between sheared and non-sheared systems at fixed pressure. | cond-mat |
Profiles of near-resonant population-imbalanced trapped Fermi gases: We investigate the density profiles of a partially polarized trapped Fermi
gas in the BCS-BEC crossover region using mean field theory within the local
density approximation. Within this approximation the gas is phase separated
into concentric shells. We describe how the structure of these shells depends
upon the polarization and the interaction strength. A Comparison with
experiments yields insight into the possibility of a polarized superfluid
phase. | cond-mat |
On super-Poissonian behavior of the Rosenzweig-Porter model in the
non-ergodic extended regime: The Rosenzweig-Porter model has seen a resurgence in interest as it exhibits
a non-ergodic extended phase between the ergodic extended metallic phase and
the localized phase. Such a phase is relevant to many physical models from the
Sachdev-Ye-Kitaev model in high-energy physics and quantum gravity, to the
interacting many-body localization in condensed matter physics and quantum
computing. This phase is characterized by fractal behavior of the
wavefunctions, and a postulated correlated mini-band structure of the energy
spectrum. Here we will seek evidence for the latter in the spectrum. Since this
behavior is expected on intermediate energy scales spectral rigidity is a
natural way to tease it out. Nevertheless, due to the Thouless energy and
ambiguities in the unfolding procedure, the results are inconclusive. On the
other hand, by using the singular value decomposition method, clear evidence
for a super-Poissonian behavior in this regime emerges, consistent with a
picture of correlated mini-bands. | cond-mat |
Hot 2DHG states in tellurium: Element semiconductor Te is very popular in both fundamental electronic
structure study, and device fabrication research area due to its unique band
structure. Specifically, in low temperatures, Te possesses strong quantum
oscillations with magnetic field applied in basal plane, either following
Shubnikov-de Haas (SdH) oscillation rule or following log-periodic oscillation
rule. With magnetic field applied along the [001] direction, the SdH
oscillations are attributed to the two-dimensional hole gas (2DHG) surface
states. Here we reported an interesting SdH oscillation in Te-based single
crystals, with the magnetic field applied along the [001] direction of the
crystals, showing the maximum oscillation intensity at ~ 75 K, and still
traceable at 200 K, which indicates a rather hot 2DHG state. The nontrivial
Berry phase can be also obtained from the oscillations, implying the
contribution from topological states. More importantly, the high temperature
SdH oscillation phenomena are observed in different Te single crystals samples,
and Te single crystals with nonmagnetic/magnetic dopants, showing robustness to
bulk defects. Therefore, the oscillation may be contributed by the bulk
symmetry protected hot 2DHG states, which will offer a new platform for
high-temperature quantum transport studies. | cond-mat |
Subsets and Splits
No saved queries yet
Save your SQL queries to embed, download, and access them later. Queries will appear here once saved.