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Bulk-like viscosity and shear thinning during dynamic compression of a
nanoconfined liquid: The viscosity of liquids under nanoconfinement remains controversial. Reports
range from spontaneous solidification to no change in the viscosity at all.
Here, we present thorough measurements with a small-amplitude linear atomic
force microscopy technique and careful consideration of the confinement
geometry, to show that in a weakly interacting liquid, average viscosity
remains bulk like, except for strong shear thinning once the liquid is confined
to less than four molecular layers. Overlaid over this bulk-like viscous
behavior are stiffness and damping oscillations, indicating non-continuum
behavior, as well as an elastic response when the liquid is allowed to order in
the confinement gap. | cond-mat |
Nonequilibrium Invariant Measure under Heat Flow: We provide an explicit representation of the nonequilibrium invariant measure
for a chain of harmonic oscillators with conservative noise in the presence of
stationary heat flow. By first determining the covariance matrix, we are able
to express the measure as the product of Gaussian distributions aligned along
some collective modes that are spatially localized with power-law tails.
Numerical studies show that such a representation applies also to a purely
deterministic model, the quartic Fermi-Pasta-Ulam chain. | cond-mat |
Localization as an entanglement phase transition in boundary-driven
Anderson models: The Anderson localization transition is one of the most well studied examples
of a zero temperature quantum phase transition. On the other hand, many open
questions remain about the phenomenology of disordered systems driven far out
of equilibrium. Here we study the localization transition in the prototypical
three-dimensional, noninteracting Anderson model when the system is driven at
its boundaries to induce a current carrying non-equilibrium steady state.
Recently we showed that the diffusive phase of this model exhibits extensive
mutual information of its non-equilibrium steady-state density matrix. We show
that that this extensive scaling persists in the entanglement and at the
localization critical point, before crossing over to a short-range (area-law)
scaling in the localized phase. We introduce an entanglement witness for
fermionic states that we name the mutual coherence, which, for fermionic
Gaussian states, is also a lower bound on the mutual information. Through a
combination of analytical arguments and numerics, we determine the finite-size
scaling of the mutual coherence across the transition. These results further
develop the notion of entanglement phase transitions in open systems, with
direct implications for driven many-body localized systems, as well as
experimental studies of driven-disordered systems. | cond-mat |
The random force in molecular dynamics with electronic friction: The Langevin equation includes a random force to maintain equilibrium and
prevent friction from bringing motion to a standstill; but for ballistic
motion, the random force is often neglected. Here, we use the Langevin equation
for molecular dynamics simulations of 2.76 eV H-atoms experiencing electronic
friction in collisions with 300 K metals, where a random force arises from
thermal electron-hole pairs. Simulations without the random force fail
dramatically to reproduce experiment, although the incidence energy is much
larger than $k_\text{B}T$. We analyze the Ornstein-Uhlenbeck process to show
that this is a general property of ballistic particles experiencing friction
under the influence of thermal fluctuations. | cond-mat |
Low-energy quasiparticle states at superconductor-CDW interfaces: Quasiparticle bound states are found theoretically on transparent interfaces
of d-wave superconductors (dSC) with charge density wave solids (CDW), as well
as s-wave superconductors (sSC) with d-density waves (DDW). These bound states
represent a combined effect of Andreev reflection from the superconducting side
and an unconventional quasiparticle Q-reflection from the density wave solid.
If the order parameter for a density wave state is much less than the Fermi
energy, bound states with almost zero energy take place for an arbitrary
orientation of symmetric interfaces. For larger values of the order parameter,
dispersionless zero-energy states are found only on (110) interfaces. Two
dispersive energy branches of subgap quasiparticle states are obtained for
(100) symmetric interfaces. Andreev low-energy bound states, taking place in
junctions with CDW or DDW interlayers, result in anomalous junction properties,
in particular, the low-temperature behavior of the Josephson critical current. | cond-mat |
Theory and Simulation of Multiphase Polymer Systems: The theory of multiphase polymer systems has a venerable tradition. The
'classical' theory of polymer demixing, the Flory-Huggins theory, was developed
already in the forties of the last century. It is still the starting point for
most current approaches -- be they improved theories for polymer
(im)miscibility that take into account the microscopic structure of blends more
accurately, or sophisticated field theories that allow to study inhomogeneous
multicomponent systems of polymers with arbitrary architectures in arbitrary
geometries. In contrast, simulations of multiphase polymer systems are
relatively young. They are still limited by the fact that one must simulate a
large number of large molecules in order to obtain meaningful results. Both
powerful computers and smart modeling and simulation approaches are necessary
to overcome this problem.
This article gives an overview over the state-of-the art in both areas,
theory and simulation. While the theory has reached a fairly mature stage by
now, and many aspects of it are covered in textbooks on polymer physics, the
information on simulations is much more scattered. This is why some effort has
been invested into putting together a representative list of references in this
area (up to the year of 2008) -- which is of course still far from complete. | cond-mat |
Stirring by swimmers in confined microenvironments: We consider the tracer diffusion $D_{rr}$ that arises from the run-and-tumble
motion of low Reynolds number swimmers, such as bacteria. In unbounded dilute
suspensions, where the dipole swimmers move in uncorrelated runs of length
$\lambda$, an exact solution showed that $D_{rr}$ is independent of $\lambda$.
Here we verify this result in numerical simulations for a particular model
swimmer, the spherical squirmer. We also note that in confined
microenvironments, such as microscopic droplets, microfluidic devices and
bacterial microzones in marine ecosystems, the size of the system can be
comparable to $\lambda$. We show that this effect alone reduces the value of
$D_{rr}$ in comparison to its bulk value, and predict a scaling form for its
relative decrease. | cond-mat |
Universality of One-Dimensional Heat Conductivity: We show analytically that the heat conductivity of oscillator chains diverges
with system size N as N^{1/3}, which is the same as for one-dimensional fluids.
For long cylinders, we use the hydrodynamic equations for a crystal in one
dimension. This is appropriate for stiff systems such as nanotubes, where the
eventual crossover to a fluid only sets in at unrealistically large N. Despite
the extra equation compared to a fluid, the scaling of the heat conductivity is
unchanged. For strictly one-dimensional chains, we show that the dynamic
equations are those of a fluid at all length scales even if the static order
extends to very large N. The discrepancy between our results and numerical
simulations on Fermi-Pasta-Ulam chains is discussed. | cond-mat |
Nonadiabatic Channels in the Superconducting Pairing of Fullerides: We show the intrinsic inconsistency of the conventional phonon mediated
theory of superconductivity in relation to the observed properties of
Rb$_3$C$_{60}$. The recent, highly accurate measurement of the carbon isotope
coefficient $\alpha_{\rm C}=0.21$, together with the high value of $T_c$ (30 K)
and the very small Fermi energy $E_{\rm F}$ (0.25 eV), unavoidably implies the
opening of nonadiabatic channels in the superconducting pairing. We estimate
these effects and show that they are actually the key elements for the high
value of $T_c$ in these materials compared to the very low values of graphite
intercalation compounds. | cond-mat |
Effects of turbulent transfer on the critical behaviour: Critical behaviour of two systems, subjected to the turbulent mixing, is
studied by means of the field theoretic renormalization group. The first
system, described by the equilibrium model A, corresponds to relaxational
dynamics of a non-conserved order parameter. The second one is the strongly
nonequilibrium reaction-diffusion system, known as Gribov process or directed
percolation process. The turbulent mixing is modelled by the stochastic
Navier-Stokes equation with random stirring force with the correlator \propto
\delta(t-t') p^{4-d-y}, where p is the wave number, d is the space dimension
and y the arbitrary exponent. It is shown that, depending on the relation
between y and d, the systems exhibit various types of critical behaviour. In
addition to known regimes (original systems without mixing and passively
advected scalar field), existence of new strongly nonequilibrium universality
classes is established, and the corresponding critical dimensions are
calculated to the first order of the double expansion in y and \epsilon=4-d
(one-loop approximation). | cond-mat |
Isostructural Metal-Insulator Transition Driven by Dimensional-Crossover
in SrIrO3 Heterostructures: Dimensionality reduction induced metal-insulator transitions in oxide
heterostructures are usually coupled with structural and magnetic phase
transitions, which complicate the interpretation of the underlying physics.
Therefore, achieving isostructural MIT is of great importance for fundamental
physics and even more for applications. Here, we report an isostructural
metal-insulator transition driven by dimensional-crossover in spin-orbital
coupled SrIrO3 films. By using in-situ pulsed laser deposition and
angle-resolved photoemission spectroscopy, we synthesized and investigated the
electronic structure of SrIrO3 ultrathin films with atomic-layer precision.
Through inserting orthorhombic CaTiO3 buffer layers, we demonstrate that the
crystal structure of SrIrO3 films remains bulk-like with similar oxygen
octahedra rotation and tilting when approaching the ultrathin limit. We observe
that a dimensional-crossover metal-insulator transition occurs in isostructural
SrIrO3 films. Intriguingly, we find the bandwidth of Jeff=3/2 states reduces
with lowering the dimensionality and drives the metal-insulator transition. Our
results establish a bandwidth controlled metal-insulator transition in the
isostructural SrIrO3 thin films. | cond-mat |
Role of interactions in 87Rb-40K Bose-Fermi mixtures in a 3d optical
lattice: We investigate the effect of interspecies interaction on a degenerate mixture
of bosonic 87Rb and fermionic 40K atoms in a three-dimensional optical lattice
potential. Using a Feshbach resonance, the 87Rb-40K interaction is tuned over a
wide range. Through an analysis of the 87Rb momentum distribution, we find a
pronounced asymmetry between strong repulsion and strong attraction. In the
latter case, the Bose-Hubbard parameters are renormalized due to self-trapping,
leading to a marked shift in the superfluid to Mott insulator transition with
increasing Bose-Fermi interaction. | cond-mat |
BEC phase diagram of a $^{87}$Rb trapped gas in terms of macroscopic
thermodynamic parameters: We measure the phase diagram of a $^{87}$Rb Bose gas in a harmonic trap in
terms of macroscopic parameters obtained from the spatial distribution of
atoms. Considering the relevant variables as size of the cloud ${\cal V}$,
number of atoms $N$ and temperature $T$, a novel parameter $\Pi = \Pi(N,{\cal
V},T)$ is introduced to characterize the overall pressure of the system. We
construct the phase diagram ($\Pi$ vs $T$) identifying new features related to
Bose-Einstein condensation (BEC) transition in a trapped gas. A thermodynamic
description of the phase transition based on purely macroscopic parameters,
provide us with properties that do not need the local density approximation. An
unexpected consequence of this analysis is the suggestion that BEC appears as a
continuous third-order phase transition instead of being a second-order one. | cond-mat |
Topological critical states and anomalous electronic transmittance in
one dimensional quasicrystals: Due to the absence of periodic length scale, electronic states and their
topological properties in quasicrystals have been barely understood. Here, we
focus on one dimensional quasicrystal and reveal that their electronic critical
states are topologically robust. Based on tiling space cohomology, we exemplify
the case of one dimensional aperiodic tilings especially Fibonacci quasicrystal
and prove the existence of topological critical states at zero energy.
Furthermore, we also show exotic electronic transmittance behavior near such
topological critical states. Within the perturbative regime, we discuss lack of
translational symmetries and presence of topological critical states lead to
unconventional scaling behavior in transmittance. Considering both analytic
analysis and numerics, electronic transmittance is computed in cases where the
system is placed in air or is connected by semi-infinite periodic leads.
Finally, we also discuss generalization of our analysis to other quasicrystals.
Our findings open a new class of topological quantum states which solely exist
in quasicrystals due to exotic tiling patterns in the absence of periodic
length scale, and their anomalous electronic transport properties applicable to
many experiments. | cond-mat |
Synthesis and characterization of the infinite-layer superconductor
Sr_{0.9}La_{0.1}CuO_{2}: We report the high-pressure synthesis of the electron-doped infinite-layer
superconductor Sr_{0.9}La_{0.1}CuO_{2}. A Rietveld analysis using X-ray powder
diffraction data showed that, within the resolution of the measurement, the
sample was purely an infinite-layer structure without any discernible
impurities. The superconducting volume fraction and the transition width were
greatly improved compared to those in the previous reports. Also the
irreversibility field line was much higher than that of (La,Sr)_{2}CuO_{4}. The
higher value seems to originate from the strong interlayer coupling due to the
reduced average distance between the CuO_{2} planes. | cond-mat |
Preemptive nematic order, pseudogap, and orbital order in the iron
pnictides: Starting from a microscopic itinerant model, we derive and analyze the
effective low-energy model for collective magnetic excitations in the iron
pnictides. We show that the stripe magnetic order is generally preempted by an
Ising-nematic order which breaks $C_{4}$ lattice symmetry but preserves O(3)
spin-rotational symmetry. This leads to a rich phase diagram as function of
doping, pressure, and elastic moduli, displaying split magnetic and nematic
tri-critical points. The nematic transition may instantly bring the system to
the verge of a magnetic transition, or it may occur first, being followed by a
magnetic transition at a lower temperature. In the latter case, the preemptive
nematic transition is accompanied by either a jump or a rapid increase of the
magnetic correlation length, triggering a pseudogap behavior associated with
magnetic precursors. Furthermore, due to the distinct orbital character of each
Fermi pocket, the nematic transition also induces orbital order. We compare our
results to various experiments, showing that they correctly address the changes
in the character of the magneto-structural transition across the phase diagrams
of different compounds, as well as the relationship between the orthorhombic
and magnetic order parameters. | cond-mat |
Optical probing of correlation driven liquid-to-insulator transition in
2D electron gas: We study the quantum Hall liquid and the metal-insulator transition in a high
mobility two dimensional electron gas, by means of photoluminescence and
magneto-transport. In the integer and fractional regime at nu > 1/3, analyzing
the emission energy dispersion we probe the magneto-Coulomb screening and the
hidden symmetry of the electron liquid. In the fractional regime above above nu
=1/3 the system undergoes the metal-to-insulator transition, and in the
insulating phase the dispersion becomes linear with evidence of an increased
renormalized mass. | cond-mat |
Colossal pressure-induced softening in scandium fluoride: The counter-intuitive phenomenon of pressure-induced softening in materials
is likely to be caused by the same dynamical behaviour that produces negative
thermal expansion. Through a combination of molecular dynamics simulation on an
idealised model and neutron diffraction at variable temperature and pressure,
we show the existence of extraordinary and unprecedented pressure-induced
softening in the negative thermal expansion material scandium fluoride,
ScF$_3$, with values of the pressure-derivative of the bulk modulus $B$,
$B^\prime = \partial B / \partial P$, reaching as low as $-40 \pm 1$. | cond-mat |
First-principles study of magnetic structures of triangular
antiferromagnets NaYbS$_2$ and NaYbO$_2$: We investigate the magnetic interactions in triangular rare-earth
delafossites materials NaYbO$_2$ and NaYbS$_2$ via first-principles
calculations. The calculated Curie-Weiss temperatures are in good agreement
with experiments. We perform classical Monte Carlo simulations of the two
compounds using the extracted exchange parameters. We find that if only the
nearest neighbor interactions are considered, the magnetic ground states of
NaYbO$_2$ and NaYbS$_2$ are a stripe and a planar 120\degree~ N\'{e}el state,
respectively. The simulated transition temperatures are much higher than the
lowest experimental temperatures, where no magnetic ordering was observed.
However, we show by adding suitable second neighbor interactions, the {\it
classical} magnetic ground state of NaYbO$_2$ becomes to the $Z_2$ vortex
phase, and the simulated specific heat $C_v$ are very similar to the
experimental observations, with no obvious phase transition down to the
extremely low temperature. | cond-mat |
Superconductivity From Confinement of Singlets in Metal Oxides: The Yang-Mills description of phonons and the consequent structure of
electron liquids in strongly anharmonic crystals such as metal oxides is shown
to yield an attractive electron-phonon interaction, and thus an instability
towards the formation of bound states, which can condense to form a
superconductor. This mechanism differs significantly from the pairing mechanism
of conventional superconductivity: the ground state from which
superconductivity emerges is a many-body state of paired electrons and holes
which is not amenable to a quasiparticle description, and whose properties are
similar to those seen in the Cuprate high temperature superconductors.
Confinement arises because the electron liquid structure acts as a source for
Yang-Mills bosons, and not the traditional longitudinal density waves of BCS
pairing. | cond-mat |
Magnetic and magneto-transport characterization of (Ga,Mn)(Bi,As)
epitaxial layers: High-quality layers of the (Ga,Mn)(Bi,As) quaternary compound semiconductor
have been grown by the low-temperature molecular-beam epitaxy technique. An
effect of Bi incorporation into the (Ga,Mn)As ferromagnetic semiconductor and
the post-growth annealing treatment of the layers have been investigated
through examination of their magnetic and magneto-transport properties.
Significant enhancement of the planar Hall effect magnitude upon addition of Bi
into the layers is interpreted as a result of increased spin-orbit coupling in
the (Ga,Mn)(Bi,As) layers. | cond-mat |
Point tension in adsorption at a chemically inhomogeneous substrate in
two dimensions: We study adsorption of liquid at a one-dimensional substrate composed of a
single chemical inhomogeneity of width $2L$ placed on an otherwise homogeneous,
planar, solid surface. The excess point free energy $\eta (L,T)$ associated
with the adsorbed layer's inhomogeneity induced by the substrate's chemical
structure is calculated within exact continuum transfer-matrix approach. It is
shown that the way $\eta (L,T)$ varies with $L$ depends sensitively on the
temperature regime. It exhibits logarithmic divergence as a function of $L$ in
the limit $L\to\infty$ for temperatures such that the chemical inhomogeneity is
completely wetted by the liquid. In the opposite case $\eta (L,T)$ converges
for large $L$ to $2\eta_0$, where $\eta_0$ is the corresponding point tension,
and the dominant $L$-dependent correction to $2\eta_0$ decays exponentially.
The interaction between the liquid layer inhomogeneities at $-L$ and $L$ for
the two temperature regimes is discussed and compared to earlier mean-field
theory predictions. | cond-mat |
Antivortices due to competing orbital and paramagnetic pair-breaking
effects: Thermodynamically stable vortex-antivortex structures in a
quasi-two-dimensional superconductor in a tilted magnetic field are predicted.
For this geometry, both orbital and spin pair-breaking effects exist, with
their relative strength depending on the tilt angle \Theta. The spectrum of
possible states contains as limits the ordinary vortex state (for large \Theta)
and the Fulde-Ferrell-Larkin-Ovchinnikov state (for \Theta=0). The
quasiclassical equations are solved near H_{c2} for arbitrary \Theta and it is
shown that stable states with coexisting vortices and antivortices exist in a
small interval close to \Theta=0. The results are compared with recent
predictions of antivortices in mesoscopic samples. | cond-mat |
Interplay of two $E_g$ orbitals in Superconducting La$_3$Ni$_2$O$_7$
Under Pressure: The discovery of high-$T_c$ superconductivity (SC) in La$_3$Ni$_2$O$_7$ (LNO)
has aroused a great deal of interests. Previously, it was proposed that the
Ni-$3d_{z^2}$ orbital is crucial to realize the high-$T_c$ SC in LNO: The
preformed Cooper pairs therein acquire coherence via hybridization with the
$3d_{x^2-y^2}$ orbital to form the SC. However, we held a different viewpoint
that the interlayer pairing $s$-wave SC is induced by the $3d_{x^2-y^2}$
orbital, driven by the strong interlayer superexchange interaction. To include
effects from both $E_g$-orbitals , we establish a two-orbital bilayer $t$-$J$
model. Our calculations reveal that due to the no-double-occupancy constraint,
the $3d_{x^2-y^2}$ band and the $3d_{z^2}$ bonding band are flattened by a
factor of about 2 and 10, respectively, which is consistent with recent
angle-resolved-photo-emission-spectroscopy measurements. Consequently, a high
temperature SC can be hardly induced in the $3d_{z^2}$-orbital due to the
difficulty to develop phase coherence. However, it can be easily achieved by
the $3d_{x^2-y^2}$ orbital under realistic interaction strength. With electron
doping, the $3d_{z^2}$-band gradually dives below the Fermi level, but $T_c$
continues to enhance, suggesting that it is not necessary for the high-$T_c$ SC
in LNO. With hole doping, $T_c$ initially drops and then rises, accompanied by
the crossover from the BCS to BEC-type superconducting transitions. | cond-mat |
Percolation and jamming of linear $k$-mers on square lattice with
defects: effect of anisotropy: We study the percolation and jamming of rods ($k$-mers) on a square lattice
that contains defects. The point defects are placed randomly and uniformly on
the substrate before deposition of the rods. The general case of unequal
probabilities for orientation of depositing of rods along different directions
of the lattice is analyzed. Two different models of deposition are used. In the
relaxation random sequential adsorption model (RRSA), the deposition of rods is
distributed over different sites on the substrate. In the single cluster
relaxation model (RSC), the single cluster grows by the random accumulation of
rods on the boundary of the cluster. For both models, a suppression of growth
of the infinite cluster at some critical concentration of defects $d_c$ is
observed. In the zero defect lattices, the jamming concentration $p_j$ (RRSA)
and the density of single clusters $p_s$ (RSC) decrease with increasing length
rods and with a decrease in the order parameter. For the RRSA model, the value
of $d_c$ decreases for short rods as the value of $s$ increases. For longer
rods, the value of $d_c$ is almost independent of $s$. Moreover, for short
rods, the percolation threshold is almost insensitive to the defect
concentration for all values of $s$. For the RSC model, the growth of clusters
with ellipse-like shapes is observed for non-zero values of $s$. The density of
the clusters $p_s$ at the critical concentration of defects $d_c$ depends in a
complex manner on the values of $s$ and $k$. For disordered systems, the value
of $p_s$ tends towards zero in the limits of the very long rods and very small
critical concentrations $d_c \to 0$. In this case, the introduction of defects
results in a suppression of rods stacking and in the formation of `empty' or
loose clusters with very low density. On the other hand, denser clusters are
formed for ordered systems. | cond-mat |
Detection of short DNA sequences with DNA nanopores: Several studies suggest strong correlation between different types of cancer
and the relative concentration of short circulating RNA sequences (miRNA).
Because of short length and low concentration, miRNA detection is not easy.
Standard methods such as RT-PCR require both the standard PCR amplification
step and a preliminary additional step of reverse transcription. In this paper,
we investigate the use of DNA nanopores as a tool to detect short
oligonucleotide sequences at the single molecule level. These nanostructures
show two different conformations depending on the presence of DNA analogues of
miRNA sequences. By monitoring current across a lipid bilayer, we show that
this change of conformation translates to different levels of conductivity. | cond-mat |
Testing self-energy embedding theory in combination with GW: We present a theoretical framework and implementation details for self-energy
embedding theory (SEET) with the GW approximation for the treatment of weakly
correlated degrees of freedom and configuration interactions solver for handing
the strongly correlated degrees. On a series of molecular examples, for which
the exact results are known within a given basis, we demonstrate that
SEET(CI/GW) is a systematically improvable and well controlled method capable
of giving accurate results and well behaved causal self-energies and Green's
functions. We compare the theoretical framework of SEET(CI/GW) to that of
GW+DMFT and comment on differences between these to approaches that aim to
treat both strongly and weakly correlated simultaneously. | cond-mat |
Synchronization in dynamical networks of locally coupled self-propelled
oscillators: Systems of mobile physical entities exchanging information with their
neighborhood can be found in many different situations. The understanding of
their emergent cooperative behaviour has become an important issue across
disciplines, requiring a general conceptual framework in order to harvest the
potential of these systems. We study the synchronization of coupled oscillators
in time-evolving networks defined by the positions of self-propelled agents
interacting in real space. In order to understand the impact of mobility in the
synchronization process on general grounds, we introduce a simple model of
self-propelled hard disks performing persistent random walks in 2$d$ space and
carrying an internal Kuramoto phase oscillator. For non-interacting particles,
self-propulsion accelerates synchronization. The competition between agent
mobility and excluded volume interactions gives rise to a richer scenario,
leading to an optimal self-propulsion speed. We identify two extreme dynamic
regimes where synchronization can be understood from theoretical
considerations. A systematic analysis of our model quantifies the departure
from the latter ideal situations and characterizes the different mechanisms
leading the evolution of the system. We show that the synchronization of
locally coupled mobile oscillators generically proceeds through coarsening
verifying dynamic scaling and sharing strong similarities with the phase
ordering dynamics of the 2$d$ XY model following a quench. Our results shed
light into the generic mechanisms leading the synchronization of mobile agents,
providing a efficient way to understand more complex or specific situations
involving time-dependent networks where synchronization, mobility and excluded
volume are at play. | cond-mat |
Scaling of 1/f noise in tunable break-junctions: We have studied the $1/f$ voltage noise of gold nano-contacts in
electromigrated and mechanically controlled break-junctions having resistance
values $R$ that can be tuned from 10 $\Omega$ (many channels) to 10 k$\Omega$
(single atom contact). The noise is caused by resistance fluctuations as
evidenced by the $S_V\propto V^2$ dependence of the power spectral density
$S_V$ on the applied DC voltage $V$. As a function of $R$ the normalized noise
$S_V/V^2$ shows a pronounced cross-over from $\propto R^3$ for low-ohmic
junctions to $\propto R^{1.5}$ for high-ohmic ones. The measured powers of 3
and 1.5 are in agreement with $1/f$-noise generated in the bulk and reflect the
transition from diffusive to ballistic transport. | cond-mat |
Statistical scattering of waves in disordered waveguides: Universal
Properties: The statistical theory of certain complex wave interference phenomena, like
the statistical fluctuations of transmission and reflection of waves, is of
considerable interest in many fields of physics. In this article we shall be
mainly interested in those situations where the complexity derives from the
quenched randomness of scattering potentials, as in the case of disordered
conductors, or, more in general, disordered waveguides. In studies performed in
such systems one has found remarkable statistical regularities, in the sense
that the probability distribution for various macroscopic quantities involves a
rather small number of relevant physical parameters, while the rest of the
microscopic details serves as mere "scaffolding". We shall review past work in
which this feature was captured following a maximum-entropy approach, as well
as later studies in which the existence of a limiting distribution, in the
sense of a generalized central-limit theorem, has been actually demonstrated.
We then describe a microscopic potential model that was developed recently,
which gives rise to a further generalization of the central-limit theorem and
thus to a limiting macroscopic statistics. | cond-mat |
Multilayer Pt/Al Based Ohmic contacts for AlGaN/GaN Heterostructures
Stable up to 600oC Ambient Air: In this paper, we present a Pt/Al multilayer stack-based ohmic contact
metallization for AlGaN/GaN heterostructures. CTLM structures were fabricated
to assess the electrical properties of the proposed metallization. The
fabricated stack shows excellent stability after more than 100 hours of
continuous aging at 600oC in air. Measured I-V characteristics of the
fabricated samples show excellent linearity after the aging. The Pt/Al-based
metallization shows great potential for future device and sensor applications
in extreme environment conditions. | cond-mat |
Simulating Met-Enkephalin With Population Annealing Molecular Dynamics: Met-enkephalin, one of the smallest opiate peptides and an important
neurotransmitter, is a widely used benchmarking problem in the field of
molecular simulation. Through its range of possible low-temperature
conformations separated by free-energy barriers it was previously found to be
hard to thermalize using straight canonical molecular dynamics simulations.
Here, we demonstrate how one can use the recently proposed population annealing
molecular dynamics scheme to overcome these difficulties. We show how the use
of multi-histogram reweighting allows one to accurately estimate the density of
states of the system and hence derive estimates such as the potential energy as
quasi continuous functions of temperature. We further investigate the
free-energy surface as a function of end-to-end distance and radius-of-gyration
and observe two distinct basins of attraction. | cond-mat |
Structural transformations in porous glasses under mechanical loading.
I. Tension: The evolution of porous structure and mechanical properties of binary glasses
under tensile loading were examined using molecular dynamics simulations. We
consider vitreous systems obtained in the process of phase separation after a
rapid isochoric quench of a glass-forming liquid to a temperature below the
glass transition. The porous structure in undeformed samples varies from a
connected porous network to a random distribution of isolated pores upon
increasing average glass density. We find that at small strain, the elastic
modulus follows a power-law dependence on the average glass density and the
pore size distribution remains nearly the same as in quiescent samples. Upon
further loading, the pores become significantly deformed and coalesce into
larger voids that leads to formation of system-spanning empty regions
associated with breaking of the material. | cond-mat |
NMR relaxation and rattling phonons in type-I Ba8Ga16Sn30 clathrate: Atomic motion of guest atoms inside semiconducting clathrate cages is
considered as an important source for the glasslike thermal behavior.69Ga and
71Ga Nuclear Magnetic Resonance (NMR) studies on type-I Ba8Ga16Sn30 show a
clear low temperature relaxation peak attributed to the influence of Ba
rattling dynamics on the framework-atom resonance, with a quadrupolar
relaxation mechanism as the leading contribution. The data are analyzed using a
two-phonon Raman process, according to a recent theory involving localized
anharmonic oscillators. Excellent agreement is obtained using this model, with
the parameters corresponding to a uniform array of localized oscillators with
very large anharmonicity. | cond-mat |
Residual entropy from temperature incremental Monte Carlo method: Residual entropy, indicative of the degrees of freedom in a system at
absolute zero, is a cornerstone for understanding quantum and classical ground
states. Despite its critical role in elucidating low-temperature phenomena and
ground state degeneracy, accurately quantifying residual entropy remains a
formidable challenge due to significant computational hurdles. In this Letter,
we introduce the Temperature Incremental Monte Carlo (TIMC) method, our novel
solution crafted to surmount these challenges. The TIMC method incrementally
calculates the partition function ratio of neighboring temperatures within
Monte Carlo simulations, enabling precise entropy calculations and providing
insights into a spectrum of other temperature-dependent observables in a single
computational sweep of temperatures. We have rigorously applied TIMC to a
variety of complex systems, such as the frustrated antiferromagnetic Ising
model on both C60 and 2D triangular lattices, the Newman-Moore spin glass
model, and a 2D quantum transverse field Ising model. Notably, our method
surmounts the traditional obstacles encountered in partition function
measurements when mapping $d$-dimensional quantum models to $d+1$-dimensional
classical counterparts. The TIMC method's finesse in detailing entropy across
the entire temperature range enriches our comprehension of critical phenomena
in condensed matter physics. This includes insights into spin glasses, phases
exhibiting spontaneous symmetry breaking, topological states of matter and
fracton phases. Our approach not only advances the methodology for probing the
entropic landscape of such systems but also paves the way for exploring their
broader thermodynamic and quantum mechanical properties. | cond-mat |
Extracting the Dispersion of Periodic Lossless LC Circuits Using White
Noise: The spectral energy density (SED) method is used to obtain the phonon
dispersion of materials in molecular dynamics codes, e.g., LAMMPS. We show how
the electric analog of the SED method can be done using commercial circuit
simulators to find the dispersion of periodic lossless LC circuits. The purpose
of this article is (a) to demonstrate how SED proves useful, should the
analytic methods of calculating dispersion of a circuit render difficult e.g.,
due to nonlinearity or having large number of elements in each unit-cell, and
(b) to show how the concepts like Brillouin zone (BZ), dispersion (or band
structure), zone folding, gap formation, and avoided crossing can be taught to
students of electrical engineering by highlighting the analogies between
phonons and periodic circuits. This analogy also suggests that thermal devices,
e.g., heat rectifiers can be simulated and understood using commercial circuit
simulators. | cond-mat |
First observation of bright solitons in bulk superfluid He-4: The existence of bright solitons in bulk superfluid He-4 is demonstrated by
time-resolved shadowgraph i maging experiments and density functional theory
(DFT) calculations. The initial liquid compression that leads to the creation
of non-linear waves is produced by rapidly expanding plasma from laser
ablation. Af ter the leading dissipative period, these waves transform into
bright solitons, which exhibit three chara cteristic features: dispersionless
propagation, negligible interaction in two-wave collision, and direct
dependence between soliton amplitude and the propagation velocity. The
experimental observations are supp orted by DFT calculations, which show rapid
evolution of the initially compressed liquid into bright soli tons. At high
amplitudes, solitons become unstable and break down into dispersive shock
waves. | cond-mat |
Thermodynamics of the multi-component dimerizing hard-sphere Yukawa
mixture in the associative mean spherical approximation: Explicit analytical expressions for Helmholtz free energy, chemical
potential, entropy and pressure of the multi-component dimerizing Yukawa
hard-sphere fluid are presented. These expressions are written in terms of the
Blum's scaling parameter $\Gamma$, which follows from the solution of the
associative mean spherical approximation (AMSA) for the model with factorized
Yukawa coefficients. In this case solution of the AMSA reduces to the solution
of only one nonlinear algebraic equation for $\Gamma$. This feature enables the
theory to be used in the description of the thermodynamical properties of
associating fluids with arbitrary number of components, including the limiting
case of polydisperse fluids. | cond-mat |
Electrical Transport Property of ZnO Thin Films in High H2 Pressure up
to 20 bar: We have investigated the H2 pressure-dependent (from vacuum to 20 bar)
current-voltage characteristics of ZnO thin films prepared by spin coating
method. The gas pressure effect on conductance (G) was subtracted using He gas.
The G increased as applying 2 bar of H2 pressure, and then it monotonously
decreased with the further increment of H2 pressure. Using X-ray diffraction
patterns and X-ray photoelectron spectroscopy before and after H2 exposure, we
found that the H2 spillover effect plays an important role in the variation of
G of ZnO film. | cond-mat |
Ferromagnetism and Canted Spin Phase in AlAs/GaMnAs Single Quantum
Wells: Monte Carlo Simulation: The magnetic order resulting from a confinement-adapted
Ruderman-Kittel-Kasuya-Yosida indirect exchange between magnetic moments in the
metallic phase of a AlAs/Ga(1-x)Mn(x)As quantum well is studied by Monte Carlo
simulation. This coupling mechanism involves magnetic moments and carriers
(holes), both coming from the same Mn(2+) ions. It leads to a paramagnetic, a
ferromagnetic, or a canted spin phase, depending on the carrier concentration,
and on the magnetic layer width. It is shown that high transition temperatures
may be obtained. | cond-mat |
Fate of spinons at the Mott point: Gapless spin liquids have recently been observed in several frustrated Mott
insulators, with elementary spin excitations - "spinons" - reminiscent of
degenerate Fermi systems. However, their precise role at the Mott point, where
charge fluctuations begin to proliferate, remains controversial and
ill-understood. Here we present the simplest theoretical framework that treats
the dynamics of emergent spin and charge excitations on the same footing,
providing a new physical picture of the Mott metal-insulator transition at half
filing. We identify a generic orthogonality mechanism leading to strong damping
of spinons, arising as soon as the Mott gap closes. Our results indicates that
spinons should not play a significant role within the high-temperature quantum
critical regime above the Mott point - in striking agreement with all available
experiments. | cond-mat |
Overcoming correlation fluctuations in two-photon interference
experiments with differently bright and independently blinking remote quantum
emitters: As a fundamental building block for quantum computation and communication
protocols, the correct verification of the two-photon interference (TPI)
contrast between two independent quantum light sources is of utmost importance.
Here, we experimentally demonstrate how frequently present blinking dynamics
and changes in emitter brightness critically affect the Hong-Ou-Mandel-type
(HOM) correlation histograms of remote TPI experiments measured via the
commonly utilized setup configuration. We further exploit this qualitative and
quantitative explanation of the observed correlation dynamics to establish an
alternative interferometer configuration, which is overcoming the discussed
temporal fluctuations, giving rise to an error-free determination of the remote
TPI visibility. We prove full knowledge of the obtained correlation by
reproducing the measured correlation statistics via Monte-Carlo simulations. As
exemplary system, we make use of two pairs of remote semiconductor quantum
dots, however, the same conclusions apply for TPI experiments with flying
qubits from any kind of remote solid state quantum emitters. | cond-mat |
Dissipationless counterflow currents above T_c in bilayer
superconductors: We report the existence of dissipationless currents in bilayer
superconductors above the critical temperature $T_c$, assuming that the
superconducting phase transition is dominated by phase fluctuations. Using a
semiclassical $U(1)$ lattice gauge theory, we show that thermal fluctuations
cause a transition from the superconducting state at low temperature to a
resistive state above $T_c$, accompanied by the proliferation of unbound
vortices. Remarkably, while the proliferation of vortex excitations causes
dissipation of homogeneous in-plane currents, we find that counterflow
currents, flowing in opposite direction within a bilayer, remain
dissipationless. The presence of a dissipationless current channel above $T_c$
is attributed to the inhibition of vortex motion by local superconducting
coherence within a single bilayer, in the presence of counterflow currents. Our
theory presents a possible scenario for the pseudogap phase in bilayer
cuprates. | cond-mat |
Scaling Relations for Temperature Dependences of the Surface
Self-Diffusion Coefficient in Crystallized Molecular Glasses: Crystallization kinetics has features that are universal and independent of
the type of crystallized system. The possibility of using scaling relations to
describe the temperature dependences of the surface self-diffusion coefficient
$D_s$, which is one of the key characteristics of crystallization kinetics, has
been demonstrated in application to various crystallized molecular glasses. It
has been shown that the surface self-diffusion coefficient $D_s$ as a function
of the dimensionless temperature is reproduced by a power law and is
universally scaled for all considered systems. The analysis of experimental
data has revealed a correlation between the crystallization kinetic
characteristics, index of fragility, and criterion of the glass-forming ability
of a liquid. It has been shown that this correlation can be obtained within the
generalized Einstein-Stokes relation. | cond-mat |
Tunneling current characteristics in bilayer quantum Hall systems: Weakly disordered bilayer quantum Hall systems at filling factor $\nu=1$ show
spontaneous interlayer phase coherence if the layers are sufficiently close
together. We study the collective modes in the system, the current-voltage
characteristics and their evolution with an in-plane magnetic field in the
phase-coherent regime. | cond-mat |
Dirac states in armchair- and zigzag-edged graphene Möbius strips: Edge structure plays an essential role in the nature of electronic states in
graphene nanoribbons. By focusing on the interplay between this feature and
non-trivial topology in the domain of the Dirac confinement problem, this paper
proposes to examine how effects associated with edge shape manifest themselves
in conjunction with the topological signature typical of M\"{o}bius strips
within a low-energy regime. Aiming to provide an alternative to prevailing
tight-binding approaches, zigzag and armchair M\"{o}bius strips are modeled by
proposing compatible sets of boundary conditions, prescribing profiles of
terminations in both transverse and longitudinal directions which are
demonstrated to be coherent in describing consistently transverse edge patterns
in combination with a proper M\"{o}bius periodicity. Of particular importance
is the absence of constraints on the solution, in contrast with infinite mass
analogues, as well as an energy spectrum with a characteristic dual structure
responding exclusively to the parity associated with the transverse quantum
number. Zigzag ribbons are predicted to possess an intrinsic mechanism for
parity inversion, while the armchair ones carry the possibility of a coexistent
gapless and gapped band structure. We also inspect the influence of the edge
structure on persistent currents. In zigzag-edged configurations they are found
to be sensitive to a length-dependent term which behaves as an effective flux.
Armchair rings show a quite distinctive property: alternation of constant and
flux-dependent currents according to the width of the ring, for a fixed
transverse quantum number. In the flux-free case the effects of topology are
found to be entirely suppressed, and conventional odd and even currents become
undistinguishable. | cond-mat |
Lagrange statistics in systems (markets) with price constraints:
Analysis of property, car sales, marriage and job markets by the Boltzmann
function and the Pareto distribution: Statistical models of economic distributions lead to Boltzmann distributions
rather than a Pareto power law. This result is supported by two facts: 1. the
distributions of income, car sales, marriages or jobs are a matter of chances
and luck and not of reason! 2. Data for property, automobile sales, marriages
and job markets were analyzed by two models: the Pareto law and the Boltzmann
distribution of stochastic systems. In all cases the best fits to data were
obtained by the Boltzmann function. This may indicate that the principles of
stochastic systems like in physics, chemistry, thermodynamics may also be
applied to economic systems. | cond-mat |
A Paradox in the Langevin Equation with Long-Time Noise Correlations: We solve the generalized Langevin equation driven by a stochastic force with
power-law autocorrelation function. A stationary Markov process has been
applied as a model of the noise. However, the resulting velocity variance does
not stabilizes but diminishes with time. It is shown that algebraic
distributions can induce such non-stationary affects. Results are compared to
those obtained with a deterministic random force. Consequences for the
diffusion process are also discussed. | cond-mat |
Assessing the potential of perfect screw dislocations in SiC for
solid-state quantum technologies: Although point defects in solids are one of the most promising physical
systems to build functioning qubits, it remains challenging to position them in
a deterministic array and to integrate them into large networks. By means of
advanced ab initio calculations we show that undissociated screw dislocations
in cubic 3C-SiC, and their associated strain fields, could be used to create a
deterministic pattern of relevant point defects. Specifically, we present a
detailed analysis of the formation energies and electronic structure of the
divacancy in 3C-SiC when located in the vicinity of this type of dislocations.
Our results show that the divacancy is strongly attracted towards specific and
equivalent sites inside the core of the screw dislocations, and would form a
one-dimensional arrays along them. Furthermore, we show that the same strain
that attracts the divacancy allows the modulation of the position of its
electronic states and of its charge transition levels. In the case of the
neutral divacancy, we find that these modulations result in the loss of its
potential as a qubit. However, these same modulations could transform defects
with no potential as qubits when located in bulk, into promising defects when
located inside the core of the screw dislocations. Since dislocations are still
mostly perceived as harmful defects, our findings represent a technological
leap as they show that dislocations can be used as active building blocks in
future defect-based quantum computers. | cond-mat |
The effect of layer number and substrate on the stability of graphene
under MeV proton beam irradiation: The use of graphene electronics in space will depend on the radiation
hardness of graphene. The damage threshold of graphene samples, subjected to 2
MeV proton irradiation, was found to increase with layer number and also when
the graphene layer was supported by a substrate. The thermal properties of
graphene as a function of the number of layers or as influenced by the
substrate argue against a thermal model for the production of damage by the ion
beam. We propose a model of intense electronically-stimulated surface
desorption of the atoms as the most likely process for this damage mechanism. | cond-mat |
Ferrodistortive instability at the (001) surface of half-metallic
manganites: We present the structure of the fully relaxed (001) surface of the
half-metallic manganite La0.7Sr0.3MnO3, calculated using density functional
theory within the generalized gradient approximation (GGA). Two relevant
ferroelastic order parameters are identified and characterized: The tilting of
the oxygen octahedra, which is present in the bulk phase, oscillates and
decreases towards the surface, and an additional ferrodistortive Mn
off-centering, triggered by the surface, decays monotonically into the bulk.
The narrow d-like energy band that is characteristic of unrelaxed manganite
surfaces is shifted down in energy by these structural distortions, retaining
its uppermost layer localization. The magnitude of the zero-temperature
magnetization is unchanged from its bulk value, but the effective spin-spin
interactions are reduced at the surface. | cond-mat |
Dynamics of Successive Minor Hysteresis Loops: Cumulative growth of successive minor hysteresis loops in Co/Pd multilayers
with perpendicular anisotropy was studied in the context of time dependent
magnetization reversal dynamics. We show that in disordered ferromagnets, where
magnetization reversal involves nucleation, domains' expansion and
annihilation, differences between the time dependencies of these processes are
responsible for accumulation of nuclei for rapid domain expansion, for the
asymmetry of forward and backward magnetization reversals and for the
respective cumulative growth of hysteresis loops. Loops stop changing and
become macroscopically reproducible when populations of upward and downward
nucleation domains balance each other and the respective upward and downward
reversal times stabilize. | cond-mat |
Observation of Weyl nodes in TaAs: In 1929, H. Weyl proposed that the massless solution of Dirac equation
represents a pair of new type particles, the so-called Weyl fermions [1].
However the existence of them in particle physics remains elusive for more than
eight decades. Recently, significant advances in both topological insulators
and topological semimetals have provided an alternative way to realize Weyl
fermions in condensed matter as an emergent phenomenon: when two non-degenerate
bands in the three-dimensional momentum space cross in the vicinity of Fermi
energy (called as Weyl nodes), the low energy excitation behaves exactly the
same as Weyl fermions. Here, by performing soft x-ray angle-resolved
photoemission spectroscopy measurements which mainly probe bulk band structure,
we directly observe the long-sought-after Weyl nodes for the first time in
TaAs, whose projected locations on the (001) surface match well to the Fermi
arcs, providing undisputable experimental evidence of existence of Weyl fermion
quasiparticles in TaAs. | cond-mat |
Acoustic interactions between inversion symmetric and asymmetric
two-level systems: Amorphous solids, as well as many disordered lattices, display remarkable
universality in their low temperature acoustic properties. This universality is
attributed to the attenuation of phonons by tunneling two-level systems (TLSs),
facilitated by the interaction of the TLSs with the phonon field. TLS-phonon
interaction also mediates effective TLS-TLS interactions, which dictates the
existence of a glassy phase and its low energy properties. Here we consider
KBr:CN, the archetypal disordered lattice showing universality. We calculate
numerically, using conjugate gradients method, the effective TLS-TLS
interactions for inversion symmetric (CN flips) and asymmetric (CN rotations)
TLSs, in the absence and presence of disorder, in two and three dimensions. The
observed dependence of the magnitude and spatial power law of the interaction
on TLS symmetry, and its change with disorder, characterizes TLS-TLS
interactions in disordered lattices in both extreme and moderate dilutions. Our
results are in good agreement with the two-TLS model, recently introduced to
explain long-standing questions regarding the quantitative universality of
phonon attenuation and the energy scale of $\approx 1-3$ K below which
universality is observed. | cond-mat |
Stable diagonal stripes in the t-J model at $\bar{n}_h$=1/8 doping from
fPEPS calculations: We investigate the 2D t-J model at a hole doping of $\bar{n}_h$=1/8 using
recently developed high accuracy fermionic projected entangled pair
states(fPEPS) method. By applying stochastic gradient descent method combined
with Monte Carlo sampling technique, we obtain the ground state hole energy
$E_{\rm hole}$=-1.6186 for $J/t$=0.4. We show that the ground state has stable
diagonal stripes instead of vertical stripes with width of 4 unit cells, and
stripe filling $\rho_l$=0.5. We further show that the long range
superconductivity order is suppressed at this point. | cond-mat |
Phase change materials for nano-polaritonics: a case study of hBN/VO2
heterostructures: Polaritonic excitation and control in van der Waals (vdW) materials exhibit
superior merits than conventional materials and thus hold new promise for
exploring light matter interactions. In this work, we created vdW
heterostructures combining hexagonal boron nitride (hBN) and a representative
phase change material - vanadium dioxide (VO2). Using infrared
nano-spectroscopy and nano-imaging, we demonstrated the dynamic tunability of
hyperbolic phonon polaritons in hBN/VO2 heterostructures by temperature control
in a precise and reversible fashion. The dynamic tuning of the polaritons stems
from the change of local dielectric properties of the VO2 sublayer through
insulator to metal transition by the temperature control. The high
susceptibility of polaritons to electronic phase transitions opens
possibilities for applications of vdW materials in combination with correlated
phase change materials. | cond-mat |
Detecting sign-changing superconducting gap in LiFeAs using
quasiparticle interference: Using a realistic ten-orbital tight-binding model Hamiltonian fitted to the
angle-resolved photoemission (ARPES) data on LiFeAs, we analyze the
temperature, frequency, and momentum dependencies of quasiparticle interference
(QPI) to identify gap sign changes in a qualitative way, following our original
proposal [Phys. Rev. B 92, 184513 (2015)]. We show that all features present
for the simple two-band model for the sign-changing $s_{+-}$-wave
superconducting gap employed previously are still present in the realistic
tight-binding approximation and gap values observed experimentally. We discuss
various superconducting gap structures proposed for LiFeAs, and identify
various features of these superconducting gaps functions in the quasiparticle
interference patterns. On the other hand, we show that it will be difficult to
identify the more complicated possible sign structures of the hole pocket gaps
in LiFeAs, due to the smallness of the pockets and the near proximity of two of
the gap energies. | cond-mat |
Dissipationless Anomalous Hall Current in $Fe_{100-x}(SiO_2)_x$ Films: The observation of dissipationless anomalous Hall current is one of the
experimental evidences to confirm the intrinsic origin of anomalous Hall
effect. To study the origin of anomalous Hall effect in iron,
Fe$_{100-x}$(SiO$_{2}$)$_{x}$ granular films with volume fraction of SiO$_{2}$
0\le x \le 40.51 were fabricated using co-sputtering. Hall and longitudinal
resistivities were measured in the temperature range 5 to 350 K with magnetic
fields up to 5 Tesla. As x increased from 0 to 40.51, the anomalous Hall
resistivity and longitudinal resistivity increased about 4 and 3 orders in
magnitude, respectively. Analysis of the results revealed that the normalized
anomalous Hall conductivity is a constant for all the samples, the evidence of
dissipationless anomalous Hall current in Fe. | cond-mat |
Steady state extensional rheology of a dilute suspension of spheres in a
dilute polymer solution: We investigate the steady-state extensional rheology of a dilute suspension
of spherical particles in a dilute polymer solution. For a particle-free
polymeric fluid, in addition to the solvent viscosity, the extensional
viscosity due to the polymers, $\mu^\text{poly}$, contributes to the total
non-dimensionalized extensional viscosity $1+\mu^\text{poly}$. When a small
volume fraction, $\phi$, of spheres is added to a polymeric fluid, the stress
is altered by the Einstein viscosity of 2.5$\phi$ and two additional stress
contributions: the interaction stresslet and the particle-induced polymer
stress (PIPS). The net interaction stress is positive at lower Deborah numbers
(product of extension rate and polymer relaxation time), $De\lesssim0.5$, and
negative at large $De$. Relative to undisturbed flow, the presence of spheres
in uniaxial extensional flow creates larger and smaller local stretching
regions. Below the coil-stretch transition ($De<0.5$), the polymers far from
the particles are in a coiled state, and a wake of stretched polymers forms
downstream of the particle as they are stretched by the large stretching
regions around the particle. This leads to a positive interaction stresslet
(surface) and the PIPS (stretched wake). Beyond the coil-stretch transition,
polymers far from the particle are highly stretched, but they collapse closer
to the coiled state as they arrive at the low-stretching regions near the
particle surface. Therefore, a negative PIPS results from the regions of
collapsed polymers. When $De\gtrsim0.6$, the changes in extensional viscosity
from the interaction stresslet and the PIPS are $\phi\mu^\text{poly}$ and
approximately -1.85$\phi\mu^\text{poly}$, respectively. At large $De$, the
polymer extensional viscosity, $\mu^\text{poly}$, is large. Therefore, adding
particles reduces the extensional viscosity of the suspension
($(2.5-0.85\mu^\text{poly})\phi<0$). | cond-mat |
Instabilities of a Filled Vortex in a Two-Component Bose-Einstein
Condensate: A two-component Bose-Einstein condensate of cold atoms with a strong
intercomponent repulsion leading to the spatial separation of the components
has been numerically studied. Configurations with a multiple quantized vortex
in one component, where the vortex core is filled with the other component, are
considered. The effective radius of the core can exceed the width of the
transition layer between components, and then an analogy with a filled
cylindrical vortex in the classical hydrodynamics of two immiscible ideal
fluids appears. This analogy allows one to analyze the longitudinal "sausage"
instability and the transverse instability of the filled vortex in the
condensate caused by the "tangential discontinuity," as well as the stable
regime in the parametric gap between them. The presence of long-lived coherent
structures formed in some cases at the nonlinear stages of both instabilities
is numerically discovered. | cond-mat |
Voltage Control of Exchange Coupling in Phosphorus Doped Silicon: Motivated by applications to quantum computer architectures we study the
change in the exchange interaction between neighbouring phosphorus donor
electrons in silicon due to the application of voltage biases to surface
control electrodes. These voltage biases create electro-static fields within
the crystal substrate, perturbing the states of the donor electrons and thus
altering the strength of the exchange interaction between them. We find that
control gates of this kind can be used to either enhance, or reduce the
strength of the interaction, by an amount that depends both on the magnitude
and orientation of the donor separation. | cond-mat |
Dynamical slowing down in an ultrafast photo-induced phase transition: Complex systems, which consist of a large number of interacting constituents,
often exhibit universal behavior near a phase transition. A slowdown of certain
dynamical observables is one such recurring feature found in a vast array of
contexts. This phenomenon, known as critical slowing down, is well studied
mostly in thermodynamic phase transitions. However, it is less understood in
highly nonequilibrium settings, where the time it takes to traverse the phase
boundary becomes comparable to the timescale of dynamical fluctuations. Using
transient optical spectroscopy and femtosecond electron diffraction, we studied
a photo-induced transition of a model charge-density-wave (CDW) compound,
LaTe$_3$. We observed that it takes the longest time to suppress the order
parameter at the threshold photoexcitation density, where the CDW transiently
vanishes. This finding can be quantitatively captured by generalizing the
time-dependent Landau theory to a system far from equilibrium. The experimental
observation and theoretical understanding of dynamical slowing down may offer
insight into other general principles behind nonequilibrium phase transitions
in many-body systems. | cond-mat |
Quantum Mechanics with a Momentum-Space Artificial Magnetic Field: The Berry curvature is a geometrical property of an energy band which acts as
a momentum space magnetic field in the effective Hamiltonian describing
single-particle quantum dynamics. We show how this perspective may be exploited
to study systems directly relevant to ultracold gases and photonics. Given the
exchanged roles of momentum and position, we demonstrate that the global
topology of momentum space is crucially important. We propose an experiment to
study the Harper-Hofstadter Hamiltonian with a harmonic trap that will
illustrate the advantages of this approach and that will also constitute the
first realization of magnetism on a torus. | cond-mat |
Optical properties of an effective one-band Hubbard model for the
cuprates: We study the Cu and O spectral density of states and the optical conductivity
of CuO_2 planes using an effective generalized one-band Hubbard model derived
from the extended three-band Hubbard model. We solve exactly a square cluster
of 10 unit cells and average the results over all possible boundary conditions,
what leads to smooth functions of frequency. Upon doping, the Fermi energy
jumps to Zhang-Rice states which are connected to the rest of the valence band
(in contrast to an isolated new band in the middle of the gap). The transfer of
spectral weight depends on the parameters of the original three-band model not
only through the one-band effective parameters but also through the relevant
matrix elements. We discuss the evolution of the gap upon doping. The optical
conductivity of the doped system shows a mid-infrared peak due to intraband
transitions, a pseudogap and a high frequency part related to interband
transitions. Its shape and integrated weight up to a given frequency (including
the Drude weight) agree qualitatively with experiments in the cuprates for low
to moderate doping levels, but significant deviations exist for doping $x>0.3$. | cond-mat |
Bose-Einstein Condensation and quasicrystals: We consider interacting Bose particles in an external local potential. It is
shown that large class of external quasicrystal potentials cannot sustain any
type of Bose-Einstein condensates. Accordingly, at spatial dimensions $D\leq 2$
in such quasicrystal potentials a supersolid is not possible via Bose-Einstein
condensates at finite temperatures. The latter also hold true for the
two-dimensional Fibonacci tiling. However, supersolids do arise at $D\leq 2$
via Bose-Einstein condensates from infinitely long-range, nonlocal
interparticle potentials. | cond-mat |
Optical properties of $MgCNi_3$ in the normal state: We present the optical reflectance and conductivity spectra for non-oxide
antiperovskite superconductor $MgCNi_{3}$ at different temperatures. The
reflectance drops gradually over a large energy scale up to 33,000 cm$^{-1}$,
with the presence of several wiggles. The reflectance has slight temperature
dependence at low frequency but becomes temperature independent at high
frequency. The optical conductivity shows a Drude response at low frequencies
and four broad absorption features in the frequency range from 600 $cm^{-1}$ to
33,000 $cm^{-1}$. We illustrate that those features can be well understood from
the intra- and interband transitions between different components of Ni 3d
bands which are hybridized with C 2p bands. There is a good agreement between
our experimental data and the first-principle band structure calculations. | cond-mat |
Competition between the structural phase transition and
superconductivity in Ir$_{1-x}$Pt$_x$Te$_2$ as revealed by pressure effects: Pressure-dependent transport measurements of Ir$_{1-x}$Pt$_x$Te$_2$ are
reported. With increasing pressure, the structural phase transition at high
temperatures is enhanced while its superconducting transition at low
temperatures is suppressed. These pressure effects make Ir$_{1-x}$Pt$_x$Te$_2$
distinct from other studied $T$X$_2$ systems exhibiting a charge density wave
(CDW) state, in which pressure usually suppresses the CDW state and enhances
the superconducting state. The results reveal that the emergence of
superconductivity competes with the stabilization of the low temperature
monoclinic phase in Ir$_{1-x}$Pt$_x$Te$_2$. | cond-mat |
Topological flat bands in a kagomé lattice multiorbital system: Flat bands and dispersive Dirac bands are known to coexist in the electronic
bands in a two-dimensional kagome lattice. Including the relativistic
spin-orbit coupling, such systems often exhibit nontrivial band topology,
allowing for gapless edge modes between flat bands at several locations in the
band structure, and dispersive bands or at the Dirac band crossing. Here, we
theoretically demonstrate that a multiorbital system on a kagome lattice is a
versatile platform to explore the interplay between nontrivial band topology
and electronic interaction. Specifically, here we report that the multiorbital
kagome model with the atomic spin-orbit coupling naturally supports topological
bands characterized by nonzero Chern numbers $\cal C$, including a flat band
with $|{\cal C}| =1$. When such a flat band is $1/3$ filled, the non-local
repulsive interactions induce a fractional Chern insulating state. We also
discuss the possible realization of our findings in real kagome materials. | cond-mat |
Nearest level spacing statistics in open chaotic systems: a
generalization of the Wigner Surmise: We investigate the nearest level spacing statistics of open chaotic wave
systems. To this end we derive the spacing distributions for the three Wigner
ensembles in the one-channel case. The theoretical results give a clear
physical meaning of the modifications on the spacing distributions produced by
the coupling to the environment. Based on the analytical expressions obtained,
we then propose general expressions of the spacing distributions for any number
of channels, valid from weak to strong coupling. The latter expressions contain
one free parameter. The surmise is successfully compared with numerical
simulations of non-Hermitian random matrices and with experimental data
obtained with a lossy electromagnetic chaotic cavity. | cond-mat |
Theoretical Understanding of Photon Spectroscopies in Correlated
Materials In and Out of Equilibrium: Photon-based spectroscopies have had a significant impact on both fundamental
science and applications by providing an efficient approach to investigate the
microscopic physics of materials. Together with the development of synchrotron
X-ray techniques, theoretical understanding of the spectroscopies themselves
and the underlying physics that they reveal has progressed through advances in
numerical methods and scientific computing. In this review, we provide an
overview of theories for angle-resolved photoemission spectroscopy and resonant
inelastic X-ray scattering applied to quantum materials. First, we discuss
methods for studying equilibrium spectroscopies, including first-principles
approaches, numerical many-body methods and a few analytical advances. Second,
we assess the recent development of ultrafast techniques for out-of-equilibrium
spectroscopies, from characterizing equilibrium properties to generating
transient or metastable states, mainly from a theoretical point of view.
Finally, we identify the main challenges and provide an outlook for the future
direction of the field. | cond-mat |
Chaos-assisted tunneling resonances in a synthetic Floquet superlattice: The field of quantum simulation, which aims at using a tunable quantum system
to simulate another, has been developing fast in the past years as an
alternative to the all-purpose quantum computer. In particular, the use of
temporal driving has attracted a huge interest recently as it was shown that
certain fast drivings can create new topological effects, while a strong
driving leads to e.g. Anderson localization physics. In this work, we focus on
the intermediate regime to observe a quantum chaos transport mechanism called
chaos-assisted tunneling which provides new possibilities of control for
quantum simulation. Indeed, this regime generates a rich classical phase space
where stable trajectories form islands surrounded by a large sea of unstable
chaotic orbits. This mimics an effective superlattice for the quantum states
localized in the regular islands, with new controllable tunneling properties.
Besides the standard textbook tunneling through a potential barrier,
chaos-assisted tunneling corresponds to a much richer tunneling process where
the coupling between quantum states located in neighboring regular islands is
mediated by other states spread over the chaotic sea. This process induces
sharp resonances where the tunneling rate varies by orders of magnitude over a
short range of parameters. We experimentally demonstrate and characterize these
resonances for the first time in a quantum system. This opens the way to new
kinds of quantum simulations with long-range transport and new types of control
of quantum systems through complexity. | cond-mat |
Spacial Modulation of the Magnetization in Cobalt Nanowires: Cobalt nanowires with a diameter in the range between 50 to 100nm can be
prepared as single-crystal wires with the easy axis (the c-axis) perpendicular
to the wire axis. The competition between the crystal anisotropy and
demagnetization energy frustrates the magnetization direction. A periodic
modulation of the angle between M and the wire axis yields a lower energy. | cond-mat |
Learning hidden elasticity with deep neural networks: We introduce a de novo elastography method to learn the elasticity of solids
from measured strains. The deep neural network in our new method is supervised
by the theory of elasticity and does not require labeled data for training.
Results show that the proposed method can learn the hidden elasticity of solids
accurately and is robust when it comes to noisy and missing measurements. A
probable elasticity distribution for areas without measurements may also be
reconstructed by the neural network based on the elasticity distribution in
nearby regions. The neural network learns the hidden elasticity of solids as a
function of positions and thus it can generate elasticity images with an
arbitrary resolution. This feature is applied to create super-resolution
elasticity images in this study. We demonstrate that the neural network can
also learn the hidden physics when strain and elasticity distributions are both
given. The proposed method has various unique features and can be applied to a
broad range of elastography applications. | cond-mat |
Mott Insulator to Superfluid Phase Transition in Bravais Lattices via
the Jaynes-Cummings-Hubbard Model: The Properties of the Mott insulator to superfluid phase transition are
obtained through the fermionic approximation in the Jaynes-Cummings-Hubbard
model on linear, square, SC, FCC, and BCC Bravais lattices. For varying
excitation number and atom-cavity frequency detuning. We find that the Mott
lobes and the critical hopping are not scalable only for the FCC lattice. At
the large excitation number regime, the critical hopping is scalable for all
the lattices and it does not depend on the detuning. | cond-mat |
Spin Hall Effect in a Thin Pt Film: A density-functional-theory based relativistic scattering formalism is used
to study charge transport through thin Pt films with room temperature lattice
disorder. A Fuchs-Sondheimer specularity coefficient $p \sim 0.5$ is needed to
describe the suppression of the charge current at the surface even in the
absence of surface roughness. The charge current drives a spin Hall current
perpendicular to the surface. Analysing the latter with a model that is
universally used to interpret the spin Hall effect in thin films and layered
materials, we are unable to recover values of the spin-flip diffusion length
$l_{\rm sf}$ and spin Hall angle $\Theta_{\rm sH}$ that we obtain for bulk Pt
using the same approximations. We trace this to the boundary conditions used
and develop a generalized model that takes surface effects into account. A
reduced value of $\Theta_{\rm sH}$ at the surface is then found to describe the
first-principles transport results extremely well. The in-plane spin Hall
effect is substantially enhanced at the surface. | cond-mat |
Density fluctuations of polymers in disordered media: We study self avoiding random walks in an environment where sites are
excluded randomly, in two and three dimensions. For a single polymer chain, we
study the statistics of the time averaged monomer density and show that these
are well described by multifractal statistics. This is true even far from the
percolation transition of the disordered medium. We investigate solutions of
chains in a disordered environment and show that the statistics cease to be
multifractal beyond the screening length of the solution. | cond-mat |
Orbital structure and magnetic ordering in stoichiometric and doped
crednerite CuMnO2: The exchange interactions and magnetic structure in layered system CuMnO2
(mineral crednerite) and in nonstoichiometric system Cu1.04Mn0.96O2, with
triangular layers distorted due to orbital ordering of the Mn3+ ions, are
studied by ab-initio band-structure calculations, which were performed within
the GGA+U approximation. The exchange interaction parameters for the Heisenberg
model within the Mn-planes and between the Mn-planes were estimated. We explain
the observed in-plane magnetic structure by the dominant mechanism of the
direct d-d exchange between neighboring Mn ions. The superexchange via O ions,
with 90 degree Mn-O-Mn bonds, plays less important role for the in-plane
exchange. The interlayer coupling is largely dominated by one exchange path
between the half-filled 3z^2-r^2 orbitals of Mn3+. The change of interlayer
coupling from antiferromagnetic in pure CuMnO2 to ferromagnetic in doped
material is also explained by our calculations. | cond-mat |
Theory of Electronic Ferroelectricity: We present a theory of the linear and nonlinear optical characteristics of
the insulating phase of the Falicov-Kimball model within the self-consistent
mean-field approximation. The Coulomb attraction between the itinerant
d-electrons and the localized f-holes gives rise to a built-in coherence
between the d and f-states, which breaks the inversion symmetry of the
underlying crystal, leading to: (1) electronic ferroelectricity, (2)
ferroelectric resonance, and (3) a nonvanishing susceptibility for
second-harmonic generation. As experimental tests of such a built-in coherence
in mixed-valent compounds we propose measurements of the static dielectric
constant, the microwave absorption spectrum, and the dynamic second-order
susceptibility. | cond-mat |
Kohn-Sham computation and the bivariate view of density functional
theory: Informed by an abstraction of Kohn-Sham computation called a KS machine, a
functional analytic perspective is developed on mathematical aspects of density
functional theory. A natural semantics for the machine is bivariate, consisting
of a sequence of potentials paired with a ground density. Although the question
of when the KS machine can converge to a solution (where the potential
component matches a designated target) is not resolved here, a number of
related ones are. For instance: Can the machine progress toward a solution?
Barring presumably exceptional circumstances, yes in an energetic sense, but
using a potential-mixing scheme rather than the usual density-mixing variety.
Are energetic and function space distance notions of proximity-to-solution
commensurate? Yes, to a significant degree. If the potential components of a
sequence of ground pairs converges to a target density, do the density
components cluster on ground densities thereof? Yes, barring particle number
drifting to infinity. | cond-mat |
Spin-triplet Superconductivity in Nonsymmorphic crystals: Spin-triplet superconductivity is known to be a rare quantum phenomenon. Here
we show that nonsymmorphic crystalline symmetries can dramatically assist
spin-triplet superconductivity in the presence of spin-orbit coupling. Even
with a weak spin-orbit coupling, the spin-triplet pairing can be the leading
pairing instability in a lattice with a nonsymmorphic symmetry. The underlining
mechanism is the spin-sublattice-momentum lock on electronic bands that are
protected by the nonsymmorphic symmetry. We use the nonsymmorphic space group
P4/nmm to demonstrate these results and discuss related experimental
observables. Our work paves a new way in searching for spin-triplet
superconductivity. | cond-mat |
Shot noise and tunnel magnetoresistance in multilevel quantum dots:
Effects of cotunneling: Spin-dependent transport through a multilevel quantum dot weakly coupled to
ferromagnetic leads is analyzed theoretically by means of the real-time
diagrammatic technique. Both the sequential and cotunneling processes are taken
into account, which makes the results on tunnel magnetoresistance (TMR) and
shot noise applicable in the whole range of relevant bias and gate voltages.
Suppression of the TMR due to inelastic cotunneling and super-Poissonian shot
noise have been found in some of the Coulomb blockade regions. Furthermore, in
the Coulomb blockade regime there is an additional contribution to the noise
due to bunching of cotunneling processes involving the spin-majority electrons.
On the other hand, in the sequential tunneling regime TMR oscillates with the
bias voltage, while the current noise is generally sub-Poissonian. | cond-mat |
A microstructural analysis of 2D halide perovskites: Stability and
functionality: Recent observations indicated that the photoelectric conversion properties of
perovskite materials are intimately related to the presence of superlattice
structures and other unusual nanoscale features in them. The low dimensional or
mixed dimensional halide perovskite family are found to be more efficient
materials for device application compared to 3-dimensional halide perovskites.
The emergence of perovskite solar cell has revolutionized the solar cell
industry because of their flexible architecture and rapidly increased
efficiency. Tuning the dielectric constant, charge separation are the main
objective in designing a photovoltaic device that can be explored using
2-dimensional perovskite family. Thus, revisiting the fundamental properties of
perovskite crystals could reveal further possibilities for recognizing these
improvements towards device functionality. In this context, this review
discusses the material properties of 2-dimensional halide perovskite and
related optoelectronic devices aiming particularly for solar cell application. | cond-mat |
Maximum entropy analytic continuation for frequency-dependent transport
coefficients with non-positive spectral weight: The computation of transport coefficients, even in linear response, is a
major challenge for theoretical methods that rely on analytic continuation of
correlations functions obtained numerically in Matsubara space. While maximum
entropy methods can be used for certain correlation functions, this is not
possible in general, important examples being the Seebeck, Hall, Nernst and
Reggi-Leduc coefficients. Indeed, positivity of the spectral weight on the
positive real-frequency axis is not guaranteed in these cases. The spectral
weight can even be complex in the presence of broken time-reversal symmetry.
Various workarounds, such as the neglect of vertex corrections or the study of
the infinite frequency or Kelvin limits have been proposed. Here, we show that
one can define auxiliary response functions that allow to extract the desired
real-frequency susceptibilities from maximum entropy methods in the most
general multiorbital cases with no particular symmetry. As a benchmark case, we
study the longitudinal thermoelectric response and corresponding Onsager
coefficient in the single-band two-dimensional Hubbard model treated with
dynamical mean-field theory (DMFT) and continuous-time quantum Monte Carlo
(CTQMC). We thereby extend to transport coefficients the maximum entropy
analytic continuation with auxiliary functions (MaxEntAux method), developed
for the study of the superconducting pairing dynamics of correlated materials. | cond-mat |
Boson pairing and unusual criticality in a generalized XY model: We discuss the unusual critical behavior of a generalized XY model containing
both 2\pi-periodic and \pi-periodic couplings between sites. The presence of
vortices and half-vortices allows for single-particle condensate and
pair-condensate phases. Using a field theoretic formulation and worm algorithm
Monte Carlo simulations, we show that in two dimensions it is possible for the
system to pass directly from the disordered (high temperature) phase to the
single particle (quasi)-condensate via an Ising transition, a situation
reminiscent of the `deconfined criticality' scenario. | cond-mat |
Real-time measurement of laser induced photoconductivity in sol-gel
derived Al doped ZnO thin films: In this paper Al doped ZnO (AZO) thin films with 0, 3, 6 and 12 at. % Al
concentration were prepared by sol-gel method on glass substrates. The
deposited films were annealed at different temperatures of 300, 350, 400, 450
and 500 {\deg}C for 1 h in air. X-ray diffraction (XRD) showed wurtzite
crystalline structure for the films annealed above 400 {\deg}C. The films were
subsequently irradiated by beams of excimer (KrF, {\lambda}=248 nm) laser. The
evolution of crystal structure, surface morphology and optical properties were
studied using XRD, filed emission scanning electron microscope (FE-SEM) and
UV-Vis spectrophotometer, respectively. Real-time measurement of electrical
conductivity during laser irradiation showed a transient or persistent
photoconductivity effect. The effect of laser energy on this photoconductivity
was also investigated. Based on the observed photoluminescence (PL) and X-ray
photoelectron spectroscopy (XPS), the observed photoconductivity effect was
described. | cond-mat |
Quotient symmetry protected topological phenomena: Topological phenomena are commonly studied in phases of matter which are
separated from a trivial phase by an unavoidable quantum phase transition. This
can be overly restrictive, leaving out scenarios of practical relevance --
similar to the distinction between liquid water and vapor. Indeed, we show that
topological phenomena can be stable over a large part of parameter space even
when the bulk is strictly speaking in a trivial phase of matter. In particular,
we focus on symmetry-protected topological phases which can be trivialized by
extending the symmetry group. The topological Haldane phase in spin chains
serves as a paradigmatic example where the $SO(3)$ symmetry is extended to
$SU(2)$ by tuning away from the Mott limit. Although the Haldane phase is then
adiabatically connected to a product state, we show that characteristic
phenomena -- edge modes, entanglement degeneracies and bulk phase transitions
-- remain parametrically stable. This stability is due to a separation of
energy scales, characterized by quantized invariants which are well-defined
when a subgroup of the symmetry only acts on high-energy degrees of freedom.
The low-energy symmetry group is a quotient group whose emergent anomalies
stabilize edge modes and unnecessary criticality, which can occur in any
dimension. | cond-mat |
Topological charge distributions of an interacting two-spin system: Quantum systems are often described by parameter-dependent Hamiltonians.
Points in parameter space where two levels are degenerate can carry a
topological charge. Here we theoretically study an interacting two-spin system
where the degeneracy points form a nodal loop or a nodal surface in the
magnetic parameter space, similarly to such structures discovered in the band
structure of topological semimetals. We determine the topological charge
distribution along these degeneracy geometries. We show that these
non-point-like degeneracy patterns can be obtained not only by fine-tuning, but
they can be stabilized by spatial symmetries. Since simple spin systems such as
the one studied here are ubiquitous in condensed-matter setups, we expect that
our findings, and the physical consequences of these nontrivial degeneracy
geometries, are testable in experiments with quantum dots, molecular magnets,
and adatoms on metallic surfaces. | cond-mat |
Power-law temporal auto-correlations in day-long records of human
physical activity and their alteration with disease: We investigate long-duration time series of human physical activity under
three different conditions: healthy individuals in (i) a constant routine
protocol and (ii) in regular daily routine, and (iii) individuals diagnosed
with multiple chemical sensitivities. We find that in all cases human physical
activity displays power law decaying temporal auto-correlations. Moreover, we
find that under regular daily routine, time correlations of physical activity
are significantly different during diurnal and nocturnal periods but that no
difference exists under constant routine conditions. Finally, we find
significantly different auto-correlations for diurnal records of patients with
multiple chemical sensitivities. | cond-mat |
Pseudogap transition within the superconducting phase in the three-band
Hubbard model: The onset of the pseudogap in high-$T_c$ superconducting cuprates (HTSC) is
marked by the $T^*$ line in the doping-temperature phase diagram, which ends at
a point $p^*$ at zero temperature within the superconducting dome. Although
various theoretical and experimental studies indicate a competition between the
pseudogap and superconductivity, there is no general consensus on the effects
of the pseudogap within the superconducting phase. We use cluster dynamical
mean field theory on a three-band Hubbard model for the HTSC to study the
superconducting phase at $T=0$, obtained when doping the charge-transfer
insulator, for several values of $U$. We observe a first-order transition
within the superconducting phase, which separates the underdoped and overdoped
solutions. The transition to the underdoped solution is marked by a
discontinuous increase in the spectral gap, and on further underdoping the
spectral gap increases while the superconducting order parameter decreases. We
conclude that this is due to the onset of the pseudogap in the underdoped
region, which contributes to the increasing spectral gap; this is further
consistent with the appearance of a pole in the normal component of the
self-energy, in the antinodal region, in the underdoped solution. This is
accompanied by a change in the source of the condensation energy from potential
energy, in the overdoped region, to kinetic energy in the underdoped region.
Further, we also observe that the $d$-wave node vanishes smoothly within the
superconducting phase at low values of hole doping, within the underdoped
region. We see this as a manifestation of Mott physics operating at very low
doping. Various aspects of the results and their implications are discussed. | cond-mat |
High-energy long-lived resonance of electrons in fractal-like
semiconductor heterostructures: A fractal-like alignment of quantum wells is shown to accommodate resonant
states with long lifetimes. For the parameters of the semiconductor
heterostructure GaAs/Al$_{0.4}$Ga$_{0.6}$As with the well depth 300meV, a
resonant state of the energy as high as 44meV with the lifetime as long as
2.8\{mu}s is shown to be achievable. | cond-mat |
Power laws, Pareto distributions and Zipf's law: When the probability of measuring a particular value of some quantity varies
inversely as a power of that value, the quantity is said to follow a power law,
also known variously as Zipf's law or the Pareto distribution. Power laws
appear widely in physics, biology, earth and planetary sciences, economics and
finance, computer science, demography and the social sciences. For instance,
the distributions of the sizes of cities, earthquakes, solar flares, moon
craters, wars and people's personal fortunes all appear to follow power laws.
The origin of power-law behaviour has been a topic of debate in the scientific
community for more than a century. Here we review some of the empirical
evidence for the existence of power-law forms and the theories proposed to
explain them. | cond-mat |
Van Hove Exciton-Cageons and High-T$_c$ Superconductivity: XB: Polaronic
Coupling in the Doped Material: A purely ionic interpretation of the tilting mode instabilities in
La$_{2-x}$A$ _x$CuO$_4$ (A=Sr,Ba) is shown to be not self-consistent: the
dominant factor influencing the doping dependence of the interlayer mismatch is
the large change in the Cu-O bond length, which in turn leads to a strong
electron-phonon coupling. This coupling is closely related to the vHs-JT
effect. This new insight clarifies the role of the tilt-mode instabilities. The
main JT coupling is {\it not} to these modes, but to the in-plane O-O bond
stretching modes which split the vHs degeneracy. However, as these modes
soften, they couple to the lower-lying tilt modes, so that the ultimate
instability has a finite tilt component. The bond stretch modes have a large,
linear coupling to electrons, with clear polaronic effects. A striking result
of this is that there will be a large polaronic band narrowing near the vHs,
whether or not the vHs is near the Fermi level. This vHs-localized band
narrowing provides a natural explanation for the common occurence of extended
vHs's. | cond-mat |
NMR properties of a one-dimesional Cu-O model: We obtain the Knight shifts and the relaxation rates related to the Fermi
contact interaction term for a one-dimensional Cu-O model using bosonization
technique. We consider the small interaction limit at half-filling and away
from half-filling. In this framework we predict that the antiferromagnetic
contribution to the relaxation rate of the nuclear oxygen spin is completely
suppressed even away from half-filling, when the temperature is low enough. In
the strong interaction limit at half-filling we compute the effective Fermi
contact interaction performing a Gutzwiller projection. Both limits suggest
that the one-dimensional versions of the Mila-Rice and of the Shastry scenarios
of transferred hyperfine couplings which were proposed to explain the NMR
measurements for High-T_c cuprates fail in a one-dimensional situation. | cond-mat |
Topological Transitions in a Model for Proximity Induced
Superconductivity: Using a prototype model for proximity induced superconductivity on a bilayer
square lattice, we show that interlayer tunneling can drive change in topology
of the Bogoliubov quasiparticle bands. Starting with topologically trivial
superconductors, transitions to a non-trivial $p_x + {\rm i} p_y$ state and
back to another trivial state are discovered. We characterize these phases in
terms of edge-state spectra and Chern indices. We show that these transitions
can also be controlled by experimentally viable control parameters, the
bandwidth of the metallic layer and the gate potential. Insights from our
results on a simple model for proximity induced superconductivity may open up a
new route to discover topological superconductors. | cond-mat |
Instabilities of micro-phase separated Coulombic systems in constant
electric fields: Mixtures of near-symmetric oppositely charged components with strong
attractive short range interactions exhibit ordered lamellar phases at low
temperatures. In the strong segregation limit the state of these systems can be
described by the location of the interfaces between the components. It has
previously been shown that these systems are stable against small deformations
of the interfaces. We examine their stability in the presence of a uniform
electric field. When the field is perpendicular to the lamellae, the system is
unstable against long wavelength deformations for all non-zero values of the
external field. A field parallel to the lamellae produces deformed but
persistent interfaces. In a finite thickness system, onset of an external
perpendicular field modifies the ground state. Flow between the old and new
ground states requires the destruction of the original interfaces; this
destruction proceeds through the instabilities identified in the bulk case. We
examine the possibility of dynamical stabilization of structures by means of
oscillating fields. | cond-mat |
Signatures of a topological Weyl loop in Co$_3$Sn$_2$S$_2$: The search for novel topological phases of matter in quantum magnets has
emerged as a frontier of condensed matter physics. Here we use state-of-the-art
angle-resolved photoemission spectroscopy (ARPES) to investigate single
crystals of Co$_3$Sn$_2$S$_2$ in its ferromagnetic phase. We report for the
first time signatures of a topological Weyl loop. From fundamental symmetry
considerations, this magnetic Weyl loop is expected to be gapless if spin-orbit
coupling (SOC) is strictly zero but gapped, with possible Weyl points, under
finite SOC. We point out that high-resolution ARPES results to date cannot
unambiguously resolve the SOC gap anywhere along the Weyl loop, leaving open
the possibility that Co$_3$Sn$_2$S$_2$ hosts zero Weyl points or some non-zero
number of Weyl points. On the surface of our samples, we further observe a
possible Fermi arc, but we are unable to clearly verify its topological nature
using the established counting criteria. As a result, we argue that from the
point of view of photoemission spectroscopy the presence of Weyl points and
Fermi arcs in Co$_3$Sn$_2$S$_2$ remains ambiguous. Our results have
implications for ongoing investigations of Co$_3$Sn$_2$S$_2$ and other
topological magnets. | cond-mat |
Bilayer Isotropic Thermal Cloak: Invisibility has attracted intensive research in various communities, e.g.,
optics, electromagnetics, acoustics, thermodynamics, etc. However, the most of
them have only been experimentally achieved by virtue of simplified approaches
due to their inhomogeneous and extreme parameters imposed by
transformation-optic method, and usually require challenging realization with
metamaterials. In this paper, we demonstrate an advanced bilayer thermal cloak
with naturally available materials first time. This scheme, directly from
thermal conduction equation, has been validated as an exact cloak rather than a
reduced one, and we experimentally confirmed its perfect performance
(heat-front maintenance and heat protection) in an actual setup. The proposed
scheme may open a new avenue to control the diffusive heat flow in ways
inconceivable with phonons. | cond-mat |
Eigenvector Dreaming: Among the performance-enhancing procedures for Hopfield-type networks that
implement associative memory, Hebbian Unlearning (or dreaming) strikes for its
simplicity and its clear biological interpretation. Yet, it does not easily
lend itself to a clear analytical understanding. Here we show how Hebbian
Unlearning can be effectively described in terms of a simple evolution of the
spectrum and the eigenvectors of the coupling matrix. We use these ideas to
design new dreaming algorithms that are effective from a computational point of
view, and are analytically far more transparent than the original scheme. | cond-mat |
Dynamic Length Scale and Weakest Link Behavior in Crystal Plasticity: Plastic deformation of heterogeneous solid structures is often characterized
by random intermittent local plastic events. On the mesoscale this feature can
be represented by a spatially fluctuating local yield threshold. Here we study
the validity of such an approach and the ideal choice for the size of the
representative volume element for crystal plasticity in terms of a discrete
dislocation model. We find that the number of links representing possible
sources of plastic activity exhibits anomalous (super-extensive) scaling which
tends to extensive scaling (often assumed in weakest-link models) if quenched
short-range interactions are introduced. The reason is that the interplay
between long-range dislocation interactions and short-range quenched disorder
destroys scale-free dynamical correlations leading to event localization with a
characteristic length-scale. Several methods are presented to determine the
dynamic length-scale that can be generalized to other types of heterogeneous
materials. | cond-mat |
Nonlinear twistoptics at symmetry-broken interfaces: Broken symmetries induce strong nonlinear optical responses in materials and
at interfaces. Twist angle can give complete control over the presence or lack
of inversion symmetry at a crystal interface, and is thus an appealing knob for
tuning nonlinear optical systems. In contrast to conventional nonlinear
crystals with rigid lattices, the weak interlayer coupling in van der Waals
(vdW) heterostructures allows for arbitrary selection of twist angle, making
nanomechanical manipulation of fundamental interfacial symmetry possible within
a single device. Here we report highly tunable second harmonic generation (SHG)
from nanomechanically rotatable stacks of bulk hexagonal boron nitride (BN)
crystals, and introduce the term twistoptics to describe studies of optical
properties in dynamically twistable vdW systems. We observe SHG intensity
modulated by a factor of more than 50, polarization patterns determined by
moir\'e interface symmetry, and enhanced conversion efficiency for bulk
crystals by stacking multiple pieces of BN joined by symmetry-broken
interfaces. Our study provides a foundation for compact twistoptics
architectures aimed at efficient, scalable, and tunable frequency-conversion,
and demonstrates SHG as a robust probe of buried vdW interfaces. | cond-mat |
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