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Isotropic contact forces in arbitrary representation: heterogeneous
few-body problems and low dimensions: The Bethe-Peierls asymptotic approach which models pairwise short-range
forces by contact conditions is introduced in arbitrary representation for
spatial dimensions less than or equal to 3. The formalism is applied in various
situations and emphasis is put on the momentum representation. In the presence
of a transverse harmonic confinement, dimensional reduction toward
two-dimensional (2D) or one-dimensional (1D) physics is derived within this
formalism. The energy theorem relating the mean energy of an interacting system
to the asymptotic behavior of the one-particle density matrix illustrates the
method in its second quantized form. Integral equations that encapsulate the
Bethe-Peierls contact condition for few-body systems are derived. In three
dimensions, for three-body systems supporting Efimov states, a nodal condition
is introduced in order to obtain universal results from the Skorniakov
Ter-Martirosian equation and the Thomas collapse is avoided. Four-body bound
state eigenequations are derived and the 2D '3+1' bosonic ground state is
computed as a function of the mass ratio. | cond-mat |
Modeling neutron and X-ray scattering by liquids: We review exact formalisms for describing the dynamics of liquids in terms of
static parameters. We discuss how these formalisms are prone to suffer from
imposing restrictions that appear to adhere to common sense but which are
overly restrictive, resulting in a flawed description of the dynamics of
liquids. We detail a fail-safe way for modeling the scattering data of liquids
that is free from any unwarranted restriction and that models the scattering
using the fewest possible number of free parameters. We also list some common
habits in analyzing data and how these habits do not do justice to the accuracy
of the results obtained in scattering experiments, and how these habits may
stand in the way of rejecting some models used in describing the dynamics of
liquids. | cond-mat |
Origin of the abnormal diffusion of transition metal in rutile: Diffusion of dopants in rutile is the fundamental process that determines the
performance of many devices in which rutile is used. The diffusion behavior is
known to be highly sample-dependent, but the reasons for this are less well
understood. Here, rutile is studied by using first-principles calculations, in
order to unravel the microscopic origins of the diverse diffusion behaviors for
different doping elements. Anomalous diffusion behavior in the open channel
along [001] direction is found: larger atoms include Sc and Zr have lower
energy barrier for diffusion via interstitial mechanism, apparently
contradicting their known slow diffusion rate. To resolve this, we present an
alternate model for the overall diffusion rate of the large-size dopants in
rutile, showing that parallel to the [001] channel, it is limited by the
formation of the interstitial states, whereas in the direction perpendicular to
[001], it proceeds via a kick-out mechanism. By contrast, Co and Ni, prefer to
stay in the interstitial site of rutile, and have conventional diffusion with a
very small migration barrier in the [001] channel. This leads to highly
anisotropic and fast diffusion. The diffusion mechanisms found in the present
study can explain the diffusion data measured by experiments, and these
findings provide novel understanding for the classic diffusion topic. | cond-mat |
Effect of traps on the current impulse from X-ray induced conductivity
in wide-gap semiconductors: This article presents a theoretical model for the calculation of the current
impulse from X-ray induced conductivity in wide-gap semiconductors that contain
different types of traps and recombination centres. The absorption of one X-ray
photon in a semiconductor with ohmic contacts was investigated. The influence
of the main parameters of the traps and recombination centres on the shape and
amplitude of the current impulse was determined. | cond-mat |
Theoretical analysis of optimization problems - Some properties of
random k-SAT and k-XORSAT: This thesis is divided in two parts. The first presents an overview of known
results in statistical mechanics of disordered systems and its approach to
random combinatorial optimization problems. The second part is a discussion of
two original results.
The first result concerns DPLL heuristics for random k-XORSAT, which is
equivalent to the diluted Ising p-spin model. It is well known that DPLL is
unable to find the ground states in the clustered phase of the problem, i.e.
that it leads to contradictions with probability 1. However, no solid argument
supports this is general. A class of heuristics, which includes the well known
UC and GUC, is introduced and studied. It is shown that any heuristic in this
class must fail if the clause to variable ratio is larger than some constant,
which depends on the heuristic but is always smaller than the clustering
threshold.
The second result concerns the properties of random k-SAT at large clause to
variable ratios. In this regime, it is well known that the uniform distribution
of random instances is dominated by unsatisfiable instances. A general
technique (based on the Replica method) to restrict the distribution to
satisfiable instances with uniform weight is introduced, and is used to
characterize their solutions. It is found that in the limit of large clause to
variable ratios, the uniform distribution of satisfiable random k-SAT formulas
is asymptotically equal to the much studied Planted distribution.
Both results are already published and available as arXiv:0709.0367 and
arXiv:cs/0609101 . A more detailed and self-contained derivation is presented
here. | cond-mat |
Exploring quantum signatures of chaos on a Floquet synthetic lattice: Ergodicity and chaos play an integral role in the dynamical behavior of
many-particle systems and are crucial to the formulation of statistical
mechanics. Still, a general understanding of how randomness and chaos emerge in
the dynamical evolution of closed quantum systems remains elusive. Here, we
develop an experimental platform for the realization of canonical quantum
chaotic Hamiltonians based on quantum simulation with synthetic lattices. We
map the angular momentum projection states of an effective quantum spin onto
the linear momentum states of a $^{87}$Rb Bose-Einstein condensate, which can
alternatively be viewed as lattice sites in a synthetic dimension. This
synthetic lattice, with local and dynamical control of tight-binding lattice
parameters, enables new capabilities related to the experimental study of
quantum chaos. In particular, the capabilities of our system let us tune the
effective size of our spin, allowing us to illustrate how classical chaos can
emerge from a discrete quantum system. Moreover, spectroscopic control over our
synthetic lattice allows us to explore unique aspects of our spin's dynamics by
measuring the out-of-time-ordered correlation function, and enables future
investigations into entirely new classes of chaotic systems. | cond-mat |
Charge accumulation at the boundaries of a graphene strip induced by a
gate voltage: Electrostatic approach: Distribution of charge induced by a gate voltage in a graphene strip is
investigated. We calculate analytically the charge profile and demonstrate a
strong(macroscopic) charge accumulation along the boundaries of a
micrometers-wide strip. This charge inhomogeneity is especially important in
the quantum Hall regime where we predict the doubling of the number of edge
states and coexistence of two different types of such states. Applications to
graphene-based nanoelectronics are discussed. | cond-mat |
Quantum Dynamics of Coupled Bosonic Wells within the Bose-Hubbard
Picture: We relate the quantum dynamics of the Bose-Hubbard model (BHM) to the
semiclassical nonlinear equations that describe an array of interacting Bose
condensates by implementing a standard variational procedure based on the
coherent state method. We investigate the dynamics of the two-site BHM from the
purely quantum viewpoint by recasting first the model within a spin picture and
using then the related dynamical algebra. The latter allows us to study
thoroughly the energy spectrum structure and to interpret quantally the
classical symmetries of the two-site dynamics. The energy spectrum is also
evaluated through various approximations relying on the coherent state
approach. | cond-mat |
Electronic Transport and quantum oscillation of Topological Semimetals: Three-dimensional (3D) topological semimetals represent a new class of
topological matters. The study of this family of materials has been at the
frontiers of condensed matter physics, and many breakthroughs have been made.
Several topological semimetal phases, including Dirac semimetals (DSMs), Weyl
semimetals (WSMs), nodal-line semimetals (NLSMs), and triple-point semimetals,
have been theoretically predicted and experimentally demonstrated. The
low-energy excitation around the Dirac/Weyl nodal points, nodal line, or triply
degenerated nodal point can be viewed as emergent relativistic fermions.
Experimental studies have shown that relativistic fermions can result in a rich
variety of exotic transport properties, e.g., extremely large
magnetoresistance, the chiral anomaly, and the intrinsic anomalous Hall effect.
In this review, we first briefly introduce band structural characteristics of
each topological semimetal phase, then review the current studies on quantum
oscillations and exotic transport properties of various topological semimetals,
and finally provide a perspective of this area. | cond-mat |
Antisite Disorder-induced Exchange Bias Effect in Multiferroic Y2CoMnO6: Exchange bias effect in the ferromagnetic double perovskite compound
Y$_2$CoMnO$_6$, which is also a multiferroic, is reported. The exchange bias,
observed below 8~K, is explained as arising due to the interface effect between
the ferromagnetic and antiferromagnetic clusters created by {\it antisite}
disorder in this material. Below 8~K, prominent ferromagnetic hysteresis with
metamagnetic "steps" and significant coercive field, $H_c \approx$ 10~kOe are
observed in this compound which has a $T_c \approx$ 75~K. A model based on
growth of ferromagnetic domains overcoming the elastic energy of structurally
pinned magnetic interfaces, which closely resembles martensitic-like
transitions, is adapted to explain the observed effects. The role of {\it
antisite} disorder in creating the domain structure leading to exchange bias
effect is highlighted in the present work. | cond-mat |
Giant anomalous Hall and Nernst conductivities in magnetic all-$d$ metal
Heusler alloys: All-$d$ Heuslers are a category of novel compounds combining versatile
functionalities such as caloric responses and spintronics with enhanced
mechanical properties. Despite the promising transport properties (anomalous
Hall (AHC) and anomalous Nernst (ANC) conductivities) shown in the conventional
Co$_2$XY Heuslers with $p$-$d$ hybridization, the all-$d$ Heuslers with only
$d$-$d$ hybridization open a new horizon to search for new candidates with
outstanding transport properties. In this work, we evaluate the AHC and ANC for
thermodynamically stable ferro/ferri-magnetic all-$d$-metal regular Heusler
compounds based on high-throughput first-principles calculations. It is
observed that quite a few materials exhibit giant AHCs and ANCs, such as cubic
Re$_2$TaMn with an AHC of 2011 S/cm, and tetragonal Pt$_2$CrRh with an AHC of
1966 S/cm and an ANC of 7.50 A/mK. Comprehensive analysis on the electronic
structure reveals that the high AHC can be attributed to the occurrence of the
Weyl nodes or gapped nodal lines in the neighbourhood of the Fermi level. The
correlations between such transport properties and the number of valence
electrons are also thoroughly investigated, which provides a practical guidance
to tailor AHC and ANC via chemical doping for transverse thermoelectric
applications. | cond-mat |
Charge Transport in a Quantum Electromechanical System: We describe a quantum electromechanical system(QEMS) comprising a single
quantum dot harmonically bound between two electrodes and facilitating a
tunneling current between them. An example of such a system is a fullerene
molecule between two metal electrodes [Park et al., Nature, 407, 57 (2000)].
The description is based on a quantum master equation for the density operator
of the electronic and vibrational degrees of freedom and thus incorporates the
dynamics of both diagonal (population) and off diagonal (coherence) terms. We
derive coupled equations of motion for the electron occupation number of the
dot and the vibrational degrees of freedom, including damping of the vibration
and thermo-mechanical noise. This dynamical description is related to
observable features of the system including the stationary current as a
function of bias voltage. | cond-mat |
Enhanced current noise correlations in a Coulomb-Majorana device: Majorana bound states (MBSs) nested in a topological nanowire are predicted
to manifest nonlocal correlations in the presence of a finite energy splitting
between the MBSs. However, the signal of the nonlocal correlations has not yet
been detected in experiments. A possible reason is that the energy splitting is
too weak and seriously affected by many system parameters. Here we investigate
the charging energy induced nonlocal correlations in a hybrid device of MBSs
and quantum dots. The nanowire that hosts the MBSs is assumed in proximity to a
mesoscopic superconducting island with a finite charging energy. Each end of
the nanowire is coupled to one lead via a quantum dot with resonant levels.
With a floating superconducting island, the devices shows a negative
differential conductance and giant super-Poissonian shot noise, due to the
interplay between the nonlocality of the MBSs and dynamical Coulomb blockade
effect. When the island is strongly coupled to a bulk superconductor, the
current cross correlations at small lead chemical potentials are negative by
tuning the dot energy levels. In contrast, the cross correlation is always
positive in a non-Majorana setup. This difference may provide a signature for
the existence of the MBSs. | cond-mat |
Five-loop renormalization-group expansions for the three-dimensional
n-vector cubic model and critical exponents for impure Ising systems: The renormalization-group (RG) functions for the three-dimensional n-vector
cubic model are calculated in the five-loop approximation. High-precision
numerical estimates for the asymptotic critical exponents of the
three-dimensional impure Ising systems are extracted from the five-loop RG
series by means of the Pade-Borel-Leroy resummation under n = 0. These
exponents are found to be: \gamma = 1.325 +/- 0.003, \eta = 0.025 +/- 0.01, \nu
= 0.671 +/- 0.005, \alpha = - 0.0125 +/- 0.008, \beta = 0.344 +/- 0.006. For
the correction-to-scaling exponent, the less accurate estimate \omega = 0.32
+/- 0.06 is obtained. | cond-mat |
On the first Sonine correction for granular gases: We consider the velocity distribution for a granular gas of inelastic hard
spheres described by the Boltzmann equation. We investigate both the free of
forcing case and a system heated by a stochastic force. We propose a new method
to compute the first correction to Gaussian behavior in a Sonine polynomial
expansion quantified by the fourth cumulant $a_2$. Our expressions are compared
to previous results and to those obtained through the numerical solution of the
Boltzmann equation. It is numerically shown that our method yields very
accurate results for small velocities of the rescaled distribution. We finally
discuss the ambiguities inherent to a linear approximation method in $a_2$. | cond-mat |
Cavity-mediated superconductor$\unicode{x2013}$ferromagnetic insulator
coupling: A recent proof of concept showed that cavity photons can mediate
superconducting (SC) signatures to a ferromagnetic insulator (FI) over a
macroscopic distance [Phys. Rev. B, 102, 180506(R) (2020)]. In contrast with
conventional proximity systems, this facilitates long-distance
FI$\unicode{x2013}$SC coupling, local subjection to different drives and
temperatures, and studies of their mutual interactions without proximal
disruption of their orders. Here we derive a microscopic theory for these
interactions, with an emphasis on the leading effect on the FI, namely, an
induced anisotropy field. In an arbitrary practical example, we find an
anisotropy field of $14 \unicode{x2013} 16$ $\mu$T, which is expected to yield
an experimentally appreciable tilt of the FI spins for low-coercivity FIs such
as Bi-YIG. We discuss the implications and potential applications of such a
system in the context of superconducting spintronics. | cond-mat |
Optical Conductivity of the t-J model within Cluster Dynamical Mean
Field Theory: We study the evolution of the optical conductivity in the t-J model with
temperature and doping using the Extended Dynamical Cluster Approximation. The
cluster approach results in an optical mass which is doping independent near
half filling. The transition to the superconducting state in the overdoped
regime is characterized by a decrease in the hole kinetic energy, in contrast
to the underdoped side where kinetic energy of holes increases upon superfluid
condensation. In both regimes, the optical conductivity displays anomalous
transfers of spectral weight over a broad frequency region. | cond-mat |
Stochastic statistical theory of nucleation and evolution of nano-sized
precipitates in alloys with application to precipitation of copper in iron: The consistent and computationally efficient stochastic statistical approach
(SSA) is suggested to study kinetics of nucleation and evolution of nano-sized
precipitates in alloys. An important parameter of the theory is the size of
locally equilibrated regions at the nucleation stage which is estimated using
the "maximum thermodynamic gain" principle suggested.
For several realistic models of iron-copper alloys studied, the results of
the SSA-based simulations of precipitation kinetics agree well with the kinetic
Monte Carlo simulation results for all main characteristics of microstructure.
The approach developed is also used to study kinetics of nucleation and changes
in microstructural evolution under variations of temperature or concentration. | cond-mat |
Field-induced transition of the magnetic ground state from A-type
antiferromagnetic to ferromagnetic order in CsCo2Se2: We report on the magnetic properties of CsCo$_2$Se$_2$ with ThCr$_2$Si$_2$
structure, which we have characterized through a series of magnetization and
neutron diffraction measurements. We find that CsCo$_2$Se2$_2$ undergoes a
phase transition to an antiferromagnetically ordered state with a N\'eel
temperature of $T_{\rm N} \approx$ 66 K. The nearest neighbour interactions are
ferromagnetic as observed by the positive Curie-Weiss temperature of $\Theta
\approx$ 51.0 K. We find that the magnetic structure of CsCo$_2$Se$_2$ consists
of ferromagnetic sheets, which are stacked antiferromagnetically along the
tetragonal \textit{c}-axis, generally referred to as A-type antiferromagnetic
order. The observed magnitude of the ordered magnetic moment at $T$ = 1.5 K is
found to be only 0.20(1)$\mu_{\rm Bohr}$/Co. Already in comparably small
magnetic fields of $\mu_0 H_{MM}$(5K) $\approx$ 0.3 T, we observe a
metamagnetic transition that can be attributed to spin-rearrangements of
CsCo$_2$Se$_2$, with the moments fully ferromagnetically saturated in a
magnetic field of $\mu_0 H_{\rm FM}$(5K) $\approx$ 6.4 T. We discuss the entire
experimentally deduced magnetic phase diagram for CsCo$_2$Se$_2$ with respect
to its unconventionally weak magnetic coupling. Our study characterizes
CsCo$_2$Se$_2$, which is chemically and electronically posed closely to the
$A_xFe_{2-y}Se_2$ superconductors, as a host of versatile magnetic
interactions. | cond-mat |
Observation of Fermi surface deformation in a dipolar quantum gas: The deformation of a Fermi surface is a fundamental phenomenon leading to a
plethora of exotic quantum phases. Understanding these phases, which play
crucial roles in a wealth of systems, is a major challenge in atomic and
condensed-matter physics. Here, we report on the observation of a Fermi surface
deformation in a degenerate dipolar Fermi gas of erbium atoms. The deformation
is caused by the interplay between strong magnetic dipole-dipole interaction
and the Pauli exclusion principle. We demonstrate the many-body nature of the
effect and its tunability with the Fermi energy. Our observation provides basis
for future studies on anisotropic many-body phenomena in normal and superfluid
phases. | cond-mat |
Ground state and Spin-Wave dynamics in Brownmillerite SrCoO2.5, A
combined Hybrid Functional and LSDAU study: We theoretically investigate the ground state magnetic properties of the
brownmillerite phase of SrCoO2.5. Strong correlations within Co d electrons are
treated within the local spin density approximations of Density Functional
theory (DFT) with Hubbard U corrections (LSDAU) and results are compared with
the Heyd Scuzeria Ernzerhof (HSE) functional. The parameters computed with a U
value of 7.5 eV are found to match closely to those computed within the HSE
functional. A G type antiferromagnetic structure is found to be the most stable
one, consistent with experimental observation. By mapping the total energies of
different magnetic configurations onto a Heisenberg Hamiltonian we compute the
magnetic exchange interaction parameters, J, between the nearest neighbor Co
atoms. The J s obtained are then used to compute the spin wave frequencies and
inelastic neutron scattering intensities. Among four spin wave branches, the
lowest energy mode was found to have the largest scattering intensity at the
magnetic zone center, while the other modes becomes dominant at different
momenta. These predictions can be tested by experimentally. | cond-mat |
Finite-element dynamic-matrix approach for propagating spin waves:
Extension to mono- and multilayers of arbitrary spacing and thickness: In our recent work [AIP Adv. 11, 095006], we presented an efficient numerical
method to compute dispersions and spatial mode profiles of spin waves
propagating in waveguides with translationally invariant equilibrium
magnetization. Using a finite-element method (FEM) allowed to model
two-dimensional waveguide cross sections of arbitrary shape but only finite
size. Here, we extend our FEM propagating-wave dynamic-matrix approach from
finite waveguides to the important practical cases of infinitely-extended mono-
and multilayers of arbitrary spacing and thickness. To obtain the mode profiles
and frequencies, the linearized equation of motion of magnetization is solved
as an eigenvalue problem only on a one-dimensional line-trace mesh, defined
along the normal direction of the layers. Being an important contribution in
multilayer systems, we introduce interlayer-exchange interaction into our FEM
approach. With the calculation of dynamic dipolar fields being the main focus
of this paper, we also extend the previously presented plane-wave
Fredkin-Koehler method to calculate the dipolar potential of spin waves in
infinite layers. The major benefit of this method is that it avoids the
discretization of any non-magnetic material, such as non-magnetic spacers in
multilayers. Therefore, the computational effort becomes completely independent
on the spacer thicknesses. Furthermore, it keeps the resulting discretized
eigenvalue problem sparse, which therefore, inherits a comparably low
arithmetic complexity. As a validation of our method (implemented into the
open-source finite-element micromagnetic package TetraX), we present results
for various systems and compare them with theoretical predictions as well as
with established finite-difference numerical methods. We believe this method
offers an efficient and versatile tool to calculate spin-wave dispersions in
layered magnetic systems. | cond-mat |
Time-domain pumping a quantum-critical charge-density-wave-ordered
material: We determine the exact time-resolved photoemission spectroscopy for a nesting
driven charge-density-wave (described by the spinless Falicov-Kimball model
within dynamical mean-field theory). The pump-probe experiment involves two
light pulses: the first is an ultrashort intense pump pulse that excites the
system into nonequilibrium, and the second is a lower amplitude higher
frequency probe pulse that photoexcites electrons. We examine three different
cases: the strongly correlated metal, the quantum-critical charge density wave
and the critical Mott insulator. Our results show that the quantum critical
charge density wave has an ultra efficient relaxation channel that allows
electrons to be de-excited during the pump pulse, resulting in little net
excitation. In contrast, the metal and the Mott insulator show excitations that
are closer to what one expects from these systems. In addition, the pump field
produces spectral band narrowing, peak sharpening, and a spectral gap
reduction, all of which rapidly return to their field free values after the
pump is over. | cond-mat |
A cavity approach to optimization and inverse dynamical problems: In these two lectures we shall discuss how the cavity approach can be used
efficiently to study optimization problems with global (topological)
constraints and how the same techniques can be generalized to study inverse
problems in irreversible dynamical processes. These two classes of problems are
formally very similar: they both require an efficient procedure to trace over
all trajectories of either auxiliary variables which enforce global
constraints, or directly dynamical variables defining the inverse dynamical
problems. We will mention three basic examples, namely the Minimum Steiner Tree
problem, the inverse threshold linear dynamical problem, and the patient-zero
problem in epidemic cascades. All these examples are root problems in
optimization and inference over networks. They appear in many modern
applications and in a variety of different contexts. Credit for these results
should be shared with A. Braunstein, A. Ramezanpour, F. Altarelli, L.
Dall'Asta, I. Biazzo and A. Lage-Castellanos. | cond-mat |
Saddles, Twists, and Curls: Shape Transitions in Freestanding
Nanoribbons: Efforts to modulate the electronic properties of atomically thin crystalline
nanoribbons requires precise control over their morphology. Here, we perform
atomistic simulations on freestanding graphene nanoribbons (GNRs) to first
identify the minimal shapes, and then employ a core-edge framework based on
classical plate theory to quantify the width dependence in more general
systems. The elastic edge-edge interactions force ultra-narrow ribbons to be
flat, which then bifurcate to twisted and bent shapes at critical widths that
vary inversely with edge stress. Compressive edge stresses results in twisted
and saddle shapes that are energetically indistinguishable in the vicinity of
the bifurcation. Increasing widths favor the saddle shapes with (longitudinal)
ribbon curvatures that vary non-linearly with width and edge stress. Positive
edge stresses result in a flat-to-curled transition with similar scalings. At
large widths with negligible edge-edge interactions, rippling instabilities set
in, i.e. edge ripples and midline dimples for compressive and tensile edge
stresses. Our results highlight the utility of the core-edge framework in
developing a unified understanding of the interplay between geometry and
mechanics that sets the morphology of crystalline nanoribbons. | cond-mat |
The stationary SQUID: In the customary mode of operation of a SQUID, the electromagnetic field in
the SQUID is an oscillatory function of time. In this situation,
electromagnetic radiation is emitted, and couples to the sample. This is a
back-action that can alter the state that we intend to measure.
A circuit that could perform as a stationary SQUID consists of a loop of
superconducting material that encloses the magnetic flux, connected to a
superconducting and to a normal electrode. This circuit does not contain
Josephson junctions, or any other miniature feature. We study the evolution of
the order parameter and of the electrochemical potential in this circuit; they
converge to a stationary regime and the voltage between the electrodes depends
on the enclosed flux. We obtain expressions for the power dissipation and for
the heat transported by the electric current; the validity of these expressions
does not rely on a particular evolution model for the order parameter. We
evaluate the influence of fluctuations. For a SQUID perimeter of the order of
1$\mu$m and temperature $0.9T_c$, we obtain a flux resolution of the order of
$10^{-5}\Phi_0/$Hz$^{1/2}$; the resolution is expected to improve as the
temperature is lowered. | cond-mat |
Disorder Induced Suppression of CDW Long Range Order: STM Study of
Pd-intercalated ErTe3: Pd-intercalated ErTe3 is studied as a model system for the interplay between
a bidirectional two component charge density wave (CDW) state and disorder.
Using scanning tunneling microscopy (STM), we show that introducing
Pd-intercalants (i.e. disorder) disrupts the long-range order of both CDW
states via the creation of dislocations, which appear associated with each CDW
separately. While for weak disorder both CDW states continue to coexist
throughout the sample, with no "domains" of one CDW direction or another,
increasing Pd concentration has a stronger effect on the secondary CDW state,
manifested in higher density of dislocations. Vestiges of the two distinct CDW
phases persist to intercalation levels much above where signatures of the
original phase transition are totally suppressed. This study therefore presents
a first look into the disruption of multiple 2D strong-coupling CDW states by
the proliferation of dislocations. | cond-mat |
Bias dependent spin injection into graphene on YIG through bilayer hBN
tunnel barriers: We study the spin injection efficiency into single and bilayer graphene on
the ferrimagnetic insulator Yttrium-Iron-Garnet (YIG) through an exfoliated
tunnel barrier of bilayer hexagonal boron nitride (hBN). The contacts of two
samples yield a resistance-area product between 5 and 30 k$\Omega\mu$m$^2$.
Depending on an applied DC bias current, the magnitude of the non-local spin
signal can be increased or suppressed below the noise level. The spin injection
efficiency reaches values from -60% to +25%. The results are confirmed with
both spin valve and spin precession measurements. The proximity induced
exchange field is found in sample A to be (85 $\pm$ 30) mT and in sample B
close to the detection limit. Our results show that the exceptional spin
injection properties of bilayer hBN tunnel barriers reported by Gurram et al.
are not limited to fully encapsulated graphene systems but are also valid in
graphene/YIG devices. This further emphasizes the versatility of bilayer hBN as
an efficient and reliable tunnel barrier for graphene spintronics. | cond-mat |
Quasi One-Dimensional Bosons in Three-dimensional Traps: From Strong
Coupling to Weak Coupling Regime: We analyze a recent experiment on a Tonks-Girardeau gas of $^{87}$Rb atoms
(T. Kinoshita, T. Wenger, and D.S. Weiss, Science {\bf 305}, 1125 (2004)). We
find that the experimental data are compatible with the one-dimensional theory
of Lieb, Seiringer and Yngvason (Phys. Rev. Lett. {\bf 91}, 150401 (2003)) but
are better described by a theory that takes into account variations in the
transverse width of the atomic cloud. By using this theory we investigate also
the free axial expansion of the $^{87}$Rb gas in different regimes:
Tonks-Girardeau gas, one-dimensional Bose-Einstein condensate and
three-dimensional Bose-Einstein condensate. | cond-mat |
The Electron Pairing of K$_x$Fe$_{2-y}$Se$_2$: We studied the pairing instabilities in K$_x$Fe$_{2-y}$Se$_2$ using a two
stage functional renormalization group (FRG) method. Our results suggest the
leading and subleading pairing symmetries are nodeless $d_{x^2-y^2}$ and nodal
extended $s$ respectively. In addition, despite having no Fermi surfaces we
find the buried hole bands make important contributions to the final effective
interaction. From the bandstructure, spin susceptibility and the FRG results we
conclude that the low energy effective interaction in K$_x$Fe$_{2-y}$Se$_2$ is
well described by a $J_1-J_2$ model with dominant nearest-neighbor
antiferromagnetic interaction $J_1$ (at least as far as the superconducting
pairing is concerned). In the end we briefly mention several obvious
experiments to test whether the pairing symmetry is indeed $d_{x^2-y^2}$. | cond-mat |
Exact results in the large system size limit for the dynamics of the
Chemical Master Equation, a one dimensional chain of equations: We apply the Hamilton-Jacobi equation (HJE) formalism to solve the dynamics
of the Chemical Master Equation (CME). We found exact analytical expressions
(in large system-size limit) for the probability distribution, including
explicit expression for the dynamics of variance of distribution. We also give
the solution for some simple cases of the model with time-dependent rates. We
derived the results of Van Kampen method from HJE approach using a special
ansatz. Using the Van Kampen method, we give a system of ODE to define the
variance in 2-d case. We performed numerics for the CME with stationary noise.
We give analytical criteria for the disappearance of bi-stability in case of
stationary noise in 1-d CME. | cond-mat |
Relaxation of Fermionic Excitations in a Strongly Attractive Fermi Gas
in an Optical Lattice: We theoretically study the relaxation of high energy single particle
excitations into molecules in a system of attractive fermions in an optical
lattice, both in the superfluid and the normal phase. In a system characterized
by an interaction scale $U$ and a tunneling rate $t$, we show that the
relaxation rate scales as $\sim Ct\exp(-\alpha U^2/t^2)$ in the large $U/t$
limit. We obtain explicit expressions for the exponent $\alpha$, both in the
low temperature superfluid phase and the high temperature phase with pairing
but no coherence between the molecules. We find that the relaxation rate
decreases both with temperature and deviation of the fermion density from
half-filling. We show that quasiparticle and phase degrees of freedom are
effectively decoupled within experimental timescales allowing for observation
of ordered states even at high total energy of the system. | cond-mat |
Electrodynamics of highly spin-polarized tunnel Josephson junctions: The continuous development of superconducting electronics is encouraging
several studies on hybrid Josephson junctions (JJs) based on
superconductor/ferromagnet/superconductor (SFS) heterostructures, as either
spintronic devices or switchable elements in quantum and classical circuits.
Recent experimental evidence of macroscopic quantum tunneling and of an
incomplete 0-pi transition in tunnel-ferromagnetic spin-filter JJs could
enhance the capabilities of SFS JJs also as active elements. Here, we provide a
self-consistent electrodynamic characterization of NbN/GdN/NbN spin-filter JJs
as a function of the barrier thickness, disentangling the high-frequency
dissipation effects due to the environment from the intrinsic low-frequency
dissipation processes. The fitting of the IV characteristics at 4.2K and at
300mK by using the Tunnel Junction Microscopic model allows us to determine the
subgap resistance Rsg, the quality factor Q and the junction capacitance C.
These results provide the scaling behavior of the electrodynamic parameters as
a function of the barrier thickness, which represents a fundamental step for
the feasibility of tunnel ferromagnetic JJs as active elements in classical and
quantum circuits, and are of general interest for tunnel junctions other than
conventional SIS JJs. | cond-mat |
Studies of YBCO Strip Lines under Voltage Pulses: Optimisation of the
Design of Fault Current Limiters: We present experimental results on the behaviour of a superconducting YBCO/Au
meander of length L submitted to short circuit tests with constant voltage
pulses. The meander, at the beginning of the short-circuit, is divided in two
regions; one, with a length L1 proportional to the applied voltage, which first
switches into a highly dissipative state (HDS) while the rest remains
superconducting. Then the rest of the meander will progressively switch into
the normal state due to the propagation of this HDS (few m/s) from both ends.
The part L1 has to initially support a power density proportional to r.Jp^2 (r
is the resistivity of the bilayer and Jp the peak current density). To avoid
local excessive dissipation of power and over heating on one part of the wafer
in the initial period, we have developed a novel design in order to distribute
the dissipating section of the meander into many separated small dissipative
zones. Furthermore the apparent propagation velocity of these dissipative zones
is increased by the number of propagation fronts. We will show results obtained
on 3kW (300V, 10A) FCL on a 2" wafer which confirm the benefits of this new
design. | cond-mat |
Critical properties of the prethermal Floquet Time Crystal: The critical properties characterizing the formation of the Floquet time
crystal in the prethermal phase are investigated analytically in the
periodically driven $O(N)$ model. In particular, we focus on the critical line
separating the trivial phase with period synchronized dynamics and absence of
long-range spatial order from the non-trivial phase where long-range spatial
order is accompanied by period-doubling dynamics. In the vicinity of the
critical line, with a combination of dimensional expansion and exact solution
for $N\to\infty$, we determine the exponent $\nu$ that characterizes the
divergence of the spatial correlation length of the equal-time correlation
functions, the exponent $\beta$ characterizing the growth of the amplitude of
the order-parameter, as well as the initial-slip exponent $\theta$ of the aging
dynamics when a quench is performed from deep in the trivial phase to the
critical line. The exponents $\nu, \beta, \theta$ are found to be identical to
those in the absence of the drive. In addition, the functional form of the
aging is found to depend on whether the system is probed at times that are
small or large compared to the drive period. The spatial structure of the
two-point correlation functions, obtained as a linear response to a perturbing
potential in the vicinity of the critical line, is found to show algebraic
decays that are longer ranged than in the absence of a drive, and besides being
period-doubled, are also found to oscillate in space at the wave-vector
$\omega/(2 v)$, $v$ being the velocity of the quasiparticles, and $\omega$
being the drive frequency. | cond-mat |
Ferroelectric nanodomains in epitaxial PbTiO3 films grown on SmScO3 and
TbScO3 substrates: Domain structures of 320 nm thin epitaxial films of ferroelectric PbTiO3
grown by MOCVD technique in identical conditions on SmScO3 and TbScO3
perovskite sub- strates have been investigated by Raman spectroscopy and
piezoresponse force microscopy techniques. Phonon frequency shifts and typical
domain structure motifs are discussed. The results reveal strikingly different
domain structure architecture: domain structures of the PbTiO3 film grown on
SmScO3 have dominantly a-domain orientation while strongly preferential
c-domain orientation was found in the PbTiO3 film grown on the TbScO3
substrate. Differences between the two cases are traced back to the
film-substrate lattice mismatch at the deposition temperature. | cond-mat |
High Contrast X-ray Speckle from Atomic-Scale Order in Liquids and
Glasses: The availability of ultrafast pulses of coherent hard x-rays from the Linac
Coherent Light Source opens new opportunities for studies of atomic-scale
dynamics in amorphous materials. Here we show that single ultrafast coherent
x-ray pulses can be used to observe the speckle contrast in the high-angle
diffraction from liquid Ga and glassy Ni2Pd2P and B2O3. We determine the
thresholds above which the x-ray pulses disturb the atomic arrangements.
Furthermore, high contrast speckle is observed in scattering patterns from the
glasses integrated over many pulses, demonstrating that the source and optics
are sufficiently stable for x-ray photon correlation spectroscopy studies of
dynamics over a wide range of time scales. | cond-mat |
Comparative evaluation of catalyst materials using a binary choice model: Advances in algorithms and hardware have enabled computers to design new
materials atom-by-atom. However, in order for these computer-generated
materials to truly address problems of societal importance, such as clean
energy generation, it is not enough for them to have superior physical
properties. It is also important for them to be adopted by as many users as
possible. In this paper, we present a simple binary choice model for comparing
catalyst materials on the basis of consumer preferences. This model considers a
population of utility maximisers who select one of two materials by comparing
catalytic turnover rates with sales prices. Through a mixture of numerical
simulation and analytic theorems, we characterise the predictions of the model
in a variety of regimes of consumer behavior. We also show how the model can be
used as a guide for crafting policies for lowering catalyst prices in order to
improve their market shares. This work represents a first step towards
understanding how material properties should be balanced against production
costs and consumer demand when designing new materials, an intellectual advance
which may facilitate the spread of green materials in society. | cond-mat |
Fast computation of magnetostatic fields by Non-uniform Fast Fourier
Transforms: The bottleneck of micromagnetic simulations is the computation of the
long-ranged magnetostatic fields. This can be tackled on regular N-node grids
with Fast Fourier Transforms in time N logN, whereas the geometrically more
versatile finite element methods (FEM) are bounded to N^4/3 in the best case.
We report the implementation of a Non-uniform Fast Fourier Transform algorithm
which brings a N logN convergence to FEM, with no loss of accuracy in the
results. | cond-mat |
A Frequency-Controlled Magnetic Vortex Memory: Using the ultra low damping NiMnSb half-Heusler alloy patterned into
vortex-state magnetic nano-dots, we demonstrate a new concept of non-volatile
memory controlled by the frequency. A perpendicular bias magnetic field is used
to split the frequency of the vortex core gyrotropic rotation into two distinct
frequencies, depending on the sign of the vortex core polarity $p=\pm1$ inside
the dot. A magnetic resonance force microscope and microwave pulses applied at
one of these two resonant frequencies allow for local and deterministic
addressing of binary information (core polarity). | cond-mat |
Emergent isotropy of a wave-turbulent cascade in the Gross-Pitaevskii
model: The restoration of symmetries is one of the most fascinating properties of
turbulence. We report a study of the emergence of isotropy in the
Gross-Pitaevskii model with anisotropic forcing. Inspired by recent
experiments, we study the dynamics of a Bose-Einstein condensate in a
cylindrical box driven along the symmetry axis of the trap by a spatially
uniform force. We introduce a measure of anisotropy $A(k,t)$ defined on the
momentum distributions $n(\boldsymbol{k},t)$, and study the evolution of
$A(k,t)$ and $n(\boldsymbol{k},t)$ as turbulence proceeds. As the system
reaches a steady state, the anisotropy, large at low momenta because of the
large-scale forcing, is greatly reduced at high momenta. While
$n(\boldsymbol{k},t)$ exhibits a self-similar cascade front propagation,
$A(k,t)$ decreases without such self-similar dynamics. Finally, our numerical
calculations show that the isotropy of the steady state is robust with respect
to the amplitude of the drive. | cond-mat |
First-principles prediction of mechanical and bonding characteristics of
new T2 superconductor Ta5GeB2: In the present paper, DFT (Density Functional Theory) based first-principles
methods are applied to investigate the mechanical and bonding properties of
newly synthesized T2 phase superconductor Ta5GeB2 for the first time. The
calculated lattice constants are in reasonable agreement with the experiment.
The elastic constants (Cij), bulk modulus (B), shear modulus (G), Young's
modulus (Y), Poisson ratio (nu), Pugh ratio (G/B), and elastic anisotropy
factor, A, of Ta5GeB2 are calculated to explore the mechanical behavior of the
compound.To give an explanation of the bonding nature of this new ternary
tetragonal system, the band structure, density of states, and Mulliken atomic
population are investigated. The estimated Debye temperature and Vickers
hardness are also used to justify both the mechanical and bonding properties of
Ta5GeB2. | cond-mat |
Giant effective Zeeman splitting in a monolayer semiconductor realized
by spin-selective strong light-matter coupling: Strong coupling between light and the fundamental excitations of a
two-dimensional electron gas (2DEG) are of foundational importance both to pure
physics and to the understanding and development of future photonic
nanotechnologies. Here we study the relationship between spin polarization of a
2DEG in a monolayer semiconductor, MoSe$_2$, and light-matter interactions
modified by a zero-dimensional optical microcavity. We find robust
spin-susceptibility of the 2DEG to simultaneously enhance and suppress
trion-polariton formation in opposite photon helicities. This leads to
observation of a giant effective valley Zeeman splitting for trion-polaritons
(g-factor >20), exceeding the purely trionic splitting by over five times.
Going further, we observe robust effective optical non-linearity arising from
the highly non-linear behaviour of the valley-specific strong light-matter
coupling regime, and allowing all-optical tuning of the polaritonic Zeeman
splitting from 4 to >10 meV. Our experiments lay the groundwork for engineering
quantum-Hall-like phases with true unidirectionality in monolayer
semiconductors, accompanied by giant effective photonic non-linearities rooted
in many-body exciton-electron correlations. | cond-mat |
Kinetic Monte Carlo modelling of Helium Bubble Nucleation onto Oxides in
the Fe-Ti-Y-O System: A Kinetic Monte Carlo (KMC) model was created to simulate the insertion of
transmutation He atoms into nanostructured ferritic alloys (NFAs) under neutron
irradiation. Interstitial He atoms migrate through the NFA until becoming
trapped in bubbles of other He atoms and vacancies created from irradiation.
The Y-Ti-O nano-oxides in the NFAs were found to be effective in capturing
these He atoms and preventing bubbles from forming at the grain boundary and
appear to replicate the characteristics (size and number density) observed in
other experiments. The bubbles were found to prefer the <111> oxide interface
as a nucleation site and the stable bubbles have a He/Vac ratio between 1.3 and
1.8 He/Vac. The influence of He bubbles on the segregation of solutes to the
grain boundaries or on the stability of the nano-oxides were negligible. | cond-mat |
Probing the role of single defects on the thermodynamics of
electric-field induced phase transitions: The kinetics and thermodynamics of first order transitions is universally
controlled by defects that act as nucleation sites and pinning centers. Here we
demonstrate that defect-domain interactions during polarization reversal
processes in ferroelectric materials result in a pronounced fine structure in
electromechanical hysteresis loops. Spatially-resolved imaging of a single
defect center in multiferroic BiFeO3 thin film is achieved, and the defect size
and built-in field are determined self-consistently from the single-point
spectroscopic measurements and spatially-resolved images. This methodology is
universal and can be applied to other reversible bias-induced transitions
including electrochemical reactions. | cond-mat |
The origin of insulating and non-ferromagnetic SrRuO3 monolayers: The electro-magnetic properties of ultrathin epitaxial ruthenate films have
long been the subject of debate. Here we combine experimental with theoretical
investigations of (SrTiO3)5-(SrRuO3)n-(SrTiO3)5 (STO5-SROn-STO5)
heterostructures with n = 1 and 2 unit cells, including extensive
atomic-resolution scanning-transmission-electron-microscopy imaging,
electron-energy-loss-spectroscopy chemical mapping, as well as transport and
magneto-transport measurements. The experimental data demonstrate that the
STO5-SRO2-STO5 heterostructure is stoichiometric, metallic, and ferromagnetic
with TC ~ 128 K, even though it lacks the characteristic bulk-SRO octahedral
tilts and matches the cubic STO structure. In contrast, the STO5-SRO1-STO5
heterostructure features Ru-Ti intermixing in the RuO2 layer, also without
octahedral tilts, but is accompanied by a loss of metallicity and
ferromagnetism. Density-functional-theory calculations show that stoichiometric
n = 1 and n = 2 heterostructures are metallic and ferromagnetic with no
octahedral tilts, while non-stoichiometry in the Ru sublattice in the n = 1
case opens an energy gap and induces antiferromagnetic ordering. Thus, the
results indicate that the observed non-stoichiometry is the cause of the
observed loss of metallicity and ferromagnetism in the n = 1 case. | cond-mat |
Control of Multi-level Voltage States in a Hysteretic SQUID
Ring-Resonator System: In this paper we study numerical solutions to the quasi-classical equations
of motion for a SQUID ring-radio frequency (rf) resonator system in the regime
where the ring is highly hysteretic. In line with experiment, we show that for
a suitable choice of of ring circuit parameters the solutions to these
equations of motion comprise sets of levels in the rf voltage-current dynamics
of the coupled system. We further demonstrate that transitions, both up and
down, between these levels can be controlled by voltage pulses applied to the
system, thus opening up the possibility of high order (e.g. 10 state),
multi-level logic and memory. | cond-mat |
PT breaking and RG flows between multicritical Yang-Lee fixed points: We study a novel class of Renormalization Group flows which connect
multicritical versions of the two-dimensional Yang-Lee edge singularity
described by the conformal minimal models M(2,2n+3). The absence in these
models of an order parameter implies that the flows towards and between
Lee-Yang edge singularities are all related to the spontaneous breaking of PT
symmetry and comprise a pattern of flows in the space of PT symmetric theories
consistent with the c-theorem and the counting of relevant directions.
Additionally, we find that while in a part of the phase diagram the domains of
unbroken and broken PT symmetry are separated by critical manifolds of class
M(2,2n+3), other parts of the boundary between the two domains are not
critical. | cond-mat |
Electric field-induced creation and directional motion of domain walls
and skyrmion bubbles: Magnetization dynamics driven by an electric field could provide long-term
benefits to information technologies because of its ultralow power consumption.
Meanwhile, the Dzyaloshinskii-Moriya interaction in interfacially asymmetric
multilayers consisting of ferromagnetic and heavy-metal layers can stabilize
topological spin textures, such as chiral domain walls, skyrmions, and skyrmion
bubbles. These topological spin textures can be controlled by an electric
field, and hold promise for building advanced spintronic devices. Here, we
present an experimental and numerical study on the electric field-induced
creation and directional motion of topological spin textures in magnetic
multilayer films and racetracks with thickness gradient and interfacial
Dzyaloshinskii-Moriya interaction at room temperature. We find that the
electric field-induced directional motion of chiral domain wall is accompanied
with the creation of skyrmion bubbles at certain conditions. We also
demonstrate that the electric field variation can induce motion of skyrmion
bubbles. Our findings may provide opportunities for developing skyrmion-based
devices with ultralow power consumption. | cond-mat |
Magnetoelastic study on the frustrated quasi-one-dimensional spin-1/2
magnet LiCuVO$_4$: We investigated the magnetoelastic properties of the quasi-one-dimensional
spin-1/2 frustrated magnet LiCuVO$_4$. Longitudinal-magnetostriction
experiments were performed at 1.5 K in high magnetic fields of up to 60 T
applied along the $b$ axis, i.e., the spin-chain direction. The
magnetostriction data qualitatively resemble the magnetization results, and
saturate at $H_{\text{sat}} \approx 54$ T, with a relative change in sample
length of $\Delta L/L \approx 1.8\times10^{-4}$. Remarkably, both the
magnetostriction and the magnetization evolve gradually between $H_{\text{c3}}
\approx 48$ T and $H_{\text{sat}}$, indicating that the two quantities
consistently detect the spin-nematic phase just below the saturation. Numerical
analyses for a weakly coupled spin-chain model reveal that the observed
magnetostriction can overall be understood within an exchange-striction
mechanism. Small deviations found may indicate nontrivial changes in local
correlations associated with the field-induced phase transitions. | cond-mat |
Nanotransfer Printing of Organic and Carbon Nanotube Thin-Film
Transistors on Plastic Substrates: A printing process for high-resolution transfer of all components for organic
electronic devices on plastic substrates has been developed and demonstrated
for pentacene (Pn), poly (3-hexylthiophene) and carbon nanotube (CNT) thin-film
transistors (TFTs). The nanotransfer printing process allows fabrication of an
entire device without exposing any component to incompatible processes and with
reduced need for special chemical preparation of transfer or device substrates.
Devices on plastic substrates include a Pn TFT with a saturation, field-effect
mobility of 0.09 cm^2 (Vs)^-1 and on/off ratio approximately 10^4 and a CNT TFT
which exhibits ambipolar behavior and no hysteresis. | cond-mat |
Atomistic Modelling of Energy Dissipation in Nanoscale Gears: Molecule- and solid-state gears build the elementary constituents of
nanoscale mechanical machineries. Recent experimental advances in fabrication
technologies in the field have strongly contributed to better delineate the
roadmap towards the ultimate goal of engineering molecular-scale mechanical
devices. To complement experimental studies, computer simulations play an
invaluable role, since they allow to address, with atomistic resolution,
various fundamental issues such as the transmission of angular momentum in
nanoscale gear trains and the mechanisms of energy dissipation at such length
scales. We review in this chapter our work addressing the latter problem. Our
computational approach is based on classical atom-istic Molecular Dynamics
simulations. Two basic problems are discussed: (i) the dominant energy
dissipation channels of a rotating solid-state nanogear adsorbed on a surface,
and (ii) the transmission of rotational motion and frictional processes in a
heterogeneous gear pair consisting of a graphene nanodisk and a molecular-scale
gear. | cond-mat |
Finite-temperature trapped dipolar Bose gas: We develop a finite temperature Hartree theory for the trapped dipolar Bose
gas. We use this theory to study thermal effects on the mechanical stability of
the system and density oscillating condensate states. We present results for
the stability phase diagram as a function of temperature and aspect ratio. In
oblate traps above the critical temperature for condensation we find that the
Hartree theory predicts significant stability enhancement over the
semiclassical result. Below the critical temperature we find that thermal
effects are well described by accounting for the thermal depletion of the
condensate. Our results also show that density oscillating condensate states
occur over a range of interaction strengths that broadens with increasing
temperature. | cond-mat |
Interactions and screening in gated bilayer graphene nanoribbons: The effects of Coulomb interactions on the electronic properties of bilayer
graphene nanoribbons (BGNs) covered by a gate electrode are studied
theoretically. The electron density distribution and the potential profile are
calculated self-consistently within the Hartree approximation. A comparison to
their single-particle counterparts reveals the effects of interactions and
screening. Due to the finite width of the nanoribbon in combination with
electronic repulsion, the gate-induced electrons tend to accumulate along the
BGN edges where the potential assumes a sharp triangular shape. This has a
profound effect on the energy gap between electron and hole bands, which
depends nonmonotonously on the gate voltage and collapses at intermediate
electric fields. We interpret this behavior in terms of interaction-induced
warping of the energy dispersion. | cond-mat |
Effect of pressure on the electronic and magnetic properties of
CdV$_2$O$_4$: Density functional theory studies: We investigate the effect of pressure on the electronic and magnetic states
of CdV$_2$O$_4$ by using ab initio electronic structure calculations. The
Coulomb correlation and spin-orbit coupling play important role in deciding the
structural, electronic and magnetic properties of the compound. The total
magnetic moment of V ion is found to be $\sim$1.3 $\mu_B$ and making an angle
of $\sim$9.5 degree with the z-axis. In the tetragonal phase, the ground state
is the orbital ordered state where V $d_{xz}$ and $d_{yz}$ obtitals are mainly
occupied at the neighbouring sites. This work predicts the electronic phase
transition from orbital-ordered-insulator to orbital-ordered-metal to
orbital-disordered-metal with increasing pressure. The pressure induced
broadening of lower and upper Hubbard bands gives rise to metal-insulator
transition above 35 GPa. The simple mean-field theory used in the present work
is able to describe the pressure dependent variation of the antiferromagnetic
transition temperature suggesting the applicability of the method in the study
of the magnetic behaviour of similar geometrically frustrated systems. | cond-mat |
Enhanced magnetism, memory and aging in Gold-Iron oxide nanoparticle
composites: In this report we present systematic magnetic studies of pure iron oxide
nanoparticles and gold iron oxide nanocomposite with increasing Au particle
size/content. For the magnetic studies of these samples we have measured: (1)
zero field cooled (ZFC) and field cooled (FC) magnetization, (2) ac
susceptibility, (3) magnetization vs field at various temperatures, (4)
thermoremanant magnetization relaxation (TRM) and zero field cooled
magnetization relaxation (ZFCM) at fixed temperature for various wait times tw
for studying the aging effect, (5) magnetization memory effect and (6) exchange
bias as a function of cooling field. The detailed magnetic measurement analysis
indicates that the pure Fe3O4 nanoparticles sample behaves like a
superparamagnet and on incorporation of gold (Au) nanoparticles the
nanocomposite system slowly evolves from superparamagnetic to superspin glass
state. The memory and aging effect enhances with the increase of the Au
nanoparticle size/content. The most important observation in this study is the
enhancement of magnetization with the incorporation of Au nanoparticles. The
enhancement increases with the increase in the Au content in the nanocomposite.
We have explained the cause of this enhancement of magnetization as due to
large orbital magnetic moment formation at the Au/magnetic particle interface. | cond-mat |
Anderson Localization of cold atomic gases with effective spin-orbit
interaction in a quasiperiodic optical lattice: We theoretically investigate the localization properties of a spin-orbit
coupled spin-1/2 particle moving in a one-dimensional quasiperiodic potential,
which can be experimentally implemented using cold atoms trapped in a
quasiperiodic optical lattice potential and external laser fields. We present
the phase diagram in the parameter space of the disorder strength and those
related to the spin-orbit coupling. The phase diagram is verified via
multifractal analysis of the atomic wavefunctions and the numerical simulation
of diffusion dynamics. We found that spin-orbit coupling can lead to the
spectra mixing (coexistence of extended and localized states) and the
appearance of mobility edges. | cond-mat |
Mesoscopic Tunneling Magnetoresistance: We study spin-dependent transport through
ferromagnet/normal-metal/ferromagnet double tunnel junctions in the mesoscopic
Coulomb blockade regime. A general transport equation allows us to calculate
the conductance in the absence or presence of spin-orbit interaction and for
arbitrary orientation of the lead magnetizations. The tunneling
magnetoresistance (TMR), defined at the Coulomb blockade conductance peaks, is
calculated and its probability distribution presented. We show that mesoscopic
fluctuations can lead to the optimal value of the TMR. | cond-mat |
Level compressibility in a critical random matrix ensemble: The second
virial coefficient: We study spectral statistics of a Gaussian unitary critical ensemble of
almost diagonal Hermitian random matrices with off-diagonal entries
$<|H_{ij}|^{2} > \sim b^{2} |i-j|^{-2}$ small compared to diagonal ones
$<|H_{ii}|^{2} > \sim 1$. Using the recently suggested method of {\it virial
expansion} in the number of interacting energy levels (J.Phys.A {\bf 36},8265
(2003)), we calculate a coefficient $\propto b^{2}\ll 1$ in the level
compressibility $\chi(b)$. We demonstrate that only the leading terms in
$\chi(b)$ coincide for this model and for an exactly solvable model suggested
by Moshe, Neuberger and Shapiro (Phys.Rev.Lett. {\bf 73}, 1497 (1994)), the
sub-leading terms $\sim b^{2}$ being different. Numerical data confirms our
analytical calculation. | cond-mat |
Tuning the Hysteresis of a Metal-Insulator Transition via Lattice
Compatibility: Structural phase transitions serve as the basis for many functional
applications including shape memory alloys (SMAs), switches based on
metal-insulator transitions (MITs), etc. In such materials, lattice
incompatibility between phases often results in a thermal hysteresis, which is
intimately tied to degradation of reversibility of the transformation. The
non-linear theory of martensite suggests that the hysteresis of a martensitic
phase transformation is solely determined by the lattice constants, and the
conditions proposed for geometrical compatibility have been successfully
applied to minimizing the hysteresis in SMAs. In this work, we apply the
non-linear theory to a strongly correlated oxide system (W doped VO2), and show
that the hysteresis of the MIT in the system can be directly tuned by adjusting
the lattice constants of the phases. The results underscore the profound
influence structural compatibility has on intrinsic electronic properties, and
indicate that the theory provides a universal guidance for optimizing phase
transforming materials. | cond-mat |
Oscillating spin-orbit interaction as a source of spin-polarized wave
packets in two-terminal nanoscale devices: Ballistic transport through nanoscale devices with time-dependent Rashba-type
spin-orbit interaction (SOI) can lead to spin-polarized wave packets that
appear even for completely unpolarized input. The SOI that oscillates in a
finite domain generates density and spin polarization fluctuations that leave
the region as propagating waves. Particularly, spin polarization has space and
time dependence even in regions without SOI. Our results are based on an
analytic solution of the time-dependent Schr\"odinger equation. The relevant
Floquet quasi-energies that are obtained appear in the energy spectrum of both
the transmitted and reflected waves. | cond-mat |
Dopant-Induced Local Pairing Inhomogeneity in
Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$: A new theoretical model is presented to study the nanoscale electronic
inhomogeneity in high-$T_c$ cuprates. In this model, we argue that the randomly
distributed out-of-plane interstitial oxygen dopants induces locally the
off-diagonal (i.e., hopping integral) disorder. This disorder modulates the
superexchange interaction resulting from a large-$U$ Hubbard model, which in
turns changes the local pairing interaction. The microscopic self-consistent
calculations shows that the large gap regions are registered to the locations
of dopants. Large gap regions exhibit small and broader coherence peaks. These
results are qualitatively consistent with recent STM observations on optimally
doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. | cond-mat |
Measuring out quasi-local integrals of motion from entanglement: Quasi-local integrals of motion are a key concept underpinning the modern
understanding of many-body localisation, an intriguing phenomenon in which
interactions and disorder come together. Despite the existence of several
numerical ways to compute them - and astoundingly in the light of the
observation that much of the phenomenology of many properties can be derived
from them - it is not obvious how to directly measure aspects of them in real
quantum simulations; in fact, the smoking gun of their experimental observation
is arguably still missing. In this work, we propose a way to extract the
real-space properties of such quasi-local integrals of motion based on a
spatially-resolved entanglement probe able to distinguish Anderson from
many-body localisation from non-equilibrium dynamics. We complement these
findings with a new rigorous entanglement bound and compute the relevant
quantities using tensor networks. We demonstrate that the entanglement gives
rise to a well-defined length scale that can be measured in experiments. | cond-mat |
Cumulative geometric frustration in physical assemblies: Geometric frustration arises whenever the constituents of a physical assembly
locally favor an arrangement that cannot be realized globally. Recently, such
frustrated assemblies were shown to exhibit filamentation, size limitation,
large morphological variations and other exotic response properties. While
these unique characteristics can be shown to be a direct outcome of the
geometric frustration, some geometrically frustrated systems do not exhibit any
of the above phenomena. In this work we exploit the intrinsic approach to
provide a framework for directly addressing the frustration in physical
assemblies. The framework highlights the role of the compatibility conditions
associated with the intrinsic fields describing the physical assembly. We show
that the structure of the compatibility conditions determines the behavior of
small assemblies, and in particular predicts their super-extensive energy
growth exponent. We illustrate the use of this framework to several well known
frustrated assemblies. | cond-mat |
Uniaxial extensional viscosity of semidilute DNA solutions: The extensional rheology of polymeric liquids has been extensively examined
through experiments and theoretical predictions. However, a systematic study of
the extensional rheology of polymer solutions in the semidilute regime, in
terms of examining the effects of concentration and molecular weight, has not
been carried out so far. Prior studies of the shear rheology of semidilute
polymer solutions have demonstrated that their behaviour is distinctively
different from that observed in the dilute and concentrated regimes. This
difference in behaviour is anticipated to be even more pronounced in
extensional flows. In this work, the extensional rheology of linear,
double-stranded DNA molecules, spanning an order of magnitude of molecular
weights (25 to 289 kilobasepairs) and concentrations (0.03 to 0.3 mg/ml), has
been investigated. DNA solutions are now used routinely as model polymeric
systems due to their near-perfect monodispersity. Measurements have been
carried out with a filament stretching rheometer since it is the most reliable
method for obtaining an estimate of the elongational stress growth of a polymer
solution. Transient and steady-state uniaxial extensional viscosities of DNA
dissolved in a solvent under excess salt conditions, with a high concentration
of sucrose in order to achieve a sufficiently high solvent viscosity, have been
determined in the semidilute regime at room temperature. The dependence of the
steady state uniaxial extensional viscosity on molecular weight, concentration
and extension rate is measured with a view to determining if data collapse can
be observed with an appropriate choice of variables. Steady state shear
viscosity measurements suggest that sucrose-DNA interactions might play a role
in determining the observed rheological behaviour of semidilute DNA solutions
with sucrose as a component in the solvent. | cond-mat |
Magnetization curves of deposited finite spin chains: The characterization and manipulation of deposited magnetic clusters or
molecules on surfaces is a prerequisite for their future utilization. In recent
years techniques like spin-flip inelastic electron tunneling spectroscopy using
a scanning tunneling microscope proved to be very precise in determining e.g.
exchange constants in deposited finite spin chains in the meV range. In this
article we tackle the problem numerically by investigating the transition from
where a pure spin Hamiltonian is sufficient to the point where the interaction
with the surface significantly alters the magnetic properties. To this end we
study the static, i.e. equilibrium impurity magnetization of antiferromagnetic
chains for varying couplings to a conduction electron band of a metal
substrate. We show under which circumstances the screening of a part of the
system enables one to deduce molecular parameters of the remainder from level
crossings in an applied field. | cond-mat |
Classical stochastic approach to quantum mechanics and quantum
thermodynamics: We derive the equations of quantum mechanics and quantum thermodynamics from
the assumption that a quantum system can be described by an underlying
classical system of particles. Each component $\phi_j$ of the wave vector is
understood as a stochastic complex variable whose real and imaginary parts are
proportional to the coordinate and momentum associated to a degree of freedom
of the underlying classical system. From the classical stochastic equations of
motion, we derive a general equation for the covariance matrix of the wave
vector which turns out to be of the Lindblad type. When the noise changes only
the phase of $\phi_j$, the Schr\"odinger and the quantum Liouville equation are
obtained. The component $\psi_j$ of the wave vector obeying the Schr\"odinger
equation is related to stochastic wave vector by
$|\psi_j|^2=\langle|\phi_j|^2\rangle$. | cond-mat |
The electron many-body problem in graphene: We give a brief summary of the current status of the electron many-body
problem in graphene. We claim that graphene has intrinsic dielectric properties
which should dress the interactions among the quasiparticles, and may explain
why the observation of electron-electron renormalization effects has been so
elusive in the recent experiments. We argue that the strength of Coulomb
interactions in graphene may be characterized by an effective fine structure
constant given by
$\alpha^{\star}(\mathbf{k},\omega)\equiv2.2/\epsilon(\mathbf{k},\omega)$, where
$\epsilon(\mathbf{k},\omega)$ is the dynamical dielectric function. At long
wavelengths, $\alpha^{\star}(\mathbf{k},\omega)$ appears to have its smallest
value in the static regime, where $\alpha^{\star}(\mathbf{k}\to0,0)\approx1/7$
according to recent inelastic x-ray measurements, and the largest value in the
optical limit, where $\alpha^{\star}(0,\omega)\approx2.6$. We conclude that the
strength of Coulomb interactions in graphene is not universal, but depends
highly on the scale of the phenomenon of interest. We propose a prescription in
order to reconcile different experiments. | cond-mat |
Off-diagonal Long-Range Order and Supersolidity in a Quantum Solid with
Vacancies: We consider a lattice of bosonic atoms, whose number N may be smaller than
the number of lattice sites M. We study the Hartree-Fock wave function built up
from localized wave functios w(\mathbf{r}) of single atoms, with nearest
neighboring overlap. The zero-momentum particle number is expressed in terms of
permanents of matrices. In one dimension, it is analytically calculated to be
\alpha*N(M-N+1)/M, with \alpha=|\int w(\mathbf{r})d\Omega|^2/[(1+2a)l], where a
is the nearest-neighboring overlap, l is the lattice constant. \alpha is of the
order of 1. The result indicates that the condensate fraction is proportional
to and of the same order of magnitude as that of the vacancy concentration,
hence there is off-diagonal long-range order or Bose-Einstein condensation of
atoms when the number of vacancies M-N is a finite fraction of the number of
the lattice sites M. | cond-mat |
Inverse design of two-dimensional structure by self-assembly of patchy
particles: We propose an optimisation method for the inverse structural design of
self-assembly of anisotropic patchy particles. The anisotropic interaction can
be expressed by the spherical harmonics of the surface pattern on a patchy
particle, and thus arbitrary symmetry of the patch can be treated. The pairwise
interaction potential includes several to-be-optimised parameters, which are
the coefficient of each term in the spherical harmonics. We use the
optimisation method based on the relative entropy approach and generate
structures by Brownian Dynamics simulations. Our method successfully estimates
the parameters in the potential for the target structures, such as square
lattice, kagome lattice, and dodecagonal quasicrystal. | cond-mat |
A comparative study of some models of incoherence at the mesoscopic
scale: The pre-existing literature on phenomena at the mesoscopic scale is concerned
among other things with phase coherent transport. Phase coherent transport
dominates at very low temperatures. With increase in temperature, as the system
size becomes comparable to the inelastic mean free path phase incoherence sets
in. This incoherence further leads to dephasing, and as a consequence purely
quantum effects in electron transport give way to classical macroscopic
behavior. In this work we consider two distinct phenomenological models of
incoherent transport, the Coherent Absorption and Wave Attenuation models. We
reveal some physical problems in the Coherent Absorption model as opposed to
the Wave Attenuation model. We also compare our proposed model with experiments
in case of the much studied peak to valley ratios in resonant tunneling diodes,
magneto-conductance oscillations and Fano resonances in case of Aharonov-Bohm
rings. | cond-mat |
Coarse grained models of stripe forming systems: phase diagrams,
anomalies and scaling hypothesis: Two coarse-grained models which capture some universal characteristics of
stripe forming systems are stud- ied. At high temperatures, the structure
factors of both models attain their maxima on a circle in reciprocal space, as
a consequence of generic isotropic competing interactions. Although this is
known to lead to some universal properties, we show that the phase diagrams
have important differences, which are a consequence of the particular k
dependence of the fluctuation spectrum in each model. The phase diagrams are
computed in a mean field approximation and also after inclusion of small
fluctuations, which are shown to modify drastically the mean field behavior.
Observables like the modulation length and magnetization profiles are computed
for the whole temperature range accessible to both models and some important
differences in behavior are observed. A stripe compression modulus is computed,
showing an anomalous behavior with temperature as recently reported in related
models. Also, a recently proposed scaling hypothesis for modulated systems is
tested and found to be valid for both models studied. | cond-mat |
Intentionally disordered superlattices with high dc conductance: We study disordered quantum-well-based semiconductor superlattices where the
disorder is intentional and short-range correlated. Such systems consist of
quantum-wells of two different thicknesses randomly distributed along the
growth direction, with the additional constraint that wells of one kind always
appears in pairs. Imperfections due to interface roughness are considered by
allowing the quantum-well thicknesses to fluctuate around their {\em ideal}
values. As particular examples, we consider wide-gap
(GaAs-Ga$_{1-x}$Al$_{x}$As) and narrow-gap (InAs-GaSb) superlattices. We show
the existence of a band of extended states in perfect correlated disordered
superlattices, giving rise to a strong enhancement of their finite-temperature
dc conductance as compared to usual random ones whenever the Fermi level
matches this band. This feature is seen to survive even if interface roughness
is taken into account. Our predictions can be used to demonstrate
experimentally that structural correlations inhibit the localization effects of
disorder, even in the presence of imperfections. This effect might be the basis
of new, filter-like or other specific-purpose electronic devices. | cond-mat |
Modulation Doping near Mott-Insulator Heterojunctions: We argue that interesting strongly correlated two-dimensional electron
systems can be created by modulation doping near a heterojunction between Mott
insulators. Because the dopant atoms are remote from the carrier system, the
electronic system will be weakly disordered. We argue that the competition
between different ordered states can be engineered by choosing appropriate
values for the dopant density and the setback distance of the doping layer. In
particular larger setback distances favor two-dimensional antiferromagnetism
over ferromagnetism. We estimate some key properties of modulation-doped Mott
insulator heterojunctions by combining insights from Hartree-Fock-Theory and
Dynamical-Mean-Field-Theory descriptions and discuss potentially attractive
material combinations. | cond-mat |
Field exposed water in a nanopore: liquid or vapour?: We study the behavior of ambient temperature water under the combined effects
of nanoscale confinement and applied electric field. Using molecular
simulations we analyze the thermodynamic causes of field-induced expansion at
some, and contraction at other conditions. Repulsion among parallel water
dipoles and mild weakening of interactions between partially aligned water
molecules prove sufficient to destabilize the aqueous liquid phase in isobaric
systems in which all water molecules are permanently exposed to a uniform
electric field. At the same time, simulations reveal comparatively weak
field-induced perturbations of water structure upheld by flexible hydrogen
bonding. In open systems with fixed chemical potential, these perturbations do
not suffice to offset attraction of water into the field; additional water is
typically driven from unperturbed bulk phase to the field-exposed region. In
contrast to recent theoretical predictions in the literature, our analysis and
simulations confirm that classical electrostriction characterizes usual
electrowetting behavior in nanoscale channels and nanoporous materials. | cond-mat |
Anomalous thermal Hall effect in the topological antiferromagnetic state: The anomalous Hall effect (AHE), a Hall signal occurring without an external
magnetic field, is one of the most significant phenomena. However,
understanding the AHE mechanism has been challenging and largely restricted to
ferromagnetic metals. Here, we investigate the recently discovered AHE in the
chiral antiferromagnet Mn3Sn by measuring a thermal analog of the AHE, known as
an anomalous thermal Hall effect (ATHE). The amplitude of the ATHE scales with
the anomalous Hall conductivity of Mn3Sn over a wide temperature range,
demonstrating that the AHE of Mn3Sn arises from a dissipationless intrinsic
mechanism associated with the Berry curvature. Moreover, we find that the
dissipationless AHE is significantly stabilized by shifting the Fermi level
toward the magnetic Weyl points. Thus, in Mn3Sn, the Berry curvature emerging
from the proposed magnetic Weyl fermion state is a key factor for the observed
AHE and ATHE. | cond-mat |
Spin exchange in quantum rings and wires in the Wigner-crystal limit: We present a controlled method for computing the exchange coupling in
strongly correlated one-dimensional electron systems. It is based on the
asymptotically exact relation between the exchange constant and the
pair-correlation function of spinless electrons. Explicit results are obtained
for thin quantum rings with realistic Coulomb interactions, by calculating this
function via a many-body instanton approach. | cond-mat |
Negative and Positive Magnetoresistance in Bilayer Graphene: Effects of
Weak Localization and Charge Inhomogeneity: We report measurements of magnetoresistance in bilayer graphene as a function
of gate voltage (carrier density) and temperature. We examine multiple
contributions to the magnetoresistance, including those of weak localization
(WL), universal conductance fluctuations (UCF), and inhomogeneous charge
transport. A clear WL signal is evident at all measured gate voltages (in the
hole doped regime) and temperature ranges (from 0.25 K to 4.3 K), and the phase
coherence length extracted from WL data does not saturate at low temperatures.
The WL data is fit to demonstrate that electron-electron Nyquist scattering is
the major source of phase decoherence. A decrease in UCF amplitude with
increasing gate voltage and temperature is shown to be consistent with a
corresponding decrease in the phase coherence length. In addition, a weak
positive magnetoresistance at higher magnetic fields is observed, and
attributed to inhomogeneous charge transport. | cond-mat |
Effect of chirality imbalance on Hall transport of PrRhC$_2$: Much has been learned about the topological transport in real materials. We
investigate the interplay between magnetism and topology in the
magneto-transport of PrRhC$_2$. The four-fold degeneracy reduces to two-fold
followed by non-degenerate Weyl nodes when the orientation of the magnetic
quantization axis is changed from easy axis to body-diagonal through
face-diagonal. This engenders chirality imbalance between positive and negative
chirality Weyl nodes around the Fermi energy. We observe a significant
enhancement in the chiral anomaly mediated response such as planar Hall
conductivity and longitudinal magneto-conductivity, due to the emergence of
chirality imbalance upon orienting the magnetic quantization axis to
body-diagonal. The angular variations of the above quantities for different
magnetic quantization axis clearly refer to the typical signature of planar
Hall effect in Weyl semimetals. We further investigate the profiles of
anomalous Hall conductivities as a function of Fermi energy to explore the
effects of symmetries as well as chirality imbalance on Berry curvature. | cond-mat |
Magnetic coherent tunnel junctions with periodic grating barrier: A new spintronic theory has been developed for the magnetic tunnel junction
(MTJ) with single-crystal barrier. The barrier will be treated as a diffraction
grating with intralayer periodicity, the diffracted waves of tunneling
electrons thus contain strong coherence, both in charge and especially in spin.
The theory can answer the two basic problems present in MgO-based MTJs: (1) Why
does the tunneling magnetoresistance (TMR) oscillate with the barrier
thickness? (2) Why is the TMR still far away from infinity when the two
electrodes are both half-metallic? Other principal features of TMR can also be
explained and reproduced by the present work. It also provides possible ways to
modulate the oscillation of TMR, and to enhance TMR so that it can tend to
infinity. Within the theory, the barrier, as a periodic diffraction grating,
can get rid of the confinement in width, it can vary from nanoscale to
microscale. Based on those results, a future-generation MTJ is proposed where
the three pieces can be fabricated separately and then assembled together, it
is especially appropriate for the layered materials, e.g., MoS2 and graphite,
and most feasible for industries. | cond-mat |
Transient dynamics of strongly coupled spin vortex pairs: effects of
anharmonicity and resonant excitation on inertial switching: Spin vortices in magnetic nanopillars are used as GHz oscillators, with
frequency however essentially fixed in fabrication. We demonstrate a model
system of a two-vortex nanopillar, in which the resonance frequency can be
changed by an order of magnitude, without using high dc magnetic fields. The
effect is due to switching between the two stable states of the vortex pair,
which we show can be done with low-amplitude fields of sub-ns duration. We
detail the relevant vortex-core dynamics and explain how field anharmonicity
and phase control can be used to enhance the performance. | cond-mat |
Cooper Instability in the Occupation Dependent Hopping Hamiltonians: A generic Hamiltonian, which incorporates the effect of the orbital
contraction on the hopping amplitude between the nearest sites, is studied both
analytically at the weak coupling limit and numerically at the intermediate and
strong coupling regimes for finite atomic cluster. The effect of the orbital
contraction due to hole localization at atomic sites is specified with two
coupling parameters V and W (multiplicative and additive contraction terms).
The singularity of the vertex part of the two-particle Green's function
determines the critical temperature Tc and the relaxation rate Gamma(T) of the
order parameter at temperature above Tc. Unlike in conventional BCS
superconductors, Gamma has a non-zero imaginary part which may influence the
fluctuation conductivity of superconductor above Tc. We compute the ground
state energy as a function of the particle number and magnetic flux through the
cluster, and show the existence of the parity gap Delta appearing at the range
of system parameters consistent with the appearance of Cooper instability.
Numeric calculation of the Hubbard model (with U>0) at arbitrary occupation
does not show any sign of superconductivity in small cluster. | cond-mat |
Composite fermion dynamics in half-filled Landau levels of graphene: We report on exact-diagonalization studies of correlated many-electron states
in the half-filled Landau levels of graphene, including pseudospin (valley)
degeneracy. We demonstrate that the polarized Fermi sea of non-interacting
composite fermions remains stable against a pairing transition in the lowest
two Landau levels. However, it undergoes spontaneous depolarization, which is
unprotected owing to the lack of single-particle pseudospin splitting. These
results suggest the absence of the Pfaffian phase in graphene. | cond-mat |
Traveling waves in reaction-diffusion system: A new asymptotic method is presented for the analysis of the traveling waves
in the one-dimensional reaction-diffusion system with the diffusion with a
finite velocity and Kolmogorov-Petrovskii-Piskunov kinetics. The analysis makes
use of the path-integral approach, scaling procedure and the singular
perturbation techniques involving the large deviations theory for the Poisson
random walk. The exact formula for the position and speed of reaction front is
derived. It is found that the reaction front dynamics is formally associated
with the relativistic Hamiltonian/Lagrangian mechanics. | cond-mat |
Temperature- and Force-Induced beta-Sheet Unfolding in an Exactly
Solvable Model: The stability of a $\beta$-sheeted conformation and its transition into a
random coil are studied with a 2D lattice biopolymer model. At low temperature
and low external force, the polymer folds back and forth on itself and forms a
$\beta$-sheet. Our analytical calculation and Monte Carlo simulation reveal
that a co-operative $\beta$-sheet--random coil transition takes places when the
temperature or force is increased, with a dramatic decrease in the contact
number. These predictions are in good agreement with experiments on titin
protein. This transition is not a real phase-transition, indicating that
backbone hydrogen-bonding alone is unable to stabilize a distinct $\beta$-sheet
phase. | cond-mat |
Landau Levels in Strained Optical Lattices: We propose a hexagonal optical lattice system with spatial variations in the
hopping matrix elements. Just like in the valley Hall effect in strained
Graphene, for atoms near the Dirac points the variations in the hopping matrix
elements can be described by a pseudo-magnetic field and result in the
formation of Landau levels. We show that the pseudo-magnetic field leads to
measurable experimental signatures in momentum resolved Bragg spectroscopy,
Bloch oscillations, cyclotron motion, and quantization of in-situ densities.
Our proposal can be realized by a slight modification of existing experiments.
In contrast to previous methods, pseudo-magnetic fields are realized in a
completely static system avoiding common heating effects and therefore opening
the door to studying interaction effects in Landau levels with cold atoms. | cond-mat |
Quasi-one-dimensional charge density wave in electromagnetic field
arbitrarily oriented to conducting chains: generalized Frohlich relations: We derive equations for the collective CDW-current transverse conducting
chains in a quasi-one-dimensional CDW-conductor. Generalized Frohlich relations
between the transverse currents and phase gradients are due to the polarization
corrections to the 1+1 chiral anomaly Lagrangean. The CDW Hall constant is
calculated. | cond-mat |
Surface Engineering for Phase Change Heat Transfer: A Review: Among numerous challenges to meet the rising global energy demand in a
sustainable manner, improving phase change heat transfer has been at the
forefront of engineering research for decades. The high heat transfer rates
associated with phase change heat transfer are essential to energy and industry
applications; but phase change is also inherently associated with poor
thermodynamic efficiencies at low heat flux, and violent instabilities at high
heat flux. Engineers have tried since the 1930's to fabricate solid surfaces
that improve phase change heat transfer. The development of micro and
nanotechnologies has made feasible the high-resolution control of surface
texture and chemistry over length scales ranging from molecular levels to
centimeters. This paper reviews the fabrication techniques available for
metallic and silicon-based surfaces, considering sintered and polymeric
coatings. The influence of such surfaces in multiphase processes of high
practical interest, e.g., boiling, condensation, freezing, and the associated
physical phenomena are reviewed. The case is made that while engineers are in
principle able to manufacture surfaces with optimum nucleation or thermofluid
transport characteristics, more theoretical and experimental efforts are needed
to guide the design and cost-effective fabrication of surfaces that not only
satisfy the existing technological needs, but also catalyze new discoveries. | cond-mat |
Shear Banding and Spatiotemporal Oscillations in Vortex Matter in
Nanostructured Superconductors: We propose a simple nanostructured pinning array geometry where a rich
variety of complex vortex shear banding phenomena can be realized. A single row
of pinning sites is removed from a square pinning array. Shear banding effects
arise when vortex motion in the pin-free channel nucleates motion of vortices
in the surrounding pinned regions, creating discrete steps in the vortex
velocity profile away from the channel. Near the global depinning transition,
the width of the band of moving vortices undergoes oscillations or fluctuations
that can span the entire system. We use simulations to show that these effects
should be observable in the transport properties of the system. Similar large
oscillations and shear banding effects are known to occur for sheared complex
fluids in which different dynamical phases coexist. | cond-mat |
Angle-dependent ultrasonic transmission through plates with
subwavelength hole arrays: We study sound transmission in perforated plates as a function of incident
angle and conclude that it holds distinctive properties that make it unique and
essentially different from optical transmission through perforated metallic
plates. More precisely, we conclude the following: (a) similar to its optical
counterpart, acoustic transmission minima respond to Wood anomalies in which
the periodicity plays a central role; (b) in contrast to both the optical case
and the acoustical case with slits, homogeneous-plate modes (Lamb and
Scholte-Stoneley modes) are strongly coupled to lattice and Fabry-P\'erot
resonances. This gives rise to unique transmission behavior, thus opening new
perspectives for exotic wave phenomena. | cond-mat |
Analysis of the valence band photoemission spectrum of
Sr$_2$CuO$_2$Cl$_2$ along the high-symmetry directions: Band structure calculations have been used to identify the different bands
contributing to the polarisation-dependent photoemission spectra of the undoped
model cuprate Sr$_2$CuO$_2$Cl$_2$ at the high-symmetry points of the CuO$_2$
plane $\Gamma$, $(\pi/a,0)$ and $(\pi/a,\pi/a)$ and along the high-symmetry
directions $\Gamma - (\pi/a,\pi/a)$ and $\Gamma - (\pi/a,0)$. Results from
calculations within the local density approximation (LDA) have been compared
with calculations taking into account the strong electron correlations by
LDA+U, with the result that the experimental order of energy levels at the
high-symmetry points is better described by the LDA+U calculation than by the
simple LDA. All the main peaks in the photoemission spectra at the high
symmetry points could be assigned to different Cu 3$d$ and O 2$p$ orbitals
which we have classified according to their point symmetries. The dispersions
along the high-symmetry directions were compared with an 11-band tight-binding
model which was fitted both to the LDA+U band structure calculation and the
angle-resolved photoemission data. The mean field treatment successfully
describes the oxygen derived bands but shows discrepancies for the copper ones. | cond-mat |
Observation of charged excitons in hole-doped carbon nanotubes using
photoluminescence and absorption spectroscopy: We report the first observation of trions (charged excitons), three-particle
bound states consisting of one electron and two holes, in hole-doped carbon
nanotubes at room temperature. When p-type dopants are added to carbon nanotube
solutions, the photoluminescence and absorption peaks of the trions appear far
below the E11 bright exciton peak, regardless of the dopant species. The
unexpectedly large energy separation between the bright excitons and the trions
is attributed to the strong electron-hole exchange interaction in carbon
nanotubes. | cond-mat |
X-ray cross-correlation analysis of disordered systems: potentials and
limitations: Angular x-ray cross-correlation analysis (XCCA) is an approach to study the
structure of disordered systems using the results of x-ray scattering
experiments. In this paper we summarize recent theoretical developments related
to the Fourier analysis of the cross-correlation functions. Results of our
simulations demonstrate the application of XCCA to two- and three-dimensional
(2D and 3D) disordered systems of particles. We show that the structure of a
single particle can be recovered using x-ray data collected from a 2D
disordered system of identical particles. We also demonstrate that valuable
structural information about the local structure of 3D systems, inaccessible
from a standard small-angle x-ray scattering experiment, can be resolved using
XCCA. | cond-mat |
Some words on the "phase transitions" in magnetic mesoscopic system: "Phase transitions" between quantum and classical behaviour in large spin
magnetic systems discused. | cond-mat |
Pairs of Bloch electrons and magnetic translation groups: A product of irreducible representations of magnetic translation group is
considered. It leads to irreducible representations which were previously
rejected as nonphysical. A very simple example indicates a possible application
of these representations. In particular, they are important in descriptions of
pairs of electrons in a magnetic field and a periodic potential. The
periodicity of some properties with respect to the charge of a particle is
briefly discussed. | cond-mat |
A Simple Method to Make the Wang-Landau Sampling Converge: We show that a histogram maintained throughout the Wang-Landau (WL) sampling
for the energy entries visited during the simulation could be used to make the
simulated density of states (DOS) converge. The method is easy to be
implemented to the WL sampling with no extra computational cost and bears the
advantages of both the WL method and the multicanonical method. | cond-mat |
Replica Cluster Variational Method: the Replica Symmetric solution for
the 2D random bond Ising model: We present and solve the Replica Symmetric equations in the context of the
Replica Cluster Variational Method for the 2D random bond Ising model
(including the 2D Edwards-Anderson spin glass model). First we solve a
linearized version of these equations to obtain the phase diagrams of the model
on the square and triangular lattices. In both cases the spin-glass transition
temperatures and the tricritical point estimations improve largely over the
Bethe predictions. Moreover, we show that this phase diagram is consistent with
the behavior of inference algorithms on single instances of the problem.
Finally, we present a method to consistently find approximate solutions to the
equations in the glassy phase. The method is applied to the triangular lattice
down to T=0, also in the presence of an external field. | cond-mat |
Lack of an equation of state for the nonequilibrium chemical potential
of gases of active particles in contact: We discuss the notion of nonequilibrium chemical potential in gases of
non-interacting active particles filling two compartments separated by a
potential energy barrier. Different types of active particles are considered:
run-and-tumble particles, active Brownian particles, and active Brownian
particles with a stochastic reorientation along an external field. After
recalling some analytical results for run-and-rumble particles in one
dimension, we focus on the two-dimensional case and obtain a perturbative
expression of the density profile in the limit of a fast reorientation
dynamics, for the three models of active particles mentioned above. Computing
the chemical potentials of the non-equilibrium systems in contact from the
knowledge of the stationary probability distribution of the whole system
---which agrees with a recently proposed general definition of the chemical
potential in non-equilibrium systems in contact--- we generically find that the
chemical potential lacks an equation of state, in the sense that it depends on
the detailed shape of the potential energy barrier separating the compartments
and not only on bulk properties, at odds with equilibrium. This situation is
reminiscent of the properties of the mechanical pressure in active systems. We
also argue that the Maxwell relation is no longer valid and cannot be used to
infer the nonequilibrium chemical potential from the knowledge of the
mechanical pressure. | cond-mat |
Universality class of Ising critical states with long-range losses: We show that spatial resolved dissipation can act on $d$-dimensional spin
systems in the Ising universality class by qualitatively modifying the nature
of their critical points. We consider power-law decaying spin losses with a
Lindbladian spectrum closing at small momenta as $\propto q^\alpha$, with
$\alpha$ a positive tunable exponent directly related to the power-law decay of
the spatial profile of losses at long distances, $1/r^{(\alpha+d)}$. This
yields a class of soft modes asymptotically decoupled from dissipation at small
momenta, which are responsible for the emergence of a critical scaling regime
ascribable to the non-unitary counterpart of the universality class of
long-range interacting Ising models. For $\alpha<1$ we find a non-equilibrium
critical point ruled by a dynamical field theory described by a Langevin model
with coexisting inertial ($\sim {\partial^2_t}$) and frictional ($\sim
{\partial_t}$) kinetic coefficients, and driven by a gapless Markovian noise
with variance $\propto q^\alpha$ at small momenta. This effective field theory
is beyond the Halperin-Hohenberg description of dynamical criticality, and its
critical exponents differ from their unitary long-range counterparts. Our work
lays out perspectives for a revision of universality in driven-open systems by
employing dark states taylored by programmable dissipation. | cond-mat |
Correspondence between winding numbers and skin modes in non-hermitian
systems: We establish exact relations between the winding of "energy" (eigenvalue of
Hamiltonian) on the complex plane as momentum traverses the Brillouin zone with
periodic boundary condition, and the presence of "skin modes" with open
boundary condition in non-hermitian systems. We show that the nonzero winding
with respect to any complex reference energy leads to the presence of skin
modes, and vice versa. We also show that both the nonzero winding and the
presence of skin modes share the common physical origin that is the
non-vanishing current through the system. | cond-mat |
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