title:
Temperature behavior of the magnon modes of the square lattice
  antiferromagnet

abstract:
  A spin-wave theory of short-range order in the square lattice Heisenberg
antiferromagnet is formulated. With growing temperature from T=0 a gapless mode
is shown to arise simultaneously with opening a gap in the conventional
spin-wave mode. The spectral intensity is redistributed from the latter mode to
the former. For low temperatures the theory reproduces results of the modified
spin-wave theory by M.Takahashi, J.E.Hirsch et al. and without fitting
parameters gives values of observables in good agreement with Monte Carlo
results in the temperature range 0 <= T < 0.8J where J is the exchange
constant.


title:
Density-functional theory study of the catalytic oxidation of CO over
  transition metal surfaces

abstract:
  In recent years due to improvements in calculation methods and increased
computer power, it has become possible to perform first-principles
investigations for ``simple'' chemical reactions at surfaces. We have carried
out such studies for the catalytic oxidation of CO at transition metal
surfaces, in particular, at the ruthenium surface for which unusual behavior
compared to other transition metal catalysts has been reported. High gas
pressure catalytic reactor experiments have revealed that the reaction rate
over Ru for oxidizing conditions is the highest of the transition metals
considered -- in contrast, under ultra high vacuum conditions, the rate is by
far the lowest. We find that important for understanding the pressure
dependence of the reaction is the fact that Ru(0001) can support high
concentrations of oxygen at the surface. Under these conditions, the O-metal
bond is atypically weak compared to that at lower coverages. We have
investigated a number of possible reaction pathways for CO oxidation for the
conditions of high oxygen coverages, including scattering reactions of
gas-phase CO at the oxygen covered surface (Eley-Rideal mechanism) as well as
the Langmuir-Hinshelwood mechanism involving reaction between adsorbed CO
molecules and O atoms.


title:
Electric Field Induced Converse Piezoeffect in Screw Dislocated YBCO
  Thin Films

abstract:
  A possible scenario for electric-field induced critical current enhancement
in screw dislocated YBCO thin films is proposed based on the converse
piezoelectric effect originated from a heavily defected medium of the
superconducting films in an applied electric field. The magnitude of the effect
is found to depend concurrently on the film thickness and the number of
dislocations inside a superconductor.


title: Evolution of speckle during spinodal decomposition

abstract:
  Time-dependent properties of the speckled intensity patterns created by
scattering coherent radiation from materials undergoing spinodal decomposition
are investigated by numerical integration of the Cahn-Hilliard-Cook equation.
For binary systems which obey a local conservation law, the characteristic
domain size is known to grow in time $\tau$ as $R = [B \tau]^n$ with n=1/3,
where B is a constant. The intensities of individual speckles are found to be
nonstationary, persistent time series. The two-time intensity covariance at
wave vector ${\bf k}$ can be collapsed onto a scaling function $Cov(\delta
t,\bar{t})$, where $\delta t = k^{1/n} B |\tau_2-\tau_1|$ and $\bar{t} =
k^{1/n} B (\tau_1+\tau_2)/2$. Both analytically and numerically, the covariance
is found to depend on $\delta t$ only through $\delta t/\bar{t}$ in the
small-$\bar{t}$ limit and $\delta t/\bar{t} ^{1-n}$ in the large-$\bar{t}$
limit, consistent with a simple theory of moving interfaces that applies to any
universality class described by a scalar order parameter. The speckle-intensity
covariance is numerically demonstrated to be equal to the square of the
two-time structure factor of the scattering material, for which an analytic
scaling function is obtained for large $\bar{t}.$ In addition, the two-time,
two-point order-parameter correlation function is found to scale as
$C(r/(B^n\sqrt{\tau_1^{2n}+\tau_2^{2n}}),\tau_1/\tau_2)$, even for quite large
distances $r$. The asymptotic power-law exponent for the autocorrelation
function is found to be $\lambda \approx 4.47$, violating an upper bound
conjectured by Fisher and Huse.


title: Dipole-dipole interaction of Josephson diamagnetic moments

abstract:
  The role of dipole-dipole interaction between Josephson diamagnetic moments
is considered within a model system of two clusters (each cluster contains
three weakly connected superconducting grains). The sign of the resulting
critical current is shown to depend on the orientation between clusters,
allowing for both 0 and $\pi$ type junctions behavior. The possibility of the
experimental verification of the model predictions is discussed.


title:
Ab initio Hartree-Fock computation of electronic static structure factor
  of crystalline insulators: benchmark results on LiF

abstract:
  In this paper we present a fully ab initio Hartree-Fock approach aimed at
calculating the static structure factor of crystalline insulators at arbitrary
values of momentum transfer. In particular, we outline the computation of the
incoherent scattering function, the component of the structure factor which
governs the incoherent x-ray scattering from solids. The presented theory is
applied to crystalline LiF to obtain benchmark Hartree-Fock values for its
incoherent scattering function. Benchmark theoretical values such as this, can
be combined with the experimentally measured static structure factor, to
understand the influence of electron correlation effects on cohesive properties
of solids.


title:
Proton NMR for Measuring Quantum-Level Crossing in the Magnetic
  Molecular Ring Fe10

abstract:
  The proton nuclear spin-lattice relaxation rate 1/T1 has been measured as a
function of temperature and magnetic field (up to 15 T) in the molecular
magnetic ring Fe10. Striking enhancement of 1/T1 is observed around magnetic
field values corresponding to a crossing between the ground state and the
excited states of the molecule. We propose that this is due to a
cross-relaxation effect between the nuclear Zeeman reservoir and the reservoir
of the Zeeman levels of the molecule. This effect provides a powerful tool to
investigate quantum dynamical phenomena at level crossing.


title:
A new method for detection of exciton Bose condensation using stimulated
  two-photon emission

abstract:
  Stimulated two-photon emission by Bose-condensed excitons accompanied by a
coherent two-exciton recombination, i.e., by simultaneous recombination of two
excitons with opposite momenta leaving unchanged the occupation numbers of
excitonic states with nonzero momenta, is investigated. Raman light scattering
accompanied by a similar two-exciton recombination (or generation of two
excitons) is also analyzed. The processes under consideration can occur only if
a system contains Bose condensate, therefore, their detection can be used as a
new method to reveal Bose condensation of excitons. The recoil momentum, which
corresponds to a change in the momentum of the electromagnetic field in the
processes, is transferred to phonons or impurities. If the recoil momentum is
transmitted to optical phonons with frequency $\omega_0^s$, the stimulated
two-photon emission with the coherent two-exciton recombination leads to the
appearance of a line at $2\Omega'-\omega$, where $\Omega'=\Omega-\omega_0^s$
and $\Omega$ is the light frequency corresponding to the recombination of an
exciton with zero momentum. Formulas for the cross sections at finite
temperatures are obtained for the processes under consideration. Our estimates
indicate that a spectral line, corresponding to the stimulated two-photon
emission accompanied by the coherent optical phonon-assisted two-exciton
recombination can be experimentally detected in Cu$_2$O.


title: Soliton-Magnon Scattering in Two-Dimensional Isotropic Ferromagnets

abstract:
  It is studied the scattering of magnons by the 2d topological
Belavin-Polyakov soliton in isotropic ferromagnet. Analytical solutions of the
scattering problem are constructed: (i) exactly for any magnon wave vectors for
the partial wave with the azimuthal number m=1 (translational mode), and (ii)
in the long- and short-wave limits for the rest modes. The magnon mode
frequencies are found for the finite size magnets. An effective equation of the
soliton motion is constructed. The magnon density of states, connected with the
soliton-magnon interaction, is found in a long-wave approximation.


title: Magnetic Impurities in d-wave Superconductors

abstract:
  We solve the problem of a magnetic impurity in a $d_{x^2-y^2}$-wave
superconductor by a variational method. A moment is found to exist in the
superconducting state only if the Kondo-temperature in the normal state is
larger than the maximum in the superconducting gap-function. If a moment exists
in the superconducting state, and provided spin-orbit coupling is non-zero, it
induces a time-reversal breaking superconducting state locally around the
inpurity which is a linear combination of the $d_{x^2-y^2}$ and $d_{xy}$
states. The current pattern around the impurity in this state are evaluated.


title:
Electromagnetic absorption of a pinned Wigner crystal at finite
  temperatures

abstract:
  We investigate the microwave absorption of a pinned, two-dimensional Wigner
crystal in a strong magnetic field at finite temperatures. Using a model of a
uniform commensurate pinning potential, we analyze thermal broadening of the
electromagnetic absorption resonance. Surprisingly, we find that the pinning
resonance peak should remain sharp even when the temperature is comparable or
greater than the peak frequency. This result agrees qualitatively with recent
experimental observations of the ac conductivity in two-dimensional hole
systems in a magnetically induced insulating state. It is shown, in analogy
with Kohn's theorem, that the electron-electron interaction does not affect the
response of a harmonically pinned Wigner crystal to a spatially uniform
external field at any temperature. We thus focus on anharmonicity in the
pinning potential as a source of broadening. Using a 1/N expansion technique,
we show that the broadening is introduced through the self-energy corrections
to the magnetophonon Green's functions.


title:
Filling dependence of the Mott transition in the degenerate Hubbard
  model

abstract:
  Describing the doped Fullerenes using a generalized Hubbard model, we study
the Mott transition for different integer fillings of the t_1u band. We use the
opening of the energy-gap E_g as a criterion for the transition. E_g is
calculated as a function of the on-site Coulomb interaction U using fixed-node
diffusion Monte Carlo. We find that for systems with doping away from
half-filling the Mott transitions occurs at smaller U than for the half-filled
system. We give a simple model for the doping dependence of the Mott
transition.


title:
Numerical renormalization-group study of spin correlations in
  one-dimensional random spin chains

abstract:
  We calculate the ground-state two-spin correlation functions of spin-1/2
quantum Heisenberg chains with random exchange couplings using the real-space
renormalization group scheme. We extend the conventional scheme to take account
of the contribution of local higher multiplet excitations in each decimation
step. This extended scheme can provide highly accurate numerical data for large
systems. The random average of staggered spin correlations of the chains with
random antiferromagnetic (AF) couplings shows algebraic decay like $1/r^2$,
which verifies the Fisher's analytic results. For chains with random
ferromagnetic (FM) and AF couplings, the random average of generalized
staggered correlations is found to decay more slowly than a power-law, in the
form close to $1/\ln(r)$. The difference between the distribution functions of
the spin correlations of the random AF chains and of the random FM-AF chains is
also discussed.


title: Field-tilt Anisotropy Energy in Quantum Hall Stripe States

abstract:
  Recently reported giant anisotropy in the longitudinal resistivity of a 2D
electron system with valence Landau level index $N \ge 2$ has been interpreted
as a signal of unidirectional charge density wave (UCDW) ground states. We
report on detailed Hartree-Fock calculations of the UCDW orientation energy
induced by a tilted magnetic field. We find that for current experimental
samples stripes are oriented perpendicular to the in-plane field, consistent
with experiment. For wider 2D electron systems we predict tilt-induced stripe
states with variable anisotropy energy sign.


title: The Link Overlap and Finite Size Effects for the 3D Ising Spin Glass

abstract:
  We study the link overlap between two replicas of an Ising spin glass in
three dimensions using the Migdal-Kadanoff approximation and scaling arguments
based on the droplet picture. For moderate system sizes, the distribution of
the link overlap shows the asymmetric shape and large sample-to-sample
variations found in Monte Carlo simulations and usually attributed to replica
symmetry breaking. However, the scaling of the width of the distribution, and
the link overlap in the presence of a weak coupling between the two replicas
are in agreement with the droplet picture. We also discuss why it is impossible
to see the asymptotic droplet-like behaviour for moderate system sizes and
temperatures not too far below the critical temperature.


title:
Stretching Single Domain Proteins: Phase Diagram and Kinetics of
  Force-Induced Unfolding

abstract:
  Single molecule force spectroscopy reveals unfolding of domains in titin upon
stretching. We provide a theoretical framework for these experiments by
computing the phase diagrams for force-induced unfolding of single domain
proteins using lattice models. The results show that two-state folders (at zero
force) unravel cooperatively whereas stretching of non-two-state folders occurs
through intermediates. The stretching rates of individual molecules show great
variations reflecting the heterogeneity of force-induced unfolding pathways.
The approach to the stretched state occurs in a step-wise "quantized" manner.
Unfolding dynamics depends sensitively on topology. The unfolding rates
increase exponentially with force f till an optimum value which is determined
by the barrier to unfolding when f=0. A mapping of these results to proteins
shows qualitative agreement with force-induced unfolding of Ig-like domains in
titin. We show that single molecule force spectroscopy can be used to map the
folding free energy landscape of proteins in the absence of denaturants.


title: Coexistence of Single and Double-Quantum Vortex Lines

abstract:
  We discuss the configurations in which singly and doubly quantized vortex
lines may coexist in a rotating superfluid. General principles of energy
minimization lead to the conclusion that in equilibrium the two vortex species
segregate within a cylindrical vortex cluster in two coaxial domains where the
singly quantized lines are in the outer annular region. This is confirmed with
simulation calculations on discrete vortex lines. Experimentally the
coexistence can be studied in rotating superfluid $^3$He-A. With cw NMR
techniques we find the radial distribution of the two vortex species to depend
on how the cluster is prepared: (i) By cooling through $T_c$ in rotation,
coexistence in the minimum energy configuration is confirmed. (ii) A glassy
agglomerate is formed if one starts with an equilibrium cluster of
single-quantum vortex lines and adds to it sequentially double-quantum lines,
by increasing the rotation velocity in the superfluid state. This proves that
the energy barriers, which separate different cluster configurations, are too
high for metastabilities to anneal.


title: A Model for the Thermodynamics of Globular Proteins

abstract:
  Comments: 6 pages RevTeX, 6 Postscript figures. We review a statistical
mechanics treatment of the stability of globular proteins based on a simple
model Hamiltonian taking into account protein self interactions and
protein-water interactions. The model contains both hot and cold folding
transitions. In addition it predicts a critical point at a given temperature
and chemical potential of the surrounding water. The universality class of this
critical point is new.


title: Models for adatom diffusion on fcc(001) metal surfaces

abstract:
  We present a class of models that describe self diffusion on several fcc(001)
metal substrates within a common framework. The models are found to apply well
for Cu(001), Ag(001), Au(001), Ni(001) and Pd(001).For each of these metals the
models can be used to estimate the activation energy of any diffusion process
using a few basic parameters which may be obtained from experiments, ab-initio
or semi-empirical calculations. To demonstrate the approach, the parameters of
the models are optimized to describe self-diffusion on the (001) surface, by
comparing the energy barriers to a full set of barriers obtained from
semi-empirical potentials via the embedded atom method (EAM). It is found that
these models with at most four parameters,provide a good description of the
full landscape of hopping energy barriers on FCC(001) surfaces. The main
features of the diffusion processes revealed by EAM calculations are
quantitatively reproducible by the models.


title: Phase transition in inelastic disks

abstract:
  This letter investigates the molecular dynamics of inelastic disks without
external forcing. By introducing a new observation frame with a rescaled time,
we observe the virtual steady states converted from asymptotic energy
dissipation processes. System behavior in the thermodynamic limit is carefully
investigated. It is found that a phase transition with symmetry breaking occurs
when the magnitude of dissipation is greater than a critical value.


title: Quantum stochastic processes in mesoscopic conductors

abstract:
  We show an equivalence between the approach of Buttiker and the Fermi quantum
stochastic calculus for mesoscopic systems. To illustrate the method we derive
the current fluctuations in a two terminal mesoscopic circuit with two tunnel
barriers containing a single quasi bound state on the well. The method enables
us to focus on either the incoming/outgoing Fermi fields in the leads, or on
the irreversible dynamics of the well state itself. The quantum stochastic
calculus we use is the Fermi analogue of the input/output methods of quantum
optics.


title:
Spectral Densities of Response Functions for the O(3) Symmetric Anderson
  and Two Channel Kondo Models

abstract:
  The O(3) symmetric Anderson model is an example of a system which has a
stable low energy marginal Fermi liquid fixed point for a certain choice of
parameters. It is also exactly equivalent, in the large U limit, to a localized
model which describes the spin degrees of freedom of the linear dispersion two
channel Kondo model. We first use an argument based on conformal field theory
to establish this precise equivalence with the two channel model. We then use
the numerical renormalization group (NRG) approach to calculate both
one-electron and two-electron response functions for a range of values of the
interaction strength U. We compare the behaviours about the marginal Fermi
liquid and Fermi liquid fixed points and interpret the results in terms of a
renormalized Majorana fermion picture of the elementary excitations. In the
marginal Fermi liquid case the spectral densities of all the Majorana fermion
modes display a |omega| dependence on the lowest energy scale, and in addition
the zero Majorana mode has a delta function contribution. The weight of this
delta function is studied as a function of the interaction U and is found to
decrease exponentially with U for large U. Using the equivalence with the two
channel Kondo model in the large U limit, we deduce the dynamical spin
susceptibility of the two channel Kondo model over the full frequency range. We
use renormalized perturbation theory to interpret the results and to calculate
the coefficient of the ln omega divergence found in the low frequency behaviour
of the T=0 dynamic susceptibility.


title:
Ergodic versus nonergodic behavior in oxygen deficient high-T_c
  superconductors

abstract:
  The oxygen defects induced phase transition from nonergodic to ergodic state
in superconductors with intragrain granularity is considered within the
superconductive glass model. The model predictions are found to be in a
qualitative agreement with some experimental observations in deoxygenated
high-T_c single crystals.


title: Flicker-noise spectrum in weak-links-containing superconductors

abstract:
  Low-frequency magnetic noise spectrum is calculated within the
superconductive glass model. The model predicts the existence of both white
noise and flicker-like noise f^{-z} with z strongly dependent on applied
magnetic field and temperature. It is shown, in particular, that z increases
with field from 0.5 to 1, and that in the critical region (near T_{c}(H)) the
1/sqrt{f} law should prevail in the noise spectrum.


title: Strong magnetoresistance induced by long-range disorder

abstract:
  We calculate the semiclassical magnetoresistivity $\rho_{xx}(B)$ of
non-interacting fermions in two dimensions moving in a weak and smoothly
varying random potential or random magnetic field. We demonstrate that in a
broad range of magnetic fields the non-Markovian character of the transport
leads to a strong positive magnetoresistance. The effect is especially
pronounced in the case of a random magnetic field where $\rho_{xx}(B)$ becomes
parametrically much larger than its B=0 value.


title:
Photoluminescence Detected Doublet Structure in the Integer and
  Fractional Quantum Hall Regime

abstract:
  We present here the results of polarized magneto-photoluminescence
measurements on a high mobility single-heterojunction. The presence of a
doublet structure over a large magnetic field range (2>nu>1/6) is interpreted
as possible evidence for the existence of a magneto-roton minima of the charged
density waves. This is understood as an indication of strong electronic
correlation even in the case of the IQHE limit.


title:
Coherence between superconducting edge states in superconducting
  periodic arrays of artificial defects

abstract:
  The transition line of superconducting arrays of holes exhibits a rich field
structure due to the interference of superconducting states nucleated at the
holes edges. We studied by means of resistance measurements their effect on the
T*c(H) line as a function of transverse magnetic field using regular arrays of
nanofabricated micron size holes. The arrays transition fields are higher than
for the bulk. Moreover we found a nontrivial field modulation of the T*c(H)
line with an inversion, with increasing field, of the modulation concavity
which we assigned to a crossover from a collective to an isolated edge state
regime. The high field regime is well described by the nucleation at a single
hole in an infinite film. The modulation at low fields was found to be
dominated by the interference of neighbor edge states when the inter-hole
distance w becomes comparable to the temperature dependent coherence length. A
comparison between arrays of different hole shape shows the influence of
geometry on the type of interaction established, which can described either as
a superconducting wire network or as a weak link array.


title: A numerical exact solution of the Bose-Hubbard model

abstract:
  In this paper we report results from a systematic strong-coupling expansion
of the Bose-Hubbard model in one and two spatial dimensions. We obtain
numerically exact results for the structure factor and the spectrum of single
particle and single hole excitations in the Mott insulator. This enables the
determination of the zero-temperature phase diagram and the location of the
critical endpoints of the Mott lobes. In one dimension we confirm the
occurrence of reentrance behavior from the compressible to the insulating phase
in a region close to the critical point.


title: Electrostatics of Inhomogeneous Quantum Hall Liquid

abstract:
  The distribution of electron density in the quantum Hall liquid is considered
in the presence of macroscopic density gradient caused by side electrodes or
inhomogeneous doping. In this case different Landau levels are occupied in
different regions of a sample. These regions are separated by incompressible
liquid. It is shown that the applicability of the approach by Chklovskii et al.
is substantially restricted if the density gradient is not very large and
disorder is important. Due to the fluctuations of the remote donor's density
the liquid in the transition region can not be considered as completely
incompressible. In the typical situation, when the gap between Landau levels is
not much larger than the energy of disorder, the transition region is a wide
band where electron density, averaged over the fluctuations, is independent of
magnetic field. The band is a random mixture of regions occupied by electrons
of upper level, by holes of lower level and by incompressible liquid. The width
of this band is calculated and an analytical expression for the fraction of
incompressible liquid in different parts of this band is given.


title:
Supression of magnetic subbands in semiconductor superlattices driven by
  a laser field

abstract:
  The effect of strong laser radiation on magnetic subbands in semiconductor
superlattices is investigated. Due to the presence of a magnetic field
perpendicular to the growth direction, non-linear effects such as band
supression and electron localization become relevant at relatively lower
intensities and for any polarization perpendicular to the magnetic field.
Electron quasienergies and density of states are calculated in the
Kramers-Henneberger approximation, whose validity is discussed. The conditions
under which collapse of magnetic subbands and quenching of N-photon emission or
absorption processes occur are discussed. We conclude that at laser frequencies
close to cyclotronic frequency and intensities typical of c.w. lasers, magnetic
subbands become flat, magnetotunneling is inhibited and multiphotonic processes
dominate optical absorption.


title: Tunneling Spectroscopy of Tl2Ba2CuO6

abstract:
  New results from tunneling spectroscopies on near optimally-doped single
crystals of Tl_{2}Ba_{2}CuO_{6} (Tl-2201) junctions are presented. The
superconductor-insulator-normal metal (SIN) tunnel junctions are obtained using
the point-contact technique with a Au tip. The tunneling conductances
reproducibly show a sharp cusp-like subgap, prominent quasiparticle peaks with
a consistent asymmetry, and weakly decreasing backgrounds. A rigorous analysis
of the SIN tunneling data is performed using two different models for the
$d_{x^{2}-y^{2}}$ (d-wave) density of states (DOS). Based on these and earlier
results, the tunneling DOS of Tl-2201 have exhibited the most reproducible data
that are consistent with a d-wave gap symmetry. We show that the dip feature at
$2\Delta$ that is clearly seen in SIN tunneling data of
Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} is also present in Tl-2201, but at a weaker
level. The gap values for crystals with a bulk T_c = 86 K are in the range of
19-25 meV.


title:
Tunneling into Ferromagnetic Quantum Hall States: Observation of a Spin
  Bottleneck

abstract:
  We explore the characteristics of equilibrium tunneling of electrons from a
3D electrode into a high mobility 2D electron system. For most 2D Landau level
filling factors, we find that tunneling can be characterized by a single,
well-defined tunneling rate. However, for spin-polarized quantum Hall states
(nu = 1, 3 and 1/3) tunneling occurs at two distinct rates that differ by up to
2 orders of magnitude. The dependence of the two rates on temperature and
tunnel barrier thickness suggests that slow in-plane spin relaxation creates a
bottleneck for tunneling of electrons.


title:
Momentum Dependence of Resonant Inelastic X-Ray Scattering Spectrum in
  Insulating Cuprates

abstract:
  The resonant inelastic x-ray scattering spectrum in insulating cuprates is
examined by using the exact diagonalization technique on small clusters in the
two-dimensional Hubbard model with second and third neighbor hopping terms.
When the incident photon energy is tuned near the Cu K absorption edges, we
find that the features of the unoccupied upper Hubbard band can be extracted
from the spectrum through an anisotropic momentum dependence. They provide an
opportunity for the understanding of the different behavior of hole- and
electron-doped superconductors.


title:
Two-site polaron problem: a perturbation approach with variational basis
  states

abstract:
  A convergent perturbation method using modified Lang Firsov transformation is
developed for a two-site single-polaron system. The method is applicable for
the entire range of the electron-phonon coupling strength from the
antiadiabatic limit to the intermediate region of hopping. The single-electron
energies, oscillator wave functions and correlation functions, calculated using
this method, are in good agreement with the exact results.


title:
A simple method to create a vortex in Bose-Einstein condensate of alkali
  atoms

abstract:
  Bose-Einstein condensation in alkali atoms has materialized quite an
interesting system, namely a condensate with a spin degree of freedom. In
analogy with the A-phase of the superfluid $^3$He, numerous textures with
nonvanishing vorticity have been proposed. In the present paper, interesting
properties of such spin textures are analyzed. We propose a remarkably simple
method to create a vortex state of a BEC in alkali atoms.


title: Core-softened potentials and the anomalous properties of water

abstract:
  We study the phase diagram of a system of spherical particles interacting in
three dimensions through a potential consisting of a strict hard core plus a
linear repulsive shoulder at larger distances. The phase diagram (obtained
numerically, and analytically in a limiting case) shows anomalous properties
that are similar to those observed in water. Specifically, we find maxima of
density and isothermal compressibility as a function of temperature, melting
with volume contraction, and multiple stable crystalline structures. If in
addition a long range attraction between the particles is included, the usual
liquid-gas coexistence curve with its critical point is obtained. But more
interestingly, a first order line in the metastable fluid branch of the phase
diagram appears, ending in a new critical point, as it was suggested to occur
in water. In this way the model provides a comprehensive, consistent and
unified picture of most of the anomalous thermodynamical properties of water,
showing that all of them can be qualitatively explained by the existence of two
competing equilibrium values for the interparticle distance.


title: DOF phase separation of the Lennard-Jones fcc(111) surface

abstract:
  Recent lattice model calculations have suggested that a full-layered crystal
surface may undergo, under canonical (particle-conserving) conditions, a
preroughening-driven two-dimensional phase separation into two disordered flat
(DOF) regions, of opposite order parameter. We have carried out extensive
classical molecular dynamics (MD) simulations of the Lennard-Jones fcc(111)
surface, to check whether these predictions are relevant or not for a realistic
continuous system. Very long simulation times, a grid of temperatures from
(2/3)Tm to Tm, and unusually large system sizes are employed to ensure full
equilibrium and good statistics. By examining layer-by-layer occupancies,
height fluctuations, sublattice order parameter and X-ray structure factors, we
find a clear anomaly at ~0.83Tm. The anomaly is distinct from roughening (whose
incipiency is also detected at ~0.94Tm), and is seen to be consistent with the
preroughening plus phase separation scenario.


title:
Metal-Insulator Transition of the Quasi-One Dimensional Luttinger Liquid
  Due to the Long-Range Character of the Coulomb Interaction

abstract:
  An instability of the quasi-1D Luttinger liquid associated with the metal -
insulator transition is considered. The homogeneous metal ground state of this
liquid is demonstrated to be unstable and the charge-density wave arises in the
system. The wavevector of this wave has nonzero component both along the
direction of the chains and in the perpendicular direction. The ground state of
the system has a dielectric gap at the Fermi surface, the value of this gap
being calculated.


title:
Defects in Lamellar Diblock Copolymers: Chevron- and Omega-shaped Tilt
  Boundaries

abstract:
  We investigate symmetric grain boundaries in a lamellar diblock copolymer
system. The form of the interface between two grains strongly depends on the
angle $\theta$, between the normals of the grains. When this angle is small,
the lamellae transform smoothly from one orientation to the other, creating the
chevron morphology. As $\theta$ increases, a gradual transition is observed to
an omega morphology characterized by a protrusion of the lamellae along the
interface between the two phases. We present a theoretical approach to find
these tilt boundaries in two-dimensional systems, based on a Ginzburg-Landau
expansion of the free energy. Calculated order parameter profiles and energies
agree well with transmission electron microscope experiments, and with full
numerical solution of the same problem.


title:
Quantum Phase Transition of Randomly-Diluted Heisenberg Antiferromagnet
  on a Square Lattice

abstract:
  Ground-state magnetic properties of the diluted Heisenberg antiferromagnet on
a square lattice are investigated by means of the quantum Monte Carlo method
with the continuous-time loop algorithm. It is found that the critical
concentration of magnetic sites is independent of the spin size S, and equal to
the two-dimensional percolation threshold. However, the existence of quantum
fluctuations makes the critical exponents deviate from those of the classical
percolation transition. Furthermore, we found that the transition is not
universal, i.e., the critical exponents significantly depend on S.


title: A Raman Investigation of PbZr0.94Ti0.06O3 Ceramics Under High-Pressures

abstract:
  We have investigated the behavior of the PbZr(0.94}Ti(0.06)O3 Raman spectra
as a function of Hydrostatic pressures. The new structural phases were
identified based on previous works performed in PZT system with different Ti
concentrations at room- and high pressures. We showed that PbZr(0.94}Ti(0.06}O3
exhibits a rich phase sequence up to 3.7 GPa: rhombohedral(LT) {0.3 GPa}
orthorhombic(I) {2.9 GPa} orthorhombic(I'). This sequence is different from
that exhibited by PbZr(0.90)Ti(0.10)O3 suggesting a very interesting
concentration-pressure phase diagram for rich Zr PZT system.


title:
Determination of the order of phase transitions in Potts model by the
  graph-weight approach

abstract:
  We examine the order of the phase transition in the Potts model by using the
graph representation for the partition function, which allows treating a
non-integer number of Potts states. The order of transition is determined by
the analysis of the shape of the graph-weight probability distribution. The
approach is illustrated on special cases of the one-dimensional Potts model
with long-range interactions and on its mean-field limit.


title: Coefficient of restitution of colliding viscoelastic spheres

abstract:
  We perform a dimension analysis for colliding viscoelastic spheres to show
that the coefficient of normal restitution $\epsilon$ depends on the impact
velocity g as \epsilon=1-\gamma_1g^{1/5}+\gamma_2g^{2/5}\mp..., in accordance
with recent findings. We develop a simple theory to find explicit expressions
for coefficients \gamma_1 and \gamma_2. Using these and few next expansion
coefficients for \epsilon(g) we construct a Pad\'e-approximation for this
function which may be used for a wide range of impact velocities where the
concept of the viscoelastic collision is valid. The obtained expression
reproduces quite accurately the existing experimental dependence \epsilon(g)
for ice particles.


title: Spontaneous symmetry breaking in single and molecular quantum dots

abstract:
  Classes of spontaneous symmetry breaking at zero and low magnetic fields in
single quantum dots (QD's) and quantum dot molecules (QDM's) are discussed in
relation to the ratio R_W between the interelectron Coulomb repulsion and the
harmonic confinement, using spin-and-Space unrestricted Hartree-Fock
calculations. These include: Wigner crystallization for R_W > 1, and formation
of non-crystallized electron puddles localized on the individual dots in QDM's,
as well as spin-density waves in single QD's, for R_W < 1.
  Erratum:
  Subsequent to the the publication of our Letter, we have performed further
systematic spin-and-space unrestricted Hartree-Fock calculations. While the
behavior and magnitudes of the addition energies shown in our Letter are
maintained, as well as our finding pertaining to the prevalent violation of
Hund's first rule, our improved calculations yield in certain instances
different spin polarizations.


title: Dynamics of the diluted Ising antiferromagnet Fe0.31Zn0.69F2

abstract:
  The diluted Ising antiferromagnet, Fe_{0.31}Zn_{0.69}F_{2}, has been
investigated by dynamic susceptibility measurements in zero and and finite
applied dc-fields. In zero field, there is a para- to antiferromagnetic phase
transition at $T_{N} \approx {20.0 K}$, followed by a dramatic slowing down of
the dynamics at low temperatures. The latter described by a pure Arrhenius law.
The effect of a superposed dc-field on the antiferromagnetic phase transition
and on the low-temperature dynamics is investigated, and a comprehensive static
and dynamic phase diagram in the H-T plane is derived. In agreement with
earlier results on the same system, $T_{N}(H)$ follows a random-exchange Ising
model to random-field Ising model crossover scaling for fields $H \leq 1.5 T$.
A random-field induced glassy dynamics appears for higher values of H, where
the antiferromagnetic phase transition is destroyed. The low-temperature
dynamics shows striking similarities with the behavior observed in reentrant
antiferromagnets.


title: Random Surfaces that Suppress Single Scattering

abstract:
  We present a method for generating numerically a one-dimensional random
surface, defined by the equation $x_3 = \zx$, that suppresses single-scattering
processes in the scattering of light from it within a specified range of
scattering angles. Rigorous numerical calculations of the scattering of light
from surfaces generated by this approach show that the single-scattering
contribution to the mean scattered intensity is indeed suppressed within that
range of angles.


title:
Microwave controlled phase coherent transport in mesoscopic S-N-S
  structures

abstract:
  We show that transport through a superconducting quantum point contact biased
at subgap voltages is strongly affected by a microwave field. The subgap
current is increased by several orders of magnitude. Quantum interference among
resonant scattering events involving photon absorption is reflected as an
oscillating structure in the I-V curve. We also discuss how the same
interference effect can be applied for detecting weak electromagnetic signals
up to the gap frequency, and how it is affected by dephasing and relaxation.


title: Self-organized evolution in socio-economic environments

abstract:
  We propose a general scenario to analyze social and economic changes in
modern environments. We illustrate the ideas with a model that incorporating
the main trends is simple enough to extract analytical results and, at the same
time, sufficiently complex to display a rich dynamic behavior. Our study shows
that there exists a macroscopic observable that is maximized in a regime where
the system is critical, in the sense that the distribution of events follow
power-laws. Computer simulations show that, in addition, the system always
self-organizes to achieve the optimal performance in the stationary state.


title: Disordered d-wave superconductors with chiral symmetry

abstract:
  A two-dimensional lattice model for d-wave superconductor with chiral
symmetry is studied. The field theory at the band center is shown to be in the
universality class of U(2n)/O(2n) and U(2n) nonlinear sigma model for the
system with broken and unbroken time-reversal symmetry, respectively. Vanishing
of the beta function implies extended states at the band center. Density of
state vanishes as a cubic function of the energy at the band center for the
former case, while linear for the latter.


title: Accuracy of equation of state formulations

abstract:
  The accuracy of equation-of-state formulations is compared for theoretical
total energies or experimental pressure-volume measurements for H$_{2}$, Ne,
Pt, and Ta. This spans the entire range of compression found for minerals and
vo latiles in the Earth. The Vinet equation is found to be most accurate. The
origin of the behavior of different equation-of-state formulations is
discussed. It is shown that subtle phase transitions can be detected by
examining the residuals from an equation-of-state fit. A change in the
electronic structure of Ta is found at high pressures using this procedure, and
a possible new transition in H$_{2}$.


title:
Universality of vortex avalanches in a type II superconductor with
  periodic pinning

abstract:
  In this work the robustness of a simple cellular automaton developed by
Bassler and Paczuski to describe the critical state in type-II superconductors
is studied. Two different configurations of pinning centers are introduced and
a new universality class is found. The numerical values of the critical
exponents were calculated following two scaling techniques to ensure the
validity of our results.


title:
Spin-Peierls Transition in CuGeO_3: Critical, Tricritical, or Mean
  Field?

abstract:
  The spin-Peierls phase transition in CuGeO_3 has been extensively studied
utilizing a variety of experimental techniques. Interpretations of the phase
transition behavior vary from tricritical to mean field to Ising critical to XY
critical. We show that the behavior in the vicinity of the phase transition of
each of the order parameter, the magnetic energy gap and the heat capacity can
be quantitatively fitted with few adjustable parameters with a mean field model
incorporating a tricritical to mean field critical crossover in the transition
region.


title:
Noise induced transition from an absorbing phase to a regime of
  stochastic spatiotemporal intermittency

abstract:
  We introduce a stochastic partial differential equation capable of
reproducing the main features of spatiotemporal intermittency (STI).
Additionally the model displays a noise induced transition from laminarity to
the STI regime. We show by numerical simulations and a mean-field analysis that
for high noise intensities the system globally evolves to a uniform absorbing
phase, while for noise intensities below a critical value spatiotemporal
intermittence dominates. A quantitative computation of the loci of this
transition in the relevant parameter space is presented.


title: The Mesostructure of Polymer Collapse and Fractal Smoothing

abstract:
  We investigate the internal structure of a polymer during collapse from an
expanded coil to a compact globule. Collapse is more probable in local regions
of high curvature, so a smoothing of the fractal polymer structure occurs that
proceeds systematically from the shortest to the longest length scales. A
proposed universal scaling relationship is tested by comparison with Monte
Carlo simulations. We speculate that the universal form applies to various
fractal systems with local processes that promote smoothness over time. The
results complement earlier work showing that on the macroscale polymer collapse
proceeds by driven diffusion of the polymer ends.


title:
Non-k-diagonality in the interlayer pair-tunneling model of
  high-temperature superconductivity

abstract:
  We investigate the effect of k-space broadening of the interlayer pairing
kernel on the critical temperature T_c and the k-dependence of the gap function
in a one-dimensional version of the interlayer pair-tunneling model of high-T_c
superconductivity. We consider constant as well as k-dependent intralayer
pairing kernels. We find that the sensitivity to k-space broadening is larger
the smaller the width of the peak of the Fermi-level gap calculated for zero
broadening. This width increases with the overall magnitude of the interlayer
tunneling matrix element, and decreases with the bandwidth of the
single-electron intralayer excitation spectrum. The width also increases as the
Fermi level is moved towards regions where the excitation spectrum flattens
out. We argue that our qualitative conclusions are valid also for a
two-dimensional model. This indicates that at or close to half-filling in two
dimensions, when the Fermi-surface gap for zero broadening attains its peaks at
$(\pm \pi/a,0)$ and $(0,\pm\pi/a)$ where the excitation spectrum is flat, these
peaks should be fairly robust to moderate momentum broadening.


title:
High-precision determination of the critical exponents for the
  lambda-transition of 4He by improved high-temperature expansion

abstract:
  We determine the critical exponents for the XY universality class in three
dimensions, which is expected to describe the $\lambda$-transition in ${}^4$He.
They are obtained from the analysis of high-temperature series computed for a
two-component $\lambda\phi^4$ model. The parameter $\lambda$ is fixed such that
the leading corrections to scaling vanish. We obtain $\nu = 0.67166(55)$,
$\gamma = 1.3179(11)$, $\alpha=-0.0150(17)$. These estimates improve previous
theoretical determinations and agree with the more precise experimental results
for liquid Helium.


title:
Influence of spin-flip scattering on the stability of ferromagnetism in
  a two-band Hubbard model

abstract:
  We investigate the influence of an interband exchange interaction on
magnetism in a two-band Hubbard model. Our main emphasis lies on spin-flip
scattering which is often neglected but is neccessary to retain the full
rotational symmetry of the Hamiltonian. We find a striking dependence of the
magnetization on the interband exchange coupling constant J and a substantial
suppression of ferromagnetic order for a large range of values of J. The onset
of an RKKY-like magnetic ordering mechanism is also observed.


title:
Strong-Coupling Calculation of Fluctuation Pressure of a Membrane
  Between Walls

abstract:
  We calculate analytically the proportionality constant in the pressure law of
a membrane between parallel walls from the strong-coupling limit of variational
perturbation theory up to third order. Extrapolating the zeroth to third
approximations to infinity yields the pressure constant alpha=0.0797149. This
result lies well within the error bounds of the most accurate available
Monte-Carlo result 0.0798 +- 0.0003.


title: Charge Density Wave Ratchet

abstract:
  We propose to operate a locally-gated charge density wave as an electron
pump. Applying an oscillating gate potential with frequency $f$ causes equally
spaced plateaux in the sliding charge density wave current separated by $\Delta
I=2eNf,$ where $N$ is the number of parallel chains. The effects of thermal
noise are investigated.


title:
Fermi Liquid Properties of a Two Dimensional Electron System With the
  Fermi Level Near a van Hove Singularity

abstract:
  We use a diagrammatic approach to study low energy physics of a two
dimensional electron system where the Fermi level is near van-Hove singularies
in the energy spectrum. We find that in most regions of the $\epsilon_F-T$
phase diagram the system behaves as a normal Fermi liquid rather than a
marginal Fermi liquid. Particularly, the imaginary part of the self energy is
much smaller than the excitation energy, which implies well defined
quasiparticle excitations, and single particle properties are only weakly
affected by the presence of the van-Hove singularities. The relevance to high
temperature superconductivity is also discussed.


title: How to determine Fermi vectors by angle resolved photoemission?

abstract:
  Angle resolved photoemission spectroscopy (ARPES) has been commonly applied
to evaluate the shape of Fermi surfaces by employing simple criteria for the
determination of the Fermi vector k_F parallel to the surface such as maximum
photoemission intensity at the Fermi level or discontinuity in the momentum
distribution function. Here we show that these criteria may lead to large
uncertainties in particular for narrow band systems. We develop a reliable
method for the determination of Fermi vectors employing high resolution ARPES
at different temperatures.
  The relevance and accuracy of the method is discussed on data of the quasi
two dimensional system TiTe_2.


title:
Further evidence of the absence of Replica Symmetry Breaking in Random
  Bond Potts Models

abstract:
  In this short note, we present supporting evidence for the replica symmetric
approach to the random bond q-state Potts models. The evidence is statistically
strong enough to reject the applicability of the Parisi replica symmetry
breaking scheme to this class of models. The test we use is a generalization of
one formerly proposed by Dotsenko et al. and consists in measuring scaling laws
of disordered-averaged moments of the spin-spin correlation functions.
Numerical results, obtained via Monte Carlo simulations for several values of
q, are shown to be in fair agreement with the replica symmetric values computed
by using perturbative CFT for the second and third moments of the q=3 model.
RSB effects, which should increase in strength with moment, are unobserved.


title:
Universal conductance fluctuations and low temperature 1/f noise in
  mesoscopic AuFe spin glasses

abstract:
  We report on intrinsic time-dependent conductance fluctuations observed in
mesoscopic AuFe spin glass wires. These dynamical fluctuations have a 1/f-like
spectrum and appear below the measured spin glass freezing temperature of our
samples. The dependence of the fluctuation amplitude on temperature, magnetic
field, voltage and Fe concentration allows a consistent interpretation in terms
of quantum interference effects which are sensitive to the slowly fluctuating
spin configuration.


title: Entropy Revisited: the Plausible Role of Gravitation

abstract:
  We first present open questions related to the foundations of thermodynamics
and statistical physics. We then argue that in principle one can not have
"closed systems", and that a universal background should exist. We propose that
the gravitational field plays this role, due to its vanishing energy-momentum
tensor. This leads to a new possible picture, in which entropy and
irreversibility in macroscopic systems emerge from their coupling to the
background gravitational field.


title: Bubbling and Large-Scale Structures in Avalanche Dynamics

abstract:
  Using a simple lattice model for granular media, we present a scenario of
self-organization that we term self-organized structuring where the steady
state has several unusual features: (1) large scale space and/or time
inhomogeneities and (2) the occurrence of a non-trivial peaked distribution of
large events which propagate like ``bubbles'' and have a well-defined frequency
of occurrence. We discuss the applicability of such a scenario for other models
introduced in the framework of self-organized criticality.


title:
Do correlations create an energy gap in electronic bilayers? Critical
  analysis of different approaches

abstract:
  This paper investigates the effect of correlations in electronic bilayers on
the longitudinal collective mode structure. We employ the dielectric
permeability constructed by means of the classical theory of moments. It is
shown that the neglection of damping processes overestimates the role of
correlations. We conclude that the correct account of damping processes leads
to an absence of an energy gap.


title:
Optical Susceptibilities of Polymers: Current-Current versus
  Dipole-Dipole Correlation

abstract:
  The static current operator leads to definitional zero frequency divergence
and unphysical results in studying nonlinear optical susceptibilities of
polymers. A well-defined dipole-dipole correlation is superior to the
complicated current-current correlation to solve this problem. As illustrative
examples, optical susceptibilities under both SSH and TLM models of
trans-(CH)_x are studied. New analytical results are obtained. The reasons of
previous improper results are analyzed.


title: The Dynamics of Structural Transitions in Sodium Chloride Clusters

abstract:
  In recent experiments on sodium chloride clusters structural transitions
between nanocrystals with different cuboidal shapes were detected. Here we
presents results for the thermodynamics and dynamics of one of these clusters,
(NaCl)35Cl-. As the time scales for the structural transitions can be much
longer than those accessible by conventional dynamics simulations, we use a
master equation to describe the probability flow within a large sample of
potential energy minima. We characterize the processes contributing to
probability flow between the different nanocrystals, and obtain rate constants
and activation energies for comparison with the experimental values.


title:
"Exhaustion" Physics in the Periodic Anderson Model using Iterated
  Perturbation Theory

abstract:
  We discuss the "exhaustion" problem in the context of the Periodic Anderson
Model using Iterated Perturbation Theory(IPT) within the Dynamical Mean Field
Theory. We find that, despite its limitations, IPT captures the exhaustion
physics, which manifests itself as a dramatic, strongly energy dependent
suppression of the effective Anderson impurity problem. As a consequence, low
energy scales in the lattice case are strongly suppressed compared to the
"Kondo scale" in the single-impurity picture. The IPT results are in
qualitative agreement with recent Quantum Monte Carlo results for the same
problem.


title: Interference of Bose-Einstein condensates in momentum space

abstract:
  We suggest an experiment to investigate the linear superposition of two
spatially separated Bose-Einstein condensates. Due to the coherent combination
of the two wave functions, the dynamic structure factor, measurable through
inelastic photon scattering at high momentum transfer $q$, is predicted to
exhibit interference fringes with frequency period $\Delta\nu = q/md$ where $d$
is the distance between the condensates. We show that the coherent
configuration corresponds to an eigenstate of the physical observable measured
in the experiment and that the relative phase of the condensates is hence
created through the measurement process.


title:
Activated mechanisms in amorphous silicon: an
  activation-relaxation-technique study

abstract:
  At low temperatures, dynamics in amorphous silicon occurs through a sequence
of discrete activated events that locally reorganize the topological network.
Using the activation-relaxation technique, a data base containing over 8000
such events is generated, and the events are analyzed with respect to their
energy barrier and asymmetry, displacement and volume expansion/contraction.
Special attention is paid to those events corresponding to diffusing
coordination defects. The energetics is not clearly correlated with the
displacement, nor with the defect density in well relaxed configurations. We
find however some correlation with the local volume expansion: it tends to
increase by about 4 eV/\AA$^3$. The topological properties of these events are
also studied; they show an unexpectedly rich diversity.


title: Noise-Induced Phase Separation: Mean-Field Results

abstract:
  We present a study of a phase-separation process induced by the presence of
spatially-correlated multiplicative noise. We develop a mean-field approach
suitable for conserved-order-parameter systems and use it to obtain the phase
diagram of the model. Mean-field results are compared with numerical
simulations of the complete model in two dimensions. Additionally, a comparison
between the noise-driven dynamics of conserved and nonconserved systems is made
at the level of the mean-field approximation.


title: Spectral correlations: understanding oscillatory contributions

abstract:
  We give a transparent derivation of a relation obtained using a
supersymmetric non-linear sigma model by Andreev and Altshuler [Phys. Rev.
Lett. 72, 902, (1995)], which connects smooth and oscillatory components of
spectral correlation functions. We show that their result is not specific to
random matrix theory. Also, we show that despite an apparent contradiction, the
results obtained using their formula are consistent with earlier perspectives
on random matrix models. In particular, the concept of resurgence is not
required.


title: Theory of Financial Risk: Basic notions in probability

abstract:
  Risk control has become one of the major concern of financial institutions.
The need for adequate statistical tools to measure and anticipate the amplitude
of the potential moves of financial markets is clearly expressed, in particular
for derivative markets. Classical theories, however, are based on simplified
assumptions -- such as Gaussian statistics -- and lead to a systematic (and
sometimes dramatic) underestimation of real risks.
  We summarize a few basic notions in probability theory which are useful in a
financial context: Statistics of Extremes, Sums of Random Variables, Central
Limit Theorems and Deviations, Correlations and Dependence, and a brief
introduction to Random Matrix Theory. This self-contained text corresponds to
the first chapter of our book, `Theory of Financial Risk'.


title:
Identification of the Chemical Potential of an Open System at 0 K with
  Functional Derivatives of the Integer-State Energy Density Functionals

abstract:
  Open-system density functional theory may be formulated in terms of ensemble
averages arising from interaction with a bath. The system is allowed to
exchange particles with the bath and the states in the ensemble average are
those corresponding to integer numbers of particles. The weights in the
ensemble average are typically equated with time-averaged values of the
occupation numbers of the various states comprising the open system. As a
result, there are two constraints on the occupation numbers: (1) Their sum must
be unity so that the ensemble average is a probability function and (2) The sum
of the occupation numbers times the number of particles for the associated
state must equal the time-averaged number of particles.
  By solving explicitly the first constraint we arrive at an expression for the
energy having a form structurally equivalent to a Gibbs thermodynamic function.
  From this form follows both chemical potential equalization and the
identification of the functional derivative of the energy for a particular
state with respect to its integer-state density as the chemical potential. We
highlight a distinction between functional derivatives of the time-averaged and
the integer-state energies with respect to integer-state densities. From this
we can restate the jump discontinuity behavior of the exchange-correlation
functional in terms of integer-state behavior.


title:
Spin transport properties of the quantum one-dimensional non-linear
  sigma model : an application to Haldane gap systems

abstract:
  Spin transport properties of the quantum one-dimensional non-linear sigma
model are studied based upon the Bethe ansatz exact solution for the O(3) sigma
model and the 1/N-expansion approach for the O(N) sigma model. It is shown that
the spin transport at finite temperatures is ballistic and there is no spin
diffusion in this system. A possible mechanism for the spin diffusion observed
in the NMR experiment of the Haldane gap system AgVP_2S_6 by Takigawa et al.
[Phys. Rev. Lett.{\bf 76}, 2173 (1996)] is also discussed. The inclusion of the
spin-phonon interaction accounts for the experimental results qualitatively.


title:
Magnetic behaviour of Eu_2CuSi_3: Large negative magnetoresistance above
  Curie temperature

abstract:
  We report here the results of magnetic susceptibility,
electrical-resistivity, magnetoresistance (MR), heat-capacity and ^{151}Eu
Mossbauer effect measurements on the compound, Eu_2CuSi_3, crystallizing in an
AlB_2-derived hexagonal structure. The results establish that Eu ions are
divalent, undergoing long-range ferromagnetic-ordering below (T_C=) 37 K. An
interesting observation is that the sign of MR is negative even at temperatures
close to 3T_C, with increasing magnitude with decreasing temperature exhibiting
a peak at T_C. This observation, being made for a Cu containing magnetic
rare-earth compound for the first time, is of relevance to the field of
collosal magnetoresistance.


title:
Spatial distribution of atoms in gas-covered Pd-X nanoparticles (X= Ag,
  Cu, Ni, Pt)

abstract:
  A Monte-carlo (MC) simulation procedure has been developed where the pair
bond energies are allowed to take into account the various coordination numbers
of surface atoms and the presence of adsorbates. The pair bond energies are
calculated from partial bond energies of atoms which, in turn, are calculated
from modified tight binding model in the second moment approximation. The model
has been applied to study the role of adsorption of hydrogen, oxygen, carbon
monoxide and nitric oxide on the surface composition and surface bond geometry
of bimetallic Pd-X (X = Ag, Cu, Ni, Pt) nanoparticles having fcc
cubo-octahedral geometry with 201, 586, 1289 and 2406 atoms. The results are
compared with the known experimental results. Importance of the results in
studying reactions on supported bimetallic catalysts has been highlighted.


title:
Born Effective Charge Reversal and Metallic Threshold State at a Band
  Insulator-Mott Insulator Transition

abstract:
  We study the quantum phase transition between a band (``ionic'') insulator
and a Mott-Hubbard insulator, realized at a critical value U=Uc in a bipartite
Hubbard model with two inequivalent sites, whose on-site energies differ by an
offset Delta. The study is carried out both in D=1 and D=2 (square and
honeycomb lattices), using exact Lanczos diagonalization, finite-size scaling,
and Berry's phase calculations of the polarization. The Born effective charge
jump from positive infinity to negative infinity previously discovered in D=1
by Resta and Sorella is confirmed to be directly connected with the transition
from the band insulator to the Mott insulating state, in agreement with recent
work of Ortiz et al.. In addition, symmetry is analysed, and the transition is
found to be associated with a reversal of inversion symmetry in the ground
state, of magnetic origin. We also study the D=1 excitation spectrum by Lanczos
diagonalization and finite-size scaling. Not only the spin gap closes at the
transition, consistent with the magnetic nature of the Mott state, but also the
charge gap closes, so that the intermediate state between the two insulators
appears to be metallic. This finding, rationalized within unrestricted
Hartree-Fock as due to a sign change of the effective on-site energy offset
Delta for the minority spin electrons, underlines the profound difference
between the two insulators. The band-to-Mott insulator transition is also
studied and found in the same model in D=2. There too we find an associated,
although weaker, polarization anomaly, with some differences between square and
honeycomb lattices. The honeycomb lattice, which does not possess an inversion
symmetry, is used to demonstrate the possibility of an inverted piezoelectric
effect in this kind of ionic Mott insulator.


title: Energetics of the oxidation and opening of a carbon nanotube

abstract:
  We apply first principles calculations to study the opening of single-wall
carbon nanotubes (SWNT's) by oxidation. We show that an oxygen rim can
stabilize the edge of the open tube. The sublimation of CO$_2$ molecules from
the rim with the subsequent closing of the tube changes from endothermic to
exothermic as the tube radius increases, within the range of experimental
feasible radii. We also obtain the energies for opening the tube at the cap and
at the wall, the latter being significantly less favorable.


title:
Excitation-assisted inelastic processes in trapped Bose-Einstein
  condensates

abstract:
  We find that inelastic collisional processes in Bose-Einstein condensates
induce local variations of the mean-field interparticle interaction and are
accompanied by the creation/annihilation of elementary excitation. The physical
picture is demonstrated for the case of three body recombination in a trapped
condensate. For a high trap barrier the production of high energy trapped
single particle excitations results in a strong increase of the loss rate of
atoms from the condensate.


title: Ground state of the hard-core Bose gas on lattice I. Energy estimates

abstract:
  We investigate the properties of the ground state of a system of interacting
bosons on regular lattices with coordination number $k\geq 2$. The interaction
is a pure, infinite, on-site repulsion. Our concern is to give an improved
upper bound on the ground state energy per site. For a density $\rho$ a trivial
upper bound is known to be $-k\rho(1-\rho)$. We obtain a smaller variational
bound within a reasonably large family of trial functions. The estimates make
use of a large deviation principle for the energy of the Ising model on the
same lattice.


title:
Efficient Real Space Solution of the Kohn-Sham Equations with Multiscale
  Techniques

abstract:
  We present a multigrid algorithm for self consistent solution of the
Kohn-Sham equations in real space. The entire problem is discretized on a real
space mesh with a high order finite difference representation. The resulting
self consistent equations are solved on a heirarchy of grids of increasing
resolution with a nonlinear Full Approximation Scheme, Full Multigrid
algorithm. The self consistency is effected by updates of the Poisson equation
and the exchange correlation potential at the end of each eigenfunction
correction cycle. The algorithm leads to highly efficient solution of the
equations, whereby the ground state electron distribution is obtained in only
two or three self consistency iterations on the finest scale.


title: Lyapunov Potential Description for Laser Dynamics

abstract:
  We describe the dynamical behavior of both class A and class B lasers in
terms of a Lyapunov potential. For class A lasers we use the potential to
analyze both deterministic and stochastic dynamics. In the stochastic case it
is found that the phase of the electric field drifts with time in the steady
state. For class B lasers, the potential obtained is valid in the absence of
noise. In this case, a general expression relating the period of the relaxation
oscillations to the potential is found. We have included in this expression the
terms corresponding to the gain saturation and the mean value of the
spontaneously emitted power, which were not considered previously. The validity
of this expression is also discussed and a semi-empirical relation giving the
period of the relaxation oscillations far from the stationary state is proposed
and checked against numerical simulations.


title:
NMR studies of the original magnetic properties of cuprates: effect of
  impurities and defects

abstract:
  Substitutional impurities in the CuO2 planes of the cuprates allow us to
probe the electronic properties of the host material. The pseudo-gap in the
underdoped regime is unmodified far from the impurities even though Tc is
greatly reduced. The spin polarisation induced by magnetic impurities has an
oscillatory behaviour reflecting the existing AF correlations between the Cu
spins. Its influence on the NMR spectra opens a way to determine the q
dependence of the static spin susceptibility and the T dependence of the AF
correlation length. NMR measurements demonstrate that non-magnetic impurities
such as Zn induce a local moment behaviour on the neighbouring Cu sites. This
magnetism revealed by spin-less sites can be understood on theoretical grounds
in the case of undoped quantum spin systems, while here the carriers greatly
complicate the situation. Susceptibility data show that the magnitude of the
local moment decreases with increasing hole doping. This experimental evidence
directly reflects the influence of AF correlations and the interference between
the carriers and the Cu hole spins in the cuprates. The anomalously large
scattering of the carriers on spinless defects is another indication of the
originality of the electronic properties of the cuprates, which apparently
extends even to the overdoped regime.


title:
Theory of superradiant scattering of laser light from Bose-Einstein
  condensates

abstract:
  In a recent MIT experiment, a new form of superradiant Rayleigh scattering
was observed in Bose-Einstein condensates. We present a detailed theory of this
phenomena in which the directional dependence of the scattering rate and
condensate depletion lead to mode competition which is ultimately responsible
for superradiance. The nonlinear response of the system is highly sensitive to
initial quantum fluctuations which cause large run to run variations in the
observed superradiant pulses.


title:
Testing quantum correlations in a confined atomic cloud by scattering
  fast atoms: Direct and time reversed processes

abstract:
  We suggest measuring the one-particle density matrix of a trapped ultracold
atomic cloud by scattering fast atoms in a pure momentum state off the cloud.
The lowest-order probability for the process, resulting in a pair of outcoming
fast atoms for each incoming one, as well as of its time reversed counterpart,
turns out to be given by the Fourier transform of the density matrix.
Accordingly, important information about quantum correlations can be deduced
directly from the differential scattering cross-section of these processes.
Several most interesting cases of scattering - from a single condensate
containing a vortex, and from a split condensate characterized by some phase
difference - are discussed.


title:
Hall Effect and Resistivity in High-Tc Superconductors: The Conserving
  Approximation

abstract:
  The Hall coefficient (R_H) of high-Tc cuprates in the normal state shows the
striking non-Fermi liquid behavior: R_H follows a Curie-Weiss type temperature
dependence, and |R_H|>>1/|ne| at low temperatures in the under-doped compounds.
Moreover, R_H is positive for hole-doped compounds and is negative for
electron-doped ones, although each of them has a similar hole-like Fermi
surface. In this paper, we give the explanation of this long-standing problem
from the standpoint of the nearly antiferromagnetic (AF) Fermi liquid. We
consider seriously the vertex corrections for the current which are
indispensable to satisfy the conservation laws, which are violated within the
conventional Boltzmann transport approximation. The obtained total current J_k
takes an enhanced value and is no more perpendicular to the Fermi surface due
to the strong AF fluctuations. By virtue of this mechanism, the anomalous
behavior of R_H in high-Tc cuprates is neutrally explained. We find that both
the temperature and the (electron, or hole) doping dependences of R_H in
high-T_c cuprates are reproduced well by numerical calculations based on the
fluctuation-exchange (FLEX) approximation, applied to the single-band Hubbard
model. We also discuss the temperature dependence of R_H in other nearly AF
metals, e.g., V_2O_3, kappa-BEDT-TTF organic superconductors, and heavy fermion
systems close to the AF phase boundary.


title:
Theory of Hall Effect and Electrical Transport in High-Tc Cuprates:
  Effects of Antiferromagnetic Spin Fluctuations

abstract:
  In the normal state of high-Tc cuprates, the Hall coefficient shows
remarkable temperature dependence, and its absolute value is enhanced in
comparison with that value simply estimated on the basis of band structure. It
has been recognized that this temperature dependence of the Hall coefficient is
due to highly anisotropic quasiparticle damping rate on the Fermi surface. In
this paper we further take account of the vertex correction to the current
vertex arising from quasiparticle interactions. Then the transport current is
transformed to a large extent from the quasiparticle velocity, and is no longer
proportional to the latter. As a consequence some pieces of the Fermi surface
outside of the antiferromagnetic Brillouin zone make negative contribution to
the Hall conductivity, even if the curvature of the Fermi surface is hole-like.
The Hall coefficient is much larger at low temperatures than the estimate made
without the vertex correction. Temperature dependence of the antiferromagnetic
spin correlation length is also crucial to cause remarkable temperature
dependence of the Hall coefficient. In our treatment the Hall coefficient of
the electron-doped cuprates can be negative despite hole-like curvature of the
Fermi surface.


title: Spectral Functions of 1D Hubbard Rings with Varying Boundary Conditions

abstract:
  We study the effect of varying the boundary condition on the spectral
function of a finite 1D Hubbard chain, which we compute using direct (Lanczos)
diagonalization of the Hamiltonian. By direct comparison with the two-body
response functions and with the exact solution of the Bethe Ansatz equations,
we can identify both spinon and holon features in the spectra. At half-filling
the spectra have the well-known structure of a low energy holon band and its
shadow---which span the whole Brillouin zone---and a spinon band present for
momenta less than the Fermi momentum. New features related to the twisted
boundary condition are cusps in the spinon band. We show that the spectral
building principle, adapted to account for both the finite system size and the
twisted boundary condition, describes the spectra well in terms of single
spinon and holon excitations. We argue that these finite-size effects are a
signature of spin-charge separation and that their study should help establish
the existence and nature of spin-charge separation in finite-size systems.


title: The transition temperature of the dilute interacting Bose gas

abstract:
  We show that the critical temperature of a uniform dilute Bose gas must
increase linearly with the s-wave scattering length describing the repulsion
between the particles. Because of infrared divergences, the magnitude of the
shift cannot be obtained from perturbation theory, even in the weak coupling
regime; rather, it is proportional to the size of the critical region in
momentum space. By means of a self-consistent calculation of the quasiparticle
spectrum at low momenta at the transition, we find an estimate of the effect in
reasonable agreement with numerical simulations.


title:
Ground State Entropy of Potts Antiferromagnets on Cyclic Polygon Chain
  Graphs

abstract:
  We present exact calculations of chromatic polynomials for families of cyclic
graphs consisting of linked polygons, where the polygons may be adjacent or
separated by a given number of bonds. From these we calculate the (exponential
of the) ground state entropy, $W$, for the q-state Potts model on these graphs
in the limit of infinitely many vertices. A number of properties are proved
concerning the continuous locus, ${\cal B}$, of nonanalyticities in $W$. Our
results provide further evidence for a general rule concerning the maximal
region in the complex q plane to which one can analytically continue from the
physical interval where $S_0 > 0$.


title:
Electromigration-Induced Flow of Islands and Voids on the Cu(001)
  Surface

abstract:
  Electromigration-induced flow of islands and voids on the Cu(001) surface is
studied at the atomic scale. The basic drift mechanisms are identified using a
complete set of energy barriers for adatom hopping on the Cu(001) surface,
combined with kinetic Monte Carlo simulations. The energy barriers are
calculated by the embedded atom method, and parameterized using a simple model.
The dependence of the flow on the temperature, the size of the clusters, and
the strength of the applied field is obtained. For both islands and voids it is
found that edge diffusion is the dominant mass-transport mechanism. The rate
limiting steps are identified. For both islands and voids they involve
detachment of atoms from corners into the adjacent edge. The energy barriers
for these moves are found to be in good agreement with the activation energy
for island/void drift obtained from Arrhenius analysis of the simulation
results. The relevance of the results to other FCC(001) metal surfaces and
their experimental implications are discussed.


title:
Theory of Non-linear Susceptibility and Correlation Length in Glasses
  and Liquids

abstract:
  Within the framework of the effective potential theory of the structural
glass transition, we calculate for the p-spin model a static nonlinear
susceptibility related to a four-point density correlation function, and show
that it grows and diverges in mean field with exponent $\gamma=1/2$ as the mode
coupling critical temperature T_c is approached from below. When T_c is
approached from above, we calculate within the mode coupling framework a
dynamic nonlinear susceptibility and show that there is a characteristic time
where the susceptibility is a maximum, and that this time grows with decreasing
T. We find that this susceptibility diverges as T_c is approached from above,
and has key features in common with the ``displacement-displacement
susceptibility'' recently introduced to measure correlated particle motion in
simulations of glass-forming liquids.


title:
Renormalization Group Transformations under strong mixing conditions:
  gibbsianess and convergence of renormalized interactions

abstract:
  In this paper we study a renormalization-group map: the block averaging
transformation applied to Gibbs measures relative to a class of finite range
lattice gases, when suitable strong mixing conditions are satisfied. Using
block decimation procedure, cluster expansion (like in [HK]) and detailed
comparison between statistical ensembles, we are able to prove Gibbsianess and
convergence to a trivial (i.e. Gaussian and product) fixed point. Our results
apply to 2D standard Ising model at any temperature above the critical one and
arbitrary magnetic field.


title:
Field induced phase segregation and collective excitations of a trapped
  spinor Bose-Einstein condensate

abstract:
  A hydrodynamic description is used to study the zero-temperature properties
of a trapped spinor Bose-Einstein condensate in the presence of a uniform
magnetic field. We show that, in the case of antiferromagnetic spin-spin
interaction, the polar and ferromagnetic configurations of the ground state can
coexist in the trap. These two phases are spatially segregated in such a way
that the polar state occupies the inner part while the ferromagnetic state
occupies the outer part of the atomic cloud. We also derive a set of coupled
hydrodynamic equations for the number density and spin density excitations of
the system. It is shown that these equations can be analytically solved for the
system in an isotropic harmonic trap and a constant magnetic field. Remarkably,
the related low lying excitation spectra are completely determined by the
solutions in the region occupied by the polar state. We find that, within the
Thomas-Fermi approximation, the presence of a constant magnetic field does not
change the excitation spectra which still possess the similar form of that
obtained by Stringari.


title: The SO(5) Theory as a Critical Dynamical Symmetry

abstract:
  We use generalized coherent states to analyze the SO(5) theory of
high-temperature superconductivity and antiferromagnetism. We demonstrate that
the SO(5) symmetry can be embedded in a larger algebra that allows it to be
interpreted as a critical dynamical symmetry interpolating between
antiferromagnetic and superconducting phases. This dynamical interpretation
suggests that SO(5) defines a phase with the character of a spin-glass for a
significant range of doping.


title: Magnetoelastic mechanism of spin-reorientation transitions at step-edges

abstract:
  The symmetry-induced magnetic anisotropy due to monoatomic steps at strained
Ni films is determined using results of first - principles relativistic
full-potential linearized augmented plane wave (FLAPW) calculations and an
analogy with the N\'eel model. We show that there is a magnetoelastic
anisotropy contribution to the uniaxial magnetic anisotropy energy in the
vicinal plane of a stepped surface. In addition to the known spin-direction
reorientation transition at a flat Ni/Cu(001) surface, we propose a
spin-direction reorientation transition in the vicinal plane for a stepped
Ni/Cu surface due to the magnetoelastic anisotropy. We show that with an
increase of Ni film thickness, the magnetization in the vicinal plane turns
perpendicular to the step edge at a critical thickness calculated to be in the
range of 16-24 Ni layers for the Ni/Cu(1,1,13) stepped surface.


title: Chaos in neural networks with a nonmonotonic transfer function

abstract:
  Time evolution of diluted neural networks with a nonmonotonic transfer
function is analitically described by flow equations for macroscopic variables.
The macroscopic dynamics shows a rich variety of behaviours: fixed-point,
periodicity and chaos. We examine in detail the structure of the strange
attractor and in particular we study the main features of the stable and
unstable manifolds, the hyperbolicity of the attractor and the existence of
homoclinic intersections. We also discuss the problem of the robustness of the
chaos and we prove that in the present model chaotic behaviour is fragile
(chaotic regions are densely intercalated with periodicity windows), according
to a recently discussed conjecture. Finally we perform an analysis of the
microscopic behaviour and in particular we examine the occurrence of damage
spreading by studying the time evolution of two almost identical initial
configurations. We show that for any choice of the parameters the two initial
states remain microscopically distinct.


title: Impurity-Induced Bound Excitations on the Surface of Bi2Sr2CaCu2O8

abstract:
  We have probed the effects of atomic-scale impurities on superconductivity in
Bi_{2}Sr_{2}CaCu_{2}O_{8} by performing low-temperature tunneling spectroscopy
measurements with a scanning tunneling microscope. Our results show that
non-magnetic defect structures at the surface create localized low-energy
excitations in their immediate vicinity. The impurity-induced excitations occur
over a range of energies including the middle of the superconducting gap, at
the Fermi level. Such a zero bias state is a predicted feature for strong
non-magnetic scattering in a d-wave superconductor.


title: Ensemble Dependence of the Transient Fluctuation Theorem

abstract:
  The Fluctuation Theorem gives an analytical expression for the probability of
observing second law violating dynamical fluctuations, in nonequilibrium
systems. At equilibrium statistical mechanical fluctuations are known to be
ensemble dependent. In this paper we generalise the Transient and Steady State
Fluctuation Theorems to various nonequilibrium dynamical ensembles. The
Transient and Steady State Fluctuation Theorem for an isokinetic ensemble of
isokinetic trajectories is tested using nonequilibrium molecular dynamics
simulations of shear flow.


title:
Two-dimensional behavior of the sublattice magnetization in
  three-dimensional Ising antiferromagnets

abstract:
  A three-dimensional layered Ising-Antiferromagnet with a ferromagnetic
intra-layer coupling to z neighbors, zJ > 0, and an antiferromagnetic
interlayer coupling to z' neighbors, z'J' < 0, is investigated by Monte Carlo
simulations on a hexagonal lattice. The physical nature of the anomalous
temperature bahavior of the sublattice magnetizations, which is found for
certain values of r=zJ/z'J' and z' in magnetic fields is explained in terms of
successive phase transitions. They take place on the ferromagnetic
2-dimensional spin-down sublattice at T = T_c^{2d}, smeared by a finite
stabilizing molecular field, and on both antiferromagnetically coupled
sublattices at T_c^{3d} > T_c^{2d}.


title:
One-Dimensional Electron Liquid in an Antiferromagnetic Environment:
  Spin Gap from Magnetic Correlations

abstract:
  We study a one-dimensional electron liquid coupled by a weak spin-exchange
interaction to an antiferromagnetic spin-S ladder with n legs. A perturbative
renormalization group analysis in the semiclassical limit reveals the opening
of a spin gap, driven by the local magnetic correlations on the ladder. The
effect, which we argue is present for any gapful ladder or gapless ladder with
$nS\gg 1$, is enhanced by the repulsive interaction among the conduction
electrons but is insensitive to the sign of the spin exchange interaction with
the ladder. Possible implications for the striped phases of the cuprates are
discussed.


title: Metallic xenon. Polarizability. Equation of state

abstract:
  It is shown that some of the physical properties of inert gas (IG)
condensates (polarizability, compressibility, metallization under pressure,
equation of state) may be well described taking into account the first excited
state of atoms only. The Herzfeld criterion of metallization well corresponds
to the Mott transition criterion and to the percolation threshold. For metallic
xenon concentration of the molecular type excitations corresponds to the Bose
condensation temperature $T_C\sim 4000K$. The BCS formula gives $T_c\sim
5000K$. If phonons are changed by fluctuations of interatomic interaction
energy. A simple relations between the parameters of atoms at metallization has
been found.


title: Persistent Currents in Quantum Chaotic Systems

abstract:
  The persistent current of ballistic chaotic billiards is considered with the
help of the Gutzwiller trace formula. We derive the semiclassical formula of a
typical persistent current $I^{typ}$ for a single billiard and an average
persistent current $<I>$ for an ensemble of billiards at finite temperature.
These formulas are used to show that the persistent current for chaotic
billiards is much smaller than that for integrable ones. The persistent
currents in the ballistic regime therefore become an experimental tool to
search for the quantum signature of classical chaotic and regular dynamics.


title:
Current dependence of grain boundary magnetoresistance in
  La_0.67Ca_0.33MnO_3 films

abstract:
  We prepared epitaxial ferromagnetic manganite films on bicrystal substrates
by pulsed laser ablation. Their low- and high-field magnetoresistance (MR) was
measured as a function of magnetic field, temperature and current. At low
temperatures hysteretic changes in resistivity up to 70% due to switching of
magnetic domains at the coercitive field are observed. The strongly non-ohmic
behavior of the current-voltage leads to a complete suppression of the MR
effect at high bias currents with the identical current dependence at low and
high magnetic fields. We discuss the data in view of tunneling and mesoscale
magnetic transport models and propose an explicit dependence of the spin
polarization on the applied current in the grain boundary region.


title: Charge density wave and quantum fluctuations in a molecular crystal

abstract:
  We consider an electron-phonon system in two and three dimensions on square,
hexagonal and cubic lattices. The model is a modification of the standard
Holstein model where the optical branch is appropriately curved in order to
have a reflection positive Hamiltonian. Using infrared bounds together with a
recent result on the coexistence of long-range order for electron and phonon
fields, we prove that, at sufficiently low temperatures and sufficiently strong
electron-phonon coupling, there is a Peierls instability towards a period two
charge-density wave at half-filling. Our results take into account the quantum
fluctuations of the elastic field in a rigorous way and are therefore
independent of any adiabatic approximation. The strong coupling and low
temperature regime found here is independent of the strength of the quantum
fluctuations of the elastic field.


title: Quasi-particles for quantum Hall edges

abstract:
  We discuss a quasi-particle formulation of effective edge theories for the
fractional quantum Hall effect. Fundamental quasi-particles for the Laughlin
state with filling fraction \nu =1/3 are edge electrons of charge -e and edge
quasi-holes of charge +e/3. These quasi-particles satisfy exclusion statistics
in the sense of Haldane. We exploit algebraic properties of edge electrons to
derive a kinetic equation for charge transport between a \nu=1/3 fractional
quantum Hall edge and a normal metal.


title:
Interplay between orbital ordering and lattice distortions in LaMnO3,
  YVO3, and YTiO3

abstract:
  We have studied the interplay between orbital ordering, Jahn-Teller and
GdFeO3-type lattice distortions in perovskite-type transition-metal oxides
using model Hartree-Fock calculations. It has been found that the covalency
between A-site cations and oxygens causes interaction between the Jahn-Teller
and GdFeO3-type distortions. The present calculations explain why the d-type
Jahn-Teller distortion and orbital ordering compatible with it are realized in
LaMnO3, YVO3 and YTiO3


title: Error-Correcting Codes That Nearly Saturate Shannon's Bound

abstract:
  Gallager-type error-correcting codes that nearly saturate Shannon's bound are
constructed using insight gained from mapping the problem onto that of an Ising
spin system. The performance of the suggested codes is evaluated for different
code rates in both finite and infinite message length.


title:
Positive and negative Hanbury-Brown and Twiss correlations in normal
  metal-superconducting devices

abstract:
  In the light of the recent analogs of the Hanbury--Brown and Twiss
experiments in mesoscopic beam splitters, negative current noise correlations
are recalled to be the consequence of an exclusion principle. Here, positive
(bosonic) correlations are shown to exist in a fermionic system, composed of a
superconductor connected to two normal reservoirs. In the Andreev regime, the
correlations can either be positive or negative depending on the reflection
coefficient of the beam splitter. For biases beyond the gap, the transmission
of quasiparticles favors fermionic correlations. The presence of disorder
enhances positive noise correlations. Potential experimental applications are
discussed.


title: Incipient Spanning Clusters in Square and Cubic Percolation

abstract:
  The analysis of extensive numerical data for the percolation probabilities of
incipient spanning clusters in two dimensional percolation at criticality are
presented. We developed an effective code for the single-scan version of the
Hoshen-Kopelman algorithm. We measured the probabilities on the square lattice
forming samples of rectangular strips with widths from 8 to 256 sites and
lengths up to 3200 sites. At total of more than $10^{15}$ random numbers are
generated for the sampling procedure. Our data confirm the proposed exact
formulaes for the probability exponents conjectured recently on the base of 2D
conformal field theory. Some preliminary results for 3D percolation are also
discussed.


title: Configuration space in electron glasses

abstract:
  We study numerically the configuration space at low energy of electron
glasses. We consider systems with Coulomb interactions, short-range
interactions and no interactions. First, we calculate the integrated density of
configurations as a function of energy. At a given energy, this density is
smaller for Coulomb glasses than for short-range systems, which in turn is
smaller than for non-interacting systems. We analyze how the site occupancy
varies with the number of configurations. Through this study we estimate the
number of particles involved in a typical low-energy transition between
configurations. This number increases with system size for long range
interactions, while it is basically constant for a short-range interaction.
Finally we calculate the density of metastable configurations, i.e. valleys,
classified according to their degree of stability.


title:
Zero frequency divergence and gauge phase factor in the optical response
  theory

abstract:
  The static current-current correlation leads to the definitional zero
frequency divergence (ZFD) in the optical susceptibilities. Previous
computations have shown nonequivalent results between two gauges (${\bf p\cdot
A}$ and ${\bf E \cdot r}$) under the exact same unperturbed wave functions. We
reveal that those problems are caused by the improper treatment of the
time-dependent gauge phase factor in the optical response theory. The gauge
phase factor, which is conventionally ignored by the theory, is important in
solving ZFD and obtaining the equivalent results between these two gauges. The
Hamiltonians with these two gauges are not necessary equivalent unless the
gauge phase factor is properly considered in the wavefunctions. Both
Su-Shrieffer-Heeger (SSH) and Takayama-Lin-Liu-Maki (TLM) models of
trans-polyacetylene serve as our illustrative examples to study the linear
susceptibility $\chi^{(1)}$ through both current-current and dipole-dipole
correlations. Previous improper results of the $\chi^{(1)}$ calculations and
distribution functions with both gauges are discussed. The importance of gauge
phase factor to solve the ZFD problem is emphasized based on SSH and TLM
models. As a conclusion, the reason why dipole-dipole correlation favors over
current-current correlation in the practical computations is explained.


title:
Analytical solutions to the third-harmonic generation in
  trans-polyacetylene: Application of dipole-dipole correlation on the single
  electron models

abstract:
  The analytical solutions for the third-harmonic generation (THG) on infinite
chains in both Su-Shrieffer-Heeger (SSH) and Takayama-Lin-Liu-Maki (TLM) models
of trans-polyacetylene are obtained through the scheme of dipole-dipole ($DD$)
correlation. They are not equivalent to the results obtained through static
current-current ($J_0J_0$) correlation or under polarization operator
$\hat{P}$. The van Hove singularity disappears exactly in the analytical forms,
showing that the experimentally observed two-photon absorption peak (TPA) in
THG may not be directly explained by the single electron models.


title: Algorithmic Complexity in Minority Game

abstract:
  In this paper we introduce a new approach for the study of the complex
behavior of Minority Game using the tools of algorithmic complexity, physical
entropy and information theory. We show that physical complexity and mutual
information function strongly depend on memory size of the agents and yields
more information about the complex features of the stream of binary outcomes of
the game than volatility itself.

categories: cond-mat.stat-mech adap-org chao-dyn cs.CC nlin.AO nlin.CD


title:
On mesoscopic forces and quantized conductance in model metallic
  nanowires

abstract:
  Energetics and conductance in jellium modelled nanowires are investigated
using the local-density-functional-based shell correction method. In analogy
with studies of other finite-size fermion systems, e.g., simple-metal clusters
or He-3 clusters, we find that the energetics of the wire as a function of its
radius (transverse reduced dimension) leads to formation of self-selecting
magic wire configurations (MWC's, i.e., discrete sequence of wire radii with
enhanced stability), originating from quantization of the electronic spectrum,
namely formation of subbands which are the analogs of electronic shells in
clusters. These variations in the energy result in oscillations in the force
required to affect a transition from one MWC of the nanowire to another, and
are correlated directly with step-wise variations of the quantized conductance
of the nanowire in units of 2*e^2/h.


title: The one-dimensional Bose-Hubbard Model with nearest-neighbor interaction

abstract:
  We study the one-dimensional Bose-Hubbard model using the Density-Matrix
Renormalization Group (DMRG).For the cases of on-site interactions and
additional nearest-neighbor interactions the phase boundaries of the
Mott-insulators and charge density wave phases are determined. We find a direct
phase transition between the charge density wave phase and the superfluid
phase, and no supersolid or normal phases. In the presence of nearest-neighbor
interaction the charge density wave phase is completely surrounded by a region
in which the effective interactions in the superfluid phase are repulsive. It
is known from Luttinger liquid theory that a single impurity causes the system
to be insulating if the effective interactions are repulsive, and that an even
bigger region of the superfluid phase is driven into a Bose-glass phase by any
finite quenched disorder. We determine the boundaries of both regions in the
phase diagram. The ac-conductivity in the superfluid phase in the attractive
and the repulsive region is calculated, and a big superfluid stiffness is found
in the attractive as well as the repulsive region.


title:
Responses of a Hodgkin-Huxley Neuron to Various Types of Spike-Train
  Inputs

abstract:
  Numerical investigations have been made of responses of a Hodgkin-Huxley (HH)
neuron to spike-train inputs whose interspike interval (ISI) is modulated by
deterministic, semi-deterministic (chaotic) and stochastic signals. As
deterministic one, we adopt inputs with the time-independent ISI and with
time-dependent ISI modulated by sinusoidal signal. The R\"{o}ssler and Lorentz
models are adopted for chaotic modulations of ISI. Stochastic ISI inputs with
the Gamma distribution are employed. It is shown that distribution of output
ISI data depends not only on the mean of ISIs of spike-train inputs but also on
their fluctuations. The distinction of responses to the three kinds of inputs
can be made by return maps of input and output ISIs, but not by their
histograms. The relation between the variations of input and output ISIs is
shown to be different from that of the integrate and fire (IF) model because of
the refractory period in the HH neuron.