title: Mesoscopic Cooperative Emission From a Disordered System

abstract:
  We study theoretically the cooperative light emission from a system of $N\gg
1$ classical oscillators confined within a volume with spatial scale, $L$, much
smaller than the radiation wavelength, $\lambda_0=2\pi c/\omega_0$. We assume
that the oscillators frequencies are randomly distributed around a central
frequency, $\omega_0$, with some characteristic width, $\Omega\ll\omega_0$. In
the absence of disorder, that is $\Omega=0$, the cooperative emission spectrum
is composed of a narrow subradiant peak superimposed on a wide superradiant
band. When $\Omega\neq 0$, we demonstrate that if $N$ is large enough, the
subradiant peak is not simply broadened by the disorder but rather splits into
a system of random narrow peaks. We estimate the spectral width of these peaks
as a function of $N, L, \Omega$, and $\lambda_0$. We also estimate the
amplitude of this mesoscopic structure in the emission spectrum.


title:
Interlayer Transport of Quasiparticles and Cooper pairs in
  Bi2Sr2CaCu2O8+y Superconductors

abstract:
  We study the c-axis transport of stacked, intrinsic junctions in
Bi2Sr2CaCu2O8+y single crystals, fabricated by the double-sided ion beam
processing technique from single crystal whiskers. Measurements of the I-V
characteristics of these samples allow us to obtain the temperature and voltage
dependence of the quasiparticle c-axis conductivity in the superconducting
state, the Josephson critical current, and the superconducting gap. We show
that the BCS d-wave model in the clean limit for resonant impurity scattering
with a significant contribution from coherent interlayer tunneling, describes
satisfactorily the low temperature and low energy c-axis transport of both
quasiparticles and Cooper pairs.


title:
On the global bifurcation diagram for the one-dimensional Ginzburg
  Landau model of superconductivity

abstract:
  Some new global results are given about solutions to the boundary value
problem for the Euler-Lagrange equations for the Ginzburg-Landau model of a
one-dimensional superconductor. The main advance is a proof that in some
parameter range there is a branch of asymmetric solutions connecting the branch
of symmetric solutions to the normal state. Also, simplified proofs are
presented for some local bifurcation results of Bolley and Helffer. These
proofs require no detailed asymptotics for the solutions of the linear
equations. Finally, an error in Odeh's work on this problem is discussed.


title: Charge Density Wave Ordering in Half Filled High Landau Levels

abstract:
  We report on numerical studies of two-dimensional electron systems in the
presence of a perpendicular magnetic field, with a high Landau level index (N >
1) half filled by electrons. Strong and sharp peaks are found in the wave
vector dependence of both static density susceptibility and equal-time
density-density correlation function, in finite-size systems with up to twelve
electrons. Qualitatively different from partially filled lowest (N=0) Landau
level, these results are suggestive of a tendency toward charge density wave
ordering in these systems. The ordering wave vector is found to decrease with
increasing N.


title:
Beyond Wavelets: Exactness theorems and algorithms for physical
  calculations

abstract:
  This paper develops the use of wavelets as a basis set for the solution of
physical problems exhibiting behavior over wide-ranges in length scale. In a
simple diagrammatic language, this article reviews both the mathematical
underpinnings of wavelet theory and the algorithms behind the fast wavelet
transform. This article underscores the fact that traditional wavelet bases are
fundamentally ill-suited for physical calculations and shows how to go beyond
these limitations by the introduction of the new concept of semicardinality,
which leads to the profound, new result that basic physical couplings may be
computed {\em without approximatation} from very sparse information, thereby
overcoming the limitations of traditional wavelet bases in the treatment of
physical problems. The paper then explores the convergence rate of conjugate
gradient solution of the Poisson equation in both semicardinal and lifted
wavelet bases and shows the first solution of the Kohn-Sham equations using a
novel variational principle.


title: Apparent finite-size effects in the dynamics of supercooled liquids

abstract:
  Molecular dynamics simulations are performed for a supercooled simple liquid
with changing the system size from N=108 to $10^4$ to examine possible
finite-size effects. Although almost no systematic deviation is detected in the
static pair correlation functions, it is demonstrated that the structural
$\alpha$ relaxation in a small system becomes considerably slower than that in
larger systems for temperatures below $T_c$ at which the size of the
cooperative particle motions becomes comparable to the unit cell length of the
small system. The discrepancy increases with decreasing temperature.


title:
Doping Dependence of the Electronic Structure of Ba_{1-x}K_{x}BiO_{3}
  Studied by X-Ray Absorption Spectroscopy

abstract:
  We have performed x-ray absorption spectroscopy (XAS) and x-ray photoemission
spectroscopy (XPS) studies of single crystal Ba_{1-x}K_{x}BiO_{3} (BKBO)
covering the whole composition range $0 \leq x \leq 0.60$. Several features in
the oxygen 1\textit{s} core XAS spectra show systematic changes with $x$.
Spectral weight around the absorption threshold increases with hole doping and
shows a finite jump between $x=0.30$ and 0.40, which signals the
metal-insulator transition. We have compared the obtained results with
band-structure calculations. Comparison with the XAS results of
BaPb_{1-x}Bi_{x}O_{3} has revealed quite different doping dependences between
BKBO and BPBO. We have also observed systematic core-level shifts in the XPS
spectra as well as in the XAS threshold as functions of $x$, which can be
attributed to a chemical potential shift accompanying the hole doping. The
observed chemical potential shift is found to be slower than that predicted by
the rigid band model based on the band-structure calculations.


title:
Theory of spiral wave dynamics in weakly excitable media: asymptotic
  reduction to a kinematic model and applications

abstract:
  In a weakly excitable medium, characterized by a large threshold stimulus,
the free end of an isolated broken plane wave (wave tip) can either rotate
(steadily or unsteadily) around a large excitable core, thereby producing a
spiral pattern, or retract causing the wave to vanish at boundaries. An
asymptotic analysis of spiral motion and retraction is carried out in this
weakly excitable large core regime starting from the free-boundary limit of the
reaction-diffusion models, valid when the excited region is delimited by a thin
interface. The wave description is shown to naturally split between the tip
region and a far region that are smoothly matched on an intermediate scale.
This separation allows us to rigorously derive an equation of motion for the
wave tip, with the large scale motion of the spiral wavefront slaved to the
tip. This kinematic description provides both a physical picture and exact
predictions for a wide range of wave behavior, including: (i) steady rotation
(frequency and core radius), (ii) exact treatment of the meandering instability
in the free-boundary limit with the prediction that the frequency of unstable
motion is half the primary steady frequency (iii) drift under external actions
(external field with application to axisymmetric scroll ring motion in
three-dimensions, and spatial or/and time-dependent variation of excitability),
and (iv) the dynamics of multi-armed spiral waves with the new prediction that
steadily rotating waves with two or more arms are linearly unstable. Numerical
simulations of FitzHug-Nagumo kinetics are used to test several aspects of our
results. In addition, we discuss the semi-quantitative extension of this theory
to finite cores and pinpoint mathematical subtleties related to the thin
interface limit of singly diffusive reaction-diffusion models.

categories: cond-mat.soft cond-mat.stat-mech nlin.PS patt-sol


title: Quasicrystals. The Technique for Constructing Quasiperiodic Structures

abstract:
  In this paper, a technique for constructing quasiperiodic structures is
suggested, which allows one by the assigned matching to restore the atoms
density distribution formula of a corresponding quasicrystal. The algorithm to
restore the atom density distribution has been considered on the example of the
Penrose matching. The analytical record of a Penrose quasicrystal is given.


title:
Discrete random walk models for symmetric Levy-Feller diffusion
  processes

abstract:
  We propose a variety of models of random walk, discrete in space and time,
suitable for simulating stable random variables of arbitrary index $\alpha$
($0< \alpha \le 2$), in the symmetric case. We show that by properly scaled
transition to vanishing space and time steps our random walk models converge to
the corresponding continuous Markovian stochastic processes, that we refer to
as Levy-Feller diffusion processes.


title:
Unusually wide plateau of quantized Hall resistance in a quasi bilayer
  hole system inside the p-GeSi / Ge / p-GeSi quantum well

abstract:
  An unusually wide plateau in the quantized Hall resistance has been revealed
for a MQW heterostructure of wide p-GeSi / Ge / p-GeSi quantum wells with the
Fermi energy comparable to the well bottom bending amplitude. This plateau
exists in one of two metastable states of the sample, for which a symmetric
quasi-double-quantum-well system is formed inside the Ge layer, and corresponds
to the filling factor nu = 1 for each of two sublayers in the Ge layer. The
plateau exists not only within a magnetic field range related to the
quantum-Hall liquid, but extends beyond it into a so-called quantized Hall
insulator phase. For the other metastable state only a weak plateau is observed
in the Hall resistivity, corresponding to nu = 1 per Ge layer as a whole.
According to the existing theories, the extra wide plateau may be indicative of
a kind of disorder in a conducting layer, characterized by a uniform
distribution of puddles in their carrier density and by their small average
size. We attribute the differences between these two states to the existence of
two metastable modes in the self consistent potential profile that settle
spontaneously in the multilayer system, which are characterized by different
distributions of holes in the quantum well cross-section.


title:
The magnetic susceptibility of disordered non-diffusive mesoscopic
  systems

abstract:
  Disorder-induced spectral correlations of mesoscopic quantum systems in the
non-diffusive regime and their effect on the magnetic susceptibility are
studied. We perform impurity averaging for non-translational invariant systems
by combining a diagrammatic perturbative approach with semiclassical
techniques. This allows us to study the entire range from clean to diffusive
systems. As an application we consider the magnetic response of non-interacting
electrons in microstructures in the presence of weak disorder. We show that in
the ballistic case (elastic mean free path $\ell$ larger than the system size)
there exist two distinct regimes of behaviour depending on the relative
magnitudes of $\ell$ and an inelastic scattering length $L_{\phi}$. We present
numerical results for square billiards and derive approximate analytical
results for generic chaotic geometries. The magnetic field dependence and
$L_{\phi}$ dependence of the disorder-induced susceptibility is qualitatively
similar in both types of geometry.


title: Generation of short and long range temporal correlated noises

abstract:
  We present the implementation of an algorithm to generate Gaussian random
noises with prescribed time correlations that can be either long or short
ranged. Examples of Langevin dynamics with short and long range noises are
presented and discussed.


title: Dilational Viscosity of Langmuir monolayers

abstract:
  The dilational viscosity $\epsilon'$ of Langmuir monolayer is considered in a
theoretical model taking into account an orientational effect of the dilational
wave on surface molecules. This orientational order is described by the surface
order parameter Q; the orientational part of the free surface energy is given
by Landau's expansion in powers of Q. The magnitude of surface viscosity,
driven by the surface tension derivative $\partial\gamma/\partial Q$, is in
good accord with the experimentally observed $\epsilon'$. The sign of
$\epsilon'$ is positive that indicates that increased in-plane ordering
increases the surface tension.


title: Fluctuations and correlations in sandpile models

abstract:
  We perform numerical simulations of the sandpile model for non-vanishing
driving fields $h$ and dissipation rates $\epsilon$. Unlike simulations
performed in the slow driving limit, the unique time scale present in our
system allows us to measure unambiguously response and correlation functions.
We discuss the dynamic scaling of the model and show that
fluctuation-dissipation relations are not obeyed in this system.


title: Multifractal scaling in the Bak-Tang-Wiesenfeld Sandpile and edge events

abstract:
  An analysis of moments and spectra shows that, while the distribution of
avalanche areas obeys finite size scaling, that of toppling numbers is
universally characterized by a full, nonlinear multifractal spectrum. Rare,
large avalanches dissipating at the border influence the statistics very
sensibly. Only once they are excluded from the sample, the conditional toppling
distribution for given area simplifies enough to show also a well defined,
multifractal scaling. The resulting picture brings to light unsuspected, novel
physics in the model.


title:
Field induced $d_{x^2-y^2}+id_{xy}$ state and marginal stability of
  high-Tc superconductors

abstract:
  It is shown that the {\em complex} $d_{xy}$ component is generated in d-wave
superconductor in the magnetic field. As one enters superconducting state at
finite field the normal to superconducting transition occurs into bulk
$d_{x^2-y^2}+i d_{xy}$ state . The driving force for the transition is the
linear coupling between magnetic field and non zero magnetization of the
$d_{x^2-y^2}+i d_{xy}$ condensate. The external magnetic field violates parity
and time reversal symmetries and the nodal quasiparticle states respond by
generating the $id_{xy}$ component of the order parameter, with the magnitude
estimated to be on the order of few Kelvin. Parity (P) and time reversal (T)
symmetries are violated in this state.


title: Melting of the Electron Glass

abstract:
  A model of spinless interacting electrons in presence of randomness is
examined using an extended dynamical mean-field formulation. When the
interaction strength is large as compared to the Fermi energy, a low
temperature glassy phase is identified, which in our formulation corresponds to
a replica-symmetry breaking instability. The glassy phase is characterized by a
pseudo-gap in the single particle density of states, reminiscent of the Coulomb
gap of Efros and Shklovskii. Due to ergodicity breaking, the ``zero-field
cooled'' compressibility of this electron glass vanishes at T=0, consistent
with absence of screening. When the Fermi energy exceeds a critical value, the
glassy phase is suppressed, and normal metallic behavior is recovered.


title:
The Dynamical Cluster Approximation: Non-Local Dynamics of Correlated
  Electron Systems

abstract:
  We recently introduced the dynamical cluster approximation(DCA), a new
technique that includes short-ranged dynamical correlations in addition to the
local dynamics of the dynamical mean field approximation while preserving
causality. The technique is based on an iterative self-consistency scheme on a
finite size periodic cluster. The dynamical mean field approximation (exact
result) is obtained by taking the cluster to a single site (the thermodynamic
limit). Here, we provide details of our method, explicitly show that it is
causal, systematic, $\Phi$-derivable, and that it becomes conserving as the
cluster size increases. We demonstrate the DCA by applying it to a Quantum
Monte Carlo and Exact Enumeration study of the two-dimensional Falicov-Kimball
model. The resulting spectral functions preserve causality, and the spectra and
the CDW transition temperature converge quickly and systematically to the
thermodynamic limit as the cluster size increases.


title: Extended integrability regime for the supersymmetric U model

abstract:
  An extension of the supersymmetric U model for correlated elctrons is given
and integrability is established by demonstrating that the model can be
constructed through the Quantum Inverse Scattering Method using an R-matrix
without the difference property. Some general symmetry properties of the model
are discussed and from the Bethe ansatz solution an expression for the energies
is presented.


title: Binding Transition in Quantum Hall Edge States

abstract:
  We study a class of Abelian quantum Hall (QH) states which are topologically
unstable (T-unstable). We find that the T-unstable QH states can have a phase
transition on the edge which causes a binding between electrons and reduces the
number of gapless edge branches. After the binding transition, the
single-electron tunneling into the edge gains a finite energy gap, and only
certain multi-electron co-tunneling (such as three-electron co-tunneling for
$\nu=9/5$ edges) can be gapless. Similar phenomenon also appear for edge state
on the boundary between certain QH states. For example edge on the boundary
between $\nu=2$ and $\nu=1/5$ states only allow three-electron co-tunneling at
low energies after the binding transition.


title: Population Dynamics and Non-Hermitian Localization

abstract:
  We review localization with non-Hermitian time evolution as applied to simple
models of population biology with spatially varying growth profiles and
convection. Convection leads to a constant imaginary vector potential in the
Schroedinger-like operator which appears in linearized growth models. We
illustrate the basic ideas by reviewing how convection affects the evolution of
a population influenced by a simple square well growth profile. Results from
discrete lattice growth models in both one and two dimensions are presented. A
set of similarity transformations which lead to exact results for the spectrum
and winding numbers of eigenfunctions for random growth rates in one dimension
is described in detail. We discuss the influence of boundary conditions, and
argue that periodic boundary conditions lead to results which are in fact
typical of a broad class of growth problems with convection.


title: Microscopic Motion of Particles Flowing through a Porous Medium

abstract:
  We use Stokesian Dynamics simulations to study the microscopic motion of
particles suspended in fluids passing through porous media. We construct model
porous media with fixed spherical particles, and allow mobile ones to move
through this fixed bed under the action of an ambient velocity field. We first
consider the pore scale motion of individual suspended particles at pore
junctions. The relative particle flux into different possible directions
exiting from a single pore, for two and three dimensional model porous media is
found to approximately equal the corresponding fractional channel width or
area. Next we consider the waiting time distribution for particles which are
delayed in a junction, due to a stagnation point caused by a flow bifurcation.
The waiting times are found to be controlled by two-particle interactions, and
the distributions take the same form in model porous media as in two-particle
systems. A simple theoretical estimate of the waiting time is consistent with
the simulations. We also find that perturbing such a slow-moving particle by
another nearby one leads to rather complicated behavior. We study the stability
of geometrically trapped particles. For simple model traps, we find that
particles passing nearby can ``relaunch'' the trapped particle through its
hydrodynamic interaction, although the conditions for relaunching depend
sensitively on the details of the trap and its surroundings.

categories: cond-mat.dis-nn comp-gas cond-mat.stat-mech nlin.CG


title: Exclusion statistics: A resolution of the problem of negative weights

abstract:
  We give a formulation of the single particle occupation probabilities for a
system of identical particles obeying fractional exclusion statistics of
Haldane. We first derive a set of constraints using an exactly solvable model
which describes an ideal exclusion statistics system and deduce the general
counting rules for occupancy of states obeyed by these particles. We show that
the problem of negative probabilities may be avoided with these new counting
rules.


title:
Relation between the transmission rates and the wave functions in the
  carbon nanotube junctions

abstract:
  Electron transmission and wave functions through junctions with a pair of a
pentagonal defect and a heptagonal defect connecting two metallic carbon
nanotubes are analyzed by the analytical calculation with the effective mass
equation. The energy region $|E| < E_c$ is considered where the channel number
is kept to two. Close relation between the transmission rate and the wave
function is found; the transmission rate is given by the inverse squared
absolute value of the wave function. The dependence of the transmission rates
on the energy and on the size of the junction is clearly explained by the
nature of the wave function. Though the wave function and the transmission rate
calculated by the tight binding model agree well with the corresponding
analytical results by the effective mass approximation, the discrepancy becomes
considerable when $|E| \simeq E_c$. To study the origin of this discrepancy, an
efficient numerical calculation method is developed with a generalized transfer
matrix for the tight binding model. Their numerical results are compared with
the corresponding analytical ones and the results show that the origin of the
discrepancy comes from the evanescent waves with the longest decay length in
the tube parts.


title:
Coherent dynamics of localized spins coupled with a two-dimensional hole
  gas in diluted-magnetic quantum wells

abstract:
  Spin dynamics in a quantum well made of diluted-magnetic semiconductor is
studied theoretically. The exchange interaction of the ensemble of localized
spins with two-dimensional heavy-hole gas is shown to affect time evolution of
the spin system in an in-plane magnetic field. The Larmor frequency of
localized spins is reduced under influence of an oscillating effective field of
holes.


title:
Low energy magnetic excitations of the Mn_{12}-acetate spin cluster
  observed by neutron scattering

abstract:
  We performed high resolution diffraction and inelastic neutron scattering
measurements of Mn_{12}-acetate. Using a very high energy resolution, we could
separate the energy levels corresponding to the splitting of the lowest S
multiplet. Data were analyzed within a single spin model (S=10 ground state),
using a spin Hamiltonian with parameters up to 4^{th} order.
  The non regular spacing of the transition energies unambiguously shows the
presence of high order terms in the anisotropy (D= -0.457(2) cm^{-1}, B_4^0 =
-2.33(4) 10^{-5}cm^{-1}).
  The relative intensity of the lowest energy peaks is very sensitive to the
small transverse term, supposed to be mainly responsible for quantum tunneling.
  This allows an accurate determination of this term in zero magnetic field
(B_4^4 = \pm 3.0(5) 10^{-5} cm^{-1}). The neutron results are discussed in view
of recent experiments and theories.


abstract:
  Force fluctuations in granular materials are investigated. A continuum
equation is derived starting from a discrete model proposed in the literature.
The influence of boundary conditions is investigated. For periodic boundary
conditions the average weight is found to increase linearly with depth while it
saturates to a constant value for absorbing boundary conditions, which models
the existence of walls. The scale dependencies of the saturation weight, the
saturation depth and the average squared fluctuations are obtained. The
analytical results are compared with previous works and with numerical
simulations in one dimension.


title: Discrete instability in nonlinear lattices

abstract:
  The discrete multiscale analysis for boundary value problems in nonlinear
discrete systems leads to a first order discrete modulational instability above
a threshold amplitude for wave numbers beyond the zero of group velocity
dispersion. Applied to the electrical lattice [Phys. Rev. E, 51 (1995) 6127 ],
this acurately explains the experimental instability at wave numbers beyond
1.25 . The theory is also briefly discussed for sine-Gordon and Toda lattices.


title: Interlayer Josephson vortices in the high-T_c superconducting cuprates

abstract:
  We comment on the work by Moler, et al., [Science 279, 1193 (1998)]
concerning determination of the c-axis penetration depth in the single-layer
compound Tl-2201 through measuring the magnetic flux due to isolated interlayer
Josephson vortices.


title:
Reply to: --- Interlayer Josephson vortices in the high-T_c
  superconducting cuprates

abstract: Here we reply to Farid's comment.


title: Layer dynamics of a freely standing smectic-A film

abstract:
  We study the hydrodynamics of a freely-standing smectic-A film in the
isothermal, incompressible limit theoretically by analyzing the linearized
hydrodynamic equations of motion with proper boundary conditions. The dynamic
properties for the system can be obtained from the response functions for the
free surfaces. Permeation is included and its importance near the free surfaces
is discussed. The hydrodynamic mode structure for the dynamics of the system is
compared with that of bulk systems. We show that to describe the dynamic
correlation functions for the system, in general, it is necessary to consider
the smectic layer displacement $u$ and the velocity normal to the layers,
$v_z$, together. Finally, our analysis also provides a basis for the
theoretical study of the off-equilibrium dynamics of freely-standing smectic-A
films.


title: On the electron-energy loss spectra and plasmon resonance in cuprates

abstract:
  The consequences of the non-Drude charge response in the normal state of
cuprates and the effect of the layered structure on electron-energy loss
spectra are investigated, both for experiments in the transmission and the
reflection mode. It is shown that in the intermediate doping regime the plasmon
resonance has to be nearly critically damped as a result of the anomalous
frequency dependence of the relaxation rate. This also implies an unusual
low-energy dependence of the loss function. Both facts are consistent with
experiments in cuprates. Our study based on the t-J model shows good agreement
with measured plasmon frequencies.


title:
Critical finite-size scaling with constraints: Fisher renormalization
  revisited

abstract:
  The influence of a thermodynamic constraint on the critical finite-size
scaling behavior of three-dimensional Ising and XY models is analyzed by
Monte-Carlo simulations. Within the Ising universality class constraints lead
to Fisher renormalized critical exponents, which modify the asymptotic form of
the scaling arguments of the universal finite-size scaling functions. Within
the XY universality class constraints lead to very slowly decaying corrections
inside the scaling arguments, which are governed by the specific heat exponent
alpha. If the modification of the scaling arguments is properly taken into
account in the scaling analysis of the data, finite-size scaling functions are
obtained, which are independent of the constraint as anticipated by analytic
theory.


title:
Binary data corruption due to a Brownian agent II: two dimensions,
  competing agents, and generalized couplings

abstract:
  This work is a continuation of our previous investigation of binary data
corruption due to a Brownian agent [T. J. Newman and W. Triampo, preprint
cond-mat/9811237]. We extend our study in three main directions which allow us
to make closer contact with real bistable systems. These are i) a detailed
analysis of two dimensions, ii) the case of competing agents, and iii) the
cases of asymmetric and quenched random couplings. Most of our results are
obtained by extending our original phenomenological model, and are supported by
extensive numerical simulations.


title:
Two new topologically ordered glass phases of smectics confined in
  anisotropic random media

abstract:
  We show that smectic liquid crystals confined in_anisotropic_ porous
structures such as e.g.,_strained_ aerogel or aerosil exhibit two new glassy
phases. The strain both ensures the stability of these phases and determines
their nature. One type of strain induces an ``XY Bragg glass'', while the other
creates a novel, triaxially anisotropic ``m=1 Bragg glass''. The latter
exhibits anomalous elasticity, characterized by exponents that we calculate to
high precision. We predict the phase diagram for the system, and numerous other
experimental observables.


title:
Anomalies of the infrared-active phonons in underdoped YBCO as an
  evidence for the intra-bilayer Josephson effect

abstract:
  The spectra of the far-infrared c-axis conductivity of underdoped YBCO
crystals exhibit dramatic changes of some of the phonon peaks when going from
the normal to the superconducting state. We show that the most striking of
these anomalies can be naturally explained by changes of the local fields
acting on the ions arising from the onset of inter- and intra-bilayer Josephson
effects.


title: Novel Phenomena in Dilute Electron Systems in Two Dimensions

abstract:
  We review recent experiments that provide evidence for a transition to a
conducting phase in two dimensions at very low electron densities. The nature
of this phase is not understood, and is currently the focus of intense
theoretical and experimental attention.


title:
Kinetic theory for dissipative particle dynamics: the importance of
  collisions

abstract:
  Kinetic theory of dissipative particle dynamics is developed in terms of a
Boltzmann pair collision theory. The kinetic transport coefficients are
computed from explicit collision integrals and compared favourably with
detailed simulations. Previous theory is found to correspond to a weak
scattering limit, or Vlasov theory, and previously reported discrepancies with
simulations are thereby resolved. In the large dissipation limit, we find
qualitatively new scaling properties for the transport coefficients.


title:
Dual Vortex Theory of Strongly Interacting Electrons: Non-Fermi Liquid
  to the (Hard) Core

abstract:
  As discovered in the quantum Hall effect, a very effective way for
strongly-repulsive electrons to minimize their potential energy is to aquire
non-zero relative angular momentum. We pursue this mechanism for interacting
two-dimensional electrons in zero magnetic field, by employing a representation
of the electrons as composite bosons interacting with a Chern-Simons gauge
field. This enables us to construct a dual description in which the fundamental
constituents are vortices in the auxiliary boson fields. The resulting
formalism embraces a cornucopia of possible phases. Remarkably,
superconductivity is a generic feature, while the Fermi liquid is not --
prompting us to conjecture that such a state may not be possible when the
interactions are sufficiently strong. Many aspects of our earlier discussions
of the nodal liquid and spin-charge separation find surprising incarnations in
this new framework.


title:
The role of imaginary vector potential in composite fermion pairing
  theory

abstract:
  We show that the imaginary vector potential causes a pair-breaking effect in
the composite fermion theory, and discuss its irrelevance in the pairing state.
The Hamiltonian for pairing states of composite fermions is proposed. The
advantage of non-unitary transformations and the meaning of the non-Hermitian
term are discussed.


title: Noncollinear cluster magnetism in the framework of the Hubbard model

abstract:
  Noncollinear magnetic states in clusters are studied by using the single-band
Hubbard Hamiltonian. The unrestricted Hartree-Fock (UHF) approximation is
considered without imposing any symmetry constraints neither to the size or
orientation of the local magnetic moments $<\vec S_l>$ nor to the local charge
densities $<n_l>$. A variety of qualitatively different selfconsistent
solutions is obtained as a function of cluster size, structure, number of
valence electrons $\nu$ and Coulomb interaction strength $U/t$. This includes
inhomogeneous density distributions, paramagnetic solutions, magnetic solutions
with collinear moments and noncollinear spin arrangements that show complex
antiferromagnetic and ferromagnetic-like orders. The environment dependence of
the magnetic properties is analyzed giving emphasis to the effects of
antiferromagnetic frustrations in compact structures close to half-band
filling. Electron correlation effects are quantified by comparing UHF and exact
results for the local magnetic moments, total spin, spin-correlation functions
and structural stability of 13-atom clusters. Goals and limitations of the
present noncollinear approach are discussed.


title:
Enumeration of the self-avoiding polygons on a lattice by the
  Schwinger-Dyson equations

abstract:
  We show how to compute the generating function of the self-avoiding polygons
on a lattice by using the statistical mechanics Schwinger-Dyson equations for
the correlation functions of the $N$-vector spin model on that lattice.


title:
An augmented space recursion study of the electronic structure of rough
  epitaxial overlayers

abstract:
  In this communication we propose the use of the Augmented Space Recursion as
an ideal methodology for the study of electronic and magnetic structures of
rough surfaces, interfaces and overlayers. The method can take into account
roughness, short-ranged clustering effects, surface dilatation and
interdiffusion. We illustrate our method by an application of Fe overlayer on
Ag (100) surface.


title:
Towards an Explanation of the Mesoscopic Double-Slit Experiment: a new
  model for charging of a Quantum Dot

abstract:
  For a quantum dot (QD) in the intermediate regime between integrable and
fully chaotic, the widths of single-particle levels naturally differ by orders
of magnitude. In particular, the width of one strongly coupled level may be
larger than the spacing between other, very narrow, levels. In this case many
consecutive Coulomb blockade peaks are due to occupation of the same broad
level. Between the peaks the electron jumps from this level to one of the
narrow levels and the transmission through the dot at the next resonance
essentially repeats that at the previous one. This offers a natural explanation
to the recently observed behavior of the transmission phase in an
interferometer with a QD.


title: On the Exact Solution of Models based on Non-Standard Representations

abstract:
  The algebraic Bethe ansatz is a powerful method to diagonalize
transfer-matrices of statistical models derived from solutions of (graded) Yang
Baxter equations, connected to fundamental representations of Lie
(super-)algebras and their quantum deformations respectively. It is, however,
very difficult to apply it to models based on higher dimensional
representations of these algebras in auxiliary space, which are not of fusion
type. A systematic approach to this problem is presented here. It is
illustrated by the diagonalization of a transfer-matrix of a model based on the
product of two different four-dimensional representations of
$U_{q}(\hat{gl}'(2,1;C))$.


title:
Classical nucleation theory for the nucleation of the solid phase of
  spherical particles with a short-ranged attraction

abstract:
  Classical nucleation theory is used to estimate the free-energy barrier to
nucleation of the solid phase of particles interacting via a potential which
has a short-ranged attraction. Due to the high interfacial tension between the
fluid and solid phases, this barrier is very large, much larger than in hard
spheres. It is divergent in the limit that the range of the attraction tends to
zero. We predict an upper limit on nucleation in good agreement with the
results of experiments on the crystallisation of proteins.


title: Field dependence of the temperature at the peak of the ZFC magnetization

abstract:
  The effect of an applied magnetic field on the temperature at the maximum of
the ZFC magnetization, $M_{ZFC}$, is studied using the recently obtained
analytic results of Coffey et al. (Phys. Rev. Lett. {\bf 80}(1998) 5655) for
the prefactor of the N\'{e}el relaxation time which allow one to precisely
calculate the prefactor in the N\'{e}el-Brown model and thus the blocking
temperature as a function of the coefficients of the Taylor series expansion of
the magnetocrystalline anisotropy. The present calculations indicate that even
a precise determination of the prefactor in the N\'{e}el-Brown theory, which
always predicts a monotonic decrease of the relaxation time with increasing
field, is insufficient to explain the effect of an applied magnetic field on
the temperature at the maximum of the ZFC magnetization. On the other hand, we
find that the non linear field-dependence of the magnetization along with the
magnetocrystalline anisotropy appears to be of crucial importance to the
existence of this maximum.


title:
Surface Reconstructions and Bonding via the Electron Localization
  Function: The Case of Si(001)

abstract:
  The bonding pattern of a covalent semiconductor is disrupted when a surface
is cut while keeping a rigid (truncated bulk) geometry. The covalent bonds are
partly reformed (with a sizeable energy gain) when reconstruction is allowed.
We show that the ``electron localization function'' (ELF)---applied within a
first--principles pseudopotential framework---provides un unprecedented insight
into the bonding mechanisms. In the unreconstructed surface one detects a
partly metallic character, which disappears upon reconstruction. In the surface
reformed bonds, the ELF sharply visualizes strongly paired electrons, similar
in character to those of the bulk bonds.


title:
Phenomenological Model of the Nonlinear Microwave Response of a
  Superconductor Containing Weak Links

abstract:
  A phenomenological model is proposed which describes the effect of both dc
and rf magnetic fields, $H$, on the microwave surface impedance, Z_s=R_s+jX_s,
of a superconductor containing weak link. Two cases are considered; a weak link
between two grains, shunted by another grain, and a non-shunted weak link. In
both cases the dependences of R_s and X_s on applied field H were found to be
anomalous. Under certain conditions, the values of Z_s(H) can fall below the
zero-field values. Comparison with experiment is performed, and very good
qualitative and (in some cases) quantitative agreement is found.


title:
Evolution of the Potential Energy Surface with Size for Lennard-Jones
  Clusters

abstract:
  Disconnectivity graphs are used to characterize the potential energy surfaces
of Lennard-Jones clusters containing 13, 19, 31, 38, 55 and 75 atoms. This set
includes members which exhibit either one or two `funnels' whose low-energy
regions may be dominated by a single deep minimum or contain a number of
competing structures. The graphs evolve in size due to these specific size
effects and an exponential increase in the number of local minima with the
number of atoms. To combat the vast number of minima we investigate the use of
monotonic sequence basins as the fundamental topographical unit. Finally, we
examine disconnectivity graphs for a transformed energy landscape to explain
why the transformation provides a useful approach to the global optimization
problem.


title:
Unconventional MBE Strategies from Computer Simulations for Optimized
  Growth Conditions

abstract:
  We investigate the influence of step edge diffusion (SED) and desorption on
Molecular Beam Epitaxy (MBE) using kinetic Monte-Carlo simulations of the
solid-on-solid (SOS) model. Based on these investigations we propose two
strategies to optimize MBE growth. The strategies are applicable in different
growth regimes: During layer-by-layer growth one can exploit the presence of
desorption in order to achieve smooth surfaces. By additional short high flux
pulses of particles one can increase the growth rate and assist layer-by-layer
growth. If, however, mounds are formed (non-layer-by-layer growth) the SED can
be used to control size and shape of the three-dimensional structures. By
controlled reduction of the flux with time we achieve a fast coarsening
together with smooth step edges.


title:
Modelling the Nonlinear High-Frequency Response of a Short Josephson
  Junction under Two-Frequency Irradiation

abstract:
  The nonlinear response of a short Josephson Junction (JJ), being irradiated
simultaneously with two high-frequency signals, has been studied in the
framework of the nonlinear Resistively-Shunted Junction (RSJ) Model. One of the
signals, hereafter referred to as "probe signal", has a small amplitude
$I_{pr}<I_c$ ($I_c$ is the critical current of the JJ) and frequency $f_{pr}$,
and is used to monitor the response of the junction to the other high-power
signal with amplitude $I_{pm}$ and frequency $f_{pm}$, hereafter referred to as
"pump signal". Varying the frequency ratio $f_{pm}/f_{pr}$ from 0.5 to 100, and
the current amplitude of the probe signal from 0.01 to 0.9 of $I_c$, we found
that the dependence of the junction impedance at the frequency $f_{pr}$,
$Z_s^{f_{pr}}$, versus $I_{pm}$ preserves its general features, independent of
$f_{pm}/f_{pr}$ and $I_{pr}/I_c$ values. At the same time, some particular
features, like negative values of $Re(Z_s^{f_{pr}})$ and "fine" structure of
the steps in $Z_s^{f_{pr}}(I_{pm})$ are observed for $f_{pm}/f_{pr}<1$ and for
particular values of $I_{pr}/I_c$. In general, the behavior of
$Z_s^{f_{pr}}(I_{pm})$ is rather different from that predicted by the nonlinear
RSJ model for a short JJ in the regime of single-frequency irradiation, when
one and the same signal plays the roles of the pump and the probe signals
simultaneously. Possible applications of the model are briefly discussed.


title: The Force Exerted by a Molecular Motor

abstract:
  The stochastic driving force exerted by a single molecular motor (e.g., a
kinesin, or myosin) moving on a periodic molecular track (microtubule, actin
filament, etc.) is discussed from a general viewpoint open to experimental
test. An elementary "barometric" relation for the driving force is introduced
that (i) applies to a range of kinetic and stochastic models, (ii) is
consistent with more elaborate expressions entailing explicit representations
of externally applied loads and, (iii) sufficiently close to thermal
equilibrium, satisfies an Einstein-type relation in terms of the velocity and
diffusion coefficient of the (load-free) motor. Even in the simplest two-state
models, the velocity-vs.-load plots exhibit a variety of contrasting shapes
(including nonmonotonic behavior). Previously suggested bounds on the driving
force are shown to be inapplicable in general by analyzing discrete jump models
with waiting time distributions.


title:
Non-linear Microwave Surface Impedance of Epitaxial HTS Thin Films in
  Low DC Magnetic Fields

abstract:
  We have carried out non-linear microwave (8 GHz) surface impedance
measurements of three YBaCuO thin films in dc magnetic fields $H_{dc}$
(parallel to c axis) up to 12 mT using a coplanar resonator technique. In zero
dc field the three films, deposited by the same method, show a spread of
low-power residual surface resistance, $R_{res}$ and penetration depth,
$\lambda$ (T=15 K) within a factor of 1.9. However, they exhibit dramatically
different microwave field, $H_{rf}$ dependences of the surface resistance,
$R_s$, but universal $X_s(H_{rf})$ dependence. Application of a dc field was
found to affect not only absolute values of $R_s$ and $X_s$, but the functional
dependences $R_s(H_{rf})$ and $X_s(H_{rf})$ as well. For some of the samples
the dc field was found to decrease $R_s$ below its zero-field low-power value.


title: A Hierarchical Approach to Protein Molecular Evolution

abstract:
  Biological diversity has evolved despite the essentially infinite complexity
of protein sequence space. We present a hierarchical approach to the efficient
searching of this space and quantify the evolutionary potential of our approach
with Monte Carlo simulations. These simulations demonstrate that non-homologous
juxtaposition of encoded structure is the rate-limiting step in the production
of new tertiary protein folds. Non-homologous ``swapping'' of low energy
secondary structures increased the binding constant of a simulated protein by
$\approx10^7$ relative to base substitution alone. Applications of our approach
include the generation of new protein folds and modeling the molecular
evolution of disease.

categories: cond-mat.stat-mech cond-mat.soft physics.bio-ph q-bio


title: Crystal Field Triplets: A New Route to Non-Fermi Liquid Physics

abstract:
  A model for crystal field triplet ground states on rare earth or actinide
ions with dipolar and quadrupolar couplings to conduction electrons is studied
for the first time with renormalization group methods. The quadrupolar coupling
leads to a new nontrivial, non-Fermi liquid fixed point, which survives in an
intermediate valence Anderson model. The calculated magnetic susceptibility
displays one parameter scaling, going as $T^{-\alpha}$ ($\alpha \approx 0.4$)
at intermediate temperatures, reminiscent of the non-Fermi liquid alloy
UCu_{5-x}Pd_x.


title: Magnetism Controlled Vortex Matter

abstract:
  We discuss a new class of phenomena based on strong interaction between
magnetic superstructures and vortices in superconductors in combined
heterogeneous structures. An inhomogeneous magnetization can pin vortices or
create them spontaneously changing drastically properties of the
superconductor. On the other hand, the interaction between magnetic moments
mediated by vortices can result in specific types of magnetic ordering. The
same interaction can create coupled magnetic-superconducting defects. We
discuss possible experimental observation of magnetism controlled vortex matter
in superconducting films with magnetic nanoscale dots or stripes and layered
systems with alternating superconducting and magnetic layers.


title:
Real-space local polynomial basis for solid-state electronic-structure
  calculations: A finite-element approach

abstract:
  We present an approach to solid-state electronic-structure calculations based
on the finite-element method. In this method, the basis functions are strictly
local, piecewise polynomials. Because the basis is composed of polynomials, the
method is completely general and its convergence can be controlled
systematically. Because the basis functions are strictly local in real space,
the method allows for variable resolution in real space; produces sparse,
structured matrices, enabling the effective use of iterative solution methods;
and is well suited to parallel implementation. The method thus combines the
significant advantages of both real-space-grid and basis-oriented approaches
and so promises to be particularly well suited for large, accurate ab initio
calculations. We develop the theory of our approach in detail, discuss
advantages and disadvantages, and report initial results, including the first
fully three-dimensional electronic band structures calculated by the method.


title: Comparison of superconductivity in Sr_2RuO_4 and copper oxides

abstract:
  To compare the superconductivity in strongly correlated electron systems with
the antiferromagnetic fluctuations in the copper oxides and with the
ferromagnetic fluctuations in Sr_2RuO_4 a t-J-I model is proposed. The
antiferromagnetic coupling J results in the superconducting state of
d_{x^2-y^2} symmetry and the ferromagnetic coupling constant I results in the
spin-triplet p-type state. The difference in the gap anisotropies provides the
large difference in T_c values, for the typical values of the coupling
constants: T_c of order of 1K for the ruthenate and T_c of order of 100K for
the cuprates.


title: Universality and Crossover of Directed Polymers and Growing Surfaces

abstract:
  We study KPZ surfaces on Euclidean lattices and directed polymers on
hierarchical lattices subject to different distributions of disorder, showing
that universality holds, at odds with recent results on Euclidean lattices.
Moreover, we find the presence of a slow (power-law) crossover toward the
universal values of the exponents and verify that the exponent governing such
crossover is universal too. In the limit of a 1+epsilon dimensional system we
obtain both numerically and analytically that the crossover exponent is 1/2.


title:
Associative memory storing an extensive number of patterns based on a
  network of oscillators with distributed natural frequencies in the presence
  of external white noise

abstract:
  We study associative memory based on temporal coding in which successful
retrieval is realized as an entrainment in a network of simple phase
oscillators with distributed natural frequencies under the influence of white
noise. The memory patterns are assumed to be given by uniformly distributed
random numbers on $[0,2\pi)$ so that the patterns encode the phase differences
of the oscillators. To derive the macroscopic order parameter equations for the
network with an extensive number of stored patterns, we introduce the effective
transfer function by assuming the fixed-point equation of the form of the TAP
equation, which describes the time-averaged output as a function of the
effective time-averaged local field. Properties of the networks associated with
synchronization phenomena for a discrete symmetric natural frequency
distribution with three frequency components are studied based on the order
parameter equations, and are shown to be in good agreement with the results of
numerical simulations. Two types of retrieval states are found to occur with
respect to the degree of synchronization, when the size of the width of the
natural frequency distribution is changed.


title: Remnant Fermi Surfaces in Photoemission

abstract:
  Recent experiments have introduced a new concept for analyzing the
photoemission spectra of correlated electrons -- the remnant Fermi surface
(rFs), which can be measured even in systems which lack a conventional Fermi
surface. Here, we analyze the rFs in a number of interacting electron models,
and find that the results fall into two classes. For systems with pairing
instabilities, the rFs is an accurate replica of the true Fermi surface. In the
presence of nesting instabilities, the rFs is a map of the resulting
superlattice Brillouin zone. The results suggest that the gap in Ca_2CuO_2Cl_2
is of nesting origin.


title: Broken Symmetry Ground States in \nu=2 Bilayer Quantum Hall Systems

abstract:
  We report on a study of a bilayer two-dimensional electron gas at Landau
level filling factor $\nu=2$. The system exhibits both magnetic and spontaneous
interlayer phase coherence broken symmetries. We propose a 3-parameter Slater
determinant variational wavefunction which describes the ground state over the
full range of bias potential ($\Delta_V$) Zeeman coupling ($\Delta_z$) and
interlayer tunneling amplitude ($\Delta_t$) strengths. Broken symmetry states
occur inside a volume in this three-dimensional phase diagram near the
$\Delta_z^2=\Delta_V^2+\Delta_t^2$ surface. We have obtained analytic results
for the intersections of the phase diagram with the $\Delta_t=0$, $\Delta_z=0$,
and $\Delta_V=0$, planes and show that the differential capacitance of the
bilayer system is singular at the phase boundary.


title:
Self-Organised Optimality in Driven Systems with Symmetrical
  Interactions

abstract:
  Extremal principles are fundamental in our interpretation of phenomena in
nature. One of the best known examples is the second law of thermodynamics,
governing most physical and chemical systems and stating the continuous
increase of entropy in closed systems. Biological and social systems, however,
are usually open and characterised by self-organised structures. Being results
of an evolutionary optimisation process, one may conjecture that such systems
use resources like energy very efficiently, but there is no proof for this.
Recent results on driven systems indicate that systems composed of competing
entities tend to reach a state of self-organised optimality associated with
minimal interaction or minimal dissipation, respectively. Using concepts from
non-equilibrium thermodynamics and game theoretical ideas, we will show that
this is universal to an even wider class of systems which, generally speaking,
have the ability to reach a state of maximal overall ``success''. This
principle is expected to be relevant for driven systems in physics, but its
main significance concerns biological and social systems, for which only a
limited number of quantitative principles are available yet.


title: Quantum inelastic conductance through molecular wires

abstract:
  We calculate non-perturbatively the inelastic effects on the conductance
through a conjugated molecular wire-metal heterojunction, including realistic
electron-phonon coupling. We show that at sub-band-gap energies the current is
dominated by quantum coherent transport of virtual polarons through the
molecule. In this regime, the tunneling current is strongly increased relative
to the case of elastic scattering. It is essential to describe the full quantum
coherence of the polaron formation and transport in order to obtain correct
physics. Our results are generally applicable to one-dimensional atomic or
molecular wires.


title: Predicting Financial Crashes Using Discrete Scale Invariance

abstract:
  We present a synthesis of all the available empirical evidence in the light
of recent theoretical developments for the existence of characteristic
log-periodic signatures of growing bubbles in a variety of markets including 8
unrelated crashes from 1929 to 1998 on stock markets as diverse as the US,
Hong-Kong or the Russian market and on currencies. To our knowledge, no major
financial crash preceded by an extended bubble has occurred in the past 2
decades without exhibiting such log-periodic signatures.


title: Optical Investigations of Charge Gap in Orbital Ordered La1/2Sr3/2MnO4

abstract:
  Temperature and polarization dependent electronic structure of La1/2Sr3/2MnO4
were investigated by optical conductivity analyses. With decreasing
temperature, for E//ab, a broad mid-infrared (MIR) peak of La1/2Sr3/2MnO4
becomes narrower and moves to the higher frequency, while that of
Nd1/2Sr3/2MnO4 nearly temperature independent. We showed that the MIR peak in
La1/2Sr3/2MnO4 originates from orbital ordering associated with CE-type
magnetic ordering and that the Jahn-Teller distortion has a significant
influence on the width and the position of the MIR peak.


title: Comment on ``Small-world networks: Evidence for a crossover picture''

abstract:
  In a recent letter (cond-mat/9903108), Barthelemy and Nunes Amaral discuss
the crossover phenomenon between regular and ``small-world'' networks, as a
function of the network size $n$ and of the disorder $p$. They claim that the
average distance $\ell$ between vertices of the network scales with $n / n^*$,
with $n^*(p \ll 1) sim p^{-\tau}$ and $\tau \approx 2/3$. We show analytically
that $\tau$ cannot be lower than 1 and perform numerical simulations showing
that $\tau = 1$.


title: Do cylinders exhibit a cubatic phase?

abstract:
  We investigate the possibility that freely rotating cylinders with an aspect
ratio $L/D=0.9$ exhibit a cubatic phase similar to the one found for a system
of cut-spheres. We present theoretical arguments why a cubatic phase might
occur in this particular system. Monte Carlo simulations do not confirm the
existence of a cubatic phase for cylinders. However, they do reveal an
unexpected phase behavior between the isotropic and crystalline phase.


title: New Magnetically Ordered State in the Electron Plasma of Metals

abstract:
  Within the simplest model of metals, namely a gas of electrons with Coulomb
interactions, in the presence of a uniform background of positive charge to
enforce electric neutrality of the system, we have derived a mechanism, by
which the Coulomb interaction between the electrons generates a new kind of
magnetism. The ground state of the metal is represented by a magnetically
ordered state described by a non-local magnetic field. This non-local magnetic
field does not produce spin polarisation of electrons, but induces a special
long range correlation between electrons of opposite spin. This mechanism
results in a theoretical value for the binding energy per electron, which is
more than twice the corresponding value for the unmagnetised state of the
metal. The new magnetic order proposed and analysed theoretically here, can in
principle be experimentally tested.


title:
Debye-Waller factor in He => Cu(001) collisions revisited: the role of
  the interaction potentials

abstract:
  Following the recently accumulated information on the vibrational properties
of the Cu(001) surface acquired through single- and multiphonon He atom
scattering experiments and the concomitant theoretical investigations, we have
reexamined the properties of the Debye-Waller factor (DWF) characteristic of
the He \to Cu(001) collisions using the recently developed fully quantal and
three-dimensional model of inelastic He atom scattering from surfaces. We have
focused our attention on the role which the various He-surface model potentials
with their characteristic interaction parameters (range of the interaction,
momentum and energy transfer cut-offs etc.) employed in the interpretation of
the scattering data may play in determining the magnitude of the DWF. By
combining the He-Cu(001) potential whose repulsive and attractive components
are both allowed to vibrate with the substrate phonon density of states
encompassing anharmonic effects, we obtain the values of the DWF which agree
nicely with the experimental data without invoking additional fitting
parameters. On the other hand, by taking the phonon momentum transfer cut-off
Q_c as an adjustable parameter, as has been frequently exploited in the
literature, all the considered potentials can produce agreement with
experiments by varying Q_c. The magnitudes of such best fit Q_c values are
compared with those available in the literature and their physical significance
is discussed.


title: Short-time behaviour of the two-dimensional hard-disk model

abstract:
  Starting from the ordered state, we investigate the short-time behaviour of
the hard-disk model. For the positional order, we determine the critical
exponents eta and z from the dynamic relaxation of the order parameter and the
cumulant with molecular dynamics simulations. The results are compared with
previous Monte Carlo (MC) simulations. The bond orientational order is studied
with MC dynamics.


title:
Neutron-diffraction study of the evolution of antiferromagnetic order in
  UPt3 doped with Pd

abstract:
  Neutron-diffraction experiments have been carried out on U(Pt1-xPdx)3 single
crystals (x<= 0.05). The small-moment antiferromagnetic order (SMAF) reported
for pure UPt3 is robust upon doping and persists till at least x= 0.005. The
ordered moment grows from 0.018(2) mB/U-atom for pure UPt3 to 0.048(8)
mB/U-atom for x= 0.005. The Neel temperature, TN, is approximately 6 K and does
not vary with Pd contents. The order parameter for the SMAF has an unusual
quasi-linear temperature variation. For x>= 0.01 a large moment
antiferromagnetic phase (LMAF) is found. For this phase at optimum doping (x=
0.05) TN attains a maximum value of 5.8 K and the ordered moment equals 0.63(5)
mB/U-atom. TN(x) for the LMAF follows a Doniach-type phase diagram. We infer
that the antiferromagnetic instability in U(Pt1-xPdx)3 is located in the range
0.5-1 at.% Pd.


title: Spin and Statistics for Quantum Hall Quasi-Particles

abstract:
  In two space dimensions the possibilities of fractional spin as well as
fractional statistics exist. I examine the relation between fractional spin and
statistics for Laughlin quasi-particles in a two-dimensional electron system
with spherical geometry. The relevance of this for quasi-particles in a planar
system is discussed.


title:
Kondo resonance effect on persistent currents through a quantum dot in a
  mesoscopic ring

abstract:
  The persistent current through a quantum dot inserted in a mesoscopic ring of
length L is studied. A cluster representing the dot and its vicinity is exactly
diagonalized and embedded into the rest of the ring. The Kondo resonance
provides a new channel for the current to flow. It is shown that due to scaling
properties, the persistent current at the Kondo regime is enhanced relative to
the current flowing either when the dot is at resonance or along a perfect ring
of same length. In the Kondo regime the current scales as $L^{-1/2}$, unlike
the $L^{-1}$ scaling of a perfect ring. We discuss the possibility of detection
of the Kondo effect by means of a persistent current measurement.


title:
Effective-field approximations, including DMRG method, for classical
  inhomogeneous 2D spin lattice models

abstract:
  A new approach to derivation of various effective-field approximation for
lattice spin models is presented. It is shown that it can give a number of
methods, including the DMRG method, that can be used to find generally
inhomogeneous solutions of 2D classical lattice problems. A method, closely
related the DMRG method but without necessity to perform any renormalization,
is derived, yielding results practically not different from the DMRG ones. The
matrix-product wave function of Rommer and \"Ostlund can be constructed from
the output of the method. The computational costs of all the derived methods
are smaller than those of the DMRG. Most of the results are applicable to the
1D quantum systems, as well.


title:
A Density Matrix Renormalization Group Study of Ultrasmall
  Superconducting Grains

abstract:
  We apply the DMRG method to the BCS pairing Hamiltonian which describes
ultrasmall superconducting grains. Our version of the DMRG uses the particle
(hole) states around the Fermi level as the system block (environment). We
observe a smooth logarithmic-like crossover between the few electron regime and
the BCS-bulk regime.


title:
Superconductivity in heavy-fermion U(Pt,Pd)3 and its interplay with
  magnetism

abstract:
  The effect of Pd doping on the superconducting phase diagram of the
unconventional superconductor UPt3 has been measured by (magneto)resistance,
specific heat, thermal expansion and magnetostriction. Experiments on single-
and polycrystalline U(Pt1-xPdx)3 for x<= 0.006 show that the superconducting
transition temperatures of the A phase, Tc+, and of the B phase, Tc-, both
decrease, while the splitting DTc increases at a rate of 0.30(2)K/at.%Pd. We
find that DTc(x) correlates with an increase of the weak magnetic moment m(x)
upon Pd doping. This provides further evidence for Ginzburg-Landau scenarios
with magnetism as the symmetry breaking field, i.e. the 2D E representation and
the 1D odd parity model. Only for small splittings DTc is proportional to
m^2(Tc+) (DTc<= 0.05 K) as predicted. The results at larger splittings call for
Ginzburg-Landau expansions beyond 4th order. The tetracritical point in the B-T
plane persists till at least x= 0.002 for B perpendicular to c, while it is
rapidly suppressed for B||c. Upon alloying the A and B phases gain stability at
the expense of the C phase.


title: Clustering of volatility as a multiscale phenomenon

abstract:
  The dynamics of prices in financial markets has been studied intensively both
experimentally (data analysis) and theoretically (models). Nevertheless, a
complete stochastic characterization of volatility is still lacking. What it is
well known is that absolute returns have memory on a long time range, this
phenomenon is known as clustering of volatility. In this paper we show that
volatility correlations are power-laws with a non-unique scaling exponent. This
kind of multiscale phenomenology, which is well known to physicists since it is
relevant in fully developed turbulence and in disordered systems, is recently
pointed out for financial series. Starting from historical returns series, we
have also derived the volatility distribution, and the results are in agreement
with a log-normal shape. In our study we consider the New York Stock Exchange
(NYSE) daily composite index closes (January 1966 to June 1998) and the US
Dollar/Deutsch Mark (USD-DM) noon buying rates certified by the Federal Reserve
Bank of New York (October 1989 to September 1998).


title:
Roughening Transition of a Restricted Solid-on-Solid Model in the
  Directed Percolation Universality Class

abstract:
  We carried out a finite-size scaling analysis of the restricted
solid-on-solid version of a recently introduced growth model that exhibits a
roughening transition accompanied by spontaneous symmetry breaking. The dynamic
critical exponent of the model was calculated and found to be consistent with
the universality class of the directed percolation process in a symmetry-broken
phase with a crossover to Kardar-Parisi-Zhang behavior in a rough phase. The
order parameter of the roughening transition together with the string order
parameter was calculated, and we found that the flat, gapped phase is
disordered with an antiferromagnetic spin-fluid structure of kinks, although
strongly dominated by the completely flat configuration without kinks. A
possible interesting extension of the model is mentioned.


title: Shear induced alignment in a model of a block copolymer

abstract:
  A mesoscopic model of a diblock copolymer is used to study the stability of a
lamellar structure under a uniform shear flow. We first obtain the nonlinear
lamellar solutions under both steady and oscillatory shear flows. Regions of
existence of these solutions are determined as a function of the parameters of
the model and of the flow. Finally, we address the stability of the lamellar
solution against long wavelength perturbations.

categories: cond-mat.soft cond-mat.mtrl-sci nlin.PS patt-sol physics.flu-dyn


title: Geometrical Defects in Josephson Junction Arrays

abstract:
  Dislocations and disclinations in a lattice of Josephson junctions will
affect the dynamics of vortex excitations within the array. These defects
effectively distort the space in which the excitations move and interact. The
interaction energy between such defects and excitations are determined and
vortex trajectories in twisted lattices are calculated. Finally, possible
experiments observing these effects are presented.


title: Another critique of the replica trick

abstract:
  Kamenev and Mezard, and Yurkevich and Lerner, have recently shown how to
reproduce the large-frequency asymptotics of the energy level correlations for
disordered electron systems, by doing perturbation theory around the saddles of
the compact nonlinear sigma model derived from fermionic replicas. We present a
critical review of their procedure and argue that its validity is limited to
the perturbative regime of large frequency. The miraculous exactness of the
saddle-point answer for beta = 2 (unitary symmetry) in the universal limit, is
shown to be a special feature due to the Duistermaat-Heckman theorem.


title: Quantum opening of the Coulomb gap in two dimensions

abstract:
  For a constant density of spinless fermions in strongly disordered two
dimensional clusters, the energy level spacing between the ground state and the
first excitation is studied for increasing system sizes. The average indicates
a smooth opening of the gap when the Coulomb energy to Fermi energy ratio $r_s$
increases from 0 to 3, while the distribution exhibits a sharp
Poisson-Wigner-like transition at $r_s \approx 1$. The results are related to
the transition from Mott to Efros-Shklovskii hopping conductivity recently
observed at a similar ratio $r_s$.


title:
A possible new phase of commensurate insulators with disorder: the Mott
  Glass

abstract:
  A new thermodynamic phase resulting from the competition between a
commensurate potential and disorder in interacting fermionic or bosonic systems
is predicted. It requires interactions of finite extent. This phase,
intermediate between the Mott insulator and the Anderson insulator, is both
incompressible and has no gap in the conductivity. The corresponding phase is
also predicted for commensurate classical elastic systems in presence of
correlated disorder.


title: A 2-D asymmetric exclusion model for granular flows

abstract:
  A 2-D version of the asymmetric exclusion model for granular sheared flows is
presented. The velocity profile exhibits two qualitatively different behaviors,
dependent on control parameters. For low friction, the velocity profile follows
an exponential decay while for large friction the profile is more accurately
represented by a Gaussian law. The phase transition occurring between these two
behavior is identified by the appearance of correlations in the cluster size
distribution. Finally, a mean--field theory gives qualitative and quantitative
good agreement with the numerical results.


title:
Transports of Disordered Carbon Nanotubes with Long Range Coulomb
  Interaction

abstract:
  Transport properties of disordered carbon nanotubes are investigated with
including long range Coulomb interactions. The resistivity and optical
conductivity are calculated by using the memory functional method. In addition,
the effect of localization is taken into account by use of the renormalization
group analysis and it is shown that the backward scattering of the intra-valley
and that of the inter-valley cannot coexist in the localized regime.


title: Superconductivity from Repulsion: A Variational View

abstract:
  This work present a new class of variational wave functions for fermi systems
in any dimension. These wave functions introduce correlations between Cooper
pairs in different momentum states and the relevant correlations can be
computed analytically. At half filling we have a ground state with critical
superconducting correlations, that is argued to be exact for the model
considered. We find large enhancements in a Cooper pair correlation function
caused purely by the interplay between the uncertainty principle, repulsion and
the proximity of half filling. This is surprising since there is no
accompanying signature in usual charge and spin response functions, and
typifies a novel kind of many body cooperative behaviour.


title: On the theory of Josephson effect in a diffusive tunnel junction

abstract:
  Specific features of the equilibrium current-carrying state of a Josephson
tunnel junction between diffusive superconductors are studied theoretically in
the 1D geometry. It is found that the Josephson current induces localized
states of electron excitations in the vicinity of the tunnel barrier, which are
a continuous analog of Andreev levels in a ballistic junction. The depth of the
corresponding ``potential well'' is much greater than the separation between an
Andreev level and the continuous energy spectrum boundary for the same
transmissivity of the barrier. In contrast to a ballistic junction in which the
Josephson current is transported completely by localized excitations, the
contribution to current in a diffusive junction comes from whole spectral
region near the energy gap boundary, where the density of states differs
considerably from its unperturbed value. The correction to the Josephson
current in the second order of the barrier transmissivity, which contains the
second harmonic of the phase jump, is calculated and it is found that the true
expansion parameter of the perturbation theory for a diffusive junction is not
the tunneling probability $\Gamma$ itself, but a much larger parameter $W =
(3\xi_0/4l)\Gamma$.


title: Charge Distribution on Annealed Polyelectrolytes

abstract:
  We investigate the equilibrium charge distribution along a single annealed
polyelectrolyte chain under different conditions. The coupling between the
conformation of the chain and the local charge distribution is described for
various solvent qualities and salt concentration. In salt free solution, we
find a slight charge depletion in the central part of the chain: the charges
accumulate at the ends. The effect is less important if salt is added to the
solution since the charge inhomogeneity is localized close to the chain ends
over a distance of order of the Debye length. In the case of poor solvent
conditions we find a different charging of beads and strings in the framework
of the necklace model. This inhomogeneity leads to a charge instability and a
first order transition between spherical globules and elongated chains.


title: Self-organized criticality and interface depinning transitions

abstract:
  We discuss the relation between self-organized criticality and depinning
transitions by mapping sandpile models to equations that describe driven
interfaces in random media. This equivalence yields a continuum description and
gives insight about various ways of reaching the depinning critical point: slow
drive (self-organized criticality), fixed density simulations, tuning the
interface velocity (extremal drive criticality), or tuning the driving force.
We obtain a scaling relation for the correlation length exponent for sandpiles.


title:
Diffusion, localization and dispersion relations on ``small-world''
  lattices

abstract:
  The spectral properties of the Laplacian operator on ``small-world''
lattices, that is mixtures of unidimensional chains and random graphs
structures are investigated numerically and analytically. A transfer matrix
formalism including a self-consistent potential a la Edwards is introduced. In
the extended region of the spectrum, an effective medium calculation provides
the density of states and pseudo relations of dispersion for the eigenmodes in
close agreement with the simulations. Localization effects, which are due to
connectivity fluctuations of the sites are shown to be quantitatively described
by the single defect approximation recently introduced for random graphs.


title: Magnetism and magnetic asphericity in NiFe alloys

abstract:
  We here study magnetic properties of Ni$_{x}$Fe$_{1-x}$ using Augmented space
recursion technique coupled with tight-binding linearized muffin tin orbital
method. Also the spectral properties of this alloy has been studied here.


title:
Intrinsic Resistivity via Quantum Nucleation of Phase Slips in a
  One-Dimensional Josephson Junction Array

abstract:
  The resistivity of a one-dimensional Josephson junction array at zero
temperature is calculated by estimating the nucleation rate of quantum phase
slips. We choose a certain collective coordinate which describes the nucleation
process and estimate the corresponding effective mass. In the strong coupling
regime, where Josephson coupling energy exceeds the charging energy, we
calculate the nucleation rate by means of WKB method. Our estimation is in good
agreement with recent experimental data. The superconductor-insulator
transition point is also discussed.


title: Kinetical theory beyond conventional approximations and 1/f-noise

abstract:
  The 1/f-noise is considered which has no relation to long-lasting processes
but originates from the same dynamical mechanisms as what are responsible for
the loss of memory, fast relaxation and usual shot noise.
  The universal long-range statistics of memoryless random flows of events and
of related Brownian motion is analysed whose peculiarity is close connection
between spectral properties and non-Gaussian probabilistic properties both
determined by only short-range characteristic time scales.
  The exact relations between equilibrium and non-equilibrium 1/f-noise in
thermodynamical systems are presented. The existence of long-living
higher-order correlations and flicker noise in the Kac's ring model is
demonstrated.
  It is shown that conventional Boltzmannian gas kinetics losses 1/f-noise
because incorrectly takes into account the conservation of particles and
probabilities in collision events. The correctly performed translation of BBGKY
hierarchy into kinetical language, under Boltzmann-Grad limit, results in
infinite chain of kinetical equations which describe probability distributions
on the hypersurfaces corresponding to encounters and collisions of particles.
These equations can be reduced to Boltzmann's equation only in the limit of
spatially uniform gas, but generally forbid the molecular chaos. If being
applied to Brownian motion of gas particles, the theory yields 1/f fluctuations
of self-diffusivity and mobility. This is example of Hamiltonian dynamics which
produces 1/f-noise without any slow processes and long life-times.


title: Transverse dipole spin modes in quantum dots

abstract:
  We have carried out a systematic analysis of the transverse dipole spin
response of a large size quantum dot within time-dependent current density
functional theory. Results for magnetic fields corresponding to integer filling
factors are reported, as well as a comparison with the longitudinal dipole spin
response. As in the two dimensional electron gas, the spin response at high
spin magnetization is dominated by a low energy transverse mode.


title: Observation of out-of-phase bilayer plasmons in YBa_2Cu_3O_7-delta

abstract:
  The temperature dependence of the c-axis optical conductivity \sigma(\omega)
of optimally and overdoped YBa_2Cu_3O_x (x=6.93 and 7) is reported in the far-
(FIR) and mid-infrared (MIR) range. Below T_c we observe a transfer of spectral
weight from the FIR not only to the condensate at \omega = 0, but also to a new
peak in the MIR. This peak is naturally explained as a transverse out-of-phase
bilayer plasmon by a model for \sigma(\omega) which takes the layered crystal
structure into account. With decreasing doping the plasmon shifts to lower
frequencies and can be identified with the surprising and so far not understood
FIR feature reported in underdoped bilayer cuprates.


title: Stability of vortices in rotating taps: a 3d analysis

abstract:
  We study the stability of vortex-lines in trapped dilute gases subject to
rotation. We solve numerically both the Gross-Pitaevskii and the Bogoliubov
equations for a 3d condensate in spherically and cilyndrically symmetric
stationary traps, from small to very large nonlinearities. In the stationary
case it is found that the vortex states with unit and $m=2$ charge are
energetically unstable. In the rotating trap it is found that this energetic
instability may only be suppressed for the $m=1$ vortex-line, and that the
multicharged vortices are never a local minimum of the energy functional, which
implies that the absolute minimum of the energy is not an eigenstate of the
$L_z$ operator, when the angular speed is above a certain value, $\Omega >
\Omega_2$.


title: X-ray photoelectron 3s spectra of transition metal oxides

abstract:
  We present metal 3s x-ray photoelectron spectra of different transition metal
oxides. The values of the 3s exchange splitting for the 3d metal oxides are
given as a function of the 3d electron number in the ground state. The spectra
were analysed using the simple two-configuration model of the interatomic
configuration mixing. The change of the 3s spectra is ascribed to the change of
the charge-transfer energy.


title: New skyrmions in the attractive Hubbard model with broken SO(4) symmetry

abstract:
  The coexistence of superconducting and charge-density-wave order in the
half-filled attractive Hubbard model is interpreted as a consequence of the
pseudospin SU(2) symmetry spontaneously broken to a `hidden' subgroup U(1). By
topological arguments we show that there must exist new skyrmion textures
associated with this symmetry breakdown. This fact is illustrated via a
non-linear $\sigma$-model. Unlike the spin textures previously known in an
antiferromagnetic background, doping the model away from half-filling leads the
new skyrmions to unwrap.


title:
Nonextensive statistics: Theoretical, experimental and computational
  evidences and connections

abstract:
  The domain of validity of standard thermodynamics and Boltzmann-Gibbs
statistical mechanics is discussed and then formally enlarged in order to
hopefully cover a variety of anomalous systems. The generalization concerns
{\it nonextensive} systems, where nonextensivity is understood in the
thermodynamical sense. This generalization was first proposed in 1988 inspired
by the probabilistic description of multifractal geometries, and has been
intensively studied during this decade. In the present effort, after
introducing some historical background, we briefly describe the formalism, and
then exhibit the present status in what concerns theoretical, experimental and
computational evidences and connections, as well as some perspectives for the
future. In addition to these, here and there we point out various (possibly)
relevant questions, whose answer would certainly clarify our current
understanding of the foundations of statistical mechanics and its
thermodynamical implications


title: Renormalization group analysis of the small-world network model

abstract:
  We study the small-world network model, which mimics the transition between
regular-lattice and random-lattice behavior in social networks of increasing
size. We contend that the model displays a normal continuous phase transition
with a divergent correlation length as the degree of randomness tends to zero.
We propose a real-space renormalization group transformation for the model and
demonstrate that the transformation is exact in the limit of large system size.
We use this result to calculate the exact value of the single critical exponent
for the system, and to derive the scaling form for the average number of
"degrees of separation" between two nodes on the network as a function of the
three independent variables. We confirm our results by extensive numerical
simulation.

categories: cond-mat.stat-mech adap-org cond-mat.dis-nn nlin.AO


title: Coherent stochastic resonance in the case of two absorbing boundaries

abstract:
  The coherent stochastic resonance is observed and studied with multi-step
periodic signal in continuous medium having two absorbing boundaries. The
general features of this process are exihibited. The universal features at the
resonance point are demonstrated. The kinetic behaviors around the resonance
point are also presented.


title:
Comment on "Superconducting phases in the presence of Coulomb
  interaction: From weak to strong correlations''

abstract:
  We examine the paper basic equations of T. Doma\'nski and K.I. Wysoki\'nski
[Phys. Rev. B {\bf 59}, 173 (1999)], who calculated the critical
superconducting temperature, T_c, in function of Coulomb correlations (U) for
s- and d-wave order parameter symmetries. We give physical arguments, based on
general theory of perturbation, that in their gap equation the Coulomb
repulsion is counted twice. Because of that, we write down the right mean-field
gap equation and solve it by using a normal state one-particle Green function
which gives a Mott metal-insulator transition for $T > T_c$ and which is valid
for any value of U. In consequence, we are not allowed to use mean-field theory
for both U and pairing (V). Our numerical results for T_c vs U shows that U is
detrimental to superconductivity which agrees with the numerical results of
Doma\'nski and Wysoki\'nski. Our results can be extended to any value of the
pairing interaction if superconducting pairing fluctuations are taken into
account.


title:
Transport properties of vortices in easy flow channels: a
  Frenkel-Kontorova study

abstract:
  Incommensurate easy flow channels in an otherwise perfect vortex lattice are
investigated. The associated (point) defects in the lattice inside the channel
cause an almost vanishing critical current, as shown by molecular dynamics
simulations and a comparison with the Frenkel-Kontorova model. In addition to
the normal flux flow behavior we find a low mobility regime at small drives
associated with defect motion. We treat this situation analytically for the
case of a single defective vortex row. We also briefly discuss the relation to
existing experiments on artificial vortex channels.


title: SiC(0001): a surface Mott-Hubbard insulator

abstract:
  We present ab-initio electronic structure calculations for the Si-terminated
SiC(0001)$\sqrt{3}\times\sqrt{3}$ surface. While local density approximation
(LDA) calculations predict a metallic ground state with a half-filled narrow
band, Coulomb effects, included by the spin-polarized LDA+U method, result in a
magnetic (Mott-Hubbard) insulator with a gap of 1.5 eV, comparable with the
experimental value of 2.0 eV. The calculated value of the inter-site exchange
parameter, J=30K, leads to the prediction of a paramagnetic Mott state, except
at very low temperatures. The observed Si 2p surface core level doublet can
naturally be explained as an on-site exchange splitting.


title: Spin Echo Decay in a Stochastic Field Environment

abstract:
  We derive a general formalism with which it is possible to obtain the time
dependence of the echo size for a spin in a stochastic field environment. Our
model is based on ``strong collisions''. We examine in detail three cases
where: (I) the local field is Ising-like, (II) the field distribution is
continuous and has a finite second moment, and (III) the distribution is
Lorentzian. The first two cases show a T2 minimum effect and are exponential in
time cubed for short times. The last case can be approximated by a
phenomenological stretched exponential.


title: Phase diagram of three-leg ladders at strong coupling along the rungs

abstract:
  A phase diagram of the t-J three-leg ladder as a function of hole dopping is
derived in the limit where the coupling parameters along the rungs, $t_{\perp}$
and $J_{\perp}$, are taken to be much larger than those along the legs,
$t_{||}$ and $J_{||}$ At large exchange coupling along the rungs,
$J_{\perp}/t_{\perp}> 3 / \sqrt{2}$, there is a transition from a low-dopping
Luttinger liquid phase into a Luther-Emery liquid at a critical hole
concentration $n_{crit}\approx 1/3$. In the opposite case,
$J_{\perp}/t_{\perp}< 3 / \sqrt{2}$, there as a sequence of three Luttinger
liquid phases (LLI, LLII and LLIII) as a function of hole dopping.


title: Conformations of closed DNA

abstract:
  We examine the conformations of a model for a short segment of closed DNA.
The molecule is represented as a cylindrically symmetric elastic rod with a
constraint corresponding to a specification of the linking number. We obtain
analytic expressions leading to the spatial configuration of a family of
solutions representing distortions that interpolate between the circular form
of DNA and a figure-eight form that represents the onset of interwinding. We
are also able to generate knotted loops. We suggest ways to use our approach to
produce other configurations relevant to studies of DNA structure. The
stability of the distorted configurations is assessed, along with the effects
of fluctuations on the free energy of the various configurations.


title: Inclusions induced phase separation in mixed lipid film

abstract:
  The effect of rigid inclusions on the phase behavior of a film containing a
mixture of lipid molecules is investigated. In the proposed model, the
inclusion-induced deformation of the film, and the resulting energy cost are
strongly dependent upon the spontaneous curvature of the mixed film. The
spontaneous curvature is in turn strongly influenced by the composition of
film. This coupling between the film composition and the energy per inclusion
leads to a lateral modulation of the composition, which follows the local
curvature of the membrane. In particular, it is shown that the inclusion may
induce a global phase separation in a film which would otherwise be
homogeneously mixed. The mixed film is then composed of patches of different
average composition, separated by the inclusions. This process may be of
relevance to explain some aspects of lipid-protein association in biological
membranes.


title:
First-principles and semi-empirical calculations for bound hole polarons
  in KNbO3

abstract:
  The ab initio linear muffin-tin-orbital (LMTO) formalism and the
semi-empirical method of the Intermediate Neglect of the Differential Overlap
(INDO) based on the Hartree-Fock formalism are combined for the study of the
hole polarons (a hole trapped nearby the cation vacancy) in a cubic phase of
KNbO3 perovskite crystals. The 40-atom and 320-atom supercells were used,
respectively. We predict existence of both, one-site and two-site (molecular)
polarons with close optical absorption energies (0.9 eV and 0.95 eV). The
relevant experimental data are discussed.


title:
Metal-Insulator Transition in a 2-Dimensional System with an Easy Spin
  Axis

abstract:
  The low-temperature resistivity of a SiGe 2-dimensional hole gas has been
studied using the gate controlled carrier density as a parameter. A
metal-insulator transition is seen both in the temperature and in the electric
field behaviour. Values of 1 for the dynamical exponent and 2.85 for the
correlation length exponent are obtained from scaling plots. No quenching of
the metallic phase in a parallel magnetic field is observed. Since in our
system there is an easy axis for magnetization, this result supports the
hypothesis that the transition is related to spin interactions.


title:
Algebraic description of a two-dimensional system of charged particles
  in an external magnetic field and periodic potential

abstract:
  Properties of the magnetic translation operators for a charged particle
moving in a crystalline potential and a uniform magnetic field show that it is
necessary to consider all inequivalent irreducible projective representations
of the the crystal lattice translation group. These considerations lead to the
concept of magnetic cells and indicate the periodicity of physical properties
with respect to the charge. It is also proven that a direct product of such
representations describe a system of two (many, in general) particles.
Therefore, they can be applied in description of interacting electrons in a
magnetic field, for example in the fractional quantum Hall effect.


title: The statistical properties of the volatility of price fluctuations

abstract:
  We study the statistical properties of volatility---a measure of how much the
market is likely to fluctuate. We estimate the volatility by the local average
of the absolute price changes. We analyze (a) the S&P 500 stock index for the
13-year period Jan 1984 to Dec 1996 and (b) the market capitalizations of the
largest 500 companies registered in the Trades and Quotes data base,
documenting all trades for all the securities listed in the three major stock
exchanges in the US for the 2-year period Jan 1994 to Dec 1995. For the S&P 500
index, the probability density function of the volatility can be fit with a
log-normal form in the center. However, the asymptotic behavior is better
described by a power-law distribution characterized by an exponent 1 + \mu
\approx 4. For individual companies, we find a power law asymptotic behavior of
the probability distribution of volatility with exponent 1 + \mu \approx 4,
similar to the S&P 500 index. In addition, we find that the volatility
distribution scales for a range of time intervals. Further, we study the
correlation function of the volatility and find power law decay with long
persistence for the S&P 500 index and the individual companies with a crossover
at approximately 1.5 days. To quantify the power-law correlations, we apply
power spectrum analysis and a recently-developed modified root-mean-square
analysis, termed detrended fluctuation analysis (DFA). For the S&P 500 index,
DFA estimates for the exponents characterizing the power law correlations are
\alpha_1=0.66 for short time scales (within \approx 1.5 days) and \alpha_2=0.93
for longer time scales (up to a year). For individual companies, we find
\alpha_1=0.60 and \alpha_2=0.74, respectively. The power spectrum gives
consistent estimates of the two power-law exponents.


title:
The response of glassy systems to random perturbations: A bridge between
  equilibrium and off-equilibrium

abstract:
  We discuss the response of aging systems with short-range interactions to a
class of random perturbations. Although these systems are out of equilibrium,
the limit value of the free energy at long times is equal to the equilibrium
free energy. By exploiting this fact, we define a new order parameter function,
and we relate it to the ratio between response and fluctuation, which is in
principle measurable in an aging experiment. For a class of systems possessing
stochastic stability, we show that this new order parameter function is
intimately related to the static order parameter function, describing the
distribution of overlaps between clustering states. The same method is applied
to investigate the geometrical organization of pure states. We show that the
ultrametric organization in the dynamics implies static ultrametricity, and we
relate these properties to static separability, i.e., the property that the
measure of the overlap between pure states is essentially unique. Our results,
especially relevant for spin glasses, pave the way to an experimental
determination of the order parameter function.


title:
Single Spin Measurement using Single Electron Transistors to Probe Two
  Electron Systems

abstract:
  We present a method for measuring single spins embedded in a solid by probing
two electron systems with a single electron transistor (SET). Restrictions
imposed by the Pauli Principle on allowed two electron states mean that the
spin state of such systems has a profound impact on the orbital states
(positions) of the electrons, a parameter which SET's are extremely well suited
to measure. We focus on a particular system capable of being fabricated with
current technology: a Te double donor in Si adjacent to a Si/SiO2 interface and
lying directly beneath the SET island electrode, and we outline a measurement
strategy capable of resolving single electron and nuclear spins in this system.
We discuss the limitations of the measurement imposed by spin scattering
arising from fluctuations emanating from the SET and from lattice phonons. We
conclude that measurement of single spins, a necessary requirement for several
proposed quantum computer architectures, is feasible in Si using this strategy.


title: The electronic structure of the heavy fermion metal $LiV_2O_4$

abstract:
  The electronic structure of the first reported heavy fermion compound without
f-electrons LiV_2O_4 was studied by an ab-initio calculation method. In the
result of the trigonal splitting and d-d Coulomb interaction one electron of
the $d^{1.5}$ configuration of V ion is localized and the rest partially fills
a relatively broad conduction band. The effective Anderson impurity model was
solved by Non-Crossing-Approximation method, leading to an estimation for the
single-site Kondo energy scale T_K. Then, we show how the so-called exhaustion
phenomenon of Nozi\`eres for the Kondo lattice leads to a remarkable decrease
of the heavy-fermion (or coherence) energy scale $T_{coh}\equiv {T_K}^2/D$ (D
is the typical bandwidth), comparable to the experimental result.


title: Kondo effect in a Luttinger liquid: nonuniversality of the Wilson ratio

abstract:
  Using a precise coset Ising-Bose representation, we show how backscattering
of electrons off a magnetic impurity destabilizes the two-channel Kondo fixed
point and drives the system to a new fixed point, in agreement with previous
results. In addition, we verify the scaling proposed by Furusaki and Nagaosa
and prove that the other possible critical fixed point, namely the local Fermi
liquid class, is not completely universal when backscattering is included
because the Wilson ratio is not well-defined in the spinon basis.


title: Spin Fluctuation-Induced Superconductivity in \kappa-BEDT-TTF Compounds

abstract:
  Spin fluctuation-induced superconductivity in quasi-two dimensional organic
compounds, \kappa-BEDT-TTF salts, is investigated within a fluctuation exchange
(FLEX) approximation using a half-filled Hubbard model with a right-angled
isosceles triangular lattice (transfer matrices -\tau, -\tau^\prime), extending
a previous work above T_c. An energy gap of A_2 or (x^2-y^2)-type develops with
decreasing temperature below T_c more rapidly than in the BCS model. The
calculated dynamical susceptibilities enough below T_c show sharp resonance
peaks like those in certain cuprates superconductors. The calculated nuclear
spin-lattice relaxation rate 1/T_1 shows a T^3 behavior below T_c in accordance
with experiment. Estimated values of 1/T_1 are roughly consistent with
experimental results. A prediction is made for the doping concentration
dependence of T_c and the antiferromagnetic and superconductive instability
points are calculated in the U/\tau vs. \tau^\prime/\tau plane.


title: Quantum Monte Carlo Study on Magnetization Processes

abstract:
  A quantum Monte Carlo method combining update of the loop algorithm with the
global flip of the world line is proposed as an efficient method to study the
magnetization process in an external field, which has been difficult because of
inefficiency of the update of the total magnetization. The method is
demonstrated in the one dimensional antiferromagnetic Heisenberg model and the
trimer model. We attempted various other Monte Carlo algorithms to study
systems in the external field and compared their efficiency.


title: On A Local Carnot Engine

abstract:
  Starting from a master equation in a quantum Hamilton form we study
analytically a nonequilibrium system which is coupled locally to two heat
bathes at different temperatures. Based on a lattice gas description an
evolution equation for the averaged density in the presence of a temperature
gradient is derived. Firstly, the case is analysed where a particle is removed
from a heat bath at a fixed temperature and is traced back to the bath at
another temperature. The stationary solution and the relaxation time is
discussed. Secondly, a collective hopping process between different heat bathes
is studied leading to an evolution equation which offers a bilinear coupling
between density and temperature gradient contrary to the conventional approach.
Whereas in case of a linear decreasing static temperature field the relaxtion
time offers a continuous spectrum it results a discrete spectrum for a
quadratically decreasing temperature profile.