title:
Ab Initio Quantum Chemistry using the Density Matrix Renormalization
  Group

abstract:
  In this paper we describe how the density matrix renormalization group (DMRG)
can be used for quantum chemical calculations for molecules, as an alternative
to traditional methods, such as configuration interaction or coupled cluster
approaches. As a demonstration of the potential of this approach, we present
results for the H$_2$O molecule in a standard gaussian basis. Results for the
total energy of the system compare favorably with the best traditional quantum
chemical methods.


title: Coarsening Dynamics of Crystalline Thin Films

abstract:
  The formation of pyramid-like structures in thin-film growth on substrates
with a quadratic symmetry, e.g., {001} surfaces, is shown to exhibit
anisotropic scaling as there exist two length scales with different time
dependences. Analytical and numerical results indicate that for most
realizations coarsening of mounds is described by an exponent n=0.2357.
However, depending on material parameters, n may lie between 0 (logarithmic
coarsening) and 1/3. In contrast, growth on substrates with triangular
symmetries ({111} surfaces) is dominated by a single length scale and an
exponent n=1/3.


title:
Functional relations for the order parameters of the chiral Potts model:
  low-temperature expansions

abstract:
  This is the third in a series of papers in which we set up and discuss the
functional relations for the ``split rapidity line'' correlation function in
the N - state chiral Potts model. The order parameters of the model can be
obtained from this function. Here we consider the case N = 3 and write the
equations explicitly in terms of the hyperelliptic functions parametrization.
We also present four-term low-temperature series expansions, which we hope will
cast light on the analyticity properties needed to solve the relations. The
problem remains unsolved, but we hope that this will prove to be a step in the
right direction.


title:
Phase Transitions of an Oscillator Neural Network with a Standard Hebb
  Learning Rule

abstract:
  Studies have been made on the phase transition phenomena of an oscillator
network model based on a standard Hebb learning rule like the Hopfield model.
The relative phase informations---the in-phase and anti-phase, can be embedded
in the network. By self-consistent signal-to-noise analysis (SCSNA), it was
found that the storage capacity is given by $\alpha_c = 0.042$, which is better
than that of Cook's model. However, the retrieval quality is worse. In
addition, an investigation was made into an acceleration effect caused by
asymmetry of the phase dynamics. Finally, it was numerically shown that the
storage capacity can be improved by modifying the shape of the coupling
function.


title:
A new bond fluctuation method for a polymer undergoing gel
  electrophoresis

abstract:
  We present a new computational methodology for the investigation of gel
electrophoresis of polyelectrolytes. We have developed the method initially to
incorporate sliding motion of tight parts of a polymer pulled by an electric
field into the bond fluctuation method (BFM). Such motion due to tensile force
over distances much larger than the persistent length is realized by non-local
movement of a slack monomer at an either end of the tight part. The latter
movement is introduced stochastically. This new BFM overcomes the well-known
difficulty in the conventional BFM that polymers are trapped by gel fibers in
relatively large fields. At the same time it also reproduces properly
equilibrium properties of a polymer in a vanishing filed limit. The new BFM
thus turns out an efficient computational method to study gel electrophoresis
in a wide range of the electric field strength.


title:
Josephson array of mesoscopic objects. Modulation of system properties
  through the chemical potential

abstract:
  The phase diagram of a two-dimensional Josephson array of mesoscopic objects
is examined. Quantum fluctuations in both the modulus and phase of the
superconducting order parameter are taken into account within a lattice boson
Hubbard model. Modulating the average occupation number $n_0$ of the sites in
the system leads to changes in the state of the array, and the character of
these changes depends significantly on the region of the phase diagram being
examined. In the region where there are large quantum fluctuations in the phase
of the superconducting order parameter, variation of the chemical potential
causes oscillations with alternating superconducting (superfluid) and normal
states of the array. On the other hand, in the region where the bosons interact
weakly, the properties of the system depend monotonically on $n_0$. Lowering
the temperature and increasing the particle interaction force lead to a
reduction in the width of the region of variation in $n_0$ within which the
system properties depend weakly on the average occupation number. The phase
diagram of the array is obtained by mapping this quantum system onto a
classical two-dimensional XY model with a renormalized Josephson coupling
constant and is consistent with our quantum Path-Integral Monte Carlo
calculations.


title:
Effects of Electron Correlation, Orbital Degeneracy and Jahn-Teller
  Coupling in Perovskite Manganites

abstract:
  Roles of Coulomb interaction, orbital degeneracy and Jahn-Teller coupling in
double-exchange models are examined for Mn perovskite oxides. We study the
undoped Mott insulator as well as metal-insulator transitions by hole doping,
and especially strong incoherence of ferromagnetic metal. We derive models
where all the spins are fully polarized in two-dimensional planes as in the
experimental indications, and investigate their ground-state properties by
quantum Monte Carlo method. At half filling where the number of $e_{g}$
electron is one per site on average, the Coulomb interaction opens a Mott gap
and induces a staggered orbital ordering. The opening of the Mott gap is,
however, substantially slower than the mean-field results if the Jahn-Teller
coupling is absent. The synergy between the strong correlation and the
Jahn-Teller coupling largely enhances the Mott gap amplitude and reproduces
realistic amplitudes and stabilization energy of the Jahn-Teller distortion.
Upon doping, the orbital ordering stabilized by the Coulomb interaction is
destroyed immediately. Toward the metal-insulator transition, the short-ranged
orbital correlation is critically enhanced in metals, which should be related
to strong incoherence of charge dynamics observed in experiments. Our model,
moreover, exhibits a uniform ordering of $d_{x^{2}-y^{2}}$ orbital in a wide
region of doping in agreement with experimental indications.


title:
Low-temperature renormalization group study of uniformly frustrated
  models for type-II superconductors

abstract:
  We study phase transitions in uniformly frustrated SU(N)-symmetric
$(2+\epsilon)$-dimensional lattice models describing type-II superconductors
near the upper critical magnetic field $H_{c2}(T)$. The low-temperature
renormalization-group approach is employed for calculating the beta-function
$\beta(T,f)$ with $f$ an arbitrary rational magnetic frustration. The
phase-boundary line $H_{c2}(T)$ is the ultraviolet-stable fixed point found
from the equation $\beta(T,f)=0$, the corresponding critical exponents being
identical to those of the non-frustrated continuum system. The critical
properties of the SU(N)-symmetric complex Ginzburg-Landau (GL) model are then
examined in $(4+\epsilon)$ dimensions. The possibility of a continuous phase
transition into the mixed state in such a model is suggested.


title:
The Influence of Structural Inhomogeneities on Intragranular Properties
  of Y- and Bi-based Superconductors

abstract:
  The measurements of temperature dependence of AC magnetic susceptibility were
used to study the influence of inhomogeneities on intragranular superconducting
properties of YBa2Cu3-xMxOy (M=Fe,x=0.005; Al,x=0.01; Cr,x=0.01),
Bi1.7Pb0.2Sb0.1Sr2Ca2Cu3Oy and Bi1.5Pb0.3Sb0.2Sr2Ca2Cu3Oy ceramics. It is found
that step-like regions are revealed on AC magnetic field dependence curves
Tmg(ho) of hysteresis losses peak temperature for superconducting granuls. The
steps at temperature scale depend on sample's composition as well as on the
type of atoms sabstituted for the copper. They range from 1 to 2 K for Y-based
and about 5 k for Bi-basedsamples. Possible interpretation of the obtained
results is presented.


title: Electrons in High-Tc Compounds: Ab-Initio Correlation Results

abstract:
  Electronic correlations in the ground state of an idealized infinite-layer
high-Tc compound are computed using the ab-initio method of local ansatz.
Comparisons are made with the local-density approximation (LDA) results, and
the correlation functions are analyzed in detail. These correlation functions
are used to determine the effective atomic-interaction parameters for model
Hamiltonians. On the resulting model, doping dependencies of the relevant
correlations are investigated. Aside from the expected strong atomic
correlations, particular spin correlations arise. The dominating contribution
is a strong nearest neighbor correlation that is Stoner-enhanced due to the
closeness of the ground state to the magnetic phase. This feature depends
moderately on doping, and is absent in a single-band Hubbard model. Our
calculated spin correlation function is in good qualitative agreement with that
determined from the neutron scattering experiments for a metal.


title:
Giant Shapiro steps for two-dimensional Josephson-junction arrays with
  time-dependent Ginzburg-Landau dynamics

abstract:
  Two-dimensional Josephson junction arrays at zero temperature are
investigated numerically within the resistively shunted junction (RSJ) model
and the time-dependent Ginzburg-Landau (TDGL) model with global conservation of
current implemented through the fluctuating twist boundary condition (FTBC).
Fractional giant Shapiro steps are found for {\em both} the RSJ and TDGL cases.
This implies that the local current conservation, on which the RSJ model is
based, can be relaxed to the TDGL dynamics with only global current
conservation, without changing the sequence of Shapiro steps. However, when the
maximum widths of the steps are compared for the two models some qualitative
differences are found at higher frequencies. The critical current is also
calculated and comparisons with earlier results are made. It is found that the
FTBC is a more adequate boundary condition than the conventional uniform
current injection method because it minimizes the influence of the boundary.


title:
Quadratic response theory for the interaction of charged particles with
  an electron gas

abstract:
  A survey is presented of the theoretical status of quadratic response
theories for the understanding of nonlinear aspects in the interaction of
charged particles with matter. In the frame of the many-body perturbation
theory we study the interaction of charged particles with the electron gas,
within the random-phase approximation (RPA). In particular, nonlinear
corrections to the stopping power of an electron gas for ions are analyzed, and
special emphasis is made on the contribution to the stopping power coming from
the excitation of single and double plasmons. Double plasmon mean free paths of
swift electrons passing through an electron gas are also discussed.


title:
Finite-difference methods for simulation models incorporating
  non-conservative forces

abstract:
  We discuss algorithms applicable to the numerical solution of second-order
ordinary differential equations by finite-differences. We make particular
reference to the solution of the dissipative particle dynamics fluid model, and
present extensive results comparing one of the algorithms discussed with the
standard method of solution. These results show the successful modeling of
phase separation and surface tension in a binary immiscible fluid mixture.

categories: cond-mat.soft cs.NA math.NA physics.chem-ph physics.comp-ph physics.flu-dyn


title: Short-time Critical Dynamics of the 3-Dimensional Ising Model

abstract:
  Comprehensive Monte Carlo simulations of the short-time dynamic behaviour are
reported for the three-dimensional Ising model at criticality. Besides the
exponent $\theta$ of the critical initial increase and the dynamic exponent
$z$, the static critical exponents $\nu$ and $\beta$ as well as the critical
temperature are determined from the power-law scaling behaviour of observables
at the beginning of the time evolution. States of very high temperature as well
as of zero temperature are used as initial states for the simulations.


title:
New Universality Class at the Magnetic Field Tuned
  Superconductor-Insulator Transition?

abstract:
  The superconductor-insulator transition in ultrathin films of amorphous Bi
was tuned by an applied magnetic field. A finite size scaling analysis has been
carried out to determine the critical exponent product vz=0.7 for five films of
different thicknesses and normal state resistances. This result differs from
the exponents found in previous experiments on InOx and MoGe films. We discuss
several possible reasons for this disagreement.


title: On the Description of Quantum Vortices in Superfluid Films

abstract:
  The dynamics of vortices in a superfluid film at the absolute zero of
temperature is studied. The quantum-mechanical, i.e., ``first-quantized''
description is compared to a recently proposed quantum-field-theoretic, i.e.,
``second-quantized'' description. The theory, consisting of the nonrelativistic
effective action of phonons linearly coupled to a Chern-Simons term, is used to
calculate the one-loop amplitude for the elastic scattering of phonons from a
vortex.


title:
Parity breaking and phase transition induced by a magnetic field in high
  $T_c$ superconductors

abstract:
  We suggest that the 2+1 dimensional Gross-Neveu model can give an effective
field theory description of low-energy quasiparticles in high temperature
superconductors. The magnetic catalysis of dynamical symmetry breaking is
examined. The model shows that a magnetic field can induce a phase transition
associated with parity breaking. In particular, it is intended to give an
explanation of a second phase transition observed in a recent experiment by
Krishana et al.


title: Jamming and Stress Propagation in Particulate Matter

abstract:
  We present simple models of particulate materials whose mechanical integrity
arises from a jamming process. We argue that such media are generically
"fragile", that is, they are unable to support certain types of incremental
loading without plastic rearrangement. In such models, fragility is naturally
linked to the marginal stability of force chain networks (granular skeletons)
within the material. Fragile matter exhibits novel mechanical responses that
may be relevant to both jammed colloids and cohesionless assemblies of poured,
rigid grains.


title:
Two interacting particles in a disordered chain III: Dynamical aspects
  of the interplay disorder-interaction

abstract:
  The interplay between the quantum interferences responsible for one particle
localization over a length L_1, and the partial dephasing induced by a local
interaction of strength U with another particle leading to partial
delocalization over a length L_2 > L_1, is illustrated by a study of the motion
of two particles put close to each other at the initial time. Localization is
reached in two steps. First, before the time t_1 necessary to propagate over
L_1, the interaction slows down the ballistic motion. On the contrary, after
t_1 the interaction favors a very slow delocalization, characterized by a
$\log(t)$ spreading of the center of mass, until L_2 is reached. This slow
motion is related to the absence of quantum chaos in this one dimensional
model, the interaction being only able to induce weaker chaos with critical
spectral statistics. Under appropriate initial conditions, the motion remains
invariant under the duality transformation mapping the behavior at small U onto
the behavior at large U.


title: Metal-insulator transition in VO$_{2}$: a Peierls-Mott-Hubbard mechanism

abstract:
  The electronic structure of VO$_2$ is studied in the frameworks of local
density approximation (LDA) and LDA+$U$ to give a quantitative description of
the metal-insulator (MI) transition in this system. It is found that, both
structural distortion and the local Coulomb interaction, play important roles
in the transition. An optical gap, comparable to the experimental value has
been obtained in the monoclinic structure by using the LDA+$U$ method. Based on
our results, we believe that both, the Peierls and the Mott-Hubbard mechanism,
are essential for a description of the MI transition in this system.


title:
Two interacting particles in a disordered chain IV: Scaling of level
  curvatures

abstract:
  The curvatures of two-particle energy levels with respect to the enclosed
magnetic flux in mesoscopic disordered rings are investigated numerically. We
find that the typical value of the curvatures is increased by interactions in
the localised regime and decreased in the metallic regime. This confirms a
prediction by Akkermans and Pichard (Eur. Phys. J. B 1, 223 (1998)). The
interaction-induced changes of the typical curvatures at different energies and
disorder strengths exhibit one-parameter scaling with a conductance-like single
parameter. This suggests that interactions could influence the conductance of
mesoscopic systems similarly.


title:
Critical spectral statistics in two-dimensional interacting disordered
  systems

abstract:
  The effect of Coulomb and short-range interactions on the spectral properties
of two-dimensional disordered systems with two spinless fermions is
investigated by numerical scaling techniques. The size independent universality
of the critical nearest level-spacing distribution $P(s)$ allows one to find a
delocalization transition at a critical disorder $W_{\rm c}$ for any non-zero
value of the interaction strength. At the critical point the spacings
distribution has a small-$s$ behavior $P_c(s)\propto s$, and a Poisson-like
decay at large spacings.


title: Evidence for the droplet/scaling picture of spin glasses

abstract:
  We have studied the Parisi overlap distribution for the three dimensional
Ising spin glass in the Migdal-Kadanoff approximation. For temperatures T
around 0.7Tc and system sizes upto L=32, we found a P(q) as expected for the
full Parisi replica symmetry breaking, just as was also observed in recent
Monte Carlo simulations on a cubic lattice. However, for lower temperatures our
data agree with predictions from the droplet or scaling picture. The failure to
see droplet model behaviour in Monte Carlo simulations is due to the fact that
all existing simulations have been done at temperatures too close to the
transition temperature so that sytem sizes larger than the correlation length
have not been achieved.


title: Melting of Flux Lines in an Alternating Parallel Current

abstract:
  We use a Langevin equation to examine the dynamics and fluctuations of a flux
line (FL) in the presence of an {\it alternating longitudinal current}
$J_{\parallel}(\omega)$. The magnus and dissipative forces are equated to those
resulting from line tension, confinement in a harmonic cage by neighboring FLs,
parallel current, and noise. The resulting mean-square FL fluctuations are
calculated {\it exactly}, and a Lindemann criterion is then used to obtain a
nonequilibrium `phase diagram' as a function of the magnitude and frequency of
$J_{\parallel}(\omega)$. For zero frequency, the melting temperature of the
mixed phase (a lattice, or the putative "Bose" or "Bragg Glass") vanishes at a
limiting current. However, for any finite frequency, there is a non-zero
melting temperature.


title: Free energy landscape in a dense hard sphere system

abstract:
  The topography of the free energy landscape in phase space of a dense hard
sphere system characterized by a discretized free energy functional of the
Ramakrishnan-Yussouff form is investigated numerically using a
``microcanonical'' Monte Carlo procedure. We locate a considerable number of
glassy local minima of the free energy and analyze the distributions of the
free energy at a minimum and an appropriately defined phase-space ``distance''
between different minima. We find evidence for the existence of pairs of
closely related glassy minima (``two-level systems''). We also investigate the
way the system makes transitions as it moves from the basin of attraction of a
minimum to that of another one after a start under nonequilibrium conditions.
This allows us to determine the effective height of free energy barriers that
separate a glassy minimum from the others. The dependence of the height of free
energy barriers on the density is investigated in detail. The general
appearance of the free energy landscape resembles that of a putting green:
relatively deep minima separated by a fairly flat structure. We discuss the
connection of our results with the Vogel-Fulcher law and relate our
observations to other work on the glass transition.


title: Phase diagram of 2D array of mesoscopic granules

abstract:
  A lattice boson model is used to study ordering phenomena in regular 2D array
of superconductive mesoscopic granules, Josephson junctions or pores filled
with a superfluid helium. Phase diagram of the system, when quantum
fluctuations of both the phase and local superfluid density are essential, is
analyzed both analytically and by quantum Monte Carlo technique. For the system
of strongly interacting bosons it is found that as the boson density $n_0$ is
increased the boundary of ordered superconducting state shifts to {\it lower
temperatures} and at $n_0 > 8$ approaches its limiting position corresponding
to negligible relative fluctuations of moduli of the order parameter (as in an
array of "macroscopic" granules). In the region of weak quantum fluctuations of
phases mesoscopic phenomena manifest themselves up to $n_0 \sim 10$. The mean
field theory and functional integral $1/n_0$ - expansion results are shown to
agree with that of quantum Monte Carlo calculations of the boson Hubbard model
and its quasiclassical limit, the quantum XY model.


title: Multiple Histogram Method for Quantum Monte Carlo

abstract:
  An extension to the multiple-histogram method (sometimes referred to as the
Ferrenberg-Swendsen method) for use in quantum Monte Carlo simulations is
presented. This method is shown to work well for the 2D repulsive Hubbard
model, allowing measurements to be taken over a continuous region of
parameters. The method also reduces the error bars over the range of parameter
values due the overlapping of multiple histograms. A continuous sweep of
parameters and reduced error bars allow one to make more difficult
measurements, such as Maxwell constructions used to study phase separation.
Possibilities also exist for this method to be used for other quantum systems.


title: Absence of Floating Delocalized States in a Two-Dimensional Hole Gas

abstract:
  By tracking the delocalized states of the two-dimensional hole gas in a
p-type GaAs/AlGaAs heterostructure as a function of magnetic field, we mapped
out a phase diagram in the density-magnetic-field plane. We found that the
energy of the delocalized state from the lowest Landau level flattens out as
the magnetic field tends toward zero. This finding is different from that for
the two-dimensional electron system in an n-type GaAs/AlGaAs heterostructure
where delocalized states diverge in energy as B goes to zero indicating the
presence of only localized states below the Fermi energy. The possible
connection of this finding to the recently observed metal-insulator transition
at B = 0 in the two-dimensional hole gas systems is discussed.


title: Replica analysis of the p-spin interactions Ising spin-glass model

abstract:
  The thermodynamics of the infinite-range Ising spin glass with p-spin
interactions in the presence of an external magnetic field h is investigated
analytically using the replica method. We give emphasis to the analysis of the
transition between the replica symmetric and the one-step replica symmetry
breaking regimes. In particular, we derive analytical conditions for the onset
of the continuous transition, as well as for the location of the tricritical
point at which the transition between those two regimes becomes discontinuous.


title:
Thermodynamic Depth of Causal States: When Paddling around in Occam's
  Pool Shallowness Is a Virtue

abstract:
  Thermodynamic depth is an appealing but flawed structural complexity measure.
It depends on a set of macroscopic states for a system, but neither its
original introduction by Lloyd and Pagels nor any follow-up work has considered
how to select these states. Depth, therefore, is at root arbitrary.
Computational mechanics, an alternative approach to structural complexity,
provides a definition for a system's minimal, necessary causal states and a
procedure for finding them. We show that the rate of increase in thermodynamic
depth, or {\it dive}, is the system's reverse-time Shannon entropy rate, and so
depth only measures degrees of macroscopic randomness, not structure. To fix
this we redefine the depth in terms of the causal state
representation---$\epsilon$-machines---and show that this representation gives
the minimum dive consistent with accurate prediction. Thus, $\epsilon$-machines
are optimally shallow.

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


title: Exciting, Cooling And Vortex Trapping In A Bose-Condensed Gas

abstract:
  A straight forward numerical technique, based on the Gross-Pitaevskii
equation, is used to generate a self-consistent description of
thermally-excited states of a dilute boson gas. The process of evaporative
cooling is then modelled by following the time evolution of the system using
the same equation. It is shown that the subsequent rethermalisation of the
thermally-excited state produces a cooler coherent condensate. Other results
presented show that trapping vortex states with the ground state may be
possible in a two-dimensional experimental environment.


title: The role of step edge diffusion in epitaxial crystal growth

abstract:
  The role of step edge diffusion (SED) in epitaxial growth is investigated. To
this end we revisit and extend a recently introduced simple cubic
solid-on-solid model, which exhibits the formation and coarsening of pyramid or
mound like structures. By comparing the limiting cases of absent, very fast
(significant), and slow SED we demonstrate how the details of this process
control both the shape of the emerging structures as well as the scaling
behavior. We find a sharp transition from significant SED to intermediate
values of SED, and a continuous one for vanishing SED. We argue that one should
be able to control these features of the surface in experiments by variation of
the flux and substrate temperature.


title:
Ergodic properties of a generic non-integrable quantum many-body system
  in thermodynamic limit

abstract:
  We study a generic but simple non-integrable quantum {\em many-body} system
of {\em locally} interacting particles, namely a kicked $t-V$ model of spinless
fermions on 1-dim lattice (equivalent to a kicked Heisenberg XX-Z chain of 1/2
spins). Statistical properties of dynamics (quantum ergodicity and quantum
mixing) and the nature of quantum transport in {\em thermodynamic limit} are
considered as the kick parameters (which control the degree of
non-integrability) are varied. We find and demonstrate {\em ballistic}
transport and non-ergodic, non-mixing dynamics (implying infinite conductivity
at all temperatures) in the {\em integrable} regime of zero or very small kick
parameters, and more generally and important, also in {\em non-integrable}
regime of {\em intermediate} values of kicked parameters, whereas only for
sufficiently large kick parameters we recover quantum ergodicity and mixing
implying normal (diffusive) transport. We propose an order parameter (charge
stiffness $D$) which controls the phase transition from non-mixing/non-ergodic
dynamics (ordered phase, $D>0$) to mixing/ergodic dynamics (disordered phase,
D=0) in the thermodynamic limit. Furthermore, we find {\em exponential decay of
time-correlation function} in the regime of mixing dynamics.
  The results are obtained consistently within three different numerical and
analytical approaches: (i) time evolution of a finite system and direct
computation of time correlation functions, (ii) full diagonalization of finite
systems and statistical analysis of stationary data, and (iii) algebraic
construction of quantum invariants of motion of an infinite system, in
particular the time averaged observables.


title: Electron Localization in the Insulating State

abstract:
  The insulating state of matter is characterized by the excitation spectrum,
but also by qualitative features of the electronic ground state. The insulating
ground wavefunction in fact: (i) sustains macroscopic polarization, and (ii) is
localized. We give a sharp definition of the latter concept, and we show how
the two basic features stem from essentially the same formalism. Our approach
to localization is exemplified by means of a two--band Hubbard model in one
dimension. In the noninteracting limit the wavefunction localization is
measured by the spread of the Wannier orbitals.


title:
Density Waves and Jamming Transition in Cellular Automaton Models for
  Traffic Flow

abstract:
  In this paper computer simulation results of higher order density correlation
for cellular automaton models of traffic flow are presented. The examinations
show the jamming transition as a function of both the density and the magnitude
of noise and allow to calculate the velocity of upstream moving jams. This
velocity is independent of the density and decreases with growing noise. The
point of maximum flow in the fundamental diagram determines its value. For that
it is not necessary to define explicitly jams in the language of the selected
model, but only based upon the well defined characteristic density profiles
along the line.


title:
Stationary definition of persistence for finite temperature phase
  ordering

abstract:
  For the two dimensional kinetic Ising model at finite temperature, the local
mean magnetisation $M_t=t^{-1}\int_{0}^t\sigma(t')\d t'$, simply related to the
fraction of time spent by a given spin in the positive direction, has a
limiting distribution, singular at $\pm m_0(T)$, the Onsager spontaneous
magnetization. The exponent of this singularity defines the persistence
exponent $\theta$. We also study first passage exponents associated to
persistent large deviations of $M_t$, and their temperature dependence.


title: A study of energy minimization techniques applied to protein design

abstract:
  We present a detailed study of the performance and reliability of design
procedures based on energy minimization. The analysis is carried out for model
proteins where exact results can be obtained through exhaustive enumeration.
The efficiency of design techniques is assessed as a function of protein
lengths and number of classes into which amino acids are coarse grained. It
turns out that, while energy minimization strategies can identify correct
solutions in most circumstances, it may be impossible for numerical
implementations of design algorithms to meet the efficiency required to yield
correct solutions in realistic contexts. We also investigated how the design
efficiency varies when putative solutions are required to obey some external
constraints and found that a restriction of the sequence space impairs the
design performance rather than boosting it. Finally some alternate design
strategies based on a correct treatment of the free energy are discussed. These
are shown to be significantly more efficient than energy-based methods while
requiring nearly the same CPU time.


title: Continuous-Time Quantum Monte Carlo Algorithm for the Lattice Polaron

abstract:
  An efficient continuous-time path-integral Quantum Monte Carlo algorithm for
the lattice polaron is presented. It is based on Feynman's integration of
phonons and subsequent simulation of the resulting single-particle
self-interacting system. The method is free from the finite-size and
finite-time-step errors and works in any dimensionality and for any range of
electron-phonon interaction. The ground-state energy and effective mass of the
polaron are calculated for several models. The polaron spectrum can be measured
directly by Monte Carlo, which is of general interest.


title:
Electron-hole correlation effects in the emission of light from quantum
  wires

abstract:
  We present a self-consistent treatment of the electron-hole correlations in
optically excited quantum wires within the ladder approximation, and using a
contact potential interaction. The limitations of the ladder approximation to
the excitonic low-density region are largely overcome by the introduction of
higher order correlations through self consistency. We show relevance of these
correlations in the low-temperature emission, even for high density relevant in
lasing, when large gain replaces excitonic absorption.


title:
Evidence for Stimulated Scattering of Excitons into Microcavity
  Polaritons

abstract:
  The wavefunction of a collection of identical quantum particles of integer
spin (bosons) is even under exchange of the coordinates of any two of them.
This symmetrization rule of quantum mechanics leads to stimulation of the
scattering rate into final states whose population is larger than one. For
massless photons, it is routinely observed in the laser. For massive bosons,
most previous efforts concentrated in realizing the largely populated
equilibrium Bose-Einstein condensate, and observing stimulated emission into
it. Here we show that we do not need to produce such a condensate to observe
final state stimulation. Instead, we take advantage of the highly
non-equilibrium exciton reservoir and of the large polariton populations
produced by external laser action on a semiconductor microcavity, and measure
final state stimulation of the scattering rate of these excitons into the
polaritons.


title:
Anomalous spin-splitting of two-dimensional electrons in an AlAs Quantum
  Well

abstract:
  We measure the effective Lande g-factor of high-mobility two-dimensional
electrons in a modulation-doped AlAs quantum well by tilting the sample in a
magnetic field and monitoring the evolution of the magnetoresistance
oscillations. The data reveal that |g| = 9.0, which is much enhanced with
respect to the reported bulk value of 1.9. Surprisingly, in a large range of
magnetic field and Landau level fillings, the value of the enhanced g-factor
appears to be constant.


title: Time-Reversal Symmetry Breaking Superconductivity in Sr2RuO4

abstract:
  We report muon spin relaxation measurements on the superconductor Sr2RuO4
that reveal the spontaneous appearance of an internal magnetic field below the
transition temperature: the appearance of such a field indicates that the
superconducting state in this material is characterized by the breaking of
time-reversal symmetry. These results, combined with other symmetry
considerations, suggest that superconductivity in Sr2RuO4 is of p-wave
(odd-parity) type, analogous to superfluid 3He.


title:
Coexistence of localized and itinerant carriers near Tc in calcium-doped
  manganites

abstract:
  We explore the possibility that polaronic distortions in the paramagnetic
phase of La$_{0.67}$Ca$_{0.33}$MnO$_3$ manganites persist in the ferromagnetic
phase by considering the observed electrical resistivity to arise from
coexisting field- and temperature-dependent polaronic and band-electron
fractions. We use an effective medium approach to compute the total resistivity
of the two-component system, and find that a limit with low percolation
threshold explains the data rather well. To test the validity of this model, we
apply it to the thermoelectric coefficient. We propose a plausible mean-field
model that reproduces the essential features of a microscopic model and
provides a comparison with the experimental mixing fraction, as well as the
magnetization and magnetic susceptibility.


title: Quantum Chaos in Quantum Wells

abstract:
  We develop a quantitative semiclassical theory for the resosnant tunneling
through a quantum well in a tilted magnetic field. It is shown, that in the
leading semiclassical approximation the tunneling current depends only on
periodic orbits within the quantum well. Further corrections (due to e.g.
"ghost" effect) can be expressed in terms of closed, but non-periodic orbits,
started at the "injection point". The results of the semiclassical theory are
shown to be in good agreement with both the experimental data and numerical
calculations.


title: Maxwell Model of Traffic Flows

abstract:
  We investigate traffic flows using the kinetic Boltzmann equations with a
Maxwell collision integral. This approach allows analytical determination of
the transient behavior and the size distributions. The relaxation of the car
and cluster velocity distributions towards steady state is characterized by a
wide range of velocity dependent relaxation scales, $R^{1/2}<\tau(v)<R$, with
$R$ the ratio of the passing and the collision rates. Furthermore, these
relaxation time scales decrease with the velocity, with the smallest scale
corresponding to the decay of the overall density. The steady state cluster
size distribution follows an unusual scaling form $P_m \sim < m>^{-4} \Psi(m/<
m>^2)$. This distribution is primarily algebraic, $P_m\sim m^{-3/2}$, for $m\ll
< m>^2$, and is exponential otherwise.


title: Detection of nanocracks on double fluoride rare earth crystal surface

abstract:
  Predicted earlier, microcracks on the crystal surface of both finely
dispersed $LiYF_4$ powders and single crystals of the Van Vleck paramagnet $%
LiTmF_4$ were detected by using the NMR Cryoporometry and Atomic-Force
Microscopy technique.


title:
Hall Anomaly and Vortex-Lattice Melting in Superconducting Single
  Crystal YBa2Cu3O7-d

abstract:
  Sub-nanovolt resolution longitudinal and Hall voltages are measured in an
ultra pure YBa2Cu3O7-d single crystal. The Hall anomaly and the first-order
vortex-lattice melting transition are observed simultaneously. Changes in the
dynamic behavior of the vortex solid and liquid are correlated with features of
the Hall conductivity sxy. With the magnetic field oriented at an angle from
the twin-boundaries, the Hall conductivity sharply decreases toward large
negative values at the vortex-lattice melting transition.


title:
High frequency dielectric and magnetic anomaly at the phase transition
  in NaV2O5

abstract:
  We found anomalies in the temperature dependence of the dielectric and the
magnetic susceptibiliy of NaV_2O_5 in the microwave and far infrared frequency
ranges. The anomalies occur at the phase transition temperature T_c, at which
the spin gap opens. The real parts of the dielectric constants epsilon_a and
epsilon_c decrease below T_c. The decrease of epsilon_a (except for the narrow
region close to T_c) is proportional to the intensity of the x-ray reflection
appearing at T_c. The dielectric constant anomaly can be explained by the
zigzag charge ordering in the ab-plane appearing below T_c. The anomaly of the
microwave magnetic losses is probably related to the coupling between the spin
and charge degrees of freedom in vanadium ladders.


title: Changing the Electronic Spectrum of a Quantum Dot by Adding Electrons

abstract:
  The temperature dependence of Coulomb blockade peak height correlation is
used to investigate how adding electrons to a quantum dot alters or "scrambles"
its electronic spectrum. Deviations from finite-temperature random matrix
theory with an unchanging spectrum indicate spectral scrambling after a small
number of electrons are added. Enhanced peak-to-peak correlations at low
temperature are observed. Peak height statistics show similar behavior in
several dot configurations despite significant differences in correlations.


title: SO(N) symmetries in the two--chain model of correlated fermions

abstract:
  The two-chain model of correlated fermions, appropriate e.g. for doped spin
ladders or carbon nanotubes, is investigated in the weak-interaction limit. It
is shown that the model scales to situations with high internal symmetries,
generically SO(6), whereas on critical lines there is SO(5) or SO(3) symmetry.
Consequences for the level spectroscopy of the model are discussed. In
particular, the ``pi-mode'' is shown to be sixfold degenerate and contains both
spin and charge excitations.


title: Pricing defaultable debt: some exact results

abstract:
  In this letter, I consider the issue of pricing risky debt by following
Merton's approach. I generalize Merton's results to the case where the interest
rate is modeled by the CIR term structure. Exact closed forms are provided for
the risky debt's price.


title:
Excitation spectrum of the S=1/2 quantum spin ladder with frustration:
  elementary quasiparticles and many-particle bound states

abstract:
  We study the excitation spectrum of the two-chain S=1/2 Heisenberg spin
ladder with additional inter-chain second-neighbor frustrating interactions.
The one and two-particle excitations are analyzed by using a mapping of the
model onto a Bose gas of hard-core triplets. We find that low-lying singlet and
triplet two-particle bound states are present and their binding energy
increases with increasing frustration. In addition, many-particle bound states
are found by a combination of variational and exact diagonalization techniques.
We prove that the larger the number of bound quasiparticles the larger the
binding energy. Thus the excitation spectrum has a complex structure and
consists of elementary triplets and collective many-particle singlet and
triplet excitations which generally mix with the elementary ones.
  The model exhibits a quantum phase transition from an antiferromagnetic
ladder phase (small frustration) into Haldane phase (effectively ferromagnetic
ladder for large frustration). We argue that near the transition point the
spectrum in both triplet and singlet channels becomes gapless. The excitation
wave function is dominated by large-size bound states which leads to the
vanishing of the quasiparticle residue.


title: Solidity of Viscous Liquids

abstract:
  Recent NMR experiments on supercooled toluene and glycerol by Hinze and
Bohmer show that small rotation angles dominate with only few large molecular
rotations. These results are here interpreted by assuming that viscous liquids
are solid-like on short length scales. A characteristic length, the "solidity
length", separates solid-like behavior from liquid-like behavior.


title: Condensed matter physics with trapped atomic Fermi gases

abstract:
  We present an overview of the various phase transitions that we anticipate to
occur in trapped fermionic alkali gases. We also discuss the prospects of
observing these transitions in (doubly) spin-polarized Li-6 and K-40 gases,
which are now actively being studied by various experimental groups around the
world.


title: Competing Glauber and Kawasaki Dynamics

abstract:
  Using a quantum formulation of the master equation we study a kinetic Ising
model with competing stochastic processes: the Glauber dynamics with
probability $p$ and the Kawasaki dynamics with probability $1 - p$. Introducing
explicitely the coupling to a heat bath and the mutual static interaction of
the spins the model can be traced back exactly to a Ginzburg Landau functional
when the interaction is of long range order. The dependence of the correlation
length on the temperature and on the probability $p$ is calculated. In case
that the spins are subject to flip processes the correlation length disappears
for each finite temperature. In the exchange dominated case the system is
strongly correlated for each temperature.


title: Exact General Solutions to Extraordinary N-body Problems

abstract:
  We solve the N-body problems in which the total potential energy is any
function of the mass-weighted root-mean-square radius of the system of N point
masses. The fundamental breathing mode of such systems vibrates non-linearly
for ever. If the potential is supplemented by any function that scales as the
inverse square of the radius there is still no damping of the fundamental
breathing mode. For such systems a remarkable new statistical equilibrium is
found for the other coordinates and momenta, which persists even as the radius
changes continually.


title:
Effective kp-Hamiltonian and correct boundary conditions for envelope
  functions in A3B5-heterostructures. Method of invariants

abstract:
  Method of invariants is used to obtain effective kp-Hamiltonian with
position-dependent band parameters and correct boundary conditions for electron
and hole envelope functions in A3B5-heterostructures with arbitrary interface
orientation. It is shown that the presence of heterointerface yields additional
quadratic-in-k and linear-in-k terms in kinetic energy operator of electron and
hole due to non-commutativity of position-dependent band parameters and
momentum operator. In particular, the conduction band Hamiltonian is determined
by two position-dependent band parameters. Similarly for description of the
valence band kp-Hamiltonian the three Luttinger parameters are no longer
sufficient, and introducing of two additional parameters that determine
quadratic-in-k terms, and additional nonrelativistic linear-in-k term, is
necessary. The additional terms appear in kp-Hamiltonians due to interface
effective spin-orbit interaction and variation of the Bloch functions across
the interface. In the framework of this approach the two band hole
kp-Hamiltonian is derived. Presence of heterointerface gives rise to short
range interface corrections, which are determined by the method of invariants.
The new form of the Hamiltonian allows us to obtain the correct boundary
conditions for envelope functions of electron and hole in A3B5-heterostructures
with arbitrary orientated abrupt heterointerfaces.


title: Linear Aggregation Revisited: Rods, Rings and Worms

abstract:
  The problem of ring formation in solutions of cylindrical micelles is
reinvestigated theoretically, taking into account a finite bending rigidity of
the self-assembled linear objects. Transitions between three regimes are found
when the scission energy is sufficiently large. At very low densities only
spherical and very short, rod-like micelles form. Beyond a critical density,
mainly rings but also rod-like chains appear in (virtually) fixed relative
amounts. Above a second transition both the length of the linear chains and the
relative amount of material taken up by them increase rapidly with increasing
concentration. The mass accumulated into long, semi-flexible worms then
overwhelms that in rings. The ring-dominated regime is very narrow for
semi-flexible chains, confirming that the presence of rings may be difficult to
observe in many micellar systems, and indeed disappears completely for
sufficiently low scission energy and/or large persistence length.


title:
Thickness-Magnetic Field Phase Diagram at the Superconductor-Insulator
  Transition in 2D

abstract:
  The superconductor-insulator transition in ultrathin films of amorphous Bi
was tuned by changing the film thickness, with and without an applied magnetic
field. The first experimentally obtained phase diagram is mapped as a function
of thickness and magnetic field in the T=0 limit. A finite size scaling
analysis has been carried out to determine the critical exponent product vz,
which was found to be 1.2 for the zero field transition, and 1.4 for the finite
field transition. Both results are different from the exponents found for the
magnetic field tuned transition in the same system, 0.7.


title: Effective action for the Kondo lattice model. New approach for S=1/2

abstract:
  In the partition function of the Kondo lattice, spin matrices are exactly
replaced by bilinear combinations of Fermi operators with the purely imaginary
chemical potential lambda=-i.pi.T/2 (Popov representation). This new
representation of spin operators allows one to introduce new Green's functions
with Matsubara frequencies 2.pi.T(n+1/4) for S=1/2. A simple temperature
diagram technique is constructed with the path integral method. This technique
is standard and does not contain the complicated combinatoric rules
characteristic of most of the known variants of the diagram techniques for spin
systems. The effective action for the almost antiferromagnetic Kondo lattice is
derived.


title: Ultrasonic attenuation in clean d-wave superconductors

abstract:
  We consider the attenuation of longitudinal ultrasonic waves in a clean
two-dimensional d-wave superconductor. We show that the attenuation coefficient
is linear in temperature at low temperatures for all in-plane directions of the
propagation of the ultrasound, and that the coefficient of the linear term can
be used to determine the parameters crucial for the low temperature transport
in these compounds.


title: Thermodynamics of quantum Heisenberg spin chains

abstract:
  Thermodynamic properties of the quantum Heisenberg spin chains with S = 1/2,
1, and 3/2 are investigated using the transfer-matrix renormalization-group
method. The temperature dependence of the magnetization, susceptibility,
specific heat, spin-spin correlation length, and several other physical
quantities in a zero or finite applied field are calculated and compared. Our
data agree well with the Bethe ansatz, exact diagonalization, and quantum Monte
Carlo results and provide further insight into the quantum effects in the
antiferromagnetic Heisenberg spin chains.


title:
Statistical and Scaling Properties of the $ac$ Conductivity in Thin
  Metal-Dielectric Composites

abstract:
  We Study in this paper the scaling and statistical properties of the $ac$
conductivity of thin metal-dielectric films in different regions of the loss in
the metallic components and particularly in the limit of vanishing loss. We
model the system by a 2D $RL-C$ network and calculate the effective
conductivity by using a real space renormalization group method. It is found
that the real conductivity strongly fluctuates for very small losses. The
correlation length, which seems to be equivalent to the localization length,
diverges for vanishing losses confirming our previous results for the decay of
the real conductivity with the loss. We found also that the distribution of the
real conductivity becomes log-normal below a certain critical loss $R_{c}$
which is size dependent for finite systems. For infinite systems this critical
loss vanishes and corresponds to the phase transition between localized modes
for finite losses and the extended ones at zero loss.


title: Wetting transitions of Ne

abstract:
  We report studies of the wetting behavior of Ne on very weakly attractive
surfaces, carried out with the Grand Canonical Monte Carlo method. The Ne-Ne
interaction was taken to be of Lennard-Jones form, while the Ne-surface
interaction was derived from an ab initio calculation of Chizmeshya et al.
Nonwetting behavior was found for Li, Rb, and Cs in the temperature regime
explored (i.e., T < 42 K). Drying behavior was manifested in a depleted fluid
density near the Cs surface. In contrast, for the case of Mg (a more attractive
potential) a prewetting transition was found near T= 28 K. This temperature was
found to shift slightly when a corrugated potential was used instead of a
uniform potential. The isotherm shape and the density profiles did not differ
qualitatively between these cases.


title: A Farey Fraction Spin Chain

abstract:
  We introduce a new number-theoretic spin chain and explore its thermodynamics
and connections with number theory. The energy of each spin configuration is
defined in a translation-invariant manner in terms of the Farey fractions, and
is also expressed using Pauli matrices. We prove that the free energy exists
and exhibits a unique phase transition at inverse temperature beta = 2. The
free energy is the same as that of a related, non translation-invariant
number-theoretic spin chain. Using a number-theoretic argument, the
low-temperature (beta > 3) state is shown to be completely magnetized for long
chains. The number of states of energy E = log(n) summed over chain length is
expressed in terms of a restricted divisor problem. We conjecture that its
asymptotic form is (n log n), consistent with the phase transition at beta = 2,
and suggesting a possible connection with the Riemann zeta function. The spin
interaction coefficients include all even many-body terms and are translation
invariant. Computer results indicate that all the interaction coefficients,
except the constant term, are ferromagnetic.


title: The Computational Complexity of Sandpiles

abstract:
  Given an initial distribution of sand in an Abelian sandpile, what final
state does it relax to after all possible avalanches have taken place? In d >=
3, we show that this problem is P-complete, so that explicit simulation of the
system is almost certainly necessary. We also show that the problem of
determining whether a sandpile state is recurrent is P-complete in d >= 3.
  In d=1, we give two algorithms for predicting the sandpile on a lattice of
size n, both faster than explicit simulation: a serial one that runs in time
O(n log n), and a parallel one that runs in time O(log^3 n), i.e. in the class
NC^3. The latter is based on a more general problem we call Additive Ranked
Generability. This leaves the two-dimensional case as an interesting open
problem.

categories: cond-mat adap-org nlin.AO nlin.PS patt-sol


title:
Universal Relationship Between Giant Magnetoresistance and Anisotropic
  Magnetoresistance in Spin Valve Multilayers

abstract:
  We measure the giant magnetoresistance (GMR) with the current both parallel
and perpendicular to the direction of the magnetization in the ferromagnetic
(FM) layers and thus probe the anisotropy of the effective mean free paths for
the spin-up and spin-down electrons, seen in the anisotropic magnetoresistance.
We find that the difference of the GMR in the two configurations, when
expressed in terms of the sheet conductance, displays a nearly universal
behavior as a function of GMR. On interpreting the results within the Boltzmann
transport formalism we demonstrate the importance of bulk scattering for GMR.


title: Orbital Kondo Effect in Ce$_x$La$_{1-x}$B$_6$: Scaling Analysis

abstract:
  Peculiarity of the Kondo effect in Ce$_x$La$_{1-x}$B$_6$ is investigated on
the basis of the scaling equations up to third order. For the case where the
$f^1$-$f^2$ charge fluctuation enters in addition to the $f^1$-$f^0$ one, the
effective exchange interaction becomes anisotropic with respect to the orbital
pseudospins which represent the two different orbitals in the $\Gamma_8$ ground
state. Because of different characteristic energies for electric and magnetic
tensors, scaling with the single Kondo temperature does not apply to physical
quantities such as the resistivity and magnetic susceptibility. Possibility of
a bizzare phase is pointed out where the RKKY interaction leads to the spin
ordering without orbital ordering. This phase serves as a candidate of the
phase IV which is observed to be isotropic magnetically.


title: Itinerant Electron Ferromagnetism in the Quantum Hall Regime

abstract:
  We report on a study of the temperature and Zeeman-coupling-strength
dependence of the one-particle Green's function of a two-dimensional (2D)
electron gas at Landau level filling factor $\nu =1$ where the ground state is
a strong ferromagnet. Our work places emphasis on the role played by the
itinerancy of the electrons, which carry the spin magnetization and on
analogies between this system and conventional itinerant electron ferromagnets.
We discuss the application to this system of the self-consistent Hartree-Fock
approximation, which is analogous to the band theory description of metallic
ferromagnetism and fails badly at finite temperatures because it does not
account for spin-wave excitations. We go beyond this level by evaluating the
one-particle Green's function using a self-energy, which accounts for
quasiparticle spin-wave interactions. We report results for the temperature
dependence of the spin magnetization, the nuclear spin relaxation rate, and
2D-2D tunneling conductances. Our calculations predict a sharp peak in the
tunneling conductance at large bias voltages with strength proportional to
temperature. We compare with experiment, where available, and with predictions
based on numerical exact diagonalization and other theoretical approaches.


title: Anomalous Magnetism of UPT_{3}:

abstract:
  UPt${}_{3}$ exhibits a paradoxical behavior in its antiferromagnetism.
Namely, the neutron scattering experiments surely detect the antifferomagnetic
ordering below the transition temperature ${\rm T}_{N}\sim 5K$, while no trace
of the transition is seen in the NMR experiments and in the specific heat at
${\rm T}_{N}$. For resolution of this paradox, we propose the ``SDW state''
which oscillates with a finite frequency larger than those of the NMR and
smaller than those of the neutron scattering. For demonstrating such
possibility we seek the divergent point of susceptibility, and offer a
preliminary calculation of the frequency dependence of ``magnetization''.


title:
Resistivity saturation revisited: results from a dynamical mean field
  theory

abstract:
  We use the dynamical mean field method to study the high-temperature
resistivity of electrons strongly coupled to phonons. The results reproduce the
qualtiative behavior of the temperature and disorder dependence of the
resistivity of the 'A-15' materials, which is commonly described in terms of
saturation, but imply that the resistivity does not saturate. Rather, a change
in temperature dependence occurs when the scattering becomes strong enough to
cause a breakdown of the Migdal approximation.


title: The Kondo-Hubbard model at half-filling

abstract:
  We have analyzed the antiferromagnetic (J>0) Kondo-Hubbard lattice with the
band at half-filling by means of a perturbative approach in the strong coupling
limit, the small parameter is an arbitrary tight-binding band. The results are
valid for any band shape and any dimension. We have obtained the energies of
elementary charge and spin excitations as well as the magnetic correlations in
order to elucidate the magnetic and charge behavior of the Kondo lattice at
half-filling. Finally, we have briefly analyzed the ferromagnetic case (J<0),
which is shown to be equivalent to an effective antiferromagnetic Heisenberg
model.


title: Quantum-Hall to insulator transition

abstract:
  The crossover from the quantum Hall regime to the Hall-insulator is
investigated by varying the strength of the diagonal disorder in a 2d
tight-binding model. The Hall and longitudinal conductivities and the behavior
of the critical states are calculated numerically. We find that with increasing
disorder the current carrying states close to the band center disappear first.
Simultaneously, the quantized Hall conductivity drops monotonically to zero
also from higher quantized values.


title: A coalescence model for freely decaying two-dimensional turbulence

abstract:
  We propose a ballistic coalescence model (punctuated-Hamiltonian approach)
mimicking the fusion of vortices in freely decaying two-dimensional turbulence.
A temporal scaling behaviour is reached where the vortex density evolves like
$t^{-\xi}$. A mean-field analytical argument yielding the approximation
$\xi=4/5$ is shown to slightly overestimate the decay exponent $\xi$ whereas
Molecular Dynamics simulations give $\xi =0.71\pm 0.01$, in agreement with
recent laboratory experiments and simulations of Navier-Stokes equation.


title: Two interacting particles at the metal-insulator transition

abstract:
  To investigate the influence of electronic interaction on the metal-insulator
transition (MIT), we consider the Aubry-Andr\'{e} (or Harper) model which
describes a quasiperiodic one-dimensional quantum system of non-interacting
electrons and exhibits an MIT. For a two-particle system, we study the effect
of a Hubbard interaction on the transition by means of the transfer-matrix
method and finite-size scaling. In agreement with previous studies we find that
the interaction localizes some states in the otherwise metallic phase of the
system. Nevertheless, the MIT remains unaffected by the interaction. For a
long-range interaction, many more states become localized for sufficiently
large interaction strength and the MIT appears to shift towards smaller
quasiperiodic potential strength.


title: Spin-density-functional theory of circular and elliptical quantum dots

abstract:
  Using spin-density-functional theory, we study the electronic states of a
two-dimensional parabolic quantum dot with up to N=58 electrons. We observe a
shell structure for the filling of the dot with electrons. Hund's rule
determines the spin configuration of the ground state, but only up to 22
electrons. At specific N, the ground state is degenerate, and a small
elliptical deformation of the external potential induces a rotational
charge-density-wave (CDW) state. Previously identified spin-density-wave (SDW)
states are shown to be artifacts of broken spin symmetry in density-functional
theory.


title:
Microwave Nonlinearities in High Tc Superconductors: The Truth Is Out
  There

abstract:
  This paper discusses some of the major experimental features of microwave
nonlinearity in high temperature superconductors, both intrinsic and extrinsic.
The case is made for solving the problem of extrinsic nonlinearity through the
use of localized measurements of microwave surface impedance and
electromagnetic fields. Along these lines, a brief introduction is given to our
work on scanning near-field microwave microscopy


title:
Low Power Superconducting Microwave Applications and Microwave
  Microscopy

abstract:
  We briefly review some non-accelerator high-frequency applications of
superconductors. These include the use of high-Tc superconductors in front-end
band-pass filters in cellular telephone base stations, the High Temperature
Superconductor Space Experiment, and high-speed digital electronics. We also
present an overview of our work on a novel form of near-field scanning
microscopy at microwave frequencies. This form of microscopy can be used to
investigate the microwave properties of metals and dielectrics on length scales
as small as 1 mm. With this microscope we have demonstrated quantitative
imaging of sheet resistance and topography at microwave frequencies. An
examination of the local microwave response of the surface of a heat-treated
bulk Nb sample is also presented.


title:
Hopping Conductivity of a Nearly-1d Fractal: a Model for Conducting
  Polymers

abstract:
  We suggest treating a conducting network of oriented polymer chains as an
anisotropic fractal whose dimensionality D=1+\epsilon is close to one.
Percolation on such a fractal is studied within the real space renormalization
group of Migdal and Kadanoff. We find that the threshold value and all the
critical exponents are strongly nonanalytic functions of \epsilon as \epsilon
tends to zero, e.g., the critical exponent of conductivity is \epsilon^{-2}\exp
(-1-1/\epsilon). The distribution function for conductivity of finite samples
at the percolation threshold is established. It is shown that the central body
of the distribution is given by a universal scaling function and only the
low-conductivity tail of distribution remains $\epsilon $-dependent. Variable
range hopping conductivity in the polymer network is studied: both DC
conductivity and AC conductivity in the multiple hopping regime are found to
obey a quasi-1d Mott law. The present results are consistent with electrical
properties of poorly conducting polymers.


title: Ground-state behavior of the 3d +/-J random-bond Ising model

abstract:
  Large numbers of ground states of the three-dimensional $\pm J$ random-bond
Ising model are calculated for sizes up to $14^3$ using a combination of a
genetic algorithm and Cluster-Exact Approximation. Several quantities are
calculated as function of the concentration $p$ of the antiferromagnetic bonds.
The critical concentration where the ferromagnetic order disappears is
determined using the Binder cumulant of the magnetization. A value of
$p_c=0.222\pm 0.005$ is obtained. From the finite-size behavior of the Binder
cumulant and the magnetization critical exponents $\nu=1.1 \pm 0.3$ and
$\beta=0.2 \pm 0.1$ are calculated.


title: Mobile small polaron

abstract:
  Extending the Froehlich polaron problem to a discrete ionic lattice we study
a polaronic state with a small radius of the wave function but a large size of
the lattice distortion. We calculate the energy dispersion and the effective
mass of the polaron with the 1/\lambda perturbation theory and with the exact
Monte Carlo method in the nonadiabatic and adiabatic regimes, respectively. The
``small'' Froehlich polaron is found to be lighter than the small Holstein
polaron by one or more orders of magnitude.


title: Synchronization of Coupled Systems with Spatiotemporal Chaos

abstract:
  We argue that the synchronization transition of stochastically coupled
cellular automata, discovered recently by L.G. Morelli {\it et al.} (Phys. Rev.
{\bf 58 E}, R8 (1998)), is generically in the directed percolation universality
class. In particular, this holds numerically for the specific example studied
by these authors, in contrast to their claim. For real-valued systems with
spatiotemporal chaos such as coupled map lattices, we claim that the
synchronization transition is generically in the universality class of the
Kardar-Parisi-Zhang equation with a nonlinear growth limiting term.


title: Diffusion of Pt dimers on Pt(111)

abstract:
  We report the results of a density-functional study of the diffusion of Pt
dimers on the (111) surface of Pt. The calculated activation energy of 0.37 eV
is in {\em exact} agreement with the recent experiment of Kyuno {\em et al.}
\protect{[}Surf. Sci. {\bf 397}, 191 (1998)\protect{]}. Our calculations
establish that the dimers are mobile at temperatures of interest for adatom
diffusion, and thus contribute to mass transport. They also indicate that the
diffusion path for dimers consists of a sequence of one-atom and (concerted)
two-atom jumps.


title: Interplay between superconductivity and flux phase in the t-J model

abstract:
  We study the phase diagram of the t-J model using a mean field type
approximation within the Baym-Kadanoff perturbation expansion for Hubbard
$X$-operators. The line separating the normal state from a d-wave flux or
bond-order state starts near optimal doping at T=0 and rises quickly with
decreasing doping. The transition temperature $T_c$ for d-wave
superconductivity increases monotonically in the overdoped region towards
optimal doping. Near optimaldoping a strong competition between the two d-wave
order parameters sets in leading to a strong suppression of $T_c$ in the
underdoped region. Treating for simplicity the flux phase as commensurate the
superconducting and flux phases coexist in the underdoped region below $T_c$,
whereas a pure flux phase exists above $T_c$ with a pseudo-gap of d-wave
symmetry in the excitation spectrum. We also find that incommensurate
charge-density-wave ground states due to Coulomb interactions do not modify
strongly the above phase diagram near the superconducting phase, at least, as
long as the latter exists at all.


title: Fractal Analysis of Protein Potential Energy Landscapes

abstract:
  The fractal properties of the total potential energy V as a function of time
t are studied for a number of systems, including realistic models of proteins
(PPT, BPTI and myoglobin). The fractal dimension of V(t), characterized by the
exponent \gamma, is almost independent of temperature and increases with time,
more slowly the larger the protein. Perhaps the most striking observation of
this study is the apparent universality of the fractal dimension, which depends
only weakly on the type of molecular system. We explain this behavior by
assuming that fractality is caused by a self-generated dynamical noise, a
consequence of intermode coupling due to anharmonicity. Global topological
features of the potential energy landscape are found to have little effect on
the observed fractal behavior.


title:
Robustness of a Local Fermi Liquid against Ferromagnetism and Phase
  Separation II

abstract:
  This is a response to a recent paper by van Dongen, Uhrig, and E.
M\"uller-Hartmann. We clarify the issues, which the above authors apparently
did not fully appreciate, by referring to the microscopic foundations of
Landau's Fermi Liquid Theory, its general application to the theory of metals
and the theory of local Fermi Liquids.


title:
Finite temperature molecular dynamics study of unstable stacking fault
  free energies in silicon

abstract:
  We calculate the free energies of unstable stacking fault (USF)
configurations on the glide and shuffle slip planes in silicon as a function of
temperature, using the recently developed Environment Dependent Interatomic
Potential (EDIP). We employ the molecular dynamics (MD) adiabatic switching
method with appropriate periodic boundary conditions and restrictions to atomic
motion that guarantee stability and include volume relaxation of the USF
configurations perpendicular to the slip plane. Our MD results using the EDIP
model agree fairly well with earlier first-principles estimates for the
transition from shuffle to glide plane dominance as a function of temperature.
We use these results to make contact to brittle-ductile transition models.


title: Topological Excitations of One-Dimensional Correlated Electron Systems

abstract:
  Properties of low-energy excitations in one-dimensional superconductors and
density-wave systems are examined by the bosonization technique. In addition to
the usual spin and charge quantum numbers, a new, independently measurable
attribute is introduced to describe elementary, low-energy excitations. It can
be defined as a number w which determines, in multiple of $\pi$, how many times
the phase of the order parameter winds as an excitation is transposed from far
left to far right. The winding number is zero for electrons and holes with
conventional quantum numbers, but it acquires a nontrivial value w=1 for
neutral spin-1/2 excitations and for spinless excitations with a unit electron
charge. It may even be irrational, if the charge is irrational. Thus, these
excitations are topological, and they can be viewed as composite particles made
of spin or charge degrees of freedom and dressed by kinks in the order
parameter.


title: Back to basics: historical option pricing revisited

abstract:
  We reconsider the problem of option pricing using historical probability
distributions. We first discuss how the risk-minimisation scheme proposed
recently is an adequate starting point under the realistic assumption that
price increments are uncorrelated (but not necessarily independent) and of
arbitrary probability density. We discuss in particular how, in the Gaussian
limit, the Black-Scholes results are recovered, including the fact that the
average return of the underlying stock disappears from the price (and the
hedging strategy). We compare this theory to real option prices and find these
reflect in a surprisingly accurate way the subtle statistical features of the
underlying asset fluctuations.


title: Parity effect in a small superconducting grain: A rigorous result

abstract:
  The parity effect in an ultra-small superconducting grain is examined. By
applying a generalized version of Lieb's spin-reflection positivity technique,
we show rigorously that the parity parameter $\Delta_P$ is nonvanishing in such
a system. A positive lower bound for $\Delta_P$ is derived.


title: Unstable Growth and Coarsening in Molecular-Beam Epitaxy

abstract:
  The coarsening dynamics of three-dimensional islands on a growing film is
discussed. It is assumed that the origin of the initial instability of a planar
surface is the Ehrlich-Schwoebel step-edge barrier for adatom diffusion. Two
mechanisms of coarsening are identified: (i) surface diffusion driven by an
uneven distribution of bonding energies, and (ii) mound coalescence driven by
random deposition. Semiquantitative estimates of the coarsening time are given
in each case. When the surface slope saturates, an asymptotic dynamical
exponent $z=4$ is obtained.


title: Pairing in the Three-Band Hubbard Model of the Cu-O Plane

abstract:
  By a canonical transformation of the three-band Hubbard model, we introduce
an effective Hamiltonian for the propagation of two holes doped into the ground
state of the Cu-O plane. When the pair belongs to the $^{1}B_{2}$ or $%
^{1}A_{2}$ Irreducible Representations of the C$_{4v}$ Group, the bare holes do
not interact by the on-site repulsion; the effective interaction between the
dressed holes is obtained analytically in terms of renormalized matrix
elements, and generalizes earlier findings from cluster calculations. The Fermi
liquid is unstable and numerical estimates with reasonable parameters of the
binding energy of the pair are in the range of tens of meV. Our scheme
naturally lends itself to embody phonon-mediated and other interactions which
cannot occur in the Hubbard model but may give important contributions.


title: Collective Modes in High-Temperature Superconductors

abstract:
  The role of collective modes in various experiments on the cuprates is
investigated. We calculate the neutron scattering, photoemission (ARPES), and
Raman scattering intensities below $T_c$ within the fluctuation-exchange (FLEX)
approximation for the two-dimensional Hubbard model. It is shown that the large
peak in the dynamical spin susceptibility arises from a weakly damped
spin-density-wave collective mode. This gives rise to a dip between the sharp
low energy peak and the higher binding energy hump in the ARPES spectrum.
Furthermore, we show that the collective mode of the amplitude fluctuation of
the d-wave gap yields a broad peak above the pair-breaking threshold in the
$B_{1g}$ Raman spectrum.


title:
Simulations of mechanics and structure of nanomaterials --- from
  nanoscale to coarser scales

abstract:
  We discuss how simulations of mechanical properties of materials require
descriptions at many different length scales --- from the nanoscale where an
atomic description is appropriate, through a mesoscale where dislocation based
descriptions may be useful, to macroscopic length scales. In some materials,
such as nanocrystalline metals, the range of length scales is compressed and a
polycrystalline material may be simulated at the atomic scale. The first part
of the paper describes such simulations of nanocrystalline copper. We observe
how the grain boundaries contribute actively to the deformation. At grain sizes
below 10-15 nm deformation in the grain boundaries dominate over the
traditional dislocation-based deformation mechanisms. This results in a
reversal of the normal grain size dependence of the yield stress: we observe
that the material becomes softer when the grain size is reduced. The second
part of the paper gives an overview over simulation techniques appropriate for
problems too large to be treated in atomic-scale simulations, and how they can
be combined to describe the interplay between phenomena at different length
scales through multiscale modelling.


title: The Quantum Hall effect, Skyrmions and Anomalies

abstract:
  We discuss the properties of Skyrmions in the Fractional Quantum Hall effect
(FQHE). We begin with a brief description of the Chern-Simons-Landau-Ginzburg
description of the FQHE, which provides the framework in which to understand a
new derivation of the properties of FQHE Skyrmions (S. Baez, A.P. Balachandran,
A. Stern and A. Travesset cond-mat/9712151) from anomaly and edge
considerations.


title: Reptation Quantum Monte Carlo

abstract:
  We present an elementary and self-contained account of the analogies existing
between classical diffusion and the imaginary-time evolution of quantum
systems. These analogies are used to develop a new quantum simulation method
which allows to calculate the ground-state expectation values of local
observables without any mixed estimates nor population-control bias, as well as
static and dynamic (in imaginary time) response functions. This method, which
we name Reptation Quantum Monte Carlo, is demonstrated with a few case
applications to 4He, including the calculation of total and potential energies,
static and imaginary-time dependent density response functions, and low-lying
excitation energies. Finally, we discuss the relations of our technique with
other simulation schemes.


title: The tubular phase of self-avoiding anisotropic crystalline membranes

abstract:
  We analyze the tubular phase of self-avoiding anisotropic crystalline
membranes. A careful analysis using renormalization group arguments together
with symmetry requirements motivates the simplest form of the large-distance
free energy describing fluctuations of tubular configurations. The
non-self-avoiding limit of the model is shown to be exactly solvable. For the
full self-avoiding model we compute the critical exponents using an
epsilon-expansion about the upper critical embedding dimension for general
internal dimension D and embedding dimension d. We then exhibit various methods
for reliably extrapolating to the physical point (D=2,d=3). Our most accurate
estimates are nu=0.62 for the Flory exponent and zeta=0.80 for the roughness
exponent.


title: Incoherent Pair Tunneling as a Probe of the Cuprate Pseudogap

abstract:
  We argue that incoherent pair tunneling in a cuprate superconductor junction
with an optimally doped superconducting and an underdoped normal lead can be
used to detect the presence of pairing correlations in the pseudogap phase of
the underdoped lead. We estimate that the junction characteristics most
suitable for studying the pair tunneling current are close to recently
manufactured cuprate tunneling devices.


title:
Adaptive Mesh Refinement Computation of Solidification Microstructures
  using Dynamic Data Structures

abstract:
  We study the evolution of solidification microstructures using a phase-field
model computed on an adaptive, finite element grid. We discuss the details of
our algorithm and show that it greatly reduces the computational cost of
solving the phase-field model at low undercooling. In particular we show that
the computational complexity of solving any phase-boundary problem scales with
the interface arclength when using an adapting mesh. Moreover, the use of
dynamic data structures allows us to simulate system sizes corresponding to
experimental conditions, which would otherwise require lattices greater that
$2^{17}\times 2^{17}$ elements. We examine the convergence properties of our
algorithm. We also present two dimensional, time-dependent calculations of
dendritic evolution, with and without surface tension anisotropy. We benchmark
our results for dendritic growth with microscopic solvability theory, finding
them to be in good agreement with theory for high undercoolings. At low
undercooling, however, we obtain higher values of velocity than solvability
theory at low undercooling, where transients dominate, in accord with a
heuristic criterion which we derive.


title: Nonlinearity effect on 1D periodic and disordered lattices

abstract:
  The Kronig-Penney model is used to Study the effect of nonlinear interaction
on the transmissive properties of both ordered and disordered chains. In the
ordered case, the nonlinearity can either localize or delocalize the electronic
states depending on both its sign and strength but there is a critical strength
above which all the states are localized. In the disordered case, however, we
found that the transmission decays as $T\sim L^{-\gamma}$ around the band edge
of the corresponding periodic system. The exponent $\gamma$ is independent of
the strength of the nonlinearity in the case of disordered barrier potentials,
while it varies with the strength for mixed potentials.


title: Surface barrier dominated transport in NbSe$_2$

abstract:
  Transport current distribution in clean 2$H$-NbSe$_{2}$ crystals is studied
by measuring the self-induced magnetic field across the sample. Below $T_c$
most of the current flows at the edges of the crystals due to strong surface
barriers, which are found to dominate the transport properties and the
resistive transition. The measured critical current is determined by the
critical current for vortex penetration through the surface barrier rather than
by bulk pinning.


title: Incommensuration in quantum antiferromagnetic chain

abstract:
  A dimerized quantum Heisenberg or XY antiferromagnetic chain has a gap in the
spectrum. We show that a weak incommensurate modulation around a dimerized
chain produces a zero temperature quantum critical point. As the
incommensuration wavelength is varied, there is a transition to a modulated
gapless state. The critical behaviour is in the universality class of the
classical commensurate-incommensurate (Pokrovsky-Talapov) transition. An
analogous metal-insulator transition can also take place for an incommensurate
chain.


title:
A multi-species asymmetric simple exclusion process and its relation to
  traffic flow

abstract:
  Using the matrix product formalism we formulate a natural p-species
generalization of the asymmetric simple exclusion process. In this model
particles hop with their own specific rate and fast particles can overtake slow
ones with a rate equal to their relative speed. We obtain the algebraic
structure and study the properties of the representations in detail. The
uncorrelated steady state for the open system is obtained and in the ($p \to
\infty)$ limit, the dependence of its characteristics on the distribution of
velocities is determined. It is shown that when the total arrival rate of
particles exceeds a certain value, the density of the slowest particles rises
abroptly.


title: Jastrow-Luttinger Fractional Liquids

abstract:
  In this paper, we present a description of Haldane's Luttinger liquid which
parallels Laughlin's theory of the Fractional Quantum Hall (FQH) incompressible
fluid, both exhibiting similar ground states as well as fractional excitations.
These two non-Fermi liquids are instances of a generic structure for
low-dimensional quantum liquids which we propose to dub Jastrow-Luttinger
Fractional Liquids. An important feature of such liquids is the complete
fractionalization of the parent particle. In particular, in both one and two
dimensions spin-charge separation can be achieved and is indeed suggested to
occur for unpolarized quantum Hall systems both at the edge and in the bulk.


title: Collinear N\'eel-type ordering in partially frustrated lattices

abstract:
  We consider two partially frustrated S = 1/2 antiferromagnetic spin systems
on the triangular and pentagonal lattices. In an elementary plaquette of the
two lattices, one bond has exchange interaction strength $\alpha$ ($\alpha \leq
1$) whereas all other bonds have exchange interaction strength unity. We show
that for $\alpha$ less than a critical value $\alpha_{c}$, collinear
N\'eel-type ordering is possible in the ground state. The ground state energy
and the excitation spectrum have been determined using linear spin wave theory
based on the Holstein-Primakoff transformation.


title: Velocity Distribution for Strings in Phase Ordering Kinetics

abstract:
  The continuity equations expressing conservation of string defect charge can
be used to find an explicit expression for the string velocity field in terms
of the order parameter in the case of an O(n) symmetric time-dependent
Ginzburg-Landau model. This expression for the velocity is used to find the
string velocity probability distribution in the case of phase-ordering kinetics
for a nonconserved order parameter. For long times $t$ after the quench,
velocities scale as $t^{-1/2}$. There is a large velocity tail in the
distribution corresponding to annihilation of defects which goes as
$V^{-(2d+2-n)}$ for both point and string defects in $d$ spatial dimensions.


title:
Barkhausen noise in soft amorphous magnetic materials under applied
  stress

abstract:
  We report experimental measurements of Barkhausen noise on
Fe_{64}Co_{21}B_{15} amorphous alloy under tensile stress. We interpret the
scaling behavior of the noise distributions in terms of the depinning
transition of the domain walls. We show that stress induced anisotropy enhance
the effect of short-range elastic interactions that dominate over long-range
dipolar interactions. The universality class is thus different from the one
usually observed in Barkhausen noise measurements and is characterized by the
exponents \tau = 1.3 and \alpha = 1.5, for the decay of the distributions of
jump sizes and durations.


title: Polymer depletion effects near mesoscopic particles

abstract:
  The behavior of mesoscopic particles dissolved in a dilute solution of long,
flexible, and nonadsorbing polymer chains is studied by field-theoretic
methods. For spherical and cylindrical particles the solvation free energy for
immersing a single particle in the solution is calculated explicitly. Important
features are qualitatively different for self-avoiding polymer chains as
compared with ideal chains. The results corroborate the validity of the
Helfrich-type curvature expansion for general particle shapes and allow for
quantitative experimental tests. For the effective interactions between a small
sphere and a wall, between a thin rod and a wall, and between two small spheres
quantitative results are presented. A systematic approach for studying
effective many-body interactions is provided. The common Asakura-Oosawa
approximation modelling the polymer coils as hard spheres turns out to fail
completely for small particles and still fails by about 10% for large
particles.


title: Competition between Magnetic and Structural Transition in CrN

abstract:
  CrN is observed to undergo a paramagnetic to antiferromagnetic transition
accompanied by a shear distortion from cubic NaCl-type to orthorhombic
structure. Our first-principle plane wave and ultrasoft pseudopotential
calculations confirm that the distorted antiferromagnetic phase with spin
configuration arranged in double ferromagnetic sheets along [110] is the most
stable. Antiferromagnetic ordering leads to a large depletion of states around
Fermi level, but it does not open a gap. Simultaneous occurence of structural
distortion and antiferromagnetic order is analyzed.


title:
Evidence for an Anisotropic State of Two-Dimensional Electrons in High
  Landau Levels

abstract:
  Magneto-transport experiments on high mobility two-dimensional electron gases
in GaAs/AlGaAs heterostructures have revealed striking anomalies near
half-filling of several spin-resolved, yet highly excited, Landau levels. These
anomalies include strong anisotropies and non-linearities of the longitudinal
resistivity rho_xx which commence only below about 150mK. These phenomena are
not seen in the ground or first excited Landau level but begin abruptly in the
third level. Although their origin remains unclear, we speculate that they
reflect the spontaneous development of a generic anisotropic many-electron
state.


title: Sonoluminescing Gas Bubbles

abstract:
  We draw attention to the fact that the popular but unproven hypothesis of
shock-driven sonoluminescence is incompatible with the reported synchronicity
of the single bubble sonoluminescence (SBSL) phenomenon. Moreover, it is not a
necessary requirement, since we show that the sub-shock dynamic heating in gas
bubble cavitation can lead to conditions required to generate intense 100ps
light pulses. To wit we study the dynamics of the interior of a cavitating gas
bubble subject to conditions suitable for sonoluminescence. We explore
quantitatively the transfer of energy from the sound wave to the bubble
interior, the frequency of atomic collisions in the bubble, the limits of
quasi-stability of the non-linear bubble oscillations driven by an acoustical
field, and obtain the implied reaction time scales.


title: Interstitial He and Ne in Nanotube Bundles

abstract:
  We explore the properties of atoms confined to the interstitial regions
within a carbon nanotube bundle. We find that He and Ne atoms are of ideal size
for physisorption interactions, so that their binding energies are much greater
there than on planar surfaces of any known material. Hence high density phases
exist at even small vapor pressure. There can result extraordinary anisotropic
liquids or crystalline phases, depending on the magnitude of the corrugation
within the interstitial channels.


title:
Mixed-state quasiparticle transport in high-T_c cuprates: localization
  by magnetic field

abstract:
  Theory of quasiparticle transport in the mixed state of a d-wave
superconductor is developed under the assumption of disordered vortex array. A
novel universal regime is identified at fields above H*= c*H_{c2}(T/T_c)^2,
characterized by a field-independent longitudinal thermal conductivity. It is
argued that this behavior is responsible for the high-field plateau in the
thermal conductivity experimentally observed in cuprates by Krishana, Ong and
co-workers.


title:
Measurements of the Magnetic Field Dependence of Lambda in
  YBa_2Cu_3O_6.95: Results as a Function of Temperature and Field Orientation

abstract:
  We present measurements of the magnetic field dependence of the penetration
depth Lambda(H) for untwinned YBa_2Cu_3O_6.95 for temperatures from 1.2 to 70 K
in dc fields up to 42 gauss and directions 0, 45 and 90 degrees with respect to
the crystal b-axis. The experiment uses an ac susceptometer with fields applied
parallel to the ab-plane of thin platelet samples. The resolution is about 0.15
Angstroms in zero dc field, degrading to 0.2 or 0.3 Angstroms at the higher
fields. At low temperatures the field dependencies are essentially linear in H,
ranging from 0.04 Angstroms/gauss for Delta-Lambda_a to 0.10 Angstroms/gauss
for Delta-Lambda_b, values comparable to the T=0 Yip and Sauls prediction for a
d-wave superconductor. However, the systematics versus temperature and
orientation do not agree with the d-wave scenario probably due, in part, to
residual sample problems.


title:
Low temperature hcp to monoclinic structural transition in solid
  C$_{70}$ : Is there an intermediate phase?"

abstract:
  We follow the structural transformation in solid C$_{70}$ from the high
temperature hcp to a low temperature monoclinic phase using x-ray diffraction
studies at controlled cooling-rates from 0.0033 to 0.42 K/min. Rapid cooling of
the sample gives the signature of a two-step transformation which disappears
when the system is transformaed quasi-statically. These experimental results
can be rationalised using a simple mean field, Langevin dynamical theory using
a free energy functional with minima corresponding to the parent and two
competing product phases such that one of these product phases remains
metastable throughout. The implication of our results on the existence of the
intermediate phase in the sequence, hcp-dhcp-monoclinic, of structural
transitions in solid C$_{70}$ with the lowering of temperature is discussed.


title:
Fluctuation correction to the ground state energy density of a dilute
  Bose gas in the functional Schr\"{o}dinger picture

abstract:
  A dilute Bose gas system is studied using the functional Schr\"{o}dinger
picture theory. The ground state properties are obtained by solving the
infinite dimensional Schr\"{o}dinger equation variationally. It is shown that a
shifted Gaussian trial wavefunctional enables us to calculate a higher order
correction, which corresponds to the fluctuation contribution from the
condensate. The obtained term is compared with the quantum correction arising
from the low energy $3 \to 3$ scattering.


title:
Ground State Wavefunctions of General Filling Factors in the Lowest
  Landau Level

abstract:
  We present a set of explicit trial wavefunctions for the filling factors
\nu=n/(2n\pm 1) and \nu=1/2 in the symmetric gauge. We show that the zeroes of
the wavefunction, except those dictated by the Fermi statistics, are detached
from the particles. The evolution of zeroes as the filling factor is varied is
examined. We show that the wavefunction at half-filling exhibits a 2k_F-like
oscillation in its occupation number profile. The center-of-mass motion of the
ground state droplet is described in terms of the intra-Landau- level
excitations of composite fermions.


title:
Suppression of antiferromagnetic correlations by quenched dipole--type
  impurities

abstract:
  The effect of quenched random ferromagnetic bonds on the antiferromagnetic
correlation length of a two--dimensional Heisneberg model is studied, applying
the renormalization group method to the classical non--linear sigma model with
quenched random dipole moments. It is found that the antiferromagnetic long
range order is destroyed for any non--zero concentration, of the dipolar
defects, even at zero temperature. Below a line T ~ concentration, the
correlation length is independent of T, and decreases exponentially with
concentration. At higher temperatures, itdecays exponentially with an effective
stiffness constant which decreases with concentration/T. The results are used
to estimate the three--dimensional N\'{e}el temperature, which decays linearly
with $x$ at small concentrations, and drops precipitously at a critical
concentration. These predictions are compared with experiments on doped copper
oxides, and are shown to reproduce successfully some of the prominent features
of the data.


title: Spin Reorientations Induced by Morphology Changes in Fe/Ag(001)

abstract:
  By means of magneto-optical Kerr effect we observe spin reorientations from
in-plane to out-of-plane and vice versa upon annealing thin Fe films on Ag(001)
at increasing temperatures. Scanning tunneling microscopy images of the
different Fe films are used to quantify the surface roughness. The observed
spin reorientations can be explained with the experimentally acquired roughness
parameters by taking into account the effect of roughness on both the magnetic
dipolar and the magnetocrystalline anisotropy.


title:
Fluctuations in photon local delay time and their relation to phase
  spectra in random media

abstract:
  The temporal evolution of microwave pulses transmitted through random
dielectric samples is obtained from the Fourier transform of field spectra.
Large fluctuations are found in the local or single channel delay time, which
is the first temporal moment of the transmitted pulse at a point in the output
speckle pattern. Both positive and negative values of local delay time are
observed. The widest distribution is found at low intensity values near a phase
singularity in the transmitted speckle pattern. In the limit of long duration,
narrow-bandwidth incident pulses, the single channel delay time equals the
spectral derivative of the phase of the transmitted field. Fluctuations of the
phase of the transmitted field thus reflect the underlying statistics of
dynamics in mesoscopic systems.


title: Aperiodic and correlated disorder in XY-chains: exact results

abstract:
  We study thermodynamic properties, specific heat and susceptibility, of XY
quantum chains with coupling constants following arbitrary substitution rules.
Generalizing an exact renormalization group transformation, originally
formulated for Ising quantum chains, we obtain exact relevance criteria of
Harris-Luck type for this class of models. For two-letter substitution rules, a
detailed classification is given of sequences leading to irrelevant, marginal
or relevant aperiodic modulations. We find that the relevance of the same
aperiodic sequence of couplings in general will be different for XY and Ising
quantum chains. By our method, continuously varying critical exponents may be
calculated exactly for arbitrary (two-letter) substitution rules with marginal
aperiodicity. A number of examples are given, including the period-doubling,
three-folding and precious mean chains. We also discuss extensions of the
renormalization approach to a special class of long-range correlated random
chains, generated by random substitutions.


title:
Chiral Solitons in a Current Coupled Schr\"odinger Equation With Self
  Interaction

abstract:
  Recently non-topological chiral soliton solutions were obtained in a
derivatively coupled non-linear Schr\"odinger model in 1+1 dimensions. We
extend the analysis to include a more general self-coupling potential (which
includes the previous cases) and find chiral soliton solutions. Interestingly
even the magnitude of the velocity is found to be fixed. Energy and U(1) charge
associated with this non-topological chiral solitons are also obtained.


title:
Scaling Laws for the Market Microstructure of the Interdealer Broker
  Markets

abstract:
  We propose a series of simple models for the microstructure of a double
auction market without intermediaries. We specialize to those markets, such
interdealer broker markets, which are dominated by professional traders, who
trade mainly through limit orders, watch markets closely, and move their limit
order prices frequently. We model these markets as a set of buyers and a set of
sellers diffusing in price space and interacting through an annihilation
interaction. We seek to compute the purely statistical effects of the presence
of large numbers of traders, as scaling laws on various measures of liquidity,
and to this end we allow our model very few parameters. We find that the
bid-offer spread scales as $\sqrt{1/{\rm Deal Rate}}$.In addition we
investigate the scaling of other intuitive relationships, such as the relation
between fluctuations of the best bid/offer and the density of buyers/sellers.
We then study this model and its scaling laws under the influence of random
disturbances to trader drift, trader volatility, and entrance rate. We also
study possible extensions to the model, such as the addition of market order
traders, and an interaction that models momentum-type trading. Finally, we
discuss how detailed simulations may be carried out to study scaling in all of
these settings, and how the models may be tested inactual markets.