title:
Generalized nonlinear sigma model approach to alternating spin chains
  and ladders

abstract:
  We generalize the nonlinear sigma model treatment of quantum spin chains to
cases including ferromagnetic bonds. When these bonds are strong enough, the
classical ground state is no longer the standard Neel order and we present an
extension of the known formalism to deal with this situation. We study the
alternating ferromagnetic-antiferromagnetic spin chain introduced by Hida. The
smooth crossover between decoupled dimers and the Haldane phase is
semi-quantitatively reproduced. We study also a spin ladder with diagonal
exchange couplings that interpolates between the gapped phase of the two-leg
spin ladder and the Haldane phase. Here again we show that there is good
agreement between DMRG data and our analytical results.


title: Spin dynamics of quantum and classical Heisenberg dimers

abstract:
  Analytical solutions for the time-dependent autocorrelation function of the
classical and quantum mechanical spin dimer with arbitrary spin are presented
and compared. For large spin quantum numbers or high temperature the classical
and the quantum dimer become more and more similar, yet with the major
difference that the quantum autocorrelation function is periodic in time
whereas the classical is not.


title: Collisional Cooling of a Charged Granular Medium

abstract:
  The dissipation rate due to inelastic collisions between equally charged,
insulating particles in a dilute granular medium is calculated. It is equal to
the known dissipation rate for uncharged granular media multiplied by a
Boltzmann-like factor, that originates from Coulomb interactions. We include
particle correlations by introducing an effective potential, that replaces the
bare Coulomb potential in the Boltzmann factor. All results are confirmed by
computer simulations.


title: Functional Renormalization Description of the Roughening Transition

abstract:
  We reconsider the problem of the static thermal roughening of an elastic
manifold at the critical dimension $d=2$ in a periodic potential, using a
perturbative Functional Renormalization Group approach. Our aim is to describe
the effective potential seen by the manifold below the roughening temperature
on large length scales. We obtain analytically a flow equation for the
potential and surface tension of the manifold, valid at all temperatures. On a
length scale $L$, the renormalized potential is made up of a succession of
quasi parabolic wells, matching onto one another in a singular region of width
$\sim L^{-6/5}$ for large $L$. We also obtain numerically the step energy as a
function of temperature, and relate our results to the existing experimental
data on $^4$He. Finally, we sketch the scenario expected for an arbitrary
dimension $d<2$ and examine the case of a non local elasticity which is
realized physically for the contact line.


title:
Electrodynamics of Nd0.7Sr0.3MnO3 Single Crystal investigated by Optical
  Conductivity Analyses

abstract:
  We investigated temperature dependent optical conductivity spectra of
Nd0.7Sr0.3MnO3 single crystal. We found that polishing and surface scattering
effects on the Nd0.7Sr0.3MnO3 crystal surfaces could significantly distort the
optical responses, especially in the mid-infrared region. However, oxygen
annealing and gold normalization processes seemed to remedy the distortions. We
found that the spectral weight of Nd0.7Sr0.3MnO3 in the metallic state might be
composed of a Drude carrier term and a strong incoherent mid-infrared
absorptions. The temperature dependence of the spectral weight suggests that
the electron-lattice coupling should be important in optical properties of
colossal magnetoresistance manganites.


title: Non-Fermi behavior of the disorder electronic system

abstract:
  We showed that an electronic system with weak disorder considered in the
finite-charge infinite U Hubbard model can present a non-Fermi behavior. The
imaginary pert of the self-energy has been calculated and a linear temperature
dependence was obtained. This result is in agreement with the non-Fermi
behavior observed at the insulator-metal crossover in La_{2-x}Sr_xCuO_4.


title: Step fluctuations and random walks

abstract:
  The probability distribution p(l) of an atom to return to a step at distance
l from the detachment site, with a random walk in between, is exactly
enumerated. In particular, we study the dependence of p(l) on step roughness,
presence of other reflecting or absorbing steps, interaction between steps and
diffusing atom, as well as concentration of defects on the terrace neighbouring
the step. Applying Monte Carlo techniques, the time evolution of equilibrium
step fluctuations is computed for specific forms of return probabilities.
Results are compared to previous theoretical and experimental findings.


title: Optimal Path in Two and Three Dimensions

abstract:
  We apply the Dijkstra algorithm to generate optimal paths between two given
sites on a lattice representing a disordered energy landscape. We study the
geometrical and energetic scaling properties of the optimal path where the
energies are taken from a uniform distribution. Our numerical results for both
two and three dimensions suggest that the optimal path for random uniformly
distributed energies is in the same universality class as the directed
polymers. We present physical realizations of polymers in disordered energy
landscape for which this result is relevant.


title:
Probability Distribution of the Shortest Path on the Percolation
  Cluster, its Backbone and Skeleton

abstract:
  We consider the mean distribution functions Phi(r|l), Phi(B)(r|l), and
Phi(S)(r|l), giving the probability that two sites on the incipient percolation
cluster, on its backbone and on its skeleton, respectively, connected by a
shortest path of length l are separated by an Euclidean distance r. Following a
scaling argument due to de Gennes for self-avoiding walks, we derive analytical
expressions for the exponents g1=df+dmin-d and g1B=g1S-3dmin-d, which determine
the scaling behavior of the distribution functions in the limit x=r/l^(nu) much
less than 1, i.e., Phi(r|l) proportional to l^(-(nu)d)x^(g1), Phi(B)(r|l)
proportional to l^(-(nu)d)x^(g1B), and Phi(S)(r|l) proportional to
l^(-(nu)d)x^(g1S), with nu=1/dmin, where df and dmin are the fractal dimensions
of the percolation cluster and the shortest path, respectively. The theoretical
predictions for g1, g1B, and g1S are in very good agreement with our numerical
results.


title:
Quantum Monte Carlo simulations of the half-filled two-dimensional Kondo
  lattice model

abstract:
  The 2D half-filled Kondo lattice model with exchange J and nearest neighbor
hopping t is considered. It is shown that this model belongs to a class of
Hamiltonians for which zero-temperature auxiliary field Monte Carlo methods may
be efficiently applied. We compute the staggered moment, spin and quasiparticle
gaps on lattice sizes up to 12 X 12. The competition between the RKKY
interaction and Kondo effect leads to a continuous quantum phase transition
between antiferromagnetic and spin-gaped insulators. This transition occurs at
J/t = 1.45 \pm 0.05.


title: One-dimensional transport in bundles of single-walled carbon nanotubes

abstract:
  We report measurements of the temperature and gate voltage dependence for
individual bundles (ropes) of single-walled nanotubes. When the conductance is
less than about e^2/h at room temperature, it is found to decrease as an
approximate power law of temperature down to the region where Coulomb blockade
sets in. The power-law exponents are consistent with those expected for
electron tunneling into a Luttinger liquid. When the conductance is greater
than e^2/h at room temperature, it changes much more slowly at high
temperatures, but eventually develops very large fluctuations as a function of
gate voltage when sufficiently cold. We discuss the interpretation of these
results in terms of transport through a Luttinger liquid.


title: Sandpiles on fractal bases: pile shape and phase segregation

abstract:
  Sandpiles have become paradigmatic systems for granular flow studies in
statistical physics. New directions of investigations are discussed here.
Rather than varying the nature of the pile (sand, salt, rice,..) we have
investigated changes in the boundary conditions. We have investigated
experimentally and numerically sandpile structures on bases having a fractal
perimeter. This type of perimeter induces the formation of a quite complex set
of ridges and valleys. A screening effect of the valleys is observed and
depends on the angle of repose. Binary granular systems have also been
investigated on such bases: a spectacular demixing is obtained along the
valleys. This type of phase segregation is discussed with respect to numerical
studies.


title: Electronic structure and dimerization of a single monatomic gold wire

abstract:
  The electronic structure of a single monatomic gold wire is presented for the
first time. It has been obtained with state-of-the-art ab-initio full-potential
density-functional (DFT) LMTO (linearized muffin-tin orbital) calculations
taking into account relativistic effects. For stretched structures in the
experimentally accessible range the conduction band is exactly half-filled,
whereas the band structures are more complex for the optimized structure. By
studying the total energy as a function of unit-cell length and of a possible
bond-length alternation we find that the system can lower its total energy by
letting the bond lengths alternate leading to a structure containing separated
dimers with bond lengths of about 2.5 \AA, largely independent of the
stretching. However, first for fairly large unit cells (above roughly 7 \AA),
is the total-energy gain upon this dimerization comparable with the energy
costs upon stretching. We propose that this together with band-structure
effects is the reason for the larger interatomic distances observed in recent
experiments. We find also that although spin-orbit couplings lead to
significant effects on the band structure, the overall conclusions are not
altered, and that finite Au_2, Au_4, and Au_6 chains possess electronic
properties very similar to those of the infinite chain.


title:
Exact solution for two interacting electrons on artificial atoms and
  molecules in solids

abstract:
  We present a general scheme for finding the exact eigenstates of two
electrons, with on-site repulsive potentials U_i, on I impurities in a
macroscopic crystal. The model describes impurities in doped semiconductors and
artificial molecules in quantum dots. For quantum dots, the energy cost for
adding two electrons is bounded by the single-electron spectrum, and does not
diverge when U_i approaches infinity, implying limitations on the validity of
the Coulomb blockade picture. Analytic applications on a one-dimensional chain
yield quantum delocalization and magnetic transitions.


title: Freezing transition of the vortex liquid in anisotropic superconductors

abstract:
  We study the solid-liquid transition of a model of pancake vortices in
laminar superconductors using a density functional theory of freezing. The
physical properties of the system along the melting line are discussed in
detail. We show that there is a very good agreement with experimental data in
the shape and position of the first order transition in the phase diagram and
in the magnitude and temperature dependence of the magnetic induction jump at
the transition. We analyze the validity of the Lindemann melting criterion and
the Hansen-Verlet freezing criterion. Both criteria are shown to be good to
predict the phase diagram in the region where a first order phase transition is
experimentally observed.


title: Liquid-Gas phase transition in Bose-Einstein Condensates

abstract:
  We study the effects of a repulsive three-body interaction on a system of
trapped ultra-cold atoms in a Bose-Einstein condensed state. The corresponding
$s-$wave non-linear Schr\"{o}dinger equation is solved numerically and also by
a variational approach. A first-order liquid-gas phase transition is observed
for the condensed state up to a critical strength of the effective three-body
force.


title:
Quantum Dynamics of Pseudospin Solitons in Double-Layer Quantum Hall
  Systems

abstract:
  Pseudospin solitons in double-layer quantum Hall systems can be introduced by
a magnetic field component coplanar with the electrons and can be pinned by
applying voltages to external gates. We estimate the temperature below which
depinning occurs predominantly via tunneling and calculate low-temperature
depinning rates for realistic geometries. We discuss the local changes in
charge and current densities and in spectral functions that can be used to
detect solitons and observe their temporal evolution.


title:
Paramagnetic limiting of the upper critical field of the layered organic
  superconductor $\kappa -(BEDT-TTF)_2Cu(SCN)_2$

abstract:
  We report detailed measurements of the interlayer magnetoresistance of the
layered organic superconductor $\kappa -(BEDT-TTF)_2Cu(SCN)_2$ for temperatures
down to 0.5 K and fields up to 30 tesla. The upper critical field is determined
from the resistive transition for a wide range of temperatures and field
directions. For magnetic fields parallel to the layers, the upper critical
field increases approximately linearly with decreasing temperature. The upper
critical field at low temperatures is compared to the Pauli paramagnetic limit,
at which singlet superconductivity should be destroyed by the Zeeman splitting
of the electron spins. The measured value is comparable to a value for the
paramagnetic limit calculated from thermodynamic quantities but exceeds the
limit calculated from BCS theory. The angular dependence of the upper critical
field shows a cusp-like feature for fields close to the layers, consistent with
decoupled layers.


title: Learning, competition and cooperation in simple games

abstract:
  The minority model was introduced to study the competition between agents
with limited information. It has the remarkable feature that, as the amount of
information available increases, the collective gain made by the agents is
reduced. This crowd effect arises from the fact that only a minority can profit
at each moment, while all agents make their choices using the same input. We
show that the properties of the model change drastically if the agents make
choices based on their individual stories, keeping all remaining rules
unaltered. This variation reduces the intrinsic frustration of the model, and
improves the tendency towards cooperation and self organization. We finally
study the stable mixing of individual and collective behavior.


title: Phonon mediated drag in double layer two dimensional electron systems

abstract:
  Experiments studying phonon mediated drag in the double layer two dimensional
electron gas system are reported. Detailed measurements of the dependence of
drag on temperature, layer spacing, density ratio, and matched density are
discussed. Comparisons are made to theoretical results [M. C. Bonsager et al.,
Phys. Rev. B 57, 7085 (1998)] which propose the existence of a new coupled
electron-phonon collective mode. The layer spacing and density dependence at
matched densities for samples with layer spacings below 2600 A do not support
the existence of this mode, showing behavior expected for independent electron
and phonon systems. The magnitude of the drag, however, suggests the alternate
limit; one in which electrons and phonons are strongly coupled. The results for
still larger layer spacing show significant discrepancies with the behavior
expected for either limit.


title: Elongation of confined ferrofluid droplets under applied fields

abstract:
  Ferrofluids are strongly paramagnetic liquids. We study the behavior of
ferrofluid droplets confined between two parallel plates with a weak applied
field parallel to the plates. The droplets elongate under the applied field to
reduce their demagnetizing energy and reach an equilibrium shape where the
magnetic forces balance against the surface tension. This elongation varies
logarithmically with aspect ratio of droplet thickness to its original radius,
in contrast to the behavior of unconfined droplets. Experimental studies of a
ferrofluid/water/surfactant emulsion confirm this prediction.


title: Minimal Potentials with Very Many Minima

abstract:
  We demonstrate, by construction, that simple renormalizable matrix potentials
with S_N, as opposed to O(N), symmetry can exhibit an exponentially large
number of inequivalent deep local minima.


title: Irreversible Magnetization of Pin-Free Type II Superconductors

abstract:
  The magnetization curve of a type II superconductor in general is hysteretic
even when the vortices exhibit no volume or surface pinning. This geometric
irreversibility, caused by an edge barrier for flux penetration, is absent only
when the superconductor has precisely ellipsoidal shape or is a wedge with a
sharp edge where the flux lines can penetrate. A quantitative theory of this
irreversibility is presented for pin-free disks and strips with constant
thickness. The resulting magnetization loops are compared with the reversible
magnetization curves of ideal ellipsoids.


title: QHE of Bilayer Systems in the Presence of Tunneling -- $\nu=1/q$ case --

abstract:
  Transport properties of bilayer quantum Hall systems at $\nu=1/q$, where $q$
is an odd integer, are investigated. The edge theory is used for the
investigation, since tunneling between the two layers is assumed to occur on
the edge of the sample because of the bulk incompressibility. It is shown that
in the case of the independent Laughlin state tunneling is irrelevant when
$\nu<1/2$ in the low temperature and long wave length limit. The temperature
dependence of two-terminal conductance of the system in which only one of the
two layers is contacted with electrode is discussed.


title: Macroscopic Finite Size Effects in Relaxational Processes

abstract:
  We present results on dynamical processes that exhibit a stretched
exponential relaxation. When the relaxation is a result of two competing
exponential processes, the size of the system, although macroscopic, play a
dominant role. There exist a crossover time tx that depends logarithmically on
the size of the system, above which, the relaxation changes from a stretched
exponential to a simple exponential decay. The decay rate also depends
logarithmically on the size of the system. The results are relevant to
large-scale Monte-Carlo simulations and should be amenable to experiments in
low-dimensional macroscopic systems and mesoscopic systems.


title:
Magnetic clusters formation in Li_{1-x}Ni_{1+x}O_2 compounds :
  experiments and numerical simulations

abstract:
  The magnetic properties of Li_{1-x}Ni_{1+x}O_2 compounds with x ranging
between 0.02 and 0.2 are investigated. Magnetization and ac susceptibility
measured at temperatures between 2 K and 300 K reveal a high sensitivity to x,
the excess Nickel concentration. We introduce a percolation model describing
the formation of Ni clusters and use an Ising model to simulate their magnetic
properties. Numerical results, obtained by a Monte-Carlo technique, are
compared to the experimental data. We show the existence of a critical
concentration, x_c = 0.136, locating the Ni percolation threshold. The system
is superparamagnetic for x<x_c, while it is ferrimagnetic for x>x_c. The 180
Ni-O-Ni inter-plane super-exchange coupling J_\perp \simeq -110K is confirmed
to be the predominant magnetic interaction. From the low temperature behavior,
we find a clear indication of a 90 Ni-O-Ni intra-plane antiferromagnetic
interaction $J_\parallel \simeq -1.5K$ which implies magnetic frustration.


title: Anomalous Size Dependence of Relaxational Processes

abstract:
  We consider relaxation processes that exhibit a stretched exponential
behavior. We find that in those systems, where the relaxation arises from two
competing exponential processes, the size of the system may play a dominant
role. Above a crossover time tx that depends logarithmically on the size of the
system the relaxation changes from a stretched exponential to a simple
exponential decay, where the decay rate also depends logarithmically on the
size of the system. This result is relevant to large-scale Monte-Carlo
simulations and should be amenable to experimental verification in
low-dimensional and mesoscopic systems.


title: Anomalous Transient Current in Nonuniform Semiconductors

abstract:
  Nonequilibrium processes in semiconductors are considered with highly
nonuniform initial densities of charge carriers. It is shown that there exist
such distributions of charge densities under which the electric current through
a sample displays quite abnormal behaviour flowing against the applied voltage.
The appearance of this negative electric current is a transient phenomenon
occurring at the initial stage of the process. After this anomalous negative
fluctuation, the electric current becomes normal, i.e. positive as soon as the
charge density becomes more uniform. Several possibilities for the practical
usage of this effect are suggested.


title:
Field-Theoretical description of the crossover between BCS and BEC in a
  non-Fermi superconductor

abstract:
  Using the field-theoretical methods we studied the evolution from BCS
description of a non-Fermi superconductor to that of Bose-Einstein condensation
(BEC) in one loop approximation. We showed that the repulsive interaction
between composite bosons is determined by the exponent $\alpha$ of the Anderson
propagator in a two dimensional model. For $\a\neq 0$ the crossover is also
continous and for $\a=0$ we obtain the case of the Fermi liquid.


title:
Spin-Vacancy-Induced Long-Range Order in a New Haldane-Gap
  Antiferromagnet

abstract:
  Magnetic susceptibility, high-field magnetization and inelastic neutron
scattering experiments are used to study the magnetic properties of a new S=1
quasi-1-dimensional antiferromagnet PbNi2V2O8. Inter-chain interactions are
shown to be almost, but not quite, strong enough to destroy the nonmagnetic
singlet ground state and the energy gap in the magnetic excitation spectrum.
Substituting nonmagnetic Mg$^{2+}$ (S=0) ions for Ni$^{2+}$ (S=1) induces a
magnetically ordered state at low temperatures. To our knowledge this is the
first observation of doping-induced long-range order in a Haldane-gap system.


title:
Theory of Low Temperature Electron Spin Resonance in Half-integer Spin
  Antiferromagnetic Chains

abstract:
  A theory of low temperature (T) electron spin resonance (ESR) in half-integer
spin antiferromagnetic chains is developed using field theory methods and
avoiding previous approximations. It is compared to experiments on Cu benzoate.
Power laws are predicted for the line-width broadening due to various types of
anisotropy. At T -> 0, zero width absorption peaks occur in some cases. The
second ESR peak in Cu benzoate, observed at T<.76K, is argued not to indicate
Neel order as previously claimed, but to correspond to a sine-Gordon "breather"
excitation.


title: Density of states in coupled chains with off-diagonal disorder

abstract:
  We compute the density of states (d.o.s.) in N coupled chains with random
hopping. At zero energy, the d.o.s. shows a singularity that strongly depends
on the parity of N. For odd N, the d.o.s. is proportional to 1/(E (\ln |E|)^3),
with and without time-reversal symmetry. For even N, the d.o.s. is proportional
to \ln |E| in the presence of time-reversal symmetry, while there is a
pseudogap, d.o.s. proportional to E \ln |E|, in the absence of time-reversal
symmetry.


title:
Nonuniversality in quantum wires with off-diagonal disorder: a geometric
  point of view

abstract:
  It is shown that, in the scaling regime, transport properties of quantum
wires with off-diagonal disorder are described by a family of scaling equations
that depend on two parameters: the mean free path and an additional continuous
parameter. The existing scaling equation for quantum wires with off-diagonal
disorder [Brouwer et al., Phys. Rev. Lett. 81, 862 (1998)] is a special point
in this family. Both parameters depend on the details of the microscopic model.
Since there are two parameters involved, instead of only one, localization in a
wire with off-diagonal disorder is not universal. We take a geometric point of
view and show that this nonuniversality follows from the fact that the group of
transfer matrices is not semi-simple. Our results are illustrated with
numerical simulations for a tight-binding model with random hopping amplitudes.


title: Electronic versus Phononic Friction of Xenon on Silver

abstract:
  Molecular dynamics simulations of a Xe monolayer sliding on Ag(001) and
Ag(111) are carried out in order to ascertain the microscopic origin of
friction. For several values of the electronic contribution to the friction of
individual Xe atoms, the intra-overlayer phonon dissipation is calculated as a
function of the corrugation amplitude of the substrate potential, which is a
pertinent parameter to consider. Within the accuracy of the numerical results
and the uncertainty with which the values of the relevant parameters are known
at present, we conclude that electronic and phononic dissipation channels are
of similar importance. While phonon friction gives rise to the rapid variation
with coverage, the electronic friction provides a roughly coverage-independent
contribution to the overall sliding friction.


title:
Negative electrostatic contribution to the bending rigidity of charged
  membranes and polyelectrolytes screened by multivalent counterions

abstract:
  Bending rigidity of a charged membrane or a charged polyelectrolyte screened
by monovalent counterions is known to be enhanced by electrostatic effects. We
show that in the case of screening by multivalent counterions the electrostatic
effects reduce the bending rigidity. This inversion of the sign of the
electrostatic contribution is related to the formation of two-dimensional
strongly correlated liquids (SCL) of counterions at the charged surface due to
strong lateral repulsion between them. When a membrane or a polyelectrolyte is
bent, SCL is compressed on one side and stretched on the other so that
thermodynamic properties of SCL contribute to the bending rigidity.
Thermodynamic properties of SCL are similar to those of Wigner crystal and are
anomalous in the sense that the pressure, compressibility and screening radius
of SCL are negative. This brings about substantial negative correction to the
bending rigidity. For the case of DNA this effect qualitatively agrees with
experiment.


title: Nonlinear Maxwell Theory and Electrons in Two Dimensions

abstract:
  We consider a system of nonlinear equations that extends the Maxwell theory.
It was pointed out in a previous paper that symmetric solutions of these
equations display properties characteristic of magnetic oscillations. In this
paper I study a discrete model of the equations in two dimensions. This leads
to the discovery of a new mechanism of vortex lattice formation. Namely, when a
parameter corresponding to a magnetic field normal to the surface increases
above a certain critical level, the trivial uniform-magnetic-field solution
becomes, in a certain sense, unstable and a periodic vortex lattice solution
emerges. The discrete vortex solutions are proven to exist, and can also be
found numerically with high accuracy. Description of magnetic vortices given by
the equations is optical in spirit, and may be particularly attractive in the
context of high-$T_c$ superconductivity and the quantum Hall effects. Moreover,
analysis of parameters involved in the discrete theory suggests existence of
continuous domain solutions---a conjecture that seems unobvious on grounds of
the current topological and variational methods.


title:
Microscopic Simulation of Reaction-Diffusion Processes and Applications
  to Population Biology and Product Marketing

abstract:
  We simulate reaction-diffusion processes with discrete fields. We use a novel
algorithm to simulate different autocatalytic processes with trace densities.
Anderson localization with a diffusive potential is studied. A
reaction-diffusion process with dynamic localization is discussed in a
marketing context.


title: Trivial Ground State Structure in the Two-Dimensional Ising Spin Glass

abstract:
  We study how the ground state of the two-dimensional Ising spin glass with
Gaussian interactions in zero magnetic field changes on altering the boundary
conditions. The probability that relative spin orientations change in a region
far from the boundary goes to zero with the (linear) size of the system L like
L^{-lambda}, where lambda = -0.70 +/- 0.08. We argue that lambda is equal to
d-d_f where d (=2) is the dimension of the system and d_f is the fractal
dimension of a domain wall induced by changes in the boundary conditions. Our
value for d_f is consistent with earlier estimates. These results show that, at
zero temperature, there is only a single pure state (plus the state with all
spins flipped) in agreement with the predictions of the droplet model.


title:
Effective potential for classical field theories subject to stochastic
  noise

abstract:
  Classical field theories coupled to stochastic noise provide an extremely
powerful tool for modeling phenomena as diverse as turbulence,
pattern-formation, and the structural development of the universe itself. In
this Letter we sketch a general formalism that maps such systems into a field
theory language, and demonstrate how to extract the one-loop physics for an
arbitrary classical field theory coupled to Gaussian noise. The amplitude of
the noise two-point function serves as the loop-counting parameter and is the
analog of Planck's constant hbar in quantum field theory. We define the
effective action and the effective potential, and derive a general formula for
the one-loop effective potential of a classical field theory coupled to
translation-invariant Gaussian noise.

categories: cond-mat.stat-mech chao-dyn hep-th nlin.CD


title:
Blocking and Persistence in the Zero-Temperature Dynamics of Homogeneous
  and Disordered Ising Models

abstract:
  A ``persistence'' exponent theta has been extensively used to describe the
nonequilibrium dynamics of spin systems following a deep quench: for
zero-temperature homogeneous Ising models on the d-dimensional cubic lattice,
the fraction p(t) of spins not flipped by time t decays to zero like
t^[-theta(d)] for low d; for high d, p(t) may decay to p(infinity)>0, because
of ``blocking'' (but perhaps still like a power). What are the effects of
disorder or changes of lattice? We show that these can quite generally lead to
blocking (and convergence to a metastable configuration) even for low d, and
then present two examples --- one disordered and one homogeneous --- where p(t)
decays exponentially to p(infinity).


title:
A Consistent Picture of Electronic Raman Scattering and Infrared
  Conductivity in the Cuprates

abstract:
  Calculations are presented for electronic Raman scattering and infrared
conductivity in a $d_{x^{2}-y^{2}}$ superconductor including the effects of
elastic scattering via anisotropic impurities and inelastic spin-fluctuation
scattering. A consistent description of experiments on optimally doped Bi-2212
is made possible by considering the effects of correlations on both inelastic
and elastic scattering.


title: One-Particle Spectral Properties of Mott-Hubbard Insulators

abstract:
  We use an exact holon and spinon Landau-liquid functional which describes the
holon - spinon interactions at all scattering orders, to study correlation
functions of integrable multicomponent many-particle problems showing both
linear and non-linear energy bands. We consider specific cases when the
dominant non-linear band terms are quadratic and apply our results to the
evaluation of the 1D Hubbard model correlation functions beyond conformal-field
theory.


title:
Excess Currents Larger than the Point Contact Limit in Normal Metal
  -Superconducting Junctions

abstract:
  In a point contact NS junction, perfect Andreev reflection occurs over a
range of voltages equal to the superconducting energy gap, producing an excess
current of $I_{exc} = (4/3)(2e\Delta/h)$. If the superconductor has a finite
width, rather than the infinite width of the point contact, one cannot neglect
superfluid flow inside the superconducting contact. The energy range available
for perfect Andreev reflections then becomes larger than the superconducting
gap, since superfluid flow alters the dispersion relation inside the finite
width superconductor. We find a maximum excess current of approximately
$(7/3)(2e\Delta/h)$ when the width of the superconductor is approximately 7/3
times the width of the normal metal.


title:
Ballistic Transport and Andreev Resonances in Nb-In Superconducting
  Contacts to InAs and LTG-GaAs

abstract:
  We have formed superconducting contacts in which Cooper pairs incident from a
thick In layer must move through a thin Nb layer to reach a semiconductor,
either InAs or low temperature grown (LTG) GaAs. The effect of pair tunneling
through the Nb layer can be seen by varying the temperature through the
critical temperature of In. Several of the In/Nb-InAs devices display a peak in
the differential conductance near zero-bias voltage, which is strong evidence
of ballistic transport across the NS interface. The differential conductance of
the In/Nb-(LTG) GaAs materials system displays conductance resonances of
McMillan-Rowell type. These resonant levels exist within a band of conducting
states inside the energy gap, formed from excess As incorporation into the
(LTG) GaAs during growth. Electrons propagating in this band of states multiply
reflect between the superconductor and a potential barrier in the GaAs
conduction band to form the conductance resonances. A scattering state theory
of the differential conductance, including Andreev reflections from the
composite In/Nb contact, accounts for most qualitative features in the data.


title: Possible Ordered States in the 2D Extended Hubbard Model

abstract:
  Possible ordered states in the 2D extended Hubbard model with on-site (U>0)
and nearest-neighbor (V) interaction are examined near half filling, with
emphasis on the effect of finite V. First, the phase diagram at absolute zero
is determined in the mean field approximation. For $V<0$, a state where
d_{x^{2}-y^{2}}-wave superconductivity (dSC), commensurate spin-density-wave
(SDW) and $\pi$-triplet pair coexist is seen to be stabilized. Here, the
importance of $\pi$-triplet pair on the coexistence of dSC and SDW is
indicated. This coexistent state is hampered by the phase separation (PS),
which is generally expected to occur in the presence of finite-range attractive
interaction, but survives. For V>0, a state where commensurate
charge-density-wave (CDW), SDW and ferromagnetism (FM) coexist is seen to be
stabilized. Here, the importance of FM on the coexistence of CDW and SDW is
indicated. Next, in order to examine the effects of fluctuation on each mean
field ordered state, the renormalization group method for the special case that
the Fermi level lies just on the saddle points, ($\pi$,0) and (0,$\pi$), is
applied. The crucial difference from the mean field result is that
superconductivity can arise even for U>0 and $V\geq0$, where the
superconducting gap symmetry is d_{x^{2}-y^{2}}-wave for U>4V and s-wave for
U<4V. Finally, the possibilities that the mean field coexistent states survive
in the presence of fluctuation are discussed.


title:
Frictional Drag Between Coupled 2D Hole Gases in GaAs/AlGaAs
  Heterostructures

abstract:
  We report on the first measurements of the drag effect between coupled
2D-hole gases. We investigate the coupling by changing the carrier densities in
the quantum wells, the widths of the barriers between the gases and the
perpendicular magnetic field. From the data we are able to attribute the
frictional drag to phonon coupling, because the non-parabolicity allows to tune
the Fermi wavevector and the Fermi velocity separately and, thereby, to
distinguish between phonon- and plasmon-dominated coupling.


title: Effective action for stochastic partial differential equations

abstract:
  Stochastic partial differential equations (SPDEs) are the basic tool for
modeling systems where noise is important. In this paper we set up a functional
integral formalism and demonstrate how to extract all the one-loop physics for
an arbitrary SPDE subject to arbitrary Gaussian noise. It is extremely
important to realize that Gaussian noise does not imply that the field
variables undergo Gaussian fluctuations, and that these non-quantum field
theories are fully interacting. Experience with quantum field theories (QFTs)
has taught us that one-loop physics is often quite adequate to give a good
description of the salient issues, and furthermore offers marked technical
advantages: We can sidestep the complications inherent in the
Martin-Siggia-Rose formalism (the SPDE analog of the BRST formalism used in
QFT) and instead focus attention on a minimalist approach that uses only the
physical fields (this ``direct approach'' is the SPDE analog of canonical
quantization using physical fields.) We show how to define the effective action
to all loops, and then focus on the one-loop effective action, and its
specialization to constant fields: the effective potential. An important result
is that the amplitude of the two-point function governing the noise acts as the
loop-counting parameter and is the analog of Planck's constant hbar in this
SPDE context. We derive a general expression for the one-loop effective
potential of an arbitrary SPDE subject to translation-invariant Gaussian noise,
and compare this with the one-loop potential for QFT.


title:
Field Theory of Critical Behaviour in Driven Diffusive Systems with
  Quenched Disorder

abstract:
  We present a field theoretic renormalization group study for the critical
behaviour of a uniformly driven diffusive system with quenched disorder, which
is modelled by different kinds of potential barriers between sites. Due to
their symmetry properties, these different realizations of the random potential
barriers lead to three different models for the phase transition to transverse
order and to one model for the phase transition to longitudinal order all
belonging to distinct universality classes. In these four models that have
different upper critical dimensions d_{c} we find the critical scaling
behaviour of the vertex functions in spatial dimensions d < d_{c} . Its
deviation from purely diffusive behaviour is characterized by the
anomaly-exponent \eta that we calculate at first and second order, respectively
in $\epsilon = d_{c} -d$. In each model \eta turns out to be positive which
means superdiffusive spread of density fluctuations in the driving force
direction.


title: Towards Strong-Coupling Generalization of the Bogoliubov Model

abstract:
  The well-known results concerning a dilute Bose gas with the short-range
repulsive interaction should be reconsidered due to a thermodynamic
inconsistency of the method being basic to much of the present understanding of
this subject and nonrelevant behaviour of the pair distribution function at
small boson separations. The aim of our paper is to propose a new way of
treating the dilute Bose gas with an arbitrary strong interaction. Using the
reduced density matrix of the second order and a variational procedure, this
way allows us to escape the inconsistency mentioned and operate with singular
potentials like the Lennard-Jones one. All the consideration concerns the zero
temperature.


title:
Multi-Chain Mean-Field Theory of Quasi One-Dimensional Quantum Spin
  Systems

abstract:
  A multi-chain mean-field theory is developed and applied to a two-dimensional
system of weakly coupled S=1/2 Heisenberg chains. The environment of a chain
C_0 is modeled by a number of neighbor chains C_d, d = +/-1,...,+/-n, with the
edge chains (d=+/-n) coupled to a staggered field. Using a quantum Monte Carlo
method, the effective (2n+1)-chain Hamiltonian is solved self-consistently for
$n$ up to 4. The results are compared with simulation results for the original
Hamiltonian on large rectangular lattices. Both methods show that the staggered
magnetization M for small interchain couplings alpha behaves as M=A*alpha^1/2
enhanced by a multiplicative logarithmic correction.


title: Instantons in the Langevin dynamics: an application to spin glasses

abstract:
  We develop a general technique to calculate the probability of transitions
over the barriers in spin-glasses in the framework of the dynamical theory. We
use Lagrangian formulation of the instanton dynamics in which the transitions
are represented by instantons. We derive the full set of the equations that
determine the instantons but instead of solving them directly we prove that an
instanton process can be mapped into a usual process going back in time which
simplifies the problem significantly. We apply this general considerations to a
simple example of the spherical Sherrington-Kirkpatrick model and we find the
probability of the transition between the metastable states which is in
agreement with physical expectations.


title:
Ground state parameters, finite-size scaling, and low-temperature
  properties of the two-dimensional S=1/2 XY model

abstract:
  We present high-precision quantum Monte Carlo results for the S=1/2 XY model
on a two-dimensional square lattice, in the ground state as well as at finite
temperature. The energy, the spin stiffness, the magnetization, and the
susceptibility are calculated and extrapolated to the thermodynamic limit. For
the ground state, we test a variety of finite-size scaling predictions of
effective Lagrangian theory and find good agreement and consistency between the
finite-size corrections for different quantities. The low-temperature behavior
of the susceptibility and the internal energy is also in good agreement with
theoretical predictions.


title: Short-range antiferromagnetic correlations in Kondo insulators

abstract:
  We study the influence of short range antiferromagnetic correlations between
local $f$-electrons on the transport and thermodynamic properties of Kondo
insulators, as first proposed by Coqblin et al. for metallic heavy fermions.
The inter-site magnetic correlations produce an effective bandwidth for the
$f$-electrons. They are treated on the same footing as the local Kondo
correlations such that two energy scales appear in our approach. We discuss the
competition between these two scales on the physical properties.


title:
Magnetic and quantum disordered phases in triangular-lattice Heisenberg
  antiferromagnets

abstract:
  We study, within the Schwinger-boson approach, the ground-state structure of
two Heisenberg antiferromagnets on the triangular lattice: the J1-J2 model,
which includes a next-nearest-neighbor coupling J2, and the
spatially-anisotropic J1-J'1 model, in which the nearest-neighbor coupling
takes a different value J'1 along one of the bond directions. The motivations
for the study of these systems range from general theoretical questions
concerning frustrated quantum spin models to the concrete description of the
insulating phase of some layered molecular crystals. For both models, the
inclusion of one-loop corrections to saddle-point results leads to the
prediction of nonmagnetic phases for particular values of the parameters J1/J2
and J'1/J1. In the case of the J1-J2 model we shed light on the existence of
such disordered quantum state, a question which is controversial in the
literature. For the J1-J'1 model our results for the ground-state energy,
quantum renormalization of the pitch in the spiral phase, and the location of
the nonmagnetic phases, nicely agree with series expansions predictions.


title: The structure of colloid-polymer mixtures

abstract:
  We investigate the structure of colloid-polymer mixtures by calculating the
structure factors for the Asakura-Oosawa model in the PY approximation. We
discuss the role of potential range, polymer concentration and polymer-polymer
interactions on the colloid-colloid structure. Our results compare reasonably
well with the recent experiments of Moussa\"{i}d et. al. for small wavenumber
$k$, but we find that the Hansen-Verlet freezing criterion is violated when the
liquid phase becomes marginal.


title:
Electron-Ion Structure Factors and the General Accuracy of Linear
  Response

abstract:
  We show that electron-ion structure factors in fluid metallic systems can be
well understood from an application of linear response in the electron system,
combined with hard-sphere like correlation for the ionic component. In
particular, we predict that electron-ion structure factors fall into two
general classes, one for high ($Z>3$) and one for low ($Z\leq2$) valence
metals, and make suggestions for experiments to test these ideas. In addition,
we show how the general success of electronic linear response for most metallic
systems stems in part from an intrinsic interference between atomic and
electronic length scales which weakens the nonlinear response. The main
exception to this is metallic hydrogen.


title:
Extending Linear Response: Inferences from Electron-Ion Structure
  Factors

abstract:
  Linear response methods applied to electron systems often display a level of
accuracy which is notable when viewed in terms of the strengths of perturbing
interactions. Neglect of higher response terms is in fact justifiable in many
cases and it can be shown to stem from an intrinsic interference between atomic
and electronic length scales. For fluid metallic systems it can be further
shown that electron-ion structure (increasingly accessible experimentally) can
be understood from an application of linear response in the electron system,
combined with hard-sphere like correlation for the ionic component.


title: Devil's staircase for a nonconvex interaction

abstract:
  We study ground-state orderings of particles in classical lattice-gas models
of adsorption on crystal surfaces. In the considered models, the energy of
adsorbed particles is a sum of two components, each one representing the energy
of a one-dimensional lattice gas with two-body interactions in one of the two
orthogonal lattice directions. This feature reduces the two-dimensional problem
to a one-dimensional one. The interaction energy in each direction is repulsive
and strictly convex only from distance 2 on, while its value at distance 1 can
be positive or negative, but close to zero.
  We show that if the decay rate of the interactions is fast enough, then
particles form 2-particle lattice-connected aggregates which are distributed in
the same most homogeneous way as particles whose interaction is strictly convex
everywhere. Moreover, despite the lack of convexity, the density of particles
versus the chemical potential appears to be a fractal curve known as the
complete devil's staircase.


title: Fractional statistic

abstract:
  We improve Haldane's formula which gives the number of configurations for $N$
particles on $d$ states in a fractional statistic defined by the coupling
$g=l/m$. Although nothing is changed in the thermodynamic limit, the new
formula makes sense for finite $N=pm+r$ with $p$ integer and $0<r\leq m.$ A
geometrical interpretation of fractional statistic is given in terms of
''composite particles''.


title:
Nonergodicity transitions in colloidal suspensions with attractive
  interactions

abstract:
  The colloidal gel and glass transitions are investigated using the idealized
mode coupling theory (MCT) for model systems characterized by short-range
attractive interactions. Results are presented for the adhesive hard sphere and
hard core attractive Yukawa systems. According to MCT, the former system shows
a critical glass transition concentration that increases significantly with
introduction of a weak attraction. For the latter attractive Yukawa system, MCT
predicts low temperature nonergodic states that extend to the critical and
subcritical region. Several features of the MCT nonergodicity transition in
this system agree qualitatively with experimental observations on the colloidal
gel transition, suggesting that the gel transition is caused by a low
temperature extension of the glass transition. The range of the attraction is
shown to govern the way the glass transition line traverses the phase diagram
relative to the critical point, analogous to findings for the fluid-solid
freezing transition.


title: Low energy transition in spectral statistics of 2D interactingfermions

abstract:
  We study the level spacing statistics $P(s)$ and eigenstate properties of
spinless fermions with Coulomb interaction on a two dimensional lattice at
constant filling factor and various disorder strength. In the limit of large
lattice size, $P(s)$ undergoes a transition from the Poisson to the
Wigner-Dyson distribution at a critical total energy independent of the number
of fermions. This implies the emergence of quantum ergodicity induced by
interaction and delocalization in the Hilbert space at zero temperature.


title: A symmetry group of a Thue-Morse quasicrystal

abstract:
  We present a method of coding general self-similar structures. In particular,
we construct a symmetry group of a one-dimensional Thue-Morse quasicrystal,
i.e., of a nonperiodic ground state of a certain translation-invariant,
exponentially decaying interaction.


title: Spin Liquid State around a Doped Hole in Insulating Cuprates

abstract:
  The numerically exact diagonalization study on small clusters of the t-J
model with the second- and third- neighbor hopping terms shows that a novel
spin liquid state is realized around a doped hole with momentum k=(pi,0) and
energy \sim 2J compared with that with (pi/2,pi/2) in insulating cuprates,
where the spin and charge degrees of freedom are approximately decoupled. Our
finding implies that the excitations in the insulating cuprates are mapped onto
the the d-wave resonating valence bond state.


title: Specific heat and magnetic order in LaMnO_{3+\delta}

abstract:
  Magnetic and specific-heat measurements are performed in three different
samples of LaMnO_{3+\delta}, with \delta=0.11, 0.15 and 0.26, presenting
important disorder effects, such as carrier localization, due to high amounts
of La and Mn vacancies. For the samples with \delta =0.11 and 0.15, magnetic
measurements show signatures of a two-step transition: as the temperature is
lowered, the system enters a ferromagnetic phase followed by a disorder-induced
cluster-glass state. Spin-wave-like contributions and an unexpected large
linear term are observed in the specific heat as a function of temperature. In
the sample with the highest vacancy content, \delta=0.26, the disorder is
sufficient to suppress even short-range ferromagnetic order and yield a
spin-glass-like state.


title:
Weak Field Hall Resistance and Effective Carrier Density Through
  Metal-Insulator Transition in Si-MOS Structures

abstract:
  We studied the weak field Hall voltage in 2D electron layers in Si-MOS
structures with different mobilities, through the metal-insulator transition.
In the vicinity of the critical density on the metallic side of the transition,
we have found weak deviations (about 6-20 %) of the Hall voltage from its
classical value. The deviations do not correlate with the strong temperature
dependence of the diagonal resistivity rho_{xx}(T). The smallest deviation in
R_{xy} was found in the highest mobility sample exhibiting the largest
variation in the diagonal resistivity \rho_{xx} with temperature (by a factor
of 5).


title: Self-Organized Criticality on Quasiperiodic Graphs

abstract:
  Self-organized critical models are used to describe the 1/f-spectra of rather
different physical situations like snow avalanches, noise of electric currents,
luminosities of stars or topologies of landscapes. The prototype of the
SOC-models is the sandpile model of Bak, Tang and Wiesenfeld (Phys. Rev. Lett.
59, (1987) 351). We implement this model on non-periodic graphs where it can
become either isotropic or anisotropic and compare its properties with the
periodic counterpart on the square lattice.


title: Compressibility of the Two-Dimensional infinite-U Hubbard Model

abstract:
  We study the interactions between the coherent quasiparticles and the
incoherent Mott-Hubbard excitations and their effects on the low energy
properties in the $U=\infty$ Hubbard model. Within the framework of a
systematic large-N expansion, these effects first occur in the next to leading
order in 1/N. We calculate the scattering phase shift and the free energy, and
determine the quasiparticle weight Z, mass renormalization, and the
compressibility. It is found that the compressibility is strongly renormalized
and diverges at a critical doping $\delta_c=0.07\pm0.01$. We discuss the nature
of this zero-temperature phase transition and its connection to phase
separation and superconductivity.


title: Phase Separation of Rigid-Rod Suspensions in Shear Flow

abstract:
  We analyze the behavior of a suspension of rigid rod-like particles in shear
flow using a modified version of the Doi model, and construct diagrams for
phase coexistence under conditions of constant imposed stress and constant
imposed strain rate, among paranematic, flow-aligning nematic, and log-rolling
nematic states. We calculate the effective constitutive relations that would be
measured through the regime of phase separation into shear bands. We calculate
phase coexistence by examining the stability of interfacial steady states and
find a wide range of possible ``phase'' behaviors.


title: Temperature scaling in a dense vibro-fluidised granular material

abstract:
  The leading order "temperature" of a dense two dimensional granular material
fluidised by external vibrations is determined. An asymptotic solution is
obtained where the particles are considered to be elastic in the leading
approximation. The velocity distribution is a Maxwell-Boltzmann distribution in
the leading approximation. The density profile is determined by solving the
momentum balance equation in the vertical direction, where the relation between
the pressure and density is provided by the virial equation of state. The
predictions of the present analysis show good agreement with simulation results
at higher densities where theories for a dilute vibrated granular material,
with the pressure-density relation provided by the ideal gas law, are in error.
The theory also predicts the scaling relations of the total dissipation in the
bed reported by McNamara and Luding (PRE v 58, p 813).


title: An Adiabatic Quantum Electron Pump

abstract:
  A quantum pumping mechanism which produces dc current or voltage in response
to a cyclic deformation of the confining potential in an open quantum dot is
reported. The voltage produced at zero current bias is sinusoidal in the phase
difference between the two ac voltages deforming the potential and shows random
fluctuations in amplitude and direction with small changes in external
parameters such as magnetic field. The amplitude of the pumping response
increases linearly with the frequency of the deformation. Dependencies of
pumping on the strength of the deformations, temperature, and breaking of
time-reversal symmetry are also investigated.


title:
Universality class for bootstrap percolation with $m=3$ on the cubic
  lattice

abstract:
  We study the $m=3$ bootstrap percolation model on a cubic lattice, using
Monte Carlo simulation and finite-size scaling techniques. In bootstrap
percolation, sites on a lattice are considered occupied (present) or vacant
(absent) with probability $p$ or $1-p$, respectively. Occupied sites with less
than $m$ occupied first-neighbours are then rendered unoccupied; this culling
process is repeated until a stable configuration is reached. We evaluate the
percolation critical probability, $p_c$, and both scaling powers, $y_p$ and
$y_h$, and, contrarily to previous calculations, our results indicate that the
model belongs to the same universality class as usual percolation (i.e.,
$m=0$). The critical spanning probability, $R(p_c)$, is also numerically
studied, for systems with linear sizes ranging from L=32 up to L=480: the value
we found, $R(p_c)=0.270 \pm 0.005$, is the same as for usual percolation with
free boundary conditions.


title: Density mismatch in thin diblock copolymer films

abstract:
  Thin films of diblock copolymer subject to gravitational field are simulated
by means of a cell dynamical system model. The difference in density of the two
sides of the molecule and the presence of the field causes the formation of
lamellar patterns with orientation parallel to the confining walls even when
they are neutral. The concentration profile of those films is analyzed in the
weak segregation regime and a functional form for the profile is proposed.


title:
Is the `Finite Bias Anomaly' in planar GaAs-Superconductor junctons
  caused by point-contact like structures?

abstract:
  We correlate transmission electron microscope (TEM) pictures of
superconducting In contacts to an AlGaAs/GaAs heterojunction with differential
conductance spectroscopy performed on the same heterojunction. Metals deposited
onto a (100) AlGaAs/GaAs heterostructure do not form planar contacts but,
during thermal annealing, grow down into the heterostructure along
crystallographic planes in pyramid-like `point contacts'. Random surface
nucleation and growth gives rise to a different interface transmission for each
superconducting point contact. Samples annealed for different times, and
therefore having different contact geometry, show variations in $dI/dV$
characteristic of ballistic transport of Cooper pairs, wave interference
between different point emitters, and different types of weak localization
corrections to Giaever tunneling. We give a possible mechanism whereby the
`finite bias anomaly' of Poirier et al. (Phys. Rev. Lett., {\bf 79}, 2105
(1997)), also observed in these samples, can arise by adding the conductance of
independent superconducting point emitters in parallel.


title: Low-lying excitations in superconducting bilayer systems

abstract:
  The ground and first excited state of two superconducting layers in
interaction are studied considering two different coupling terms, one
represented by the standard Josephson interaction, and one new, which is a
superexchange pairing force between bilayer pairs. It is shown that a
moderate-to strong Josephson interaction produces a low-lying collective state,
pictured as an out-of-phase oscillation of the BCS gauge angles of the two
layers. This antisymmetric angular oscillation might explain the 41 meV
resonance observed in the neutron scattering experiments. The bilayer pairs are
formed by electrons from different layers with an antiparallel orientation of
the spins, being related to the antiferromagnetic arrangement. The pair
operators within the layers together with the bilayer pairs generate by
commutation an so(5) algebra. It is shown that the transition between the
superconducting and antiferromagnetic phases can be explained assuming the
dependence on concentration of the bilayer pairing strength, with maximum at
half-filling.


title:
The sublattice magnetizations of the spin-s quantum Heisenberg
  antiferromagnets

abstract:
  We study the spin-$s$ quantum Heisenberg antiferromagnetic models in a magnon
theory free of any unphysical magnon state. Because the unphysical magnon
states are completely removed in the magnon Hamiltonians and during the
approximation process, we derive spin-s (s>1/2) reliable Neel temperature $T_N$
and reasonable sublattice magnetization unified for $T\leq T_N$.


title: Analytical ground state for the three-band Hubbard model

abstract:
  For the calculation of charge excitations as those observed in, e.g.,
photo-emission spectroscopy or in electron-energy loss spectroscopy, a correct
description of ground-state charge properties is essential. In strongly
correlated systems like the undoped cuprates this is a highly non-trivial
problem. In this paper we derive a non-perturbative analytical approximation
for the ground state of the three-band Hubbard model on an infinite, half
filled CuO_2 plane. By comparison with Projector Quantum Monte Carlo
calculations it is shown that the resulting expressions correctly describe the
charge properties of the ground state. Relations to other approaches are
discussed. The analytical ground state preserves size consistency and can be
generalized for other geometries, while still being both easy to interpret and
to evaluate.


title: Dynamical response of a weakly pinned two-dimensional Wigner crystal

abstract:
  Motivated by puzzling results of recent experiments, we re-examine the
response of a weakly pinned two-dimensional Wigner crystal to a uniform AC
electric field. We confirm that at some disorder and magnetic field dependent
frequency $\omega_p$, an inhomogeneously broadened absorption line emerges.
Although the line is conventionally broad in zero magnetic field, in strong
fields it appears as a sharp resonance whose width is related to the density of
states in the low-frequency tail of the zero-field phonon spectrum. This
behavior originates due to the long-range Coulomb interactions.


title: Universal Finite Size Scaling Functions in the 3D Ising Spin Glass

abstract:
  We study the three-dimensional Edwards-Anderson model with binary
interactions by Monte Carlo simulations. Direct evidence of finite-size scaling
is provided, and the universal finite-size scaling functions are determined.
Monte Carlo data are extrapolated to infinite volume with an iterative
procedure up to correlation lengths xi \approx 140. The infinite volume data
are consistent with a conventional power law singularity at finite temperature
Tc. Taking into account corrections to scaling, we find Tc = 1.156 +/- 0.015,
nu = 1.8 +/- 0.2 and eta = -0.26 +/- 0.04. The data are also consistent with an
exponential singularity at finite Tc, but not with an exponential singularity
at zero temperature.


title:
Precursor of Non-Fermi Liquid Behaviour in the One-Dimensional Periodic
  Anderson Model with Disorder

abstract:
  We have studied the one-dimensional periodic, symmetric Anderson model at
half filling in the presence of disorder using finite-temperature quantum Monte
Carlo techniques. We have examined for the first time the disorder both in
hybridization between the local $f$-orbitals and the conduction electrons and
in the local $f$-site energy, using a uniform distribution of width $\Delta$.
The $f$-orbital local magnetic moment, the uniform magnetic susceptibility, the
charge compressibility, and the nearest-neighbor magnetic correlation function
have been calculated as a function of the disorder distribution width $\Delta$.
We find that the disorder in hybridization has a dramatic effect on the
low-temperature magnetic properties exhibiting a non-Fermi-liquid behaviour,
and that for the range of temperature studied the magnetic susceptibility can
be scaled by a power law with an exponent that is in agreement with recent
experiments. On the other hand, disorder in the local $f$-orbital energy level
does not show a non-Fermi liquid behaviour.


title:
Tunneling Via Individual Electronic States in Ferromagnetic
  Nanoparticles

abstract:
  We measure electron tunneling via discrete energy levels in ferromagnetic
cobalt particles less than 4 nm in diameter, using non-magnetic electrodes. Due
to magnetic anisotropy, the energy of each tunneling resonance shifts as an
applied magnetic field rotates the particle's magnetic moment. We see both
spin-increasing and decreasing tunneling transitions, but we do not observe the
spin degeneracy at small magnetic fields seen previously in non-magnetic
materials. The tunneling spectrum is denser than predicted for independent
electrons, possibly due to spin-wave excitations.


title: Bcc $^4$He as a Coherent Quantum Solid

abstract:
  In this work we investigate implications of the quantum nature of bcc $^{4}$%
He. We show that it is a unique solid phase with both a lattice structure and
an Off-Diagonal Long Range Order of coherently oscillating local electric
dipole moments. These dipoles arise from the local motion of the atoms in the
crystal potential well, and oscillate in synchrony to reduce the dipolar
interaction energy. The dipolar ground-state is therefore found to be a
coherent state with a well defined global phase and a three-component complex
order parameter. The condensation energy of the dipoles in the bcc phase
stabilizes it over the hcp phase at finite temperatures. We further show that
there can be fermionic excitations of this ground-state and predict that they
form an optical-like branch in the (110) direction. A comparison with
'super-solid' models is also discussed.


title:
Temperature-dependent logarithmic corrections in the spin-1/2 Heisenberg
  chain

abstract:
  We obtain the logarithmic corrections to the dynamic response function and
NMR T_1 and T_{2G} rates in the spin-1/2 antiferromagnetic Heisenberg chain
using perturbative renormalization group in the leading irrelevant operator.
The result is compared with NMR experiments in Sr_2CuO_3.


title:
Orientational phase transitions in anisotropic rare-earth magnets at low
  temperatures

abstract:
  Orientational phase transitions are investigated within the Heisenberg model
with single-site anisotropy. The temperature dependence of the cone angle is
calculated within the spin-wave theory. The role of the quantum
renormalizations of anisotropy constants is discussed. A comparison with the
experimental data on the cone-plane orientational transition in holmium is
performed.


title: Hidden Order in $URu_2Si_2$

abstract:
  We review current attempts to characterize the underlying nature of the
hidden order in $URu_2Si_2$. A wide variety of experiments point to the
existence of two order parameters: a large primary order parameter of unknown
character which co-exists with secondary antiferromagnetic order. Current
theories can be divided into two groups determined by whether or not the
primary order parameter breaks time-reversal symmetry. We propose a series of
experiments designed to test the time-reversal nature of the underlying primary
order in $URu_2Si_2$ and to characterize its local single-ion physics.


title: Spin-Charge Binding Mechanism for Superconductivity in Cuprates

abstract:
  A spin-charge recombination route to superconductivity, proposed earlier
(1992)], is examined using the Schwinger boson representation of the t-J model.
The representation is known to work well at half-filling. It was shown in the
earlier paper that, away from half filling, spin-charge recombination into
electrons occurs directly through the hopping term as the system tries to
recover the kinetic energy lost in forming the "spins" (magnetic moments).
Recombination is strong enough in 2-D to (1) destroys long-range magnetic order
and (2) restores the large Fermi surface. But the normal state remains
incoherent, characterize by a Fermi liquid of holons and bosonic spinons which
are paired into singlets. The singlets evolve out of half-filling and are
responsible for the spin-gap behavior. They induce a pairing of holons and bind
with the latter (to recover additional kinetic energy). In this paper we show
that $d_{x^2-y^2}$ symmetry of the order parameter and the shape of the T_c vs
doping curve, observed in cuprate superconductors, emerge naturally as a
consequence of the symmetry of the spinon pairs. The latter symmetry (p-wave),
however, is already determined at half-filling. Thus the nature of the
superconducting state is intimately connected with that of quantum spin
fluctuations in the insulator!


title: Anisotropy of the optical conductivity of high Tc cuprates

abstract:
  The optical conductivity along and perpendicular to the planes is calculated
assuming strong k-dependence of the scattering rate and the c-axis hopping
parameter. Closed analytical expressions for the optical conductivy along these
directions are obtained, and are shown to be integrable at low and high
frequencies. A large and qualitatively different frequency dependence for both
polarizations follows directly from the model. The expression for the in-plane
conductivity has an effective scattering rate proportional to frequency, and
can be easily generalized to provide a simple analytical expression, which may
replace the Drude formula in the case of non-Fermi liquids.


title:
Adaptive Sampling Approach to the Negative Sign Problem in the Auxiliary
  Field Quantum Monte Carlo Method

abstract:
  We propose a new sampling method to calculate the ground state of interacting
quantum systems. This method, which we call the adaptive sampling quantum monte
carlo (ASQMC) method utilises information from the high temperature density
matrix derived from the monte carlo steps. With the ASQMC method, the negative
sign ratio is greatly reduced and it becomes zero in the limit $\Delta \tau$
goes to zero even without imposing any constraint such like the constraint path
(CP) condition. Comparisons with numerical results obtained by using other
methods are made and we find the ASQMC method gives accurate results over wide
regions of physical parameters values.


title: Effect of zero point phase fluctuations on Josephson tunneling

abstract:
  In the presence of phase fluctuations the dc Josephson effect is modified and
the supercurrent at zero voltage is replaced by a peak at small but finite
voltages. It is shown that at zero temperature this peak is determined by two
complementary expansions of finite radius of convergence. The leading order
expressions are related to results known from the regimes of Coulomb blockade
and of macroscopic quantum tunneling. The peak positions and the suppression of
the critical current by quantum fluctuations are discussed.


title: Screening of a point charge: a fixed-node diffusion Monte Carlo study

abstract:
  We study the static screening in a Hubbard-like model using fixed-node
diffusion Monte Carlo. We find that the random phase approximation is
surprisingly accurate even for metallic systems close to the Mott transition.
As a specific application we discuss the implications of the efficient
screening for the superconductivity in the doped Fullerenes. In the Monte Carlo
calculations we use trial functions with two Gutzwiller-type parameters. To
deal with such trial functions, we introduce a method for efficiently
optimizing the Gutzwiller parameters, both in variational and in fixed-node
diffusion Monte Carlo.


title: Ferromagnetism in single-band Hubbard models with a partially flat band

abstract:
  A Hubbard model with a single, partially flat band has ferromagnetic ground
states. It is shown that local stability of ferromagnetism implies its global
stability in such a model: The model has only ferromagnetic ground states if
there are no single spin-flip ground states. Since a single-band Hubbard model
away from half filling describes a metal, this result may open a route to
metallic ferromagnetism in single band Hubbard models.


title:
Inter-molecular structure factors of macromolecules in solution:
  integral equation results

abstract:
  The inter-molecular structure of semidilute polymer solutions is studied
theoretically. The low density limit of a generalized Ornstein-Zernicke
integral equation approach to polymeric liquids is considered. Scaling laws for
the dilute-to-semidilute crossover of random phase (RPA) like structure are
derived for the inter-molecular structure factor on large distances when
inter-molecular excluded volume is incorporated at the microscopic level. This
leads to a non-linear equation for the excluded volume interaction parameter.
For macromolecular size-mass scaling exponents, $\nu$, above a
spatial-dimension dependent value, $\nu_c=2/d$, mean field like density scaling
is recovered, but for $\nu<\nu_c$ the density scaling becomes non-trivial in
agreement with field theoretic results and justifying phenomenological
extensions of RPA. The structure of the polymer mesh in semidilute solutions is
discussed in detail and comparisons with large scale Monte Carlo simulations
are added. Finally a new possibility to determine the correction to scaling
exponent $\omega_{12}$ is suggested.


title:
Numerical analysis of the dissipative two-state system with the
  density-matrix Hilbert-space-reduction algorithm

abstract:
  Ground state of the dissipative two-state system is investigated by means of
the Lanczos diagonalization method. We adopted the Hilbert-space-reduction
scheme proposed by Zhang, Jeckelmann and White so as to reduce the overwhelming
reservoir Hilbert space to being tractable in computers. Both the
implementation of the algorithm and the precision applied for the present
system are reported in detail. We evaluate the dynamical susceptibility
(resolvent) with the continued-fraction-expansion formula. Through analysing
the resolvent over a frequency range, whose range is often called `interesting'
frequency, we obtain the damping rate and the oscillation frequency. Our
results agree with those of a recent quantum Monte-Carlo study, which concludes
that the critical dissipation from oscillatory to over-damped behavior
decreases as the tunneling amplitude is strengthened.


title: Comment on the anisotropic impurity scattering in superconductors

abstract:
  We discuss a case of a strong anisotropic impurity scattering within the
model introduced in our previous paper [Phys. Rev. B54, 15463 (1996)] and
clarify our former statement about a possible enhancement of the critical
temperature in this scattering regime. We show that for the anisotropy of the
impurity potential determined by the functions from the non-identity
irreducible representations of the crystal point group, the critical
temperature decreases with the impurity scattering rate which is consistent
with a generally understood role of disorder in superconductors.


title:
The phase transition phenomena in anisotropic superconductors: effect of
  the orthorhombic crystal field and the potential impurity scattering

abstract:
  A combined effect of the orthorhombic crystal field and potential impurity
scattering on several superconducting states of a tetragonal symmetry is
studied within a weak-coupling mean field approach. It is shown that the
nonmagnetic impurities stabilize the states belonging to the identity
irreducible representation. The electronic specific heat jump at the phase
transition is analyzed. Its dependence on the potential scattering rate for
large impurity concentration is shown to be remarkably different for the states
with a nonzero value of the Fermi surface averaged order parameter than for
those with a vanishing one. In particular, very distinct signals from
d_{x^2-y^2} state in YBCO and d_{xy} state in BSCCO compound are predicted.
This effect may be used as a test for the presence of these states in the above
cuprates.


title: Effect of anisotropic impurity scattering in superconductors

abstract:
  We discuss the weak-coupling BCS theory of a superconductor with the
impurities, accounting for their anisotropic momentum-dependent potential. The
impurity scattering process is considered in the t-matrix approximation and its
influence on the superconducting critical temperature is studied in the Born
and unitary limit for a d- and (d+s)-wave superconductors. We observe a
significant dependence of the pair-breaking strength on the symmetry of the
scattering potential and classify the impurity potentials according to their
ability to alter T_c. A good agreement with the experimental data for Zn doping
and oxygen irradiation in the overdoped cuprates is found.


title: Adiabatic transport of Cooper pairs in arrays of Josephson junctions

abstract:
  We have developed a quantitative theory of Cooper pair pumping in gated
one-dimensional arrays of Josephson junctions. The pumping accuracy is limited
by quantum tunneling of Cooper pairs out of the propagating potential well and
by direct supercurrent flow through the array. Both corrections decrease
exponentially with the number N of junctions in the array, but give a serious
limitation of accuracy for any practical array. The supercurrent at resonant
gate voltages decreases with N only as sin(v/N)/N, where v is the Josephson
phase difference across the array.


title:
Magnetic Properties of Finite Systems: Microcanonical Finite Size
  Scaling

abstract:
  In the microcanonical ensemble, suitably defined observables show
non-analyticities and power law behaviour even for finite systems. For these
observables, a microcanonical finite-size scaling theory is established which
facilitates an approach to the critical exponents of the infinite system.


title:
Comment on `Universal relation between the Kolmogorov-Sinai entropy and
  the thermodynamic entropy in simple liquids'

abstract:
  The intriguing relations between Kolmogorov-Sinai entropy and self diffusion
coefficients and the excess (thermodynamic) entropy found by Dzugutov and
collaborators do not appear to hold for hard sphere and hard disks systems.


title: Dynamic transverse susceptibility in Au-Fe-Au "nano-onions"

abstract:
  A precision radio-frequency (RF) transverse modulation technique based on a
resonant tunnel diode oscillator (TDO) has been used to study the dynamic
susceptibility in novel ring-shaped single domain Fe nanoparticles synthesized
using reverse-micelle chemistry. Hysteresis loops measured using a SQUID
magnetometer indicate a large coercivity (Hc = 400Oe) below the blocking
temperature (TB ~52K). The dynamic susceptibility shows remarkable consistency
with the M-H data and exhibits peaks at characteristic anisotropy fields.
Overall, these RF experiments are well suited to probe the magnetic anisotropy
in nanophase materials.


title: Theory of ferroelectrics: A vision for the next decade and beyond

abstract:
  In the last ten years significant advances have been made in our
understanding and ability to compute and predict properties of ferroelectrics
and piezoelectrics using fundamental physics. Phase diagrams, electromechanical
and elastic properties, and effects of defects and surfaces are now amenable to
computation. Most importantly, new techniques have been developed, and new
understanding of the meaning of polarization in dielectrics has been developed.
Prospects for the future are discussed.


title:
Vortex stability of interacting Bose-Einstein condensates confined in
  anisotropic harmonic traps

abstract:
  Vortex states of weakly-interacting Bose-Einstein condensates confined in
three-dimensional rotating harmonic traps are investigated numerically at zero
temperature. The ground state in the rotating frame is obtained by propagating
the Gross-Pitaevskii equation for the condensate in imaginary time. The total
energies between states with and without a vortex are compared, yielding
critical rotation frequencies that depend on the anisotropy of the trap and the
number of atoms. Vortices displaced from the center of nonrotating traps are
found to have long lifetimes for sufficiently large numbers of atoms. The
relationship between vortex stability and bound core states is explored.


title:
Ring-Pattern Dynamics in Smectic-C* and Smectic-C_A* Freely Suspended
  Liquid Crystal Films

abstract:
  Ring patterns of concentric 2pi-solitons in molecular orientation, form in
freely suspended chiral smectic-C films in response to an in-plane rotating
electric field. We present measurements of the zero-field relaxation of ring
patterns and of the driven dynamics of ring formation under conditions of
synchronous winding, and a simple model which enables their quantitative
description in low polarization DOBAMBC. In smectic C_A* TFMHPOBC we observe an
odd-even layer number effect, with odd number layer films exhibiting order of
magnitude slower relaxation rates than even layer films. We show that this rate
difference is due to much larger spontaneous polarization in odd number layer
films.


title:
Robust gamma oscillations in networks of inhibitory Hippocampal
  interneurons

abstract:
  Recent experiments suggest that inhibitory networks of interneurons can
synchronize the neuronal discharge in in vitro hippocampal slices. Subsequent
theoretical work has shown that strong synchronization by mutual inhibition is
only moderately robust against neuronal heterogeneities in the current drive,
provided by activation of metabotropic glutamate receptors. In vivo neurons
display greater variability in the interspike intervals due to the presence of
synaptic noise. Noise and heterogeneity affect synchronization properties
differently. In this paper we study using model simulations how robust
synchronization can be in the presence of synaptic noise and neuronal
heterogeneity. We find that with at least a minimum amount of noise stochastic
weak synchronization (SWS) (i.e. when neurons spike within a short interval
from each other, but not necessarily at each period) is produced that is much
more robust than strong synchronization (i.e. when neurons spike each period).
The statistics of the SWS population discharge are consistent with previous
experimental data. We find robust SWS in the gamma frequency range (20-80 Hz)
for a stronger synaptic coupling compared to previous models and for networks
with 10 -- 1000 neurons.


title: Effect of Coulomb blockade on STM current through a granular film

abstract:
  The electron transport through an array of tunnel junctions consisting of an
  STM tip and a granular film is studied both theoretically and experimentally.
When the tunnel resistance between the tip and a granule on the surface is much
larger than those between granules, a bottleneck of the tunneling current is
created in the array. It is shown that the period of the Coulomb staircase(CS)
is given by the capacitance at the bottleneck.
  Our STM experiments on Co-Al-O granular films show the CS with a single
period at room temperature. This provides a new possibility for
single-electron-spin-electronic devices at room temperature.


title:
The ``Mixed'' Green's Function Approach to Quantum Kinetics with Initial
  Correlations

abstract:
  A method for deriving quantum kinetic equations with initial correlations is
developed on the basis of the nonequilibrium Green's function formalism. The
method is applicable to a wide range of correlated initial states described by
nonequilibrium statistical thermodynamics. Initial correlations and the
real-time evolution are treated by a unified technique employing many-component
``mixed'' Green's functions. The Dyson equation for the mixed Green's function
leads to a set of equations for real-time Green's functions and new (cross)
components linking initial correlations with dynamical processes. These
equations are used to formulate a generalized Kadanoff-Baym ansatz for
correlated initial states. A non-Markovian short-time kinetic equation is
derived within the T-matrix approximation for the self-energies. The properties
of the memory kernels in this equation are considered in detail in Born
approximation for the T-matrices. The kinetic equation is demonstrated to
conserve the total energy of the system. An explicit expression for the
time-dependent correlation energy is obtained.


title:
Low-Temperature Saturation of the Dephasing Time and Effects of
  Microwave Radiation on Open Quantum Dots

abstract:
  The dephasing time of electrons in open semiconductor quantum dots, measured
using ballistic weak localization, is found to saturate below ~ 100 mK, roughly
twice the electron base temperature, independent of dot size. Microwave
radiation deliberately coupled to the dots affects quantum interference
indistinguishably from elevated temperature, suggesting that direct dephasing
due to radiation is not the cause of the observed saturation. Coulomb blockade
measurements show that the applied microwaves create sufficient source drain
voltages to account for dephasing due to Joule heating.


title:
Nuclear spin-spin coupling in La_{2-x}Sr_{x}CuO_{4} studied by
  stimulated echo decay

abstract:
  We have performed copper NQR experiments in high temperature superconductors
YBa_{2}Cu_{4}O_{8}, YBa_{2}Cu_{3}O_{7}, and La_{2-x}Sr_{x}CuO_{4} (x=0.12 and
0.15), using the stimulated echo technique. The stimulated echo intensity is
analyzed by a model that includes the spin-lattice relaxation process (T_ {1
}-process) and the fluctuating local field due to nuclear spin-spin coupling.
The model gives quantitative account of the experimental results in Y-based
compounds using the known values of 1/T_{1} and 1/T_{2G}, the gaussian decay
rate of the spin echo intensity. The same model applied to LSCO enables us to
extract the value of T_{2G}. Our results indicate that T_{1}T/T_{2G} is
independent of temperature, implying that the dynamic exponent is one in
La_{2-x}Sr_{x}CuO_{4}.


title:
Nonequilibrium Josephson effect in mesoscopic ballistic multiterminal
  SNS junctions

abstract:
  We present a detailed study of nonequilibrium Josephson currents and
conductance in ballistic multiterminal SNS-devices. Nonequilibrium is created
by means of quasiparticle injection from a normal reservoir connected to the
normal part of the junction. By applying a voltage at the normal reservoir the
Josephson current can be suppressed or the direction of the current can be
reversed. For a junction longer than the thermal length, $L\gg\xi_T$, the
nonequilibrium current increases linearly with applied voltage, saturating at a
value equal to the equilibrium current of a short junction. The conductance
exhibits a finite bias anomaly around $eV \sim \hbar v_F/L$. For symmetric
injection, the conductance oscillates $2\pi$-periodically with the phase
difference $\phi$ between the superconductors, with position of the minimum
($\phi=0$ or $\pi$) dependent on applied voltage and temperature. For
asymmetric injection, both the nonequilibrium Josephson current and the
conductance becomes $\pi$-periodic in phase difference. Inclusion of barriers
at the NS-interfaces gives rise to a resonant behavior of the total Josephson
current with respect to junction length with a period $\sim \lambda_F$. Both
three and four terminal junctions are studied.


title: Statistical Mechanics of Torque Induced Denaturation of DNA

abstract:
  A unifying theory of the denaturation transition of DNA, driven by
temperature T or induced by an external mechanical torque Gamma is presented.
Our model couples the hydrogen-bond opening and the untwisting of the
helicoidal molecular structure. We show that denaturation corresponds to a
first-order phase transition from B-DNA to d-DNA phases and that the
coexistence region is naturally parametrized by the degree of supercoiling
sigma. The denaturation free energy, the temperature dependence of the twist
angle, the phase diagram in the T,Gamma plane and isotherms in the sigma, Gamma
plane are calculated and show a good agreement with experimental data.


title:
Fast global oscillations in networks of integrate-and-fire neurons with
  low firing rates

abstract:
  We study analytically the dynamics of a network of sparsely connected
inhibitory integrate-and-fire neurons in a regime where individual neurons emit
spikes irregularly and at a low rate. In the limit when the number of neurons N
tends to infinity,the network exhibits a sharp transition between a stationary
and an oscillatory global activity regime where neurons are weakly
synchronized. The activity becomes oscillatory when the inhibitory feedback is
strong enough. The period of the global oscillation is found to be mainly
controlled by synaptic times, but depends also on the characteristics of the
external input. In large but finite networks, the analysis shows that global
oscillations of finite coherence time generically exist both above and below
the critical inhibition threshold. Their characteristics are determined as
functions of systems parameters, in these two different regimes. The results
are found to be in good agreement with numerical simulations.


title: Nucleation of Stable Superconductivity in YBCO-Films

abstract:
  By means of the linear dynamic conductivity, inductively measured on
epitaxial films between 30mHz and 30 MHz, the transition line $T_g (B)$ to
generic superconductivity is studied in fields between B=0 and 19T. It follows
closely the melting line $T_m (B)$ described recently in terms of a blowout of
thermal vortex loops in clean materials. The critical exponents of the
correlation length and time near $T_g (B)$, however, seem to be dominated by
some intrinsic disorder. Columnar defects produced by heavy-ion irradiation up
to field-equivalent-doses of $B_{\phi} = 10T$ lead to a disappointing reduction
of $T_g (B \to 0)$ while for $B>B_{\phi}$ the generic line of the pristine film
is recovered. These novel results are also discussed in terms of a loop-driven
destruction of generic superconductivity.


title:
Random quantum magnets with long-range correlated disorder: Enhancement
  of critical and Griffiths-McCoy singularities

abstract:
  We study the effect of spatial correlations in the quenched disorder on
random quantum magnets at and near a quantum critical point. In the random
transverse field Ising systems disorder correlations that decay algebraically
with an exponent rho change the universality class of the transition for small
enough rho and the off-critical Griffiths-McCoy singularities are enhanced. We
present exact results for 1d utilizing a mapping to fractional Brownian motion
and generalize the predictions for the critical exponents and the generalized
dynamical exponent in the Griffiths phase to d>=2.


title: Dynamical studies of the response function in a Spin Glass

abstract:
  Experiments on the time dependence of the response function of a Ag(11 at%Mn)
spin glass at a temperature below the zero field spin glass temperature are
used to explore the non-equilibrium nature of the spin glass phase. It is found
that the response function is only governed by the thermal history in the very
neighbourhood of the actual measurement temperature. The thermal history
outside this narrow region is irrelevant to the measured response. A result
that implies that the thermal history during cooling (cooling rate, wait times
etc.) is imprinted in the spin structure and is always retained when any higher
temperature is recovered. The observations are discussed in the light of a real
space droplet/domain phenomenology. The results also emphasise the importance
of using controlled cooling procedures to acquire interpretable and
reproducible experimental results on the non-equilibrium dynamics in spin
glasses.


title: Equations of structural relaxation

abstract:
  In the mode coupling theory of the liquid to glass transition the long time
structural relaxation follows from equations solely determined by equilibrium
structural parameters. The present extension of these structural relaxation
equations to arbitrarily short times on the one hand allows calculations
unaffected by model assumptions about the microscopic dynamics and on the other
hand supplies new starting points for analytical studies. As a first
application, power-law like structural relaxation at a glass-transition
singularity is explicitly proven for a special schematic MCT model.


title: Interchain interactions and magnetic properties of Li2CuO2

abstract:
  An effective Hamiltonian is constructed for an insulating cuprate with
edge-sharing chains Li2CuO2.The Hamiltonian contains the nearest and
next-nearest neighboring intrachain and zigzag-type interchain interactions.The
values of the interactions are obtained from the analysis of the magnetic
susceptibility, and this system is found to be described as coupled frustrated
chains.We calculate the dynamical spin correlation function S(q,\omega) by
using the exact diagonalization method, and show that the spectra of
S(q,\omega) are characterized by the zigzag-type interchain interactions. The
results of the recent inelastic neutron scattering experiment are discussed in
the light of the calculated spectra.


title: Plasmon excitation by charged particles interacting with metal surfaces

abstract:
  Recent experiments (R. A. Baragiola and C. A. Dukes, Phys. Rev. Lett. {\bf
76}, 2547 (1996)) with slow ions incident at grazing angle on metal surfaces
have shown that bulk plasmons are excited under conditions where the ions do
not penetrate the surface, contrary to the usual statement that probes exterior
to an electron gas do not couple to the bulk plasmon. We here use the quantized
hydrodynamic model of the bounded electron gas to derive an explicit expression
for the probability of bulk plasmon excitation by external charged particles
moving parallel to the surface. Our results indicate that for each ${\bf q}$
(the surface plasmon wave vector) there exists a continuum of bulk plasmon
excitations, which we also observe within the semi-classical infinite-barrier
(SCIB) model of the surface.


title:
Numerical investigation of the thermodynamic limit for ground states in
  models with quenched disorder

abstract:
  The effect of open boundary conditions for four models with quenched disorder
are studied in finite samples by numerical ground state calculations.
Extrapolation to the infinite volume limit indicates that the configurations in
``windows'' of fixed size converge to a unique configuration, up to global
symmetries. The scaling of this convergence is consistent with calculations
based on the fractal dimension of domain walls. These results provide strong
evidence for the ``two-state'' picture of the low temperature behavior of these
models. Convergence in three-dimensional systems can require relatively large
windows.


title:
The dimer-RVB State of the Four-Leg Heisenberg Ladder: Interference
  among Resonances

abstract:
  We study the ground state of the 4-leg spin ladder using a dimer-RVB ansatz
and the Lanczos method. Besides the well known resonance mechanism between
valence bond configurations we find novel interference effects among nearby
resonances.


title: Ground state wavefunction of the quantum Frenkel-Kontorova model

abstract:
  The wavefunction of an incommensurate ground state for a one-dimensional
discrete sine-Gordon model -- the Frenkel-Kontorova (FK) model -- at zero
temperature is calculated by the quantum Monte Carlo method. It is found that
the ground state wavefunction crosses over from an extended state to a
localized state when the coupling constant exceeds a certain critical value.
So, although the quantum fluctuation has smeared out the breaking of
analyticity transition as observed in the classical case, the remnant of this
transition is still discernible in the quantum regime.


title:
Thermodynamics of the Superconducting Phase Transition in
  Ba_{0.6}K_{0.4}BiO_3

abstract:
  We suggest that the transition to superconductivity in a single crystal of
Ba_{0.6}K_{0.4}BiO_3 with a T_c = 32K, and having critical fields with
anomalous temperature dependencies and vanishing discontinuities in specific
heat and magnetic susceptibility, may well be an example of a fourth order (in
Ehrenfest's sense) phase transition. We have derived a free energy functional
for a fourth order transition and calculated (for the temperature range ${T_c/2
< T \simeq T_c}$) the temperature dependence of the critical fields. We find
$H_{c1}(T)\propto (1-T/T_c)^3, H_0(T)\propto (1-T/T_c)^2$ and $H_{c2}(T)\propto
(1-T/T_c)^{1}$ in general agreement with experiments.


title: Geometry of fully coordinated, two-dimensional percolation

abstract:
  We study the geometry of the critical clusters in fully coordinated
percolation on the square lattice. By Monte Carlo simulations (static
exponents) and normal mode analysis (dynamic exponents), we find that this
problem is in the same universality class with ordinary percolation statically
but not so dynamically. We show that there are large differences in the number
and distribution of the interior sites between the two problems which may
account for the different dynamic nature.


title: Dynamics and Transport in Random Antiferromagnetic Spin Chains

abstract:
  We present the first results on the low-frequency dynamical and transport
properties of random antiferromagnetic spin chains at low temperature ($T$). We
obtain the momentum and frequency dependent dynamic structure factor in the
Random Singlet (RS) phases of both spin-1/2 and spin-1 chains, as well as in
the Random Dimer phase of spin-1/2 chains. We also show that the RS phases are
unusual `spin-metals' with divergent low-frequency conductivity at T=0, and
follow the spin conductivity through `metal-insulator' transitions tuned by the
strength of dimerization or Ising anisotropy in the spin-1/2 case, and by the
strength of disorder in the spin-1 case.


title:
Schwinger boson mean field theory of the Heisenberg Ferrimagnetic Spin
  Chain

abstract:
  The Schwinger boson mean field theory is applied to the quantum ferrimagnetic
Heisenberg chain. There is a ferrimagnetic long range order in the ground
state. We observe two branches of the low lying excitation and calculate the
spin reduction, the gap of the antiferromagnetic branch, and the spin
fluctuation at $T=0K$. These results agree with the established numerical
results quite well. At finite temperatures, the long range order is destroyed
because of the disappearance of the Bose condensation. The thermodynamic
observables, such as the free energy, magnetic susceptibility, specific heat,
and the spin correlation at $T>0K$, are calculated. The $T\chi_{uni}$ has a
minimum at intermediate temperatures and the spin correlation length behaves as
$T^{-1}$ at low temperatures. These qualitatively agree with the numerical
results and the difference is small at low temperatures.


title:
Charge Kondo effect toward a non-Fermi-liquid fixed point in the
  orbitally degenerate exchange model

abstract:
  We show that a Kondo-type model with an orbital degeneracy has a new
non-Fermi-liquid fixed point. Near the fixed point the spin degrees of freedom
are completely quenched, and the residual charge degrees of freedom lead to the
multi-channel Kondo effect. Anomalous behavior appears in electric and thermal
properties, but the magnetic susceptibility should show the local Fermi-liquid
behavior. The non-Fermi-liquid fixed point becomes unstable against
perturbations breaking the particle-hole symmetry. We derive these results
using the third-order scaling for a spherically symmetric model with a
fictitious spin. In contrast to the Coqblin-Schrieffer model, the present model
respects different time-reversal properties of multipole operators.