title: Supercooling of the disordered vortex lattice in Bi_2Sr_2CaCu_2O_8+d

abstract:
  Time-resolved local induction measurements near to the vortex lattice
order-disorder transition in optimally doped
Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ single crystals shows that the
high-field, disordered phase can be quenched to fields as low as half the
transition field. Over an important range of fields, the electrodynamical
behavior of the vortex system is governed by the co-existence of the two phases
in the sample. We interpret the results in terms of supercooling of the
high-field phase and the possible first order nature of the order-disorder
transition at the ``second peak''.


title:
Reply to the Comment on: Quantum Monte Carlo study of the dipole moment
  of CO [J. Chem. Phys. 110, 11700 (1999)]

abstract:
  In our previous publication we have mistakenly claimed that the applicability
of the Hellmann-Feynman theorem in fixed-node quantum Monte Carlo calculations
is not subject to the manner how the nodal boundary depends on an external
parameter. This statement is not correct in general, except where the
Hellmann-Feynman force is calculated for a nodal boundary which coincides with
that of the unconstrained exact eigenfunction. We point out the error in our
arguments and present an explicit expression for the correction term which
supplements the Hellmann-Feynman force. This term can be interpreted as the
linear response of the energy with respect to variations of the the nodal
region.


title: Mechanical Mixing in Nonlinear Nanomechanical Resonators

abstract:
  Nanomechanical resonators, machined out of Silicon-on-Insulator wafers, are
operated in the nonlinear regime to investigate higher-order mechanical mixing
at radio frequencies, relevant to signal processing and nonlinear dynamics on
nanometer scales. Driven by two neighboring frequencies the resonators generate
rich power spectra exhibiting a multitude of satellite peaks. This nonlinear
response is studied and compared to $n^{th}$-order perturbation theory and
nonperturbative numerical calculations.


title: Colloidal gelation and non-ergodicity transitions

abstract:
  Within the framework of the mode coupling theory (MCT) of structural
relaxation, mechanisms and properties of non-ergodicity transitions in rather
dilute suspensions of colloidal particles characterized by strong short-ranged
attractions are studied. Results building on the virial expansion for particles
with hard cores and interacting via an attractive square well potential are
presented, and their relevance to colloidal gelation is discussed.


title: Supercooling of the high field vortex phase in single crystalline BSCCO

abstract:
  Time resolved magneto-optical images show hysteresis associated with the
transition at the so-called ``second magnetization peak'' at B_sp in
single-crystalline Bi_2Sr_2CaCu_2O_8+d. By rapid quenching of the high-field
phase, it can be made to persist metastably in the sample down to fields that
are nearly half B_sp.


title:
Microscopic model of phase transition in the crystals of DMAAlS and
  DMAGaS type

abstract:
  The four-state model is proposed for description of phase transition in
ferroelectric crystals of DMAGaS and DMAAlS type. Thermodynamical functions of
the model are obtained in the mean field approximation. The phase transition
between paraelectric and ferroelectric phases is investigated. It is
established that order of the phase transitions depends on relations between
model parameters.


title: Magnetic relaxation in the "Bragg-glass" phase in BSCCO

abstract:
  Magnetic relaxation in the Bragg-glass phase of overdoped Bi_2Sr_2CaCu_2O_8
crystals was investigated using time-resolved magneto-optical visualisation of
the flux distribution. This has permitted us to extract the current-voltage
characteristic, which can be well described by a power-law, although fits to a
stretched exponential E \sim \exp(- j_{c} / j)^{\mu} with 0.3 < \mu < 0.8 are
possible at long times in excess of 100 s.


title: Magnetization Decay due to Vortex Phase Boundary Motion in BSCCO

abstract:
  We identify a new regime of decay of the irreversible magnetization in clean
Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ crystals, at induction values close to the
``second peak field'' at which the bulk critical current density steeply
increases. A time window is identified during which the decay of the induction
is controlled by the slow propagation of the phase transformation front across
the sample.


title: Melting of regular and decoupled vortex lattices in BSCCO crystals

abstract:
  The angular dependence of the first-order phase transition (FOT) in the
vortex lattice in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ crystals was investigated
by a low frequency AC shielding technique (with the AC field $\parallel c$), in
which the static-field component parallel to $c$- ($H_{\perp}$) was varied with
the in-plane field $H_{\parallel}$ held constant. The linear decrease of the
FOT field $H_{\perp}^{FOT}$ with increasing $H_{\parallel}$ ends at a
temperature--dependent critical value of $H_{\parallel}$. A new transition,
marked by the abrupt drop of the $ab$-plane shielding current, appears at this
point. We draw a new phase diagram with $H_{\parallel}$ and $H_{\perp}$ field
components as coordinates; this features at least two distinct regions in the
vortex solid phase, that are determined by the different interplay between the
pancake vortex-- and Josephson vortex lattice.


title:
Order-disorder model of phase transitions in the DMAGaS-DMAAlS family
  crystals

abstract:
  A four-state model is proposed for description of the sequence of phase
transitions in ferroelectric crystals of DMAGaS and DMAAlS type. The ordering
processes in the subsystems of DMA groups are considered as a main reason of
such transformations. The interaction between groups in their various
orientational states is taken into account in the dipole-dipole approximation.
Obtained thermodynamical characteristics of the model (spontaneous
polarization, occupancy of orientational states, dielectric susceptibility,
phase diagram) are in good agreement with experimental data. The experimental
fact of suppression of ferroelectric phase at increase of hydrostatic pressure
is explained under assumption that pressure changes the difference between
energies of various orientational states of DMA groups mostly.


title: Viability of competing field theories for the driven lattice gas

abstract:
  It has recently been suggested that the driven lattice gas should be
described by a novel field theory in the limit of infinite drive. We review the
original and the new field theory, invoking several well-documented key
features of the microscopics. Since the new field theory fails to reproduce
these characteristics, we argue that it cannot serve as a viable description of
the driven lattice gas. Recent results, for the critical exponents associated
with this theory, are re-analyzed and shown to be incorrect.


title: A twisted conformal field theory description of the Quantum Hall Effect

abstract:
  We construct an effective conformal field theory by using a procedure which
induces twisted boundary conditions for the fundamental scalar fields. That
allows to describe a quantum Hall fluid at Jain hierarchical filling,
nu=m/(2pm+1), in terms of one charged scalar field and m-1 neutral ones. Then
the resulting algebra of the chiral primary fields is U(1)xW_m. Finally the
ground state wave functions are given as correlators of appropriate composite
fields (a-electrons).


title: Anomalous Tien-Gordon scaling in a 1d tunnel junction

abstract:
  We investigate the nonlinear ac transport through a quantum wire with an
impurity in the presence of finite range electron-electron interactions. We
discuss the influence of the spatial shape of the ac electric field onto
transport properties of the system and find that the scaling behavior of the
occupation probability of the sidebands depends on the range of the voltage
drop.


title:
The joys and pitfalls of Fermi surface mapping in Bi2212 using angle
  resolved photoemission

abstract:
  On the basis of angle-scanned photoemission data recorded using unpolarised
radiation, with high (E,k) resolution, and an extremely dense sampling of
k-space, we resolve the current controversy regarding the normal state Fermi
surface (FS) in Bi_2Sr_2CaCu_2O_8 (Bi2212). The true picture is simple,
self-consistent and robust: the FS is hole-like, with the form of rounded tubes
centred on the corners of the Brillouin zone. Two further types of features are
also clearly observed: shadow FSs, which are most likely to be due to short
range antiferromagnetic spin correlations, and diffraction replicas of the main
FS caused by passage of the photoelectrons through the modulated Bi-O planes.


title: Disorder-Induced Time-Dependent Diffusion in Zeolites

abstract:
  We suggest that disordered framework aluminums and non-framework cations can
create a disordered electrostatic potential in zeolites that can lead to a
discrepancy between diffusivities measured by microscopic and macroscopic
experimental techniques. We calculate the value of the discrepancy and the
characteristic time scale at which it occurs for several ionic and polarizable
species diffusing in zeolites. For ionic species, a discrepancy is almost
inevitable. For polarizable species, a significant discrepancy may occur in
some zeolites only for long alkanes or large species such as benzene.


title: Finite Temperature Ordering in the Three-Dimensional Gauge Glass

abstract:
  We present results of Monte Carlo simulations of the gauge glass model in
three dimensions using exchange Monte Carlo. We show for the first time clear
evidence of the vortex glass ordered phase at finite temperature. Using finite
size scaling we obtain estimates for the correlation length exponent, nu = 1.39
+/- 0.20, the correlation function exponent, eta = -0.47 +/- 0.07, and the
dynamic exponent z = 4.2 +/- 0.6. Using our values for z and nu we calculate
the resistivity exponent to be s = 4.5 +/- 1.1. Finally, we provide a plausible
lower bound on the the zero-temperature stiffness exponent, theta >= 0.18.


title: Deformation of Fermi Surface Due to Antiferromagnetic Correlation

abstract:
  It is investigated, on the basis of the fluctuation exchange approximation,
how the shape of the Fermi surface is modified in two-dimensional $t-t'-U$
Hubbard model at half filling as strength of the onsite Coulomb interaction $U$
is increased. The antiferromagnetic (AF) correlation length is shown to be
enhanced as the Coulomb interaction get closer to the critical value $U_{\rm
c}$ for the onset AF order. At the same time, the shape of the renormalized
Fermi surface is deformed showing the tendency of nesting near $U_{\rm c}$.


title: Particle Systems with Stochastic Passing

abstract:
  We study a system of particles moving on a line in the same direction.
  Passing is allowed and when a fast particle overtakes a slow particle, it
acquires a new velocity drawn from a distribution P_0(v), while the slow
particle remains unaffected. We show that the system reaches a steady state if
P_0(v) vanishes at its lower cutoff; otherwise, the system evolves
indefinitely.


title:
Limits on Phase Separation for Two-Dimensional Strongly Correlated
  Electrons

abstract:
  From calculations of the high temperature series for the free energy of the
two-dimensional t-J model we construct series for ratios of the free energy per
hole. The ratios can be extrapolated very accurately to low temperatures and
used to investigate phase separation. Our results confirm that phase separation
occurs only for J/t greater than 1.2. Also, the phase transition into the phase
separated state has Tc of approximately 0.25J for large J/t.


title:
An Attempt to Calculate Energy Eigenvalues in Quantum Systems of Large
  Sizes

abstract:
  We report an attempt to calculate energy eigenvalues of large quantum systems
by the diagonalization of an effectively truncated Hamiltonian matrix. For this
purpose we employ a specific way to systematically make a set of orthogonal
states from a trial wavefunction and the Hamiltonian. In comparison with the
Lanczos method, which is quite powerful if the size of the system is within the
memory capacity of computers, our method requires much less memory resources at
the cost of the extreme accuracy.
  In this paper we demonstrate that our method works well in the systems of
one-dimensional frustrated spins up to 48 sites, of bosons on a chain up to 32
sites and of fermions on a ladder up to 28 sites. We will see this method
enables us to study eigenvalues of these quantum systems within reasonable
accuracy.


title:
Two-Impurity Kondo Effect in Double-Quantum-Dot Systems - Effect of
  Interdot Kinetic Exchange Coupling

abstract:
  Tunneling conductance through two quantum dots, which are connected in series
to left and right leads, is calculated by using the numerical renormalization
group method. As the hopping between the dots increases from very small value,
the following states continuously appear; (i) Kondo singlet state of each dot
with its adjacent-site lead, (ii) singlet state between the local spins on the
dots, and (iii) double occupancy in the bonding orbital of the two dots. The
conductance shows peaks at the transition regions between these states.
Especially, the peak at the boundary between (i) and (ii) has the unitarity
limit value of $2e^{2}/h$ because of coherent connection through the
lead-dot-dot-lead. For the strongly correlated cases, the characteristic energy
scale of the coherent peak shows anomalous decrease relating to the quantum
critical transition known for the two-impurity Kondo effect. The two dots
systems give the new realization of the two-impurity Kondo problem.


title: Unzipping DNA - towards the first step of replication

abstract:
  The opening of the Y-fork - the first step of DNA replication - is shown to
be a critical phenomenon under an external force at one of its ends. From the
results of an equivalent delocalization in a non-hermitian quantum-mechanics
problem we show the different scaling behavior of unzipping and melting. The
resultant long-range critical features within the unzipped part of Y might play
a role in the highly correlated biochemical functions during replication.


title: Berry phase induced persistent current in mesoscopic systems

abstract:
  Since the discovery of the Berry phase, there has been much interest in the
study of topological effects in the fields of quantum mechanics and condensed
matter physics. In this paper we investigate the persistent charge current in
mesoscopic disorderd rings and compute the disorder averaged persistent current
using the diagrammatic method. We find that within the adiabatic regime the
average current oscillates as a function of the geometric flux which is related
to the Berry phase.


title:
Interdependence of magnetism and superconductivity in the borocarbide
  TmNi2B2C

abstract:
  We have discovered a new antiferromagnetic phase in TmNi2B2C by neutron
diffraction. The ordering vector is Q_A = (0.48,0,0) and the phase appears
above a critical in-plane magnetic field of 0.9 T. The field was applied in
order to test the assumption that the zero-field magnetic structure at Q_F =
(0.094,0.094,0) would change into a c-axis ferromagnet if superconductivity
were destroyed. We present theoretical calculations which show that two effects
are important: A suppression of the ferromagnetic component of the RKKY
exchange interaction in the superconducting phase, and a reduction of the
superconducting condensation energy due to the periodic modulation of the
moments at the wave vector Q_A.


title: Dynamical susceptibilities in strong coupling approach

abstract:
  A general scheme to calculate dynamical susceptibilities of strongly
correlated electron systems within the dynamical mean field theory is
developed. Approach is based on an expansion over electron hopping around the
atomic limit (within the diagrammatic technique for site operators: projection
and Hubbard ones) in infinite dimensions. As an example, the Falicov-Kimball
and simplified pseudospin-electron models are considered for which an
analytical expressions for dynamical susceptibilities are obtained.


title:
Nonlocal interactions prevent collapse in negative scattering length
  Bose-Einstein gases

abstract:
  We study the effect of nonlocality on the collapse properties of a
self-focusing Nonlinear Schr\"odinger system related to Bose-Einstein
condensation problems.
  Using a combination of moment techniques, time dependent variational methods
and numerical simulations we present evidences in support of the hypothesis
that nonlocal attractively interacting condensates cannot collapse when the
dominant interaction term is due to finite range interactions. Instead there
apppear oscillations of the wave packet with a localized component whose size
is of the order of the range of interactions.
  We discuss the implications of the results to collapse phenomena in negative
scattering length Bose-Einstein condensates.


title:
Measurements of critical current diffraction patterns in annular
  Josephson junctions

abstract:
  We report systematic measurements of the critical current versus magnetic
field patterns of annular Josephson junctions in a wide magnetic field range. A
modulation of the envelope of the pattern, which depends on the junction width,
is observed. The data are compared with theory and good agreement is found.


title: Avalanches at rough surfaces

abstract:
  We describe the surface properties of a simple lattice model of a sandpile
that includes evolving structural disorder. We present a dynamical scaling
hypothesis for generic sandpile automata, and additionally explore the kinetic
roughening of the sandpile surface, indicating its relationship with the
sandpile evolution. Finally, we comment on the surprisingly good agreement
found between this model, and a previous continuum model of sandpile dynamics,
from the viewpoint of critical phenomena.


title: Multiscaling of energy correlations in the random-bond Potts model

abstract:
  We numerically calculate the exponent for the disorder averaged and
fixed-sample decay of the energy-energy correlator in the q-state random-bond
Potts model. Our results are in good agreement with a two-loop expansion
(cond-mat/9910181) around q=2 recently found from perturbative renormalisation
group techniques, fulfill the correlation length bound \nu >= 2/d, and give
further evidence against replica symmetry breaking in this class of models.


title: Self-consistent cluster CPA methods and the nested CPA theory

abstract:
  The coherent potential approximation, CPA, is a useful tool to treat systems
with disorder. Cluster theories have been proposed to go beyond the translation
invariant single-site CPA approximation and include some short range
correlations. In this framework one can also treat simultaneously diagonal
disorder (in the site-diagonal elements of the Hamiltonian) and non-diagonal
disorder (in the bond energies). It proves difficult to obtain reasonable
results, free of non-analyticities, for lattices of dimension higher than one
(D>1). We show electronic structure results obtained for a Hubbard model,
treated in mean field approximation, on a square lattice and a simple cubic
lattice, with the simultaneous inclusion of diagonal and non-diagonal disorder.
We compare the results obtained using three different methods to treat the
problem: a self-consistent 2-site cluster CPA method, the
Blackman-Esterling-Berk single-site like extension of the CPA and a nested CPA
approach.


title:
Spectral properties of the one-dimensional two-channel Kondo lattice
  model

abstract:
  We have studied the energy spectrum of a one-dimensional Kondo lattice, where
the localized magnetic moments have SU(N) symmetry and two channels of
conduction electrons are present. At half filling, the system is shown to exist
in two phases: one dominated by RKKY-exchange interaction effects, and the
other by Kondo screening. A quantum phase transition point separates these two
regimes at temperature $T = 0$. The Kondo-dominated phase is shown to possess
soft modes, with spectral gaps much smaller than the Kondo temperature.


title:
Development of (4-epsilon)-dimensional theory for the density of states
  near the Anderson transition

abstract:
  The density of states for the Schroedinger equation with a Gaussian random
potential is determined by the functional integral corresponding to the phi^4
theory with a `wrong' sign of the interaction constant. The special role of the
dimension d=4 for such a problem can be seen from different viewpoints but is
fundamentally determined by the renormalizability of the theory. The
construction of an epsilon-expansion in direct analogy with the
phase-transition theory gives rise to the problem of a `spurious' pole. To
solve this problem, a proper treatment of the factorial divergency of the
perturbation series is necessary. Simplifications arising in high dimensions
can be used for the development of a (4-epsilon)-dimensional theory, but this
requires successive consideration of four types of theories: a
nonrenormalizable theory for d>4, nonrenormalizable and renormalizable theories
in the logarithmic situation (d=4), and a super-renormalizable theory for d<4.
An approximation is found for each type of theory giving asymptotically exact
results. The qualitative effect is the same in all four cases and consists in a
shifting of the phase transition point in the complex plane. This results in
the elimination of the `spurious' pole and in regularity of the density of
states for all energies. A discussion is given of the calculation of high
orders of perturbation theory and a perspective of the epsilon-expansion for
the problem of conductivity near the Anderson transition.


title:
Two-mode theory of vortex stability in multicomponent Bose-Einstein
  condensates

abstract:
  We study the stability and dynamics of vortices in two-species condensates
using a two mode model. The recent experimental results obtained at JILA (M. R.
Matthews, {\em et al.}, Phys. Rev. Lett. 83 (1999) 2498) and theoretical
predictions based on the Gross-Pitaevskii equations are verified. We also make
an exhaustive analysis of the stability properties of the system when the
relative populations of the two species and/or their relative scattering
lengths are changed and prove that stabilization of otherwise unstable
configurations can be attained by controlling the relative population of both
species.


title:
Field theoretical computations of static conductivity for a white noise
  magnetic system

abstract:
  We consider an electron coupled to a random magnetic field with local
correlations and eventually non-zero average value. Starting from the
source-term formalism of Levine, Libby and Pruisken, we define a generating
function for static transport coefficients. We use a non-hermitian
representation of the Hamiltonian, and a functionnal representation of the
generating function, where the integration variable consists of non-local
supermatrices, due to the non-locality of the Aharanov-Bohm interaction.The
conductivity coefficients are computed using a specific saddle-point procedure,
which incorporates ladder diagrams contributions. The situation with an
external uniform magnetic field is considered with the two different spin
projection possibilities. The limit of vanishingly small external magnetic
field is unstable with respect to spin flip, because of E=0 states. In
particular a quantized step is obtained when comparing the Hall conductance
between the two possible spin projections.


title:
Correlated magnetoexcitons in semiconductor quantum dots at finite
  temperatur

abstract:
  We describe computational methods for the theoretical study of explicit
correlations beyond the mean field in excitons confined in semiconductor
quantum dots in terms of the Auxiliary-Field Monte Carlo (AFMC) method. Using
AFMC, the many-body problem is formulated as a Feynman path integral at finite
temperatures and evaluated to numerical precision. This approach is ideally
suited for implementation on high-performance parallel computers. Our strategy
is to generate a set of mean-field states via the Hartree-Fock method for use
as a basis for the AFMC calculations. We present preliminary results.


title: Coulomb Drag Between Parallel Ballistic Quantum Wires

abstract:
  The Coulomb drag between parallel, {\it ballistic} quantum wires is studied
theoretically in the presence of a perpendicular magnetic field B. The
transresistance R_D shows peaks as a function of the Fermi level and splitting
energy between the 1D subbands of the wires. The sharpest peaks appear when the
Fermi level crosses the subband extrema so that the Fermi momenta are small.
Two other kinds of peaks appear when either {\it intra}- or {\it inter}-subband
transitions of electrons have maximum probability; the {\it intra}-subband
transitions correspond to a small splitting energy. R_D depends on the field B
in a nonmonotonic fashion: it decreases with B, as a result of the suppression
of backscattering, and increases sharply when the Fermi level approaches the
subband bottoms and the suppression is outbalanced by the increase of the
Coulomb matrix elements and of the density of states.


title:
Viscous fingering in liquid crystals: Anisotropy and morphological
  transitions

abstract:
  We show that a minimal model for viscous fingering with a nematic liquid
crystal in which anisotropy is considered to enter through two different
viscosities in two perpendicular directions can be mapped to a two-fold
anisotropy in the surface tension. We numerically integrate the dynamics of the
resulting problem with the phase-field approach to find and characterize a
transition between tip-splitting and side-branching as a function of both
anisotropy and dimensionless surface tension. This anisotropy dependence could
explain the experimentally observed (reentrant) transition as temperature and
applied pressure are varied. Our observations are also consistent with previous
experimental evidence in viscous fingering within an etched cell and
simulations of solidification.

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


title: Path Integral Approach to the Nonextensive Canonical Density Matrix

abstract:
  Feynman's path integral is herein generalized to the nonextensive canonical
density matrix based on Tsallis entropy. This generalization is done in two
ways by using unnormalized and normalized constraints. Firstly, we consider the
path integral formulation with unnormalized constraints, and this
generalization is worked out through two different ways, which are shown to be
equivalent. These formulations with unnormalized constraints are solutions to
two generalized Bloch equations proposed in this work. The first form of the
generalized Bloch equation is linear, but with a temperature-dependent
effective Hamiltonian; the second form is nonlinear and resembles the anomalous
correlated diffusion equation (porous medium equation). Furthermore, we can
extend these results to the prescription of field theory using integral
representations. The second development is dedicated to analyzing the path
integral formulation with normalized constraints.
  To illustrate the methods introduced here, we analyze the free particle case
and a non-interacting scalar field. The results herein obtained are expected to
be useful in the discussion of generic nonextensive contexts.


title:
Nodal Quasiparticles versus Phase Fluctuations in High Tc
  Superconductors: An Intermediate Scenario

abstract:
  We explore a BCS Bose Einstein crossover scenario for $0 \leq T \leq T_c$ and
its implications for the superfluid density and specific heat. The low lying
excitations consist of nodal (fermionic) quasi-particles as well as excited
(bosonic) pair states. Semi-quantitative comparison with cuprate data is
reasonable, with no compelling indications for Landau parameter effects.


title:
Ferromagnetic Semiconductor - Singlet (or Triplet) Superconductor -
  Ferromagnetic Semiconductor Systems as Possible Logic Circuits and Switches

abstract:
  We consider thin superconducting (S) films of thickness d $\ll \xi_{0}$,
sandwiched between two ferromagnetic semiconducting insulators (FI) with
differently orientated magnetizations - the FI-S-FI system. We calculate the
dependence of the superconducting critical temperature on the orientation of
the magnetization in the insulators and on the thickness of the superconducting
film. The calculations are done for singlet as well as triplet superconductors.
In the singlet case T_{c} depends on the relative orientation of the left and
right magnetization only, while in the triplet case T_{c} depends on the
absolute orientation of magnetization. The latter property can serve as a kind
of spin-spectroscopy of triplet and unconventional superconductors, for
instance in resolving the structure of the triplet order parameter in the
recently discovered layered superconductor Sr_{2}RuO_{4}. The possibility of
logic circuits and switches, which are based on the FI-S-FI systems with
arbitrary orientation of magnetizations in FI films, is analyzed too.


title: Decay Process for Three - Species Reaction - Diffusion System

abstract:
  We propose the deterministic rate equation of three-species in the reaction -
diffusion system. For this case, our purpose is to carry out the decay process
in our three-species reaction-diffusion model of the form $A+B+C\to D$. The
particle density and the global reaction rate are also shown analytically and
numerically on a two-dimensional square lattice with the periodic boundary
conditions. Especially, the crossover of the global reaction rate is discussed
in both early-time and long-time regimes.


title: Dephasing Times in a Non-degenerate Two-Dimensional Electron Gas

abstract:
  Studies of weak localization by scattering from vapor atoms for electrons on
a liquid helium surface are reported. There are three contributions to the
dephasing time. Dephasing by the motion of vapor atoms perpendicular to the
surface is studied by varying the holding field to change the characteristic
width of the electron layer at the surface. A change in vapor density alters
the quasi-elastic scattering length and the dephasing due to the motion of
atoms both perpendicular and parallel to the surface. Dephasing due to the
electron-electron interaction is dependent on the electron density.


title: Models of the Pseudogap State in Cuprates

abstract:
  We review a certain class of ("nearly") exactly solvable models of electronic
spectrum of two-dimensional systems with fluctuations of short range order of
"dielectric" (e.g. antiferromagnetic) or "superconducting" type, leading to the
formation of anisotropic pseudogap state on certain parts of the Fermi surface.
The models are based on recurrence procedure for one- and two-electron Green's
functions which takes into account of all Feynman diagrams in perturbation
series with the use of the approximate Ansatz for higher-order terms in this
series. These models can be applied to calculation of spectral density, density
of states and conductivity in the normal state, as well as to calculation of
some properties of superconducting state.


title: Magnetic properties of doped Heisenberg chains

abstract:
  The magnetic susceptibility of systems from a class of integrable models for
doped spin-$S$ Heisenberg chains is calculated in the limit of vanishing
magnetic field. For small concentrations $x_h$ of the mobile spin-$(S-1/2)$
charge carriers we find an explicit expression for the contribution of the
gapless mode associated to the magnetic degrees of freedom of these holes to
the susceptibility which exhibits a singularity for $x_h\to0$ for sufficiently
large $S$. We prove a sum rule for the contributions of the two gapless
magnetic modes in the system to the susceptibility which holds for arbitrary
hole concentration. This sum rule complements the one for the low temperature
specific heat which has been obtained previously.


title:
One-dimensional metallic behavior of the stripe phase in
  La$_{2-x}$Sr$_x$CuO$_4$

abstract:
  Using an exact diagonalization method within the dynamical mean-field theory
we study stripe phases in the two-dimensional Hubbard model. We find a
crossover at doping $\delta\simeq 0.05$ from diagonal stripes to vertical
site-centered stripes with populated domain walls, stable in a broad range of
doping, $0.05<\delta<0.17$. The calculated chemical potential shift $\propto
-\delta^2$ and the doping dependence of the magnetic incommensurability are in
quantitative agreement with the experimental results for doped
La$_{2-x}$Sr$_x$CuO$_4$. The electronic structure shows one-dimensional
metallic behavior along the domain walls, and explains the suppression of
spectral weight along the Brillouin zone diagonal.


title: Integrable Extended Hubbard Hamiltonians from Symmetric Group Solutions

abstract:
  We consider the most general form of extended Hubbard Hamiltonian conserving
the total spin and number of electrons, and find all the 1-dimensional
completely integrable models which can be derived from first degree polynomial
solution of the Yang-Baxter equation. It is shown that such models are 96. They
are identified with the 16-dimensional representation of a new class of
solutions of symmetric group relations, acting as generalized permutators. As
particular examples, the EKS and some other known models are obtained.


title:
Temperature and filling dependence of the superconducting $\pi$-phase in
  the Penson-Kolb-Hubbard model

abstract:
  We investigate in the Hartree Fock approximation the temperature and filling
dependence of the superconducting $\pi$-phase for the Penson-Kolb-Hubbard
model. Due to the presence of the pair-hopping term, the phase survives for
repulsive values of the on-site Coulomb interaction, exhibiting an interesting
filling and temperature dependence. The structure of the self-consistent
equations peculiar to the $\pi$-phase of the model allows to explicitly solve
them for the chemical potential. The phase diagrams are shown and discussed in
dimension 2 and 3. We also show that, when a next-nearest neighbours hopping
term is included, the critical temperature of the superconducting region
increases, and the corresponding range of filling values is shifted away from
half-filling. Comparison with known exact results is also discussed.


title: Nodal Liquids in Extended t-J Models and Dynamical Supersymmetry

abstract:
  In the context of extended t-J models, with intersite Coulomb interactions of
the form $-V\sum\limits_{< {i,j} >} {n_in_j}$, with n_i denoting the electron
number operator at site i, nodal liquids are discussed. We use the spin-charge
separation ansatz as applied to the nodes of a d-wave superconducting gap.Such
a situation may be of relevance to the physics of high-temperature
superconductivity. We point out the possibility of existence of certain points
in the parameter space of the model characterized by dynamical supersymmetries
between the spinon and holon degrees of freedom, which are quite different from
the symmetries in conventional supersymmetric t-J models. Such symmetries
pertain to the continuum effective field theory of the nodal liquid, and one's
hope is that the ancestor lattice model may differ from the continuum theory
only by renormalization-group irrelevant operators in the infrared. We give
plausible arguments that nodal liquids at such supersymmetric points are
characterized by superconductivity of Kosterlitz-Thouless type. The fact that
quantum fluctuations around such points can be studied in a controlled way,
probably makes such systems of special importance for an eventual
non-perturbative understanding of the complex phase diagram of the associated
high-temperature superconducting materials.

categories: cond-mat.str-el cond-mat.supr-con hep-lat hep-th


title:
Bilayer Quantum Hall Systems at Filling Factor \nu=2: An Exact
  Diagonalisation Study

abstract:
  We present an exact diagonalisation study of bilayer quantum Hall systems at
a filling factor of two in the spherical geometry. We find the
high-Zeeman-coupling phase boundary of the broken symmetry canted
antiferromagnet is given exactly by previous Hartree-Fock mean-field theories,
but that the state's stability at weak Zeeman coupling has been qualitatively
overestimated. In the absence of interlayer tunneling, degeneracies occur
between total spin multiplets due to the Hamiltonian's invariance under
independent spin-rotations in top and bottom two-dimensional electron layers.


title:
Dynamic critical behaviors of three-dimensional XY models related to
  superconductors/superfluids

abstract:
  The dynamic critical exponent z is determined from numerical simulations for
the three-dimensional XY model subject to two types of dynamics, i.e.
relaxational dynamics and resistively shunted junction (RSJ) dynamics, as well
as for two different treatments of the boundary, i.e., periodic boundary
condition (PBC) and fluctuating twist boundary condition (FTBC). In case of
relaxational dynamics, finite size scaling at the critical temperature gives
$z\approx 2$ for PBC and 1.5 for FTBC, while for RSJ dynamics $z\approx 1.5$ is
obtained in both cases. The results are discussed in the context of
superfluid/superconductors and vortex dynamics, and are compared with what have
been found for other related models.


title:
Critical current from dynamical boundary instability for fully
  frustrated Josephson junction arrays

abstract:
  We investigate numerically the critical current of two-dimensional fully
frustrated arrays of resistively shunted Josephson junctions at zero
temperature. It is shown that a domino-type mechanism is responsible for the
existence of a critical current lower than the one predicted from the
translationally invariant flux lattice. This domino mechanism is demonstrated
for uniform-current injection as well as for various busbar conditions. It is
also found that inhomogeneities close to the contacts makes it harder for the
domino propagation to start, which increases the critical current towards the
value based on the translational invariance. This domino-type vortex motion can
be observed in experiments as voltage pulses propagating from the contacts
through the array.


title:
Evaluation of Single Vehicle Data in Dependence of the Vehicle-Type,
  Lane, and Site

abstract:
  In this paper we study dependencies of fundamental diagrams, time gap
distributions, and velocity-distance relations on vehicle types, lanes and/or
measurement sites. We also propose measurement and aggregation methods that
have more favourable statistical properties than conventional methods.


title: Three-state neural network: from mutual information to the hamiltonian

abstract:
  The mutual information, I, of the three-state neural network can be obtained
exactly for the mean-field architecture, as a function of three macroscopic
parameters: the overlap, the neural activity and the {\em activity-overlap},
i.e. the overlap restricted to the active neurons. We perform an expansion of I
on the overlap and the activity-overlap, around their values for neurons almost
independent on the patterns. From this expansion we obtain an expression for a
Hamiltonian which optimizes the retrieval properties of this system. This
Hamiltonian has the form of a disordered Blume-Emery-Griffiths model. The
dynamics corresponding to this Hamiltonian is found. As a special
characteristic of such network, we see that information can survive even if no
overlap is present. Hence the basin of attraction of the patterns and the
retrieval capacity is much larger than for the Hopfield network. The extreme
diluted version is analized, the curves of information are plotted and the
phase diagrams are built.


title: Directed motion of Brownian particles with internal energy depot

abstract:
  A model of Brownian particles with the ability to take up energy from the
environment, to store it in an internal depot, and to convert internal energy
into kinetic energy of motion, is discussed. The general dynamics outlined in
Sect. 2 is investigated for the deterministic and stochastic particle's motion
in a non-fluctuating ratchet potential. First, we discuss the attractor
structure of the ratchet system by means of computer simulations. Dependent on
the energy supply, we find either periodic bound attractors corresponding to
localized oscillations, or one/two unbound attractors corresponding to directed
movement in the ratchet potential. Considering an ensemble of particles, we
show that in the deterministic case two currents into different directions can
occur, which however depend on a supercritical supply of energy. Considering
stochastic influences, we find the current only in one direction. We further
investigate how the current reversal depends on the strength of the stochastic
force and the asymmetry of the potential. We find both a critical value of the
noise intensity for the onset of the current and an optimal value where the net
current reaches a maximum. Eventually, the dynamics of our model is compared
with other ratchet models previously suggested.


title: Driving Force in Investment

abstract:
  We study investment strategy in different models of financial markets, where
the investors cannot reach a perfect knowledge about available assets. The
investor spends a certain effort to get information; this allows him to better
choose the investment strategy, and puts a selective pressure upon assets. The
best strategy is then a compromise between diversification and effort to get
information.


title:
Structurally constrained protein evolution: results from a lattice
  simulation

abstract:
  We simulate the evolution of a protein-like sequence subject to point
mutations, imposing conservation of the ground state, thermodynamic stability
and fast folding. Our model is aimed at describing neutral evolution of natural
proteins. We use a cubic lattice model of the protein structure and test the
neutrality conditions by extensive Monte Carlo simulations. We observe that
sequence space is traversed by neutral networks, i.e. sets of sequences with
the same fold connected by point mutations. Typical pairs of sequences on a
neutral network are nearly as different as randomly chosen sequences. The
fraction of neutral neighbors has strong sequence to sequence variations, which
influence the rate of neutral evolution. In this paper we study the
thermodynamic stability of different protein sequences. We relate the high
variability of the fraction of neutral mutations to the complex energy
landscape within a neutral network, arguing that valleys in this landscape are
associated to high values of the neutral mutation rate. We find that when a
point mutation produces a sequence with a new ground state, this is likely to
have a low stability. Thus we tentatively conjecture that neutral networks of
different structures are typically well separated in sequence space. This
results indicates that changing significantly a protein structure through a
biologically acceptable chain of point mutations is a rare, although possible,
event.


title:
The topology of the Fermi surface of Bi2212 from angle resolved
  photoemission

abstract:
  We present a study of the topology of the normal state Fermi surface (FS) of
the high Tc superconductor Bi2212 using angle-resolved photoemission. We
present FS mapping experiments, recorded using unpolarised radiation with high
(E,k) resolution, and an extremely dense sampling of k-space. In addition,
synchrotron radiation-based ARPES has been used to prove the energy
independence of the FS as seen by photoemission. We resolve the current
controversy regarding the normal state FS in Bi2212. The true picture is
simple, self-consistent and robust: the FS is hole-like, with the form of
rounded tubes centred on the corners of the Brillouin zone. Two further types
of features are also clearly observed: shadow FSs, and diffraction replicas of
the main FS caused by passage of the photoelectrons through the modulated Bi-O
planes.


title:
In-Plane and Out-of-Plane Optical Properties of NdBa2Cu3Ox Single
  Crystals close to x = 7.0

abstract:
  We present results of reflectivity measurements with E parallel to c and E
perpendicular to c on NdBa2Cu3Ox (Nd123) single crystals close to full oxygen
doping. Along the c-axis the optical conductivity shows a well developed
absorption band around 450 cm^-1 at all temperatures. The in-plane optical
properties are dominated by crystal-field excitations at low energies, a
prominent step at 400 cm^-1 and a weaker feature between 500 - 550 cm^-1. A
comparison of the c-axis optical conductivity and the in-plane scattering rate
with neutron scattering derived spin susceptibility spectra suggests that the
in-plane and out-of-plane anomalies are caused by the same mechanism, probably
electron--spin scattering.


title:
Uphill Motion of Active Brownian Particles in Piecewise Linear
  Potentials

abstract:
  We consider Brownian particles with the ability to take up energy from the
environment, to store it in an internal depot, and to convert internal energy
into kinetic energy of motion. Provided a supercritical supply of energy, these
particles are able to move in a ``high velocity'' or active mode, which allows
them to move also against the gradient of an external potential. We investigate
the critical energetic conditions of this self-driven motion for the case of a
linear potential and a ratchet potential. In the latter case, we are able to
find two different critical conversion rates for the internal energy, which
describe the onset of a directed net current into the two different directions.
The results of computer simulations are confirmed by analytical expressions for
the critical parameters and the average velocity of the net current. Further,
we investigate the influence of the asymmetry of the ratchet potential on the
net current and estimate a critical value for the asymmetry in order to obtain
a positive or negative net current.


title:
The Effect of Chemical Doping and Hydrostatic Pressure on Tc of
  Y1-yCayBa2Cu3Ox Single Crystals

abstract:
  We performed susceptibility measurements on Y1-yCayBa2Cu3Ox single crystals
under high He pressure. For each Ca content various samples with different
oxygen contents have been prepared to probe the influence of Ca on Tc(x),
dTc/dp(x) and Tc,max. Starting from the parabolic Tc(nh) behavior we calculated
nh values from Tc and Tc,max for each sample. It is shown that in the overdoped
region dTc/dp can be described by a pressure induced charge transfer with
dnh/dp = 3.7E-3 [1/GPa] and a dTc,max/dp value of 0.8 K/GPa, irrespective of
the Ca content. In the underdoped region additional pressure effects lead to a
peak in dTc/dp at approximately 0.11 holes/CuO2 plane. However, with increasing
Ca content this peak is strongly depressed. This is explained in terms of an
increasing disorder in the CuO chain system due to doping. Deviations in dTc/dp
at very low nh values can be assigned to the ortho II ordering in the CuO chain
system.


title: Jamming and Fluctuations in Granular Drag

abstract:
  We investigate the dynamic evolution of jamming in granular media through
fluctuations in the granular drag force. The successive collapse and formation
of jammed states give a stick-slip nature to the fluctuations which is
independent of the contact surface between the grains and the dragged object --
thus implying that the stress-induced collapse is nucleated in the bulk of the
granular sample. We also find that while the fluctuations are periodic at small
depths, they become "stepped" at large depths, a transition which we interpret
as a consequence of the long-range nature of the force chains.


title: Jamming, Friction and Unsteady Rheology

abstract:
  The connection between friction and jamming in granular media, molecular
glasses, and complex fluids is explored. The paper first reviews the way
friction is measured, the types of results that are observed, and what is known
about the geometry of contacts between macroscopic surfaces. Then simple models
for the origin of static friction are described. These are unable to explain
the universal observation of static friction between macroscopic objects. The
effects of surface roughness and chemical heterogeneity are discussed, and
shown to yield exponentially weak static friction in our three dimensional
world. In contrast, jamming of the "third bodies" that are present between most
surfaces is shown to produce static friction that is consistent with
macroscopic measurements. Experimental and simulation studies of jamming of
molecularly thin fluid films are described and compared to studies of bulk
glass transitions. The paper concludes by considering the unsteady stick-slip
motion that often arises when systems become unjammed by sufficiently large
stress. Different types of stick-slip motion are identified, and some of its
origins are explored.


title: Scattering Rate Gap in the IR Response of HgBa_2Ca_2Cu_3O_{8+delta}

abstract:
  The ab-plane optical spectra of one underdoped and one nearly optimally doped
single crystal of HgBa_2Ca_2Cu_3O_{8+delta} were investigated in the frequency
range from 40 to 40,000 cm^-1. The frequency dependent scattering rate was
obtained by Kramers Kronig analysis of the reflectance. Both crystals have a
scattering rate gap of about 1000 cm^-1 which is much larger than the 700 cm^-1
gap seen in optical studies of several cuprates with maximum Tc around 93 K.
There appears to be a universal scaling between scattering rate gap and maximum
Tc for the cuprate superconductors.


title: On Bose-Einstein condensate inside moving exciton-phonon droplets

abstract:
  We explore a nonlinear field model to describe the interplay between the
ability of excitons to be Bose condensed and their interaction with other modes
of a crystal. We apply our consideration to the long-living paraexcitons in
Cu2O. Taking into account the exciton-phonon interaction and introducing a
coherent phonon part of the moving condensate, we solve quasi-stationary
equations for the exciton-phonon condensate. These equations support localized
solutions, and we discuss the conditions for the inhomogeneous condensate to
appear in the crystal. Allowable values of the characteristic width of
ballistic condensates are estimated. The stability conditions of the moving
condensate are analyzed by use of Landau arguments, and Landau critical
parameters appear in the theory. It follows that, under certain conditions, the
condensate can move through the crystal as a stable droplet. To separate the
coherent and non-coherent parts of the exciton-phonon packet, we suggest to
turn off the phonon wind by the changes in design of the 3D crystal and
boundary conditions for the moving droplet.


title: Nuclear spin relaxation in ordered bimetallic chain compounds

abstract:
  A theoretical interpretation is given to recent proton spin relaxation-time
(T_1) measurements on NiCu(C_7H_6N_2O_6)(H_2O)_3$\cdot$2H_2O, which is an ideal
one-dimensional ferrimagnetic Heisenberg model system of alternating spins 1
and 1/2. The relaxation rate T_1^{-1} is formulated in temrs of the spin-wave
theory and is evaluated by the use of a quantum Monte Carlo method.
Calculations of the temperature and applied-field (H) dependences of T_1^{-1}
are in total agreement with the experimental findings. T_1 behaves as
$T_1^{-1}\propto H^{-1/2}$, which turns out an indirect observation of the
quadratic dispersion relations dominating the low-energy physics of quantum
ferrimagnets.


title: Directed polymers at finite temperatures in 1+1 and 2+1 dimensions

abstract:
  We present systematic numerical simulations for directed polymers at finite
temperatures in 1+1 and 2+1 dimensions. The transverse fluctuations and free
energy fluctuations tend to the strong coupling limit at any temperature in
both 1+1 and 2+1 dimensions for long time t. Two different definitions for
energy fluctuations at finite temperatures, which are the ensemble energy
fluctuations and the internal energy fluctuations, are investigated. Apart from
zero temperature, the behavior of the energy fluctuations and the free energy
fluctuations for directed polymers is shown to be different. At finite
temperatures, the ensemble energy fluctuations in both 1+1 and 2+1 dimensions
and internal energy fluctuations in 1+1 dimensions scale as t^{1/2} where the
free energy fluctuations in 1+1 dimensions and 2+1 dimensions scale as t^{1/3}
and t^{0.2} respectively. As a consequence of that the specific heat in both
1+1 and 2+1 dimensions scales as t and the entropy fluctuations in 1+1
dimensions scale as t^{1/2} at any finite temperature.


title: Quantum Monte Carlo Study of electrons in low dimensions

abstract:
  We report on a diffusion Monte Carlo investigation of model electron systems
in low dimensions, which should be relevant to the physics of systems
obtainable nowadays in semiconductor heterostructures. In particular, we
present results for a one dimensional electron gas, at selected values of the
coupling strength and confinement parameter, briefly analyzing the pair
correlations and relating them to predictions by Schulz for a Luttinger liquid
with long-range interactions. We find no evidence of the the Bloch instability
yielded by approximate treatments such as the STLS and DFT schemes.


title: Interactions in Quasicrystals

abstract:
  Although the effects of interactions in solid state systems still remains a
widely open subject, some limiting cases such as the three dimensional Fermi
liquid or the one-dimensional Luttinger liquid are by now well understood when
one is dealing with interacting electrons in {\it periodic} crystalline
structures. This problem is much more fascinating when periodicity is lacking
as it is the case in {\it quasicrystalline} structures. Here, we discuss the
influence of the interactions in quasicrystals and show, on a controlled
one-dimensional model, that they lead to anomalous transport properties,
intermediate between those of an interacting electron gas in a periodic and in
a disordered potential.


title:
Formation of New Fermi Surfaces in 3D Crystals at Ultra High Magnetic
  Field with Different Orientations

abstract:
  In the tight-binding approximation the Harper like equation describing an
electron in 3D crystal subject to a uniform magnetic field is obtained. It is
supposed that the vector H can be oriented along several directions in the
lattice. The Fermi surfaces relevant to a magnetic flux p/q=1/2 in a simple
cubic lattice are built. The quantization rules in magnetic fields slightly
distinguished from p/q=1/2 are investigated.


title: Metal-Insulator transitions in generalized Hubbard models

abstract:
  We study the Mott transition in Hubbard models with a degenerate band on
different 3-dimensional lattices. While for a non-degenerate band only the
half-filled system may exhibit a Mott transition, with degeneracy there can be
a transition for any integer filling. We analyze the filling dependence of the
Mott transition and find that $U_c$ (the Hubbard interaction $U$ at which the
transition takes place) decreases away from half-filling. In addition we can
change the lattice structure of the model. This allows us to study the
influence of frustration on the Mott transition. We find that frustration
increases $U_c$, compared to bipartite systems. The results were obtained from
fixed-node diffusion Monte Carlo calculations using trial functions which allow
us to systematically vary the magnetic character of the system. To gain a
qualitative understanding of the results, we have developed simple hopping
arguments that help to rationalize the doping dependence and the influence of
frustration on the Mott transition. Choosing the model parameters to describe
the doped Fullerides, we can make contact with experiment and understand why
some of the Fullerides are metals, while others, which according to density
functional theory should also be metallic, actually are insulators.


title:
Effect of the sample geometry on the second magnetization peak in single
  crystalline Ba$_{0.63}$K$_{0.37}$BiO$_3$ thick film

abstract:
  Magnetization hysteresis loop $M(H)$ measurements performed on a single
crystalline Ba$_{0.63}$K$_{0.37}$BiO$_3$ superconducting thick film reveal
pronounced sample geometry dependence of the "second magnetization peak" (SMP),
i.e. a maximum in the width of $M(H)$ occurring at the field $H_{\rm SMP}(T)$.
In particular, it is found that the SMP vanishes decreasing the film dimension.
We argue that the observed sample geometry dependence of the SMP cannot be
accounted for by models which assume a vortex pinning enhancement as the origin
of the SMP. Our results can be understood considering the thermomagnetic
instability effect and/or non-uniform current distribution at $H < H_{\rm SMP}$
in large enough samples.


title: Critical properties of the reaction - diffusion model 2A -> 3A, 2A ->0

abstract:
  The steady-state phase diagram of the one-dimensional reaction-diffusion
model 2A -> 3A, 2A -> 0 is studied through the non-hermitian density matrix
renormalization group. In the absence of single-particle diffusion the model
reduces to the pair-contact process, which has a phase transition in the
universality class of Directed Percolation (DP) and an infinite number of
absorbing steady states. When single-particle diffusion is added, the number of
absorbing steady states is reduced to two and the model does not show DP
critical behaviour anymore. The exponents $\theta=\nu_{\|}/\nu_{\perp}$ and
$\beta/\nu_{\perp}$ are calculated numerically. The value of
$\beta/\nu_{\perp}$ is close to the value of the Parity Conserving universality
class, in spite of the absence of local conservation laws.


title: Full-CI Quantum Chemistry using the Density-Matrix Renormalization Group

abstract:
  We describe how density-matrix renormalization group (DMRG) can be used to
solve the full-CI problem in quantum chemistry. As an illustration of the
potential of this method, we apply it to a paramagnetic molecule. In
particular, we show the effect of various basis set, the scaling as the fourth
power of the size of the problem, and compare the DMRG with other methods.


title: Spin-dependent transport in a quasiballistic quantum wire

abstract:
  We describe the transport properties of a 5 $\mu$m long one-dimensional (1D)
quantum wire. Reduction of conductance plateaux due to the introduction of
weakly disorder scattering are observed. In an in-plane magnetic field, we
observe spin-splitting of the reduced conductance steps. Our experimental
results provide evidence that deviation from conductance quantisation is very
small for electrons with spin parallel and is about 1/3 for electrons with spin
anti-parallel. Moreover, in a high in-plane magnetic field, a spin-polarised 1D
channel shows a plateau-like structure close to $0.3 \times e^2/h$ which
strengthens with {\em increasing} temperatures. It is suggested that these
results arise from the combination of disorder and the electron-electron
interactions in the 1D electron gas.


title: A planar diagram approach to the correlation problem

abstract:
  We transpose an idea of 't Hooft from its context of Yang and Mills' theory
of strongly interacting quarks to that of strongly correlated electrons in
transition metal oxides and show that a Hubbard model of N interacting electron
species reduces, to leading orders in N, to a sum of almost planar diagrams.
The resulting generating functional and integral equations are very similar to
those of the FLEX approximation of Bickers and Scalapino. This adds the Hubbard
model at large N to the list of solvable models of strongly correlated
electrons.
  PACS Numbers: 71.27.+a 71.10.-w 71.10.Fd


title:
Dislocations in the ground state of the solid-on-solid model on a
  disordered substrate

abstract:
  We investigate the effects of topological defects (dislocations) to the
ground state of the solid-on-solid (SOS) model on a simple cubic disordered
substrate utilizing the min-cost-flow algorithm from combinatorial
optimization. The dislocations are found to destabilize and destroy the elastic
phase, particularly when the defects are placed only in partially optimized
positions. For multi defect pairs their density decreases exponentially with
the vortex core energy. Their mean distance has a maximum depending on the
vortex core energy and system size, which gives a fractal dimension of $1.27
\pm 0.02$. The maximal mean distances correspond to special vortex core
energies for which the scaling behavior of the density of dislocations change
from a pure exponential decay to a stretched one. Furthermore, an extra
introduced vortex pair is screened due to the disorder-induced defects and its
energy is linear in the vortex core energy.


title: Phase separation in t-J ladders

abstract:
  The phase separation boundary of isotropic t-J ladders is analyzed using
density matrix renormalization group techniques. The complete boundary to phase
separation as a function of J/t and doping is determined for a chain and for
ladders with two, three and four legs. Six-chain ladders have been analyzed at
low hole doping. We use a direct approach in which the phase separation
boundary is determined by measuring the hole density in the part of the system
which contains both electrons and holes. In addition we examine the binding
energy of multi-hole clusters. An extrapolation in the number of legs suggests
that the lowest J/t for phase separation to occur in the two dimensional t-J
model is J/t~1.


title: Boolean derivatives and computation of cellular automata

abstract:
  The derivatives of a Boolean function are defined up to any order. The Taylor
and MacLaurin expansions of a Boolean function are thus obtained. The last
corresponds to the ring sum expansion (RSE) of a Boolean function, and is a
more compact form than the usual canonical disjunctive form. For totalistic
functions the RSE allows the saving of a large number of Boolean operations.
The algorithm has natural applications to the simulations of cellular automata
using the multi site coding technique. Several already published algorithms are
analized, and expressions with fewer terms are generally found.


title:
Electron spectrum, thermodynamics and transport in antiferromagnetic
  metals at low temperatures

abstract:
  Electron spectrum of 2D and 3D antiferromagnetic metals is calculated with
account of spin-fluctuation corrections within perturbation theory in the s-f
exchange model. Effects of the interaction of conduction electrons with spin
waves in thermodynamic and transport properties are investigated. At lowest
temperatures $T<T^{*}\sim (\Delta /E_F)T_N$ ($\Delta $ is the AFM splitting of
the electron spectrum) a Fermi-liquid behavior takes place, and non-analytic
T^3lnT-contributions to specific heat are present for D=3. At the same time,
for T>T^{*}, in 2D and ``nested'' 3D systems the picture corresponds to a
marginal Fermi liquid (TlnT-contributions to specific heat and nearly T-linear
dependence of resistivity). Frustrations in the spin system in the 3D case are
demonstrated to lead to similar results. The Kondo contributions to electronic
properties are analyzed and demonstrated to be strongly suppressed. The
incoherent contributions to transport properties in the presence of impurity
scattering are considered. In particular, in the 2D case T-linear terms in
resistivity are present up to T=0, and thermoelectric power demonstrates the
anomalous T\lnT-dependence.


title:
Oxygen isotope effects in high-quality thin films of manganites:
  Quantitative constraints on the physics of manganites

abstract:
  Oxygen isotope effects on the transport properties have been studied in
high-quality epitaxial thin films of La_{0.75}Ca_{0.25}MnO_{3} and
Nd_{0.7}Sr_{0.3}MnO_{3}. In the paramagnetic state, the resistivity can be well
fitted by \rho (T) = (A/\sqrt{T})\exp(E_{\rho}/k_{B}T) with the parameters A
and E_{a} depending strongly on the oxygen isotope mass. The resistivity below
80 K almost perfectly follows \rho = \rho_{o}+
B\omega_{s}/\sinh^{2}(\hbar\omega_{s}/2k_{B}T) with \hbar\omega_{s}/k_{B} \sim
100 K. Both \rho_{o} and B increase by about 15(3)% upon raplacing $^{16}$O by
$^{18}$O. The results provide quantitative constraints on the basic physics of
manganites.


title: Structure and Tc of Y0.8Ca0.2Ba2Cu3O6.92 under High Pressure

abstract:
  The lattice parameters and Tc of Y1-yCayBa2Cu3Ox have been determined under
pressure up to 10 GPa using a diamond anvil cell. The results are used to test
the extended charge transfer model that connects the pressure effect on Tc to
an intrinsic dTc,max/dp and a pressure induced hole doping dnh/dp, the latter
being caused by charge transfer from the CuO chains to the CuO2 planes. dnh/dp
and dTc,max/dp are usually assumed to be constant with respect to pressure p.
However, our experiments show that the usage of this model gives a poor
description of the experimental Tc(p) values. We connected the extended charge
transfer model to the pressure induced changes of the compressibilities. With
this ansatz the calculated Tc(p) values show an excellent agreement with the
experimentally determined values.


title:
The Effects of Chemical Doping and Hydrostatic Pressure on Tc of
  Y(1-y)CayBa2Cu3Ox and NdBa2Cu3Ox Single Crystals

abstract:
  We studied Tc of YBa2Cu3Ox (Y123), Y0.89Ca0.11Ba2Cu3Ox (YCa123) and
NdBa2Cu3Ox (Nd123) single crystals with various oxygen contents x. Compared to
Tc(x) of Y123 the Tc(x) curves of YCa123 are shifted to lower oxygen contents
and the maximum transition temperature Tc,max decreases with increasing Ca
content whereas in Nd123 Tc(x) is shifted to higher oxygen contents and Tc,max
is increased. According to the universal parabolic Tc(nh) behavior the
differences in Tc(x)of Y123, YCa123 and Nd123 can be ascribed to different hole
concentrations nh in the CuO2 planes caused by doping via changes in chemistry
or structure. In order to study the influence of structural changes on Tc we
examined the hydrostatic pressure effect dTc/dp (p up to 0.6 GPa). In the
underdoped region, at nh=0.11, the examined compounds show a peak in dTc/dp
which is very pronounced for systems with well ordered CuO chains. As this peak
occurs at the same nh in all investigated systems it is not caused by oxygen
ordering, but its origins might be found in a strong influence of lattice
deformations on the electronic structure.


title:
Breakdown of the Landau-Fermi liquid in Two Dimensions due to Umklapp
  Scattering

abstract:
  We study the renormalization group flow of the interactions in the
two-dimensional t-t' Hubbard model near half filling in a N-patch
representation of the whole Fermi surface. Starting from weak to intermediate
couplings the flows are to strong coupling with different character depending
on the choice of parameters. In a large parameter region elastic Umklapp
scatterings drive an instability which on parts of the Fermi surface exhibits
the key signatures of an insulating spin liquid (ISL), as proposed by Furukawa
et al., rather than a conventional symmetry-broken state. The ISL is
characterized by both strong d-wave pairing and antiferromagnetic correlations,
however it is insulating due to the vanishing local charge compressibility and
a spin liquid because of the spin gap arising from the pairing correlations. We
find that the ISL is a consequence of a Fermi surface close to the saddle
points at the Brillouin zone boundaries which provides an intrinsic and
mutually reinforcing coupling between pairing and Umklapp channels.


title: Compressibility and thermal expansion of YBa2Cu3Ox

abstract:
  The linear compressibilities and the thermal expansion of an untwinned,
nearly optimal doped YBa2Cu3O6.94 single crystal were measured along the three
crystallographic axes with a high-resolution dilatometer mounted in a
high-pressure cell. The measurements were performed in a temperature range from
50 K to 320 K under hydrostatic gas pressure of maximal 0.65 GPa. At a
temperature of 300 K the measured bulk modulus is reduced and shows a very
strong enhancement under increasing pressure due to pressure-induced oxygen
ordering. This ordering process can also be related to a glass transition seen
in the thermal expansion above 225 K. At lower temperatures, when oxygen
ordering is frozen, the bulk modulus is increased and shows a significantly
reduced enhancement under pressure. This pressure dependence, however, is still
anomalous high, probably caused by bonds of extremely different
compressibilities within the unit cell. The widely spread literature data of
the bulk modulus can be explained by this high pressure dependence.


title: Twirling and Whirling: Viscous Dynamics of Rotating Elastica

abstract:
  Motivated by diverse phenomena in cellular biophysics, including bacterial
flagellar motion and DNA transcription and replication, we study the overdamped
nonlinear dynamics of a rotationally forced filament with twist and bend
elasticity. Competition between twist injection, twist diffusion, and writhing
instabilities is described by a novel pair of coupled PDEs for twist and bend
evolution. Analytical and numerical methods elucidate the twist/bend coupling
and reveal two dynamical regimes separated by a Hopf bifurcation: (i)
diffusion-dominated axial rotation, or twirling, and (ii) steady-state
crankshafting motion, or whirling. The consequences of these phenomena for
self-propulsion are investigated, and experimental tests proposed.


title: Calorimetric Investigation of NdBa2Cu3Ox Single Crystals

abstract:
  The specific heat of Nd(Y)Ba2Cu3Ox single crystals without substitution of Nd
for Ba has been measured for various oxygen contents. In comparison with
YBa2Cu3Ox, the maximum attainable transition temperature of the crystals is
about 4 K higher and shifted from x = 6.92 to x = 7.0, the highest possible
oxygen content. In spite of the difference in the Tc(x) dependences, the jump
in the specific heat DeltaC/Tc in both systems increases continuously being
maximum at x = 7.0. Bond valence sum calculations based on neutron diffraction
data point to a retarded generation of holes with increasing oxygen content in
the Nd system and a different hole distribution between the chains and the
planes in comparison to YBa2Cu3Ox accounting for the different behaviour of
both systems.


title: The mechanism for the 3 x 3 distortion of Sn/ge (111)

abstract:
  We show that two distinct $3 \times 3$ ground states, one nonmagnetic,
metallic, and distorted, the other magnetic, semimetallic (or insulating) and
undistorted, compete in $\alpha$-phase adsorbates on semiconductor (111)
surfaces. In Sn/Ge(111), LSDA/GGA calculations indicate, in agreement with
experiment, that the distorted metallic ground state prevails. The reason for
stability of this state is analysed, and is traced to a sort of bond density
wave, specifically a modulation of the antibonding state filling between the
adatom and a Ge-Ge bond directly underneath.


title: Two-gap model for underdoped cuprate superconductors

abstract:
  Various properties of underdoped superconducting cuprates, including the
momentum-dependent pseudogap opening, indicate a behavior which is neither BCS
nor Bose-Einstein condensation (BEC) like. To explain this issue we introduce a
two-gap model. This model assumes an anisotropic pairing interaction among two
kinds of fermions with small and large Fermi velocities representing the
quasiparticles near the M and the nodal points of the Fermi surface
respectively. We find that a gap forms near the M points resulting into
incoherent pairing due to strong fluctuations. Instead the pairing near the
nodal points sets in with phase coherence at lower temperature. By tuning the
momentum-dependent interaction, the model allows for a continuous evolution
from a pure BCS pairing (in the overdoped and optimally doped regime) to a
mixed boson-fermion picture (in the strongly underdoped regime).


title: Josephson Effect in Fulde-Ferrell-Larkin-Ovchinnikov Superconductors

abstract:
  Due to the difference in the momenta of the superconducting order parameters,
the Josephson current in a Josephson junction between a
Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superconductor and a conventional BCS
superconductor is suppressed. We show that the Josephson current may be
recovered by applying a magnetic field in the junction. The field strength and
direction at which the supercurrent recovery occurs depend upon the momentum
and structure of the order parameter in the FFLO state. Thus the Josephson
effect provides an unambiguous way to detect the existence of an FFLO state,
and to measure the momentum of the order parameter.


title: Dissipation and Vortex Creation in Bose-Einstein Condensed Gases

abstract:
  We solve the Gross-Pitaevskii equation to study energy transfer from an
oscillating `object' to a trapped Bose-Einstein condensate. Two regimes are
found: for object velocities below a critical value, energy is transferred by
excitation of phonons at the motion extrema; while above the critical velocity,
energy transfer is via vortex formation. The second regime corresponds to
significantly enhanced heating, in agreement with a recent experiment.


title:
Zero-frequency anomaly in quasiclassical ac transport: Memory effects in
  a two-dimensional metal with a long-range random potential or random magnetic
  field

abstract:
  We study the low-frequency behavior of the {\it ac} conductivity
$\sigma(\omega)$ of a two-dimensional fermion gas subject to a smooth random
potential (RP) or random magnetic field (RMF). We find a non-analytic
$\propto|\omega|$ correction to ${\rm Re} \sigma$, which corresponds to a
$1/t^2$ long-time tail in the velocity correlation function. This contribution
is induced by return processes neglected in Boltzmann transport theory. The
prefactor of this $|\omega|$-term is positive and proportional to $(d/l)^2$ for
RP, while it is of opposite sign and proportional to $d/l$ in the weak RMF
case, where $l$ is the mean free path and $d$ the disorder correlation length.
This non-analytic correction also exists in the strong RMF regime, when the
transport is of a percolating nature. The analytical results are supported and
complemented by numerical simulations.


title:
Relation between bulk order-parameter correlation function and
  finite-size scaling

abstract:
  We study the large-$r$ behavior of the bulk order-parameter correlation
function $G(\bf{r})$ for $T>T_c$ within the lattice $\phi^4$ theory. We also
study the large-$L$ behavior of the susceptibility $\chi$ of the confined
lattice system of size $L$ with periodic boundary conditions. The large-$L$
behavior of $\chi$ is closely related to the large-$r$ behavior of $G(\bf{r})$.
Explicit results are derived for $d>2$. Finite-size scaling must be formulated
in terms of the anisotropic exponential correlation length $\xi_1$ that governs
the decay of $G(\bf{r})$ for large $r$ rather than in terms of the isotropic
correlation length $\xi$ defined via the second moment of $G(\bf{r})$. This
result modifies a recent interpretation concerning an apparent violation of
finite-size scaling in terms of $\xi \neq \xi_1$. Exact results for the $d=1$
Ising model illustrate our conclusions. Furthermore, we show that the
exponential finite-size behavior for $L/\xi\gg 1$ is not captured by the
standard perturbation approach that separates the lowest mode from the higher
modes. Consequences for the theory of finite-size effects for $d>4$ are
discussed. The two-variable finite-size scaling form predicts an approach
$\propto e^{-L/\xi_1}$ to the bulk critical behavior whereas a single-variable
scaling form implies a power-law approach $\propto L^{-d}$.


title:
A Rigorous Proof of Fermi Liquid Behavior for Jellium Two-Dimensional
  Interacting Fermions

abstract:
  Using the method of continuous constructive renormalization group around the
Fermi surface, it is proved that a jellium two-dimensional interacting system
of Fermions at low temperature $T$ remains analytic in the coupling constant
$\lambda$ for $|\lambda| |\log T| \le K $ where $K$ is some numerical constant
and $T$ is the temperature. Furthermore in that range of parameters, the first
and second derivatives of the self-energy remain bounded, a behavior which is
that of Fermi liquids and in particular excludes Luttinger liquid behavior. Our
results prove also that in dimension two any transition temperature must be
non-perturbative in the coupling constant, a result expected on physical
grounds. The proof exploits the specific momentum conservation rules in two
dimensions.


title:
Boson Realization of the SU(4) Model of High-Temperature
  Superconductivity

abstract:
  The SU(4) algebraic model of high-temperature superconductivity is studied
employing the boson mapping techniques. The bosonization of the model enables
us to get exact numerical solution of the model. The order parameters are
discussed. The situation close to the SO(5) dynamical symmetry limit is
interpreted as a modelling case for the behaviour of $D$-wave superconducting
and antiferromagnetic phases in cuprates.


title: Ferromagnetic Nanowires with Superconducting Electrodes

abstract:
  The proximity effect in mesoscopic ferromagnet/superconductor ($FS$) Ni/Al
structures of various geometries was studied experimentally on both $F$- and
$S$-sides of the structures. Samples with a wide range of interface
transparency were fabricated. The dependence of the effect on $FS$ interface
transparency was investigated. The amplitude of this effect was found to be
larger than expected from classical theory of proximity effect. Preliminary
experiments showed no phase-sensitive oscillations in Andreev interferometer
geometry. Various theoretical models are discussed.


title: Coulomb Drag between Quantum Wires

abstract:
  We study Coulomb drag in a pair of parallel one-dimensional electron systems
within the framework of the Tomanaga-Luttinger model. We find that Coulomb
coupling has a much stronger effect on one dimensional wires than on
two-dimensional layers: At zero temperature the trans-resistivity {\em
diverges}, due to the formation of locked charge density waves. At temperature
well above a cross-over temperature $T^*$ the trans-resistivity follows a power
law $\rho \propto T^x$, where the interaction-strength dependent exponent $x$
is determined by the Luttinger Liquid parameter $K_{c-}$ of the relative charge
mode. At temperature below $T^*$ relative charge displacements are enabled by
solitonic excitations, reflected by an exponential temperature dependence. The
cross-over temperature $T^*$ depends sensitively on the wire width, inter-wire
distance, Fermi wavelength and the effective Bohr radius. For wire distances
$\bar{d} \gg k_F^{-1}$ it is exponentially suppressed with $T/E_F \sim \exp[ -
\bar{d} k_F / (1-K_{c-}) ]$. The behavior changes drastically if each of the
two wires develop spin gaps. In this case we find that the trans-resistivity
{\em vanishes} at zero temperature. We discuss our results in view of possible
experimental realizations in GaAs-AlGaAs semiconductor structures.


title:
Low energy dynamics of the one dimensional multichannel Kondo-Heisenberg
  Lattice

abstract:
  We determine exactly the fixed point Hamiltonian of the one dimensional
multichannel Kondo-Heisenberg lattice model for any number of channels N>=2. An
anomalous singlet with non trivial internal dynamics is generated. We compute
the correlation functions of the various conventional and unconventional order
parameters of the system and find that for N<=4 the composite order parameter
induce the dominant instabilities.


title: Improved Spin Dynamics Simulations of Magnetic Excitations

abstract:
  Using Suzuki-Trotter decompositions of exponential operators we describe new
algorithms for the numerical integration of the equations of motion for
classical spin systems. These techniques conserve spin length exactly and, in
special cases, also conserve the energy and maintain time reversibility. We
investigate integration schemes of up to eighth order and show that these new
algorithms can be used with much larger time steps than a well established
predictor-corrector method. These methods may lead to a substantial speedup of
spin dynamics simulations, however, the choice of which order method to use is
not always straightforward.


title:
Spin dynamics simulations of excitations and critical dynamics in
  RbMnF_3

abstract:
  Spin-dynamics simulations have been used to investigate the dynamic behavior
of RbMnF_3, treating it as a classical Heisenberg antiferromagnet on a simple
cubic lattice. Time-evolutions of spin configurations were determined
numerically from coupled equations of motion for individual spins using a new
algorithm which is based on Suzuki-Trotter decompositions of exponential
operators. The dynamic structure factor was calculated from the space- and
time-displaced spin-spin correlation function. The crossover from hydrodynamic
to critical behavior of the dispersion curve and spin-wave half-width was
studied as the temperature was increased towards the critical value. The
dynamic critical exponent was estimated to be z=(1.43\pm 0.03), which is
slightly lower than the dynamic scaling prediction, but in good agreement with
a recent experimental value. Comparisons are made of both the dispersion curve
and the lineshapes obtained from our simulations with very recent experimental
results for RbMnF_3 are presented.


title:
Spin dynamics simulations - a powerful method for the study of critical
  dynamics

abstract:
  Spin-dynamics techniques can now be used to study the deterministic
time-dependent behavior of magnetic systems containing over 10^5 spins with
quite good accuracy. This approach will be introduced, including the
theoretical foundations of the methods of analysis. Then newly developed,
improved techniques based upon Suzuki-Trotter decomposition methods will be
described. The current ``state-of-the-art'' will be evaluated with specific
examples drawn from data on simple magnetic models. The examination of dynamic
critical behavior will be highlighted but the extraction of information about
excitations at low temperatures will be included.


title: Out-of-equilibrium Kondo effect in Double Quantum Dots

abstract:
  The out-of-equilibrium transport properties of a double quantum dot system in
the Kondo regime are studied theoretically by means of a two-impurity Anderson
hamiltonian with interimpurity hopping. The hamiltonian is solved by means of a
non-equilibrium generalization of the slave-boson mean-field theory. It is
demonstrated that measurements of the differential conductance dI/dV, for the
appropriate values of voltages and tunneling couplings, can give a direct
observation of the coherent superposition between the many-body Kondo states of
each dot.
  For large voltages and arbitrarily large interdot tunneling, there is a
critical voltage above which the physical behaviour of the system is again that
of two decoupled quantum dots, i.e two Kondo singularities pinned at each
chemical potential.


title:
Cooling Rate Dependence and Dynamic Heterogeneity Below the Glass
  Transition in a Lennard-Jones Glass

abstract:
  We investigate a binary Lennard-Jones mixture with molecular dynamics
simulations. We consider first a system cooled linearly in time with the
cooling rate gamma. By varying gamma over almost four decades we study the
influence of the cooling rate on the glass transition and on the resulting
glass. We find for all investigated quantities a cooling rate dependence; with
decreasing cooling rate the system falls out of equilibrium at decreasing
temperatures, reaches lower enthalpies and obtains increasing local order. Next
we study the dynamics of the melting process by investigating the most immobile
and most mobile particles in the glass. We find that their spatial distribution
is heterogeneous and that the immobile/mobile particles are surrounded by
denser/less dense cages than an average particle.

categories: cond-mat.stat-mech cond-mat.dis-nn cond-mat.mtrl-sci cond-mat.soft


title: Bloch electrons in electric and magnetic fields

abstract:
  We investigate Bloch electrons in two dimensions subject to constant electric
and magnetic fields. The model that results from our pursuit is governed by a
finite difference equation with a quasienergy spectrum that interpolates
between a butterfly-like structure and a Stark ladder structure. These findings
ensued from the use of electric and magnetic translation operators.


title:
Magnetization quantum tunneling at excited levels for a biaxial spin
  system in an arbitrarily directed magnetic field

abstract:
  The quantum tunneling of the magnetization vector between excited levels are
studied theoretically in single-domain ferromagnetic nanoparticles with biaxial
crystal symmetry placed in an external magnetic field at an arbitarily directed
angle in the ZX plane. The temperature dependences of the tunneling frequency
and the decay rate are clearly shown for each case.


title: Quantum Confinement Transition and Cuprate Criticality

abstract:
  Theoretical attempts to explain the origin of high temperature
superconductivity are challenged by the complexity of the normal state, which
exhibits three regimes with increasing hole doping: a pseudo-gap regime when
underdoped, strange power laws near optimal doping and more conventional
metallic behavior when heavily overdoped. We suggest that the origin of this
behavior is linked to a zero temperature quantum phase transition separating
the overdoped Fermi liquid from a spin-charge separated underdoped phase.
Central to our analysis is a new $Z_2$ gauge theory formulation, which supports
topological vortex excitations - dubbed visons. The visons are gapped in the
underdoped phase, splitting the electron's charge and Fermi statistics into two
separate excitations. Superconductivity occurs when the resulting charge $e$
boson condenses. The visons are condensed in the overdoped phase, thereby
confining the charge and statistics of the electron leading to a Fermi liquid
phase. Right at the quantum confinement transition the visons are in a critical
state, leading to power law behavior for both charge and spin.


title: Phase-Locking of Vortex Lattices Interacting with Periodic Pinning

abstract:
  We examine Shapiro steps for vortex lattices interacting with periodic
pinning arrays driven by AC and DC currents. The vortex flow occurs by the
motion of the interstitial vortices through the periodic potential generated by
the vortices that remain pinned at the pinning sites. Shapiro steps are
observed for fields B_{\phi} < B < 2.25B_{\phi} with the most pronouced steps
occuring for fields where the interstitial vortex lattice has a high degree of
symmetry. The widths of the phase-locked current steps as a function of the
magnitude of the AC driving are found to follow a Bessel function in agreement
with theory.


title: Schwinger boson theory of anisotropic ferromagnetic ultrathin films

abstract:
  Ferromagnetic thin films with magnetic single-ion anisotropies are studied
within the framework of Schwinger bosonization of a quantum Heisenberg model.
Two alternative bosonizations are discussed. We show that qualitatively correct
results are obtained even at the mean-field level of the theory, similar to
Schwinger boson results for other magnetic systems. In particular, the
Mermin-Wagner theorem is satisfied: a spontaneous magnetization at finite
temperatures is not found if the ground state of the anisotropic system
exhibits a continuous degeneracy. We calculate the magnetization and effective
anisotropies as functions of exchange interaction, magnetic anisotropies,
external magnetic field, and temperature for arbitrary values of the spin
quantum number. Magnetic reorientation transitions and effective anisotropies
are discussed. The results obtained by Schwinger boson mean-field theory are
compared with the many-body Green's function technique.


title: Limits of the Quantum Monte Carlo method

abstract:
  We consider the one-dimensional quantum-statistical problem of interacting
spin-less particles in an infinite deep potential valley and on a ring. Several
limits for the applicability of the Quantum Monte Carlo (QMC) methods were
revealed and discussed. We show the inapplicability of the QMC method for
ring-like geometries, realize an unphysical frustration for interacting
fermions and a minus-sign problem for interacting bosons.


title:
Kadanoff-Baym equations and non-Markovian Boltzmann equation in
  generalized T-matrix approximation

abstract:
  A recently developed method for incorporating initial binary correlations
into the Kadanoff-Baym equations (KBE) is used to derive a generalized T-matrix
approximation for the self-energies. It is shown that the T-matrix obtains
additional contributions arising from initial correlations. Using these results
and taking the time-diagonal limit of the KBE, a generalized quantum kinetic
equation in binary collision approximation is derived. This equation is a
far-reaching generalization of Boltzmann-type kinetic equations: it
selfconsistently includes memory effects (retardation, off-shell T-matrices) as
well as many-particle effects (damping, in-medium T-Matrices) and
spin-statistics effects (Pauli-blocking).


title:
Specific heat of an S=1/2 Heisenberg ladder compound
  Cu$_2$(C$_5$H$_{12}$N$_2$)$_2$Cl$_4$ under magnetic fields

abstract:
  Specific heat measurements down to 0.5 K have been performed on a single
crystal sample of a spin-ladder like compound
Cu$_{2}$(C$_{5}$H$_{12}$N$_{2}$)$_{2}$Cl$_{4}$ under magnetic fields up to 12
T. The temperature dependence of the observed data in a magnetic field below 6
T is well reproduced by numerical results calculated for the S=1/2 two-leg
ladder with $J_{\rm{rung}}$/$J_{\rm{leg}}$=5. In the gapless region above 7 T
($H_{\rm{c1}}$), the agreement between experiment and calculation is good above
about 2 K and a sharp and a round peak were observed below 2 K in a magnetic
field around 10 T, but the numerical data show only a round peak, the magnitude
of which is smaller than that of the observed one. The origin of the sharp peak
and the difference between the experimental and numerical round peak are
discussed.


title: Self-organization in BML Traffic Flow Model: Analytical Approaches

abstract:
  Analytical investigations are made on BML two-dimensional traffic flow model
with alternative movement and exclude-volume effect. Several exact results are
obtained, including the upper critical density above which there are only
jamming configurations asymptotically, and the lower critical density below
which there are only moving configurations asymptotically. The jamming
transition observed in the ensemble average velocity takes place at another
critical density $p_{c}(N)$, which is dependent on the lattice size $N$ and is
in the intermediate region between the lower and upper critical densities. It
is suggested that $p_{c}(N)$ is proportional to a power of $N$, in good
agreement with the numerical simulation. The order parameter of this jamming
transition is identified.


title: Anomalous Spreading of Power-Law Quantum Wave Packets

abstract:
  We introduce power-law tail quantum wave packets. We show that they can be
seen as eigenfunctions of a Hamiltonian with a physical potential. We prove
that the free evolution of these packets presents an asymptotic decay of the
maximum of the wave packets which is anomalous for an interval of the
characterizing power-law exponent. We also prove that the number of finite
moments of the wave packets is a conserved quantity during the evolution of the
wave packet in the free space.


title: Universality for 2D Wedge Wetting

abstract:
  We study 2D wedge wetting using a continuum interfacial Hamiltonian model
which is solved by transfer-matrix methods. For arbitrary binding potentials,
we are able to exactly calculate the wedge free-energy and interface height
distribution function and, thus, can completely classify all types of critical
behaviour. We show that critical filling is characterized by strongly universal
fluctuation dominated critical exponents, whilst complete filling is determined
by the geometry rather than fluctuation effects. Related phenomena for
interface depinning from defect lines in the bulk are also considered.


title: Optimization of Trial Wave Functions for Hamiltonian Lattice Models

abstract:
  We propose a new Monte Carlo algorithm for the numerical study of general
lattice models in Hamiltonian form. The algorithm is based on an initial Ansatz
for the ground state wave function depending on a set of free parameters which
are dynamically optimized. The method is discussed in details and results are
reported from explicit simulations of U(1) lattice gauge theory in 1+1
dimensions.


title: Magnetotransport in the low carrier density ferromagnet EuB_6

abstract:
  We present a magnetotransport study of the low--carrier density ferromagnet
EuB_6. This semimetallic compound, which undergoes two ferromagnetic
transitions at T_l = 15.3 K and T_c = 12.5 K, exhibits close to T_l a colossal
magnetoresistivity (CMR). We quantitatively compare our data to recent
theoretical work, which however fails to explain our observations. We attribute
this disagreement with theory to the unique type of magnetic polaron formation
in EuB_6.


title: Spintronic Spin Accumulation and Thermodynamics

abstract:
  The spin degree of freedom can play an essential role in determining the
electrical transport properties of spin-polarized electron systems in metals or
semiconductors. In this article, I address the dependence of spin-subsystem
chemical potentials on accumulated spin-densities. I discuss both approaches
which can be used to measure this fundamental thermodynamic quantity and the
microscopic physics which determines its value in several different systems.


title:
Bounded Luttinger liquids as a universality class of quantum critical
  behavior

abstract:
  We show that one-dimensional quantum systems with gapless degrees of freedom
and open boundary conditions form a new universality class of quantum critical
behavior, which we propose to call ``bounded Luttinger liquids''. They share
the following properties with ordinary (periodic) Luttinger liquids: absence of
fermionic quasi-particle excitations, charge-spin separation, anomalous
power-law correlations with exponents whose scaling relations are parametrized
by a single coupling constant per degree of freedom, $K_{\nu}$. The values of
$K_{\nu}$ are independent of boundary conditions, but the representation of the
critical exponents in terms of these $K_{\nu}$ depends on boundary conditions.
We illustrate these scaling relations by exploring general rules for boundary
critical exponents derived earlier using the Bethe Ansatz solution of the 1D
Hubbard model together with boundary conformal field theory, and the theory of
Luttinger liquids in finite-size systems. We apply this theory to the
photoemission properties of the organic conductors $(TMTSF)_2X$, and discuss to
what extent the assumption of finite strands with open boundaries at the sample
surface can reconcile the experimental results with independent information on
the Luttinger liquid state in these materials.


title: Evolution of Fermi Liquid Behavior with Doping in the Hubbard Model

abstract:
  We calculate the single-particle Green's function for the tight-binding band
structure, $\xi_{\vec p}=-2t\cos p_x-2t\cos p_y -\mu$, with a function of
chemical potential $\mu$ for square-lattice system. The form of the
single-particle self-energy, $\Sigma({\vec p}, E)$, is determined by the
density-density correlation function, $\chi({\vec q}, \omega)$, which develops
two peaks for $\mu \gtrsim -2.5t$ unlike parabolic band case. Near half filling
$\chi({\vec q}, \omega)$ becomes independent of $\omega$, one dimensional
behavior, at intermediate values of $\omega$ which leads to one dimensional
behavior in $\Sigma({\vec p},E)$. However $\mu \leq -0.1t$ there is no
influence on the Fermi Liquid dependences from SDW instability. The strong
$\vec p$ and $E$ dependence of the off-shell self-energy, $\Sigma(p,E)$, found
earlier for the parabolic band is recovered for $\mu \lesssim -t$ but
deviations from this develop for $\mu \gtrsim -0.1t$. The resonance peak width
of the spectral function, $A({\vec p}, E)$ has linear dependence in $\xi_{\vec
p}$ due to the $E$ dependence of the imaginary part of $\Sigma({\vec p}, E)$.
We point out that an accurate detailed form for $\Sigma({\vec p},E)$ would be
very difficult to recover from ARPES data for the spectral density.


title:
Theory of High-Tc Superconducting Cuprates Based on Experimental
  Evidence

abstract:
  A model of superconductivity in layered high-temperature superconducting
cuprates is proposed, based on the extended saddle point singularities in the
electron spectrum, weak screening of the Coulomb interaction and
phonon-mediated interaction between electrons plus a small short -range
repulsion of Hund's, or spin-fluctuation, origin. This permits to explain the
large values of $Tc$, features of the isotope effect on oxygen and copper, the
existence of two types of the order parameter, the peak in the inelastic
neutron scattering, the positive curvature of the upper critical field, as
function of temperature etc.


title:
Monte Carlo Simulations of Doped, Diluted Magnetic Semiconductors - a
  System with Two Length Scales

abstract:
  We describe a Monte Carlo simulation study of the magnetic phase diagram of
diluted magnetic semiconductors doped with shallow impurities in the low
concentration regime. We show that because of a wide distribution of
interaction strengths, the system exhibits strong quantum effects in the
magnetically ordered phase. A discrete spin model, found to closely approximate
the quantum system, shows long relaxation times, and the need for specialized
cluster algorithms for updating spin configurations. Results for a
representative system are presented.


title: Anderson impurity in a correlated conduction band

abstract:
  We investigate the physics of a magnetic impurity with spin 1/2 in a
correlated metallic host. Describing the band by a Hubbard Hamiltonian, the
problem is analyzed using dynamical mean-field-theory in combination with
Wilson's nonperturbative numerical renormalization group. We present results
for the single-particle density of states and the dynamical spin susceptibility
at zero temperature. New spectral features (side peaks) are found which should
be observable experimentally. In addition, we find a general enhancement of the
Kondo scale due to correlations. Nevertheless, in the metallic phase, the Kondo
scale always vanishes exponentially in the limit of small hybridization.