title: Hikami boxes and the Sinai billiard

abstract:
  Diagram, known in theory of the Anderson localization as the Hikami box, is
computed for the Sinai billiard. This interference effect is mostly important
for trajectories tangent to the opening of the billiard. This diagram is
universal at low frequencies, because of the particle number conservation law.
An independent parameter, which we call phase volume of diffraction, determines
the corresponding frequency range. This result suggests that level statistics
of a generic chaotic system is not universal.


title:
Bose-Glass behaviour in Bi_{2}Sr_{2}Ca_{1-x}Y_{x}Cu_{2}O_{8} crystals
  with columnar defects: experimental evidence for variable-range hopping

abstract:
  We report on vortex transport in Bi_{2}Sr_{2}Ca_{1-x}Y_{x}Cu_{2}O_{8}
crystals irradiated at different doses of heavy ions. We show evidence of a
flux-creep resistivity typical of a variable-range vortex hopping mechanism as
predicted by Nelson and Vinokur.


title:
New state of the matter between the Fermi glass and the Wigner crystal
  in two dimensions

abstract:
  Spinless fermions with Coulomb interaction in a square disordered lattice
form a new state of the matter which is nor a Fermi glass, neither a Wigner
crystal for intermediate Coulomb interaction. From a numerical study of small
clusters, we find that this new state occurs between the two critical carrier
densities where insulator-metal-insulator transitions of a hole gas have been
observed in GaAs.


title: Lifetimes of impurity states in crossed magnetic and electric fields

abstract:
  We study the quantum dynamics of localized impurity states created by a point
interaction for an electron moving in two dimensions under the influence of a
perpendicular magnetic field and an in-plane weak electric field. All impurity
states are unstable in presence of the electric field. Their lifetimes are
computed and shown to grow in a Gaussian way as the electric field tends to
zero.


title: Field-induced Ordering in Critical Antiferromagnets

abstract:
  Transfer-matrix scaling methods have been used to study critical properties
of field-induced phase transitions of two distinct two-dimensional
antiferromagnets with discrete-symmetry order parameters: triangular-lattice
Ising systems (TIAF) and the square-lattice three-state Potts model (SPAF-3).
Our main findings are summarised as follows. For TIAF, we have shown that the
critical line leaves the zero-temperature, zero -field fixed point at a finite
angle. Our best estimate of the slope at the origin is $(dT_c/dH)_{T=H=0} =
4.74 \pm 0.15$. For SPAF-3 we provided evidence that the zero-field correlation
length diverges as $\xi(T \to 0, H=0) \simeq \exp (a/T^{x})$, with $x=1.08 \pm
0.13$, through analysis of the critical curve at $H \neq 0$ plus crossover
arguments. For SPAF-3 we have also ascertained that the conformal anomaly and
decay-of-correlations exponent behave as: (a) H=0: $c=1, \eta=1/3$; (b) $H \neq
0: c=1/2, \eta=1/4$.


title: Critical properties of Bose-glass superconductors

abstract:
  We study vortex lines in high-temperature superconductors with columnar
defects produced by heavy ion irradiation. We reconsider scaling theory for the
Bose glass transition with tilted magnetic fields, and propose, e.g., a new
scaling form for the shape of the Bose glass phase boundary, which is relevant
for experiments. We also consider Monte Carlo simulations for a vortex model
with a screened interaction. Critical exponents are determined from scaling
analysis of Monte Carlo data for current-voltage characteristics and other
quantities. The dynamic critical exponent is found to be z = 4.6 +/- 0.3.


title: Superconducting Coherence and the Helicity Modulus in Vortex Line Models

abstract:
  We show how commonly used models for vortex lines in three dimensional
superconductors can be modified to include k=0 excitations. We construct a
formula for the k=0 helicity modulus in terms of fluctuations in the projected
area of vortex loops. This gives a convenient criterion for the presence of
superconducting coherence. We also present Monte Carlo simulations of a
continuum vortex line model for the melting of the Abrikosov vortex lattice in
pure YBCO.


title:
Parallelization of a Dynamic Monte Carlo Algorithm: a Partially
  Rejection-Free Conservative Approach

abstract:
  We experiment with a massively parallel implementation of an algorithm for
simulating the dynamics of metastable decay in kinetic Ising models. The
parallel scheme is directly applicable to a wide range of stochastic cellular
automata where the discrete events (updates) are Poisson arrivals. For high
performance, we utilize a continuous-time, asynchronous parallel version of the
n-fold way rejection-free algorithm. Each processing element carries an lxl
block of spins, and we employ the fast SHMEM-library routines on the Cray T3E
distributed-memory parallel architecture. Different processing elements have
different local simulated times. To ensure causality, the algorithm handles the
asynchrony in a conservative fashion. Despite relatively low utilization and an
intricate relationship between the average time increment and the size of the
spin blocks, we find that for sufficiently large l the algorithm outperforms
its corresponding parallel Metropolis (non-rejection-free) counterpart. As an
example application, we present results for metastable decay in a model
ferromagnetic or ferroelectric film, observed with a probe of area smaller than
the total system.

categories: cond-mat.stat-mech cond-mat.mtrl-sci cs.DC physics.comp-ph


title: Fermiology of a 1D heavy electron metal

abstract:
  We present a Quantum Monte Carlo (QMC) study of the 1D Kondo lattice at
non-integer filling, i.e. a one-dimensional version of a heavy electron metal.
For this special system the minus-sign problem turns out to be strongly
reduced, which allows QMC simulations for temperatures as low as 1% of the
conduction electron bandwidth. The single particle Green's function shows and
intricate network of low-energy bands at low temperature, with a Fermi surface
volume that comprises both c and f-electrons. As temperature increases the
system evolves through two distinct crossover temperatures into a very simple
band structure with one free c-electron band and a disconnected upper and lower
Hubbard band for f-electrons. The f-electrons thus `drop out' of the Fermi
surface volume as temperature increases.


title:
Damage spreading transition in glasses: a probe for the ruggedness of
  the configurational landscape

abstract:
  We consider damage spreading transitions in the framework of mode-coupling
theory. This theory describes relaxation processes in glasses in the mean-field
approximation which are known to be characterized by the presence of an
exponentially large number of meta-stable states. For systems evolving under
identical but arbitrarily correlated noises we demonstrate that there exists a
critical temperature $T_0$ which separates two different dynamical regimes
depending on whether damage spreads or not in the asymptotic long-time limit.
This transition exists for generic noise correlations such that the zero damage
solution is stable at high-temperatures being minimal for maximal noise
correlations. Although this dynamical transition depends on the type of noise
correlations we show that the asymptotic damage has the good properties of an
dynamical order parameter such as: 1) Independence on the initial damage; 2)
Independence on the class of initial condition and 3) Stability of the
transition in the presence of asymmetric interactions which violate detailed
balance. For maximally correlated noises we suggest that damage spreading
occurs due to the presence of a divergent number of saddle points (as well as
meta-stable states) in the thermodynamic limit consequence of the ruggedness of
the free energy landscape which characterizes the glassy state. These results
are then compared to extensive numerical simulations of a mean-field glass
model (the Bernasconi model) with Monte Carlo heat-bath dynamics. The freedom
of choosing arbitrary noise correlations for Langevin dynamics makes damage
spreading a interesting tool to probe the ruggedness of the configurational
landscape.


title: Rheology, and how to stop aging

abstract:
  Recent analytical developments in glass theory are equally relevant to the
understanding of anomalous rheology, with characteristic features such as the
Reynolds dilatancy and the driving-power dependence of the viscosity arising
naturally. A notion of effective temperature based on the
fluctuation-dissipation relation can be introduced in the limit of small
driving power. Within mean-field, the analogue of the Edwards compactivity can
be computed, and it coincides with this effective temperature. The approach
does not invoke any particular geometry for the constituents of the fluid,
provided it has glassy behaviour.


title: Destabilization of the 2D conducting phase by an in-plane magnetic field

abstract:
  We propose a mechanism for the recently reported destabilization by an
in-plane magnetic field of the conducting phase of low density electrons in 2D.
We apply our self-consistent approach based on the memory function formalism to
the fully spin polarized electron system. This takes into account both disorder
and exchange-correlation effects. We show that spin polarization significantly
favors localization because of the enhancement of the exchange-correlations. A
key outcome is that the conducting phase for the fully spin polarized system is
significantly suppressed. The in-plane magnetic field needed to stabilize the
fully spin polarized state lies in the range 0.1<H<1 T, depending on the
carrier density. We determine the metal-insulator phase diagram for the
unpolarized and fully polarized systems, and we estimate the dependence of the
critical magnetic field on carrier density.


title:
A density matrix renormalisation group algorithm for quantum lattice
  systems with a large number of states per site

abstract:
  A variant of White's density matrix renormalisation group scheme which is
designed to compute low-lying energies of one-dimensional quantum lattice
models with a large number of degrees of freedom per site is described. The
method is tested on two exactly solvable models---the spin-1/2
antiferromagnetic Heisenberg chain and a dimerised XY spin chain. To illustrate
the potential of the method, it is applied to a model of spins interacting with
quantum phonons. It is shown that the method accurately resolves a number of
energy gaps on periodic rings which are sufficiently large to afford an
accurate investigation of critical properties via the use of finite-size
scaling theory.


title:
Observation of p-wave Threshold Law Using Evaporatively Cooled Fermionic
  Atoms

abstract:
  We have measured independently both s-wave and p-wave cross-dimensional
thermalization rates for ultracold potassium-40 atoms held in a magnetic trap.
These measurements reveal that this fermionic isotope has a large positive
s-wave triplet scattering length in addition to a low temperature p-wave shape
resonance. We have observed directly the p-wave threshold law which, combined
with the Fermi statistics, dramatically suppresses elastic collision rates at
low temperatures. In addition, we present initial evaporative cooling results
that make possible these collision measurements and are a precursor to
achieving quantum degeneracy in this neutral, low-density Fermi system.


title:
Persistent currents in mesoscopic rings and boundary conformal field
  theory

abstract:
  A tight-binding model of electron dynamics in mesoscopic normal rings is
studied using boundary conformal field theory. The partition function is
calculated in the low energy limit and the persistent current generated as a
function of an external magnetic flux threading the ring is found. We study the
cases where there are defects and electron-electron interactions separately.
The same temperature scaling for the persistent current is found in each case,
and the functional form can be fitted, with a high degree of accuracy, to
experimental data.


title: On Superconductors,Superfluids and Torsion Vortices

abstract:
  The Meissner effect for superconductors in spacetimes with torsion is
revisited. Two new physical interpretations are presented. The first considers
the Landau-Ginzburg theory yields a new symmetry-breaking vacuum depending on
torsion. In the second interpretation a gravitational Meissner torsional effect
where when the Higgs field vanishes, torsion and electromagnetic fields need
not vanish and outside the Abrikosov tubes a torsion vector analogous to the
Maxwell potential is obtained. The analogy is even stronger if we think that in
this case the torsion vector has to be derivable from a torsion potential.
Another solution of Landau-Ginzburg equation is shown to lead naturally to the
geometrization of the electromagnetism in terms of the torsion field. As an
application we show that Landau-Ginzburg equations extended to Cartan spaces
allow us to determine torsion in the case when the order parameter is frozen to
a minimum. It is shown that the presence of torsion in solid state physics
changes the ground state of the superfluid phases.


title: Dynamical properties of the one-dimensional Holstein model

abstract:
  The spectral weight functions and the optical conductivity of the Holstein
model are studied on a one-dimensional six-site lattice with periodic boundary
conditions for three different electron concentrations: a single electron, two
electrons of opposite spins, and half filling. A density matrix approach is
used to obtain an optimal phonon basis and to truncate the phonon Hilbert space
without significant loss of accuracy. This approach allows us to calculate
spectral functions for electrons dressed locally by the optimal phonons as well
as for bare electrons. We obtain evidence for a smooth crossover from
quasi-free electrons to an heavy itinerant small polaron (single-electron case)
or bipolaron (two-electron case) as the electron-phonon coupling strength
increases. At half filling we observe a crossover from a quasi-free-electron
ground state to a quasi-degenerate Peierls charge-density-wave ground state for
a finite electron-phonon coupling. This crossover is marked by an abrupt drop
of the Drude weight which is vanishingly small in the Peierls phase.


title:
Reduction of the Superfluid Density in the Vortex-Liquid Phase of
  Bi2Sr2CaCu2Oy

abstract:
  In-plane complex surface impedance of a Bi2Sr2CaCu2Oy single crystal was
measured in the mixed state at 40.8 GHz.The surface reactance, which is
proportional to the real part of the effective penetration depth, increased
rapidly just above the first-order vortex-lattice melting transition field and
the second magnetization peak field.This increase is ascribed to the decrease
in the superfluid density rather than the loss of pinning.This result indicates
that the vortex melting transition changes the electronic structure as well as
the vortex structure.


title: Theory of Colossal Magnetoresistance in Doped Manganites

abstract:
  The exchange interaction of polaronic carriers with localized spins leads to
a ferromagnetic/paramagnetic transition in doped charge-transfer insulators
with strong electron-phonon coupling. The relative strength of the exchange and
electron-phonon interactions determines whether the transition is first or
second order. A giant drop in the number of current carriers during the
transition, which is a consequence of local bound pair (bipolaron) formation in
the paramagnetic phase, is extremely sensitive to an external magnetic field.
Below the critical temperature of the transition, $T_c$, the binding of the
polarons into immobile pairs competes with the ferromagnetic exchange between
polarons and the localized spins on Mn ions, which tends to align the polaron
moments and, therefore, breaks up those pairs. The number of carriers abruptly
increases below $T_c$ leading to a sudden drop in resistivity. We show that the
carrier density collapse describes the colossal magnetoresistance of doped
manganites close to the transition.
  Below $T_c$, transport occurs by polaronic tunneling, whereas at high
temperatures the transport is by hopping processes. The transition is
accompanied by a spike in the specific heat, as experimentally observed. The
gap feature in tunneling spectroscopy is related to the bipolaron binding
energy, which depends on the ion mass. This dependence explains the giant
isotope effect of the magnetization and resistivity upon substitution of
$^{16}$O by $^{18}$O. It is shown also that the localization of polaronic
carriers by disorder {\em cannot} explain the observed huge sensitivity of the
transport properties to the magnetic field in doped manganites.


title:
Successive transitions and intermediate chiral phase in a superfuilid
  ^3He film

abstract:
  Superfluidity ordering of thin ^3He films is studied by Monte Carlo
simulations based on a two-dimensional lattice spin model with Z_2\times
U(1)\times SO(3) symmetry. Successive phase transitions with an intermediate
`chiral' phase, in which the l-vector aligns keeping the phase of the
condensate disordered, is found. Possible experiments to detect the successive
transitions are discussed.


title:
Comment on "Universality of the 1/3 shot-noise suppression factor in
  nondegenerate diffusive conductors"

abstract:
  We argue that the nearly 1/3 suppression of shot noise in nondegenerate
diffusive contacts recently obtained by Gonzalez et al. (cond-mat/9803372) is
due to the specific choice of the energy-independent elastic scattering time.


title:
Self-Consistent Second Order Perturbation Theory for the Hubbard Model
  in Two Dimensions

abstract:
  We apply self-consistent second order perturbation theory (SCSOPT) with
respect to the on-site repulsive interaction U to study the Hubbard model in
two dimensions. We investigate single particle properties of the model over the
entire doping range at zero temperature. It is shown that as doping decreases
toward half-filling $\omega$-mass enhancement factor increases, while k-mass
enhancement factor decreases. The increase in $\omega$-mass enhancement factor
is larger than the decrease in k-mass enhancement factor, so that total-mass is
larger than that in the non-interacting case. When particle number density per
unit cell n is given by 0.64<n<1.0 interaction enhances anisotropy of the Fermi
surface, whereas at lower densities n<0.64 interaction suppresses anisotropy of
it. Due to the decrease in k-mass enhancement factor the density of states
(DOS) at the Fermi level is suppressed. It is possible to understand the
results within the framework of the weak coupling Fermi liquid theory.


title: Stationary Motion of the Adiabatic Piston

abstract:
  We consider a one-dimensional system consisting of two infinite ideal fluids,
with equal pressures but different temperatures T_1 and T_2, separated by an
adiabatic movable piston whose mass M is much larger than the mass m of the
fluid particules. This is the infinite version of the controversial adiabatic
piston problem. The stationary non-equilibrium solution of the Boltzmann
equation for the velocity distribution of the piston is expressed in powers of
the small parameter \epsilon=\sqrt{m/M}, and explicitly given up to order
\epsilon^2. In particular it implies that although the pressures are equal on
both sides of the piston, the temperature difference induces a non-zero average
velocity of the piston in the direction of the higher temperature region. It
thus shows that the asymmetry of the fluctuations induces a macroscopic motion
despite the absence of any macroscopic force. This same conclusion was
previously obtained for the non-physical situation where M=m.


title: Thermopower of atomic-size metallic contacts

abstract:
  The thermopower and conductance of atomic-size metallic contacts have been
simultaneously measured using a mechanically controllable break junction. For
contacts approaching atomic dimensions, abrupt steps in the thermopower are
observed which coincide with jumps in the conductance. The measured thermopower
for a large number of atomic-size contacts is randomly distributed around the
value for large contacts and can be either positive or negative in sign.
However, it is suppressed at the quantum value of the conductance G_0 = 2e^2/h.
We derive an expression that describes these results in terms of quantum
interference of electrons backscattered in the banks.


title: Competing Species Dynamics: Qualitative Advantage versus Geography

abstract:
  A simple cellular automata model for a two-group war over the same territory
is presented. It is shown that a qualitative advantage is not enough for a
minority to win. A spatial organization as well a definite degree of
aggressiveness are instrumental to overcome a less fitted majority. The model
applies to a large spectrum of competing groups: smoker-non smoker war,
epidemic spreading, opinion formation, competition for industrial standards and
species evolution. In the last case, it provides a new explanation for
punctuated equilibria.

categories: cond-mat.dis-nn adap-org comp-gas cond-mat.stat-mech nlin.AO nlin.CG q-bio


title:
Phase diagram of quantized vortices in a trapped Bose-Einstein condensed
  gas

abstract:
  We investigate the thermodynamic stability of quantized vortices in a dilute
Bose gas confined by a rotating harmonic trap at finite temperature.
Interatomic forces play a crucial role in characterizing the resulting phase
diagram, especially in the large $N$ Thomas-Fermi regime. We show that the
critical temperature for the creation of stable vortices exhibits a maximum as
a function of the frequency of the rotating trap and that the corresponding
transition is associated with a discontinuity in the number of atoms in the
condensate. Possible strategies for approaching the vortical region are
discussed.


title: Molecular Correlations in a Supercooled Liquid

abstract:
  We present static and dynamic properties of molecular correlation functions
S_{lmn,l'm'n'}(q,t) in a simulated supercooled liquid of water molecules, as a
preliminary effort in the direction of solving the molecular mode coupling
theory (MMCT) equations for supercooled molecular liquids. The temperature and
time dependence of various molecular correlation functions, calculated from 250
ns long molecular dynamics simulations, show the characteristic patterns
predicted by MMCT and shed light on the driving mechanism responsible for the
slowing down of the molecular dynamics. We also discuss the symmetry properties
of the molecular correlation functions which can be predicted on the basis of
the C_{2v}-symmetry of the molecule. The analysis of the MD--results for the
static correlators S_{lmn,l'm'n'}(q) reveals that additional relationships
between correlators with different signs of n and n' exist. We prove that for
molecules with C_{rv}-symmetry this unexpected result becomes exact at least
for high temperatures.


title: A modified form of the polar model of crystals

abstract:
  A model for description of materials with narrow energy bands is proposed. It
is shown that in narrow-band materials electron-hole symmetry is absent in
contrast to the Hubbard model. In this paper metal-insulator transition is
studied. The obtained results are compared with experimental data for
narrow-band materials. Some specific narrow-band effects are discussed.


title: Magnetic precursor effects in Gd alloys

abstract:
  The behaviour of electrical resistivity ($\rho$) and magnetoresistance in the
vicinity of respective magnetic ordering temperatures in a number of Gd alloys
is reported. In some compounds, e.g., GdNi_2Sn_2 and GdPt_2Ge_2, there is an
enhancement of $\rho$ prior to long range magnetic order over a wide
temperature range which can be highlighted by the suppression of $\rho$ caused
by the application of a magnetic field. However, such features are absent in
many other Gd compounds, e.g., GdCu_2Ge_2, GdAg_2Si_2, GdAu_2Si_2, GdPd_2Ge_2
and GdCo_2Si_2. Attempts to relate such features to magnetic precursor effects
in heat capacity are made. On the basis of our studies, we suggest that better
understanding of magnetic precursor effects in Gd alloys will be helpful to
throw light on some of the current trends in magnetism. Various other
interesting findings in the magnetically ordered state in some of these alloys
are also brought out.


title:
"Wigner crystal" and "stripe" models for the magnetic and
  crystallographic superstructures of La0.333Ca0 .667MnO3

abstract:
  The crystallographic (charge-ordered) and magnetic superstructures of
La0.333Ca0.667MnO3 were studied by high-resolution synchrotron x-ray and
neutron powder diffraction. In the antiferromagnetic structure, which was
refined using a non-collinear model, the a lattice parameter is tripled and the
c lattice parameter is doubled with respect to the average crystallographic
unit cell (Pnma setting). The crystallographic structure below the
charge-ordering temperature (TCO ~ 260 K) is characterized by ordering of the
dz2 orbitals of the Jahn-Teller-distorted Mn3+O6 octahedra in the orthorhombic
ac plane, and the appearance of superlattice peaks in the x-ray patterns
corresponding to a tripling of the a axis lattice parameter. The intensities of
the superlattice peaks can be accounted for satisfactorily in terms of ordering
of the Mn3+ cations in sites as far apart as possible in the ac plane ("Wigner
crystal" model) and transverse displacements of the Mn4+O6 octahedra in the c
direction. These results are not consistent with a recently proposed model
based on transmission electron microscopy (TEM) data in which the Mn3+O6
octahedra are arranged in a stripe pattern ("stripe" model). In particular, the
large longitudinal stripe modulation revealed by TEM is not observed,
suggesting that the TEM data may not be representative of the bulk sample.
Within the framework of the "Wigner crystal" model, the magnetic structure at
1.5 K and the charge-ordered superstructure at 160 K and 1.5 K were refined
from the neutron data.


title: On edge states in semi-infinite quantum Hall systems

abstract:
  We consider an electron in two dimensions submitted to a magnetic field and
to the potential of impurities. We show that when the electron is confined to a
half-space by a planar wall described by a smooth increasing potential, the
total Hamiltonian necessarily has a continuous spectrum in some intervals
in-between the Landau levels provided that both the amplitude and spatial
variation of the impurity potential are sufficiently weak. The spatial decay of
the impurity potential is not needed. In particular this proves the occurence
of edge states in semi-infinite quantum Hall systems.


title: On localization effects in underdoped cuprates

abstract:
  We comment on transport experiments in underdoped LaSrCuO in the
non-superconducting phase. The temperature dependence of the resistance
strongly resembles what is expected from standard localization theory. However
this theory fails, when comparing with experiments in more detail.


title: The Puzzling Stability of Monatomic Gold Wires

abstract:
  We have examined theoretically the spontaneous thinning process of
tip-suspended nanowires, and subsequently studied the structure and stability
of the monatomic gold wires recently observed by Transmission Electron
Microscopy (TEM). The methods used include thermodynamics, classical many-body
force simulations, Local Density (LDA) and Generalized Gradient (GGA)
electronic structure calculations as well as ab-initio simulations including
the two tips. The wire thinning is well explained in terms of a thermodynamic
tip suction driving migration of surface atoms from the wire to the tips. For
the same reason the monatomic wire becomes progressively stretched.
Surprisingly, however, all calculations so far indicate that the stretched
monatomic gold wire should be unstable against breaking, contrary to the
apparent experimental stability. The possible reasons for the observed
stability are discussed.


title:
NMR imaging of the soliton lattice profile in the spin-Peierls compound
  CuGeO_3

abstract:
  In the spin-Peierls compound CuGeO$_{3}$, the commensurate-incommensurate
transition concerning the modulation of atomic position and the local
spin-polarization is fully monitored at T=0 by the application of an external
magnetic field ($H$) above a threshold value $H_{c}\simeq $ 13 Tesla. The
solitonic profile of the spin-polarization, as well as its absolute magnitude,
has been precisely imaged from $^{65}Cu$ NMR lineshapes obtained for
$h=(H-H_{c})/H_{c}$ varying from 0.0015 to 2. This offers a unique possibility
to test quantitatively the various numerical and analytical methods developed
to solve a generic Hamiltonian in 1-D physics, namely strongly interacting
fermions in presence of electron-phonon coupling at arbitrary band filling.


title:
Thermodynamic characteristics of the classical n-vector magnetic model
  in three dimensions

abstract:
  The method of calculating the free energy and thermodynamic characteristics
of the classical n-vector three-dimensional (3D) magnetic model at the
microscopic level without any adjustable parameters is proposed. Mathematical
description is perfomed using the collective variables (CV) method in the
framework of the $\rho^4$ model approximation. The exponentially decreasing
function of the distance between the particles situated at the N sites of a
simple cubic lattice is used as the interaction potential. Explicit and
rigorous analytical expressions for entropy,internal energy, specific heat near
the phase transition point as functions of the temperature are obtained. The
dependence of the amplitudes of the thermodynamic characteristics of the system
for $T>T_c$ and $T<T_c$ on the microscopic parameters of the interaction
potential are studied for the cases $n=1,2,3$ and $n\to\infty$. The obtained
results provide the basis for accurate analysis of the critical behaviour in
three dimensions including the nonuniversal characteristics of the system.


title:
Dynamical Correlation Functions using the Density Matrix Renormalization
  Group

abstract:
  The density matrix renormalization group (DMRG) method allows for very
precise calculations of ground state properties in low-dimensional strongly
correlated systems. We investigate two methods to expand the DMRG to
calculations of dynamical properties. In the Lanczos vector method the DMRG
basis is optimized to represent Lanczos vectors, which are then used to
calculate the spectra. This method is fast and relatively easy to implement,
but the accuracy at higher frequencies is limited. Alternatively, one can
optimize the basis to represent a correction vector for a particular frequency.
The correction vectors can be used to calculate the dynamical correlation
functions at these frequencies with high accuracy. By separately calculating
correction vectors at different frequencies, the dynamical correlation
functions can be interpolated and pieced together from these results. For
systems with open boundaries we discuss how to construct operators for specific
wavevectors using filter functions.


title:
Energy distribution of maxima and minima in a one-dimensional random
  system

abstract:
  We study the energy distribution of maxima and minima of a simple
one-dimensional disordered Hamiltonian. We find that in systems with short
range correlated disorder there is energy separation between maxima and minima,
such that at fixed energy only one kind of stationary points is dominant in
number over the other. On the other hand, in the case of systems with long
range correlated disorder maxima and minima are completely mixed.


title:
Modified Double Exchange Model with Novel Spin and Orbital Coupling:
  Phase Diagram of The Manganites

abstract:
  From a general model of the Mn oxides R_{1-x}A_{x}MnO_3, we derive an
effective Hamiltonian in the low-energy subspace using the projection operator
method, in which a novel coupling between the spin and orbital degrees of
freedom is included. A phase diagram for temperature T versus doping
concentration x is computed by means of Monte Carlo simulation. Our result is
consistent with experimental observations in the Mn oxides with relatively wide
conduction band, such as Pr_{1-x}Sr_{x} MnO_{3} and La_{1-x}Sr_{x}MnO_{3}.
According to the obtained orbital ordering, we also predict that the motion of
charge carriers transforms from three-dimensional to two-dimensional as x is
increased beyond a critical value.


title:
Quasiparticle dynamics in ballistic weak links under weak voltage bias:
  An elementary treatment

abstract:
  A simple one-dimensional model for SNS weak links in the ballistic limit is
presented. In the presence of a bias voltage, the quasiparticle state at any
given instant of time is described as a superposition of that particular set of
phase-dependent Andreev bound states that belongs to the specific phase
difference present at this instant between the superconducting banks. The
treatment -- basically a form of adiabatic perturbation theory -- has a strong
formal similarity to the treatment of the k-space dynamics of an electron in a
periodic potential under perturbation by an external electric field,
sufficiently strong to cause transitions across the energy gaps between bands
(Zener tunneling). It is shown that the quasiparticle wave function retains its
phase information during analogous transitions between Andreev bands. The
experimental observation of Shapiro steps at one-half the canonical voltage
follows naturally from the model, along with some of the experimental
properties of these steps, especially their much weaker temperature dependence,
compared to the canonical steps.


title: Eutectic Colony Formation: A Stability Analysis

abstract:
  Experiments have widely shown that a steady-state lamellar eutectic
solidification front is destabilized on a scale much larger than the lamellar
spacing by the rejection of a dilute ternary impurity and forms two-phase cells
commonly referred to as `eutectic colonies'. We extend the stability analysis
of Datye and Langer for a binary eutectic to include the effect of a ternary
impurity. We find that the expressions for the critical onset velocity and
morphological instability wavelength are analogous to those for the classic
Mullins-Sekerka instability of a monophase planar interface, albeit with an
effective surface tension that depends on the geometry of the lamellar
interface and, non-trivially, on interlamellar diffusion. A qualitatively new
aspect of this instability is the occurence of oscillatory modes due to the
interplay between the destabilizing effect of the ternary impurity and the
dynamical feedback of the local change in lamellar spacing on the front motion.
In a transient regime, these modes lead to the formation of large scale
oscillatory microstructures for which there is recent experimental evidence in
a transparent organic system. Moreover, it is shown that the eutectic front
dynamics on a scale larger than the lamellar spacing can be formulated as an
effective monophase interface free boundary problem with a modified
Gibbs-Thomson condition that is coupled to a slow evolution equation for the
lamellar spacing. This formulation provides additional physical insights into
the nature of the instability and a simple means to calculate an approximate
stability spectrum. Finally, we investigate the influence of the ternary
impurity on a short wavelength oscillatory instability that is already present
at off-eutectic compositions in binary eutectics.


title:
Superconducting Gap Anisotropy and Quasiparticle Interactions: a Doping
  Dependent ARPES Study

abstract:
  Comparing ARPES measurements on Bi2212 with penetration depth data, we show
that a description of the nodal excitations of the d-wave superconducting state
in terms of non-interacting quasiparticles is inadequate, and we estimate the
magnitude and doping dependence of the Landau interaction parameter which
renormalizes the linear T contribution to the superfluid density. Furthermore,
although consistent with d-wave symmetry, the gap with underdoping cannot be
fit by the simple coskx-cosky form, which suggests an increasing importance of
long range interactions as the insulator is approached.


title:
Study of a microcanonical algorithm on the $\pm J$ spin glass model in
  d=3

abstract:
  We consider a microcanonical local algorithm to be applied on the $\pm J$
spin glass model. We have compared the results coming from a microcanonical
Monte Carlo simulation with those from a canonical one: Thermalization times,
spin glass susceptibilities and Binder parameters. For a fixed lattice size we
found different results between the two thermodynamic ensembles, which tend to
vanish at bigger volumes. Moreover, microcanonical thermalization times are
longer than the canonical ones. Finally we have checked that one of the Guerra
relations is satisfied with good precision for the two largest lattices.


title:
Effects of biquadratic exchange on the spectrum of elementary
  excitations in spin ladders

abstract:
  We investigate the influence of biquadratic exchange interactions on the
low-lying excitations of a S=1/2-ladder using perturbation theory, numerical
diagonalization of finite systems and exact results for ladders with matrix
product ground states. We consider in particular the combination of biquadratic
exchange interactions corresponding to ring exchange on the basic ladder
plaquette. We find that a moderate amount of ring exchange reduces the spin gap
substantially and makes equal bilinear exchange on legs and rungs consistent
with experimentally observed spectra.


title: Realistic Electron-Electron Interaction in a Quantum Wire

abstract:
  The form of an effective electron-electron interaction in a quantum wire with
a large static dielectric constant is determined and the resulting properties
of the electron liquid in such a one-dimensional system are described. The
exchange and correlation energies are evaluated and a possibility of a
paramagnetic-ferromagnetic phase transition in the ground state of such a
system is discussed. Low-energy excitations are briefly described.


title:
Field theory in superfluid 3He: What are the lessons for particle
  physics, gravity and high-temperature superconductivity?

abstract:
  There are several classes of homogeneous Fermi-systems which are
characterized by the topology of the energy spectrum of fermionic
quasiparticles: (1) Gapless systems with a Fermi-surface; (2) Systems with a
gap in their spectrum; (3) Gapless systems with topologically stable point
nodes (Fermi points); and (4) Gapless systems with topologically unstable lines
of nodes (Fermi lines). Superfluid 3He-A and electroweak vacuum belong to the
universality Class (3). The fermionic quasiparticles (particles) in this class
are chiral: they are left-handed or right-handed. The collective bosonic modes
of systems of Class (3) are the effective gauge and gravitational fields. The
great advantage of superfluid 3He-A is that we can perform experiments using
this condensed matter and thereby simulate many phenomena in high energy
physics, including axial anomaly, baryoproduction, and magnetogenesis. 3He-A
textures induce a nontrivial effective metrics of the space, where the free
quasiparticles move along geodesics. With 3He-A one can simulate event
horizons, Hawking radiation, rotating vacuum, etc. High-temperature
superconductors are believed to belong to Class (4). They have gapless
fermionic quasiparticles with a "relativistic" spectrum close to gap nodes,
which allows application of ideas developed for superfluid 3He-A.


title:
Field-asymmetric transverse magnetoresistance in a nonmagnetic
  quantum-size structure

abstract:
  A new phenomenon is observed experimentally in a heavily doped asymmetric
quantum-size structure in a magnetic field parallel to the quantum-well layers
- a transverse magnetoresistance which is asymmetric in the field (there can
even be a change in sign) and is observed in the case that the structure has a
built-in lateral electric field. A model of the effect is proposed. The
observed asymmetry of the magnetoresistance is attributed to an additional
current contribution that arises under nonequilibrium conditions and that is
linear in the gradient of the electrochemical potential and proportional to the
parameter characterizing the asymmetry of the spectrum with respect to the
quasimomentum.


title: Topology invariance in Percolation Thresholds

abstract:
  An universal invariant for site and bond percolation thresholds (p_{cs} and
p_{cb} respectively) is proposed. The invariant writes
{p_{cs}}^{1/a_s}{p_{cb}}^{-1/a_b}=\delta/d where a_s, a_b and \delta are
positive constants,and d the space dimension. It is independent of the
coordination number, thus exhibiting a topology invariance at any d.The formula
is checked against a large class of percolation problems, including percolation
in non-Bravais lattices and in aperiodic lattices as well as rigid percolation.
The invariant is satisfied within a relative error of \pm 5% for all the twenty
lattices of our sample at d=2, d=3, plus all hypercubes up to d=6.


title:
Temperature dependence of electric resistance and magnetoresistance of
  pressed nanocomposites of multilayer nanotubes with the structure of nested
  cones

abstract:
  Bulk samples of carbon multilayer nanotubes with the structure of nested
cones (fishbone structure) suitable for transport measurements, were prepared
by compressing under high pressure (~25 kbar) a nanotube precursor synthesized
through thermal decomposition of polyethylene catalyzed by nickel. The
structure of the initial nanotube material was studied using high-resolution
transmission electron microscopy. In the low-temperature range (4.2 - 100 K)
the electric resistance of the samples changes according to the law ln \rho ~
(T_0/T)^{1/3}, where T_0 ~ 7 K. The measured magnetoresistance is quadratic in
the magnetic field and linear in the reciprocal temperature. The measurements
have been interpreted in terms of two-dimensional variable-range hopping
conductivity. It is suggested that the space between the inside and outside
walls of nanotubes acts as a two-dimensional conducting medium. Estimates
suggest a high value of the density of electron states at the Fermi level of
about 5 10^{21} eV^{-1} cm^{-3}.


title: Coulomb Charging Effects for Finite Channel Number

abstract:
  We consider quantum fluctuations of the charge on a small metallic grain
caused by virtual electron tunneling to a nearby electrode. The average
electron number and the effective charging energy are determined by means of
perturbation theory in the tunneling Hamiltonian. In particular we discuss the
dependence of charging effects on the number N of tunneling channels. Earlier
results for N>>1 are found to be approached rather rapidly with increasing N.


title:
Theory of Scanning Tunneling Spectroscopy of Magnetic-Field-Induced
  Discrete Nodal States in a D-Wave Superconductor

abstract:
  In the presence of an external magnetic field, the low lying elementary
excitations of a d-wave superconductor have quantized energy and their momenta
are locked near the node direction. It is argued that these discrete states can
most likely be detected by a local probe, such as a scanning tunneling
microscope. The low temperature local tunneling conductance on the Wigner-Seitz
cell boundaries of the vortex lattice is predicted to show peaks spaced as $\pm
\sqrt{n}, n ={0,1,2, ...}$. The $n=0$ peak is anomalous, and it is present only
if the superconducting order parameter changes sign at certain points on the
Fermi surface. Away from the cell boundary, where the superfluid velocity is
nonzero, each peak splits, in general, into four peaks, corresponding to the
number of nodes in the order parameter.


title: Glass-Like Heat Conduction in High-Mobility Crystalline Semiconductors

abstract:
  The thermal conductivity of polycrystalline semiconductors with type-I
clathrate hydrate crystal structure is reported. Ge clathrates (doped with Sr
and/or Eu) exhibit lattice thermal conductivities typical of amorphous
materials. Remarkably, this behavior occurs in spite of the well-defined
crystalline structure and relatively high electron mobility ($\sim 100
cm^2/Vs$). The dynamics of dopant ions and their interaction with the
polyhedral cages of the structure are a likely source of the strong phonon
scattering.


title: Exact statistical properties of the zeros of complex random polynomials

abstract:
  The zeros of complex Gaussian random polynomials, with coefficients such that
the density in the underlying complex space is uniform, are known to have the
same statistical properties as the zeros of the coherent state representation
of one-dimensional chaotic quantum systems. We extend the interpretation of
these polynomials by showing that they also arise as the wave function for a
quantum particle in a magnetic field constructed from a random superposition of
states in the lowest Landau level. A study of the statistical properties of the
zeros is undertaken using exact formulas for the one and two point distribution
functions. Attention is focussed on the moments of the two-point correlation in
the bulk, the variance of a linear statistic, and the asymptotic form of the
two-point correlation at the boundary. A comparison is made with the same
quantities for the eigenvalues of complex Gaussian random matrices.


title:
Classical versus Quantum Effects in the B=0 Conducting Phase in Two
  Dimensions

abstract:
  In the dilute two-dimensional electron system in silicon, we show that the
temperature below which Shubnikov-de Haas oscillations become apparent is
approximately the same as the temperature below which an exponential decrease
in resistance is seen in B=0, suggesting that the anomalous behavior in zero
field is observed only when the system is in a degenerate (quantum) state. The
temperature dependence of the resistance is found to be qualitatively similar
in B=0 and at integer Landau level filling factors.


title: Sterols sense swelling in lipid bilayers

abstract:
  In the mimetic membrane system of phosphatidylcholine bilayers, thickening
(pre-critical behavior, anomalous swelling) of the bilayers is observed, in the
vicinity of the main transition, which is non-linear with temperature. The
sterols cholesterol and androsten are used as sensors in a time-resolved
simultaneous small- and wide angle x-ray diffraction study to investigate the
cause of the thickening. We observe precritical behavior in the pure lipid
system, as well as with sterol concentrations less than 15%. To describe the
precritical behavior we introduce a theory of precritical phenomena.The good
temperature resolution of the data shows that a theory of the influence of
fluctuations needs modification. The main cause of the critical behavior
appears to be a changing hydration of the bilayer.


title: Straight cracks in dynamic brittle fracture

abstract:
  We study the dynamics of cracks in brittle materials when the velocity of the
crack is comparable to the sound velocity by means of lattice simulations.
Inertial and damped dynamics are analyzed. It is shown that dissipation
strongly influences the shape of the crack. While inertial cracks are highly
unstable, dissipation can stabilize straight cracks. Our results can help to
explain recent experiments on PMMA.


title: Electron Correlations in Molecular Systems

abstract:
  A short review of correlated electrons in molecular systems has been
performed. Main attention has been focussed on ET salts, which are the d=2
systems. They show the Mott transition in high temperatures and the transition
from the antiferromagnetic to the superconducting phase in low temperatures,
under a (chemical) pressure. Physical properties (the electrical resistivity,
the specific heat, the magnetic susceptibility, the photoemission spectra, the
optical conductivity) of ET salts have been compared with those ones in other
strongly correlated systems. The optical conductivity is described in the
framework of the Hubbard model, with a low frequency peak as an evidence for
the Abrikosov-Suhl resonance.


title: The Charge Ordered State from Weak to Strong Coupling

abstract:
  We apply the Dynamical Mean Field Theory to the problem of charge ordering.
In the normal state as well as in the Charge Ordered (CO) state the existence
of polarons, i.e. electrons strongly coupled to local lattice deformation, is
associated to the qualitative properties of the Lattice Polarization
Distribution Function (LPDF). At intermediate and strong coupling a CO state
characterized by a certain amount of thermally activated defects arise from the
spatial ordering of preexisting randomly distributed polarons. Properties of
this particular CO state gives a qualitative understanding of the low frequency
behavior of optical conductivity of $Ni$ perovskites.


title:
The Equation of State of a Gaussian Phantom Network with Defined Cross
  Link Functionality

abstract:
  A phantom network of Gaussian chains far from the point of gelation can be
described as a gas of interacting particles represented by the cross links. The
type of particles varies with the network functionality, whereas the type of
interaction depends on the properties of the connecting chains. In a mean field
approximation the Equation of State can be calculated using Mayer's cluster
expansion. The resulting isothermal compressibility is compared for different
cross link functionalities.


title:
Spin Dependence of the Critical Concentration for the Neel State of 2D
  Impure Heisenberg Antiferromagnets

abstract:
  The spin-wave theory and the coherent potential approximation are applied to
a spin S Heisenberg antiferromagnet with nonmagnetic impurities on square
lattice. The impurity effects are taken into account by substituting S(1-x) for
S and using the coherent potential approximation to the exchange interaction,
where x is the impurity concentration. At T=0 for S=1/2 the critical impurity
concentration xc of the Neel state is 0.303 and the percolation threshold xp is
0.500. The ground state in xc<x<xp is the disordered state with the spin gap.
For S>=1 the long range Neel order vanishes at xp=0.500. These results explain
qualitatively the experimental results of La2Cu(1-x)Mg(x)O4 (S=1/2) and
K2Mn(1-x)Mg(x)F4 (S=5/2). The difference of xc between these materials is
caused by the decrease in the magnitude of the effective spin with impurity
doping. The spin gap is expected to be observed for La2Cu(1-x)Mg(x)O4 in
xc<x<xp at low temperatures.


title:
On the Implications of Discrete Symmetries for the Beta Function of
  Quantum Hall Systems

abstract:
  We argue that the large discrete symmetry group of quantum Hall systems is
insufficient in itself to determine the complete beta function for the scaling
of the conductivities, $\sigma_{xx}$ and $\sigma_{xy}$. We illustrate this
point by showing that a recent ansatz for this function is one of a
many-parameter family. A clean prediction for the delocalization exponents for
these systems therefore requires the specification of more information, such as
past proposals that the beta function is either holomorphic or
quasi-holomorphic in the variable $z = (\hbar/e^2)(\sigma_{xy} +
i\sigma_{xx})$.


title:
Temperature dependence of local states due to S=1/2 impurities and their
  correlation in a S=1 Heisenberg chain

abstract:
  We study the temperature dependence of the low temperature spin
configurations, investigating the magnetization profile of the local states due
to the impurities and the two point correlation function centered in one of the
impurities. This correlation is found to be weak against temperature effects
although the magnetization profile in the triplet state is visible up to higher
temperatures. Here we introduce a loop cluster quantum Monte-Carlo method with
a fixed magnetization Mz in order to study the correlations in the ground state
of a given value of Mz. From the population distribution of magnetization, the
very small energy gap between the quasi degenerate states due to the impurities
is obtained.


title: Stripe structure, spectral feature and soliton gap in high Tc cuprates

abstract:
  We show that the lightly doped La_{2-x}Sr_{x}CuO_{4} can be described in
terms of a stripe magnetic structure or soliton picture. The internal
relationship between the recent neutron observation of the diagonal (x=0.05) to
vertical (x >= 0.06) stripe transition, which was predicted, and the
concomitant metal-insulator transition is clarified by this solitonic physics.
The phase diagram with the unidentified transition lines between
antiferromagnetic to stripe phases, the doping dependence of the modulation
period, the origin of the mid-infrared optical absorption are investigated
comparatively with other single layer systems: La_{2-x}Sr_{x}NiO_{4} and
(La,Nd)_{2-x}Sr_{x}CuO_{4}. The novel type of quasi-particles and holes is
fully responsible for metallic conduction and ultimately superconductivity.


title:
Comment on ``Critical behavior of the chain--generating function of
  self--avoiding walks on the Sierpinski gasket family: The Euclidean limit''

abstract:
  We refute the claims made by Riera and Chalub [Phys.Rev.E {\bf 58}, 4001
(1998)] by demonstrating that they have not provided enough data (requisite in
their series expansion method) to draw reliable conclusions about criticality
of self-avoiding walks on the Sierpinski gasket family of fractals.


title: Tunneling into fractional quantum Hall liquids

abstract:
  Motivated by the recent experiment by Grayson et.al., we investigate a
non-ohmic current-voltage characteristics for the tunneling into fractional
quantum Hall liquids. We give a possible explanation for the experiment in
terms of the chiral Tomonaga-Luttinger liquid theory. We study the interaction
between the charge and neutral modes, and found that the leading order
correction to the exponent $\alpha$ $(I\sim V^\alpha)$ is of the order of
$\sqrt{\epsilon}$ $(\epsilon=v_n/v_c)$, which reduces the exponent $\alpha$. We
suggest that it could explain the systematic discrepancy between the observed
exponents and the exact $\alpha =1/\nu$ dependence.


title: Comment on "Ising Spin Glasses in a Magnetic Field"

abstract:
  In ref. cond-mat/9811419 Houdayer and Martin analyze the T=0 3d EA spin glass
with a magnetic field $B$. By using a new, powerful method, they determine an
effective critical field $B_c$ as a function of the lattice size $L$. They use
their results to deduce that the model is behaving like in the droplet approach
and not like the mean-field theory. We show here, by using some unpublished
data, that this very interesting method and numerical results are completely
compatible with the behavior implied by the Replica Symmetry Breaking theory.


title:
Chiral electromagnetic waves at the boundary of optical isomers: Quantum
  Cotton-Mouton effect

abstract:
  We demonstrate that the boundary of two optical isomers with opposite
directions of the gyration vectors (both parallel to boundary) can support
propagation of electromagnetic wave in the direction perpendicular to the
gyration axes (Cotton-Mouton geometry). The components of electromagnetic field
in this wave decay exponentially into both media. The characteristic decay
length is of the order of the Faraday rotation length for the propagation along
the gyration axis. The remarkable property of the boundary wave is its
chirality. Namely, the wave can propagate only in one direction determined by
the relative sign of non-diagonal components of the dielectric tensor in
contacting media. We find the dispersion law of the boundary wave for the cases
of abrupt and smooth boundaries. We also study the effect of asymmetry between
the contacting media on the boundary wave and generalize the result to the case
of two parallel boundaries. Finally we consider the arrangement when the
boundaries form a random network. We argue that at a point, when this network
percolates, the corresponding boundary waves undergo quantum delocalization
transition, similar to the quantum Hall transition.


title: Some general features of matrix product states in stochastic systems

abstract:
  We will prove certain general relations in Matrix Product Ansatz for one
dimensional stochastic systems, which are true in both random and sequential
updates. We will derive general MPA expressions for the currents and current
correlators and find the conditions in the MPA formalism, under which the
correlators are site-independent or completely vanishing.


title: Field Induced Transition from Metal to Insulator in the CMR Manganites

abstract:
  The gigantic reduction of the electric resistivity under the applied magnetic
field, CMR effect, is now widely accepted to appear in the vicinity of the
insulator to metal transition of the perovskite manganites. Recently, we have
discovered the first order transition from ferromagnetic metal to insulator in
$\rm La_{0.88}Sr_{0.12}MnO_3$ of the CMR manganite. This phase transition
induces the tremendous increase of the resistivity under the external magnetic
field just near above the phase transition temperature. We report here fairly
detailed results from the systematic experiments including neutron and
synchrotron X-ray scattering studies.


title:
Two Ferromagnetic States in Magnetoresistive Manganites - First Order
  Transition Driven by Orbitals -

abstract:
  A systematic study of the electronic structure in perovskite manganites is
presented. The effective Hamiltonian is derived by taking into account the
degeneracy of $e_g$ orbitals and strong electron correlation in Mn ions. The
spin and orbital orderings are examined as functions of carrier concentration
in the mean-field approximation applied to the effective Hamiltonian. We obtain
the first order phase transition between ferromagnetic metallic and
ferromagnetic insulating states in the lightly doped region. The transition is
accompanied with the orbital order-disorder one which is directly observed in
the anomalous X-ray scattering experiments. The present investigation shows a
novel role of the orbital degree of freedom on metal-insulator transition in
manganites.


title: The Kronig-Penney-Ising picture of the colossal magnetoresistance

abstract:
  From general arguments, it is shown that a magnetic Kronig-Penney model based
on the thermodynamics of an Ising model can be used for describing the Colossal
Magnetoresistance (CMR) phenomenon. The model considers a tunneling-like
transmission process of hopping electrons through a dynamic lattice
characterized by evolving magnetic clusters. In this model, correlations
between the magnetic states are considered to be more relevant than the lattice
strain effects for obtaining the CMR features. Physical arguments lead to the
theoretical description of the intrinsic temperature and field dependences of
the CMR observed in typical manganite materials.


title:
Dynamic mechanisms of the structural phase transitions in KNbO 3:
  Molecular dynamics simulations

abstract:
  The question on the dominant driving mechanism (displacive or order-disorder)
at each structural phase transition of KNbO3 is investigated by means of
molecular dynamics simulations. To this purpose, we first develop a shell model
by determining its potential parameters in order to reproduce the ferroelectric
instabilities obtained by first-principles total energy calculations. The phase
diagram as a function of temperature is obtained through constant-pressure
molecular dynamics simulations. The analysis of the dynamical structure factor
and the microscopic dynamics of the particles in the different phases allows us
to reveal the nature of the dynamics associated with each structural
transition. Correlations between local polarizations forming chain-like
precursor clusters in the paraelectric phase are examined.


title:
Electronic Density of States of Atomically Resolved Single-Walled Carbon
  Nanotubes: Van Hove Singularities and End States

abstract:
  The electronic density of states of atomically resolved single-walled carbon
nanotubes have been investigated using scanning tunneling microscopy. Peaks in
the density of states due to the one-dimensional nanotube band structure have
been characterized and compared with the results of tight-binding calculations.
In addition, tunneling spectroscopy measurements recorded along the axis of an
atomically-resolved nanotube exhibit new, low-energy peaks in the density of
states near the tube end. Calculations suggest that these features arise from
the specific arrangement of carbon atoms that close the nanotube end.


title: Phase diagram of a Heisenberg spin-Peierls model with quantum phonons

abstract:
  Using a new version of the density-matrix renormalization group we determine
the phase diagram of a model of an antiferromagnetic Heisenberg spin chain
where the spins interact with quantum phonons. A quantum phase transition from
a gapless spin-fluid state to a gapped dimerized phase occurs at a non-zero
value of the spin-phonon coupling. The transition is in the same universality
class as that of a frustrated spin chain, which the model maps to in the
anti-adiabatic limit. We argue that realistic modeling of known spin-Peierls
materials should include the effects of quantum phonons.


title:
Stability of the compressible quantum Hall state around the half-filled
  Landau level

abstract:
  We study the compressible states in the quantum Hall system using a mean
field theory on the von Neumann lattice. In the lowest Landau level, a kinetic
energy is generated dynamically from Coulomb interaction. The compressibility
of the state is calculated as a function of the filling factor $\nu$ and the
width $d$ of the spacer between the charge carrier layer and dopants. The
compressibility becomes negative below a critical value of $d$ and the state
becomes unstable at $\nu=1/2$. Within a finite range around $\nu=1/2$, the
stable compressible state exists above the critical value of $d$.


title: Phase Analysis of Thin Film Oxide Systems by AES and EELS

abstract:
  Auger electron spectroscopy (AES) and electron energy loss spectroscopy
(EELS) in a reflection mode are compared as the methods for phase composition
investigation of thin film oxide systems by ion profiling. As an example, the
Al/Al2O3/Si and YBa2Cu3O7/CeO2/Al2O3 systems are considered. The adaptation of
applied statistics methods for AES and EELS data treatment is discussed.


title:
Observation of Induced Chern-Simons Term in P- and T- violating
  Superconductors

abstract:
  We show in this letter that P- and T-odd terms of the electromagnetic
potentials (P; parity, T; time reversal) are induced and lead an unusual
phenomenon in P- and T-violating superconductors discovered recently. The
Ginzburg-Landau action includes this term in the gauge invariant manner. The
coefficient of the term is nearly topological invariant with a small correction
and equals to the fine structure constant approximately. Unusual ``Hall
effect'' {\it without external} magnetic field could be observed by SQUID.


title: Critical Exponents for Diluted Resistor Networks

abstract:
  An approach by Stephen is used to investigate the critical properties of
randomly diluted resistor networks near the percolation threshold by means of
renormalized field theory. We reformulate an existing field theory by Harris
and Lubensky. By a decomposition of the principal Feynman diagrams we obtain a
type of diagrams which again can be interpreted as resistor networks. This new
interpretation provides for an alternative way of evaluating the Feynman
diagrams for random resistor networks. We calculate the resistance crossover
exponent $\phi$ up to second order in $\epsilon=6-d$, where $d$ is the spatial
dimension. Our result $\phi=1+\epsilon /42 +4\epsilon^2 /3087$ verifies a
previous calculation by Lubensky and Wang, which itself was based on the
Potts--model formulation of the random resistor network.


title:
Critical Behavior and Griffiths-McCoy Singularities in the
  Two-Dimensional Random Quantum Ising Ferromagnet

abstract:
  We study the quantum phase transition in the two-dimensional random Ising
model in a transverse field by Monte Carlo simulations. We find results similar
to those known analytically in one-dimension. At the critical point, the
dynamical exponent is infinite and the typical correlation function decays with
a stretched exponential dependence on distance. Away from the critical point
there are Griffiths-McCoy singularities, characterized by a single,
continuously varying exponent, z', which diverges at the critical point, as in
one-dimension. Consequently, the zero temperature susceptibility diverges for a
RANGE of parameters about the transition.


title: On the symmetry of excitations in SU(2) Bethe Ansatz systems

abstract:
  Using the XXX Heisenberg chain as an example, based on the symmetry
properties of the eigenstates with respect to reversing all the spins we argue,
that the basic SU(2) symmetry of the model is inherited by the excitations with
slight modifications only.


title: Critical Exponents near a Random Fractal Boundary

abstract:
  The critical behaviour of correlation functions near a boundary is modified
from that in the bulk. When the boundary is smooth this is known to be
characterised by the surface scaling dimension $\xt$. We consider the case when
the boundary is a random fractal, specifically a self-avoiding walk or the
frontier of a Brownian walk, in two dimensions, and show that the boundary
scaling behaviour of the correlation function is characterised by a set of
multifractal boundary exponents, given exactly by conformal invariance
arguments to be $\lambda_n = 1/48 (\sqrt{1+24n\xt}+11)(\sqrt{1+24n\xt}-1)$.
This result may be interpreted in terms of a scale-dependent distribution of
opening angles $\alpha$ of the fractal boundary: on short distance scales these
are sharply peaked around $\alpha=\pi/3$. Similar arguments give the
multifractal exponents for the case of coupling to a quenched random bulk
geometry.


title:
Excited Eigenstates and Strength Functions for Isolated Systems of
  Interacting Particles

abstract:
  Eigenstates in finite systems such as nuclei, atoms, atomic clusters and
quantum dots with few excited particles are chaotic superpositions of shell
model basis states. We study criterion for the equilibrium distribution of
basis components (ergodicity, or Quantum Chaos), effects of level density
variation and transition from the Breit-Wigner to the Gaussian shape of
eigenstates and strength functions. In the model of $n$ interacting particles
distributed over $m$ orbitals, the shape is given by the Breit-Wigner function
with the width in the form of gaussian dependence on energy.

categories: cond-mat.stat-mech chao-dyn cond-mat.dis-nn cond-mat.mes-hall nlin.CD nucl-th quant-ph


title:
Impurity effect on low-temperature polarisation of the
  charge-density-waves in o-TaS$_3$

abstract:
  The temperature dependence of the low-temperature dielectric response is
studied in o-TaS$_3$ samples doped by Nb, Se, and Ni and for nominally pure
ones. It is found, that the low-temperature dielectric constant depends
anomalously on doping and is higher for doped crystals, whereas the temperature
dependence of the characteristic time of all samples follows the activation law
with nearly the same activation energy $\sim 400$ K (T>20 K). The observed
behaviour is inconsistent with all available explanations of the
low-temperature dielectric anomaly.


title: Orientational Phase Transition in Solid C_60: A Model Approach

abstract:
  A simple model for the angular dependent interaction between C_60 molecules
in face centered cubic lattice is proposed and analyzed by use of the rigorous
bifurcation approach. The quantitative results for the orientational phase
transition and the characteristics of the ordered phase are in good agreement
with the experimental data.


title:
First order transitions and multicritical points in weak itinerant
  ferromagnets

abstract:
  It is shown that the phase transition in low-T_c clean itinerant ferromagnets
is generically of first order, due to correlation effects that lead to a
nonanalytic term in the free energy. A tricritical point separates the line of
first order transitions from Heisenberg critical behavior at higher
temperatures. Sufficiently strong quenched disorder suppresses the first order
transition via the appearance of a critical endpoint. A semi-quantitative
discussion is given in terms of recent experiments on MnSi, and predictions for
other experiments are made.


title: On the reorientation transition of ultra-thin Ni/Cu(001) films

abstract:
  The reorientation transition of the magnetization of ferromagnetic films is
studied on a microscopic basis within a Heisenberg spin model. Using a modified
mean field formulation it is possible to calculate properties of magnetic thin
films with non-integer thicknesses. This is especially important for the
reorientation transition in Ni/Cu(001), as there the magnetic properties are a
sensitive function of the film thickness. Detailed phase diagrams in the
thickness-temperature plane are calculated using experimental parameters and
are compared with experimental measurements by Baberschke and Farle (J. Appl.
Phys. 81, 5038 (1997)).


title: Phonon assisted tunneling in Josephson junctions

abstract:
  The expression for additional subgap current in the presence of
electron-phonon interaction is derived. We show that the phonon assisted
tunneling leads to appearance of peaks on current-voltage characteristics at
the Josephson frequencies corresponding to the Raman-active phonons. The
relation of the obtained results to experimental observations are discussed.


title: Local modes, phonons, and mass transport in solid $^4$He

abstract:
  We propose a model to treat the local motion of atoms in solid $^{4}$He as a
local mode. In this model, the solid is assumed to be described by the Self
Consistent Harmonic approximation, combined with an array of local modes. We
show that in the bcc phase the atomic local motion is highly directional and
correlated, while in the hcp phase there is no such correlation. The correlated
motion in the bcc phase leads to a strong hybridization of the local modes with
the T$_{1}(110)$ phonon branch, which becomes much softer than that obtained
through a Self Consistent Harmonic calculation, in agreement with experiment.
In addition we predict a high energy excitation branch which is important for
self-diffusion. Both the hybridization and the presence of a high energy branch
are a consequence of the correlation, and appear only in the bcc phase. We
suggest that the local modes can play the role in mass transport usually
attributed to point defects (vacancies). Our approach offers a more overall
consistent picture than obtained using vacancies as the predominant point
defect. In particular, we show that our approach resolves the long standing
controversy regarding the contribution of point defects to the specific heat of
solid $^{4}$He.


title: Quantum kinetic theory of trapped atomic gases

abstract:
  We present a general framework in which we can accurately describe the
non-equilibrium dynamics of trapped atomic gases. This is achieved by deriving
a single Fokker-Planck equation for the gas. In this way we are able to discuss
not only the dynamics of an interacting gas above and below the critical
temperature at which the gas becomes superfluid, but also during the phase
transition itself. The last topic cannot be studied on the basis of the usual
mean-field theory and was the main motivation for our work. To show, however,
that the Fokker-Planck equation is not only of interest for recent experiments
on the dynamics of Bose-Einstein condensation, we also indicate how it can, for
instance, be applied to the study of the collective modes of a condensed Bose
gas.


title:
Huge oxygen isotope effect on local lattice fluctuations in
  La(2-x)Sr(x)CuO(4) superconductor

abstract:
  Recently a growing number of experiments have provided indications of the key
role of polarons (composite particles formed by a charge strongly coupled with
a local lattice deformation) in doped perovskites, hosting colossal
magnetoresistance (CMR) and high Tc superconductivity. While the role of
polarons is generally recognized in manganites due to the large amplitude of
the local lattice deformation, the scientific debate remains open on the
cuprates where the lattice deformation is smaller and the anomalous normal
metallic phase becomes complex due to the coexistence of polarons with
itinerant carriers. Moreover the segregation of polarons and itinerant charges
in different spatial domains forming lattice-charge stripes as well as the slow
dynamic 1D spin fluctuations have been observed. The debate on the driving
force for the stripe formation remains object of discussion since it could be
purely due to electronic interactions and/or due to strong electron-lattice
(polaronic) interactions. In order to explore the important role of the latter
in stripe charge segregation, we have studied isotope effects on the dynamical
lattice fluctuations and polaron ordering temperature. Here we report a
compelling evidence for a huge isotope effect on local lattice fluctuations of
La(2-x)Sr(x)CuO(4) high Tc superconductor by x-ray absorption spectroscopy, a
fast (~10^(-15) sec) and local probe (~5 A). Upon replacing $^{16}$O with
$^{18}$O, the characteristic temperature T^{*} for polaron ordering in
La(1.94)Sr(0.06)CuO(4) increases from about 110 K to 170 K.


title:
Giant transverse magnetoresistance in an asymmetric system of three
  GaAs/AlGaAs quantum wells in a strong magnetic field at room temperature

abstract:
  The giant transverse magnetoresistance is observed in the case of
photoinduced nonequilibrium carriers in an asymmetric undoped system of three
GaAs/AlGaAs quantum wells at room temperature. In a magnetic field of 75 kOe,
the resistance of nanostructure being studied increases by a factor of 1.85.
The magnetoresistance depends quadratically on the magnetic field in low fields
and tends to saturation in high fields. This phenomenon is attributed to the
rearrangement of the electron wave function in magnetic field. Using the fact
that the incoherent part of the scattering probability for electron scattering
on impurities and bulk defects is proportional to the integral of the forth
power of the envelope wave function, the calculated field dependence of the
magnetoresistance is shown to be similar to that observed experimentally.


title: Symmetric Anderson impurity model with a narrow band

abstract:
  The single channel Anderson impurity model is a standard model for the
description of magnetic impurities in metallic systems. Usually, the bandwidth
represents the largest energy scale of the problem. In this paper, we analyze
the limit of a narrow band, which is relevant for the Mott-Hubbard transition
in infinite dimensions. For the symmetric model we discuss two different
effects: i) The impurity contribution to the density of states at the Fermi
surface always turns out to be negative in such systems. This leads to a new
crossover in the thermodynamic quantities that we investigate using the
numerical renormalization group. ii) Using the Lanczos method, we calculate the
impurity spectral function and demonstrate the breakdown of the skeleton
expansion on an intermediate energy scale. Luttinger's theorem, as an example
of the local Fermi liquid property of the model, is shown to still be valid.


title:
Coulomb correlation effects in semiconductor quantum dots: The role of
  dimensionality

abstract:
  We study the energy spectra of small three-dimensional (3D) and
two-dimensional (2D) semiconductor quantum dots through different theoretical
approaches (single-site Hubbard and Hartree-Fock hamiltonians); in the smallest
dots we also compare with exact results. We find that purely 2D models often
lead to an inadequate description of the Coulomb interaction existing in
realistic structures, as a consequence of the overestimated carrier
localization. We show that the dimensionality of the dots has a crucial impact
on (i) the accuracy of the predicted addition spectra; (ii) the range of
validity of approximate theoretical schemes. When applied to realistic 3D
geometries, the latter are found to be much more accurate than in the
corresponding 2D cases for a large class of quantum dots; the single-site
Hubbard hamiltonian is shown to provide a very effective and accurate scheme to
describe quantum dot spectra, leading to good agreement with experiments.


title:
The Heisenberg antiferromagnet on an anisotropic triangular lattice:
  linear spin-wave theory

abstract:
  We consider the effect of quantum spin fluctuations on the ground state
properties of the Heisenberg antiferromagnet on an anisotropic triangular
lattice using linear spin-wave theory. This model should describe the magnetic
properties of the insulating phase of the kappa-(BEDT-TTF)_2 X family of
superconducting molecular crystals. The ground state energy, the staggered
magnetization, magnon excitation spectra and spin-wave velocities are computed
as a function of the ratio between the second and first neighbours, J2/J1. We
find that near J2/J1 = 0.5, i.e., in the region where the classical spin
configuration changes from a Neel ordered phase to a spiral phase, the
staggered magnetization vanishes, suggesting the possibility of a quantum
disordered state. In this region, the quantum correction to the magnetization
is large but finite. This is in contrast to the frustrated Heisenberg model on
a square lattice, for which the quantum correction diverges logarithmically at
the transition from the Neel to the collinear phase. For large J2/J1, the model
becomes a set of chains with frustrated interchain coupling. For J2 > 4 J1, the
quantum correction to the magnetization, within LSW, becomes comparable to the
classical magnetization, suggesting the possibility of a quantum disordered
state. We show that, in this regime, quantum fluctuations are much larger than
for a set of weakly coupled chains with non-frustated interchain coupling.


title:
Effect of disorder on quantum phase transitions in anisotropic XY spin
  chains in a transverse field

abstract:
  We present some exact results for the effect of disorder on the critical
properties of an anisotropic XY spin chain in a transverse field. The continuum
limit of the corresponding fermion model is taken and in various cases results
in a Dirac equation with a random mass. Exact analytic techniques can then be
used to evaluate the density of states and the localization length. In the
presence of disorder the ferromagnetic-paramagnetic or Ising transition of the
model is in the same universality class as the random transverse field Ising
model solved by Fisher using a real space renormalization group decimation
technique (RSRGDT). If there is only randomness in the anisotropy of the
magnetic exchange then the anisotropy transition (from a ferromagnet in the $x$
direction to a ferromagnet in the $y$ direction) is also in this universality
class. However, if there is randomness in the isotropic part of the exchange or
in the transverse field then in a non-zero transverse field the anisotropy
transition is destroyed by the disorder. We show that in the Griffiths' phase
near the Ising transition that the ground state energy has an essential
singularity. The results obtained for the dynamical critical exponent, the
typical correlation length, and the temperature dependence of the specific heat
near the Ising transition agree with the results of the RSRGDT and numerical
work.


title: Non-equilibrium 2-Channel Kondo model for quantum dots

abstract:
  We find that, under certain condition, a quantum dot with odd number of
electron and coupled to two leads can be described by a non-equilibrium
2-channel Kondo model, when the two leads has a large voltage bias between
them. The model is exactly soluble and can be mapped into a free fermion
system, even in the presence of external magnetic field and other relevant
perturbations. All (dynamical) correlation functions can be calculated. The
fixed point of the 2-channel Kondo model is a non-equilibrium fixed point and
is different from the usual 1-channel and 2-channel Kondo fixed point.


title: Anomalous Roughness in Dimer-Type Surface Growth

abstract:
  We point out how geometric features affect the scaling properties of
non-equilibrium dynamic processes, by a model for surface growth where
particles can deposit and evaporate only in dimer form, but dissociate on the
surface. Pinning valleys (hill tops) develop spontaneously and the surface
facets for all growth (evaporation) biases. More intriguingly, the scaling
properties of the rough one dimensional equilibrium surface are anomalous. Its
width, $W\sim L^\alpha$, diverges with system size $L$, as $\alpha={1/3}$
instead of the conventional universal value $\alpha={1/2}$. This originates
from a topological non-local evenness constraint on the surface configurations.


title: Critical behaviour in $La_{0.5}Sr_{0.5}CoO_3$

abstract:
  We have studied the critical behaviour in $La_{0.5}Sr_{0.5}CoO_{3}$ near the
paramagnetic-ferromagnetic transition temperature. We have analysed our dc
magnetisation data near the transition temperature with the help of modified
Arrott plots, Kouvel-Fisher method. We have determined the critical temperature
$T_c$ and the critical exponents, $\beta$ and $\gamma$. With these values of
$T_c$, $\beta$ and $\gamma$, we plot $M/(1-T/T_c)^{\beta}$ vs
$H/(1-T/T_c)^{\gamma}$. All the data collapse on one of the two curves. This
suggests that the data below and above $T_c$ obeys scaling, following a single
equation of state.


title:
Fermionic Integrals and Analytic Solutions for Two-Dimensional Ising
  Models

abstract:
  We review some aspects of the fermionic interpretation of the two-dimensional
Ising model. The use is made of the notion of the integral over the
anticommuting Grassmann variables. For simple and more complicated 2D Ising
lattices, the partition function can be expressed as a fermionic Gaussian
integral. Equivalently, the 2D Ising model can be reformulated as a
free-fermion theory on a lattice. For regular lattices, the analytic solution
then readily follows by passing to the momentum space for fermions. We also
comment on the effective field-theoretical (continuum-limit) fermionic
formulations for the 2D Ising models near the critical point.


title: Magnetic skyrmion lattices in heavy fermion superconductor UPt3

abstract:
  Topological analysis of nearly SO(3)_{spin} symmetric Ginzburg--Landau
theory, proposed for UPt$_{3}$ by Machida et al, shows that there exists a new
class of solutions carrying two units of magnetic flux: the magnetic skyrmion.
These solutions do not have singular core like Abrikosov vortices and at low
magnetic fields they become lighter for strongly type II superconductors.
Magnetic skyrmions repel each other as $1/r$ at distances much larger then the
magnetic penetration depth $\lambda$, forming a relatively robust triangular
lattice. The magnetic induction near $H_{c1}$ is found to increase as
$(H-H_{c1})^{2}$. This behavior agrees well with experiments.


title: Noise Stabilization of Self-Organized Memories

abstract:
  We investigate a nonlinear dynamical system which ``remembers'' preselected
values of a system parameter. The deterministic version of the system can
encode many parameter values during a transient period, but in the limit of
long times, almost all of them are forgotten. Here we show that a certain type
of stochastic noise can stabilize multiple memories, enabling many parameter
values to be encoded permanently. We present analytic results that provide
insight both into the memory formation and into the noise-induced memory
stabilization. The relevance of our results to experiments on the
charge-density wave material $NbSe_3$ is discussed.


title:
The fractional quantum Hall effect: Chern-Simons mapping, duality,
  Luttinger liquids and the instanton vacuum

abstract:
  We derive, from first principles, the complete Luttinger liquid theory of
abelian quantum Hall edge states. This theory includes the effects of disorder
and Coulomb interactions as well as the coupling to external electromagnetic
fields. We introduce a theory of spatially separated (individually conserved)
edge modes, find an enlarged dual symmetry and obtain a complete classification
of quasiparticle operators and tunneling exponents. The chiral anomaly on the
edge and Laughlin's gauge argument are used to obtain unambiguously the Hall
conductance. In resolving the problem of counter flowing edge modes, we find
that the long range Coulomb interactions play a fundamental role. In order to
set up a theory for arbitrary filling fractions $\nu$ we use the idea of a two
dimensional network of percolating edge modes. We derive an effective, single
mode Luttinger liquid theory for tunneling processes into the quantum Hall edge
which yields a continuous tunneling exponent $1/\nu$. The network approach is
also used to re-derive the instanton vacuum or $Q$-theory for the plateau
transitions.


title: Cyclotron motion of a quantized vortex in a superfluid

abstract:
  In two dimensions a microscopic theory providing a basis for the naive
analogy between a quantized vortex in a superfluid and an electron in a uniform
magnetic field is presented. Following the variational approach developed by
Peierls, Yoccoz, and Thouless, the cyclotron motion of a vortex is described by
the many-body wave function, which is a linear combination of Feynman wave
functions centered at different positions. An integral equation for the
weighting functions of the superposition is derived by minimizing the energy
functional. The matrix elements of the kernel are the overlaps between any two
displaced Feynman wave functions. A numerical study is conducted for a bosonic
superfluid based on a Hartree ground state. A one-to-one correspondence between
the rotational states of a vortex in a cylinder and the cyclotron states of an
electron in the central gauge is found. Like the Landau levels of an electron,
the energy levels of a vortex are highly degenerate. However, the gap between
two adjacent energy levels does not only depend on the quantized circulation,
but also increases with energy, and scales with the size of the vortex. The
fluid density is finite at the vortex axis and the vorticity is distributed in
the core region. The effective mass of a quantized vortex defined by the
inverse of the energy-level spacing is shown to be logarithmically divergent
with the size of the vortex.


title: Two-Dimensional Copolymers and Exact Conformal Multifractality

abstract:
  We consider in two dimensions the most general star-shaped copolymer, mixing
random (RW) or self-avoiding walks (SAW) with specific interactions thereof.
Its exact bulk or boundary conformal scaling dimensions in the plane are all
derived from an algebraic structure existing on a random lattice (2D quantum
gravity). The multifractal dimensions of the harmonic measure of a 2D RW or SAW
are conformal dimensions of certain star copolymers, here calculated exactly as
non rational algebraic numbers. The associated multifractal function f(alpha)
are found to be identical for a random walk or a SAW in 2D. These are the first
examples of exact conformal multifractality in two dimensions.


title: Anisotropic spin freezing in the S=1/2 zigzag ladder compound SrCuO2

abstract:
  Using magnetic neutron scattering we characterize an unusual low temperature
phase in orthorhombic SrCuO2. The material contains zigzag spin ladders formed
by pairs of S=1/2 chains (J=180 meV) coupled through a weak frustrated
interaction |J'|<0.1J. At T<Tc1=5.0(4)K an elastic peak develops in a gapless
magnetic excitation spectrum indicating spin freezing on a time scale larger
than 200 picoseconds. While the frozen state has long range commensurate
antiferromagnetic order along the chains with the correlation length exceeding
200 lattice periods along the c-axis and a substantial correlation length of
60(25) spacings along the a-axis perpendicular to the zigzag plane, only 2
lattice units are correlated along the b-axis which is the direction of the
frustrated interactions. The frozen magnetic moment of each Cu ion is very
small, 0.033(7) Bohr magneton even at T=0.35K, and has unusual temperature
dependence with a cusp at Tc2=1.5K reminiscent of a phase transition. We argue
that slow dynamics of stripe-like cooperative magnetic defects in tetragonal
a-c planes yield this anisotropic frozen state.


title: Breakdown of the Chiral Luttinger Liquid in One-Dimension

abstract:
  We have developed a fermionic boot-strap method to solve a class of chiral
one dimensional fermion model which cannot be tackled by bosonization. Using
this scheme, we show that Luttinger liquid behavior in a gas of four
interacting chiral Majorana fermions is highly sensitive to the velocity
degeneracy. Upon changing the velocity of a one chiral fermion, a sharp bound
(or anti-bound) state splits off from the original Luttinger liquid continuum,
cutting off the X-ray singularity to form a broad incoherent excitation with a
lifetime that grows linearly with frequency.


title: High-temperature expansion for Ising models on quasiperiodic tilings

abstract:
  We consider high-temperature expansions for the free energy of zero-field
Ising models on planar quasiperiodic graphs. For the Penrose and the octagonal
Ammann-Beenker tiling, we compute the expansion coefficients up to 18th order.
As a by-product, we obtain exact vertex-averaged numbers of self-avoiding
polygons on these quasiperiodic graphs. In addition, we analyze periodic
approximants by computing the partition function via the Kac-Ward determinant.
For the critical properties, we find complete agreement with the commonly
accepted conjecture that the models under consideration belong to the same
universality class as those on periodic two-dimensional lattices.


title:
The equation of state and specific heat of the electron gas on a one
  dimensional lattice

abstract:
  Using previous results and general thermodynamical formalism,an expression is
obtained for the specific heat per particle under constant volume of a
degenerate non-relativistic electron gas on a 1D lattice.The result is a
non-linear function of the temperature,and it could have applications in
studies of quasi one-dimensional organic metals.


title: Quantum-disordered slave-boson theory of underdoped cuprates

abstract:
  We study the stability of the spin gap phase in the U(1) slave-boson theory
of the t-J model in connection to the underdoped cuprates. We approach the spin
gap phase from the superconducting state and consider the quantum phase
transition of the slave-bosons at zero temperature by introducing vortices in
the boson superfluid. At finite temperatures, the properties of the bosons are
different from those of the strange metal phase and lead to modified gauge
field fluctuations. As a result, the spin gap phase can be stabilized in the
quantum critical and quantum disordered regime of the boson system. We also
show that the regime of quantum disordered bosons with the paired fermions can
be regarded as the strong coupling version of the recently proposed nodal
liquid theory.


title:
Andreev Reflection and Spin Injection into $s-$ and $d-$wave
  Superconductors

abstract:
  We study the effect of spin injection into $s-$ and $d-$wave superconductors,
with an emphasis on the interplay between boundary and bulk spin transport
properties. The quantities of interest include the amount of non-equilibrium
magnetization ($m$), as well as the induced spin-dependent current ($I_s$) and
boundary voltage ($V_s$). In general, the Andreev reflection makes each of the
three quantities depend on a different combination of the boundary and bulk
contributions. The situation simplifies either for half-metallic ferromagnets
or in the strong barrier limit, where both $V_s$ and $m$ depend solely on the
bulk spin transport/relaxation properties. The implications of our results for
the on-going spin injection experiments in high $T_c$ cuprates are discussed.


title:
Comment on "Theory of metal-insulator transitions in gated
  semiconductors" (B. L. Altshuler and D. L. Maslov, Phys. Rev. Lett. 82, 145
  (1999))

abstract:
  In a recent Letter, Altshuler and Maslov propose a model which attributes the
anomalous temperature and field dependence of the resistivity of
two-dimensional electron (or hole) systems to the charging and discharging of
traps in the oxide (spacer), rather than to intrinsic behavior of interacting
particles associated with a conductor-insulator transition in two dimensions.
We argue against this model based on existing experimental evidence.


title: Nonequilibrium ac Josephson effect in mesoscopic Nb-InAs-Nb junctions

abstract:
  Microwave irradiation of Nb-InAs-Nb junctions reveals frequency-doubled
Josephson currents which persist to high temperatures, in the absence of a
critical current. A nonequilibrium dynamical model, based on time-dependent
Andreev bound states, successfully accounts for the resulting half-integer
Shapiro step and an enhancement in the conductance near zero bias.


title: Multi-Species Asymmetric Exclusion Process in Ordered Sequential Update

abstract:
  A multi-species generalization of the asymmetric simple exclusion process
(ASEP) is studied in ordered sequential and sub-lattice parallel updating
schemes. In this model particles hop with their own specific probabilities to
their rightmost empty site and fast particles overtake slow ones with a
definite probability. Using Matrix Product Ansatz (MPA), we obtain the relevant
algebra, and study the uncorrelated stationary state of the model both for an
open system and on a ring. A complete comparison between the physical results
in these updates and those of random sequential introduced in [20,21] is made.


title: Exact Multifractal Exponents for Two-Dimensional Percolation

abstract:
  The harmonic measure (or diffusion field or electrostatic potential) near a
percolation cluster in two dimensions is considered. Its moments, summed over
the accessible external hull, exhibit a multifractal spectrum, which I
calculate exactly. The generalized dimensions D(n) as well as the MF function
f(alpha) are derived from generalized conformal invariance, and are shown to be
identical to those of the harmonic measure on 2D random walks or self-avoiding
walks. An exact application to the anomalous impedance of a rough percolative
electrode is given. The numerical checks are excellent. Another set of exact
and universal multifractal exponents is obtained for n independent
self-avoiding walks anchored at the boundary of a percolation cluster. These
exponents describe the multifractal scaling behavior of the average nth moment
of the probabity for a SAW to escape from the random fractal boundary of a
percolation cluster in two dimensions.


title: Jamming and Stress Propagation in Granular Materials

abstract:
  We have recently developed some simple continuum models of static granular
media which display "fragile" behaviour: they predict that the medium is unable
to support certain types of infinitesimal load (which we call "incompatible"
loads) without plastic rearrangement. We argue that a fragile description may
be appropriate when the mechanical integrity of the medium arises adaptively,
in response to a load, through an internal jamming process. We hypothesize that
a network of force chains (or "granular skeleton") evolves until it can just
support the applied load, at which point it comes to rest; it then remains
intact so long as no incompatible load is applied. Our fragile models exhibits
unusual mechanical responses involving hyperbolic equations for stress
propagation along fixed characteristics through the material. These
characteristics represent force chains; their arrangement expressly depends on
the construction history. Thus, for example, we predict a large difference in
the stress pattern beneath two conical piles of sand, one poured from a point
source and one created by sieving.


title:
Canonical phase space approach to the noisy Burgers equation:
  Probability distributions

abstract:
  We present a canonical phase space approach to stochastic systems described
by Langevin equations driven by white noise. Mapping the associated
Fokker-Planck equation to a Hamilton-Jacobi equation in the nonperturbative
weak noise limit we invoke a {\em principle of least action} for the
determination of the probability distributions. We apply the scheme to the
noisy Burgers and KPZ equations and discuss the time-dependent and stationary
probability distributions. In one dimension we derive the long-time skew
distribution approaching the symmetric stationary Gaussian distribution. In the
short-time region we discuss heuristically the nonlinear soliton contributions
and derive an expression for the distribution in accordance with the directed
polymer-replica and asymmetric exclusion model results. We also comment on the
distribution in higher dimensions.


title: Effect of Stripes on Electronic States in Underdoped La_{2-x}Sr_xCuO_4

abstract:
  We investigate the electronic states of underdoped La_{2-x}Sr_xCuO_4 (LSCO)
by using a microscopic model, i.e., t-t'-t''-J model, containing vertical
charge stripes. The numerically exact diagonalization calculation on small
clusters shows the consistent explanation of the physical properties in the
angle-resolved photoemission, neutron magnetic scattering and optical
conductivity experiments such as the antiphase domain and quasi-one-dimensional
charge transport. The pair correlation function of the d-channel is suppressed
by the stripes. These results demonstrate a crucial role of the stripes in LSCO


title: A geometrical model for Mixed cyanide crystals

abstract:
  A model of diluted random field sustained by quenched volume deformations is
shown to reproduce puzzling physical features found in X(CN)_{x}Y_{1-x} mixed
cyanide crystals. X is an alkali metal (K, Na or Rb) and Y is a spherical
halogen ion (Br, Cl or I). Critical thresholds x_c at which associated first
order ferroelastic transitions disappear are calculated exactly. The diluted
random field is shown to compete with compressibility in making the transition
first order. Transitions are then found to remain first order down to x_c
except in the case of bromine dilution where they become continuous. All the
results are in excellent agreement with available experimental data.