title:
Diffusion of single long polymers in fixed and low density matrix of
  obstacles confined to two dimensions

abstract:
  Diffusion properties of a self-avoiding polymer embedded in regularly
distributed obstacles with spacing a=20 and confined in two dimensions is
studied numerically using the extended bond fluctuation method which we have
developed recently. We have observed for the first time to our knowledge, that
the mean square displacement of a center monomer $\phi_{M/2}(t)$ exhibits four
dynamical regimes, i.e., $\phi_{M/2}(t) \sim t^{\nu_m}$ with $\nu_m\sim 0.6$,
3/8, 3/4, and 1 from the shortest to longest time regimes. The exponents in the
second and third regimes are well described by segmental diffusion in the
``self-avoiding tube''. In the fourth (free diffusion) regime, we have
numerically confirmed the relation between the reptation time $\tau_d$ and the
number of segments $M, \tau_d\propto M^3$.


title:
Quasiparticle properties around a nonmagnetic impurity in the
  superconducting state of the two-dimensional t-J model

abstract:
  The superconducting state around a well isolated nonmagnetic impurity in the
high-Tc superconductors is studied using the two-dimensional t-J model. The
spatial dependence of the order parameter and the local density of states are
obtained from the numerical diagonalization of the Bogoliubov-de Gennes
equation derived using the Gutzwiller approximation. We find a zero-energy peak
in the local density of states around the impurity. Different from the previous
results on a vortex or surfaces in the t-J model, the splitting of zero-energy
peak is not found. The zero-energy states corresponding to the zero-energy peak
is approximately localized around the impurity.


title:
Observation of a Nanoscale Metallic Dot Self-Consistently Coupled to a
  Two-Level System

abstract:
  We have observed anomalous transport properties for a 50 nm Bi dot in the
Coulomb-blockade regime. Over a range of gate voltages, Coulomb blockade peaks
are suppressed at low bias, and dramatic structure appears in the current at
higher bias. We propose that the state of the dot is determined
self-consistently with the state of a nearby two-level system (TLS) to which it
is electrostatically coupled. As a gate voltage is swept, the ground state
alternates between states of the TLS, leading to skipped Coulomb-blockade peaks
at low bias. At a fixed gate voltage and high bias, transport may occur through
a cascade of excited states connected by the dynamic switching of the TLS.


title: Scale competition in nonlinear Schrodinger models

abstract:
  Three types of nonlinear Schrodinger models with multiple length scales are
considered. It is shown that the length-scale competition universally results
into arising of new localized stationary states. Multistability phenomena with
a controlled switching between stable states become possible.


title: Equilibrium shapes and energies of coherent strained InP islands

abstract:
  The equilibrium shapes and energies of coherent strained InP islands grown on
GaP have been investigated with a hybrid approach that has been previously
applied to InAs islands on GaAs. This combines calculations of the surface
energies by density functional theory and the bulk deformation energies by
continuum elasticity theory. The calculated equilibrium shapes for different
chemical environments exhibit the {101}, {111}, {\=1\=1\=1} facets and a (001)
top surface. They compare quite well with recent atomic-force microscopy data.
Thus in the InP/GaInP-system a considerable equilibration of the individual
islands with respect to their shapes can be achieved. We discuss the
implications of our results for the Ostwald ripening of the coherent InP
islands. In addition we compare strain fields in uncapped and capped islands.


title:
Comment on ``Spin Dependent Hopping and Colossal Negative
  Magnetoresistance in Epitaxial $Nd_{0.52}Sr_{0.48}MnO_{3}$ Films in Fields up
  to 50 T''

abstract:
  Recently Wagner et al. [Phys. Rev. Lett. Vol. 81, P. 3980 (1998)] proposed
that Mott's original model be modified to incorporate a hopping barrier which
depends on the misorientation between the spins of electrons at the initial and
the final states in an elementary process. They further claimed that using the
model they can explain the observed scaling behavior--
negative-magnetoresistivity scaling proportional to the Brillouin function
$\cal{B}$ in the ferromagnetic state and to ${\cal{B}}^2$ in the paramagnetic
state. In this comment we argue that the modification needed for Mott's
original model is different from that proposed by Wagner et al. and further
show that our picture will successfully explain the observed scaling in the two
regimes.


title:
Predicting phase transition pressure in solids: a semiclassical
  possibility

abstract:
  This is a short review of the physical ideas,algorithm for calculations and
some results of a semiclassical theory of the behaviour of materials under high
pressure,proposed by P.Savic and R.Kasanin.The theory has found applications
from DAC experiments to studies of planetary interior structure.


title:
Lattice dynamics and reduced thermal conductivity of filled
  skutterudites

abstract:
  The great reduction in thermal conductivity of skutterudites upon filling the
``void'' sites with Rare Earth (RE) ions is key to their favorable
thermoelectric properties but remains to be understood. Using lattice dynamic
models based on first principles calculations, we address the most popular
microscopic mechanism, reduction via rattling ions. The model withstands
inelastic neutron scattering and specific heat measurements, and refutes
hypotheses of an anharmonic RE potential and of two distinct localized RE
vibrations of disparate frequencies. It does indicate a strong hybridization
between bare La vibrations and certain Sb-like phonon branches, suggesting
anharmonic scattering by harmonic RE motions as an important mechanism for
suppression of heat conductivity.


title:
Interplay of disorder and nonlinearity in Klein-Gordon models: Immobile
  kinks

abstract:
  We consider Klein-Gordon models with a $\delta$-correlated spatial disorder.
We show that the properties of immobile kinks exhibit strong dependence on the
assumptions as to their statistical distribution over the minima of the
effective random potential. Namely, there exists a crossover from monotonically
increasing (when a kink occupies the deepest potential well) to the
non-monotonic (at equiprobable distribution of kinks over the potential minima)
dependence of the average kink width as a function of the disorder intensity.
We show also that the same crossover may take place with changing size of the
system.


title: On the conformational structure of a stiff homopolymer

abstract:
  In this paper we complete the study of the phase diagram and conformational
states of a stiff homopolymer. It is known that folding of a sufficiently stiff
chain results in formation of a torus. We find that the phase diagram obtained
from the Gaussian variational treatment actually contains not one, but several
distinct toroidal states distinguished by the winding number. Such states are
separated by first order transition curves terminating in critical points at
low values of the stiffness. These findings are further supported by
off-lattice Monte Carlo simulation. Moreover, the simulation shows that the
kinetics of folding of a stiff chain passes through various metastable states
corresponding to hairpin conformations with abrupt U-turns.


title: Time to failure of hierarchical load-transfer models of fracture

abstract:
  The time to failure, $T$, of dynamical models of fracture for a hierarchical
load-transfer geometry is studied. Using a probabilistic strategy and
juxtaposing hierarchical structures of height $n$, we devise an exact method to
compute $T$, for structures of height $n+1$. Bounding $T$, for large $n$, we
are able to deduce that the time to failure tends to a non-zero value when $n$
tends to infinity. This numerical conclusion is deduced for both power law and
exponential breakdown rules.


title: Orthogonality Catastrophes in Carbon Nanotubes

abstract:
  Carbon nanotubes provide a remarkably versatile system in which to explore
the effects of Coulomb interactions in one dimension. The most dramatic effects
of strong electron-electron repulsion are *orthogonality catastrophes*. These
orthogonality catastrophes come in different varieties, and can be observed
both in low-bias transport and tunneling measurements on nanotubes. This
article contains a review of previous work and new material on the crossover
between Coulomb blockade and Luttinger behavior.


title: Coherence properties of an atom laser

abstract:
  We study the coherence properties of an atom laser, which operates by
extracting atoms from a gaseous Bose-Einstein condensate via a two-photon Raman
process, by analyzing a recent experiment. We obtain good agreement with the
experimental data by solving the time-dependent Gross-Pitaevskii equation in
three dimensions both numerically and with a Thomas-Fermi model. The coherence
length is strongly affected by the space-dependent phase developed by the
condensate when the trapping potential is turned off.


title: Quantum Teleportation in a Solid State System

abstract:
  We propose a practical solid-state system capable of demonstrating quantum
teleportation. The set-up exploits recent advances in the optical control of
excitons in coupled quantum dots, in order to produce maximally-entangled Bell
and Greenberger-Horne-Zeilinger (GHZ) states. Only two unitary transformations
are then required: a quantum Controlled-Not gate and a Hadamard gate. The laser
pulses necessary to generate the maximally-entangled states, and the
corresponding unitary transformations, are given explicitly.


title: Excitation of phonons in a Bose-Einstein condensate by light scattering

abstract:
  Stimulated small-angle light scattering was used to measure the structure
factor of a Bose-Einstein condensate in the phonon regime. The excitation
strength for phonons was found to be significantly reduced from that of free
particles, revealing the presence of correlated pair excitations and quantum
depletion in the condensate. The Bragg resonance line strength and line shift
agreed with predictions of a local density approximation.


title:
Low-density series expansions for directed percolation I: A new
  efficient algorithm with applications to the square lattice

abstract:
  A new algorithm for the derivation of low-density series for percolation on
directed lattices is introduced and applied to the square lattice bond and site
problems. Numerical evidence shows that the computational complexity grows
exponentially, but with a growth factor $\lambda < \protect{\sqrt[8]{2}}$,
which is much smaller than the growth factor $\lambda = \protect{\sqrt[4]{2}}$
of the previous best algorithm. For bond (site) percolation on the directed
square lattice the series has been extended to order 171 (158). Analysis of the
series yields sharper estimates of the critical points and exponents.


title:
Low-density series expansions for directed percolation II: The square
  lattice with a wall

abstract:
  A new algorithm for the derivation of low-density expansions has been used to
greatly extend the series for moments of the pair-connectedness on the directed
square lattice near an impenetrable wall. Analysis of the series yields very
accurate estimates for the critical point and exponents. In particular, the
estimate for the exponent characterizing the average cluster length near the
wall, $\tau_1=1.00014(2)$, appears to exclude the conjecture $\tau_1=1$. The
critical point and the exponents $\nu_{\parallel}$ and $\nu_{\perp}$ have the
same values as for the bulk problem.


title:
Spheres and Prolate and Oblate Ellipsoids from an Analytical Solution of
  Spontaneous Curvature Fluid Membrane Model

abstract:
  An analytic solution for Helfrich spontaneous curvature membrane model (H.
Naito, M.Okuda and Ou-Yang Zhong-Can, Phys. Rev. E {\bf 48}, 2304 (1993); {\bf
54}, 2816 (1996)), which has a conspicuous feature of representing the circular
biconcave shape, is studied. Results show that the solution in fact describes a
family of shapes, which can be classified as: i) the flat plane (trivial case),
ii) the sphere, iii) the prolate ellipsoid, iv) the capped cylinder, v) the
oblate ellipsoid, vi) the circular biconcave shape, vii) the self-intersecting
inverted circular biconcave shape, and viii) the self-intersecting nodoidlike
cylinder. Among the closed shapes (ii)-(vii), a circular biconcave shape is the
one with the minimum of local curvature energy.


title:
First order phase transition in a 1+1-dimensional nonequilibrium wetting
  process

abstract:
  A model for nonequilibrium wetting in 1+1 dimensions is introduced. It
comprises adsorption and desorption processes with a dynamics which generically
does not obey detailed balance. Depending on the rates of the dynamical
processes the wetting transition is either of first or second order. It is
found that the wet (unbound) and the non-wet (pinned) states coexist and are
both thermodynamically stable in a domain of the dynamical parameters which
define the model. This is in contrast with equilibrium transitions where
coexistence of thermodynamically stable states takes place only on the
transition line.


title: Pulsed-laser beam profiling for deposition of high-T_c YBCO films

abstract:
  A special shaping of YAG:Nd^{3+} laser beam profile has been found to improve
homogeneity and T_c of YBa_2Cu_3O_{7-\delta} films being deposited by the
pulsed laser. Targets of a considerably non-stoichiometric composition and
substrates of slanting cuts have turned to be necessary for the high-quality
film growing. Multiple droplets and solid particles ejected from the target
surface by the infrared radiation are removed successfuly from the
laser-induced plasma by use of a velocity filter.


title: Electronic properties of the doped antiferromagnet on a kagome lattice

abstract:
  Within the t-J model, we study the electronic properties of the doped
antiferromagnet on the kagome lattice based on the framework of the
self-consistent mean-field theory. At the half-filling, the spin-liquid
ground-state energy per site of the kagome antiferromagnet is Eg/NJ=-0.859,
which is in very good agreement with the numerical estimates. Away from the
half-filling, the electron photoemission spectroscopy and density of states are
discussed, and the results indicate that there is a gap in the normal-state of
the system.


title:
Optical spectroscopic study of the interplay of spin and charge in
  NaV2O5

abstract:
  We investigate the temperature dependent optical properties of NaV2O5, in the
energy range 4meV-4eV. The symmetry of the system is discussed on the basis of
infrared phonon spectra. By analyzing the optically allowed phonons at
temperatures below and above the phase transition, we conclude that a
second-order change to a larger unit cell takes place below 34 K, with a
fluctuation regime extending over a broad temperature range. In the high
temperature undistorted phase, we find good agreement with the recently
proposed centrosymmetric space group Pmmn. On the other hand, the detailed
analysis of the electronic excitations detected in the optical conductivity,
provides direct evidence for a charge disproportionated electronic
ground-state, at least on a locale scale: A consistent interpretation of both
structural and optical conductivity data requires an asymmetrical charge
distribution on each rung, without any long range order. We show that, because
of the locally broken symmetry, spin-flip excitations carry a finite electric
dipole moment, which is responsible for the detection of direct two-magnon
optical absorption processes for E parallel to the a axis. The charged-magnon
model, developed to interpret the optical conductivity of NaV2O5, is described
in detail, and its relevance to other strongly correlated electron systems,
where the interplay of spin and charge plays a crucial role in determining the
low energy electrodynamics, is discussed.


title:
Transition to Long Range Magnetic Order in the Highly Frustrated
  Insulating Pyrochlore Antiferromagnet Gd_2Ti_2O_7

abstract:
  Experimental evidence from measurements of the a.c. and d.c. susceptibility,
and heat capacity data show that the pyrochlore structure oxide, Gd_2Ti_2O_7,
exhibits short range order that starts developing at 30K, as well as long range
magnetic order at $T\sim 1$K. The Curie-Weiss temperature, $\theta_{CW}$ =
-9.6K, is largely due to exchange interactions. Deviations from the Curie-Weiss
law occur below $\sim$10K while magnetic heat capacity contributions are found
at temperatures above 20K. A sharp maximum in the heat capacity at $T_c=0.97$K
signals a transition to a long range ordered state, with the magnetic specific
accounting for only $\sim$ 50% of the magnetic entropy. The heat capacity above
the phase transition can be modeled by assuming that a distribution of random
fields acts on the $^8S_{7/2}$ ground state for Gd$^{3+}$. There is no
frequency dependence to the a.c. susceptibility in either the short range or
long range ordered regimes, hence suggesting the absence of any spin-glassy
behavior. Mean field theoretical calculations show that no long range ordered
ground state exists for the conditions of nearest-neighbor antiferromagnetic
exchange and long range dipolar couplings. At the mean-field level, long range
order at various commensurate or incommensurate wave vectors is found only upon
inclusion of exchange interactions beyond nearest-neighbor exchange and dipolar
coupling. The properties of Gd$_2Ti_2O_7 are compared with other geometrically
frustrated antiferromagnets such as the Gd_3Ga_5O_{12} gadolinium gallium
garnet, RE_2Ti_2O_7 pyrochlores where RE = Tb, Ho and Tm, and Heisenberg-type
pyrochlore such as Y_2Mo_2O_7, Tb_2Mo_2O_7, and spinels such as ZnFe_2O_4


title:
Comparison of Power Dependence of Microwave Surface Resistance of
  Unpatterned and Patterned YBCO Thin Film

abstract:
  The effect of the patterning process on the nonlinearity of the microwave
surface resistance $R_S$ of YBCO thin films is investigated. With the use of a
sapphire dielectric resonator and a stripline resonator, the microwave $R_S$ of
YBCO thin films was measured before and after the patterning process, as a
function of temperature and the rf peak magnetic field in the film. The
microwave loss was also modeled, assuming a $J_{rf}^2$ dependence of
$Z_S(J_{rf})$ on current density $J_{rf}$. Experimental and modeled results
show that the patterning has no observable effect on the microwave residual
$R_S$ or on the power dependence of $R_S$.


title: Orbital polarons in the metal-insulator transition of manganites

abstract:
  The metal-insulator transition in manganites is strongly influenced by the
concentration of holes present in the system. Based upon an orbitally
degenerate Mott-Hubbard model we analyze two possible localization scenarios to
account for this doping dependence: First, we rule out that the transition is
initiated by a disorder-order crossover in the orbital sector, showing that its
effect on charge itineracy is only small. Second, we introduce the idea of
orbital polarons originating from a strong polarization of orbitals in the
vicinity of holes. Considering this direct coupling between charge and orbital
degree of freedom in addition to lattice effects we are able to explain well
the phase diagram of manganites for low and intermediate hole concentrations.


title:
Addition Spectra of Chaotic Quantum Dots: Interplay between Interactions
  and Geometry

abstract:
  We investigate the influence of interactions and geometry on ground states of
clean chaotic quantum dots using the self-consistent Hartree-Fock method. We
find two distinct regimes of interaction strength: While capacitive energy
fluctuations $\delta \chi$ follow approximately a random matrix prediction for
weak interactions, there is a crossover to a regime where $\delta \chi$ is
strongly enhanced and scales roughly with interaction strength. This
enhancement is related to the rearrangement of charges into ordered states near
the dot edge. This effect is non-universal depending on dot shape and size. It
may provide additional insight into recent experiments on statistics of Coulomb
blockade peak spacings.


title: Pdf's of Derivatives and Increments for Decaying Burgers Turbulence

abstract:
  A Lagrangian method is used to show that the power-law with a -7/2 exponent
in the negative tail of the pdf of the velocity gradient and of velocity
increments, predicted by E, Khanin, Mazel and Sinai (1997 Phys. Rev. Lett. 78,
1904) for forced Burgers turbulence, is also present in the unforced case. The
theory is extended to the second-order space derivative whose pdf has power-law
tails with exponent -2 at both large positive and negative values and to the
time derivatives. Pdf's of space and time derivatives have the same
(asymptotic) functional forms. This is interpreted in terms of a "random Taylor
hypothesis".


title: Scale-invariance and contingent claim pricing

abstract:
  Prices of tradables can only be expressed relative to each other at any
instant of time. This fundamental fact should therefore also hold for contigent
claims, i.e. tradable instruments, whose prices depend on the prices of other
tradables. We show that this property induces local scale-invariance in the
problem of pricing contingent claims. Due to this symmetry we do not require
any martingale techniques to arrive at the price of a claim. If the tradables
are driven by Brownian motion, we find, in a natural way, that this price
satisfies a PDE. Both posses a manifest gauge-invariance. A unique solution can
only be given when we impose restrictions on the drifts of volatilities of the
tradables, i.e. the underlying market structure. We give some examples of the
application of this PDE to the pricing of claims. In the Black-Scholes world we
show the equivalence of our formulation with the standard approach. It is
stressed that the formulation in terms of tradables leads to a significant
conceptual simplification of the pricing-problem.


title: Spin Bottlenecks in the Quantum Hall Regim

abstract:
  We present a theory of time-dependent tunneling between a metal and a
partially spin-polarized two-dimensional electron system (2DES). We find that
the leakage current which flows to screen an electric field between the metal
and the 2DES is the sum of two exponential contributions whose relative weights
depend on spin-dependent tunneling conductances, on quantum corrections to the
electrostatic capacitance of the tunnel junction, and on the rate at which the
2DES spin-polarization approaches equilibrium. For high-mobility and
homogeneous 2DES's at Landau level filling factor $\nu=1$, we predict a ratio
of the fast and slow leakage rates equal to $(2K+1)^2$ where $K$ is the number
of reversed spins in the skyrmionic elementary charged excitations.


title:
Effects of noise and nonlocal interactions in nonlinear dynamics of
  molecular systems

abstract:
  We show that the NLS systems with multiplicative noise, nonlinear damping and
nonlocal dispersion exhibit a variety of interesting effects which may be
useful for modelling the dynamical behavior of one- and two-dimensional
molecular systems.


title: A relation between the resonance neutron peak and ARPES data in cuprates

abstract:
  We argue that the resonant peak observed in neutron scattering experiments on
superconducting cuprates and the peak/dip/hump features observed in ARPES
measurements are byproducts of the same physical phenomenon. We argue that both
are due to feedback effects on the damping of spin fluctuations in a $d-$wave
superconductor. We consider the spin-fermion model at strong coupling, solve a
set of coupled integral equations for fermionic and bosonic propagators and
show that the dynamical spin susceptibility below $T_c$ possesses the resonance
peak at $\Omega_{res} \propto \xi^{-1}$.
  The scattering of these magnetic excitations by electrons gives rise to a
peak/dip/hump behavior of the electronic spectral function, the peak-dip
separation is exactly $\Omega_{res}$.


title:
Universal correlations of one-dimensional interacting electrons in the
  gas phase

abstract:
  We consider dynamical correlation functions of short range interacting
electrons in one dimension at finite temperature. Below a critical value of the
chemical potential there is no Fermi surface anymore, and the system can no
longer be described as a Luttinger liquid. Its low temperature thermodynamics
is that of an ideal gas. We identify the impenetrable electron gas model as a
universal model for the gas phase and present exact and explicit expressions
for the asymptotics of correlation functions at small temperatures, in the
presence of a magnetic field.

categories: cond-mat.str-el hep-th math-ph math.MP nlin.SI solv-int


title: Microwave Absorption Peaks : Signatures of Spin Dynamics in Cuprates

abstract:
  We show that a common feature of temperature-dependent microwave absorption
is the presence of absorption peaks. $ac$ loss peaks can arise when the
internal $T$-dependent magnetic relaxation time crosses the measurement
frequency. These features are observed in the \QTR{em}{insulating}
($Sr_{x}Ca_{14-x}Cu_{24}O_{41}$, $La_{5/3}Sr_{1/3}NiO_{4}$ and
$YBa_{2}Cu_{3}O_{6.0}$), \QTR{em}{pseudo-gap} ($T>T_{c}$ in underdoped
$YBa_{2}Cu_{3}O_{7-\delta}$, $Hg:1223$ and $Hg:1201$) and
\QTR{em}{superconducting} ($T<T_{c}$) states of the oxides. The commonality of
these features suggests a magnetic (spin) mechanism, rather than a
quasiparticle origin, for the so-called ``conductivity'' peaks observed in the
cuprate superconductors.


title:
Comment on "Spin Transport properties of the quantum one-dimensional
  non-linear sigma model"

abstract:
  In a recent preprint (cond-mat/9905415), Fujimoto has used the Bethe ansatz
to compute the finite temperature, zero frequency Drude weight of spin
transport in the quantum O(3) non-linear sigma model in a magnetic field $H
\neq 0$. We show here that, contrary to his claims, the results are in accord
with earlier semiclassical results (Sachdev and Damle, cond-mat/9610115). We
also comment on his 1/N expansion, and show that it does not properly describe
the long-time correlations.


title: Disorder, pseudospins, and backscattering in carbon nanotubes

abstract:
  We address the effects of disorder on the conducting properties of metal and
semiconducting carbon nanotubes. Experimentally, the mean free path is found to
be much larger in metallic tubes than in doped semiconducting tubes. We show
that this result can be understood theoretically if the disorder potential is
long-ranged. The effects of a pseudospin index that describes the internal
sublattice structure of the states lead to a suppression of scattering in
metallic tubes, but not in semiconducting tubes. This conclusion is supported
by tight-binding calculations.


title: Optical Properties of MFe_4P_12 filled skutterudites

abstract:
  Infrared reflectance spectroscopy measurements were made on four members of
the MFe_4P_12 family of filled skutterudites, with M=La, Th, Ce and U. In
progressing from M=La to U the system undergoes a metal-insulator transition.
It is shown that, although the filling atom induces such dramatic changes in
the transport properties of the system, it has only a small effect on lattice
dynamics. We discuss this property of the compounds in the context of their
possible thermoelectric applications.


title: Rate-and-State Theory of Plastic Deformation Near a Circular Hole

abstract:
  We show that a simple rate-and-state theory accounts for most features of
both time-independent and time-dependent plasticity in a spatially
inhomogeneous situation, specifically, a circular hole in a large stressed
plate. Those features include linear viscoelastic flow at small applied
stresses, strain hardening at larger stresses, and a dynamic transition to
viscoplasticity at a yield stress. In the static limit, this theory predicts
the existence of a plastic zone near the hole for some but not all ranges of
parameters. The rate-and-state theory also predicts dynamic failure modes that
we believe may be relevant to fracture mechanics.


title: Dynamics of Interacting Neural Networks

abstract:
  The dynamics of interacting perceptrons is solved analytically. For a
directed flow of information the system runs into a state which has a higher
symmetry than the topology of the model. A symmetry breaking phase transition
is found with increasing learning rate. In addition it is shown that a system
of interacting perceptrons which is trained on the history of its minority
decisions develops a good strategy for the problem of adaptive competition
known as the Bar Problem or Minority Game.


title: Critical Behavior of the Random Potts Chain

abstract:
  We study the critical behavior of the random q-state Potts quantum chain by
density matrix renormalization techniques. Critical exponents are calculated by
scaling analysis of finite lattice data of short chains ($L \leq 16$) averaging
over all possible realizations of disorder configurations chosen according to a
binary distribution. Our numerical results show that the critical properties of
the model are independent of q in agreement with a renormalization group
analysis of Senthil and Majumdar (Phys. Rev. Lett.{\bf 76}, 3001 (1996)). We
show how an accurate analysis of moments of the distribution of magnetizations
allows a precise determination of critical exponents, circumventing some
problems related to binary disorder. Multiscaling properties of the model and
dynamical correlation functions are also investigated.


title:
Low Temperature Penetration depth of \kappa-(ET)_2Cu[N(CN)_2]Br and
  \kappa-(ET)_2Cu(NCS)_2

abstract:
  We present high precision measurements of the penetration depth \lambda of
single crystals of \kappa-(ET)_2Cu[N(CN)2]Br and \kappa-(ET)_2Cu(NCS)_2 at
temperature down to 0.4 K. We find that, at low temperatures, the in-plane
penetration depth varies as a fractional power law, \lambda\sim T^1.5. W hilst
this may be taken as evidence for novel bose excitation processes, we show that
the data are also consistent with a quasi-linear variation of the superfluid
density, as is expected for a d-wave superconductor with impurities. Our data
for the interplane penetration depth show similar features and give a direct
measurement of the absolute value, \lambda(0)=100\pm 20 \mu m.


title:
Electron transfer in the nonadiabatic regime: Crossover from
  quantum-mechanical to classical behaviour

abstract:
  We study nonadiabatic electron transfer within the biased spin-boson model.
We calculate the incoherent transfer rate in analytic form at all temperatures
for a power law form of the spectral density of the solvent coupling. In the
Ohmic case, we present the exact low temperature corrections to the zero
temperature rate for arbitrarily large bias energies between the two redox
sites. Both for Ohmic and non-Ohmic coupling, we give the rate in the entire
regime extending from zero temperature, where the rate depends significantly on
the detailed spectral behaviour, via the crossover region, up to the classical
regime. For low temperatures, the rate shows characteristic quantum features,
in particular the shift of the rate maximum to a bias value below the
reorganization energy, and the asymmetry of the rate around the maximum. We
study in detail the gradual extinction of the quantum features as temperature
is increased.


title: Driving-Induced Symmetry Breaking in the Spin-Boson System

abstract:
  A symmetric dissipative two-state system is asymptotically completely
delocalized independent of the initial state. We show that driving-induced
localization at long times can take place when both the bias and tunneling
coupling energy are harmonically modulated. Dynamical symmetry breaking on
average occurs when the driving frequencies are odd multiples of some reference
frequency. This effect is universal, as it is independent of the dissipative
mechanism. Possible candidates for an experimental observation are flux
tunneling in the variable barrier rf SQUID and magnetization tunneling in
magnetic molecular clusters.


title: Elastic constants from microscopic strain fluctuations

abstract:
  Fluctuations of the instantaneous local Lagrangian strain
$\epsilon_{ij}(\bf{r},t)$, measured with respect to a static ``reference''
lattice, are used to obtain accurate estimates of the elastic constants of
model solids from atomistic computer simulations. The measured strains are
systematically coarse- grained by averaging them within subsystems (of size
$L_b$) of a system (of total size $L$) in the canonical ensemble. Using a
simple finite size scaling theory we predict the behaviour of the fluctuations
$<\epsilon_{ij}\epsilon_{kl}>$ as a function of $L_b/L$ and extract elastic
constants of the system {\em in the thermodynamic limit} at nonzero
temperature. Our method is simple to implement, efficient and general enough to
be able to handle a wide class of model systems including those with singular
potentials without any essential modification. We illustrate the technique by
computing isothermal elastic constants of the ``soft'' and the hard disk
triangular solids in two dimensions from molecular dynamics and Monte Carlo
simulations. We compare our results with those from earlier simulations and
density functional theory.


title: Nematic phase of the two-dimensional electron gas in a magnetic field

abstract:
  The two dimensional electron gas (2DEG) in moderate magnetic fields in
ultra-clean AlAs-GaAs heterojunctions exhibits transport anomalies suggestive
of a compressible, anisotropic metallic state. Using scaling arguments and
Monte Carlo simulations, we develop an order parameter theory of an electron
nematic phase. The observed temperature dependence of the resistivity
anisotropy behaves like the orientational order parameter if the transition to
the nematic state occurs at a finite temperature, $T_c \sim 65 mK$, and is
slightly rounded by a small background microscopic anisotropy. We propose a
light scattering experiment to measure the critical susceptibility.


title: Finite-element theory of transport in ferromagnet-normal metal systems

abstract:
  We formulate a theory of spin dependent transport of an electronic circuit
involving ferromagnetic elements with non-collinear magnetizations which is
based on the conservation of spin and charge current. The theory considerably
simplifies the calculation of the transport properties of complicated
ferromagnet-normal metal systems. We illustrate the theory by considering a
novel three terminal device.


title: Atomic Scale Sliding and Rolling of Carbon Nanotubes

abstract:
  A carbon nanotube is an ideal object for understanding the atomic scale
aspects of interface interaction and friction. Using molecular statics and
dynamics methods different types of motion of nanotubes on a graphite surface
are investigated. We found that each nanotube has unique equilibrium
orientations with sharp potential energy minima. This leads to atomic scale
locking of the nanotube.
  The effective contact area and the total interaction energy scale with the
square root of the radius. Sliding and rolling of nanotubes have different
characters. The potential energy barriers for sliding nanotubes are higher than
that for perfect rolling. When the nanotube is pushed, we observe a combination
of atomic scale spinning and sliding motion. The result is rolling with the
friction force comparable to sliding.


title: Boundary critical behaviour of two-dimensional random Potts models

abstract:
  We consider random q-state Potts models for $3\le q \le 8$ on the square
lattice where the ferromagnetic couplings take two values $J_1>J_2$ with equal
probabilities. For any q the model exhibits a continuous phase transition both
in the bulk and at the boundary. Using Monte Carlo techniques the surface and
the bulk magnetizations are studied close to the critical temperature and the
critical exponents $\beta_1$ and $\beta$ are determined. In the strip-like
geometry the critical magnetization profile is investigated with free-fixed
spin boundary condition and the characteristic scaling dimension,
$\beta_1/\nu$, is calculated from conformal field theory. The critical
exponents and scaling dimensions are found monotonously increasing with q.
Anomalous dimensions of the relevant scaling fields are estimated and the
multifractal behaviour at criticality is also analyzed.


title: Dynamical Phase Transition in a Fully Frustrated Square Josephson Array

abstract:
  We study dynamical phase transitions at temperature T = 0 in a fully
frustrated square Josephson junction array subject to a driving current density
which has nonzero components parallel to both axes of the lattice. Our
numerical results show clear evidence for three dynamical phases: a pinned
vortex lattice characterized by zero time-averaged voltages, a ``plastic''
phase in which both voltages are nonzero, and a moving lattice phase in which
only one of the time-average voltage components is nonzero. The last of these
has a finite transverse critical current.


title:
Measuring the Viscosity and Time Correlation Functions in a Microscopic
  Model of a Microemulsion

abstract:
  A dynamical lattice model is used to study the viscosity and the
velocity-velocity autocorrelation function in a microemulsion phase. We find
evidence of anomalous viscosities in these phases (relative to water-rich
and/or oil-rich phases), in qualitative agreement with other results. We also
investigate the dynamic relaxation in the microemulsion phase. It has been
suggested that the temporal relaxation in the microemulsion phase may be
described by a stretched exponential Kolrausch-Williams-Watts law. In our
model, we find the velocity-velocity autocorrelation function fits this law,
showing both enhanced (b>1) and inhibited (b<1) diffusion.


title:
Superconducting Gap vs. Wave Vector: Evidence for Hot Regions on the
  Fermi Surface

abstract:
  We have used angular resolved photoemission to measure the angular dependence
of the superconducting gap in highly overdoped Bi2212 (Tc=65K). While the node
at 45 degrees is conserved, we find substantial deviation from a first order
d-wave dependence away from the node. The pairing susceptibility is peaked at
special regions on the Fermi surface. Comparing these results with a detailed
mapping of the Fermi surface we performed, we could measure the extension and
location of these hot regions. We find the hot regions to be evenly spread
about the nominal locations of hot spots. The decrease of the gap amplitude
away from these hot regions follows very closely theoretical calculations
within the spin fluctuation approach. These results strongly suggest that the
pairing susceptibility is peaked at Q=(Pi,Pi).


title: Noise of Entangled Electrons: Bunching and Antibunching

abstract:
  Addressing the feasibility of quantum communication with entangled electrons
in an interacting many-body environment, we propose an interference experiment
using a scattering set-up with an entangler and a beam splitter. It is shown
that, due to electron-electron interaction, the fidelity of the entangled
singlet and triplet states is reduced by z_F^2 in a conductor described by
Fermi liquid theory. We calculate the quasiparticle weight factor z_F for a
two-dimensional electron system. The current noise for electronic singlet
states turns out to be enhanced (bunching behavior), while it is reduced for
triplet states (antibunching). Within standard scattering theory, we find that
the Fano factor (noise-to-current ratio) for singlets is twice as large as for
independent classical particles and is reduced to zero for triplets.


title:
Direct observation of the multiple spin gap excitations in
  two-dimensional dimer system SrCu2(BO3)2

abstract:
  Various spin gap excitations have been observed in the two-dimensional dimer
system SrCu_2(BO_3)_2 by means of submillimeter wave ESR. The zero-field energy
gap of the lowest spin gap excitation shows a splitting into two triplet modes
and the energy splitting clearly depends on the magnetic field orientation when
a field is rotated in the {\mib {ac}}-plane. A zero-field splitting is also
found between the S(_z)=+1 and S(_z)=-1 branches of each triplet. These
behaviors are qualitatively explained by considering the anisotropic exchange
coupling of inter-dimer and intra-dimer, respectively. The averaged value of
the lowest spin gap energy is determined to be 722 \pm 2 GHz(34.7 K). We have
also found the second spin gap excitation at 1140 GHz(54.7 K), which indicates
that the inter-dimer coupling is significantly strong. Besides these modes, a
number of gapped ESR absorption are found and we propose that these multiple
magnetic excitations are caused by the localized nature of the excited state in
the present system.


title: Composite Fermions with Spin Freedom

abstract:
  General rule for the composite fermion transformation, when the spins of the
electrons are not polarized is derived. Condition for the quantum phase
transition between various spin states is obtained based on the rule. This rule
gives foundation for the experimental determination of the mass and $g$-factor
of the composite fermion.


title: Submillimeter Wave ESR Study of Spin Gap Excitations in CuGeO3

abstract:
  Transitions between the ground singlet state to the excited triplet state has
been observed in CuGeO3 by means of submillimeter wave electron spin resonance.
The strong absorption intensity shows the break down of the selection rule. The
energy gap at zero field is evaluated to be 570 GHz(2.36 meV) and this value is
nearly identical to the gap at the zone center observed by inelastic neutron
scattering. The absorption intensity shows strong field orientation dependence
but shows no significant dependence on magnetic field intensity. These features
have been explained by considering the existence of Dzyaloshinsky-Moriya (DM)
antisymmetric exchange interaction. The doping effect on this singlet-triplet
excitation has been also studied. A drastic broadening of the absorption line
is observed by the doping of only 0.5 % of Si.


title: Two ferromagnetic phases in La1-xSrxMnO3(x ~1/8)

abstract:
  It was discovered in La1-xSrxMnO3(x~1/8) that a field induced phase
transition occurs from a ferromagnetic metal(FM) phase to a ferromagnetic
insulator (FI) phase. The magnetization shows a sharp jump at the transition
field accompanying with a remarkable increase of magnetoresistance. Striction
measurements clarified that this transition is associated with the structural
change from a Jahn-Teller(JT) distorted orthorhombic phase to a pseudo cubic
phase. These results evidently show that the FI phase with a pseudo cubic
symmetry is more stable in high fields than the FM phase due to the double
exchange interaction. The driving force of this transition is explained by the
enhancement of the ferromagnetic superexchange interaction induced by an
antiferromagnetic type orbital ordering in the pseudo cubic phase, which was
recently found in the anomalous X-ray scattering experiments.


title:
Replica Symmetry Breaking in Short-Range Spin Glasses: Theoretical
  Foundations and Numerical Evidences

abstract:
  We discuss replica symmetry breaking (RSB) in spin glasses. We update work in
this area, from both the analytical and numerical points of view. We give
particular attention to the difficulties stressed by Newman and Stein
concerning the problem of constructing pure states in spin glass systems. We
mainly discuss what happens in finite-dimensional, realistic spin glasses.
Together with a detailed review of some of the most important features, facts,
data, and phenomena, we present some new theoretical ideas and numerical
results. We discuss among others the basic idea of the RSB theory, correlation
functions, interfaces, overlaps, pure states, random field, and the dynamical
approach. We present new numerical results for the behaviors of coupled
replicas and about the numerical verification of sum rules, and we review some
of the available numerical results that we consider of larger importance (for
example, the determination of the phase transition point, the correlation
functions, the window overlaps, and the dynamical behavior of the system).

categories: cond-mat.dis-nn adap-org cond-mat.stat-mech hep-th math-ph math.MP nlin.AO


title: Self-Organization of Complex Systems

abstract:
  The basic laws of physics are simple, so why is the world complex? The theory
of self-organized criticality posits that complex behavior in nature emerges
from the dynamics of extended, dissipative systems that evolve through a
sequence of meta-stable states into a critical state, with long range spatial
and temporal correlations. Minor disturbances lead to intermittent events of
all sizes. These events organize the system into a complex state that cannot be
reduced to a few degrees of freedom. This type of ``punctuated equilibrium''
dynamics has been observed in astrophysical, geophysical, and biological
processes, as well as in human social activity.


title: An investigation of site-bond percolation on many lattices

abstract:
  A calculation of site-bond percolation thresholds in many lattices in two to
five dimensions is presented. The line of threshold values has been
parametrized in the literature, but we show here that there are strong
deviations from the known approximate equations. We propose an alternative
parametrization that lies much closer to the numerical values


title:
Evidence of Structural Instability Near T*~250 K in the Resistivity of
  Bi_2Sr_2CaCu_2O_8 Whiskers

abstract:
  We report anomalous features in the normal-state resistivity of single
crystalline Bi_2Sr_2CaCu_2O_8 whiskers near some characteristic temperature T*
\~ 250 K. From these varied oxygen-doping samples, the resistance (R) was found
to deviate {\it upward} from the linear-T relation for T< T*, while R keeps
decreasing with T. The deviation, $\Delta R \equiv R - (a + bT)$, reaches a
maximum in a temperature range of ~ 25 K just before R-T follows the well-known
descent associated with the pseudogap opening in the underdoped samples. A
second kink was also found in some samples immediately following the first one,
resulting in an S-shaped feature in the R-T curve. The resistance then resumes
nearly linear-T decrease below the anomalies in contrast to the first case. We
interpret these features as being related to crystal structure transformation
(or lattice distortion), as was evidenced in thermal and elastic property
measurements. These structural instabilities seem to be connected to the
subsequent dynamics as T is further lowered.


title: Current noise in a irradiated point contact

abstract:
  We propose a new approach to calculate current and current correlations in a
ballistic quantum point contact interacting with a classical field. The
approach is based on the concept of scattering states for a time dependent
Hamiltonian neglecting electron-electron interaction. Using this approach we
calculated the spectra of the current noise in a biased point contact
irradiated by a weak random field. For typical radiation frequencies \nu less
than the temperature T and the bias voltage V we find a narrow peak of width
\nu on top of a broad background of width \max (T,eV).


title: Inherent Structure Entropy of Supercooled Liquids

abstract:
  We present a quantitative description of the thermodynamics in a supercooled
binary Lennard Jones liquid via the evaluation of the degeneracy of the
inherent structures, i.e. of the number of potential energy basins in
configuration space. We find that for supercooled states, the contribution of
the inherent structures to the free energy of the liquid almost completely
decouples from the vibrational contribution. An important byproduct of the
presented analysis is the determination of the Kauzmann temperature for the
studied system. The resulting quantitative picture of the thermodynamics of the
inherent structures offers new suggestions for the description of equilibrium
and out-of-equilibrium slow-dynamics in liquids below the Mode-Coupling
temperature.


title:
A generalizad Fokker-Planck equation for the ratchet problem: Memory
  effects

abstract:
  We study stochastic ratchets with inertia in the limit where the time
correlations become important. We have developed a Fokker-Planck type equation
for the ratchet problem which includes the memory effects. It is tested with
comparison to the Langevin simulations. The effect of the memory in the system
is analyzed extensively. A positive feedback regime which manifests itself as
instabilities is observed. This may be regarded as an illustration of
stochastic resonance.


title:
Exact diagonalization of the generalized supersymmetric t-J model with
  boundaries

abstract:
  We study the generalized supersymmetric $t-J$ model with boundaries in three
different gradings: FFB, BFF and FBF. Starting from the trigonometric R-matrix,
and in the framework of the graded quantum inverse scattering method (QISM), we
solve the eigenvalue problems for the supersymmetric $t-J$ model. A detailed
calculations are presented to obtain the eigenvalues and Bethe ansatz equations
of the supersymmetric $t-J$ model with boundaries in three different
backgrounds.


title: Spatial stochastic resonance in 1D Ising systems

abstract:
  The 1D Ising model is analytically studied in a spatially periodic and
oscillatory external magnetic field using the transfer-matrix method. For low
enough magnetic field intensities the correlation between the external magnetic
field and the response in magnetization presents a maximum for a given
temperature. The phenomenon can be interpreted as a resonance phenomenon
induced by the stochastic heatbath. This novel "spatial stochastic resonance"
has a different origin from the classical stochastic resonance phenomenon.


title:
Spectral Properties of Random Reactance Networks and Random Matrix
  Pencils

abstract:
  Our goal is to study statistical properies of "dielectric resonances" which
are poles of conductance of a large random $LC$ network. Such poles are a
particular example of eigenvalues $\lambda_n$ of matrix pencils ${\bf
H}-\lambda {\bf W}$, with ${\bf W}$ being positive definite matrix and ${\bf
H}$ a random real symmetric one. We first consider spectra of matrix pencils
with independent, identically distributed entries of ${\bf H}$. Then we
concentrate on an infinite-range ("full-connectivity") version of random $LC$
network. In all cases we calculate the mean eigenvalue density and the
two-point correlation function in the framework of Efetov's supersymmetry
approach. Fluctuations in spectra turn out to be the same as those provided by
Wigner-Dyson theory of usual random matrices.


title:
Level statistics in a two-dimensional system with strong spin-orbit
  coupling

abstract:
  We study level correlations in a two-dimensional system with a long-range
random potential and strong spin-orbit (SO) splitting of the spectrum. The
level correlations for sufficiently large splitting are shown to be described
by orthogonal Wigner-Dyson statistics, in contrast to the common point of view
that a system with the strong SO coupling belongs to the symplectic statistical
ensemble. We demonstrate also that in a wide energy interval the level
statistics is completely determined by transitions between two branches of the
split spectrum. A sharp resonance is obtained in the two-level correlation
function at energy equal to the value of SO splitting.


title: Theory of vortex excitation imaging via an NMR relaxation measurement

abstract:
  The temperature dependence of the site-dependent nuclear spin relaxation time
T_1 around vortices is studied in s-wave and d-wave superconductors.Reflecting
low energy electronic excitations associated with the vortex core, temperature
dependences deviate from those of the zero-field case, and T_1 becomes faster
with approaching the vortex core. In the core region, T_1^{-1} has a new peak
below T_c. The NMR study by the resonance field dependence may be a new method
to prove the spatial resolved vortex core structure in various superconductors.


title: Conductance fluctuations near Anderson transition

abstract:
  In this paper we report measurements of conductance fluctuations in single
crystal samples of Si doped with P and B close to the critical composition of
the metal insulator transition (n_c \approx 4 x 10^18 /cc). The measurements
show that the noise, which arises from bulk sources, does not diverge as the
Ioffe-Regal limit (k_{F}l -> 1) is approached from the metallic side. At room
temperatures, the magnitude of the noise shows a shallow maximum around k_{F}l
\approx 1.5 and drops sharply as the insulating state is approached.


title:
Monte Carlo simulation with time step quantification in terms of
  Langevin dynamics

abstract:
  For the description of thermally activated dynamics in systems of classical
magnetic moments numerical methods are desirable. We consider a simple model
for isolated magnetic particles in a uniform field with an oblique angle to the
easy axis of the particles. For this model, a comparison of the Monte Carlo
method with Langevin dynamics yields new insight in the interpretation of the
Monte Carlo process, leading to the implementation of a new algorithm where the
Monte Carlo step is time-quantified. The numeric results for the characteristic
time of the magnetisation reversal are in excellent agreement with asymptotic
solutions which itself are in agreement with the exact numerical results
obtained from the Fokker-Planck equation for the Neel-Brown model.


title: Electrophoresis simulated with the cage model for reptation

abstract:
  The cage model for polymer reptation is extended to simulate DC
electrophoresis. The drift velocity v of a polymer with length L in an electric
field with strength E shows three different regions: if the strength of field
is small, the drift velocity scales as E/L; for slightly larger strengths, it
scales as E^2, independent of length; for high fields, but still E much smaller
than 1, the drift velocity decreases exponentially to zero. The behaviour of
the first two regions are in agreement with earlier reports on simulations of
the Duke-Rubinstein model and with experimental work on DNA polymers in agarose
gel.


title: p-wave and d-wave superconductivity in quasi-two-dimensional metals

abstract:
  We compare predictions of the mean-field theory of superconductivity for
nearly antiferromagnetic and nearly ferromagnetic metals in two dimensions. The
calculations are based on a parametrization of the effective interaction
arising from the exchange of magnetic fluctuations. The Eliashberg equations
for the transition temperature are solved including the full momentum
dependence of the self-energy. The results show that for comparable parameters
d-wave singlet pairing in nearly antiferromagnetic metals is generally much
stronger than p-wave triplet pairing in nearly ferromagnetic metals in quasi
two dimensions. The relevance to the layered materials, and in particular
Sr2RuO4 that exhibits p-wave triplet pairing, is discussed.


title: The saddle-point method for condensed Bose gases

abstract:
  The application of the conventional saddle-point approximation to condensed
Bose gases is thwarted by the approach of the saddle-point to the ground-state
singularity of the grand canonical partition function. We develop and test a
variant of the saddle-point method which takes proper care of this
complication, and provides accurate, flexible, and computationally efficient
access to both canonical and microcanonical statistics. Remarkably, the error
committed when naively employing the conventional approximation in the
condensate regime turns out to be universal, that is, independent of the
system's single-particle spectrum. The new scheme is able to cover all
temperatures, including the critical temperature interval that marks the onset
of Bose--Einstein condensation, and reveals in analytical detail how this onset
leads to sharp features in gases with a fixed number of particles. In
particular, within the canonical ensemble the crossover from the
high-temperature asymptotics to the condensate regime occurs in an
error-function-like manner; this error function reduces to a step function when
the particle number becomes large. Our saddle-point formulas for occupation
numbers and their fluctuations, verified by numerical calculations, clearly
bring out the special role played by the ground state.


title:
Analysis of impurity-induced circular currents for the chiral
  superconductor Sr_{2}RuO_{4}

abstract:
  The microscopic mechanism of circular currents induced in the vicinity of a
non-magnetic impurity is analyzed for the (time-reversal symmetry breaking)
chiral superconducting state ${\bf d}({\bf k}) = \hat{{\bf z}} (k_x \pm i k_y)$
which is very likely realized in Sr_2RuO_4. From the analytic, not
self-consistent solution of the Bogolyubov-de Gennes equations we find two
types of quasiparticle states, a bound state and the continuum state. Through
impurity scattering the condensate transfers angular momentum to the
quasiparticle states which generate a circular current. At low temperature the
continuum part of the quasiparticle spectrum gives the main contribution. The
non-selfconsistent solution yields a state of finite angular momentum. The
comparison with the corresponding Ginzburg-Landau theory reveals the existence
of a compensating counterflow created by the superconducting condensate. The
currents and magnetic fields appearing around the impurity have possibly been
observed by muon spin relaxation measurements.


title:
Preasymptotic multiscaling in the phase-ordering dynamics of the kinetic
  Ising model

abstract:
  The evolution of the structure factor is studied during the phase-ordering
dynamics of the kinetic Ising model with conserved order parameter. A
preasymptotic multiscaling regime is found as in the solution of the
Cahn-Hilliard-Cook equation, revealing that the late stage of phase-ordering is
always approached through a crossover from multiscaling to standard scaling,
independently from the nature of the microscopic dynamics.


title: P-adic numbers and replica symmetry breaking

abstract:
  The p-adic formulation of replica symmetry breaking is presented. In this
approach ultrametricity is a natural consequence of the basic properties of the
p-adic numbers. Many properties can be simply derived in this approach and
p-adic Fourier transform seems to be an promising tool.


title:
Entropy, vortex interactions, and the phase diagram of heavy-ion
  irradiated Bi_2Sr_2CaCu_2O_8

abstract:
  Dynamic and thermodynamic magnetization experiments on heavy-ion irradiated
single crystalline Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta} are correlated in order to
clarify the nature of the mixed state phase diagram. It is shown that whereas
the entropy contribution to the free energy in the London regime plays a minor
role in unirradiated crystals and irradiated crystals at fields close to or
above the matching field $B_{\phi}$, it becomes very important at low fields in
irradiated crystals with high $B_{\phi}$. The direct determination of the
entropy contribution to the free energy from the reversible magnetization
allows one to determine not only the correct values of the pinning energy, but
also to extract quite detailed information on pancake vortex alignment. The
characteristic field $H_{int} \sim {1/6} B_{\phi}$ at which intervortex
repulsion begins to determine the vortex arrangement and the reversible
magnetization is shown to coincide with a sharp increase in the irreversibility
field $H_{irr}(T)$ and with the recoupling transition found in Josephson Plasma
Resonance. Above $H_{int}$, the repulsive interaction between vortices cause
both the vortex mobility to decrease and pancake alignement to increase. At
higher fields $\gtrsim {1/3}B_{\phi}\gg B_{c1}$, free vortices outnumber those
that are trapped on a columnar defect. This causes the decrease of c-axis
resistivity and a second crossover of the irreversibility field, to a regime
where it is determined by plastic creep.


title: Speeding Up Computer Simulations: The Transition Observable Method

abstract:
  A method is presented which allows for a tremendous speed-up of computer
simulations of statistical systems by orders of magnitude. This speed-up is
achieved by means of a new observable, while the algorithm of the simulation
remains unchanged.


title:
Imaginary Potential as a Counter of Delay Time for Wave Reflection from
  a 1D Random Potential

abstract:
  We show that the delay time distribution for wave reflection from a
one-dimensional random potential is related directly to that of the reflection
coefficient, derived with an arbitrarily small but uniform imaginary part added
to the random potential. Physically, the reflection coefficient, being
exponential in the time dwelt in the presence of the imaginary part, provides a
natural counter for it. The delay time distribution then follows
straightforwardly from our earlier results for the reflection coefficient, and
coincides with the distribution obtained recently by Texier and Comtet
[C.Texier and A. Comtet, Phys.Rev.Lett. {\bf 82}, 4220 (1999)],with all moments
infinite. Delay time distribution for a random amplifying medium is then
derived . In this case, however, all moments work out to be finite.


title:
High real-space resolution measurement of the local structure of
  Ga_1-xIn_xAs using x-ray diffraction

abstract:
  High real-space resolution atomic pair distribution functions (PDF)s from the
alloy series Ga_1-xIn_xAs have been obtained using high-energy x-ray
diffraction. The first peak in the PDF is resolved as a doublet due to the
presence of two nearest neighbor bond lengths, Ga-As and In-As, as previously
observed using XAFS. The widths of nearest, and higher, neighbor pairs are
analyzed by separating the strain broadening from the thermal motion. The
strain broadening is five times larger for distant atomic neighbors as compared
to nearest neighbors. The results are in agreement with model calculations.


title:
Ionization degree of the electron-hole plasma in semiconductor quantum
  wells

abstract:
  The degree of ionization of a nondegenerate two-dimensional electron-hole
plasma is calculated using the modified law of mass action, which takes into
account all bound and unbound states in a screened Coulomb potential.
Application of the variable phase method to this potential allows us to treat
scattering and bound states on the same footing. Inclusion of the scattering
states leads to a strong deviation from the standard law of mass action. A
qualitative difference between mid- and wide-gap semiconductors is
demonstrated. For wide-gap semiconductors at room temperature, when the bare
exciton binding energy is of the order of T, the equilibrium consists of an
almost equal mixture of correlated electron-hole pairs and uncorrelated free
carriers.


title:
Charge-order transition in the extended Hubbard model on a two-leg
  ladder

abstract:
  We investigate the charge-order transition at zero temperature in a two-leg
Hubbard ladder with additional nearest-neighbor Coulomb repulsion V using the
Density Matrix Renormalization Group technique. We consider electron densities
between quarter and half filling. For quarter filling and U=8t, we find
evidence for a continuous phase transition between a homogeneous state at small
V and a broken-symmetry state with "checkerboard" [wavevector Q=(pi,pi)] charge
order at large V. This transition to a checkerboard charge-ordered state
remains present at all larger fillings, but becomes discontinuous at
sufficiently large filling. We discuss the influence of U/t on the transition
and estimate the position of the tricritical points.


title:
Monte Carlo Simulation of Smectic Liquid Crystals and the Electroclinic
  Effect: the Role of the Molecular Shape

abstract:
  Using Monte Carlo simulation methods, we explore the role of molecular shape
in the phase behavior of liquid crystals and the electroclinic effect. We study
a "bent-rod" mesogen shaped like the letter Z, composed of seven soft spheres
bonded rigidly together with no intra-molecular degrees of freedom. For
strongly angled molecules, we find that steric repulsion alone provides the
driving force for a smectic-C phase, even without intermolecular dipole-dipole
interactions. For weakly angled (nearly rod-like) molecules, we find a stable
smectic-A (SmA) phase and a strong electroclinic effect with a saturation tilt
angle of about 19 degrees. In the SmA phase we find evidence of vortex-like
point defects. We also observe a field-induced nematic-smectic phase
transition.


title: New type of extreme value statistics

abstract:
  We investigate the extreme value statistics connected with the dilute Random
Energy Model with integer couplings. New universality class is found.


title:
Charge order, superconductivity, and a global phase diagram of doped
  antiferromagnets

abstract:
  We investigate the interplay between lattice-symmetry breaking and
superconducting order in a two-dimensional model of doped antiferromagnets,
with long-range Coulomb interactions and Sp(2N) spin symmetry, in the large-N
limit. Our results motivate the outline of a global phase diagram for the
cuprate superconductors. We describe the quantum transitions between the
phases, the evolution of their fermion excitation spectrum, and the
experimental implications.


title:
Orbital symmetry of a triplet pairing in a heavy Fermion superconductor
  UPt_3

abstract:
  The orbital symmetry of the superconducting order parameter in UPt_3 is
identified by evaluating the directionally dependent thermalconductivity and
ultrasound attenuation in the clean limit and compared with the existing data
for both basal plane and the c-axis of a hexagonal crystal. The resulting two
component orbital part expressed by (\lambda_x(k), \lambda_y(k)) is combined
with the previously determined triplet spin part, leading to clean limit and
compared with the existing data for both basal plane and the c-axis of a
hexagonal crystal. The resulting two component orbital part expressed by
(\lambda_x(k), \lambda_y(k)) is combined with the previously determined triplet
spin part, leading to the order parameter of either the non-unitary bipolar
state of the form: d(k) = b \lambda_x(k) + i j \lambda_y(k) or the unitary
planar state of the form: d(k) = b \lambda_x(k) + j \lambda_y(k) where b \perp
j = c, or a with the hexagonal unit vectors a, b and c. The d vector is
rotatable in the plane spanned by a and c perpendicular to b under weak applied
c-axis field because of the weak spin orbit coupling. Experiments are proposed
to distinguish between the equally possible these states.


title: Final State Effects in the high q response of $^3$He-$^4$He mixtures

abstract:
  A modified Gersch-Rodriguez formalism describing the leading Final State
Effects in the high momentum transfer response of low concentration
$^3$He-$^4$He mixtures is presented and discussed. The leading corrections to
the Impulse Approximation are expressed in terms of the interatomic potentials
and the semidiagonal two-body density matrices of both the mixture and its
boson-boson approximation, in which the $^3$He atoms are replaced by bosons of
the same mass and at the same partial density. Numerical calculations of the
Final State Effects functions of $^3$He and $^4$He are finally presented and
discussed.


title:
Theory and Satellite Experiment for Critical Exponent alpha of
  lambda-Transition in Superfluid Helium

abstract:
  On the basis recent seven-loop perturbation expansion for nu^{-1} = 3/(2 -
alpha) we perform a careful reinvestigation of the critical exponent alpha
governing the power behavior |T_c-T|^{- alpha} of the specific heat of
superfluid helium near the phase transition. With the help of variational
strong-coupling theory. we find alpha = - 0.01126 +- 0.0010, in very good
agreement with the space shuttle experimental value alpha = - 0.01056 +-
0.00038.


title: Spectral properties of incommensurate charge-density wave systems

abstract:
  The concept of frustrated phase separation is applied to investigate its
consequences for the electronic structure of the high T_c cuprates. The
resulting incommensurate charge density wave (CDW) scattering is most effective
in creating local gaps in k-space when the scattering vector connects states
with equal energy. Starting from an open Fermi surface we find that the
resulting CDW is oriented along the (10)- and (or) (01)-direction which allows
for a purely one-dimensional or a two-dimensional ``eggbox type'' charge
modulation. In both cases the van Hove singularities are substantially
enhanced, and the spectral weight of Fermi surface states near the M-points,
tends to be suppressed. Remarkably, a leading edge gap arises near these
points, which, in the eggbox case, leaves finite arcs of the Fermi surface
gapless. We discuss our results with repect to possible consequences for
photoemission experiments.


title: Breakup of a Dimer: a New Approach to Localization Transition

abstract:
  Within the framework of tight binding models, aperiodic systems are mapped to
a renormalized lattice with a dimer defect. In models exhibiting
metal-insulator transition, the dimer acts like a resonant cavity and explains
the existence of the ballistic transport in the system. The localization in the
model can be attributed to the vanishing of the coupling between the two sites
of the dimer. Our approach unifies Anderson transition and resonance transition
and provides a new formulation to understand localization and its absence in
aperiodic systems.


title:
A Composite Fermion Hofstader Problem: Partially Polarized Density Wave
  States in the 2/5 FQHE

abstract:
  It is well-known that the 2/5 state is unpolarized at zero Zeeman energy,
while it is fully polarized at large Zeeman energies. A novel state with
charge/spin density wave order for Composite Fermions is proposed to exist at
intermediate values of the Zeeman coupling for 2/5. This state has half the
maximum possible polarization, and can be extended to other incompressible
fractions. A Hartree-Fock calculation based on the new approach for all
fractional quantum Hall states developed by R.Shankar and the author is used to
demonstrate the stability of this state to single-particle excitations, and
compute gaps. We compare our results with a very recent experiment which shows
direct evidence for the existence of such a state, and also with more indirect
evidence from past experiments.


title: Filler-Induced Composition Waves in Phase-Separating Polymer Blends

abstract:
  The influence of immobile filler particles (spheres, fibers, platelets) on
polymer blend phase separation is investigated computationally using a
generalization of the Cahn-Hilliard-Cook (CHC) model. Simulation shows that the
selective affinity of one of the polymers for the filler surface leads to the
development of concentration waves about the filler particles at an early stage
of phase separation in near critical composition blends. These "target"
patterns are overtaken in late stage phase separation by a growing "background"
spinodal pattern characteristic of blends without filler particles. The
linearized CHC model is used to estimate the number of composition oscillations
emanating from isolated filler particles. In far-off-critical composition
blends, an "encapsulation layer" grows at the surface of the filler rather than
a target pattern. The results of these simulations compare favorably with
experiments on filled phase separating blend films.


title:
The effect of an in-plane magnetic field on the interlayer transport of
  quasiparticles in layered superconductors

abstract:
  We consider the quasiparticle c-axis conductivity in highly anisotropic
layered compounds in the presence of the magnetic field parallel to the layers.
We show that at low temperatures the quasiparticle interlayer conductivity
depends strongly on the orientation of the in-plane magnetic field if the
excitation gap has nodes on the Fermi surface. Thus measurements of the
angle-dependent c-axis (out-of-plane) magnetoresistance, as a function of the
orientation of the magnetic field in the layers, provide information on the
momentum dependence of the superconducting gap (or pseudogap) on the Fermi
surface. Clean and highly anisotropic layered superconductors seem to be the
best candidates for probing the existence and location of the nodes on the
Fermi surface.


title:
Symmetric Diblock Copolymers in Thin Films (II): Comparison of Profiles
  between Self-Consistent Field Calculations and Monte Carlo Simulations

abstract:
  The structure of lamellar phases of symmetric $AB$ diblock copolymers in a
thin film is investigated. We quantitatively compare the composition profiles
and profiles of individual segments in self-consistent field calculations with
Monte Carlo simulations in the bond fluctuation model for chain length N=32 and
$\chi N=30$. Three film thicknesses are investigated, corresponding to parallel
oriented lamellae with 2 and 4 interfaces and a perpendicular oriented
morphology. Taking account of capillary waves, we find good quantitative
agreement between the Monte Carlo simulations and the self-consistent field
calculations. However, the fluctuations of the local interfacial position are
strongly suppressed by confinement and mutual interactions between lamellae.


title:
Universality, frustration and conformal invariance in two-dimensional
  random Ising magnets

abstract:
  We consider long, finite-width strips of Ising spins with randomly
distributed couplings. Frustration is introduced by allowing both ferro- and
antiferromagnetic interactions. Free energy and spin-spin correlation functions
are calculated by transfer-matrix methods. Numerical derivatives and
finite-size scaling concepts allow estimates of the usual critical exponents
$\gamma/\nu$, $\alpha/\nu$ and $\nu$ to be obtained, whenever a second-order
transition is present. Low-temperature ordering persists for suitably small
concentrations of frustrated bonds, with a transition governed by pure--Ising
exponents. Contrary to the unfrustrated case, subdominant terms do not fit a
simple, logarithmic-enhancement form. Our analysis also suggests a vertical
critical line at and below the Nishimori point. Approaching this point along
either the temperature axis or the Nishimori line, one finds non-diverging
specific heats. A percolation-like ratio $\gamma/\nu$ is found upon analysis of
the uniform susceptibility at the Nishimori point. Our data are also consistent
with frustration inducing a breakdown of the relationship between
correlation-length amplitude and critical exponents, predicted by conformal
invariance for pure systems.


title:
Size shrinking of composite bosons for increasing density in the BCS to
  Bose-Einstein crossover

abstract:
  We consider a system of fermions in the continuum case at zero temperature,
in the strong-coupling limit of a short-range attraction when composite bosons
form as bound-fermion pairs. We examine the density dependence of the size of
the composite bosons at leading order in the density ("dilute limit"), and show
on general physical grounds that this size should decrease with increasing
density, both in three and two dimensions. We then compare with the analytic
zero-temperature mean-field solution, which indeed exhibits the size shrinking
of the composite bosons both in three and two dimensions. We argue,
nonetheless, that the two-dimensional mean-field solution is not consistent
with our general result in the "dilute limit", to the extent that mean field
treats the scattering between composite bosons in the Born approximation which
is known to break down at low energy in two dimensions.


title:
Symmetric Diblock Copolymers in Thin Films (I): Phase stability in
  Self-Consistent Field Calculations and Monte Carlo Simulations

abstract:
  We investigate the phase behavior of symmetric AB diblock copolymers confined
into a thin film. The film boundaries are parallel, impenetrable and attract
the A component of the diblock copolymer. Using a self-consistent field
technique [M.W. Matsen, J.Chem.Phys. {\bf 106}, 7781 (1997)], we study the
ordered phases as a function of incompatibility $\chi$ and film thickness in
the framework of the Gaussian chain model. For large film thickness and small
incompatibility, we find first order transitions between phases with different
number of lamellae which are parallel oriented to the film boundaries. At high
incompatibility or small film thickness, transitions between parallel oriented
and perpendicular oriented lamellae occur. We compare the self-consistent field
calculations to Monte Carlo simulations of the bond fluctuation model for chain
length N=32. In the simulations we quench several systems from $\chi N=0$ to
$\chi N=30$ and monitor the morphology into which the diblock copolymers
assemble. Three film thicknesses are investigated, corresponding to parallel
oriented lamellae with 2 and 4 interfaces and a perpendicular oriented
morphology. Good agreement between self-consistent field calculations and Monte
Carlo simulations is found.


title:
Arrested States formed on Quenching Spin Chains with Competing
  Interactions and Conserved Dynamics

abstract:
  We study the effects of rapidly cooling to T = 0 a spin chain with conserved
dynamics and competing interactions. Depending on the degree of competition,
the system is found to get arrested in different kinds of metastable states.
The most interesting of these has an inhomogeneous mixture of interspersed
active and quiescent regions. In this state, the steady-state autocorrelation
function decays as a stretched exponential $\sim
\exp(-{(t/\tau_{o})}^{{1}\over{3}})$, and there is a two-step relaxation to
equilibrium when the temperature is raised slightly.


title: Toward a Theory of Orbiton Dispersion in LaMnO_3

abstract:
  At 750K, LaMnO_3 has a cooperative Jahn-Teller (JT) distortion, with Mn atoms
in distorted oxygen octahedra. This lifts the degeneracy of the singly-occupied
$e_g$ orbitals of the Mn$^{3+}$ ions, which then become orbitally ordered. We
use a minimal model to describe the ordered phase at T=0. The on-site Coulomb
repulsion $U$ is set to infinity. There are two electronic orbitals and three
oxygen vibrational coordinates per unit cell. In addition to spin excitations
and phonons, the model has electronic excitations consisting of mis-orienting
orbitals on Mn ions. Neglecting coupling to the oxygen displacements, the gap
to such excitations is $2\Delta=16g^2/K$ where $g$ is the electron-phonon
coupling and $K$ is the oxygen spring constant. When static oxygen
displacements are coupled, this excitation becomes a self-trapped exciton with
energy $\Delta$, half the JT gap. Adding dynamic oxygen displacements in
one-phonon approximation introduces dispersion to both the (previously
Einstein-like) phonons and the orbital excitons (``orbitons''). One of the
phonon branches has zero frequency at $\vec{k}=(0,0,\pi)$. This is the
Goldstone mode of the JT broken symmetry.


title:
Coordination defects in a-Si and a-Si:H : a characterization from first
  principles calculations

abstract:
  We study by means of first-principles pseudopotential method the coordination
defects in a-Si and a-Si:H, also in their formation and their evolution upon
hydrogen interaction. An accurate analysis of the valence charge distribution
and of the ``electron localization function'' (ELF) allows to resolve possible
ambiguities in the bonding configuration, and in particular to identify clearly
three-fold (T_3) and five-fold (T_5) coordinated defects. We found that
electronic states in the gap can be associated to both kind of defects, and
that in both cases the interaction with hydrogen can reduce the density of
states in the gap.


title: Radiative Transport in a Periodic Structure

abstract:
  We derive radiative transport equations for solutions of a Schr\"odinger
equation in a periodic structure with small random inhomogeneities. We use
systematically the Wigner transform and the Bloch wave expansion. The streaming
part of the radiative transport equations is determined entirely by the Bloch
spectrum, while the scattering part by the random fluctuations.


title: Universality in short-range Ising spin glasses

abstract:
  The role of the distribution of coupling constants on the critical exponents
of the short-range Ising spin-glass model is investigated via real space
renormalization group. A saddle-point spin glass critical point characterized
by a fixed-point distribution is found in an appropriated parameter space. The
critical exponents $\beta $ and $\nu $ are directly estimated from the data of
the local Edwards-Anderson order parameters for the model defined on a diamond
hierarchical lattice of fractal dimension $d_{f}=3$. Four distinct initial
distributions of coupling constants (Gaussian, bimodal, uniform and
exponential) are considered; the results clearly indicate a universal behavior.


title:
Thermally Assisted Tunneling: An Alternative Model for the
  Thermoluminescence Process in Calcite

abstract:
  In this work we investigate the effect of gamma and ultraviolet radiations on
the thermoluminescence of Brazilian calcite. The irradiated samples presented
three TL peaks at 150, 245 and 320^oC (at a linear heating rate of 1.8^oC/s),
with a main emission band centered at 615nm due to the ^4G \to ^6S transition
of the Mn^{2+}. The irradiated samples presented, besides the Mn^{2+} signals,
three lines related related to carbonate groups. The thermal treatment and the
irradiation effects on the ESR signal lead to the conclusion that (CO_3)^{3-},
stabilized in two different symmetries, and the (CO_2)^{-} ions are the
electron trapping centers. It was found that the TL peaks follows a t^{-1}
decay as a function of the UV illumination time. An alternative model for the
TL emission of calcite is discussed, considering that the recombination of
charges is processed via a thermally assisted tunnelling mechanism.


title:
A Microscopic Model of Edge States of Fractional Quantum Hall Liquid:
  From Composite Fermions to Calogero-Sutherland Model

abstract:
  Based on the composite fermion approach, we derive a microscopic theory
describing the low-lying edge excitations in the fractional quantum Hall liquid
with $\nu=\frac{\nu^*}{\tilde\phi\nu^*+1}$. For $\nu^*>0$, it is found that the
composite fermion model reduces to an SU$(\nu^*)$ Calogero-Sutherland model in
the one-dimensional limit, whereas it is not exact soluble for $\nu^*<0$.
However, the ground states in both cases can be found and the low-lying
excitations can be shown the chiral Luttinger liquid behaviors since a gap
exists between the right- and left-moving sectors in each branch of the
azimuthal excitations.


title: Critical scaling of the a.c. conductivity for a superconductor above Tc

abstract:
  We consider the effects of critical superconducting fluctuations on the
scaling of the linear a.c. conductivity, \sigma(\omega), of a bulk
superconductor slightly above Tc in zero applied magnetic field. The dynamic
renormalization- group method is applied to the relaxational time-dependent
Ginzburg-Landau model of superconductivity, with \sigma(\omega) calculated via
the Kubo formula to O(\epsilon^{2}) in the \epsilon = 4 - d expansion. The
critical dynamics are governed by the relaxational XY-model
renormalization-group fixed point. The scaling hypothesis \sigma(\omega) \sim
\xi^{2-d+z} S(\omega \xi^{z}) proposed by Fisher, Fisher and Huse is explicitly
verified, with the dynamic exponent z \approx 2.015, the value expected for the
d=3 relaxational XY-model. The universal scaling function S(y) is computed and
shown to deviate only slightly from its Gaussian form, calculated earlier. The
present theory is compared with experimental measurements of the a.c.
conductivity of YBCO near Tc, and the implications of this theory for such
experiments is discussed.


title: Analysis of radial segregation of granular mixtures in a rotating drum

abstract:
  This paper considers the segregation of a granular mixture in a rotating
drum. Extending a recent kinematic model for grain transport on sandpile
surfaces to the case of rotating drums, an analysis is presented for radial
segregation in the rolling regime, where a thin layer is avalanching down while
the rest of the material follows rigid body rotation. We argue that segregation
is driven not just by differences in the angle of repose of the species, as has
been assumed in earlier investigations, but also by differences in the size and
surface properties of the grains. The cases of grains differing only in size
(slightly or widely) and only in surface properties are considered, and the
predictions are in qualitative agreement with observations. The model yields
results inconsistent with the assumptions for more general cases, and we
speculate on how this may be corrected.


title:
Unified theory of phase separation and charge ordering in doped
  manganite perovskites

abstract:
  A unified theory is developed to explain various types of electronic
collective behaviors in doped manganites R$_{1-x}$X$_x$MnO$_3$ (R = La, Pr,Nd
etc. and X = Ca, Sr, Ba etc.). Starting from a realistic electronic model, we
derive an effective Hamiltonianis by ultilizing the projection perturbation
techniques and develop a spin-charge-orbital coherent state theory, in which
the Jahn-Teller effect and the orbital degeneracy of e$_g$ electrons in Mn ions
are taken into account. Physically, the experimentally observed charge ordering
state and electronic phase separation are two macroscopic quantum phenomena
with opposite physical mechanisms, and their physical origins are elucidated in
this theory. Interplay of the Jahn-Teller effect, the lattice distortion as
well as the double exchange mechanism leads to different magnetic structures
and to different charge ordering patterns and phase separation.


title:
Polytype Kinetics and Quenching of Spin Chains with Competing
  Interactions using Trimer-flip Dynamics

abstract:
  We consider the effects of a quench to T = 0 on a spin system with axial
next-next nearest neighbour Ising interactions, evolving under a conserved
3-spin flip dynamics. Such a model is motivated by the kinetics of stacking
layers in polytypes near the 3C-6H transition. We find that the system
generically gets arrested in interesting metastable states which have
inhomogeneously distributed quiescent and active regions. In such arrested
states, the autocorrelation function decays as a stretched exponential $\sim
\exp(-(t/\tau_{o})^{1 \over 3})$. The latter feature can be understood in terms
of a mapping of the dynamics within active stretches to the well known simple
exclusion process of particles on a line, and bounds can be put on $\tau_o$.


title:
Stripe Structures and the Berry-Phase Connection: Concept of Geometric
  Energy

abstract:
  Electronic states of an $e_g$ electron are calculated in the system composed
of two MnO$_6$ octahedra with the inclusion of the Berry phase acquired by
parallel transport. Based on this calculation, a comment is made on the
controversy between ``Wigner-crystal'' and ``paired-stripe'' models for the the
insulating charge-ordered manganese oxides.


title:
Exact results for the one dimensional periodic Anderson model at finite
  U

abstract:
  We present exact results for the periodic Anderson model for finite Hubbard
interaction 0 <= U < +infinity on certain restricted domains of the model's
phase diagram, in d=1 dimension. Decomposing the Hamiltonian into positive
semidefinite terms we find two quantum states to be ground state, an insulating
and a metallic one. The ground state energy and several ground state
expectation values are calculated.


title:
The first-order phase transition between dimerized-antiferromagnetic and
  uniform-antiferromagnetic phases in Cu_(1-x)M_xGeO_3

abstract:
  We have performed detailed magnetic susceptibility measurements as well as
synchrotron x-ray diffraction studies to determine the temperature vs
concentration ($T$ - $x$) phase diagram of Cu${}_{1-x}$Mg${}_x$GeO${}_3$. We
observe clear double peaks in the magnetic susceptibility implying two
antiferromagnetic (AF) transition temperatures in samples with Mg
concentrations in the range 0.0237 $\le x \le$ 0.0271. We also observe a
drastic change in the inverse correlation length in this concentration range by
x-ray diffraction. The drastic change of the AF transition temperature as well
as the disappearance of the spin-Peierls (SP) phase have been clarified; these
results are consistent with a first-order phase transition between dimerized AF
(D-AF) and uniform AF (U-AF) phases as reported by T. Masuda {\it et al.}
\lbrack Phys. Rev. Lett. {\bf 80}, 4566 (1998)\rbrack. The $T$ - $x$ phase
diagram of Cu${}_{1-x}$Zn${}_x$GeO${}_3$ is similar to that of
Cu${}_{1-x}$Mg${}_x$GeO${}_3$, which suggests that the present phase transition
is universal for Cu${}_{1-x}M_{x}$GeO${}_3$.


title: The anomalous metallic ferromagnetic state of Sr doped manganites

abstract:
  We deduce a model relevant for the anomalous metallic state of Sr doped
manganites at low temperatures within the ferromagnetic phase. It provides a
natural explanation to several anomalous features observed experimentally, such
as the vanishing Drude contribution in optical conductivity, the pseudo-gap in
the density of states, an the unusual dispersion observed in photoemission.


title:
Impurity scattering and frequency dependent conductivity in spin density
  waves

abstract:
  The quasiparticle contribution to the frequency dependent electric
conductivity in the presence of randomly distributed impurities is calculated
within mean field theory for spin density waves as formed in quasi one
dimensional conductors like Bechgaard salts. Interchain hopping is taken into
account and the effects of imperfect nesting are considered. In case of an
electric field perpendicular to the chain direction there is no collective
contribution to the conductivity and our results are directly applicable to
recent measurements on both (TMTSF)2PF6 and quenched (TMTSF)2ClO4. The
experimental data are well described by the theory in the clean limit, with
somewhat larger scattering in the ClO4 salt.


title:
Oscillatory Behavior of Critical Amplitudes of the Gaussian Model on a
  Hierarchical Structure

abstract:
  We studied oscillatory behavior of critical amplitudes for the Gaussian model
on a hierarchical structure presented by a modified Sierpinski gasket lattice.
This model is known to display non-standard critical behavior on the lattice
under study. The leading singular behavior of the correlation length $\xi$ near
the critical coupling $K=K_c$ is modulated by a function which is periodic in
$\ln|\ln(K_c-K)|$. We have also shown that the common finite-size scaling
hypothesis, according to which for a finite system at criticality $\xi$ should
be of the order of the size of system, is not applicable in this case. As a
consequence of this, the exact form of the leading singular behavior of $\xi$
differs from the one described earlier (which was based on the finite-size
scaling assumption).


title:
Diamagnetic Susceptibility and Current Distributions in Granular
  Superconductors at Percolation

abstract:
  Comments: 4 pages RevTeX, 4 Postscript figures. References added. We study a
two-dimensional granular superconducting network at the percolation threshold
under the influence of an external perpendicular magnetic field. By numerical
simulations on the full nonlinear problem, we determine the scaling exponent
for the magnetic susceptibility. Further, we report on the scaling properties
of the current distribution. The scaling of the current is found to be
independent of the value of the magnetic field. Our results are in
contradiction with previous numerical results based on linearized equations. We
find a value for the susceptibility exponent which does not agree with existing
theoretical suggestions, but agrees perfectly with renormalization group
calculations.


title: Spectra of Euclidean Random Matrices

abstract:
  We study the spectrum of a random matrix, whose elements depend on the
Euclidean distance between points randomly distributed in space. This problem
is widely studied in the context of the Instantaneous Normal Modes of fluids
and is particularly relevant at the glass transition. We introduce a systematic
study of this problem through its representation by a field theory. In this way
we can easily construct a high density expansion, which can be resummed
producing an approximation to the spectrum similar to the Coherent Potential
Approximation for disordered systems.


title:
The low-energy theory for the Bose-Hubbard model and the normal ground
  state of bosons

abstract:
  A bosonic realization of the SU(2) Lie algebra and of its vector
representation is constructed, and an effective low-energy description of the
Bose-Hubbard model in the form of anisotropic theory of quantum rotors is
proposed and discussed. A possibility of a normal zero-temperature bosonic
phase with neither crystalline nor superfluid order around the tip of the
checkerboard-solid lobe at half-integer fillings is examined.


title:
Incompressible paired Hall state, stripe order and the composite fermion
  liquid phase in half-filled Landau levels

abstract:
  We consider the two lowest Landau levels at half filling. In the higher
Landau level (nu =5/2), we find a first order phase transition separating a
compressible striped phase from a paired quantum Hall state, which is
identified as the Moore-Read state. The critical point is very near the Coulomb
potential and the transition can be driven by increasing the width of the
electron layer. We find a much weaker transition (either second order or a
crossover) from pairing to the composite fermion Fermi liquid behavior. A very
similar picture is obtained for the lowest Landau level but the transition
point is not near the Coulomb potential.


title:
Extremal collision sequences of particles on a line: optimal
  transmission of kinetic energy

abstract:
  The transmission of kinetic energy through chains of inelastically colliding
spheres is investigated for the case of constant coefficient of restitution
\epsilon=const and impact-velocity dependent coefficient \epsilon(v) for
viscoelastic particles. We derive a theory for the optimal distribution of
particle masses which maximize the energy transfer along the chain and check it
numerically. We found that for \epsilon=const the mass distribution is a
monotonous function which does not depend on the value of \epsilon. In
contrast, for \epsilon(v) the mass distribution reveals a pronounced maximum,
depending on the particle properties and on the chain length. The system
investigated demonstrates that even for small and simple systems the velocity
dependence of the coefficient of restitution may lead to new effects with
respect to the same systems under the simplifying approximation \epsilon=const.


title: Stability of a vortex in a small trapped Bose-Einstein condensate

abstract:
  A second-order expansion of the Gross-Pitaevskii equation in the interaction
parameter determines the thermodynamic critical angular velocity Omega_c for
the creation of a vortex in a small axisymmetric condensate. Similarly, a
second-order expansion of the Bogoliubov equations determines the (negative)
frequency omega_a of the anomalous mode. Although Omega_c = -omega_a through
first order, the second-order contributions ensure that the absolute value
|omega_a| is always smaller than the critical angular velocity Omega_c. With
increasing external rotation Omega, the dynamical instability of the condensate
with a vortex disappears at Omega*=|omega_a|, whereas the vortex state becomes
energetically stable at the larger value Omega_c. Both second-order
contributions depend explicitly on the axial anisotropy of the trap. The
appearance of a local minimum of the free energy for a vortex at the center
determines the metastable angular velocity Omega_m. A variational calculation
yields Omega_m=|\omega_a| to first order (hence Omega_m also coincides with the
critical angular velocity Omega_c to this order). Qualitatively, the scenario
for the onset of stability in the weak-coupling limit is the same as that found
in the strong-coupling (Thomas-Fermi) limit.


title:
Systematic Cu-63 NQR studies of the stripe phase in
  La(1.6-x)Nd(0.4)Sr(x)CuO(4) for 0.07 <= x <= 0.25

abstract:
  We demonstrate that the integrated intensity of Cu-63 nuclear quadrupole
resonance (NQR) in La(1.6-x)Nd(0.4)Sr(x)CuO(4) decreases dramatically below the
charge-stripe ordering temperature T(charge). Comparison with neutron and X-ray
scattering indicates that the wipeout fraction F(T) (i.e. the missing fraction
of the integrated intensity of the NQR signal) represents the charge-stripe
order parameter. The systematic study reveals bulk charge-stripe order
throughout the superconducting region 0.07 <= x <= 0.25. As a function of the
reduced temperature t = T/T(charge), the temperature dependence of F(t) is
sharpest for the hole concentration x=1/8, indicating that x=1/8 is the optimum
concentration for stripe formation.


title:
63Cu NQR evidence of dimensional crossover to anisotropic 2d regime in
  S= 1/2 three-leg ladder Sr2Cu3O5

abstract:
  We probed spin-spin correlations up to 725 K with 63Cu NQR in the S= 1/2
three-leg ladder Sr2Cu3O5. We present experimental evidence that below 300 K,
weak inter-ladder coupling causes dimensional crossover of the spin-spin
correlation length \xi from quasi-1d (\xi ~ 1/T) to anisotropic 2d regime (\xi
\~ exp[2\pi\rho_{s}/T], where 2\pi\rho_{s} = 290 +/- 30 K is the effective spin
stiffness). This is the first experimental verification of the renormalized
classical behavior of the anisotropic non-linear sigma model in 2d, which has
been recently proposed for the striped phase in high T_{c} cuprates.