title:
Fundamental measure theory for mixtures of parallel hard cubes. II.
  Phase behavior of the one-component fluid and of the binary mixture

abstract:
  A previously developed fundamental measure fucntional [J. Chem. Phys.
vol.107, 6379 (1997)] is used to study the phase behavior of a system of
parallel hard cubes. The single-component fluid exhibits a continuous
transition to a solid with an anomalously large density of vacancies. The
binary mixture has a demixing transition for edge-length ratios below 0.1.
Freezing in this mixture reveals that at least the phase rich in large cubes
lies in the region where the uniform fluid is unstable, hence suggesting a
fluid-solid phase separation. A method is develop to study very asymmetric
binary mixtures by taking the limit of zero size ratio (scaling the density and
fugacity of the solvent as appropriate) in the semi-grand ensemble where the
chemical potential of the solvent is fixed. With this procedure the mixture is
exactly mapped onto a one-component fluid of parallel adhesive hard cubes. At
any density and solvent fugacity the large cubes are shown to collapse into a
close-packed solid. Nevertheless the phase diagram contains a large
metastability region with fluid and solid phases. Upon introduction of a slight
polydispersity in the large cubes the system shows the typical phase diagram of
a fluid with an isostructural solid-solid transition (with the exception of a
continuous freezing). Consequences about the phase behavior of binary mixtures
of hard core particles are then drawn.


title: Ordering Phenomena on Growing Films

abstract:
  In many growth processes particles are highly mobile in an active layer at
the surface, but are relatively immobile once incorporated in the bulk. We
study models in which atoms are allowed to interact, equilibrate, and order on
the surface, but are frozen in the bulk. Order parameter correlations in the
resulting bulk material are highly anisotropic, reflecting its growth history.
In a flat (layer by layer) growth mode, correlations perpendicular to the
growth direction are similar to a two dimensional system in equilibrium, while
parallel correlations reflect the dynamics of such a system. When the growing
film is rough, various couplings between height and order parameter
fluctuations are possible. Such couplings modify the dynamic scaling properties
of surface roughness, and may also change the critical behavior of the order
parameter. Even the deterministic growth of the surface profile can result in
interesting textures for the order parameter.


title:
Second harmonic generation and birefringence of some ternary pnictide
  semiconductors

abstract:
  A first-principles study of the birefringence and the frequency dependent
second harmonic generation (SHG) coefficients of the ternary pnictide
semiconductors with formula ABC$_2$ (A = Zn, Cd; B = Si, Ge; C = As, P) with
the chalcopyrite structures was carried out. We show that a simple empirical
observation that a smaller value of the gap is correlated with larger value of
SHG is qualitatively true. However, simple inverse power scaling laws between
gaps and SHG were not found. Instead, the real value of the nonlinear response
is a result of a very delicate balance between different intraband and
interband terms.


title:
Applications of the generalized gradient approximation to ferroelectric
  perovskites

abstract:
  The Perdew-Burke-Ernzerhof generalized gradient approximation to the density
functional theory is tested with respect to sensitivity to the choice of the
value of the parameter $\kappa$, which is associated to the degree of
localization of the exchange-correlation hole. A study of structural and
dynamical properties of four selected ferroelectric perovskites is presented.
The originally proposed value of $\kappa$=0.804 %(best suited for atoms and
molecules) works well for some solids, whereas for the ABO$_3$ perovskites it
must be decreased in order to predict equilibrium lattice parameters in good
agreement with experiments. The effects on the structural instabilities and
zone center phonon modes are examined. The need of varying $\kappa$ from one
system to another reflects the fact that the localization of the
exchange-correlation hole is system dependent, and the sensitivity of the
structural properties to its actual value illustrates the necessity of finding
a universal function for $\kappa$.


title:
Phase Separation in Models for Manganites: Theoretical Aspects and
  Comparison with Experiments

abstract:
  This paper is an informal review of the present status of the computational
studies of models for manganites that recently reported the existence of phase
separation at low temperatures (S. Yunoki et al., Phys. Rev. Lett. 80, 845
(1998) and cond-mat/9807149). Results for both the one-orbital and two-orbital
models, the latter with Jahn-Teller phonons, are included. A large list of
experimental results discussed in the paper are compatible with the existence
of charge-inhomogeneities in the manganites, which may be an important
ingredient for the explanation of the CMR effect in these compounds.


title: Spectral function and quasiparticle weight in the generalized t-J model

abstract:
  We extend to the spectral function an approach which allowed us to calculate
the quasiparticle weight for destruction of a real electron Z_c sigma (k) (in
contrast to that of creation of a spinless holon Z_h(k) in a generalized $t-J$
model, using the self-consistent Born approximation (SCBA). We compare our
results with those obtained using the alternative approach of Sushkov et al.,
which also uses the SCBA. The results for Z_c sigma (k) are also compared with
results obtained using the string picture and with exact diagonalizations of a
32-site square cluster. While on a qualitative level, all results look similar,
our SCBA approach seems to compare better with the ED one. The effect of
hopping beyond nearest neighbors, and that of the three-site term are
discussed.


title:
Numerical test of the damping time of layer-by-layer growth on
  stochastic models

abstract:
  We perform Monte Carlo simulations on stochastic models such as the
Wolf-Villain (WV) model and the Family model in a modified version to measure
mean separation $\ell$ between islands in submonolayer regime and damping time
$\tilde t$ of layer-by-layer growth oscillations on one dimension. The
stochastic models are modified, allowing diffusion within interval $r$ upon
deposited. It is found numerically that the mean separation and the damping
time depend on the diffusion interval $r$, leading to that the damping time is
related to the mean separation as ${\tilde t} \sim \ell^{4/3}$ for the WV model
and ${\tilde t} \sim \ell^2$ for the Family model. The numerical results are in
excellent agreement with recent theoretical predictions.


title: Quasi-particle dynamics of a strongly correlated polaron metal

abstract:
  We develop a simple diagrammatic low-energy boson exchange strong-coupling
model for an antiferromagnetically correlated polaronic metal. Important
low-temperature features of such a model are (i) a large polaronic mass
enhancement $Z$ and a strongly narrowed, low-energy, near-Fermi-level van Hove
resonance peak in the interacting density of states at temperatures $T$ well
below the lattice tunneling energy scale $\Omega_t$; (ii) a rapid thermal
suppression of $Z$ and of the van Hove resonance which occurs when the
temperature $T$ becomes comparable to the lattice tunneling excitation energy
scale $\Omega_t$; and (iii) strong isotopic mass dependence of the electronic
mass enhancement $Z$ at temperatures $T$ well below $\Omega_t$. Surprisingly,
in spite of the large low-$T$ mass enhancment, the anharmonic lattice
fluctuations in the high temperature regime $T\gsim\Omega_t$, give rise only to
a moderate, essentially $T$-independent quasi-particle damping. We suggest
possible low temperature isotope experiments to explore polaronic fluctuations
in the cuprates superconductors.


title:
Beyond paired quantum Hall states: parafermions and incompressible
  states in the first excited Landau level

abstract:
  The Pfaffian quantum Hall states, which can be viewed as involving pairing
either of spin-polarized electrons or of composite fermions, are generalized by
finding the exact ground states of certain Hamiltonians with k+1-body
interactions, for all integers k > 0. The remarkably simple wavefunctions of
these states involve clusters of k particles, and are related to correlators of
parafermion currents in two-dimensional conformal field theory. The k=2 case is
the Pfaffian. For k > 1, the quasiparticle excitations of these systems are
expected to possess nonabelian statistics, like those of the Pfaffian. For k=3,
these ground states have large overlaps with the ground states of the (2-body)
Coulomb-interaction Hamiltonian for electrons in the first excited Landau level
at total filling factors \nu=2+3/5, 2+2/5.


title: Liesegang patterns: Effect of dissociation of the invading electrolyte

abstract:
  The effect of dissociation of the invading electrolyte on the formation of
Liesegang bands is investigated. We find, using organic compounds with known
dissociation constants, that the spacing coefficient, 1+p, that characterizes
the position of the n-th band as x_n ~ (1+p)^n, decreases with increasing
dissociation constant, K_d. Theoretical arguments are developed to explain
these experimental findings and to calculate explicitly the K_d dependence of
1+p.


title:
Second-Order Phase Transition Induced by Deterministic Fluctuations in
  Aperiodic Eight-State Potts Models

abstract:
  We investigate the influence of aperiodic modulations of the exchange
interactions between nearest-neighbour rows on the phase transition of the
two-dimensional eight-state Potts model. The systems are studied numerically
through intensive Monte Carlo simulations using the Swendsen-Wang cluster
algorithm for different aperiodic sequences. The transition point is located
through duality relations, and the critical behaviour is investigated using FSS
techniques at criticality. While the pure system exhibits a first-order
transition, we show that the deterministic fluctuations resulting from the
aperiodic coupling distribution are liable to modify drastically the physical
properties in the neighbourhood of the transition point. For strong enough
fluctuations of the sequence under consideration, a second-order phase
transition is induced. The exponents $\beta/\nu$, $\gamma /\nu$ and
$(1-\alpha)/\nu$ are obtained at the new fixed point and crossover effects are
discussed. Surface properties are also studied.


title: Breathers on lattices with long range interaction

abstract:
  We analyze the properties of breathers (time periodic spatially localized
solutions) on chains in the presence of algebraically decaying interactions
$1/r^s$. We find that the spatial decay of a breather shows a crossover from
exponential (short distances) to algebraic (large distances) decay. We
calculate the crossover distance as a function of $s$ and the energy of the
breather. Next we show that the results on energy thresholds obtained for short
range interactions remain valid for $s>3$ and that for $s < 3$ (anomalous
dispersion at the band edge) nonzero thresholds occur for cases where the short
range interaction system would yield zero threshold values.


title:
Kinetics and thermodynamics across single-file pores: solute
  permeability and rectified osmosis

abstract:
  We study the effects of solute interactions on osmotic transport through
pores. By extending single-file, single-species kinetic models to include
entrance of solute into membrane pores, we model the statistical mechanics of
competitive transport of two species across membrane pores. The results have
direct applications to water transport across biomembrane pores and particle
movement in zeolites, and can be extended to study ion channel transport.
Reflection coefficients, the reduction of osmotic fluxes measured using
different solutes, are computed in terms of the microscopic kinetic parameters.
We find that a reduction in solvent flow due to solute-pore interactions can be
modelled by a Langmuir adsorption isotherm. Osmosis experiments are discussed
and proposed. Special cases and Onsager relations are presented in the
Appendices.


title: Winding angle distribution of 2D random walks with traps

abstract:
  We study analytically the asymptotic behaviour of the average probability
P(n,t) for the trajectory of a 2D Brownian particle wandering in the presence
of randomly distributed traps to wind n times around a given point after a time
t. It is shown that P(n,t)\sim\exp(-c\sqrt{t}) (1+x^2)^{-1} with x\sim
n/\sqrt{t}, where the first exponent represents a well known long-time tail of
the probability that a particle will not be trapped.


title: Current responsivity of semiconductor superlattice THz-photon detectors

abstract:
  The current responsivity of a semiconductor superlattice THz-photon detector
is calculated using an equivalent circuit model which takes into account the
finite matching efficiency between a detector antenna and the superlattice in
the presence of parasitic losses. Calculations performed for currently
available superlattice diodes show that both the magnitudes and the roll-off
frequencies of the responsivity are strongly influenced by an excitation of
hybrid plasma-Bloch oscillations which are found to be eigenmodes of the system
in the THz- frequency band. The expected room temperature values of the
responsivity (2-3 A/W in the 1-3 THz-frequency band) range up to several
percents of the quantum efficiency $e/\hbar\omega$ of an ideal superconductor
tunnel junction detector. Properly designed semiconductor superlattice
detectors may thus demonstrate better room temperature THz-photon responsivity
than conventional Schottky junction devices.


title: A microscopic model for mixed surfactant vesicles

abstract:
  A microscopic model which has proven useful in describing amphiphilic
aggregates as inhomogeneities of a fluid is extended here to study the case of
a two component surfactant mixture. We have chosen an effective interaction
between the amphiphiles that mimics the mixture of cationic--anionic
surfactants. In agreement with experiments, and other theoretical approaches,
we find regions where spherical vesicles are stable, with a well defined
radius. The experimental dependence of the radius on the chemical environment
is also qualitatively reflected in our model.


title:
Monte Carlo and Molecular Dynamics Simulation of the Glass Transition of
  Polymers

abstract:
  Two coarse-grained models for polymer chains in dense glass-forming polymer
melts are studied by computer simulation: the bond-fluctuation model on a
simple cubic lattice, where a bond-length potential favors long bonds, is
treated by dynamic Monte Carlo methods, and a bead-spring model in the
continuum with a Lennard-Jones potential between the beads is treated by
Molecular Dynamics. While the dynamics of both models differ for short length
scales and associated time scales, on mesoscopic spatial and temporal scales
both models behave similarly. In particular, the mode coupling theory of the
glass transition can be used to interpret the slowing down of the undercooled
polymer melt. For the off-lattice model, the approach to the critical point of
mode coupling is both studied for constant pressure and for constant volume.
The lattice model allows a test of the Gibbs-Di Marzio entropy theory of the
glass transition, and our finding is that although the entropy does decrease
significantly, there is no ``entropy catastrophe'', where the fluid entropy
would turn negative. Finally, an outlook on confinement effects on the glass
transition in thin film geometry is given.


title:
Point contact spectroscopy and temperature dependence of resistivity of
  metallic sodium tungsten bronzes -Role of optical phonons

abstract:
  In this paper we report the results of electrical resistivity (1.5K < T
<300K) and point contact spectroscopy (PCS) measurements on single crystals of
metallic sodium tungsten bronze with varying sodium content. We have shown that
the electron-phonon coupling function as measured through PCS can explain
quantitatively the large temperature dependence of resistivity $\rho$ seen in
these materials over the entire temperature range. The electron-phonon coupling
function shows predominatly large peaks for phonon frequency range of 30meV <
$\omega$ 100meV which match well with the calculated optical phonon modes for
WO$_6$ octahedra. The integrated electron-phonon coupling constant $\lambda$
from this data is $\approx$ 0.25-0.45.


title:
Lattice $\phi^4$ theory of finite-size effects above the upper critical
  dimension

abstract:
  We present a perturbative calculation of finite-size effects near $T_c$ of
the $\phi^4$ lattice model in a $d$-dimensional cubic geometry of size $L$ with
periodic boundary conditions for $d > 4$. The structural differences between
the $\phi^4$ lattice theory and the $\phi^4$ field theory found previously in
the spherical limit are shown to exist also for a finite number of components
of the order parameter. The two-variable finite-size scaling functions of the
field theory are nonuniversal whereas those of the lattice theory are
independent of the nonuniversal model parameters.One-loop results for
finite-size scaling functions are derived. Their structure disagrees with the
single-variable scaling form of the lowest-mode approximation for any finite
$\xi/L$ where $\xi$ is the bulk correlation length. At $T_c$, the large-$L$
behavior becomes lowest-mode like for the lattice model but not for the
field-theoretic model. Characteristic temperatures close to $T_c$ of the
lattice model, such as $T_{max}(L)$ of the maximum of the susceptibility
$\chi$, are found to scale asymptotically as $T_c - T_{max}(L) \sim L^{-d/2}$,
in agreement with previous Monte Carlo (MC) data for the five-dimensional Ising
model. We also predict $\chi_{max} \sim L^{d/2}$ asymptotically. On a
quantitative level, the asymptotic amplitudes of this large -$L$ behavior close
to $T_c$ have not been observed in previous MC simulations at $d = 5$ because
of nonnegligible finite-size terms $\sim L^{(4-d)/2}$ caused by the
inhomogeneous modes. These terms identify the possible origin of a significant
discrepancy between the lowest-mode approximation and previous MC data. MC data
of larger systems would be desirable for testing the magnitude of the
$L^{(4-d)/2}$ and $L^{4-d}$ terms predicted by our theory.


title:
Dynamic transition in vortex flow in strongly disordered Josephson
  junction arrays and superconducting thin films

abstract:
  We study the dynamics of vortices in strongly disordered $d=2$ Josephson
junction arrays and superconducting films driven by a current. We find a
dynamic phase transition in vortex flow at a current $I_p>I_c$. Below $I_p$
there is plastic flow characterized by an average-velocity correlation length
scale $\xi_v$ in the direction of motion, which diverges when approaching
$I_p$. Above $I_p$ we find a moving vortex phase with homogeneous flow and
short range smectic order. A finite size analysis shows that this phase becomes
asymptotically a liquid for large length scales.


title:
New integrable model of correlated electrons with off-diagonal
  long-range order from $so(5)$ symmetry

abstract:
  We present a new integrable model for correlated electrons which is based on
a $so(5)$ symmetry. By using an $\eta$-pairing realization we construct
eigenstates of the Hamiltonian with off-diagonal long-range order. It is also
shown that these states lie in the ground state sector. We exactly solve the
model on a one-dimensional lattice by the Bethe ansatz.


title: Optimizing Traffic Flow

abstract:
  We present an economics-based method for deciding the optimal rates at which
vehicles are allowed to enter a highway. The method exploits the naturally
occuring fluctuations of traffic flow and is flexible enough to adapt in real
time to the transient flow characteristics of road traffic. Simulations based
on realistic parameter values show that this strategy is feasible for naturally
occuring traffic, and that even far from optimality, injection policies can
improve traffic flow. Our results also allow a better understanding of the high
flows observed in ``synchronized'' congested traffic close to ramps.


title:
Predominantly Superconducting Origin of Large Energy Gaps in Underdoped
  Bi2Sr2CaCu2O8-d from Tunneling Spectroscopy

abstract:
  New tunneling data are reported in underdoped Bi2Sr2CaCu2O8-d using
superconductor-insulator-superconductor break junctions. Energy gaps, Delta, of
51+2, 54+2 and 57+3 meV are observed for three crystals with Tc=77, 74, and 70
K respectively. These energy gaps are nearly three times larger than for
overdoped crystals with similar Tc. Detailed examination of tunneling spectra
over a wide doping range from underdoped to overdoped, including the Josephson
IcRn product, indicate that these energy gaps are predominantly of
superconducting origin.


title: Far infrared giant dipole resonances in neutral quantum dots

abstract:
  A resonance behaviour of the far infrared absorption probability at a
frequency \sim N^{1/4} is predicted for clusters of N electron-hole pairs (2\le
N\le 110) confined in disk-shaped quantum dots. For radially symmetric dots,
the absorption is dominated by a Giant Dipole Resonance, which accounts for
more than 98 % of the energy-weighted photoabsorption sum rule.


title:
Effect of anisotropy on universal transport in unconventional
  superconductors

abstract:
  We investigate the universal electronic transport for a mixed
$d_{x^2-y^2}$+s-wave superconductor in the presence of an anisotropic
elliptical Fermi surface. Similar to the universal low-temperature transport
predicted in a $d_{x^2-y^2}$-wave superconductor with a circular Fermi surface,
anisotropic universal features are found in the low-temperature microwave
conductivity, and thermal conductivity in the anisotropic system. The effects
of anisotropy on the penetration depth, impurity induced $T_c$ suppression, and
the zero-frequency density of states are also considered. While a small amount
of anisotropy can lead to a strong suppression of the effective scattering rate
and hence the density of states at zero frequency, experimental data suggests
that large effects are restored by a negative $s$-component gap admixture.


title: Vortices in Quantum Spin Systems

abstract:
  We examine spin vortices in ferromagnetic quantum Heisenberg models with
planar anisotropy on two-dimensional lattices. The symmetry properties and the
time evolution of vortices built up from spin-coherent states are studied in
detail. Although these states show a dispersion typical for wave packets,
important features of classical vortices are conserved. Moreover, the results
on symmetry properties provide a construction scheme for vortex-like
excitations from exact eigenstates, which have a well-controlled time
evolution. Our approach works for arbitrary spin length both on triangular and
square lattices.


title:
Chaos and quantum interference effect in semiconductor ballistic
  micro-structures

abstract:
  We study the quantum-interference effect in the ballistic Aharonov-Bohm (AB)
billiard. The wave-number averaged conductance and the correlation function of
the non-averaged conductance are calculated by use of semiclassical theory.
Chaotic and regular AB billiards have turned out to lead to qualitatively
different semiclassical formulas for the conductance with their behavior
determined only by knowledge regarding the underlying classical scattering.


title: Josephson Current between Triplet and Singlet Superconductors

abstract:
  The Josephson effect between triplet and singlet superconductors is studied.
Josephson current can flow between triplet and singlet superconductors due to
the spin-orbit coupling in the spin-triplet superconductor but it is finite
only when triplet superconductor has $L_z=-S_z=\pm 1$, where $L_z$ and $S_z$
are the perpendicular components of orbital angular momentum and spin angular
momentum of the triplet Cooper pairs, respectively. The recently observed
temperature and orientational dependence of the critical current through a
Josephson junction between UPt$_3$ and Nb is investigated by considering a
non-unitary triplet state.


title: Atomistic modelling of large-scale metal film growth fronts

abstract:
  We present simulations of metallization morphologies under ionized sputter
deposition conditions, obtained by a new theoretical approach. By means of
molecular dynamics simulations using a carefully designed interaction
potential, we analyze the surface adsorption, reflection, and etching reactions
taking place during Al physical vapor deposition, and calculate their relative
probability. These probabilities are then employed in a feature-scale
cellular-automaton simulator, which produces calculated film morphologies in
excellent agreement with scanning-electron-microscopy data on ionized sputter
deposition.


title:
Exact diagonalization of the S=1/2 Heisenberg antiferromagnet on finite
  bcc lattices to estimate properties on the infinite lattice

abstract:
  Here we generate finite bipartite body-centred cubic lattices up to 32
vertices. We have studied the spin one half Heisenberg antiferromagnet by
diagonalizing its Hamiltonian on each of the finite lattices and hence
computing its ground state properties. By extrapolation of these data we obtain
estimates of the T = 0 properties on the infinite bcc lattice. Our estimate of
the T = 0 energy agrees to five parts in ten thousand with third order spin
wave and series expansion method estimates, while our estimate of the staggered
magnetization agrees with the spin wave estimate to within a quarter of one
percent.


title: Localization of spinwaves in the quantum Hall ferromagnet

abstract:
  The quantum Hall ferromagnet at filling fraction $\nu = 1$ has some unusual
properties due to the remarkable identity between the topological density of a
spin distortion and the associated electrical charge density. We investigate
the localization of spinwaves by coupling to a scalar disorder potential via
their topological density. A low energy description of the system in terms of a
non-linear sigma model of unitary supermatrices is derived. All states of this
model are localized in two dimensions. A possible experimental signature of
these effects in photo-luminescence is suggested.


title:
Intrinsic profiles and capillary waves at homopolymer interfaces: a
  Monte Carlo study

abstract:
  A popular concept which describes the structure of polymer interfaces by
``intrinsic profiles'' centered around a two dimensional surface, the ``local
interface position'', is tested by extensive Monte Carlo simulations of
interfaces between demixed homopolymer phases in symmetric binary (AB)
homopolymer blends, using the bond fluctuation model. The simulations are done
in an LxLxD geometry. The interface is forced to run parallel to the LxL planes
by imposing periodic boundary conditions in these directions and fixed boundary
conditions in the D direction, with one side favoring A and the other side
favoring B. Intrinsic profiles are calculated as a function of the ``coarse
graining length'' B by splitting the system into columns of size BxBxD and
averaging in each column over profiles relative to the local interface
position. The results are compared to predictions of the self-consistent field
theory. It is shown that the coarse graining length can be chosen such that the
interfacial width matches that of the self-consistent field profiles, and that
for this choice of B the ``intrinsic'' profiles compare well with the
theoretical predictions.


title: Mossbauer effect for dark solitons in Bose-Einstein condensates

abstract:
  We show that the energetic instability of dark solitons is associated with
particle-like motion, and present a simple equation of motion, based on the
M\"ossbauer effect, for dark solitons propagating in inhomogeneous Thomas-Fermi
clouds. Numerical simulations support our theory. We discuss some experimental
approaches.

categories: cond-mat nlin.PS patt-sol physics.atom-ph quant-ph


title:
Free-carrier screening of polarization fields in wurtzite GaN/InGaN
  laser structures

abstract:
  The free-carrier screening of macroscopic polarization fields in wurtzite
GaN/InGaN quantum wells lasers is investigated via a self-consistent
tight-binding approach. We show that the high carrier concentrations found
experimentally in nitride laser structures effectively screen the built-in
spontaneous and piezoelectric polarization fields, thus inducing a
``field-free'' band profile. Our results explain some heretofore puzzling
experimental data on nitride lasers, such as the unusually high lasing
excitation thresholds and emission blue-shifts for increasing excitation
levels.


title: Stability of Designed Proteins Against Mutations

abstract:
  The stability of model proteins with designed sequences is assessed in terms
of the number of sequences (obtained from the designed sequence through
mutations), which fold into 5the ``native'' conformation. By a complete
enumeration of the total number of sequences obtained by introducing up to 4
point mutations and up to 7 composition--conserving mutations (swapping of
amino acids) in a 36mers chain, it is found that there are $10^8-10^9$
sequences which in the folding process target onto the ``native'' conformation.
Consequently, proteins with designed sequences display a remarkable degree of
stability and, to a large extent, of designability.


title:
Stripe Stability in the Extended t-J Model on Planes and Four-Leg
  Ladders

abstract:
  The tendencies to phase-separation and stripe formation of the t-J model on
planes and four-leg ladders have been here reexamined including hole hopping
terms t', t'' beyond nearest-neighbor sites. The motivation for this study is
the growing evidence that such terms are needed for a quantitative description
of the cuprates. Using a variety of computational techniques it is concluded
that the stripe tendencies considerably weaken when experimentally realistic
t'<0, t''>0 for hole-doped cuprates are considered. However, a small t'>0
actually enhances the stripe formation.


title:
Finite-Size Scaling and Damage Spreading in Ising Systems with Multispin
  Interactions

abstract:
  We investigate two-dimensional Ising systems with multisping interactions of
three- (m=3) and four-body terms (m=4). The application of a new type of
finite-size algorithm of de Oliveira allow us to clearly distinguish a
first-order transition (in the m=4 case) from a continuous one (in the m=3
one). We also study the damage spreading in these systems. In this study, a
dynamical phenomenon is observed to occur at a critical point separating a
chaotic phase from a frozen one. However, the width of the interval where this
transition happens does not yield a conclusive evidence about the order of the
phase transition.


title:
Correlation and Dimerization Effects on the Physical Behavior of the
  $NR_4 [Ni(dmit)_2]_2$ Charge Transfer Salts : A DMRG Study of the
  Quarter-Filling t-J Model

abstract:
  The present work studies the quasi one-dimensional $Ni(dmit)_2$-based
compounds within a correlated model. More specifically, we focus our attention
on the composed influence of the electronic dimerization-factor and the
repulsion, on the transport properties and the localization of the electronic
density in the ground-state. Those properties are studied through the
computation of the charge gaps (difference between the ionization potential and
the electro-affinity: IP-EA) and the long- and short-bond orders of an infinite
quarter-filled chain within a $t-J(t,U)$ model. The comparison between the
computed gaps and the experimental activation energy of the semiconductor
$NH_2Me_2 [Ni(dmit)_2]_2$ allows us to estimate the on-site electronic
repulsion of the $Ni(dmit)_2$ molecule to $1.16eV$.


title: Thickness dependent magnetotransport in ultra-thin manganite films

abstract:
  To understand the near-interface magnetism in manganites, uniform, ultra-thin
films of La_{0.67}Sr_{0.33}MnO_3 were grown epitaxially on single crystal (001)
LaAlO_3 and (110) NdGaO_3 substrates. The temperature and magnetic field
dependent film resistance is used to probe the film's structural and magnetic
properties. A surface and/or interface related dead-layer is inferred from the
thickness dependent resistance and magnetoresistance. The total thickness of
the dead layer is estimated to be $\sim 30 \AA$ for films on NdGaO_3 and $\sim
50 \AA$ for films on LaAlO_3.


title:
Design of Sequences with Good Folding Properties in Coarse-Grained
  Protein Models

abstract:
  Background: Designing amino acid sequences that are stable in a given target
structure amounts to maximizing a conditional probability. A straightforward
approach to accomplish this is a nested Monte Carlo where the conformation
space is explored over and over again for different fixed sequences, which
requires excessive computational demand. Several approximate attempts to remedy
this situation, based on energy minimization for fixed structure or high-$T$
expansions, have been proposed. These methods are fast but often not accurate
since folding occurs at low $T$.
  Results: We develop a multisequence Monte Carlo procedure, where both
sequence and conformation space are simultaneously probed with efficient
prescriptions for pruning sequence space. The method is explored on
hydrophobic/polar models. We first discuss short lattice chains, in order to
compare with exact data and with other methods. The method is then successfully
applied to lattice chains with up to 50 monomers, and to off-lattice 20-mers.
  Conclusions: The multisequence Monte Carlo method offers a new approach to
sequence design in coarse-grained models. It is much more efficient than
previous Monte Carlo methods, and is, as it stands, applicable to a fairly wide
range of two-letter models.

categories: cond-mat.soft cond-mat.dis-nn hep-lat physics.bio-ph physics.chem-ph q-bio


title: The Kondo Box: A Magnetic Impurity in an Ultrasmall Metallic Grain

abstract:
  We study the Kondo effect generated by a single magnetic impurity embedded in
an ultrasmall metallic grain, to be called a ``Kondo box''. We find that the
Kondo resonance is strongly affected when the mean level spacing in the grain
becomes larger than the Kondo temperature, in a way that depends on the parity
of the number of electrons on the grain. We show that the single-electron
tunneling conductance through such a grain features Kondo-induced Fano-type
resonances of measurable size, with an anomalous dependence on temperature and
level spacing.


title: Enumeration of States in a Periodic Glass

abstract:
  We present an analytic enumeration of the metastable states, $N_s$, in a
periodic long-range Josephson array frustrated by a transverse field. We find
that the configurational entropy, $S_{conf} \equiv \ln N_s$, is extensive and
scales with frustration, confirming that the non-random system is glassy. We
also find that $S_{conf}$ is different from that of its disordered analogue,
despite that fact that the two models share the same dynamical equations.


title: Structure and Stability of an Amorphous Metal

abstract:
  Using molecular dynamics simulations, with a realistic many-body
embedded-atom potential, and a novel method to characterize local order, we
study the structure of pure nickel during the rapid quench of the liquid and in
the resulting glass. In contrast with previous simulations with pair
potentials, we find more crystalline order and fewer icosahedra for slower
quenching rates, resulting in a glass less stable against crystallization. It
is shown that there is not a specific amorphous structure, only the arrest of
the transition from liquid to crystal, resulting in small crystalline clusters
immersed in an amorphous matrix with the same structure of the liquid.


title: Comment on: Kinetic Roughening in Slow Combustion of Paper

abstract:
  We comment on a recent Letter by Maunuksela et al. [Phys. Rev. Lett. 79, 1515
(1997)].


title: Possible Stratification Mechanism in Granular Mixtures

abstract:
  We propose a mechanism to explain what occurs when a mixture of grains of
different sizes and different shapes (i.e. different repose angles) is poured
into a quasi-two-dimensional cell. Specifically, we develop a model that
displays spontaneous stratification of the large and small grains in
alternating layers. We find that the key requirement for stratification is a
difference in the repose angles of the two pure species, a prediction confirmed
by experimental findings. We also identify a kink mechanism that appears to
describe essential aspects of the dynamics of stratification.


title: Surface Flows of Granular Mixtures: III. Canonical Model

abstract:
  We present the generalization of the minimal model for surface flows of
granular mixtures, proposed by Boutreux and de Gennes [J. Phys. I France 6,
1295 (1996)]. The minimal model was valid for grains differing only in their
surface properties. The present model also takes into account differences in
the size of the grains. We apply the model to study segregation in
two-dimensional silos of mixtures of grains differing in size and/or surface
properties. When the difference in size is small, the model predicts that a
continuous segregation appears in the static phase during the filling of a
silo. When the difference in size is wide, we take into account the segregation
of the grains in the rolling phase, and the model predicts complete segregation
and stratification in agreement with experimental observations.


title:
Continuous Avalanche Segregation of Granular Mixtures in Thin Rotating
  Drums

abstract:
  We study segregation of granular mixtures in the continuous avalanche regime
(for frequencies above ~ 1 rpm) in thin rotating drums using a continuum theory
for surface flows of grains. The theory predicts profiles in agreement with
experiments only when we consider a flux dependent velocity of flowing grains.
We find the segregation of species of different size and surface properties,
with the smallest and roughest grains being found preferentially at the center
of the drum. For a wide difference between the species we find a complete
segregation in agreement with experiments. In addition, we predict a transition
to a smooth segregation regime - with an power-law decay of the concentrations
as a function of radial coordinate - as the size ratio between the grains is
decreased towards one.


title: Grain Segregation Mechanism in Aeolian Sand Ripples

abstract:
  Many sedimentary rocks are formed by migration of sand ripples. Thin layers
of coarse and fine sand are present in these rocks, and understanding how
layers in sandstone are created has been a longstanding question. Here, we
propose a mechanism for the origin of the most common layered sedimentary
structures such as inverse graded climbing ripple lamination and
cross-stratification patterns. The mechanism involves a competition between
three segregation processes: (i) size-segregation and (ii) shape-segregation
during transport and rolling, and (iii) size segregation due to different
hopping lengths of the small and large grains. We develop a discrete model of
grain dynamics which incorporates the coupling between moving grains and the
static sand surface, as well as the different properties of grains, such as
size and roughness, in order to test the plausibility of this physical
mechanism.


title: Stratification Instability in Granular Flows

abstract:
  When a mixture of two kinds of grains differing in size and shape is poured
in a vertical two-dimensional cell, the mixture spontaneously stratifies in
alternating layers of small and large grains, whenever the large grains are
more faceted than the small grains. Otherwise, the mixture spontaneously
segregates in different regions of the cell when the large grains are more
rounded than the small grains. We address the question of the origin of the
instability mechanism leading to stratification using a recently proposed set
of equations for surface flow of granular mixtures. We show that the stable
solution of the system is a segregation solution due to size (large grains tend
to segregate downhill near the substrate and small grains tend to segregate
uphill) and shape (rounded grains tend to segregate downhill and more faceted
grains tend to segregate uphill). As a result, the segregation solution of the
system is realized for mixtures of large-rounded grains and small-cubic grains
with the large-rounded grains segregating near the bottom of the pile.
Stability analysis reveals the instability mechanism driving the system to
stratification as a competition between size-segregation and shape-segregation
taking place for mixtures of large-cubic grains and small-rounded grains. The
large-cubic grains tend to size-segregate at the bottom of the pile, while at
the same time, they tend to shape-segregate near the pouring point. Thus, the
segregation solution becomes unstable, and the system evolves spontaneously to
stratification.


title: Microscopic Model for Granular Stratification and Segregation

abstract:
  We study segregation and stratification of mixtures of grains differing in
size, shape and material properties poured in two-dimensional silos using a
microscopic lattice model for surface flows of grains. The model incorporates
the dissipation of energy in collisions between rolling and static grains and
an energy barrier describing the geometrical asperities of the grains. We study
the phase diagram of the different morphologies predicted by the model as a
function of the two parameters. We find regions of segregation and
stratification, in agreement with experimental finding, as well as a region of
total mixing.


title: Kinematic Segregation of Flowing Grains in Sandpiles

abstract:
  We study the segregation of granular mixtures in two-dimensional silos using
a set of coupled equations for surface flows of grains. We study the thick flow
regime, where the grains are segregated in the rolling phase. We incorporate
this dynamical segregation process, called kinematic sieving, free-surface
segregation or percolation, into the theoretical formalism and calculate the
profiles of the rolling species and the concentration of grains in the bulk in
the steady state. Our solution shows the segregation of the mixture with the
large grains being found at the bottom of the pile in qualitative agreement
with experiments.


title: Tracer Dispersion in Porous Media with Spatial Correlations

abstract:
  We analyze the transport properties of a neutral tracer in a carrier fluid
flowing through percolation-like porous media with spatial correlations. We
model convection in the mass transport process using the velocity field
obtained by the numerical solution of the Navier-Stokes and continuity
equations in the pore space. We show that the resulting statistical properties
of the tracer can be approximated by a Levy walk model, which is a consequence
of the broad distribution of velocities plus the existence of spatial
correlations in the porous medium.


title: Dynamics of Granular Stratification

abstract:
  Spontaneous stratification in granular mixtures has been recently reported by
H. A. Makse et al. [Nature 386, 379 (1997)]. Here we study experimentally the
dynamical processes leading to spontaneous stratification. Using a high-speed
video camera, we study a rapid flow regime where the rolling grains size
segregate during the avalanche. We characterize the dynamical process of
stratification by measuring all relevant quantities: the velocity of the
rolling grains, the velocity of the kink, the wavelength of the layers, the
rate of collision between rolling and static grains, and all the angles of
repose characterizing the mixture. The wavelength of the layers behaves
linearly with the thickness of the layer of rolling grains (i.e., with the flow
rate), in agreement with theoretical predictions. The velocity profile of the
grains in the rolling phase is a linear function of the position of the grains
along the moving layer. We also find that the speed of the upward-moving kink
has the same value as the mean speed of the downward-moving grains. We measure
the shape and size of the kink, as well as the profiles of the rolling and
static phases of grains, and find agreement with recent theoretical
predictions.


title:
Wigner crystal model of counterion induced bundle formation of rod-like
  polyelectrolytes

abstract:
  A simple electrostatic theory of condensation of rod-like polyelectrolytes
under influence of polyvalent ions is proposed. It is based on the idea that
Manning condensation of ions results in formation of the Wigner crystal on a
background of a bundle of rods. It is shown that, depending on a single
dimensionless parameter, this can be the densely packed three-dimensional
Wigner crystal or the two-dimensional crystal on the rod surfaces. For DNA the
location of charge on the spiral results in a model of the one-dimensional
Wigner crystal. It is also argued that the Wigner crystal idea can be applied
to self-assembly of other polyelectrolytes, for example, colloids and DNA-lipid
complexes.


title:
Mechanisms of Granular Spontaneous Stratification and Segregation in
  Two-Dimensional Silos

abstract:
  Spontaneous stratification of granular mixtures has been reported by Makse et
al. [Nature 386, 379 (1997)] when a mixture of grains differing in size and
shape is poured in a quasi-two-dimensional heap. We study this phenomenon using
two different approaches. First, we introduce a cellular automaton model that
illustrates clearly the physical mechanism; the model displays stratification
whenever the large grains are rougher than the small grains, in agreement with
the experiments. Moreover, the dynamics are close to those of the experiments,
where the layers are built through a ``kink'' at which the rolling grains are
stopped. Second, we develop a continuum approach, based on a recently
introduced set of coupled equations for surface flows of granular mixtures that
allows us to make quantitative predictions for relevant quantities. We study
the continuum model in two limit regimes: the large flux or thick flow regime,
where the percolation effect (i.e., segregation of the rolling grains in the
flow) is important, and the small flux or thin flow regime, where all the
rolling grains are in contact with the surface of the sandpile. We find that
the wavelength of the layers behaves linearly with the flux of grains. We
obtain analytical predictions for the shape of the kink giving rise to
stratification as well as the profile of the rolling and static species when
segregation of the species is observed.


title: Modeling Urban Growth Patterns with Correlated Percolation

abstract:
  We propose and test a model that describes the morphology of cities, the
scaling of the urban perimeter of individual cities, and the area distribution
of systems of cities. The model is also consistent with observable urban growth
dynamics, our results agreeing both qualitatively and quantitatively with urban
data. The resulting growth morphology can be understood from interactions among
the constituent units forming an urban region, and can be modeled using a
correlated percolation model in the presence of a gradient.


title: Spontaneous Stratification in Granular Mixtures

abstract:
  Granular materials size segregate when exposed to external periodic
perturbations such as vibrations. Moreover, mixtures of grains of different
sizes spontaneously segregate in the absence of external perturbations: when a
mixture is simply poured onto a pile, the large grains are more likely to be
found near the base, while the small grains are more likely to be near the top.
Here, we report a spontaneous phenomenon arising when we pour a mixture between
two vertical plates: the mixture spontaneously stratifies into alternating
layers of small and large grains whenever the large grains are rougher than the
small grains. In contrast, we find only spontaneous segregation when the large
grains are more rounded than the small grains. The stratification is related to
the occurrence of avalanches; during each avalanche the grains comprising the
avalanche spontaneously stratify into a pair of layers through a "kink"
mechanism, with the small grains forming a sublayer underneath the layer of
large grains.


title: Algorithmic Complexity in Noise Induced Transport Systems

abstract:
  Time correlated fluctuations interacting with a spatial asymmetry potential
are sufficient conditions to give rise to transport of Brownian particles. The
transfer of information coming from the nonequilibrium bath, viewed as a source
of negentropy, give rise to the correlated noise. The algorithmic complexity of
an object provides a means of quantitating its information contents. The
Kolmogorov information entropy or algorithmic complexity is investigated in
order to quantitate the transfer of information that occurs in computational
models showing noise induced transport. The complexity is measured in terms of
the average number of bits per time unit necessary to specify the sequence
generated by the system.


title: On the Glassy Behavior of Parking Lot Model

abstract:
  We present a theoretical discussion of the reversible parking problem, which
appears to be one of the simplest systems exhibiting glassy behavior. The
existence of slow relaxation, nontrivial fluctuations, and an annealing effect
can all be understood by recognizing that two different time scales are present
in the problem. One of these scales corresponds to the fast filling of existing
voids, the other is associated with collective processes that overcome partial
ergodicity breaking. The results of the theory are in a good agreement with
simulation data; they provide a simple qualitative picture for understanding
recent granular compaction experiments and other glassy systems.


title: Effect of Chain Flexibility on Nematic-Smectic Transition

abstract:
  The theory of nematic-smectic phase transition in the system of uniform
semi-flexible chains with hard-core repulsion is presented. Both the general
density-functional formalism the tube-model calculation show that the
flexibility of the chains results in a strong first-order transition, in
contrast to the common weak-cristallization scenario of the nematic-smectic
transition in rigid rods. The calculated spinodal volume fraction of the
uniform nematic phase and the period of the modulation instability are
consistent with recent experimental results.


title: Theories of non-Fermi liquid behavior in heavy fermions

abstract:
  I review our incomplete understanding of non-Fermi liquid behavior in heavy
fermion systems at a quantum critical point. General considerations suggest
that critical antiferromagnetic fluctuations do not destroy the Fermi surface
by scattering the heavy electrons- but by actually breaking up the internal
structure of the heavy fermion. I contrast the weak, and strong-coupling view
of the quantum phase transition, emphasizing puzzles and questions that recent
experiments raise.


title:
Repulsion of Single-well Fundamental Edge Magnetoplasmons in Double
  Quantum Wells

abstract:
  A {\it microscopic} treatment of fundamental edge magnetoplasmons (EMPs)
along the edge of a double quantum well (DQW) is presented for strong magnetic
fields, low temperatures, and total filling factor \nu=2. It is valid for
lateral confining potentials that Landau level (LL) flattening can be
neglected. The cyclotron and Zeeman energies are assumed larger than the DQW
energy splitting \sqrt{\Delta^2 +4T^2}, where \Delta is the splitting of the
isolated wells and T the tunneling matrix element. %hen calculated unperturbed
density profile is sharp at the edge. Using a random-phase approximation (RPA),
which includes local and nonlocal contributions to the current density, it is
shown that for negligible tunnel coupling 2T << \Delta the inter-well Coulomb
coupling leads to two DQW fundamental EMPs which are strongly renormalized in
comparison with the decoupled, single-well fundamental EMP. These DQW modes can
be modified further upon varying the inter-well distance d, along the z axis,
and/or the separation of the wells' edges \Delta y along the y axis. The charge
profile of the {\it fast} and {\it slow} DQW mode varies, respectively, in an
{\it acoustic} and {\it optical} manner along the y axis and is not smooth on
the \ell_{0} scale. For strong tunneling \Delta\alt 2T these DQW modes are
essentially modified when \Delta is changed by applying a transverse electric
field to the DQW.


title:
Multilevel blocking approach to the fermion sign problem in
  path-integral Monte Carlo simulations

abstract:
  A general algorithm toward the solution of the fermion sign problem in
finite-temperature quantum Monte Carlo simulations has been formulated for
discretized fermion path integrals with nearest-neighbor interactions in the
Trotter direction. This multilevel approach systematically implements a simple
blocking strategy in a recursive manner to synthesize the sign cancellations
among different fermionic paths throughout the whole configuration space. The
practical usefulness of the method is demonstrated for interacting electrons in
a quantum dot.


title:
Improved Composite-Boson Theory of Monolayer and Bilayer Quantum Hall
  Ferromagnets

abstract:
  An improved composite-boson theory of quantum Hall ferromagnets is formulated
both for the monolayer and bilayer systems. In this scheme the field operator
describes solely the physical degrees of freedom representing the deviation
from the ground state. Skyrmions are charged excitations confined to the lowest
Landau level. By evaluating the excitation energy of one skyrmion in the
interlayer-coherent phase it is shown that the bilayer QH state becomes stabler
as the interlayer density difference becomes larger.


title:
Magnetically and thermally modulated microwave absorption in Y-Ba-Cu-O
  single crystal near T_c

abstract:
  The microwave absorption R in the Y-Ba-Cu-O single crystals was investigated
near {T_c \approx 92 K} and in the external magnetic field {0<H \leq 9 kOe}. A
modified ESR spectrometer was used in the experiment. The method of temperature
modulation, along with the usual method of magnetic-field modulation, was first
applied in studying of the microwave response of these crystals. Peaks in the
temp erature dependencies of the signals {\partial R}/ {\partial H} and
{\partial R} /{\partial T} observed in the vicinity of {T_c} were differently
shaped and slightly shifted one with respect to another. The evolution of the
peaks with variation of the magnetic field and angle {\theta} between {\bf H}
and the {\bf c}-axis was traced. It has been shown that the observed difference
of the temperature dependencies of the derivatives {\partial R}/{\partial H}
and {\partial R}/{\partial T} occures due to the field-induced broadening of
the superconducting transition, which is inherent in the high-{T_c}
superconductors.


title: Wetting of Argon on CO2

abstract:
  We have studied the wetting transition of Ar adsorbed on solid CO2 by means
of high-precision adsorption isotherms measured with a quartz microbalance. We
observe triple-point wetting. At variance with many theoretical studies based
on a model adsorption potential, which predict for this system a genuine
prewetting transition around 100K we find that a detailed density-functional
calculation employing a more realistic adsorption potential leads to
triple-point wetting of Ar on CO2, in good agreement with the experiment.


title:
Density matrix renormalization group study of dimerization of the
  Pariser-Parr-Pople model of polyacetilene

abstract:
  We apply the DMRG method to the Pariser-Parr-Pople hamiltonian and
investigate the onset of dimerization. We deduce the parameters of the hopping
term and the contribution of the sigma bonds from ab initio calculations on
ethylene. Denoting by R_{ij} the C-C distances, we perform a variational
optimization of the dimerization delta= (R_{i,i+1} - R_{i-1,i})/2 and of the
average bond length R_0 for chains up to N=50 sites. The critical value of N at
which the transition occurs is found to be between N=14 and N=18 for the
present model. The asymptotic values for large N for R_0 and delta are given by
1.408(3) angstroms and 0.036(0) angstroms.


title: Evaporation and Step Edge Diffusion in MBE

abstract:
  Using kinetic Monte-Carlo simulations of a Solid-on-Solid model we
investigate the influence of step edge diffusion (SED) and evaporation on
Molecular Beam Epitaxy (MBE). Based on these investigations we propose two
strategies to optimize MBE-growth. The strategies are applicable in different
growth regimes: during layer-by-layer growth one can reduce the desorption rate
using a pulsed flux. In three-dimensional (3D) growth the SED can help to grow
large, smooth structures. For this purpose the flux has to be reduced with time
according to a power law.


title: Tricritical Points in Random Combinatorics: the (2+p)-SAT case

abstract:
  The (2+p)-Satisfiability (SAT) problem interpolates between different classes
of complexity theory and is believed to be of basic interest in understanding
the onset of typical case complexity in random combinatorics. In this paper, a
tricritical point in the phase diagram of the random $2+p$-SAT problem is
analytically computed using the replica approach and found to lie in the range
$2/5 \le p_0 \le 0.416$. These bounds on $p_0$ are in agreement with previous
numerical simulations and rigorous results.


title:
How to handle the inelastic collapse of a dissipative hard-sphere gas
  with the TC model

abstract:
  The inelastic hard sphere model of granular material is simple, easily
accessible to theory and simulation, and captures much of the physics of
granular media. It has three drawbacks, all related to the approximation that
collisions are instantaneous: 1) The number of collisions per unit time can
diverge, i.e. the ``inelastic collapse'' can occur. 2) All interactions are
binary, multiparticle contacts cannot occur and 3) no static limit exists. We
extend the inelastic hard sphere model by defining a duration of contact t_c
such that dissipation is allowed only if the time between contacts is larger
than t_c. We name this generalized model the ``TC model'' and discuss it using
examples of dynamic and static systems. The contact duration used here does not
change the instantaneous nature of the hard sphere contacts, but accounts for a
reduced dissipation during ``multiparticle contacts''. Kinetic and elastic
energies are defined as well as forces and stresses in the system. Finally, we
present event-driven numerical simulations of situations far beyond the
inelastic collapse, possible only with the TC model.


title: Possible Exotic Phases in the One-Dimensional Extended Hubbard Model

abstract:
  We investigate numerically the ground state phase diagram of the
one-dimensional extended Hubbard model, including an on--site interaction U and
a nearest--neighbor interaction V. We focus on the ground state phases of the
model in the V >> U region, where previous studies have suggested the
possibility of dominant superconducting pairing fluctuations before the system
phase separates at a critical value V=V_PS. Using quantum Monte Carlo methods
on lattices much larger than in previous Lanczos diagonalization studies, we
determine the boundary of phase separation, the Luttinger Liquid correlation
exponent K_rho, and other correlation functions in this region. We find that
phase separation occurs for V significantly smaller than previously reported.
In addition, for negative U, we find that a uniform state re-enters from phase
separation as the electron density is increased towards half filling. For V <
V_PS, our results show that superconducting fluctuations are not dominant. The
system behaves asymptotically as a Luttinger Liquid with K_rho < 1, but we also
find strong low-energy (but gapped) charge-density fluctuations at a momentum
not expected for a standard Luttinger Liquid.


title:
Finite temperature dynamics of vortices in the two dimensional
  anisotropic Heisenberg model

abstract:
  We study the effects of finite temperature on the dynamics of non-planar
vortices in the classical, two-dimensional anisotropic Heisenberg model with
XY- or easy-plane symmetry. To this end, we analyze a generalized
Landau-Lifshitz equation including additive white noise and Gilbert damping.
Using a collective variable theory with no adjustable parameters we derive an
equation of motion for the vortices with stochastic forces which are shown to
represent white noise with an effective diffusion constant linearly dependent
on temperature. We solve these stochastic equations of motion by means of a
Green's function formalism and obtain the mean vortex trajectory and its
variance. We find a non-standard time dependence for the variance of the
components perpendicular to the driving force. We compare the analytical
results with Langevin dynamics simulations and find a good agreement up to
temperatures of the order of 25% of the Kosterlitz-Thouless transition
temperature. Finally, we discuss the reasons why our approach is not
appropriate for higher temperatures as well as the discreteness effects
observed in the numerical simulations.

categories: cond-mat.stat-mech cond-mat.mtrl-sci nlin.PS patt-sol


title: Cellular Automata

abstract:
  An introduction to cellular automata (both deterministic and probabilistic)
with examples. Definition of deterministic automata, dynamical properties,
damage spreading and Lyapunov exponents; probabilistic automata and Markov
processes, nonequilibrium phase transitions, directed percolation, diffusion;
simulation techniques, mean field. Investigation themes: life, epidemics,
forest fires, percolation, modeling of ecosystems and speciation.
  They represent my notes for the school "Dynamical Modeling in
Biotechnologies", ISI, Villa Gualino 1996.


title: BCS Theory for Trapped Ultracold Fermions

abstract:
  We develop an extension of the well-known BCS-theory to systems with trapped
fermions. The theory fully includes the quantized energy levels in the trap.
The key ingredient is to model the attractive interaction between two atoms by
a pseudo-potential which leads to a well defined scattering problem and
consequently a BCS-theory free of divergences. We present numerical results for
the BCS critical temperature and the temperature dependence of the gap. They
are used as a test of existing semi-classical approximations.


title: Ground State Properties of Anderson Impurity in a Gapless Host

abstract:
  Using the Bethe ansatz method, we study the ground state properties of a
$U\to\infty$ Anderson impurity in a ``gapless'' host, where a density of band
states vanishes at the Fermi level $\epsilon_F$ as $|\epsilon-\epsilon_F|$. As
in metals, the impurity spin is proven to be screened at arbitrary parameters
of the system. However, the impurity occupancy as a function of the bare
impurity energy is shown to acquire novel qualitative features which
demonstrate a nonuniversal behavior of the system. The latter explains why the
Kondo screening is absent (or exists only at quite a large electron-impurity
coupling) in earlier studies based on scaling arguments.


title: Solitons on the edge of a two-dimensional electron system

abstract:
  We present a study of the excitations of the edge of a two-dimensional
electron droplet in a magnetic field in terms of a contour dynamics formalism.
We find that, beyond the usual linear approximation, the non-linear analysis
yields soliton solutions which correspond to uniformly rotating shapes. These
modes are found from a perturbative treatment of a non-linear eigenvalue
problem, and as solutions to a modified Korteweg-de Vries equation resulting
from a local induction approximation to the nonlocal contour dynamics. We
discuss applications to the edge modes in the quantum Hall effect.


title:
Fractional Exclusion Statistics and the Universal Quantum of Thermal
  Conductance: A Unifying Approach

abstract:
  We introduce a generalized approach to one-dimensional (1D) conduction based
on Haldane's concept of fractional statistics (FES) and the Landauer
formulation of transport theory. We show that the 1D ballistic thermal
conductance is independent of the statistics obeyed by the carriers and is
governed by the universal quantum $ (\pi^2 k^2_B T)/(3h) $ in the degenerate
regime. By contrast, the electrical conductance of FES systems is
statistics-dependent. This work unifies previous theories of electron and
phonon systems and explains an interesting commonality in their behavior.


title: Is the broad histogram random walk dynamics correct?

abstract:
  We show explicitly that the broad histogram single-spin-flip random walk
dynamics does not give correct microcanonical average even in one dimension.
The dynamics violates detailed balance condition by an amount which decreases
with system size. As a result, in distribution different configurations with
the same energy can have different probabilities. We propose a modified
dynamics which ensures detailed balance and the histogram obtained from this
dynamics is exactly flat.


title:
The low temperature interface between the gas and solid phases of hard
  spheres with a short-ranged attraction

abstract:
  At low temperature, spheres with a very short-ranged attraction exist as a
close-packed solid coexisting with an infinitely dilute gas. We find that the
ratio of the interfacial tension between these two phases to the thermal energy
diverges as the range of the attraction goes to zero. The large tensions when
the interparticle attractions are short-ranged may be why globular proteins
only crystallise over a narrow range of conditions.


title: A Real-Space Full Multigrid study of the fragmentation of Li11+ clusters

abstract:
  We have studied the fragmentation of Li11+ clusters into the two
experimentally observed products (Li9+,Li2) and (Li10+,Li) The ground state
structures for the two fragmentation channels are found by Molecular Dynamics
Simulated Annealing in the framework of Local Density Functional theory.
Energetics considerations suggest that the fragmentation process is dominated
by non-equilibrium processes. We use a real-space approach to solve the
Kohn-Sham problem, where the Laplacian operator is discretized according to the
Mehrstellen scheme, and take advantage of a Full MultiGrid (FMG) strategy to
accelerate convergence. When applied to isolated clusters we find our FMG
method to be more efficient than state-of-the-art plane wave calculations.


title:
Investigating the influence of different thermodynamic paths on the
  structural relaxation in a glass forming polymer melt

abstract:
  We present results from Molecular Dynamics simulations of the thermal glass
transition in a dense polymer melt. In previous work we compared the simulation
data with the idealized version of mode coupling theory (MCT) and found that
the theory provides a good description of the dynamics above the critical
temperature. In order to investigate the influence of different thermodynamic
paths on the structural relaxation (alpha-process), we performed simulations
for three different pressures and are thus able to give a sketch of the
critical line of MCT in the pressure-temperature-plane [(p,T)-plane], where,
according to the idealised version of MCT, an ergodic-nonergodic transition
should occur. Furthermore, by cooling our system along two different paths (an
isobar and an isochor), with the same impact point on the critical line, we
demonstrate that neither the critical temperature nor the exponent gamma depend
on the chosen path.


title: Lattice-gas model for collective biological motion

abstract:
  A simple self-driven lattice-gas model for collective biological motion is
introduced. We find weakly first order phase transition from individual random
walks to collective migration. A mean-field theory is presented to support the
numerical results.


title:
Avalanche consumption and the stationary regions of the density profile
  around the droplets in the theory of condensation

abstract:
  The contradiction between the stationary aproach to the density profile and
the avalanche character of the metastable phase consumption is investigated.
The exact solution for the metastable phase profile is obtained. The
reconsidered value for the special parameter responsible for the hierarchy in
the structure of the density profile around the droplet is presented.


title: Electronic Transport in Unconventional Superconductors

abstract:
  We investigate the electronic transport coefficients in unconventional
superconductors at low temperatures, where charge and heat transport are
dominated by electron scattering from random lattice defects. We discuss the
features of the pairing symmetry, Fermi surface, and excitation spectrum which
are reflected in the low temperature heat transport. For temperatures $k_B T
\la \gamma \ll \Delta_0$, where $\gamma$ is the bandwidth of impurity induced
Andreev states, certain eigenvalues become {\it universal}, i.e., independent
of the impurity concentration and phase shift. Deep in the superconducting
phase ($k_B T \la \gamma$) the Wiedemann-Franz law, with Sommerfeld's value of
the Lorenz number, is recovered. We compare our results for theoretical models
of unconventional superconductivity in high-T$_c$ and heavy fermion
superconductors with experiment. Our findings show that impurities are a
sensitive probe of the low-energy excitation spectrum, and that the
zero-temperature limit of the transport coefficients provides an important test
of the order parameter symmetry.


title: Broad Histogram Method for Continuous Systems: the XY-Model

abstract:
  We propose a way of implementing the Broad Histogram Monte Carlo method to
systems with continuous degrees of freedom, and we apply these ideas to
investigate the three-dimensional XY-model with periodic boundary conditions.
We have found an excellent agreement between our method and traditional
Metropolis results for the energy, the magnetization, the specific heat and the
magnetic susceptibility on a very large temperature range. For the calculation
of these quantities in the temperature range 0.7<T<4.7 our method took less CPU
time than the Metropolis simulations for 16 temperature points in that
temperature range. Furthermore, it calculates the whole temperature range
1.2<T<4.7 using only 2.2 times more computer effort than the Histogram Monte
Carlo method for the range 2.1<T<2.2. Our way of treatment is general, it can
also be applied to other systems with continuous degrees of freedom.


title:
On the Origin of Quantum Oscillations in the Mixed State of Anisotropic
  Superconductor

abstract:
  For the vortex lattice in an anisotropic superconductor with well-separated
cores (Hc1<<B<<Hc2) it is shown that sizable de Haas-van Alphen oscillations
are caused by the levels` crossing of the energy threshold separating localized
and extended states of excitations moving in the average magnetic field,B.


title:
Critical Exponents for Nuclear Multifragmentation: dynamical lattice
  model

abstract:
  We present a dynamical and dissipative lattice model, designed to mimic
nuclear multifragmentation. Monte-Carlo simulations with this model show clear
signature of critical behaviour and reproduce experimentally observed
correlations. In particular, using techniques devised for finite systems, we
could obtain two of its critical exponents, whose values are in agreement with
those of the universality class to which nuclear multifragmentation is supposed
to belong.

categories: cond-mat.stat-mech adap-org nlin.AO nucl-th


title: Lattice Simulation of Nuclear Multifragmentation

abstract:
  Motivated by the decade-long debate over the issue of criticality supposedly
observed in nuclear multifragmentation, we propose a dynamical lattice model to
simulate the phenomenon. Its Ising Hamiltonian mimics a short range attractive
interaction which competes with a thermal-like dissipative process. The results
here presented, generated through an event-by-event analysis, are in agreement
with both experiment and those produced by a percolative (non-dynamical) model.


title: ``Superfast'' Reaction in Turbulent Flow with Potential Disorder

abstract:
  We explore the regime of ``superfast'' reactivity that has been predicted to
occur in turbulent flow in the presence of potential disorder. Computer
simulation studies confirm qualitative features of the previous renormalization
group predictions, which were based on a static model of turbulence. New
renormalization group calculations for a more realistic, dynamic model of
turbulence show that the superfast regime persists. This regime, with
concentration decay exponents greater than that for a well-mixed reaction,
appears to be a general result of the interplay among non-linear reaction
kinetics, turbulent transport, and local trapping by potential disorder.


title: Modulation of the dephasing time for a magnetoplasma in a quantum well

abstract:
  We investigate the femtosecond kinetics of optically excited 2D
magneto-plasma. We calculate the femtosecond dephasing and relaxation kinetics
of the laser pulse excited magneto-plasma due to bare Coulomb potential
scattering, because screening is under these conditions of minor importance. By
taking into account four Landau subbands in both the conduction band and the
valence band, we are now able to extend our earlier study [Phys. Rev. B {\bf
58}, 1998,in print (see also cond-mat/9808073] to lower magnetic fields. We can
also fix the magnetic field and change the detuning to further investigate the
carrier density-dependence of the dephasing time. For both cases, we predict
strong modulation in the dephasing time.


title:
Van der Waerden's Colouring Theorem and Weak-Strong Duality on the
  Lattice

abstract:
  Van der Waerden's (VDW) colouring theorem in combinatoric number theory [1]
has scope for physical applications.The solution of the two colour case has
enabled the construction of an explicit mapping of an infinite, one dimensional
antiferromagnetic Ising system to an effective pseudo- ferromagnetic one with
the coupling constants in the two cases becoming related [2].Here the three
colour problem is solved and the results are used to obtain new insights in the
theory of complex lattices, particularly those relating to ternary alloys. The
existence of these mappings of a multicolour lattice onto a monochromatic one
with different couplings illustrates {\it a new form of duality}.


title:
Orbital dynamics: The origin of the anomalous optical spectra in
  ferromagnetic manganites

abstract:
  We discuss the role of orbital degeneracy in the transport properties of
perovskite manganites, focusing in particular on the optical conductivity in
the metallic ferromagnetic phase at low temperatures. Orbital degeneracy and
strong correlations are described by an orbital t-J model which we treat in a
slave-boson approach. Employing the memory-function formalism we calculate the
optical conductivity, which is found to exhibit a broad incoherent component
extending up to bare bandwidth accompanied by a strong suppression of the Drude
weight. Further, we calculate the constant of T-linear specific heat. Our
results are in overall agreement with experiment and suggest low-energy orbital
fluctuations as the origin of the strongly correlated nature of the metallic
phase of manganites.


title: Local spin polarization in underdoped cuprates with impurities

abstract:
  We present a theory of magnetic (Ni) and nonmagnetic (Zn) impurities
substituted into planar Cu sites in the normal state of underdoped cuprates
exhibiting a spin gap. Both types of impurities induce magnetic moments on
neighboring Cu sites. In the case of Ni these moments partially screen the
inherent impurity spin, resulting in an effective S=1/2 moment. The
characteristic Kondo scale is found to have a power-law dependence on the
coupling constant. We investigate the spatial shape of the impurity-induced
spin density, taking into account the presence of short-ranged AF correlations,
and calculate the ^{17}O NMR line broadening induced by impurity doping.


title:
Anomalous Resonance of the Symmetric Single-Impurity Anderson Model in
  the Presence of Pairing Fluctuations

abstract:
  We consider the symmetric single-impurity Anderson model in the presence of
pairing fluctuations. In the isotropic limit, the degrees of freedom of the
local impurity are separated into hybridizing and non-hybridizing modes. The
self-energy for the hybridizing modes can be obtained exactly, leading to two
subbands centered at $\pm U/2$. For the non-hybridizing modes, the second order
perturbation yields a singular resonance of the marginal Fermi liquid form. By
multiplicative renomalization, the self-energy is derived exactly, showing the
resonance is pinned at the Fermi level, while its strength is weakened by
renormalization.


title:
Fluctuation contribution to the specific heat in non-Fermi models for
  superconductivity

abstract:
  We investigate the fluctuation contribution to the specific heat of a
two-dimensional superconductor with a non-Fermi normal state described by a
Anderson Green's function
$G(k,i\omega)=\omega_c^{-\alpha}/(i\omega-\epsilon_k)^{1-\alpha}$. The specific
heat corrections contain a term proportional to
$(T^{2\alpha-T_c^{2\alpha}})^{-1}$ and another logarithmic one. We defined a
coherence length as function of the non-Fermi parameter $\alpha$, which showed
that a crossover study between BCS and Bose-Einstein condensation is possible
by varying $\alpha$ in an interval $0 \div \alpha_{cr}$. By comparing our
theoretical results with the experimental data for HTSC materials, we
reobtained the value for $\alpha$, corresponding to such systems, of the order
$0.3 \div 0.45$. We also reobtained the critical temperature for such a
superconductor using the Thouless criterion.


title:
Precise Measurement of Optical Reflectivity Spectra Using Defect-Free
  Surface of La(1-x)Sr(x)MnO(3): Evidence Against Extremely Small Drude Weight

abstract:
  Optical reflectivity spectra of La(1-x)Sr(x)MnO(3) (0<=x<=0.30) were measured
on cleavage surfaces of single crystals. The optical conductivity $\sigma(w) of
ferromagnetic-metal La(0.70)Sr(0.30)MnO(3) is characterized by a Drude-like -
but not simple Drude - component with large spectral weight below 1.6 eV, which
yields a large effective carrier number N*_eff consistent with the results of
Hall coefficient and specific heat measurements. The present result
demonstrates that the previous result of "small Drude weight" originates from
the damage of the sample surface. The Sr-substitution effect on the electronic
structure and the origin of the unconventional $\sigma(w) are also discussed.


title: Thermodynamic fermion-boson symmetry in harmonic oscillator potentials

abstract:
  A remarkable thermodynamic fermion-boson symmetry is found for the canonical
ensemble of ideal quantum gases in harmonic oscillator potentials of odd
dimensions. The bosonic partition function is related to the fermionic one
extended to negative temperatures, and vice versa.


title: Ground-state landscape of 2d +-J Ising spin glasses

abstract:
  Large numbers of ground states of two-dimensional Ising spin glasses with
periodic boundary conditions in both directions are calculated for sizes up to
40^2. A combination of a genetic algorithm and Cluster-Exact Approximation is
used. For each quenched realization of the bonds up to 40 independent ground
states are obtained.
  For the infinite system a ground-state energy of e=-1.4015(3) is
extrapolated. The ground-state landscape is investigated using a finite-size
scaling analysis of the distribution of overlaps. The mean-field picture
assuming a complex landscape describes the situation better than the
droplet-scaling model, where for the infinite system mainly two ground states
exist. Strong evidence is found that the ground states are not organized in an
ultrametric fashion in contrast to previous results for three-dimensional spin
glasses.


title: The microscopic spin-phonon coupling constants in CuGeO_3

abstract:
  Using RPA results, mean field theory, and refined data for the polarization
vectors we determine the coupling constants of the four Peierls-active phonon
modes to the spin chains of CuGeO_3. We then derive the values of the coupling
of the spin system to the linear ionic displacements, the bond lengths and the
angles between bonds. Our values are consistent with microscopic theories and
various experimental results. We discuss the applicability of static approaches
to the spin-phonon coupling. The c-axis anomaly of the thermal expansion is
explained. We give the values of the coupling constants in an effective
one-dimensional Hamiltonian.


title:
Collective Effects in the Horizontal Transport of Vertically Vibrated
  Granular Layers

abstract:
  Motivated by recent advances in the investigation of fluctuation-driven
ratchets and flows in excited granular media, we have carried out experimental
and simulational studies to explore the horizontal transport of granular
particles in a vertically vibrated system whose base has a sawtooth-shaped
profile. The resulting material flow exhibits novel collective behavior, both
as a function of the number of layers of particles and the driving frequency;
in particular, under certain conditions, increasing the layer thickness leads
to a reversal of the current, while the onset of transport as a function of
frequency can occur either gradually or suddenly in a manner reminiscent of a
phase transition.


title: Droplet Spreading: Partial Wetting Regime Revisited

abstract:
  We study the time evolution of a sessile liquid droplet, which is initially
put onto a solid surface in a non-equilibrium configuration and then evolves
towards its equilibrium shape. We adapt here the standard approach to the
dynamics of mechanical dissipative systems, in which the driving force, i.e.
the gradient of the system's Lagrangian function, is balanced against the rate
of the dissipation function. In our case the driving force is the loss of the
droplet's free energy due to the increase of its base radius, while the
dissipation occurs due to viscous flows in the core of the droplet and due to
frictional processes in the vicinity of the advancing contact line, associated
with attachment of fluid particles to solid. Within this approach we derive
closed-form equations for the evolution of the droplet's base radius, and
specify several regimes at which different dissipation channels dominate. Our
analytical predictions compare very well with experimental data.


title:
Quantum critical point with competing propagating and diffusive spin
  excitations

abstract:
  Feedback effects due to spin fluctuation induced precursors in the fermionic
quasiparticle spectrum are taken into account in the description of a quantum
critical point of itinerant spin systems. A correlation length dependent spin
damping occurs, leading to a dynamical scaling with z\approx 1 which
non-trivially competes with the conventional spin wave behavior. We obtain,
within a one loop renormalization group approach, a quantitative explanation
for the scaling behavior seen in underdoped cuprate superconductors.


title:
Density matrices in O(N) electronic structure calculations: theory and
  applications

abstract:
  We analyze the problem of determining the electronic ground state within O(N)
schemes, focusing on methods in which the total energy is minimized with
respect to the density matrix. We note that in such methods a crucially
important constraint is that the density matrix must be idempotent (i.e. its
eigenvalues must all be zero or unity). Working within orthogonal tight-binding
theory, we analyze two related methods for imposing this constraint: the
iterative purification strategy of McWeeny, as modified by Palser and
Manolopoulos; and the minimization technique of Li, Nunes and Vanderbilt. Our
analysis indicates that the two methods have complementary strengths and
weaknesses, and leads us to propose that a hybrid of the two methods should be
more effective than either method by itself. This idea is tested by using
tight-binding theory to apply the proposed hybrid method to a set of condensed
matter systems of increasing difficulty, ranging from bulk crystalline C and Si
to liquid Si, and the effectiveness of the method is confirmed. The
implications of our findings for O(N) implementations of non-orthogonal
tight-binding theory and density functional theory are discussed.


title:
Exact Single-Particle Green Functions of Fermi Systems Without Using
  Bosonization or Are Hartree-Fock and Random Phase Approximations 'Controlled
  Approximations' ?

abstract:
  In this article, we revisit the question of the validity of Hartree-Fock and
random-phase approximations. We show that there is a connection between the two
and while the RPA as it is known in much of the physics literature is of
limited validity, there is a generalised sense in which the random phase
approximation is of much wider applicability including to systems that do not
possess Fermi surfaces. The main conclusion is that the Hartree-Fock
approximation is a mean-field idea applied to the density operator, and the
random-phase approximation is a mean-field idea applied to the number operator.
The generalised RPA is used to compute single-particle properties such as
momentum distribution and spectral functions. It is found that we have to go
beyond the generalised RPA and include fluctuations in the momentum
distribution in order to recover a nonzero imaginary part of the one-particle
self energy, which is also explicitly computed, all this in any number of
spatial dimensions and no bosonization is needed.


title: Equilibrium size of large ring molecules

abstract:
  The equilibrium properties of isolated ring molecules were investigated using
an off-lattice model with no excluded volume but with dynamics that preserve
the topological class. Using an efficient set of long range moves, chains of
more than 2000 monomers were studied. Despite the lack of any excluded volume
interaction, the radius of gyration scaled like that of a self avoiding walk,
as had been previously conjectured. However this scaling was only seen for
chains greater than 500 monomers.


title: Localizations in coupled electronic chains

abstract:
  We studied effects of random potentials and roles of electron-electron
interactions in the gapless phase of coupled Hubbard chains, using a
renormalization group technique. For non-interacting electrons, we obtained the
localization length proportional to the number of chains, as already shown in
the other approaches. For interacting electrons, the localization length is
longer for stronger interactions, that is, the interactions counteract the
random potentials. Accordingly, the localization length is not a simple linear
function of the number of chains. This interaction effect is strongest when
there is only a single chain. We also calculate the effects of interactions and
random potentials on charge stiffness.


title: Unified Approach to Crossover Phenomena

abstract:
  A general analytical method is developed for describing crossover phenomena
of arbitrary nature. The method is based on the algebraic self-similar
renormalization of asymptotic series, with control functions defined by
crossover conditions. The method can be employed for such difficult problems
for which only a few terms of asymptotic expansions are available, and no other
techniques are applicable. As an illustration, analytical solutions for several
important physical problems are presented.


title: Dynamics and stress in gravity driven granular flow

abstract:
  We study, using simulations, the steady-state flow of dry sand driven by
gravity in two-dimensions. An investigation of the microscopic grain dynamics
reveals that grains remain separated but with a power-law distribution of
distances and times between collisions.
  While there are large random grain velocities, many of these fluctuations are
correlated across the system and local rearrangements are very slow. Stresses
in the system are almost entirely transfered by collisions and the structure of
the stress tensor comes almost entirely from a bias in the directions in which
collisions occur.

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


title: On the microcanonical solution of a system of globally coupled rotators

abstract:
  We study the Hamiltonian Mean Field (HMF) model, a system of $N$ fully
coupled particles, in the microcanonical ensemble. We use the previously
obtained free energy in the canonical ensemble to derive entropy as a function
of energy, using Legendre transform techniques. The temperature-energy relation
is found to coincide with the one obtained in the canonical ensemble and
includes a {\it metastable} branch which represents spatially homogeneous
states below the critical energy. "Water bag" states, with removed tails
momentum distribution, lying on this branch, are shown to relax to equilibrium
on a time which diverges linearly with $N$ in an energy region just below the
phase transition.


title:
Coupled exciton model with off-diagonal disorder for optical excitations
  in extended dendrimers

abstract:
  A phenomenological coupled exciton model is proposed in order to characterize
optical excitations in extended dendrimers. An onsite exciton state is assigned
at each phenyl rings and a nearest neighbor hopping integral which obeys the
Gaussian distribution is considered between the exciton states. The decreasing
optical excitation energy with respect to the dendrimer size indicates the
presence of exciton funnels along the backbone of the dendrimers. Therefore,
the extended dendrimers can work as artificial fractal antenna systems which
capture energy of light.

categories: cond-mat physics.chem-ph physics.optics quant-ph


title: Ballistic electron transport through magnetic domain walls

abstract:
  Electron transport limited by the rotating exchange-potential of domain walls
is calculated in the ballistic limit for the itinerant ferromagnets Fe, Co, and
Ni. When realistic band structures are used, the domain wall magnetoresistance
is enhanced by orders of magnitude compared to the results for previously
studied two-band models. Increasing the pitch of a domain wall by confinement
in a nano-structured point contact is predicted to give rise to a strongly
enhanced magnetoresistance.


title:
Interfacial profiles between coexisting phases in thin films: Cahn
  Hilliard treatment versus capillary waves

abstract:
  We consider an interface between two demixed A and B phases, confined in a
thin film between two antisymmetric walls, one of which prefers A and the other
B. Above the wetting transition, the interface is stabilized in the center of
the film. Based on a suitable extension of the Cahn-Hilliard gradient-square
theory in combination with the capillary wave theory, we argue that the
confinement influences the interfacial structure in two ways: It squeezes the
intrinsic structure and cuts off the capillary wave spectrum. As a result, the
interfacial width is proportional to the film thickness D in very thin films,
and proportional to the square root of D in thicker films. These effects are
then discussed in detail for the special case of an interface between demixed
homopolymer phases. The width of the intrinsic profile is calculated
analytically as a function of film thickness in the Cahn-Hilliard approximation
(weak segregation limit) and in the Helfand theory (strong segregation limit),
and numerically in the self-consistent field approximation. The results are
confirmed by Monte Carlo simulations of a lattice model for a polymer mixture.


title:
Lattice Models for Magnetic Fluids: Correlations Between Order
  Parameters

abstract:
  Magnetic fluids are colloidal suspensions of ferromagnetic particles covered
with a surfactant layer, dispersed in a host liquid. The existence of
cooperative phenomena in such magnetic colloidal systems, makes the determining
of their physical properties a difficult task. In the framework of statistical
mechanics, taking into consideration interparticle interactions, the study of
equilibrium properties and phase transitions of magnetic fluids can be best
done. In this paper we analyze some basic aspects about the real possibility of
describing the critical properties of these systems in the language of spin-1
lattice-gas model and we also propose the use of a higher spin (3/2) model.


title: Higher order parametric level statistics in disordered systems

abstract:
  Higher order parametric level correlations in disordered systems with broken
time-reversal symmetry are studied by mapping the problem onto a model of
coupled Hermitian random matrices. Closed analytical expression is derived for
parametric density-density correlation function which corresponds to a
perturbation of disordered system by a multicomponent flux.

categories: cond-mat.dis-nn chao-dyn cond-mat.stat-mech nlin.CD


title:
Spin, Charge and Quasiparticle Gaps in the One-Dimensional Kondo Lattice
  with f^2 Configuration

abstract:
  The ground state properties of the one-dimensional Kondo lattice with an f^2
configuration at each site are studied by the density matrix renormalization
group method. At half-filling, competition between the Kondo exchange J and the
antiferromagnetic intra f-shell exchange I leads to reduction of energy gaps
for spin, quasi-particle and charge excitations. The attractive force among
conduction electrons is induced by the competition and the bound state is
formed. As J/I increases the f^2 singlet gives way to the Kondo singlet as the
dominant local correlation. The remarkable change of the quasi-particle gap is
driven by the change of the spin-1/2 excitation character from the itinerant
one to the localized one. Possible metal-insulator transition is discussed
which may occur as the ratio J/I is varied by reference to mean-field results
in the f^2 lattice system and the two impurity Kondo system.


title:
Critical packing fraction at the phase separation transition in
  hard-core mixtures

abstract:
  In this paper, I relate the phase separation transition in binary hard-core
mixtures in the limit of small size ratio to a bond percolation transition. It
allows to estimate the critical packing fraction at the transition as a
function of the size ratio and the composition of the mixture for different
shapes of objects. The theoretical predictions are in excellent quantitative
agreement with numerical simulations of binary parallel hard squares mixtures.


title: Photophysical Properties in C60 and Higher Fullerenes

abstract:
  The optical excitations in C$_{60}$ and higher fullerenes, including isomers
of C$_{76}$, C$_{78}$, and C$_{84}$, are theoretically investigated. We use a
tight binding model with long-range Coulomb interactions, treated by the
Hartree-Fock and configuration-interaction methods. We find that the optical
excitations in the energy region smaller than about 4 eV have most of their
amplitudes at the pentagons. The oscillator strengths of projected absorption
almost accord with those of the total absorption. Next, off-resonant third
order susceptibilities are investigated. We find that third order
susceptibilities of higher fullerenes are a few times larger than those of
C$_{60}$. The magnitude of nonlinearity increases as the optical gap decreases
in higher fullerenes. The nonlinearity is nearly proportional to the fourth
power of the carbon number when the onsite Coulomb repulsion is $2t$ or $4t$,
$t$ being the nearest neighbor hopping integral. This result, indicating
important roles of Coulomb interactions, agrees with quantum chemical
calculations of higher fullerenes.


title:
Ground State Entropy of Potts Antiferromagnets: Cases with Noncompact W
  Boundaries Having Multiple Points at 1/q = 0

abstract:
  We present exact calculations of the zero-temperature partition function,
$Z(G,q,T=0)$, and ground-state degeneracy (per site), $W({G},q)$, for the
$q$-state Potts antiferromagnet on a number of families of graphs ${G}$ for
which the boundary ${\cal B}$ of regions of analyticity of $W$ in the complex
$q$ plane is noncompact and has the properties that (i) in the $z=1/q$ plane,
the point $z=0$ is a multiple point on ${\cal B}$ and (ii) ${\cal B}$ includes
support for $Re(q) < 0$. These families are generated by the method of
homeomorphic expansion. Our results give further insight into the conditions
for the validity of large--$q$ series expansions for the reduced function
$W_{red.}=q^{-1}W$.


title:
Exploring structural mechanisms in disordered materials using the
  activation-relaxation technique

abstract:
  Structural mechanisms in disordered materials like amorphous semi-conductors
and glasses can be explored with the activation-relaxation technique (ART). The
application of a sequence of such mechanisms allows for the generation of
well-relaxed structures. The method and its application in the study of the
microscopic changes in amorphous silicon and silica glass are reviewed, and two
recent improvements are presented.


title: A kinetic approach to granular gases

abstract:
  We address the problem of the so-called ``granular gases'', i.e. gases of
massive particles in rapid movement undergoing inelastic collisions. We
introduce a class of models of driven granular gases for which the stationary
state is the result of the balance between the dissipation and the random
forces which inject energies. These models exhibit a genuine thermodynamic
limit, i.e. at fixed density the mean values of kinetic energy and dissipated
energy per particle are independent of the number $N$ of particles, for large
values of $N$. One has two regimes: when the typical relaxation time $\tau$ of
the driving Brownian process is small compared with the mean collision time
$\tau_c$ the spatial density is nearly homogeneous and the velocity probability
distribution is gaussian. In the opposite limit $\tau \gg \tau_c$ one has
strong spatial clustering, with a fractal distribution of particles, and the
velocity probability distribution strongly deviates from the gaussian one.
Simulations performed in one and two dimensions under the {\it Stosszahlansatz}
Boltzmann approximation confirm the scenario. Furthermore we analyze the
instabilities bringing to the spatial and the velocity clusterization. Firstly,
in the framework of a mean-field model, we explain how the existence of the
inelasticity can bring to a spatial clusterization; on the other side we
discuss, in the framework of a Langevin dynamics treating the collisions in a
mean-field way, how a non-gaussian distribution of velocity can arise. The
comparison between the numerical and the analytical results exhibits an
excellent agreement.