title:
Charge density waves and surface Mott insulators for adlayer structures
  on semiconductors: extended Hubbard modeling

abstract:
  Motivated by the recent experimental evidence of commensurate surface charge
density waves (CDW) in Pb/Ge(111) and Sn/Ge(111) sqrt{3}-adlayer structures, as
well as by the insulating states found on K/Si(111):B and SiC(0001), we have
investigated the role of electron-electron interactions, and also of
electron-phonon coupling, on the narrow surface state band originating from the
outer dangling bond orbitals of the surface. We model the sqrt{3} dangling bond
lattice by an extended two-dimensional Hubbard model at half-filling on a
triangular lattice. We include an on-site Hubbard repulsion U and a
nearest-neighbor Coulomb interaction V, plus a long-ranged Coulomb tail. The
electron-phonon interaction is treated in the deformation potential
approximation. We have explored the phase diagram of this model including the
possibility of commensurate 3x3 phases, using mainly the Hartree-Fock
approximation. For U larger than the bandwidth we find a non-collinear
antiferromagnetic SDW insulator, possibly corresponding to the situation on the
SiC and K/Si surfaces. For U comparable or smaller, a rich phase diagram
arises, with several phases involving combinations of charge and
spin-density-waves (SDW), with or without a net magnetization. We find that
insulating, or partly metallic 3x3 CDW phases can be stabilized by two
different physical mechanisms. One is the inter-site repulsion V, that together
with electron-phonon coupling can lower the energy of a charge modulation. The
other is a novel magnetically-induced Fermi surface nesting, stabilizing a net
cell magnetization of 1/3, plus a collinear SDW, plus an associated weak CDW.
Comparison with available experimental evidence, and also with first-principle
calculations is made.


title: Valence-bond states in dynamical Jahn-Teller molecular systems

abstract:
  We discuss a hopping model of electrons between idealized molecular sites
with local orbital degeneracy and dynamical Jahn-Teller effect, for crystal
field environments of sufficiently high symmetry. For the Mott-insulating case
(one electron per site and large Coulomb repulsions), in the simplest two-fold
degenerate situation, we are led to consider a particular exchange hamiltonian,
describing two isotropic spin-1/2 Heisenberg problems coupled by a quartic term
on equivalent bonds. This twin-exchange hamiltonian applies to a physical
regime in which the inter-orbital singlet is the lowest-energy intermediate
state available for hopping. This regime is favored by a relatively strong
electron-phonon coupling. Using variational arguments, a large-N limit, and
exact diagonalization data, we find that the ground state, in the one
dimensional case, is a solid valence bond state. The situation in the two
dimensional case is less clear. Finally, the behavior of the system upon hole
doping is studied in one dimension.


title:
Magnetic properties of periodic nonuniform spin-1/2 $XX$ chains in a
  random Lorentzian transverse field

abstract:
  Using continued fractions we examine the density of states, transverse
magnetization and static transverse linear susceptibility of a few periodic
nonuniform spin-1/2 $XX$ chains in a random Lorentzian transverse field.


title:
Equilibrium random-field Ising critical scattering in the
  antiferromagnet Fe(0.93)Zn(0.07)F2

abstract:
  It has long been believed that equilibrium random-field Ising model (RFIM)
critical scattering studies are not feasible in dilute antiferromagnets close
to and below Tc(H) because of severe non-equilibrium effects. The high magnetic
concentration Ising antiferromagnet Fe(0.93)Zn(0.07)F2, however, does provide
equilibrium behavior. We have employed scaling techniques to extract the
universal equilibrium scattering line shape, critical exponents nu = 0.87 +-
0.07 and eta = 0.20 +- 0.05, and amplitude ratios of this RFIM system.


title:
Resonant Tunneling between quantum Hall states at filling \nu = 1 and
  \nu = 1/3

abstract:
  We study the problem of resonant tunneling between a Fermi liquid and the
edge of a $\nu=1/3$ fractional quantum Hall state. In the limit of weak
coupling, the system is adequately described within the sequential tunneling
approximation. At low temperatures, however, the system crosses over to a
strong coupling phase, which we analyze using renormalization group techniques.
We find that at low temperatures a "perfect" can be achieved by tuning two
parameters. This resonance has a peak conductance $G = e^2/2h$ and is a
realization of the weak backscattering limit of the $g=1/2$ Luttinger liquid,
as well as the non Fermi liquid fixed point of the two channel Kondo problem.
We discuss several regimes which may be experimentally accessible as well as
the implications for resonances recently observed in cleaved edge overgrowth
structures.


title: Non-equilibrium dynamics in a 3d spin-glass

abstract:
  Non-equilibrium dynamics in a Ag(11%Mn) spin-glass has been studied by low
frequency ac-susceptibility and magnetic relaxation experiments. The results
unequivocally show that spin structures that memorize the cooling process are
imprinted in the system. These imprinted structures disclose themselves through
dramatic changes of the dynamics on re-heating the spin-glass through the
temperatures where intermittent stops or changes of the cooling rate have been
imposed. We can qualitatively interpret our results in terms of the droplet
spin-glass model developed by Fisher and Huse [Phys. Rev. B 38 (1988) 373;
386].


title:
Distribution of the quantum mechanical time-delay matrix for a chaotic
  cavity

abstract:
  We calculate the joint probability distribution of the Wigner-Smith
time-delay matrix $Q=-i\hbar S^{-1} \partial S/\partial \epsilon$ and the
scattering matrix $S$ for scattering from a chaotic cavity with ideal point
contacts. Hereto we prove a conjecture by Wigner about the unitary invariance
property of the distribution functional $P[S(\epsilon)]$ of energy dependent
scattering matrices $S(\epsilon)$. The distribution of the inverse of the
eigenvalues $\tau_1,...,\tau_N$ of $Q$ is found to be the Laguerre ensemble
from random-matrix theory. The eigenvalue density $\rho(\tau)$ is computed
using the method of orthogonal polynomials. This general theory has
applications to the thermopower, magnetoconductance, and capacitance of a
quantum dot.


title: A Cluster Expansion for Dipole Gases

abstract:
  We give a new proof of the well-known upper bound on the correlation function
of a gas of non-overlapping dipoles of fixed length and discrete orientation
working directly in the charge representation, instead of the more usual
sine-Gordon representation.


title:
Transition from quantum Hall to compressible states in the second Landau
  level: new light on the $\nu$=5/2 enigma

abstract:
  Quantum Hall states at filling fraction $\nu$=5/2 are examined by numerical
diagonalization. Spin-polarized and -unpolarized states of systems with $N\le
18$ electrons are studied, neglecting effects of Landau level mixing. We find
that the ground state is spin polarized. It is incompressible and has a large
overlap with paired states like the Pfaffian. For a given sample, the energy
gap is about 11 times smaller than at $\nu$=1/3. Evidence is presented of phase
transitions to compressible states, driven by the interaction strength at short
distance. A reinterpretation of experiments is suggested.


title:
Polaron Effective Mass, Band Distortion, and Self-Trapping in the
  Holstein Molecular Crystal Model

abstract:
  We present polaron effective masses and selected polaron band structures of
the Holstein molecular crystal model in 1-D as computed by the Global-Local
variational method over a wide range of parameters. These results are augmented
and supported by leading orders of both weak- and strong-coupling perturbation
theory. The description of the polaron effective mass and polaron band
distortion that emerges from this work is comprehensive, spanning weak,
intermediate, and strong electron-phonon coupling, and non-adiabatic, weakly
adiabatic, and strongly adiabatic regimes. Using the effective mass as the
primary criterion, the self-trapping transition is precisely defined and
located. Using related band-shape criteria at the Brillouin zone edge, the
onset of band narrowing is also precisely defined and located. These two lines
divide the polaron parameter space into three regimes of distinct polaron
structure, essentially constituting a polaron phase diagram. Though the
self-trapping transition is thusly shown to be a broad and smooth phenomenon at
finite parameter values, consistency with notion of self-trapping as a critical
phenomenon in the adiabatic limit is demonstrated. Generalizations to higher
dimensions are considered, and resolutions of apparent conflicts with
well-known expectations of adiabatic theory are suggested.


title: Effective Hamiltonian for Excitons with Spin Degrees of Freedom

abstract:
  Starting from the conventional electron-hole Hamiltonian ${\cal H}_{eh}$, we
derive an effective Hamiltonian $\tilde{\cal H}_{1s}$ for $1s$ excitons with
spin degrees of freedom. The Hamiltonian describes optical processes close to
the exciton resonance for the case of weak excitation. We show that
straightforward bosonization of ${\cal H}_{eh}$ does not give the correct form
of $\tilde{\cal H}_{1s}$, which we obtain by a projection onto the subspace
spanned by the $1s$ excitons. The resulting relaxation and renormalization
terms generate an interaction between excitons with opposite spin. Moreover,
exciton-exciton repulsive interaction is greatly reduced by the
renormalization. The agreement of the present theory with the experiment
supports the validity of the description of a fermionic system by bosonic
fields in two dimensions.


title:
Reactions at Polymer Interfaces: Transitions from Chemical to
  Diffusion-Control and Mixed Order Kinetics

abstract:
  We study reactions between end-functionalized chains at a polymer-polymer
interface. For small chemical reactivities (the typical case) the number of
diblocks formed, $R_t$, obeys 2nd order chemically controlled kinetics, $R_t
\sim t$, until interfacial saturation. For high reactivities (e.g. radicals) a
transition occurs at short times to 2nd order diffusion-controlled kinetics,
with $R_t \sim t/\ln t$ for unentangled chains while $t/\ln t$ and $t^{1/2}$
regimes occur for entangled chains. Long time kinetics are 1st order and
controlled by diffusion of the more dilute species to the interface: $R_t \sim
t^{1/4}$ for unentangled cases, while $R_t \sim t^{1/4}$ and $t^{1/8}$ regimes
arise for entangled systems. The final 1st order regime is governed by center
of gravity diffusion, $R_t \sim t^{1/2}$.


title:
Coexistence of charge density wave and spin-Peierls orders in
  quarter-filled quasi-one dimensional correlated electron systems

abstract:
  Charge and spin-Peierls instabilities in quarter-filled (n=1/2) compounds
consisting of coupled ladders and/or zig-zag chains are investigated. Hubbard
and t-J models including local Holstein and/or Peierls couplings to the lattice
are studied by numerical techniques. Next nearest neighbor hopping and magnetic
exchange, and short-range Coulomb interactions are also considered. We show
that, generically, these systems undergo instabilities towards the formation of
Charge Density Waves, Bond Order Waves and (generalized) spin-Peierls modulated
structures. Moderate electron-electron and electron-lattice couplings can lead
to a coexistence of these three types of orders. In the ladder, a zig-zag
pattern is stabilized by the Holstein coupling and the nearest-neighbor Coulomb
repulsion. In the case of an isolated chain, bond-centered and site-centered
2k_F and 4k_F modulations are induced by the local Holstein coupling. In
addition, we show that, in contrast to the ladders, a small charge ordering in
the chains, strongly enhances the spin-Peierls instability. Our results are
applied to the NaV_2O_5 compound (trellis lattice) and various phases with
coexisting charge disproportionation and spin-Peierls order are proposed and
discussed in the context of recent experiments. The role of the long-range
Coulomb potential is also outlined.


title:
Comment on ``Critical Behavior in Disordered Quantum Systems Modified by
  Broken Time--Reversal Symmetry''

abstract:
  In a recent Letter [Phys. Rev. Lett. 80, 1003 (1998)] Hussein and Pato
employed the maximum entropy principle (MEP) in order to derive interpolating
ensembles between any pair of universality classes in random matrix theory.
They apply their formalism also to the transition from random matrix to Poisson
statistics of spectra that is observed for the case of the Anderson-type
metal-insulator transition. We point out the problems with the latter
procedure.


title:
Scheme of the replica symmetry breaking for short- ranged Ising spin
  glass within the Bethe- Peierls method

abstract:
  Within the Bethe- Peierls method the for short- ranged Ising spin glass,
recently formulated by Serva and Paladin, the equation for the spin glass
parameter function near the transition to the paramagnetic phase has been
carried out. The form of this equation is qualitatively similar to that for
Sherrington- Kirpatrick model, but quantitatively the order parametr function
depends of the dimension d of the system. In the case d tends to infinity one
obtains well known Parisi solution.


title: Matter-field theory of the Casimir force

abstract:
  A matter-field theory of the Casimir force is formulated in which the
electromagnetic field and collective modes of dielectric media are treated on
an equal footing. In our theory, the Casimir force is attributed to zero-point
energies of the combined matter-field modes. We analyze why some of the
existing theories favor the interpretation of the Casimir force as originating
from zero-point energies of the electromagnetic field and others from those of
the matter.


title:
The phase diagram of magnetic ladders constructed from a composite-spin
  model

abstract:
  White's density matrix renormalization group ({DMRG}) method has been applied
to an $S= 1/2 + 1/2$ composite-spin model, which can also be considered as a
two-leg ladder model. By appropriate choices of the coupling constants this
model allows not only to study how the gap is opened around the gapless
integrable models, but also to interpolate continuously between models with
different spin lengths. We have found indications for the existence of several
different massive phases.


title:
Anomalous ferromagnetic spin fluctuations in an antiferromagnetic
  insulator Pr_{1-x}Ca_{x}MnO_{3}

abstract:
  The high temperature paramagnetic state in an antiferromagnetic (AFM)
insulator Pr_{1-x}Ca_{x}MnO_{3} is characterized by the ferromagnetic (FM) spin
fluctuations with an anomalously small energy scale. The FM fluctuations show a
precipitous decrease of the intensity at the charge ordering temperature
T_{CO}, but persist below T_{CO}, and vanish at the AFM transition temperature
T_{N}. These results demonstrate the importance of the spin ordering for the
complete switching of the FM fluctuation in doped manganites.


title: Flux quantization and superfluid weight in doped antiferromagnets

abstract:
  Doped antiferromagnets, described by a t-t'-J model and a suitable 1/N
expansion, exhibit a metallic phase-modulated antiferromagnetic ground state
close to half-filling. Here we demonstrate that the energy of latter state is
an even periodic function of the external magnetic flux threading the square
lattice in an Aharonov-Bohm geometry. The period is equal to the flux quantum
$\Phi_{0}=2\pi\hbar c/q$ entering the Peierls phase factor of the hopping
matrix elements. Thus flux quantization and a concomitant finite value of
superfluid weight D_s occur along with metallic antiferromagnetism. We argue
that in the context of the present effective model, whereby carriers are
treated as hard-core bosons, the charge q in the associated flux quantum might
be set equal to 2e. Finally, the superconducting transition temperature T_c is
related to D_s linearly, in accordance to the generic Kosterlitz-Thouless type
of transition in a two-dimensional system, signaling the coherence of the phase
fluctuations of the condensate. The calculated dependence of T_c on hole
concentration is qualitatively similar to that observed in the high-temperature
superconducting cuprates.


title: On the accuracy of the density matrix renormalization group method

abstract:
  White's density matrix renormalization group ({DMRG}) method has been applied
to the one-dimensional Ising model in a transverse field ({ITF}), in order to
study the accuracy of the numerical algorithm. Due to the exact solubility of
the {ITF} for any finite chain length, the errors introduced by the basis
truncation procedure could have been directly analysed. By computing different
properties, like the energies of the low-lying levels or the ground-state one-
and two-point correlation functions, we obtained a detailed picture, how these
errors behave as functions of the various model and algorithm parameters. Our
experience with the {ITF} contributes to a better understanding of the {DMRG}
method, and may facilitate its optimization in other applications.


title:
Problems with the definition of renormalized Hamiltonians for
  momentum-space renormalization transformations

abstract:
  For classical lattice systems with finite (Ising) spins, we show that the
implementation of momentum-space renormalization at the level of Hamiltonians
runs into the same type of difficulties as found for real-space
transformations: Renormalized Hamiltonians are ill-defined in certain regions
of the phase diagram.


title: Stability of the Haldane phase in anisotropic magnetic ladders

abstract:
  We have considered the properties of anisotropic two-leg ladder models with
S=1/2 or S=1 spins on the rungs, using White's density matrix renormalization
group method. We have generalized the method by taking into account the
symmetries of the model in order to reduce the dimensions of the matrix to be
diagonalized, thereby making possible to consider more states. The boundaries
in the parameter space of the extended region, where the Haldane phase exists,
are estimated.


title: Response of finite spin-S Heisenberg chains to local perturbations

abstract:
  We consider the properties of finite isotropic antiferromagnetic Heisenberg
chains with S=1/2, 1, 3/2 spins when a weak magnetic field is applied on a few
sites, using White's density matrix renormalization group (DMRG) method. For
the S=1 chain there exists only one length scale in the system which determines
the behavior of the one- and two-point correlation functions both around the
local perturbation and near the free boundary. For the critical,
half-odd-integer spin cases the exponent of the spin-spin correlation function
was found to be $\eta=1$, and the exponent of the decay of the site
magnetization around the perturbed site is $x_m =\eta /2 $. Close to a free
boundary, however, the behavior is completely different for S=1/2 and $S >
1/2$.


title: On the theory of phase transitions in binary fluid mixtures

abstract:
  The microscopic approach to the description of the phase behaviour and
critical phenomena in binary fluid mixtures is proposed. It is based on the
method of collective variables with a reference system. The physical nature of
the order parameter in a binary mixture is discussed. The basic density measure
(Ginsburg-Landau-Wilson Hamiltonian) is obtained in the collective variable
phase space which contains the variable connected with the order parameter of
the system. It is shown that the problem can be reduced to the 3D Ising model
in an external field.


title:
Correlated Wave-Functions and the Absence of Long Range Order in
  Numerical Studies of the Hubbard Model

abstract:
  We present a formulation of the Constrained Path Monte Carlo (CPMC) method
for fermions that uses trial wave-functions that include many-body effects.
This new formulation allows us to implement a whole family of generalized
mean-field states as constraints. As an example, we calculated superconducting
pairing correlation functions for the two-dimensional repulsive Hubbard model
using a BCS trial state as the constraint. We compared the results with the
case where a free-electron trial wave-function is used. We found that the
correlation functions are independent of which state is used as the constraint,
which reaffirms the results previously found by Zhang et. al regarding the
suppression of long range pairing correlations as the system size increases.


title: Field-induced transverse spin ordering in FeBr2

abstract:
  Weak first-order phase transitions from axial to oblique spin ordering in
FeBr2 are evidenced by SQUID magnetometry in axial fields H1(T) above the
multicritical point, Hm = 2.4 MA/m, Tm = 4.6 K, and below the
antiferro-to-paramagnetic phase line, Hc(T), in agreement with recent specific
heat data (Aruga Katori et al., 1996). The ordering of the in-plane moments is
probably due to non-diagonal coupling to the longitudinal ones, both of which
increase dis-continuously at H1(T) only under an additional symmetry-breaking
transverse field.


title:
Negative Electron Drag and Hole-Like behavior in the Integer Quantum
  Hall Regime

abstract:
  Electron drag between two two-dimensional electron gases in magnetic fields
has been observed with a polarity opposite that for zero field. This negative
drag requires that the electrons have a hole-like dispersion. Density
dependence measurements in the integer quantum Hall regime show that drag is
negative only when the upper Landau level of one layer is more than half filled
while the other is less than half filled, indicating that hole-like dispersion
is present in a half of each Landau level. Negative drag is argued to be a
consequence of disorder.


title:
Fluid membranes in hydrodynamic flow fields: Formalism and an
  application to fluctuating quasi-spherical vesicles in shear flow

abstract:
  The dynamics of a single fluid bilayer membrane in an external hydrodynamic
flow field is considered. The deterministic equation of motion for the
configuration is derived taking into account both viscous dissipation in the
surrounding liquid and local incompressibility of the membrane. For
quasi-spherical vesicles in shear flow, thermal fluctuations can be
incorporated in a Langevin-type equation of motion for the deformation
amplitudes. The solution to this equation shows an overdamped oscillatory
approach to a stationary tanktreading shape. Inclination angle and ellipiticity
of the contour are determined as a function of excess area and shear rate.
Comparisons to numerical results and experiments are discussed.


title: Exact Large Deviation Function in the Asymmetric Exclusion Process

abstract:
  By an extension of the Bethe ansatz method used by Gwa and Spohn, we obtain
an exact expression for the large deviation function of the time averaged
current for the fully asymmetric exclusion process in a ring containing $N$
sites and $p$ particles. Using this expression we easily recover the exact
diffusion constant obtained earlier and calculate as well some higher
cumulants. The distribution of the deviation $y$ of the average current is, in
the limit $N \to \infty$, skew and decays like $\exp - (A y^{5/2})$ for $y \to
+ \infty$ and $\exp - (A' |y|^{3/2})$ for $y \to -\infty$. Surprisingly, the
large deviation function has an expression very similar to the pressure (as a
function of the density) of an ideal Bose or Fermi gas in $3d$.


title:
Generalizing Merton's approach of pricing risky debt: some closed form
  results

abstract:
  In this work, I generalize Merton's approach of pricing risky debt to the
case where the interest rate risk is modeled by the CIR term structure. Closed
form result for pricing the debt is given for the case where the firm value has
non-zero correlation with the interest rate. This extends previous closed form
pricing formular of zero-correlation case to the generic one of non-zero
correlation between the firm value and the interest rate.


title: Polyelectrolyte Solutions with Multivalent Salts

abstract:
  We investigate the thermodynamic properties of a polyelectrolyte solution in
a presence of {\it multivalent} salts. The polyions are modeled as rigid
cylinders with the charge distributed uniformly along the major axis. The
solution, besides the polyions, contain monovalent and divalent counterions as
well as monovalent coions. The strong electrostatic attraction existing between
the polyions and the counterions results in formation of clusters consisting of
one polyion and a number of associated monovalent and divalent counterions. The
theory presented in the paper allows us to explicitly construct the Helmholtz
free energy of a polyelectrolyte solution. The characteristic cluster size, as
well as any other thermodynamic property can then be determined by an
appropriate operation on the free energy.


title: Nonlocal electrical conductivity coefficient sigma sub-k

abstract:
  A nonlocal functional is used to calculate superconducting properties of
barium yttrium copper oxide.


title: Biased Diffusion with Correlated Noise

abstract:
  The diffusion of hard-core particles subject to a global bias is described by
a nonlinear, anisotropic generalization of the diffusion equation with
conserved, local noise. Using renormalization group techniques, we analyze the
effect of an additional noise term, with spatially long-ranged correlations, on
the long-time, long-wavelength behavior of this model. Above an upper critical
dimension $d_{LR}$, the long-ranged noise is always relevant. In contrast, for
$d<d_{LR}$, we find a ``weak noise'' regime dominated by short-range noise. As
the range of the noise correlations increases, an intricate sequence of
stability exchanges between different fixed points of the renormalization group
occurs. Both smooth and discontinuous crossovers between the associated
universality classes are observed, reflected in the scaling exponents. We
discuss the necessary techniques in some detail since they are applicable to a
much wider range of problems.


title:
Many-body correlations probed by plasmon-enhanced drag measurements in
  double quantum well structures

abstract:
  Electron drag measurements of electron-electron scattering rates performed
close to the Fermi temperature are reported. While evidence of an enhancement
due to plasmons, as was recently predicted [K. Flensberg and B. Y.-K. Hu, Phys.
Rev. Lett. 73, 3572 (1994)], is found, important differences with the
random-phase approximation based calculations are observed. Although static
correlation effects likely account for part of this difference, it is argued
that correlation-induced multiparticle excitations must be included to account
for the magnitude of the rates and observed density dependences.


title:
One-dimensional Ising model with long-range and random short-range
  interactions

abstract:
  The one-dimensional Ising model in an external magnetic field with uniform
long-range interactions and random short-range interactions satisfying bimodal
annealed distributions is studied. This generalizes the random model discussed
by Paladin et al. (J. Phys. I France 4, 1994, p. 1597). Exact results are
obtained for the thermodynamic functions at arbitrary temperatures, and special
attention is given to the induced and spontaneous magnetization. At low
temperatures the system can exist in a ``ferrimagnetic'' phase with
magnetization 0<m<1, in addition to the usual paramagnetic, ferromagnetic and
antiferromagnetic phases. For a fixed distribution of the random variables the
system presents up to three tricritical points for different intensities of the
long-range interactions. Field-temperature diagrams can present up to four
critical points.


title: Phase diagram of random lattice gases in the annealed limit

abstract:
  An analysis of the random lattice gas in the annealed limit is presented. The
statistical mechanics of disordered lattice systems is briefly reviewed. For
the case of the lattice gas with an arbitrary uniform interaction potential and
random short-range interactions the annealed limit is discussed in detail. By
identifying and extracting an entropy of mixing term, a correct physical
expression for the pressure is explicitly given. As an application, the
one-dimensional lattice gas with uniform long-range interactions and random
short-range interactions satisfying a bimodal annealed probability distribution
is discussed. The model is exactly solved and is shown to present interesting
behavior in the presence of competition between interactions, such as the
presence of three phase transitions at constant temperature and the occurrence
of triple and quadruple points.


title:
Amplitude Ratios and the Approach to Bulk Criticality in Parallel Plate
  Geometries

abstract:
  We present analytical and numerical results for the specific heat and
susceptibility amplitude ratios in parallel plate geometries. The results are
derived using field-theoretic techniques suitable to describe the system in the
bulk limit. Advantages and drawbacks of our method are discussed on the light
of other approaches previously reported in the literature.


title: Rolling friction of a hard cylinder on a viscous plane

abstract:
  The resistance against rolling of a rigid cylinder on a flat viscous surface
is investigated. We found that the rolling-friction coefficient reveals
strongly non-linear dependence on the cylinder's velocity. For low velocity the
rolling-friction coefficient rises with velocity due to increasing deformation
rate of the surface. For larger velocity, however, it decreases with velocity
according to decreasing contact area and deformation of the surface.


title: Ab Initio Evaluation of Plasmon Lifetimes in the Alkali Metals

abstract:
  The anomalous plasmon linewidth dispersion (PLD) measured in K by vom Felde,
Sprosser-Prou, and Fink (Phys. Rev. B 40, 10181 (1989)), has been attributed to
strong dynamical electron-electron correlations. On the basis of ab initio
response calculations, and detailed comparison with experiment, we show that
the PLD of K is, in fact, dominated by decay into particle-hole excitations
involving empty states of d-symmetry. For Li, we shed new light on the physics
of the PLD. Our all-electron results illustrate the importance of ab initio
methods for the study of electronic excitations.


title:
Algebraic Bethe ansatz for integrable Kondo impurities in the
  one-dimensional supersymmetric t-J model

abstract:
  An integrable Kondo problem in the one-dimensional supersymmetric t-J model
is studied by means of the boundary supersymmetric quantum inverse scattering
method. The boundary $K$ matrices depending on the local moments of the
impurities are presented as a nontrivial realization of the graded reflection
equation algebras in a two-dimensional impurity Hilbert space. Further,the
model is solved by using the algebraic Bethe ansatz method and the Bethe ansatz
equations are obtained.


title:
Graded reflection equation algebras and integrable Kondo impurities in
  the one-dimensional t-J model

abstract:
  Integrable Kondo impurities in two cases of the one-dimensional $t-J$ model
are studied by means of the boundary ${\bf Z}_2$-graded quantum inverse
scattering method. The boundary $K$ matrices depending on the local magnetic
moments of the impurities are presented as nontrivial realizations of the
reflection equation algebras in an impurity Hilbert space. Furthermore, these
models are solved by using the algebraic Bethe ansatz method and the Bethe
ansatz equations are obtained.


title:
Evolution of a Bose-Einstein condensate of neutral atoms --- A field
  theoretical approach

abstract:
  The particle distribution in a Bose condensate under the trapping potential
and its time evolution after switching off the trapping potential suddenly are
calculated. We investigate the problem from the viewpoint of quantum field
theory,using a model of self-interacting neutral boson field. Within the
approximation of retainng the most dominant term in the Hamiltonian after
applying the Bogoliubov replacement, we can calculate analytically the particle
distribution as a function of space and time coordinates.


title:
Spin-Dependent Coulomb Blockade in Ferromagnet/Normal-Metal/Ferromagnet
  Double Tunnel Junctions

abstract:
  We study theoretically the spin-dependent transport in
ferromagnet/normal-metal/ferromagnet double tunnel junctions by special
attention to cotunneling in the Coulomb blockade region. The spin accumulation
caused by cotunneling squeezes the Coulomb blockade region when the
magnetizations in the ferromagnetic electrodes are antiparallel. Outside the
squeezed Coulomb blockade region, we propose a new anomalous region, where the
sequential tunneling in one of the spin bands is suppressed by the Coulomb
blockade and that in the other is not. In this region, the tunnel
magnetoresistance oscillates as a function of bias voltage. The temperature
dependences of the tunnel magnetoresistance and the magnitude of the spin
accumulation are calculated.


title:
Sensitivity to temperature perturbations of the ageing states in a
  re-entrant ferromagnet

abstract:
  Dynamic magnetic properties and ageing phenomena of the re-entrant
ferromagnet (Fe0.20Ni0.80)75P16B6Al3 are investigated by time dependent zero
field cooled magnetic relaxation, m (t), measurements. The influence of a
temperature cycling (perturbation), +/- *T, (prior the field application) on
the relaxation rate is investigated both in the low temperature re-entrant spin
glass 'phase' and in the ferromagnetic phase. In the ferromagnetic phase the
influence of a positive and a negative temperature cycle (of equal magnitude)
on the response is almost the same (symmetric response). The result at lower
temperatures, in the RSG 'phase' is asymmetric, with a strongly affected
response for positive, and hardly no influence on the response for negative
temperature cycles. The behaviour at low temperatures is similar to what is
observed in ordinary spin glasses.


title:
Non-Adiabatic Distortion In The Current Distribution Around A Moving
  Vortex

abstract:
  We study the phase distribution around a vortex in uniform motion. We
consider both the cases of neutral and charged superfluids. The motion of the
vortex causes the density of the system to fluctuate. This in turn produces a
compensating current, thus ensuring current conservation locally. We explicitly
calculate this current.


title: Dynamics of two interacting Bose-Einstein condensates

abstract:
  We analize the dynamics of two trapped interacting Bose-Einstein condensates
and indentify two regimes for the evolution: the regime of slow periodic
oscillations and the regime of strong non-linear mixing leading to the damping
of the relative motion of the condensates. We compare our predictions with an
experiment recently performed at JILA.


title:
Dissipative Phase Fluctuations In A Superconductor In Proximity To An
  Electron Gas

abstract:
  We study a two-dimensional superconductor in close proximity to a
two-dimensional metallic sheet. The electrons in the superconducting sheet are
coupled to those in the metallic sheet by the Coulomb interaction only. We
obtain an effective phase-only action for the superconductor by integrating out
all the electronic degrees of freedom in the problem. The Coulomb drag of the
normal electrons in the metallic sheet is found to make the spectrum of
phase-fluctuations in the superconductor, dissipative at long wavelengths. The
dissipative co-efficient $\eta$ is shown to be simply related to the normal
state conductivities of the superconducting layer ($\sigma_S$)and the metallic
sheet ($\sigma_E$) by the relation $\eta \propto {\sigma_S\sigma_E\over
\sigma_S+\sigma_E}$.


title:
Fermi Surface as the Driving Mechanism for Helical Antiferromagnetic
  Ordering in Gd-Y Alloys

abstract:
  The first direct experimental evidence for the Fermi surface (FS) driving the
helical antiferromagnetic ordering in a gadolinium-yttrium alloy is reported.
The presence of a FS sheet capable of nesting is revealed, and the nesting
vector associated with the sheet is found to be in excellent agreement with the
periodicity of the helical ordering.


title:
Ground state of the spin-1/2 Heisenberg antiferromagnet on a
  two-dimensional square-hexagonal-dodecagonal lattice

abstract:
  Up to now, the existence of the the magnetic Neel Long Range Order (NLRO) in
nearest neighbor, spin-1/2 antiferromagnetic (AF) lattice systems has been
examined for seven, from the eleven existing, two-dimensional, uniform
lattices. Plaquettes forming these uniform (Archimedean) lattices (e.g. square,
triangular, kagome) are different regular polygons. An investigation of the
NLRO in the ground state of AF spin systems on the seventh uniform (bipartite)
lattice consisting of squares, hexagons and dodecagons is presented. The NLRO
is shown to occur in this system. A simple conjecture concerning the existence
of the NLRO in the ground state of antiferromagnetic, spin-1/2 systems on two
dimensional, Archimedean lattices, is formulated.


title:
Crossover from pure Ising to random exchange dominated behavior of the
  two-dimensional antiferromagnet Rb2Co1-xMgxF4

abstract:
  The temperature dependence of the uniform susceptibility, chi, of diluted
two-dimensional Ising antiferromagnets Rb2Co1-xMgxF4,0<=x<=0.4, is investigated
in the limit of vanishing external field. Novel data for x = 0.15 are compared
with those obtained for x = 0 and 0.4 by Breed et al. (1969) and Ikeda (1983),
respectively. Whereas in the pure case, x = 0, Fishers (1962) "energetic
contribution" dominates, Aharony et al.s (1979,1986) "random contribution"
becomes increasingly important with increasing x. Taking into account both
terms not only with respect to the global ordering temperature, TN(x), but also
in relation to the "local" phase transition temperatures TN throughout the
Griffiths regime,TN(x)<=TN<=TN(0), a satisfactory quantitative description of
chi vs. T is deduced for any x above the percolation threshold.


title: Vortex matter in mesoscopic superconductors

abstract:
  Superconducting mesoscopic devices in magnetic fields present novel
properties which can only be accounted for by both the quantum confinement of
the Cooper pairs and by the interaction between the magnetic-field-induced
vortices. Sub-micrometer disks, much the same as their semiconductor
counterparts known as quantum dots, are being subject to experimental
investigation by measuring their conducting properties and, more recently,
their magnetization by using state-of-the-art ballistic Hall magnetometry. In
this work I review the main results obtained in these two types of experiments
as well as the current theoretical developments which are contributing to our
understanding of the superconducting condensate in these systems.


title:
Magnetic neutron scattering investigation of the field-induced Griffiths
  phase in FeCl2

abstract:
  Quasielastic neutron scattering has been utilized to investigate the
Griffiths phase induced by an axial magnetic field in FeCl2 at temperatures
Tc(H)<=T<=TN, where Tc(H) is the transition temperature in an external magnetic
field H. We present the temperature dependence of the integrated neutron
scattering intensity at fixed scattering vectors for various magnetic fields.
On cooling below TN the antiferromagnetic short-range fluctuations decrease due
to an increase of the volume fraction which is already antiferromagnetically
ordered. A weighted average of the local contributions to the antiferromagnetic
susceptibility quantitatively describes the scattering cross section.


title:
Characterization of a Quasi-One Dimensional Spin-1/2 Magnet which is
  Gapless and Paramagnetic for g mu_B H <= J and k_B T << J

abstract:
  High field mangetization, field-dependent specific heat measurements, and
zero field inelastic magnetic neutron scattering have been used to explore the
magnetic properties of copper pyrazine dinitrate (Cu(C4H4N2)(NO3)2). The
material is an ideal one-dimensional spin-1/2 Heisenberg antiferromagnet with
nearest neighbor exchange constant J=0.90(1) meV and chains extending along the
orthorhombic a-direction. As opposed to previosly studied molecular-based
spin-1/2 magnetic systems, coppyer pyrazine dinitrate remains gapless and
paramagnetic for g mu_B H/J at least up to 1.4 and for k_B T/J at least down to
0.03 this makes the material an excellent model system for exploring the T=0
critical line which is expected in the H - T phase diagram on the
one-dimensional spin-1/2 Heisenberg antiferromagnet. As a first example of such
a study we present accurate measurements of the Sommerfeld constant of the
spinon gas versus g mu_B H/J < 1.4 which reveal a decrease of the averages
spinon velocity by 32% in that field range. The results are in excellent
agreement with numerical calculations based on the Bethe ansatz with no
adjustable parameters.


title:
On the two- and three-dimensional Lenz-Ising-Onsager problem in presence
  of magnetic field

abstract:
  In this paper a new approach to solving the Ising-Onsager problem in external
magnetic field is investigated. The expression for free energy on one Ising
spin in external field both for the twodimensional and threedimensional Ising
model with interaction of the nearest neighbors are derived. The
representations of free energy being expressed by multidimensional integrals of
Gauss type with the appropriate dimensionality are shown. Possibility of
calculating the integrals and the critical indices on the base of the derived
representations for free energy is investigated.


title:
Realistic Surface Scattering and Surface Bound State Formation in the
  High T_c Superconductor YBa_2Cu_3O_{6+x}

abstract:
  Surface Umklapp scattering of quasiparticles, and surface roughness are shown
to play essential roles in the formation of the surface bound states in
realistic models for YBa_2Cu_3O_{6+x}. The results account for the shape, the
impurity dependence of the height, and for a proposed universal width of the
zero bias conductance peak.


title:
The theory of magneto-transport in quantum dots: 3D-0D and 2D-0D
  tunnelling and selection rules for the angular momentum

abstract:
  A study of magneto-transport through quantum dots is presented. The model
allows to analyze tunnelling both from bulk-like contacts and from 2D
accumulation layers. The fine features in the I-V characteristics due to the
quantum dot states are known to be shifted to different voltages depending upon
the value of the magnetic field. While this effect is also well reproduced by
our calculations, in this work we concentrate on the amplitude of each current
resonance as a function of the magnetic field. Such amplitudes show
oscillations reflecting the variation of the density of states at the Fermi
energy in the emitter. Furthermore the amplitude increases as a function of the
magnetic field for certain features while it decreases for others. In
particular we demonstrate that the behaviour of the amplitude of the current
resonances is linked to the value of the angular momentum of each dot level
through which tunnelling occurs. We show that a selection rule on the angular
momentum must be satisfied. As a consequence, tunnelling through specific dot
states is strongly suppresses and sometimes prohibited altogether by the
presence of the magnetic field. This will allow to extract from the
experimental curves detailed information on the nature of the quantum dot
wavefunctions involved in the electronic transport. Furthermore, when
tunnelling occurs from a 2D accumulation layer to the quantum dot, the presence
of a magnetic field hugely increases the strength of some resonant features.
This effect is predicted by our model and, to the best of our knowledge, has
never been observed.


title: Memorization of short-range potential fluctuations in Landau levels

abstract:
  We calculate energy spectra of a two-dimensional electron system in a
perpendicular magnetic field and periodic potentials of short periods. The
Coulomb interaction is included within a screened Hartree-Fock approximation.
The electrostatic screening is poor and the exchange interaction amplifies the
energy dispersion. We obtain, by numerical iterations, self-consistent
solutions that have a hysteresis-like property. With increasing amplitude of
the external potential the energy dispersion and the electron density become
periodic, and they remain stable when the external potential is reduced to
zero. We explain this property in physical terms and speculate that a real
system could memorize short-range potential fluctuations after the potential
has been turned off.


title: ESR Modes in CsCuCl3 in Pulsed Magnetic Fields

abstract:
  We present ESR results for 35-134GHz in the antiferromagnet CsCuCl3 at
T=1.5K. The external field is applied perpendicular to the hexagonal c-axis.
With our pulsed field facility we reach 50T an unprecedented field for low
temperature ESR. We observe strong resonances up to fields close to the
ferromagnetic region of ~30T. These results are discussed in a model for
antiferromagnetic modes in a two-dimensional frustrated triangular spin system.


title:
Diamagnetic response of cylindrical normal metal - superconductor
  proximity structures with low concentration of scattering centers

abstract:
  We have investigated the diamagnetic response of composite NS proximity
wires, consisting of a clean silver or copper coating, in good electrical
contact to a superconducting niobium or tantalum core. The samples show strong
induced diamagnetism in the normal layer, resulting in a nearly complete
Meissner screening at low temperatures. The temperature dependence of the
linear diamagnetic susceptibility data is successfully described by the
quasiclassical Eilenberger theory including elastic scattering characterised by
a mean free path l. Using the mean free path as the only fit parameter we found
values of l in the range 0.1-1 of the normal metal layer thickness d_N, which
are in rough agreement with the ones obtained from residual resistivity
measurements. The fits are satisfactory over the whole temperature range
between 5 mK and 7 K for values of d_N varying between 1.6 my m and 30 my m.
Although a finite mean free path is necessary to correctly describe the
temperature dependence of the linear response diamagnetic susceptibility, the
measured breakdown fields in the nonlinear regime follow the temperature and
thickness dependence given by the clean limit theory. However, there is a
discrepancy in the absolute values. We argue that in order to reach
quantitative agreement one needs to take into account the mean free path from
the fits of the linear response. [PACS numbers: 74.50.+r, 74.80.-g]


title: Fractional Calculus as a Macroscopic Manifestation of Randomness

abstract:
  We generalize the method of Van Hove so as to deal with the case of
non-ordinary statistical mechanics, that being phenomena with no time-scale
separation. We show that in the case of ordinary statistical mechanics, even if
the adoption of the Van Hove method imposes randomness upon Hamiltonian
dynamics, the resulting statistical process is described using normal calculus
techniques. On the other hand, in the case where there is no time-scale
separation, this generalized version of Van Hove's method not only imposes
randomness upon the microscopic dynamics, but it also transmits randomness to
the macroscopic level. As a result, the correct description of macroscopic
dynamics has to be expressed in terms of the fractional calculus.


title: Correlations in the impenetrable electron gas

abstract:
  We consider non-relativistic electrons in one dimension with infinitely
strong repulsive delta function interaction. We calculate the long-time,
large-distance asymptotics of field-field correlators in the gas phase. The gas
phase at low temperatures is characterized by the ideal gas law. We calculate
the exponential decay, the power law corrections and the constant factor of the
asymptotics. Our results are valid at any temperature. They simplify at low
temperatures, where they are easily recognized as products of free fermionic
correlation functions with corrections arising due to the interaction.

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


title: End-Chain Spin Effects in Haldane Gap Materials

abstract:
  This paper overviews the behavior of the end-chain spins of linear chain
systems possessing a Haldane gap. The physical properties of the end-chain
spins are described by reviewing the results obtained primarily with materials
known as NENP, Ni(C2H8N2)2NO2(ClO4), and NINAZ, Ni(C3H10N2)2N3(ClO4).


title: Are Directed Waves Multifractal?

abstract:
  Wave propagation is studied in a sufficiently anisotropic random medium that
backscattering along one direction can be neglected. A Fokker-Planck equation
is derived the solution to which would provide a complete statistical
description of such directed waves. The Fokker-Planck equation is mapped onto
an su(1,1) ferromagnet and its symmetries are identified. Using the symmetries
asymptotic wave function distributions are computed and used to show that
directed wave functions fill space uniformly and do not have multifractal
character.


title:
Reflection Symmetry and Quantized Hall Resistivity near Quantum Hall
  Transition

abstract:
  We present a direct numerical evidence for reflection symmetry of
longitudinal resistivity $\rho_{xx}$ and quantized Hall resistivity $\rho_{xy}$
near the transition between $\nu=1$ quantum Hall state and insulator, in accord
with the recent experiments. Our results show that a universal scaling behavior
of conductances, $\sigma_{xx}$ and $\sigma_{xy}$, in the transition regime
decide the reflection symmetry of $\rho_{xx}$ and quantization of $\rho_{xy}$,
independent of particle-hole symmetry. We also find that in insulating phase
away from the transition region $\rho_{xy}$ deviates from the quantization and
diverges with $\rho_{xx}$.


title: On the Generalized Kramers Problem with Exponential Memory Friction

abstract:
  The time-dependent transmission coefficient for the generalized Kramers
problem with exponential memory friction has recently been calculated by Kohen
and Tannor [D. Kohen and D. J. Tannor, J. Chem. Phys. Vol. 103, 6013 (1995)]
using a procedure based on the method of reactive flux and the phase space
distribution function. Their analysis is restricted to the high friction regime
or diffusion-limited regime. We recently developed a complementary theory for
the low-friction energy-diffusion-limited regime in the Markovian limit [Sancho
et al., cond-mat/9806001, to appear in J. Chem. Phys.]. Here we generalize our
method to the case of an exponential dissipative memory kernel. We test our
results, as well as those of Kohen and Tannor, against numerical simulations.


title: Interacting Monomer-Dimer Model with Infinitely Many Absorbing States

abstract:
  We study a modified version of the interacting monomer-dimer (IMD) model that
has infinitely many absorbing (IMA) states. Unlike all other previously studied
models with IMA states, the absorbing states can be divided into two equivalent
groups which are dynamically separated infinitely far apart. Monte Carlo
simulations show that this model belongs to the directed Ising universality
class like the ordinary IMD model with two equivalent absorbing states. This
model is the first model with IMA states which does not belong to the directed
percolation (DP) universality class. The DP universality class can be restored
in two ways, i.e., by connecting the two equivalent groups dynamically or by
introducing a symmetry-breaking field between the two groups.


title: Stripe Liquid, Crystal, and Glass Phases of Doped Antiferromagnets

abstract:
  A largely descriptive survey is given of the ordered phases of doped
antiferromagnets, and of the long wavelength properties that can be derived
from an order-parameter theory. In particular, we show that the competition
between the long-range Coulomb repulsion and the strong short-distance tendency
of doped holes to coalesce into regions of supressed antiferromagnetism leads
to a variety of self-organized charge structures on intermediate length scales,
of which ``stripes'' are the most common, both theoretically and
experimentally. These structures lead to a rich assortment of novel electronic
phases and crossover phenomena, as indicated in the title. We use the high
temperature superconductors as the experimentally best-studied examples of
doped antiferromagnets.


title: Microscopic Theory of High Temperature Superconductivity

abstract:
  It is argued that the BCS many-body theory, which is outstandingly successful
for conventional superconductors, does not apply to the high temperature
superconductors and that a realistic theory must take account of the local
electronic structure (stripes). The spin gap proximity effect is a mechanism by
which the charge carriers on the stripes and the spins in the intervening
regions acquire a spin gap at a relatively high temperature, with only strong
repulsive interactions. Superconducting phase order is achieved at a lower
temperature determined by the (relatively low) superfluid density of the doped
insulator. This picture is consistent with the phenomenology of the high
temperature superconductors. It is shown that, in momentum space, the spin gap
first arises in the neighborhood of the points $(0,\pm \pi)$ and $(\pm \pi, 0)$
and then spreads along arcs of the Fermi surface. Some of the experimental
consequences of this picture are discussed.


title:
Spectral function of the spiral spin state in the trestle and ladder
  Hubbard model

abstract:
  Eder and Ohta have found a violation of the Luttinger rule in the spectral
function for the t-t'-J model, which was interpreted as a possible breakdown of
the Tomonaga-Luttinger(TL) description in models where electrons can pass each
other. Here we have computed the spin correlation along with the spectral
function for the one-dimensional t-t' Hubbard model and two-leg Hubbard ladder.
By varying the Hubbard U we have identified that such a phenomenon is in fact a
spinless-fermion-like behavior of holes moving in a spiral spin configuration
that has a spin correlation length of the system size.


title: A Biased Monte Carlo Scheme for Zeolite Structure Solution

abstract:
  We describe a new, biased Monte Carlo scheme to determine the crystal
structures of zeolites from powder diffraction data. We test the method on all
publicly known zeolite materials, with success in all cases. We show that the
method of parallel tempering is a powerful supplement to the biased Monte
Carlo.


title: Gap's in the antiferromagnetic Heisenberg model

abstract:
  We study the one-dimensional spin-1/2 antiferromagnetic Heisenberg model
exposed to an external field, which is a superposition of a homogeneous field
$h_{3}$ and a small periodic field of strength $h_{1}$. For the case of a
transverse staggered field a gap opens, which scales with
$h_{1}^{\epsilon_{1}}$, where $\epsilon_{1}=\epsilon_{1}(h_{3})$ is given by
the critical exponent $\eta_{1}(M(h_{3}))$ defined through the transverse
structure factor of the model at $h_{1}=0$. For the case of a longitudinal
periodic field with wave vector $q=\pi/2$ and strength $h_{q}$ a plateau is
found in the magnetization curve at $M=1/4$. The difference of the upper- and
lower magnetic field scales with $h_{3}^{u}-h_{3}^{l}\sim
h_{q}^{\epsilon_{3}}$, where $\epsilon_{3}=\epsilon_{3}(h_{3})$ is given by the
critical exponent $\eta_{3}(M(h_{3}))$ defined through the longitudinal
structure factor of the model at $h_{q}=0$.


title:
Statistical and dynamical properties of the discrete Sinai model at
  finite times

abstract:
  We study the Sinai model for the diffusion of a particle in a one dimensional
quenched random energy landscape. We consider the particular case of discrete
energy landscapes made of random +/- 1 jumps on the semi infinite line Z+ with
a reflecting wall at the origin. We compare the statistical distribution of the
successive local minima of the energy landscapes, which we derive explicitly,
with the dynamical distribution of the position of the diffusing particle,
which we obtain numerically. At high temperature, the two distributions match
only in the large time asymptotic regime. At low temperature however, we find
even at finite times a clear correspondence between the statistical and
dynamical distributions, with additional interesting oscillatory behaviors.


title: Critical properties of S=1/2 Heisenberg ladders in magnetic fields

abstract:
  The critical properties of the $S=1/2$ Heisenberg two-leg ladders are
investigated in a magnetic field. Combining the exact diagonalization method
and the finite-size-scaling analysis based on conformal field theory, we
calculate the critical exponents of spin correlation functions numerically. For
a strong interchain coupling, magnetization dependence of the critical
exponents shows characteristic behavior depending on the sign of the interchain
coupling. We also calculate the critical exponents for the $S=1/2$ Heisenberg
two-leg ladder with a diagonal interaction, which is thought as a model
Hamiltonian of the organic spin ladder compound
${Cu}_2({1,4-diazacycloheptane})_2{Cl}_4$. Numerical results are compared with
experimental results of temperature dependence of the NMR relaxation rate
$1/T_1$.


title:
Contact angles on heterogeneous surfaces; a new look at Cassie's and
  Wenzel's laws

abstract:
  We consider a three dimensional liquid drop sitting on a rough and chemically
heterogeneous substrate. Using a novel minimization technique on the free
energy of this system, a generalized Young's equation for the contact angle is
found. In certain limits, the Cassie and Wenzel laws, and a new equivalent
rule, applicable in general, are derived. We also propose an equation in the
same spirit as these results but valid on a more `microscopic' level.
Throughout we work under the presence of gravity and keep account of line
tension terms.


title: Eigenvector statistics in non-Hermitian random matrix ensembles

abstract:
  We study statistical properties of the eigenvectors of non-Hermitian random
matrices, concentrating on Ginibre's complex Gaussian ensemble, in which the
real and imaginary parts of each element of an N x N matrix, J, are independent
random variables. Calculating ensemble averages based on the quantity $<
L_\alpha | L_\beta > < R_\beta | R_\alpha >$, where $< L_\alpha |$ and $|
R_\beta >$ are left and right eigenvectors of J, we show for large N that
eigenvectors associated with a pair of eigenvalues are highly correlated if the
two eigenvalues lie close in the complex plane. We examine consequences of
these correlations that are likely to be important in physical applications.


title: Interacting hard-core bosons and surface preroughening

abstract:
  The theory of the preroughening transition of an unreconstructed surface, and
the ensuing disordered flat (DOF) phase, is formulated in terms of interacting
steps. Finite terraces play a crucial role in the formulation. We start by
mapping the statistical mechanics of interacting (up and down) steps onto the
quantum mechanics of two species of one-dimensional hard-core bosons. The
effect of finite terraces translates into a number-non-conserving term in the
boson Hamiltonian, which does not allow a description in terms of fermions, but
leads to a two-chain spin problem. The Heisenberg spin-1 chain is recovered as
a special limiting case. The global phase diagram is rich. We find the DOF
phase is stabilized by short-range repulsions of like steps. On-site repulsion
of up-down steps is essential in producing a DOF phase, whereas an off-site
attraction between them is favorable but not required. Step-step correlation
functions and terrace width distributions can be directly calculated with this
method.


title:
Quasiparticle Transport in the Vortex State of Unconventional
  Superconductors

abstract:
  I consider the problem of the vortex contribution to quasiparticle transport
in unconventional superconductors with line nodes, a nd argue that the magnetic
field dependence of transport coefficients is fixed by the same scattering
processes which limit low-temperature transport. I give no detailed
calculations, but show that qualitatively correct results may be obtained in
the limit of low temperatures and fields by simple physical arguments, based on
estimates of the density of states and relaxation time in analogy with the
zero-field case. I conclude with a brief discussion of the problem of
anisotropy of the field dependence and influence of experimental geometry.


title: Hidden Kondo Effect in a Correlated Electron Chain

abstract:
  We develop a general Bethe Ansatz formalism for diagonalizing an integrable
model of a magnetic impurity of arbitrary spin coupled ferro- or
antiferromagnetically to a chain of interacting electrons. The method is
applied to an open chain, with the exact solution revealing a ``hidden'' Kondo
effect driven by forward electron scattering off the impurity. We argue that
the so-called ``operator reflection matrices'' proposed in recent Bethe Ansatz
studies of related models emulate only forward electron-impurity scattering
which may explain the absence of complete Kondo screening for certain values of
the impurity-electron coupling in these models.


title: Elementary excitations of the symmetric spin-orbital model: The XY limit

abstract:
  The elementary excitations of the 1D, symmetric, spin-orbital model are
investigated by studying two anisotropic versions of the model, the pure XY and
the dimerized XXZ case, with analytical and numerical methods. While they
preserve the symmetry between spin and orbital degrees of freedom, these models
allow for a simple and transparent picture of the low--lying excitations: In
the pure XY case, a phase separation takes place between two phases with
free--fermion like, gapless excitations, while in the dimerized case, the
low-energy effective Hamiltonian reduces to the 1D Ising model with gapped
excitations. In both cases, all the elementary excitations involve simultaneous
flips of the spin and orbital degrees of freedom, a clear indication of the
breakdown of the traditional mean-field theory.


title:
Finite Density of States in a Mixed State of d_x^2-y^2+id_xy
  Superconductor

abstract:
  We have calculated the density of states of quasiparticles in a
d_x^2-y^2+id_xy superconductor, and show that in the mixed state the
quasiparticle spectrum remains gapless because of the Doppler shift by
superflow. It was found that if the d_{xy} order gap $\Delta_1\propto \sqrt{H}$
as suggested by experiments, then thermal conductivity $\kappa \propto
\sqrt{H}$ in accord with experimental data at lowest temperatures. This is an
appended version of the paper published in Phys. Rev. {\bf B 59}, 6024, (1999).
We now also discuss the disorder effects and analyze the H log H crossover at
small fields. We argue that H log H regime is present and disorder effect is
dominant as the field-induced seconary gap is small at small fields.


title: Mobile anti-phase domains in lightly doped lanthanum cuprate

abstract:
  Light hole doping of lanthanum cuprate strongly suppresses the onset of
antiferromagnetic (AF) order. Surprisingly, it simultaneously suppresses the
extrapolated zero temperature sub-lattice magnetization. 139La NQR results in
lightly Li-doped lanthanum cuprate have demonstrated that these effects are
independent of the details of the mobility of the added holes. We propose a
model in which doped holes phase separate into charged domain walls that
surround ``anti-phase'' domains. These domains are mobile down to ~30 K where
they either become pinned to the lattice or evaporate as their constituent
holes become pinned to dopant impurities.


title:
Griffith Singularities and the Replica Instantons in the Random
  Ferromagnet

abstract:
  The problem of existence of non-analytic (Griffith-like) contributions to the
free energy of weakly disordered Ising ferromagnet is studied from the point of
view of the replica theory. The consideration is done in terms of the usual
random temperature Ginzburg-Landau Hamiltonian in space dimensions $D < 4$ in
the zero external magnetic field. It is shown that in the paramagnetic phase,
at temperatures not too close to $T_{c}$ (where the behaviour of the pure
system is correctly described by the Gaussian approximation), the free energy
of the system has additional non-perturbative contribution of the form
$\exp{-(const) \frac{\tau^{(4-D)/2}}{u}}$ (where $\tau = (T-T_{c})/T_{c}$),
which has essential singularity in the parameter $u \to 0$ which describes the
strength of the disorder. It is demonstrated that this contribution appears due
to non-linear localized (instanton-like) solutions of the mean-field stationary
equations which are characterized by the special type of the replica symmetry
breaking . It is argued that physically these replica instantons describe the
contribution from rare spatial "ferromagnetic islands" in which local (random)
temperature is below $T_{c}$


title:
Coexistence of the Electron Cooper Pair and Antiferromagnetic
  Short-Range Correlation in Copper Oxide Materials

abstract:
  Within the fermion-spin theory, the physical properties of the electron
pairing state in the copper oxide materials are discussed. According to the
common form of the electron Cooper pair, it is shown that there is a
coexistence of the electron Cooper pair and magnetic short-range correlation,
and hence the antiferromagnetic short-range correlation can persist into the
superconducting state. Moreover, the mean-field results indicate that the
electron pairing state originating from the pure magnetic interaction in the
two-dimensional t-J model is the local state, and then does not reveal the true
superconducting ground-state.


title: Electronic and magnetic states in doped LaCoO_3

abstract:
  The electronic and magnetic states in doped perovskite cobaltites, (La,
Sr)CoO_3, are studied in the numerically exact diagonalization method on
Co_2O_{11} clusters. For realistic parameter values, it is shown that a high
spin state and an intermediate spin state coexist in one-hole doped clusters
due to strong p-d mixing. The magnetic states in the doped cobaltites obtained
in the calculation explain various experimental results.


title: Symmetry Structure of the Elser-Sloane Quasicrystal

abstract:
  The 4D quasicrystal of Elser and Sloane, obtained from the root lattice E8 by
the cut-and-project method, can be parametrized by the points of an 8D torus.
This allows for an explicit analysis of its point and inflation symmetry
structure.


title: Multifractal dimension spectra in polymer physics

abstract:
  We study the multifractal properties of diffusion in the presence of an
absorbing polymer and report the numerical values of the multifractal dimension
spectra for the case of an absorbing self avoiding walk or random walk.


title:
Derivation and Improvements of the Quantum Canonical Ensemble from a
  Regularized Microcanonical Ensemble

abstract:
  We develop a regularization of the quantum microcanonical ensemble, called a
Gaussian ensemble, which can be used for derivation of the canonical ensemble
from microcanonical principles. The derivation differs from the usual methods
by giving an explanation for the, at the first sight unreasonable,
effectiveness of the canonical ensemble when applied to certain small,
isolated, systems. This method also allows a direct identification between the
parameters of the microcanonical and the canonical ensemble and it yields
simple indicators and rigorous bounds for the effectiveness of the
approximation. Finally, we derive an asymptotic expansion of the microcanonical
corrections to the canonical ensemble for those systems, which are near, but
not quite, at the thermodynamical limit and show how and why the canonical
ensemble can be applied also for systems with exponentially increasing density
of states. The aim throughout the paper is to keep mathematical rigour intact
while attempting to produce results both physically and practically
interesting.


title: Cannon for Neutral Particles

abstract:
  Dynamics of spin-polarized neutral particles, such as neutrons or neutral
atoms and molecules, in magnetic fields is studied. A new regime of motion is
found where particles move mainly in one direction forming a well-collimated
beam. This regime suggests a mechanism for creating devices emitting directed
beams of neutral particles.


title:
Comment on "Orbital Paramagnetism of Electrons in Proximity to a
  Superconductor"

abstract:
  In a recent letter [Phys. Rev. Lett. 80, 5782 (1998)], Bruder and Imry
address the reentrance effect in proximity coupled normal-metal coated
superconducting cylinders discovered by Mota et al. and challenging theoretical
understanding to date. At stake is the explanation of a low-temperature
paramagnetic reentrance of order unity. The authors claim to find a
`significant paramagnetic effect' which they strongly relate to the layout and
findings of the experiment. Here, we show that the straightforward calculation
of the paramagnetic change in the susceptibility along the lines of their
analysis is by several orders of magnitude smaller than experimentally
observed.


title: Relaxation of classical many-body hamiltonians in one dimension

abstract:
  The relaxation of Fourier modes of hamiltonian chains close to equilibrium is
studied in the framework of a simple mode-coupling theory. Explicit estimates
of the dependence of relevant time scales on the energy density (or
temperature) and on the wavenumber of the initial excitation are given. They
are in agreement with previous numerical findings on the approach to
equilibrium and turn out to be also useful in the qualitative interpretation of
them. The theory is compared with molecular dynamics results in the case of the
quartic Fermi-Pasta-Ulam potential.


title:
Metallic temperature dependence of resistivity in perchlorate doped
  polyacetylene

abstract:
  We have measured the electrical resistivity ($\rho$) and the thermoelectric
power (TEP) of the perchlorate (ClO4^-) doped stretch oriented polyacetylene
(PA) film. For the highly conducting samples ($\sigma_{RT} > 41000 S/cm$), the
temperature dependence of the 4-probe resistivity shows positive temperature
coefficient of resistivity (TCR) from T=1.5K to 300K. For the less conducting
samples, the 4-probe resistivity data show the crossover of TCR with a broad
minimum peak at T=T* > 200K. For samples of $\sigma_{RT}$$>$20000 S/cm, the
$\rho (1.5K)/\rho (300K) <1$, i.e., the resistivity at 1.5K is lower than the
room temperature resistivity value. The temperature dependence of the TEP shows
diffusive linear metallic TEP becoming temperature independent below 40K.
Unlike the others who used Cu(ClO_4)_2 for the ClO_4^- doping, the initial
doping material we used is anhydrous Fe(ClO_4)_3 which is crucial to obtain the
positive TCR from T=1.5K to 300K.


title: Optical Investigations of La_{7/8}Sr_{1/8}MnO_3

abstract:
  We investigated temperature dependent optical conductivity spectra of
La_{7/8}Sr_{1/8}MnO_3. Its phonon bending mode starts to be splitted near a
structural phase transition temperature. With further cooling, strength of a
mid-infrared small polaron peak, which is located at 0.4 eV, becomes increased.
These temperature dependent changes in the phonon spectra and the polaron
absorption peaks were explained by a phase separation and a percolation-type
transition.


title: A Dynamic Approach to the Thermodynamics of Superdiffusion

abstract:
  We address the problem of relating thermodynamics to mechanics in the case of
microscopic dynamics without a finite time scale. The solution is obtained by
expressing the Tsallis entropic index q as a function of the Levy index alpha,
and using dynamical rather than probabilistic arguments.


title:
Low-energy excitations in fermionic spin glasses: A quantum-dynamical
  image of Parisi symmetry breaking

abstract:
  We report large effects of Parisi replica permutation symmetry breaking
(RPSB) on elementary excitations of fermionic systems with frustrated magnetic
interactions. The electronic density of states is obtained exactly in the zero
temperature limit for (K=1)-step RPSB together with exact relations for
arbitrary breaking K, which lead to a new fermionic and dynamical Parisi
solution at K=\infty. The Ward identity for charge conservation indicates
RPSB-effects on the conductivity in metallic quantum spin glasses. This implies
that RPSB is essential for any fermionic system showing spin glass sections
within its phase diagram. An astonishing similarity with a neural network
problem is also observed.


title: Coordination Sequences and Critical Points

abstract:
  Coordination sequences of periodic and quasiperiodic graphs are analysed.
These count the number of points that can be reached from a given point of the
graph by a number of steps along its bonds, thus generalising the familiar
coordination number which is just the first member of this series. A possible
application to the theory of critical phenomena in lattice models is outlined.


title: Exact Results on Sinai's Diffusion

abstract:
  We study the continuum version of Sinai's problem of a random walker in a
random force field in one dimension. A method of stochastic representations is
used to represent various probability distributions in this problem (mean
probability density function and first passage time distributions). This method
reproduces already known rigorous results and also confirms directly some
recent results derived using approximation schemes. We demonstrate clearly, in
the Sinai scaling regime, that the disorder dominates the problem and that the
thermal distributions tend to zero-one laws.


title: Non-Dissipative Logic Device NOT Based on Two Coupled Quantum Dots

abstract:
  Non-dissipative dynamics of interacting electrons in two tunnel-coupled
quantum dots is studied theoretically within the framework of the Hubbard
model. Various values of intra-dot Coulomb repulsion energy $U$ and inter-dot
tunneling energy $V$ are considered, which correspond to various size of the
dots and to various distance between them. In the ground state, the average
value of the spin projection (magnetic moment) at each dot is zero. The input
signal (the local external magnetic field $H$) applied to one of the dots at a
time $t=0$ causes the electronic subsystem to evolve in such a way that
magnetic moments of quantum dots become oriented in the opposite directions at
any time $t>0$. For any set of $U$ and $V$, there exist optimal values of $H$
and $t$ which maximize the absolute values of magnetic moments at both dots,
and magnetic moments become almost saturated. Thus, the antiferromagnetic-like
spin ordering can be realized at the stage of coherent temporal evolution, well
before the relaxation to a new ground state at the sacrifice of inelastic
processes. This effect ("dynamical antiferromagnetism") may be used for
implementation of a logic function NOT in an extremely short time. A
possibility to use the arrays of quantum dots as high-speed single-electron
devices of new generation is discussed.


title:
Vortex, skyrmion and elliptical domain wall textures in the
  two-dimensional Hubbard model

abstract:
  The spin and charge texture around doped holes in the two-dimensional Hubbard
model is calculated within an unrestricted spin rotational invariant
slave-boson approach. In the first part we examine in detail the spin structure
around two holes doped in the half-filled system where we have studied cluster
sizes up to 10 x 10. It turns out that the most stable configuration
corresponds to a vortex-antivortex pair which has lower energy than the
Neel-type bipolaron even when one takes the far field contribution into
account. We also obtain skyrmions as local minima of the energy functional but
with higher total energy than the vortex solutions. Additionally we have
investigated the stability of elliptical domain walls for commensurate hole
concentrations. We find that (i) these phases correspond to local minima of the
energy functional only in case of partially filled walls, (ii) elliptical
domain walls are only stable in the low doping regime.


title: Magnetotransport in Quasilattices

abstract:
  The dc conductance and the Hall voltage of planar arrays of interconnected
quantum wires are calculated numerically. Our systems are derived from finite
patches of aperiodic graphs, with completely symmetric scatterers placed on
their vertices which are interconnected by ideal quantum wires. Already in a
periodic square lattice arrangement, quantum interference effects lead to
complicated magnetotransport properties related to the Hofstadter butterfly.
For rectangular Fibonacci grids and other quasiperiodic lattices, we obtain
still more complex fractal patterns. In particular, irrational ratios of edge
lengths and of tile areas in our samples destroy the periodicities with respect
to the Fermi wave vector and the magnetic flux, respectively.


title: Phase-transitions in isotropic extreme type-II superconductors

abstract:
  Using large scale Monte Carlo simulations on a uniformly frustrated 3DXY
model, we report a first order vortex lattice melting transition in clean,
isotropic extreme type-II $\kappa \to \infty$ superconductors. This work
clarifies an important issue: the unpinned vortex liquid is always incoherent
with no phase coherence in any directions for all anisotropies. Previous claims
of a disentangled vortex liquid for isotropic superconductors based on
simulations, are due to finite size effects. We explicitly show that the
effective vortex-line tension vanishes precisely at the superconducting phase
transition in zero magnetic field. This loss of line tension is accompanied by
an abrupt change in the connectivity of the vortex tangle across the
superconductor. We also obtain results indicating that the connectivity of the
vortex tangle changes in a similar way even in finite magnetic field, and
suggest that this could also be associated with a genuine phase-transition.


title: Quiet SDS Josephson Junctions for Quantum Computing

abstract:
  Unconventional superconductors exhibit an order parameter symmetry lower than
the symmetry of the underlying crystal lattice. Recent phase sensitive
experiments on YBCO single crystals have established the d-wave nature of the
cuprate materials, thus identifying unambiguously the first unconventional
superconductor. The sign change in the order parameter can be exploited to
construct a new type of s-wave - d-wave - s-wave Josephson junction exhibiting
a degenerate ground state and a double-periodic current-phase characteristic.
Here we discuss how to make use of these special junction characteristics in
the construction of a quantum computer. Combining such junctions together with
a usual s-wave link into a SQUID loop we obtain what we call a `quiet' qubit
--- a solid state implementation of a quantum bit which remains optimally
isolated from its environment.


title: Localization of Electronic Wave Functions on Quasiperiodic Lattices

abstract:
  We study electronic eigenstates on quasiperiodic lattices using a
tight-binding Hamiltonian in the vertex model. In particular, the
two-dimensional Penrose tiling and the three-dimensional icosahedral
Ammann-Kramer tiling are considered. Our main interest concerns the decay form
and the self-similarity of the electronic wave functions, which we compute
numerically for periodic approximants of the perfect quasiperiodic structure.
In order to investigate the suggested power-law localization of states, we
calculate their participation numbers and structural entropy. We also perform a
multifractal analysis of the eigenstates by standard box-counting methods. Our
results indicate a rather different behaviour of the two- and the
three-dimensional systems. Whereas the eigenstates on the Penrose tiling
typically show power-law localization, this was not observed for the
icosahedral tiling.


title: Phase transition in a chain of quantum vortices

abstract:
  We consider interacting vortices in a quasi-one-dimensional array of
Josephson junctions with small capacitance. If the charging energy of a
junction is of the order of the Josephson energy, the fluctuations of the
superconducting order parameter in the system are considerable, and the
vortices behave as quantum particles. Their density may be tuned by an external
magnetic field, and therefore one can control the commensurability of the
one-dimensional vortex lattice with the lattice of Josephson junctions. We show
that the interplay between the quantum nature of a vortex, and the long-range
interaction between the vortices leads to the existence of a specific
commensurate-incommensurate transition in a one-dimensional vortex lattice. In
the commensurate phase an elementary excitation is a soliton, with energy
separated from the ground state by a finite gap. This gap vanishes in the
incommensurate phase. Each soliton carries a fraction of a flux quantum; the
propagation of solitons leads to a finite resistance of the array. We find the
dependence of the resistance activation energy on the magnetic field and
parameters of the Josephson array. This energy consists of the above-mentioned
gap, and also of a boundary pinning term, which is different in the
commensurate and incommensurate phases. The developed theory allows us to
explain quantitatively the available experimental data.


title: Higher Symmetries in Condensed Matter Physics

abstract:
  We discuss some applications of higher symmetry groups to condensed matter
systems. We give special attention to the groups SO(n) (n = 4, 5, 6, 8) in the
two-dimensional Hubbard model and its generalizations, which model the high T_c
cuprate superconductors.


title: Anomalous Diffusion in Aperiodic Environments

abstract:
  We study the Brownian motion of a classical particle in one-dimensional
inhomogeneous environments where the transition probabilities follow
quasiperiodic or aperiodic distributions. Exploiting an exact correspondence
with the transverse-field Ising model with inhomogeneous couplings we obtain
many new analytical results for the random walk problem. In the absence of
global bias the qualitative behavior of the diffusive motion of the particle
and the corresponding persistence probability strongly depend on the
fluctuation properties of the environment. In environments with bounded
fluctuations the particle shows normal diffusive motion and the diffusion
constant is simply related to the persistence probability. On the other hand in
a medium with unbounded fluctuations the diffusion is ultra-slow, the
displacement of the particle grows on logarithmic time scales. For the
borderline situation with marginal fluctuations both the diffusion exponent and
the persistence exponent are continuously varying functions of the
aperiodicity. Extensions of the results to disordered media and to higher
dimensions are also discussed.


title: Information Capacity of a Hierarchical Neural Network

abstract:
  The information conveyed by a hierarchical attractor neural network is
examined. The network learns sets of correlated patterns (the examples) in the
lowest level of the hierarchical tree and can categorize them at the upper
levels. A way to measure the non-extensive information content of the examples
is formulated. Curves showing the transition from a large retrieval information
to a large categorization information behavior, when the number of examples
increase, are displayed. The conditions for the maximal information are given
as functions of the correlation between examples and the load of concepts.
Numerical simulations support the analytical results.


title: Hysteresis, Avalanches, and Noise: Numerical Methods

abstract:
  In studying the avalanches and noise in a model of hysteresis loops we have
developed two relatively straightforward algorithms which have allowed us to
study large systems efficiently. Our model is the random-field Ising model at
zero temperature, with deterministic albeit random dynamics. The first
algorithm, implemented using sorted lists, scales in computer time as O(N log
N), and asymptotically uses N (sizeof(double)+ sizeof(int)) bits of memory. The
second algorithm, which never generates the random fields, scales in time as
O(N \log N) and asymptotically needs storage of only one bit per spin, about 96
times less memory than the first algorithm. We present results for system sizes
of up to a billion spins, which can be run on a workstation with 128MB of RAM
in a few hours. We also show that important physical questions were resolved
only with the largest of these simulations.


title:
Spontaneous Breaking of Translational Invariance and Spatial
  Condensation in Stationary States on a Ring: I. The Neutral System

abstract:
  We consider a model in which positive and negative particles with equal
densities diffuse in an asymmetric, CP invariant way on a ring. The positive
particles hop clockwise, the negative counter-clockwise and oppositely-charged
adjacent particles may swap positions. The model depends on two parameters.
Analytic calculations using quadratic algebras, inhomogeneous solutions of the
mean-field equations and Monte-Carlo simulations suggest that the model has
three phases. A pure phase in which one has three pinned blocks of only
positive, negative particles and vacancies and in which translational
invariance is broken. A mixed phase in which the current has a linear
dependence on one parameter but is independent of the other one and of the
density of the charged particles. In this phase one has a bump and a fluid. The
bump (condensate) contains positive and negative particles only, the fluid
contains charged particles and vacancies uniformly distributed. The mixed phase
is separated from the disordered phase by a second-order phase-transition which
has many properties of the Bose-Einstein phase-transition observed in
equilibrium. Various critical exponents are found.


title: Momentum Distribution of the Hubbard Model

abstract:
  Using the recently perfected sea-boson method, we compute the momentum
distribution of the one-band Hubbard model in one and two spatial dimensions.
  We compute the asymptotic features of the momentum distribution explicitly
away from half filling for weak coupling in one and two dimensions. While the
results are not exact by any means, they provide the exact asymptotics, namely
they are able to reproduce the exponents obtained by Shulz in one dimension
obtained using Bethe ansatz. The corresponding results in more than one
dimension are therefore as believeable.


title:
Vortex Formation and Dynamics in Superfluid 3He and Analogies in Quantum
  Field Theory

abstract:
  The formation and dynamics of topological defects of different structure has
been of central interest in the study of the 3He superfluids. Compared to
superfluid 4He-II, the variability of the important parameters with temperature
and pressure is wider and many features are more ideal. This has made
experimentally new approaches possible. An example is the formation of
quantized vortices and other topological defects in a rapid time-dependent
phase transition from the normal state to 3He-B. This vortex formation process
is known as the Kibble-Zurek mechanism and is one the central topics of this
review. It demonstrates the use of 3He-B as a model system for quantum fields,
with detailed control from the laboratory.


title:
Some Bounds upon Nonextensivity Parameter Using the Appropriate
  Generalized Distribution Functions

abstract:
  In this study, approximate generalized quantal distribution functions and
their applications, which appeared in the literature so far, have been
summarized. Making use of the generalized Planck radiation law, which have been
obtained by the authors of the present manuscript [Physica A240 (1997) 657],
some alternative bounds for the nonextensivity parameter $q$ has been
estimated. It has been shown that these results are similar to those obtained
by Tsallis et al. [Phys. Rev. E52 (1995) 1447] and by Plastino et al. [Phys.
Lett. A207 (1995) 42].


title:
Strengthened Lindblad inequality: applications in non equilibrium
  thermodynamics and quantum information theory

abstract:
  A strengthened Lindblad inequality has been proved. We have applied this
result for proving a generalized $H$-theorem in non equilibrium thermodynamics.
Information processing also can be considered as some thermodynamic process.
  From this point of view we have proved a strengthened data processing
inequality in quantum information theory.


title: Non perturbative renormalization group approach to surface growth

abstract:
  We present a recently introduced real space renormalization group (RG)
approach to the study of surface growth.
  The method permits us to obtain the properties of the KPZ strong coupling
fixed point, which is not accessible to standard perturbative field theory
approaches. Using this method, and with the aid of small Monte Carlo
calculations for systems of linear size 2 and 4, we calculate the roughness
exponent in dimensions up to d=8. The results agree with the known numerical
values with good accuracy. Furthermore, the method permits us to predict the
absence of an upper critical dimension for KPZ contrarily to recent claims. The
RG scheme is applied to other growth models in different universality classes
and reproduces very well all the observed phenomenology and numerical results.
Intended as a sort of finite size scaling method, the new scheme may simplify
in some cases from a computational point of view the calculation of scaling
exponents of growth processes.


title: Ferron-like states in YBa_2Cu_3O_(6+x)

abstract:
  With the use of the Hubbard model bound hole states in YBa_2Cu_3O_(6+x) are
studied. For the parameters of this crystal the exchange interaction between
the spin-carrying chain ion O^- and Cu-O plane sites is shown to ensure the
formation of a large ferromagnetically ordered clusters around holes in the
plane.


title:
Small Magnetic Polaron Picture of Colossal Magnetoresistance in
  Manganites

abstract:
  We present a small-but-sizeable magnetic polaron picture where transport at
high temperatures is activated while at low temperatures it is band-like. We
show that both double exchange and finite bandwidth effects are important to
understand colossal magnetoresistance as well as the coincidence of the
metal-insulator and the ferromagnetic transitions in manganites. The magnetic
transition is explained using band-like motion of the polarons.


title: Fractionated crystallisation in a polydisperse mixture of hard spheres

abstract:
  We consider the nature of the fluid-solid phase transition in a polydisperse
mixture of hard spheres. For a sufficiently polydisperse mixture
crystallisation occurs with simultaneous fractionation. At the fluid-solid
boundary, a broad fluid diameter distribution is split into a number of
narrower fractions, each of which then crystallises. The number of crystalline
phases increases with the overall level of polydispersity. At high densities,
freezing is followed by a sequence of demixing transitions in the polydisperse
crystal.


title:
Finite Temperature Depinning of a Flux Line from a Nonuniform Columnar
  Defect

abstract:
  A flux line in a Type-II superconductor with a single nonuniform columnar
defect is studied by a perturbative diagrammatic expansion around an annealed
approximation. The system undergoes a finite temperature depinning transition
for the (rather unphysical) on-the-average repulsive columnar defect, provided
that the fluctuations along the axis are sufficiently large to cause some
portions of the column to become attractive. The perturbative expansion is
convergent throughout the weak pinning regime and becomes exact as the
depinning transition is approached, providing an exact determination of the
depinning temperature and the divergence of the localization length.


title: Inelastic Neutron Scattering Study of Mn_12-Acetate

abstract:
  We report zero-field inelastic neutron scattering experiments on a deuterated
powder sample of Mn12-Acetate consisting of a large number of nominally
identical spin-10 magnetic clusters. Our resolution enables us to see a series
of peaks corresponding to transitions between the anisotropy levels within the
spin-10 manifold. A fit to the spin Hamiltonian yields an anisotropy constant
D=0.54 K and a fourth-order diagonal anisotropy coefficient A = 1.2x10^-3 K.
Performed in the absence of a magnetic field, our experiments do not involve
the g-values as fitting parameters, thereby yielding particularly reliable
values of D and A.


title:
Magnetic and Thermodynamic Properties of the Three-Dimensional Periodic
  Anderson Hamiltonian

abstract:
  The three dimensional periodic Anderson model is studied with Quantum Monte
Carlo. We find that the cross-over to the Kondo singlet regime is remarkably
sharp at low temperatures, and that the behavior of magnetic correlations is
consistently reflected in both the thermodynamics and the density of states.
The abruptness of the transition suggests that energy changes associated with
the screening of local moments by conduction electrons might be sufficient to
drive large volume changes in systems where applied pressure tunes the ratio of
interband hybridization to correlation energy.


title: Statistical Features in Learning

abstract:
  We study some features of learning models based on "delayed" and
undifferentiated reinforcement and realized by simple algorithms which may be
considered of a very elementary nature. We show that a modification of the
Hebb-rule works well for this problem in a neural network realization and study
numerically its convergence properties. An illustration for a more "concrete"
situation is provided.


title:
Kinetic Ising model in an oscillating field: Avrami theory for the
  hysteretic response and finite-size scaling for the dynamic phase transition

abstract:
  Hysteresis is studied for a two-dimensional, spin-1/2, nearest-neighbor,
kinetic Ising ferromagnet in an oscillating field, using Monte Carlo
simulations and analytical theory. Attention is focused on large systems and
strong field amplitudes at a temperature below T_c. In this parameter regime,
the magnetization switches through random nucleation and subsequent growth of
many droplets of spins aligned with the applied field. Using a time-dependent
extension of the Kolmogorov-Johnson-Mehl-Avrami (KJMA) theory of metastable
decay, we analyze the statistical properties of the hysteresis-loop area and
the correlation between the magnetization and the applied field. This analysis
enables us to accurately predict the results of extensive Monte Carlo
simulations. The average loop area exhibits an extremely slow approach to an
asymptotic, logarithmic dependence on the product of the amplitude and the
frequency of the applied field. This may explain the inconsistent exponent
estimates reported in previous attempts to fit experimental and numerical data
for the low-frequency behavior of this quantity to a power law. At higher
frequencies we observe a dynamic phase transition. Applying standard
finite-size scaling techniques from the theory of second-order equilibrium
phase transitions to this nonequilibrium phase transition, we obtain estimates
for the transition frequency and the critical exponents (beta/nu \approx 0.11,
gamma/nu \approx 1.84 and nu \approx 1.1). In addition to their significance
for the interpretation of recent experiments on switching in ferromagnetic and
ferroelectric nanoparticles and thin films, our results provide evidence for
the relevance of universality and finite-size scaling to dynamic phase
transitions in spatially extended nonstationary systems.

categories: cond-mat.stat-mech chao-dyn cond-mat.mtrl-sci nlin.CD


title:
Magnetic Properties Of Ni-Mo Single Crystal Alloys: Theory and
  Experiment

abstract:
  The magnetization of Ni_{1-x}Mo_{x} single crystals with x=4,6,8 and 10 % by
weight have been measured at 4.2K using a vibrating sample magnetometer and a
Superconducting Quantum Interference Device (SQUID). The magnetization of the
alloy at these low concentrations and at 0 K have been theoretically determined
by using the tight-binding linearized muffin-tin orbital method coupled with
augmented space recursion. The theoretical data are compared with the
experiment.


title:
Magnetization plateau and quantum phase transition of the S=1/2
  trimerized XXZ spin chain

abstract:
  We study the plateau of the magnetization curve at $M = M_{\rm s}/3$ ($M_{\rm
s}$ is the saturation magnetization) of the $S=1/2$ trimerized $XXZ$ spin
chain. By examining the level crossing of low-lying excitations obtained from
the numerical diagonalization, we precisely determine the phase boundary
between the plateau state and the no-plateau state on the $\Delta-t$ plane,
where $\Delta$ denotes the $XXZ$ anisotropy and $t$ the magnitude of the
trimerization. This quantum phase transition is of the
Berezinskii-Kosterlitz-Thouless type.


title:
Optimization of alloy-analogy-based approaches to the
  infinite-dimensional Hubbard model

abstract:
  An analytical expression for the self-energy of the infinite-dimensional
Hubbard model is proposed that interpolates between different exactly solvable
limits. We profit by the combination of two recent approaches that are based on
the alloy-analogy (Hubbard-III) solution: The modified alloy-analogy (MAA)
which focuses on the strong-coupling regime, and the Edwards-Hertz approach
(EHA) which correctly recovers the weak-coupling regime. Investigating the
high-energy expansion of the EHA self-energy, it turns out that the EHA
reproduces the first three exactly known moments of the spectral density only.
This may be insufficient for the investigation of spontaneous magnetism. The
analysis of the high-energy behavior of the CPA self-consistency equation
allows for a new interpretation of the MAA: The MAA is the only (two-component)
alloy-analogy that correctly takes into account the first four moments of the
spectral density. For small U, however, the MAA does not reproduce Fermi-liquid
properties. The defects of the MAA as well as of the EHA are avoided in the new
approach. We discuss the prospects of the theory and present numerical results
in comparison with essentially exact quantum Monte Carlo data. The correct
high-energy behavior of the self-energy is proved to be a decisive ingredient
for a reliable description of spontaneous magnetism.


title: Closed timelike curves in superfluid $^{3}$He

abstract:
  It is shown that the curved spacetime induced in a thin film of superfluid
$^{3}$He-A by the presence of symmetric vortices with the unbroken symmetry
phase, admits the existence of closed timelike curves through which only
superfluid clusters formed by anti-$^{3}$He atoms can travel and violate
causality.