title:
Magnetic history dependence of metastable states in systems with dipolar
  interactions

abstract:
  We present the results of a Monte Carlo simulation of the ground state and
magnetic relaxation of a model of a thin film consisting on a two-dimensional
square lattice of Heisenberg spins with perpendicular anisotropy K, exchange J
and long-range dipolar interactions g. We have studied the ground state
configurations of this system for a wide range of the interaction parameters
J/g, K/g by means of the simulated annealing procedure, showing that the model
is able to reproduce the different magnetic configurations found in real
samples. We have found the existence of a certain range of K/g, J/g values for
which in-plane and out-of-plane configurations are quasi-degenerated in energy.
We show that when a system in this region of parameters is perturbed by an
external force that is subsequently removed different kinds of ordering may be
induced depending on the followed procedure. In particular, simulations of
relaxations from saturation under an a.c. demagnetizing field or in zero field
are in qualitative agreement with recent experiments on epitaxial and granular
alloy thin films, which show a wide variety of magnetic patterns depending on
their magnetic history.


title:
Impurity Substitution in Bismuth and Thallium Cuprates: Suppression of
  T_c and Estimation of Pseudogap

abstract:
  Suppression of T_c in bilayer bismuth and thallium cuprates, by substitution
of Co impurities at Cu sites, are taken for examination. T_c suppression data
on differently doped Bi2212 and Tl2212 are analysed within the unitary
pair-breaking formalism due to Abrikosov and Gorkov, by fitting data points to
a phenomenological relation valid for weak coupling d-wave superconductors.
Values of the pseudogap magnitude at each doping are thereby estimated within a
"fermi-level density of states suppression" picture. Pseudogap magnitude from
our estimation is observed to have a correspondence with a related
characteristic temperature T^\star obtained by thermoelectric power
measurements. Effects of pseudogap, on the density of states, is studied by
calculating the susceptibility which shows a broad peak at high temperature.
This peak feature in susceptibility is indicative of an unusual metallic state
which could further be explored by systematic other measurements.


title: Kondo effect in a one dimensional d-wave superconductor

abstract:
  We derive a solvable resonant-level type model, to describe an impurity spin
coupled to zero-energy bound states localized at the edge of a one dimensional
d-wave superconductor. This results in a two-channel Kondo effect with a quite
unusual low-temperature thermodynamics. For instance, the local impurity
susceptibility yields a finite maximum at zero temperature (but no
logarithmic-divergence) due to the splitting of the impurity in two Majorana
fermions. Moreover, we make comparisons with the Kondo effect occurring in a
two dimensional d-wave superconductor.


title: Aging effects in free quantum Brownian motion

abstract:
  The two-time correlation function of the displacement of a free quantum
Brownian particle with respect to its position at a given time is calculated
analytically in the framework of the Caldeira and Leggett ohmic dissipation
model (linear coupling of the particle with a thermal bath with a continuum of
modes, proportional to the frequency at low frequency). As a result, at any
temperature, the displacement correlation function exhibits aging, i.e. it
depends explicitly on both times involved and not only on their difference,
even in the limit of large age (or waiting time), in contrast with a dynamic
variable in equilibrium such as the particle velocity. The equilibrium quantum
fluctuation-dissipation theorem (QFDT) has to be modified in order to relate
the displacement response and correlation functions, since this latter quantity
takes into account even those fluctuations of the displacement which take place
during the waiting time. We describe the deviation from the equilibrium QFDT in
terms of an effective temperature which depends both on the time difference and
of the waiting time. The definition of this quantity heavily relies on the
time-dependent diffusion coefficient. The behavior of the effective temperature
as a function of the time difference for given values of the bath temperature
and of the waiting time is analyzed.


title:
Finite-size scaling and conformal anomaly of the Ising model in curved
  space

abstract:
  We study the finite-size scaling of the free energy of the Ising model on
lattices with the topology of the tetrahedron and the octahedron. Our
construction allows to perform changes in the length scale of the model without
altering the distribution of the curvature in the space. We show that the
subleading contribution to the free energy follows a logarithmic dependence, in
agreement with the conformal field theory prediction. The conformal anomaly is
given by the sum of the contributions computed at each of the conical
singularities of the space, except when perfect order of the spins is precluded
by frustration in the model.


title: Subdominant interactions and Hc2 in UBe13

abstract:
  We discuss a model based on a field-induced mixture of two odd-parity
irreducible representations to explain the unusual features of Hc2(T) in the
heavy fermion compound UBe13. We compare its predictions with recent pressure
measurements as well as with the most prominent theoretical models which have
been proposed up to now.


title:
kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br: a Fully Gapped Strong-Coupling
  Superconductor

abstract:
  High-resolution specific-heat measurements of the organic superconductor
kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br in the superconducting (B = 0) and normal (B =
14 T) state show a clearly resolvable anomaly at Tc = 11.5 K and an electronic
contribution, Ces, which can be reasonably well described by strong-coupling
BCS theory. Most importantly, Ces vanishes exponentially in the superconducting
state which gives evidence for a fully gapped order parameter.


title:
Coulomb blockade and cotunneling in single electron circuits with
  on-chip resistors: towards the implementation of R-pump

abstract:
  We report on the investigation of Al single electron structures equipped with
miniature (8 um long) on-chip Cr resistors of R > R_k = h/e^2 = 25.8 kOhm. From
the measurement of the Coulomb blockade in single-junction structures we
evaluated the self-capacitance of our resistors per unit length, c = 62 aF/um.
We demonstrate that the cotunneling current in the transistor samples in the
Coulomb blockade regime obeys the power law, $I \propto V^{3+(R/R_k)}$,
predicted by Odintsov, Bubanja and Sch\"on for a transistor having pure
ohmic-resistance leads. The concept of the three-junction single electron pump
with on-chip resistors (R-pump) is developed. We demonstrate that the
implementation of the R-pump with a relative accuracy of the electron transfer
of 10^{-8} is quite feasible with the technology available.


title:
Determining liquid structure from the tail of the direct correlation
  function

abstract:
  In important early work, Stell showed that one can determine the pair
correlation function h(r) of the hard sphere fluid for all distances r by
specifying only the "tail" of the direct correlation function c(r) at
separations greater than the hard core diameter. We extend this idea in a very
natural way to potentials with a soft repulsive core of finite extent and a
weaker and longer ranged tail. We introduce a new continuous function T(r)
which reduces exactly to the tail of c(r) outside the (soft) core region and
show that both h(r) and c(r) depend only on the "out projection" of T(r): i.e.,
the product of the Boltzmann factor of the repulsive core potential times T(r).
Standard integral equation closures can thus be reinterpreted and assessed in
terms of their predictions for the tail of c(r) and simple approximations for
its form suggest new closures. A new and very efficient variational method is
proposed for solving the Ornstein-Zernike equation given an approximation for
the tail of c. Initial applications of these ideas to the Lennard-Jones and the
hard core Yukawa fluid are discussed.


title: Molecular Crystals and High-Temperature Superconductivity

abstract:
  A simple model of the molecular crystal of $N$ atoms as a statistical mixture
in real space of $NX$ atoms in excited and $N(1-X)$ atoms in well localized
ground state is considered. The phase coherence of the atomic wave functions is
suppose to be absent. A bond energy of crystal is supposed to be a result of
the pair interaction of $NX$ excited atoms. These molecular type pair
excitations do not interact one with another before the metallization, and do
not contribute to the pressure. Nevertheless, the pressure of such kind of
crystals is determined by the interatomic distances, and by the binding energy
of pairs. The possibility of the insulator-superconductor transition of such a
``gas'' of $NX/2$ pairs, ``dissolved'' among $N(1-X)$ atoms in ground state is
discussed. This kind of transition is supposed to occur in the oxigen $O_2$, in
the sulphur $S$, and, possibly, in the xenon $Xe$ crystals under pressure. The
same kind of transition is likely to take place in HTSC materials,
metal-ammonia and hydrogen-palladium solutions under normal conditions, due to
similarity of some of their properties with the corresponding ones of molecular
crystals.


title: Power-law Distribution of Family Names in Japanese Societies

abstract:
  We study the frequency distribution of family names. From a common data base,
we count the number of people who share the same family name. This is the size
of the family. We find that (i) the total number of different family names in a
society scales as a power-law of the population, (ii) the total number of
family names of the same size decreases as the size increases with a power-law
and (iii) the relation between size and rank of a family name also shows a
power-law. These scaling properties are found to be consistent for five
different regional communities in Japan.


title: Ripples in Tapped or Blown Powder

abstract:
  We observe ripples forming on the surface of a granular powder in a container
submitted from below to a series of brief and distinct shocks. After a few
taps, the pattern turns out to be stable against any further shock of the same
amplitude. We find experimentally that the characteristic wavelength of the
pattern is proportional to the amplitude of the shocks. Starting from
consideration involving Darcy's law for air flow through the porous granulate
and avalanche properties, we build up a semi-quantitative model which fits
satisfactorily the set of experimental observations as well as a couple of
additional experiments.


title:
Electrical transport in the ferromagnetic state of manganites:
  Small-polaron metallic conduction at low temperatures

abstract:
  We report measurements of the resistivity in the ferromagnetic state of
epitaxial thin films of La_{1-x}Ca_{x}MnO_{3} and the low temperature specific
heat of a polycrystalline La_{0.8}Ca_{0.2}MnO_{3}. The resistivity below 100 K
can be well fitted by \rho - \rho_{o} = E \omega_{s}/sinh^{2}(\hbar
\omega_{s}/2k_{B}T) with \hbar \omega_{s}/k_{B} \simeq 100 K and E being a
constant. Such behavior is consistent with small-polaron coherent motion which
involves a relaxation due to a soft optical phonon mode. The specific heat data
also suggest the existence of such a phonon mode. The present results thus
provide evidence for small-polaron metallic conduction in the ferromagnetic
state of manganites.


title: The semiclassical tool in mesoscopic physics

abstract:
  Semiclassical methods are extremely valuable in the study of transport and
thermodynamical properties of ballistic microstructures. By expressing the
conductance in terms of classical trajectories, we demonstrate that quantum
interference phenomena depend on the underlying classical dynamics of
non-interacting electrons. In particular, we are able to calculate the
characteristic length of the ballistic conductance fluctuations and the weak
localization peak in the case of chaotic dynamics. Integrable cavities are not
governed by single scales, but their non-generic behavior can also be obtained
from semiclassical expansions (over isolated trajectories or families of
trajectories, depending on the system). The magnetic response of a
microstructure is enhanced with respect to the bulk (Landau) susceptibility,
and the semiclassical approach shows that this enhancement is the largest for
integrable geometries, due to the existence of families of periodic orbits. We
show how the semiclassical tool can be adapted to describe weak residual
disorder, as well as the effects of electron-electron interactions. The
interaction contribution to the magnetic susceptibility also depends on the
nature of the classical dynamics of non-interacting electrons, and is
parametrically larger in the case of integrable systems.


title:
Nanomechanical vibrating wire resonator for phonon spectroscopy in
  Helium

abstract:
  We demonstrate how to build a vibrating wire resonator for phonon excitation
in liquid helium. The resonator is designed as a nanoscopic mechanically
flexible beam machined out of a semiconductor/metal-hybrid. Quenching of the
mechanical resonance around 100 MHz by phonon excitation in liquid ^4He at 4.2
K is shown. First measurements operating the nano-resonator in a dilution of
^3He/^4He at 30 mK are presented.


title: Spintronics: electron spin coherence, entanglement, and transport

abstract:
  Prospect of building spintronic devices in which electron spins store and
transport information has attracted strong attention in recent years. Here we
present some of our representative theoretical results on three fundamental
aspects of spintronics: spin coherence, spin entanglement, and spin transport.
In particular, we discuss our detailed quantitative theory for spin relaxation
and coherence in electronic materials, resolving in the process a long-standing
puzzle of why spin relaxation is extremely fast in Al (compared with other
simple metals). In the study of spin entanglement, we consider two electrons in
a coupled GaAs double-quantum-dot structure and explore the Hilbert space of
the double dot. The specific goal is to critically assess the quantitative
aspects of the proposed spin-based quantum dot quantum computer architecture.
Furthermore, we discuss our theory of spin-polarized transport across a
semiconductor-metal interface. In particular, we study Andreev reflection,
which enables us to quantify the degree of carrier spin polarization and the
strength of interfacial scattering.


title: Weak localisation, interaction effects and the metallic phase in p-SiGe

abstract:
  Magnetoresistance results are presented for p-SiGe samples on the metallic
side of the B=0 metal-insulator transition. It was possible to separate the
weak localisation and Zeeman interaction effects but the results could not be
explained quantitatively within the framework of standard theories for quantum
corrections of a weakly interacting 2-dimensional system. Analysis using a
theory for interaction corrections at intermediate temperatures, recently
proposed by Zala, Narozhny and Aleiner, provided values of the Fermi liquid
parameter $F_0^{\sigma}$ of order -0.5. Similar values also explain the linear
increase of resistance with temperature characteristic of the metallic phase. e


title: Scenario of Superconducting Transition for quasi-2D HTS

abstract:
  We discuss the scenario of superconducing transition for quasi two dimension
HTS with spin fluctuation pairing mechanism. At mean field temperature of 2D
superconducting transition the interaction of fluctuation spin waves with holes
in copper-oxigen planes leads to the pairing of holes and to the fluctuation
generation of superconducting regions, and also to the essential temperature
dependence of the strength of the interlayer coupling. At decreasing
temperature and sufficiently small interlayer coupling the transition of the
sample to coherent superconducting state occurs.


title:
Condensation Energy and Spectral Functions in High Temperature
  Superconductors

abstract:
  If high temperature cuprate superconductivity is due to electronic
correlations, then the energy difference between the normal and superconducting
states can be expressed in terms of the occupied part of the single particle
spectral function. The latter can, in principle, be determined from angle
resolved photoemission (ARPES) data. As a consequence, the energy gain driving
the development of the superconducting state is intimately related to the
dramatic changes in the photoemission lineshape when going below Tc. These
points are illustrated in the context of the "mode" model used to fit ARPES
data in the normal and superconducting states, where the question of kinetic
energy versus potential energy driven superconductivity is explored in detail.
We use our findings to comment on the relation of ARPES data to the
condensation energy, and to various other experimental data. In particular, our
results suggest that the nature of the superconducting transition is strongly
related to how anomalous (non Fermi liquid like) the normal state spectral
function is, and as such, is dependent upon the doping level.


title:
Generalized Phase Field Model for Computer Simulation of Grain Growth in
  Anisotropic Systems

abstract:
  We study the dynamics and morphology of grain growth with anisotropic energy
and mobility of grain boundaries using a generalized phase field model. In
contrast to previous studies, both inclination and misorientation of the
boundaries are considered. The model is first validated against exact
analytical solutions for the classical problem of an island grain embedded in
an infinite matrix. It is found that grain boundary energy anisotropy has a
much stronger effect on grain shape than that of mobility anisotropy. In a
polycrystalline system with mobility anisotropy, we find that the system
evolves in a non-self-similar manner and grain shape anisotropy develops.
However, the average area of the grains grows linearly with time, as in an
isotropic system.


title: RF coupling of a Bose-Einstein condensate in a TOP trap

abstract:
  A radio frequency (RF) transition is used to convert a pure F=2,mF=2 87Rb
Bose-Einstein condensate confined in a TOP trap to a mixture of F=2,mF=2 and
F=2,mF=1 states. We show that the nature of this coupling process is strongly
influenced by the presence of the time varying field of the TOP trap, and
complicated by the presence of multiple Zeeman substates. In particular, the
effective Rabi frequency associated with the applied RF field is not constant
across the spatial extent of the cloud leading to a complex geometry for
atom-laser output coupling and `averaging out' of Rabi oscillations. Further a
time-varying detuning can give rise to complex spatial structures.


title: Corrections to scaling in the forest-fire model

abstract:
  We present a systematic study of corrections to scaling in the self-organized
critical forest-fire model. The analysis of the steady-state condition for the
density of trees allows us to pinpoint the presence of these corrections, which
take the form of subdominant exponents modifying the standard finite-size
scaling form. Applying an extended version of the moment analysis technique, we
find the scaling region of the model and compute the first non-trivial
corrections to scaling.


title: Origin of the giant magnetic moments of Fe impurities on and in Cs films

abstract:
  To explore the origin of the observed giant magnetic moments ($\sim 7 \mu_B$)
of Fe impurities on the surface and in the bulk of Cs films, we have performed
the relativistic LSDA + U calculations using the linearized muffin-tin orbital
(LMTO) band method. We have found that Fe impurities in Cs behave differently
from those in noble metals or in Pd. Whereas the induced spin polarization of
Cs atoms is negligible, the Fe ion itself is found to be the source of the
giant magnetic moment. The 3d electrons of Fe in Cs are localized as the 4f
electrons in rare-earth ions so that the orbital magnetic moment becomes as
large as the spin magnetic moment. The calculated total magnetic moment of $M =
6.43 \mu_B$, which comes mainly from Fe ion, is close to the experimentally
observed value.


title: Overlap integral for quantum skyrmions

abstract:
  We made use a simplified form for the quantum skyrmion wave function based on
the spin coherent states to obtain the analytical expression for appropriate
overlap integral.


title:
Thermodynamic Properties of Strongly Degenerate Interacting Fermi
  Systems

abstract:
  A numerical approach is presented which allows to calculate the equilibrium
properties of Fermi systems which are both, strongly coupled and strongly
degenerate. Based on a novel path integral representation of the many-particle
density operator, quantum and spin effects are included. As an illustration,
results for the pressure and energy of an electron-proton plasma are presented.


title:
Magnetoelastic coupling and spin excitations in the spin-gap system
  (VO)$_2$P$_2$O$_7$: A Raman scattering study

abstract:
  Single crystals of the spin-gap system $\rm (VO)_2P_2O_7$ have been
investigated by means of Raman scattering. At temperatures below T=75
K~$\approx 2\Delta_{01}({\bf k}=0)$, with $\Delta_{01}$ the singlet-triplet
gap, a shoulder appears at 47 cm$^{-1}$. An analysis of the polarization
selection rules and the temperature dependence of the scattering intensity
leads to its identification as a singlet bound state with very small binding
energy. Therefore in contrast to recent theoretical models the importance of
frustration for this compound must be rejected. The observation of a strong
anharmonicity of three phonons with energies close to $\Delta_{01}$ at the zone
boundary and additional quasielastic light scattering, however, both point to a
strong spin-phonon coupling in this system. We propose this coupling to be the
origin of the second bound triplet state observed in recent neutron scattering
experiments.


title: Economic Fluctuations and Diffusion

abstract:
  Stock price changes occur through transactions, just as diffusion in physical
systems occurs through molecular collisions. We systematically explore this
analogy and quantify the relation between trading activity - measured by the
number of transactions $N_{\Delta t}$ - and the price change $G_{\Delta t}$,
for a given stock, over a time interval $[t, t+\Delta t]$. To this end, we
analyze a database documenting every transaction for 1000 US stocks over the
two-year period 1994-1995. We find that price movements are equivalent to a
complex variant of diffusion, where the diffusion coefficient fluctuates
drastically in time. We relate the analog of the diffusion coefficient to two
microscopic quantities: (i) the number of transactions $N_{\Delta t}$ in
$\Delta t$, which is the analog of the number of collisions and (ii) the local
variance $w^2_{\Delta t}$ of the price changes for all transactions in $\Delta
t$, which is the analog of the local mean square displacement between
collisions. We study the distributions of both $N_{\Delta t}$ and $w_{\Delta
t}$, and find that they display power-law tails. Further, we find that
$N_{\Delta t}$ displays long-range power-law correlations in time, whereas
$w_{\Delta t}$ does not. Our results are consistent with the interpretation
that the pronounced tails of the distribution of $G_{\Delta t} are due to
$w_{\Delta t}$, and that the long-range correlations previously found for $|
G_{\Delta t} |$ are due to $N_{\Delta t}$.


title:
Torque magnetometry on single-crystal high temperature superconductors
  near the critical temperature: a scaling approach

abstract:
  Angular-dependent magnetic torque measurements performed near the critical
temperature on single crystals of HgBa_{2}CuO_{4+y}, La_{2-x}Sr{x}CuO_{4}, and
YBa_{2}Cu_{3}O_{6.93} are scaled, following the 3D XY model, in order to
determine the scaling function dG^{\pm}(z)/dz which describes the universal
critical properties near T_{c}. A systematic shift of the scaling function with
increasing effective mass anisotropy \gamma = (m_{ab}*/m_{c}*)^{1/2} is
observed, which may be understood in terms of a 3D-2D crossover. Further
evidence for a 3D-2D crossover is found from temperature-dependent torque
measurements carried out in different magnetic fields at different field
orientations \delta, which show a quasi 2D "crossing region'' (M*,T*). The
occurrence of this "crossing phenomenon'' is explained in a phenomenological
way from the weak z dependence of the scaling function around a value z = z*.
The "crossing'' temperature T* is found to be angular-dependent. Torque
measurements above T_{c} reveal that fluctuations are strongly enhanced in the
underdoped regime where the anisotropy is large, whereas they are less
important in the overdoped regime.


title:
Real-time Kadanoff-Baym approach to plasma oscillations in a correlated
  electron gas

abstract:
  A nonequilibrium Green's functions approach to the collective response of
correlated Coulomb systems at finite temperature is presented. It is shown that
solving Kadanoff-Baym type equations of motion for the two-time correlation
functions with the external perturbing field included allows to compute the
plasmon spectrum with collision effects in a systematic and consistent way. The
scheme has ``built-in'' sum rule preservation and is simpler to implement
numerically than the equivalent equilibrium approach based on the
Bethe-Salpeter equation.


title: Very long transients in globally coupled maps

abstract:
  Very long transients are found in the partially ordered phase of type II of
globally coupled logistic maps. The transients always lead the system in this
phase to a state with a few synchronous clusters. This transient behaviour is
not significantly influenced by the introduction of weak noises. However, such
noises generally favor cluster partitions with more stable periodic dynamics.


title: Thermodynamic Properties of Correlated Strongly Degenerate Plasmas

abstract:
  An efficient numerical approach to equilibrium properties of strongly coupled
systems which include a subsystem of fermionic quantum particles and a
subsystem of classical particles is presented. It uses an improved path
integral representation of the many-particle density operator and allows to
describe situations of strong coupling and strong degeneracy, where analytical
theories fail. A novel numerical method is developed, which allows to treat
degenerate systems with full account of the spin scatistics. Numerical results
for thermodynamic properties such as internal energy, pressure and pair
correlation functions are presented over a wide range of degeneracy parameter.


title:
Instability and Fluctuations of Flux Lines with Point Impurities in a
  Parallel Current

abstract:
  A parallel current can destabilize a single flux line (FL), or an array of
FLs. We consider the effects of pinning by point impurities on this
instability. The presence of impurities destroys the long-range order of a flux
lattice, leading to the so called Bragg glass (BrG) phase. We first show that
the long-range topological order of the BrG is also destroyed by a parallel
current. Nonetheless, some degree of short-range order should remain, whose
destruction by thermal and impurity fluctuations, as well as the current, is
studied here. To this end, we employ a cage model for a single FL in the
presence of impurities and current, and study it analytically (by replica
variational methods), and numerically (using a transfer matrix technique). The
results are in good agreement, and in conjunction with a Lindemann criterion,
provide the boundary in the magnetic field--temperature plane for destruction
of short-range order. In all cases, we find that the addition of impurities or
current (singly or in combination) leads to further increase in equilibrium FL
fluctuations. Thus pinning to point impurities does not stabilize FLs in a
parallel current $j_z$, although the onset of this instability is much delayed
due to large potential barriers that diverge as $j_z^{-\mu}$.


title:
Radial fluctuations induced stabilization of the ordered state in two
  dimensional classical clusters

abstract:
  Melting of two dimensional (2D) clusters of classical particles is studied
using Brownian dynamics and Langevin molecular dynamics simulations. The
particles are confined by a circular hard wall or a parabolic external
potential and interact through a dipole or a screened Coulomb potential. We
found that with decreasing strength of the inter--particle interaction clusters
with short-range inter-particle interaction which are confined by a hard wall
exhibit a re-entrant behavior in its orientational order.


title:
Quantum Kinetic Theory for Laser Plasmas. Dynamical Screening in Strong
  Fields

abstract:
  A quantum kinetic theory for correlated charged-particle systems in strong
time-dependent electromagnetic fields is developed. Our approach is based on a
systematic gauge-invariant nonequilibrium Green's functions formulation.
Extending our previous analysis (Kremp et al., Phys. Rev. E vol. 60, p. 4725
(1999) we concentrate on the selfconsistent treatment of dynamical screening
and electromagnetic fields which is applicable to arbitrary nonequilibrium
situations. The resulting kinetic equation generalizes previous results to
quantum plasmas with full dynamical screening and includes many-body effects.
It is, in particular, applicable to the interaction of dense plasmas with
strong electromagnetic fields, including laser fields and x-rays. Furthermore,
results for the modification of the plasma screening and the longitudinal field
fluctuations due to the electromagnetic field are presented.


title: Distribution of repetitions of ancestors in genealogical trees

abstract:
  We calculate the probability distribution of repetitions of ancestors in a
genealogical tree for simple neutral models of a closed population with sexual
reproduction and non-overlapping generations. Each ancestor at generation g in
the past has a weight w which is (up to a normalization) the number of times
this ancestor appears in the genealogical tree of an individual at present. The
distribution P_g(w) of these weights reaches a stationary shape P_\infty(w) for
large g, i.e. for a large number of generations back in the past. For small w,
P_\infty(w) is a power law with a non-trivial exponent which can be computed
exactly using a standard procedure of the renormalization group approach. Some
extensions of the model are discussed and the effect of these variants on the
shape of P_\infty(w) are analysed.


title: Towards a Field Theory of the Plateau Transition

abstract:
  We suggest a procedure for calculating correlation functions of the local
densities of states (DOS) at the plateau transitions in the Integer Quantum
Hall effect (IQHE). We argue that their correlation functions are appropriately
described in terms of the SL($2,{\Bbb C}$)/SU(2) WZNW model (at the usual Ka{\v
c}--Moody point and with the level $6 \leq k \leq 8$). In this model we have
identified the operators corresponding to the local DOS, and derived the
partial differential equation determining their correlation functions. The OPEs
for powers of the local DOS obtained from this equation are in agreement with
available results.


title:
From inherent structures to pure states: some simple remarks and
  examples

abstract:
  The notions of pure states and inherent structures, i.e. stable
configurations against 1-spin flip are discussed. We explain why these
different concepts accidentally coincide in mean-field models with infinite
connectivity and present an exactly solvable unidimensional model where they do
not. At zero temperature pure states are to some extent related to k-spin flip
stable configurations with k going to infinity after the thermodynamical limit
has been taken. This relationship is supported by an explicit analysis of the
TAP equations and calculation of the number of pure states and k-spin flips
stable configurations in a mean-field model with finite couplings. Finally we
discuss the relevance of the concepts of pure states and inherent structures in
finite dimensional glassy systems.


title:
Theoretical analysis of Cooper-pair phase fluctuations in underdoped
  cuprates: a spin-fluctuation exchange study

abstract:
  We study Cooper-pair phase fluctuations in cuprate superconductors for a spin
fluctuation pairing interaction. Using an electronic theory we calculate in
particular for the underdoped cuprate superconductors the superfluid density
$n_s(T)$, the superconducting transition temperature $T_c(x) \propto n_s$ below
which phase coherent Cooper-pairs occur, and $T_c^{*}(x)$ where the phase
incoherent Cooper-pairs disappear. Also we present results for the penetration
depth $\lambda(x,T)$ and for the weak pseudogap temperature $T^{*}(x)$ at which
a gap structure occurs in the spectral density. A Mei{\ss}ner effect is
obtained only for $T < T_c$. We find that phase fluctuations become
increasingly important in the underdoped regime and lead to a reduction of
$T_c$ in good agreement with the experimental situation.


title:
A study of the mechanisms of the semi-insulating conversion of InP by
  anelastic spectroscopy

abstract:
  Elastic energy absorption measurements versus temperature on semiconducting,
semi-insulating (SI) and Fe-doped InP are reported. A thermally activated
relaxation process is found only in the SI state, which is identified with the
hopping of H atoms trapped at In vacancies. It is proposed that the presence of
In vacancies in InP prepared by the liquid encapsulated Czochralski method is
due to the lowering of their energy by the saturation of the P dangling bonds
with H atoms dissolved from the capping liquid containing H2O. The conversion
of iron-free InP to the SI state following high temperature treatments would be
due to H loss with the transformation of the H-saturated In vacancies, V_In-H_4
donors, into neutral and acceptor V_In-H_n complexes with n < 4. Such complexes
would produce the observed anelastic relaxation process and may also act as
deep acceptors which neutralize unwanted donor impurities.


title:
Giant Peak of the 1/f Noise at the Metal-Insulator Transitions in Low-Tc
  CMR Manganites: Evidence of the Percolation Threshold at Tc

abstract:
  We observed a dramatic peak in the 1/f noise at the metal-insulator
transition (MIT) in low-Tc manganites. This many-orders-of-magnitude noise
enhancement is observed for both polycrystalline and single-crystal samples of
La{5/8-y}Pr{y}Ca{3/8}MnO{3} (y=0.35-0.4) in zero magnetic field and
Pr{1-x}Ca{x}MnO{3} (x=0.35-0.5) in magnetic field. This observation strongly
suggests that the microscopic phase separation in the low-Tc manganites causes
formation of a percolation network, and that the observed MIT is a percolation
threshold.


title:
A New Direct Process to Prepare YBa2Cu3O7- films on Biaxially
  Textured Ag{110}<211>

abstract:
  YBCO films were successfully prepared on biaxially textured Ag{110}<211>
substrates by using pulsed laser deposition. X-ray diffraction results showed
that the degree of preferential orientation of Ag{110}<211> substrates varied
with increasing annealing temperature. With a thin template layer deposited at
low temperature, YBCO film with c-axis orientation and in-plane biaxial
alignment could be obtained at high deposition temperature. Scanning electron
microscopy observation revealed that YBCO grains enlarged but Ag grains on the
surface of the YBCO films became smaller with increasing deposition
temperature. At optimal deposition conditions, Ag atoms diffuse into the YBCO
grain boundaries, and then fill in the weak-link regions in the YBCO film,
resulting in the conduction easier. Jc value of 5105A/cm2 was obtained
at 77K and zero magnetic field for the best YBCO film in our work.


title:
Magnetization Jump in the Spin-1/2 XXZ Chain with Next-Nearest-Neighbour
  Coupling

abstract:
  The zero-temperature magnetization process of a one-dimensional S=1/2 spin
system with next-nearest-neighbour coupling alpha and anisotropy delta is
studied. There is a region in the parameter space where the magnetization has a
jump at certain external field. The boundary of this region is determined. The
two cases are distinguished according as alpha<1/4 or alpha>1/4. In the latter
case, the energy dispersion of the low-lying excitations exhibits a double
minimum structure.


title: Influence of Wetting Properties on Diffusion in a Confined Fluid

abstract:
  We briefly discuss how the wetting properties of a fluid/solid interface can
indirectly influence the diffusion properties of fluid confined between two
solid walls. This influence is related to the variability of the hydrodynamic
boundary conditions at the interface, which correlates to the wetting
properties.


title: Classical many-particle distribution functions: some new applications

abstract:
  We present a new purely equilibrium microscopic approach to the description
of liquid-glass transition in terms of space symmetry breaking of three- and
four-particle distribution functions in the cases of two and three dimensions,
respectively. The approach has some features of spin glass theories as well as
of density-functional theories of freezing.


title: Orientational Phase Transition in solid C_60: Bifurcation Approach

abstract:
  A simple model for the angular dependent interaction between C_60 molecules
on the face-centered cubic lattice is proposed. The bifurcations of the
solutions of the nonlinear integral equations for orientational distribution
functions in the mean field approximation are analyzed and the orientational
phase transition in solid C_60 is described. The quantitative results for the
orientational phase transition are in accordance with experimental data.


title:
Non-linear response of a Kondo system: Perturbation approach to the time
  dependent Anderson impurity model

abstract:
  Nonlinear tunneling current through a quantum dot
  (an Anderson impurity system) subject to both constant and alternating
electric fields is studied in the Kondo regime. A systematic diagram technique
is developed for perturbation study of the current in physical systems out of
equilibrium governed by time - dependent Hamiltonians of the Anderson and the
Kondo models. The ensuing calculations prove to be too complicated for the
Anderson model, and hence, a mapping on an effective Kondo problem is called
for. This is achieved by constructing a time - dependent version of the
Schrieffer - Wolff transformation. Perturbation expansion of the current is
then carried out up to third order in the Kondo coupling J yielding a set of
remarkably simple analytical expressions for the current. The zero - bias
anomaly of the direct current differential conductance is shown to be
suppressed by the alternating field while side peaks develop at finite source -
drain voltage. Both the direct component and the first harmonics of the time -
dependent response are equally enhanced due to the Kondo effect, while
amplitudes of higher harmonics are shown to be relatively small. A zero
alternating bias anomaly is found in the alternating current differential
conductance, that is, it peaks around zero alternating bias. This peak is
suppressed by the constant bias. No side peaks show up in the differential
alternating - conductance but their counterpart is found in the derivative of
the alternating current with respect to the direct bias. The results pertaining
to nonlinear response are shown to be valid also below the Kondo temperature.


title:
Five-loop renormalization-group expansions for the three-dimensional
  n-vector cubic model and critical exponents for impure Ising systems

abstract:
  The renormalization-group (RG) functions for the three-dimensional n-vector
cubic model are calculated in the five-loop approximation. High-precision
numerical estimates for the asymptotic critical exponents of the
three-dimensional impure Ising systems are extracted from the five-loop RG
series by means of the Pade-Borel-Leroy resummation under n = 0. These
exponents are found to be: \gamma = 1.325 +/- 0.003, \eta = 0.025 +/- 0.01, \nu
= 0.671 +/- 0.005, \alpha = - 0.0125 +/- 0.008, \beta = 0.344 +/- 0.006. For
the correction-to-scaling exponent, the less accurate estimate \omega = 0.32
+/- 0.06 is obtained.

categories: cond-mat.stat-mech hep-lat hep-ph hep-th


title: SU(4) Dynamical Symmetry and the Origin of Pseudogaps

abstract:
  This paper has been withdrawn by the author due to incomplete interpretation
for the results.

categories: cond-mat.supr-con cond-mat.str-el nucl-th physics.atm-clus physics.atom-ph


title:
Thermodynamic Constraints on the Magnetic Field Dependence of the
  Neutron Resonance in Cuprate Superconductors

abstract:
  In order to test the recently proposed connection between the neutron
resonance and the specific heat anomaly in cuprates, the experimental specific
heat data on $YBa_2Cu_3O_{6.93}$ and a theoretical estimate of the single
particle fermionic contribution to the specific heat is used to provide an
upper bound for the intensity of the neutron peak. The deduced peak intensity
is similar in magnitude and temperature dependence to that observed in neutron
scattering experiments, and it is constrained to decrease strongly under the
influence of moderate magnetic fields oriented along the c-axis of the crystal.
An explanation is proposed for the predicted suppression, based on the
observation that the resonance intensity is very sensitive to superconducting
phase correlations.


title: Stripe dynamics in presence of disorder and lattice potentials

abstract:
  We study the influence of disorder and lattice pinning on the dynamics of a
charged stripe. Starting from a phenomenological model of a discrete quantum
string, we determine the phase diagram for this system. Three regimes are
identified, the free phase, the flat phase pinned by the lattice, and the
disorder pinned phase. In the absence of impurities, the system can be mapped
onto a 1D array of Josephson junctions. The results are compared with
measurements on nickelates and cuprates and a good qualitative agreement is
found between our results and the experimental data.


title: Dynamics of lattice pinned charge stripes

abstract:
  We study the transversal dynamics of a charged stripe (quantum string) and
show that zero temperature quantum fluctuations are able to depin it from the
lattice. If the hopping amplitude t is much smaller than the string tension J,
the string is pinned by the underlying lattice. At t>>J, the string is depinned
and allowed to move freely, if we neglect the effect of impurities. By mapping
the system onto a 1D array of Josephson junctions, we show that the quantum
depinning occurs at t/J = 2 / pi^2. Besides, we exploit the relation of the
stripe Hamiltonian to the sine-Gordon theory and calculate the infrared
excitation spectrum of the quantum string for arbitrary t/J values.


title: A model for correlations in stock markets

abstract:
  We propose a group model for correlations in stock markets. In the group
model the markets are composed of several groups, within which the stock price
fluctuations are correlated. The spectral properties of empirical correlation
matrices reported in [Phys. Rev. Lett. {\bf 83}, 1467 (1999); Phys. Rev. Lett.
{\bf 83}, 1471 (1999.)] are well understood from the model. It provides the
connection between the spectral properties of the empirical correlation matrix
and the structure of correlations in stock markets.


title: Glassy dynamics in granular compaction

abstract:
  Two models are presented to study the influence of slow dynamics on granular
compaction. It is found in both cases that high values of packing fraction are
achieved only by the slow relaxation of cooperative structures. Ongoing work to
study the full implications of these results is discussed.


title: Theory of the Transition at 0.2 K in Ni-doped Bi2Sr2CaCu2O8

abstract:
  A theory is put forward that the electronic phase transition at 0.2 K in
Ni-doped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ is result of the formation of a spin
density wave in the system of Ni impurities. The driving force for the
transition is the exchange interaction between the impurity spins and the spins
of the conduction electrons. This creates a small gap at two of the four nodes
of the superconducting gap. The effect is to reduce the thermal conductivity by
a factor of two, as observed.


title: Collective excitations in Quantum Dot

abstract:
  We investigate different types of collective excitations in a quantum dot
containing finite number of electrons at zero magnetic field. To estimate the
excitation energies analytically we follow the energy weighted sum-rule
approach. We consider the most general multipole excitation with angular
momentum l, and the breathing mode excitation (monopole excitation) of a large
quantum dot, for three different types of effective electron-electron
interaction. These are the logarithmic interaction, the short range
pseudopotential and the coulomb interaction. The ground state density of the
many-body system is calculated within Thomas-Fermi approximation. The
analytical results for the collective excitation energies and their dependence
on the system size and other external parameters are discussed in detail.


title: Edge magnetoplasmon excitations in a Quantum Dot in high magnetic fields

abstract:
  We investigate the collective magnetoplasmon excitations in a quantum dot
containing finite number of electrons in the high magnetic field limit. We
consider the electrons in the lowest Landau level and neglect mixing between
the higher Landau levels. The dispersion relation of these edge modes are
estimated following the energy weighted sum-rule approach. In this finite size
system the edge magnetoplasmon modes have different multipolarities (or angular
momemtum l). Their dependence on the magnetic field and on the system size is
investigated. With increasing magnetic field, energy of these collective modes
decreases and in the bulk limit they become gapless. We also consider the
breathing mode of a dot in the presence of a strong magnetic field, and the
energy of this mode approaches the cyclotron frequency $\hbar \omega_{c}$.


title:
What can one learn about Self-Organized Criticality from Dynamical
  Systems theory ?

abstract:
  We develop a dynamical system approach for the Zhang's model of
Self-Organized Criticality, for which the dynamics can be described either in
terms of Iterated Function Systems, or as a piecewise hyperbolic dynamical
system of skew-product type. In this setting we describe the SOC attractor, and
discuss its fractal structure. We show how the Lyapunov exponents, the
Hausdorff dimensions, and the system size are related to the probability
distribution of the avalanche size, via the Ledrappier-Young formula.

categories: cond-mat.stat-mech chao-dyn math-ph math.MP nlin.CD


title:
Non-Linear Poisson-Boltzmann Theory of a Wigner-Seitz Model for Swollen
  Clays

abstract:
  Swollen stacks of finite-size disc-like Laponite clay platelets are
investigated within a Wigner-Seitz cell model. Each cell is a cylinder
containing a coaxial platelet at its centre, together with an overall
charge-neutral distribution of microscopic co and counterions, within a
primitive model description. The non-linear Poisson-Boltzmann (PB) equation for
the electrostatic potential profile is solved numerically within a highly
efficient Green's function formulation. Previous predictions of linearised
Poisson-Boltzmann (LPB) theory are confirmed at a qualitative level, but large
quantitative differences between PB and LPB theories are found at physically
relevant values of the charge carried by the platelets. A hybrid theory
treating edge effect at the linearised level yields good potential profiles.
The force between two coaxial platelets, calculated within PB theory, is an
order of magnitude smaller than predicted by LPB theory


title: Transmission of two interacting electrons

abstract:
  The transmission of two electrons through a region where they interact is
found to be enhanced by a renormalization of the repulsive interaction. For a
specific example of the single-particle Hamiltonian, which includes a strongly
attractive potential, the renormalized interaction becomes attractive, and the
transmission has a pronounced maximum as function of the depth of the
single-electron attractive potential. The results apply directly to a simple
model of scattering of two interacting electrons by a quantum dot.


title: Coherent spin dynamics in diluted-magnetic quantum wells

abstract:
  Spins of charge carriers, paramagnetic centers, and nuclei in semiconductor
structures are known to be oriented if circular-polarized light is shined upon
the structure. This is due to transfer of angular momentum from photons to
various excitations in the semiconductor. What would happen if the structure is
anisotropic, so that the angular momentum is not conserved? Experiments
performed on diluted-magnetic II-VI quantum wells (such as CdMnTe/CdMgTe) show
that at some conditions the effect of light upon the spin state of the
semiconductor structure can be greatly amplified. Exchange field of
photoexcited heavy holes initiates a coherent evolution of a great number of
localized spins of Mn-ions. In the simplest case, it is just a precession in an
effective field, but generally the spin dynamics is more intricate. It shows
up, for instance, in long sequences of spin-flip peaks in Raman spectra, in
enhanced magnetic-polaron effect, and in modulation of optical response within
the picosecond time scale.


title: Lattice model for cold and warm swelling of polymers in water

abstract:
  We define a lattice model for the interaction of a polymer with water. We
solve the model in a suitable approximation. In the case of a non-polar
homopolymer, for reasonable values of the parameters, the polymer is found in a
non-compact conformation at low temperature; as the temperature grows, there is
a sharp transition towards a compact state, then, at higher temperatures, the
polymer swells again. This behaviour closely reminds that of proteins, that are
unfolded at both low and high temperatures.


title: Shot noise in chaotic systems: "classical" to quantum crossover

abstract:
  This paper is devoted to study of the classical-to-quantum crossover of the
shot noise value in chaotic systems. This crossover is determined by the ratio
of the particle dwell time in the system, $\tau_d$, to the characteristic time
for diffraction $t_E \simeq \lambda^{-1} |\ln \hbar|$, where $\lambda$ is the
Lyapunov exponent. The shot noise vanishes in the limit $t_E \gg \tau_d $,
while reaches its universal quantum value in the opposite limit. Thus, the
Lyapunov exponent of chaotic mesoscopic systems may be found by the shot noise
measurements.


title:
Comment on ``Inverse exciton series in the optical decay of an excitonic
  molecule''

abstract:
  Tokunaga et al. [Phys. Rev. B Vol. 59, R7837 (1999)] claim the first
successful observation of the inverse exciton M series in the emission spectrum
of excitonic molecules. We assert that actually such a series was observed as
early as 1989 in the beta-ZnP_2 crystal. We show that the objections of
Tokunaga et al. against the biexciton nature of the inverse exciton series in
beta-ZnP_2 are ungrounded. In particular, their estimations for the ratio of
the intensities of M2 and M1 emission lines in this crystal give two order
smaller value because they do not take into account the reabsorption of the M1
line photons. We report the observation of additional inverse exciton M' series
in the emission spectrum of excitonic molecules in beta-ZnP_2.


title: Lifshitz-like argument for low-lying states in a strong magnetic field

abstract:
  We propose a Lifshitz argument in order to analyse the low energy spectrum of
an electron moving in a strong magnetic field and scattered by randomly
distributed repulsive zero-range impurities. The typical configurations of
disorder which give rise to low energy states are identified as clusters of
impurities. This allows for an interpretation of low-lying states localized
around these clusters.


title: Dynamic structure factor and momentum distribution of a trapped Bose gas

abstract:
  The dynamic structure factor of a trapped Bose-Einstein condensed gas is
investigated at zero temperature in the framework of Bogoliubov theory.
Different values of momentum transfer are considered, ranging from the phonon
to the single-particle regime. Various approximated schemes are discussed,
including the local density approximation, where the system is locally
described as a uniform gas, and the impulse approximation, where the response
is fixed by the momentum distribution of the condensate. A comprehensive
approach, based on the eikonal expansion, is presented. The predictions of
theory are successfully compared with the results of recent two-photon Bragg
scattering experiments, both at low and high momentum transfer. Relevant
features of the dynamic structure factor are also discussed using the formalism
of sum rules and the concept of scaling. Particular emphasis is given to the
regime of high momentum transfer, where the dynamic structure factor is
sensitive to the behaviour of the order parameter in momentum space, and some
instructive examples showing the consequence of long-range coherence are
presented.


title:
Finite-size scaling corrections in two-dimensional Ising and Potts
  ferromagnets

abstract:
  Finite-size corrections to scaling of critical correlation lengths and free
energies of Ising and three-state Potts ferromagnets are analysed by numerical
methods, on strips of width $N$ sites of square, triangular and honeycomb
lattices. Strong evidence is given that the amplitudes of the ``analytical''
correction terms, $N^{-2}$, are identically zero for triangular-- and honeycomb
Ising systems. For Potts spins, our results are broadly consistent with this
lattice-dependent pattern of cancellations, though for correlation lengths
non-vanishing (albeit rather small) amplitudes cannot be entirely ruled out.


title:
Superconductivity with angular dependent coupling: stripes, Coulomb
  repulsion and enhanced Tc

abstract:
  We have analysed the effect of intrinsic doping inhomogeneity and the
presence of stripes in high-T_c superconductors on coupling \lambda by using a
simple analytically solvable model with an angular dependent \lambda (\phi)
represented by a square-well form. We have found that the introduction of the
Coulomb repulsion \lambda_C, increasing the ''contrast'' | \lambda | + |
\lambda_C |, or the depth of the angular modulation of \lambda (\phi), leads to
a remarkable enhancement of T_c. This effect can be optimized by combining
attractive (\lambda <0) and repulsive (\lambda >0) interactions along stripes
and perpendicular to them.


title: Geometric approach to Hamiltonian dynamics and statistical mechanics

abstract:
  This paper is a review of results which have been recently obtained by
applying mathematical concepts drawn, in particular, from differential geometry
and topology, to the physics of Hamiltonian dynamical systems with many degrees
of freedom of interest for statistical mechanics. The first part of the paper
concerns the applications of methods used in classical differential geometry to
study the chaotic dynamics of Hamiltonian systems. Starting from the identity
between the trajectories of a dynamical system and the geodesics in its
configuration space, a geometric theory of chaotic dynamics can be developed,
which sheds new light on the origin of chaos in Hamiltonian systems. In fact,
it appears that chaos can be induced not only by negative curvatures, as was
originally surmised, but also by positive curvatures, provided the curvatures
are fluctuating along the geodesics. In the case of a system with a large
number of degrees of freedom it is possible to give an analytical estimate of
the largest Lyapunov exponent by means of a geometric model independent of the
dynamics. In the second part of the paper the phenomenon of phase transitions
is addressed and it is here that topology comes into play. In fact, when a
system undergoes a phase transition, the fluctuations of the
configuration-space curvature exhibit a singular behavior at the phase
transition point, which can be qualitatively reproduced using geometric models.
In these models the origin of the singular behavior of the curvature
fluctuations appears to be caused by a topological transition in configuration
space. This leads us to put forward a Topological Hypothesis (TH). The content
of the TH is that phase transitions would be related at a deeper level to a
change in the topology of the configuration space of the system.


title: Kondo Impurity in a Mesoscopic Ring: Charge Persistent Current

abstract:
  We study the influence of a magnetic impurity or ultrasmall quantum dot on
the charge persistent current of a mesoscopic ring. The system consists of
electrons in a one-dimensional ring threaded by spin-dependent
Aharonov-Bohm/Casher fluxes, coupled via an antiferromagnetic exchange
interaction to a localized electron. By passing to a basis of electron states
with definite parities, the problem is mapped onto a Kondo model for the
even-parity channel plus free electrons in the odd-parity channel. States of
opposite parities decouple for values of the flux corresponding to periodic or
antiperiodic boundary conditions. For these special cases, the model is solved
exactly by a Bethe ansatz, allowing for an exact calculation of the charge
persistent current. In particular we show that the charge stiffness in the
special case of spin-independent fluxes is insensitive to the presence of the
magnetic impurity/quantum dot.


title:
Numerical renormalization group study of random transverse Ising models
  in one and two space dimensions

abstract:
  The quantum critical behavior and the Griffiths-McCoy singularities of random
quantum Ising ferromagnets are studied by applying a numerical implementation
of the Ma-Dasgupta-Hu renormalization group scheme. We check the procedure for
the analytically tractable one-dimensional case and apply our code to the
quasi-one-dimensional double chain. For the latter we obtain identical critical
exponents as for the simple chain implying the same universality class. Then we
apply the method to the two-dimensional case for which we get estimates for the
exponents that are compatible with a recent study in the same spirit.


title: Molecular dynamics simulation of aging in amorphous silica

abstract:
  By means of molecular dynamics simulations we examine the aging process of a
strong glass former, a silica melt modeled by the BKS potential. The system is
quenched from a temperature above to one below the critical temperature, and
the potential energy and the scattering function C(t_w,t+t_w) for various
waiting times t_w after the quench are measured. We find that both
qualitatively and quantitatively the results agree well with the ones found in
similar simulations of a fragile glass former, a Lennard-Jones liquid.


title:
Interplay of pseudo- and superconducting gaps: observable manifestations
  in cuprates

abstract:
  Angular dependence of gap, seen in photoemission, its evolution with doping
and temperature are interpreted on base t-t'-U Hubbard model in which a
pseudogap is a working function for electrons removing from dielectric segments
of zone boundary. Tunnel spectra in one-particle approximation display only
superconducting gap and too large asymmetry in regions aside from optimal
doping. Spectral functions confirm sharp well defined Fermi boundary at
$k_x=k_y$ direction and smoothed destructed one at the $\G(0,0)-M(\pi,0)-Y(\pi
,\pi)$ path. Both tunnel and ARPES data seem require spreading a model to
bilayer ones.


title:
Peculiarities in Low Temperature Properties of Doped Manganites
  A1-xBxMnO3

abstract:
  The phase diagram and low temperature properties of the doped manganites
A1-xBxMnO3 are discussed for the concentrations x < 0.4. The transition from
insulating antiferromagnetic to metallic ferromagnetic state at x_cr = 0.16 is
treated by means of percolation theory. The unifying description of insulating
and metallic states is presented. The undoped manganite is a band insulator
consisting of ferromagnetic layers, which are coupled antiferromagnetically
along the c direction with a low Neel temperature. The metallic phase can be
described by the two-band Fermi liquid picture. The behavior of conductivity,
spin wave excitations, etc. is analyzed and the comparison with experimental
data is carried out.


title:
Effects of Multi-Surface Modification on Curie temperature of
  ferroelectric films

abstract:
  Within the framework of mean field theory, we study the effects of
multi-surface modification on Curie temperature of ferroelectric films using
the transverse Ising model. The general nonlinear equations for Curie
temperature of multi-surface ferroelectric films with arbitrary exchange
constants and transverse fields are derived by the transfer matrix method. As
an example, we consider a film consisting of top surface layers, bulk layers
and bottom surface layers. Two types of surface modifications, modifications of
a surface exchange constant and a surface transverse field are taken into
account. The dependence of Curie temperature on the surface layer numbers, bulk
layer numbers, surface exchange constants, surface transverse fields and bulk
transverse fields is discussed.


title: Hole dynamics and photoemission in a t-J model for SrCu_2(BO_3)_2

abstract:
  The motion of a single hole in a t-J model for the two-dimensional spin-gap
compound SrCu_2(BO_3)_2 is investigated. The undoped Heisenberg model for this
system has an exact dimer eigenstate and shows a phase transition between a
dimerized and a Neel phase at a certain ratio of the magnetic couplings. We
calculate the photoemission spectrum in the disordered phase using a
generalized spin-polaron picture. By varying the inter-dimer hopping parameters
we find a cross-over between a narrow quasiparticle band regime known from
other strongly correlated systems and free-fermion behavior. The hole motion in
the Neel-ordered phase is also briefly considered.


title: Theory of Spin polarized Tunneling in Superconducting Sr2RuO4

abstract:
  A theory of tunneling conductance in ferromagnetic metal/insulator/triplet -
supercondcutor junctions is presented for unitary and non-unitary spin triplet
pairing states which are promising candidates for the superconducting paring
symmetry of Sr2RuO4. As the magnitude of the exchange interaction in the
ferromagnetic metal is increased, the conductance for the unitary pairing state
below the energy gap is reduced in contrast to the case for the non-unitary
pairing state.


title:
Mutual information and self-control of a fully-connected low-activity
  neural network

abstract:
  A self-control mechanism for the dynamics of a three-state fully-connected
neural network is studied through the introduction of a time-dependent
threshold. The self-adapting threshold is a function of both the neural and the
pattern activity in the network. The time evolution of the order parameters is
obtained on the basis of a recently developed dynamical recursive scheme. In
the limit of low activity the mutual information is shown to be the relevant
parameter in order to determine the retrieval quality. Due to self-control an
improvement of this mutual information content as well as an increase of the
storage capacity and an enlargement of the basins of attraction are found.
These results are compared with numerical simulations.


title: Breakdown of the Kondo Effect in Critical Antiferromagnets

abstract:
  The breakdown of the Kondo effect may be the origin of the anomalous
properties of the heavy-fermion compounds at low temperatures. We study the
dynamics of one impurity embedded in an antiferromagnetic host at the quantum
critical point and show that the impurity is not screened and develops a power
law correlation function. This suggests that the breakdown of the Kondo effect
can simply be a consequence of the system's proximity to the quantum critical
point.


title:
A possible scenario for thermally activated avalanches in type II
  superconductors

abstract:
  Using a simple cellular automaton with stochastic rules we show the possible
emergence of thermally activated avalanches (power law distributed) in type-II
superconductors. Scaling relations between the exponents characterizing these
distributions and those obtained from field driven experiments are derived and
proved through simulations. It is also shown that the conditions for the
appearance of these avalanches are independent of the pinning mechanism. The
relevance of our simulations for recently reported experimental results is also
outlined.


title:
Classical and quantum magnetisation reversal studied in single
  nanometer-sized particles and clusters using micro-SQUIDs

abstract:
  Recent progress in experiment on quantum tunnelling of the magnetic moment in
mesoscopic systems will be reviewed. The emphasis will be made on measurements
of individual nanoparticles. These nanomagnets allow one to test the border
between classical and quantum behaviour. Using the micro-SQUID magnetometer,
waiting time, switching field and telegraph noise measurements show
unambiguously that the magnetisation reversal of small enough single
crystalline nanoparticles is described by a model of thermal activation over a
single-energy barrier. Results on insulating BaFeO nanoparticles show strong
deviations from this model below 0.4 K which agree with the theory of
macroscopic quantum tunnelling in the low dissipation regime.


title: Quantum Phase Interference in Magnetic Molecular Clusters

abstract:
  The Landau Zener model has recently been used to measure very small tunnel
splittings in molecular clusters of Fe8, which at low temperature behaves like
a nanomagnet with a spin ground state of S = 10. The observed oscillations of
the tunnel splittings as a function of the magnetic field applied along the
hard anisotropy axis are due to topological quantum interference of two tunnel
paths of opposite windings. Transitions between quantum numbers M = -S and (S -
n), with n even or odd, revealed a parity effect which is analogous to the
suppression of tunnelling predicted for half integer spins. This observation is
the first direct evidence of the topological part of the quantum spin phase
(Berry or Haldane phase) in a magnetic system. We show here that the quantum
interference can also be measured by ac susceptibility measurements in the
thermal activated regime.


title: Quantum Hole Digging in Magnetic Molecular Clusters

abstract:
  Below 360 mK, Fe8 magnetic molecular clusters are in the pure quantum
relaxation regime. We showed recently that the predicted ``square-root time''
relaxation is obeyed, allowing us to develop a new method for watching the
evolution of the distribution of molecular spin states in the sample. We
measured the distribution P(H) of molecules which are in resonance at the
applied field H. Tunnelling initially causes rapid transitions of molecules,
thereby ``digging a hole'' in P(H). For small initial magnetisation values, the
hole width shows an intrinsic broadening which may be due to nuclear spins. We
present here hole digging measurements in the thermal activated regime which
may allow to study the effect of spin-phonon coupling.


title: Single nanoparticle measurement techniques

abstract:
  Various single particle measuring techniques are briefly reviewed and the
basic concepts of a new micro-SQUID technique are discussed. It allows
measurements of the magnetization reversal of single nanometer-sized particles
at low temperature. The influence of the measuring technique on the system of
interest is discussed.


title:
Orientational phase transitions in the hexagonal phase of a diblock
  copolymer melt under shear flow

abstract:
  We generalize the earlier theory by Fredrickson [J. Rheol. v.38, 1045 (1994)]
to study the orientational behaviour of the hexagonal phase of diblock
copolymer melt subjected to steady shear flow. We use symmetry arguments to
show that the orientational ordering in the hexagonal phase is a much weaker
effect than in the lamellae. We predict the parallel orientation to be stable
at low and the perpendicular orientation at high shear rates. Our analysis
reproduces the experimental results by Tepe et al. [Macromolecules v.28, 3008
(1995)] and explains the difficulties in experimental observation of the
different orientations in the hexagonal phase.


title: Coherent effects in double-barrier Josephson junctions

abstract:
  The general solution for ballistic electronic transport through
double-barrier Josephson junctions is derived. We show the existence of a
regime of phase-coherent transport in which the supercurrent is proportional to
the single barrier transparency and the way in which this coherence is
destroyed for increasing interlayer thickness. The quasiparticle dc current at
arbitrary voltage is determined.


title:
Singularities and the distribution of density in the Burgers/adhesion
  model

abstract:
  We are interested in the tail behavior of the pdf of mass density within the
one and $d$-dimensional Burgers/adhesion model used, e.g., to model the
formation of large-scale structures in the Universe after baryon-photon
decoupling. We show that large densities are localized near ``kurtoparabolic''
singularities residing on space-time manifolds of codimension two ($d \le 2$)
or higher ($d \ge 3$). For smooth initial conditions, such singularities are
obtained from the convex hull of the Lagrangian potential (the initial velocity
potential minus a parabolic term). The singularities contribute {\em
\hbox{universal} power-law tails} to the density pdf when the initial
conditions are random. In one dimension the singularities are preshocks
(nascent shocks), whereas in two and three dimensions they persist in time and
correspond to boundaries of shocks; in all cases the corresponding density pdf
has the exponent -7/2, originally proposed by E, Khanin, Mazel and Sinai (1997
Phys. Rev. Lett. 78, 1904) for the pdf of velocity gradients in one-dimensional
forced Burgers turbulence. We also briefly consider models permitting particle
crossings and thus multi-stream solutions, such as the Zel'dovich approximation
and the (Jeans)--Vlasov--Poisson equation with single-stream initial data: they
have singularities of codimension one, yielding power-law tails with exponent
-3.

categories: cond-mat.stat-mech astro-ph chao-dyn nlin.CD


title: Quasi-long-range order in nematics confined in random porous media

abstract:
  We study the effect of random porous matrices on the ordering in nematic
liquid crystals. The randomness destroys orientational lang-range order and
drives the liquid crystal into a glass state. We predict two glass phases one
of which possesses quasi-long-range order. In this state the correlation length
is infinite and the correlation function of the order parameter obeys a power
dependence on the distance. The small-angle light-scattering amplitude diverges
but slower than in the bulk nematic. In the uniaxially strained porous matrices
two new phases emerge. One type of strain induces an anisotropic
quasi-long-range-ordered state while the other stabilizes nematic long-range
order.


title: Fractal Analysis of Electrical Power Time Series

abstract:
  Fractal time series has been shown to be self-affine and are characterized by
a roughness exponent H. The exponent H is a measure of the persistence of the
fluctuations associated with the time series. We use a recently introduced
method for measuring the roughness exponent, the mobile averages analysis, to
compare electrical power demand of two different places, a touristic city and a
whole country.


title: Anomalous Quantum Diffusion and Conductivity of Quasicrystals

abstract:
  A phenomenological diffusion law $L(t)\propto t^{\beta}$, where L(t) measures
the spreading of a wave-packet in a time t, is assumed for perfect
quasicrystals. We show that it affects their conductivity with striking
differences compared to the case of periodic metals. In the absence of defects
the dissipative part of conductivity is non zero even at low frequencies
contrary to the case of crystals. Also if $\beta<1/2$ the d.c. conductivity
increases when disorder increases and the so-called Drude peak, characteristic
of metals, is replaced by a dip . Experimental results are briefly discussed.


title:
Study of Pairing Correlations in the Attractive Hubbard Model on Chains,
  Ladders, and Squares

abstract:
  We report the results of zero temperature quantum Monte Carlo simulations and
zero temperature mean-field calculations of the attractive Hubbard model on
chains, ladders, and square lattices. We investigated the predictability of the
BCS approximation, the dimensional cross-over of the pairing correlation
function from one to two dimensions as a function of the ladder width, and the
scaling of these correlations to the thermodynamic limit of the two-dimensional
model. We found that the BCS wave function is quantitatively correct only for
small values of $U/t$. For the system sizes, electron fillings, and interaction
strengths studied, we never saw the dimensional cross-over. In general our
ability to achieve the dimensional cross-over and accurate scaling to the
thermodynamic limit was limited by the size of the systems we could simulate.
For these sizes, although we saw the necessary signature of ODLRO, the
properties of the model did not vary monotonically with increasing system size
because of shell effects. We contrast this situation with the dimensional
cross-over and scaling to the thermodynamic limit of the Ising model.


title:
The N-component Ginzburg-Landau Hamiltonian with cubic anisotropy: a
  six-loop study

abstract:
  We consider the Ginzburg-Landau Hamiltonian with a cubic-symmetric quartic
interaction and compute the renormalization-group functions to six-loop order
in d=3. We analyze the stability of the fixed points using a Borel
transformation and a conformal mapping that takes into account the
singularities of the Borel transform. We find that the cubic fixed point is
stable for N>N_c, N_c = 2.89(4). Therefore, the critical properties of cubic
ferromagnets are not described by the Heisenberg isotropic Hamiltonian, but
instead by the cubic model at the cubic fixed point. For N=3, the critical
exponents at the cubic and symmetric fixed points differ very little (less than
the precision of our results, which is $\lesssim 1%$ in the case of $\gamma$
and $\nu$). Moreover, the irrelevant interaction bringing from the symmetric to
the cubic fixed point gives rise to slowly-decaying scaling corrections with
exponent $\omega_2=0.010(4)$. For N=2, the isotropic fixed point is stable and
the cubic interaction induces scaling corrections with exponent $\omega_2 =
0.103(8)$. These conclusions are confirmed by a similar analysis of the
five-loop $\epsilon$-expansion. A constrained analysis which takes into account
that $N_c = 2$ in two dimensions gives $N_c = 2.87(5)$.


title:
Density of States near the Anderson Transition in a Four-dimensional
  Space. Renormalizable Models

abstract:
  Asymptotically exact results are obtained for the average Green function and
density of states of a disordered system for a renormalizable class of models
(as opposed to the lattice models examined previously [Zh. Eksp. Teor. Fiz. 106
(1994) 560-584]). For N\sim 1 (where N is an order of the perturbation theory),
only the parquet terms corresponding to the highest powers of large logarithms
are retained. For large N, this approximation is inadequate because of the fast
growth with N of the coefficients for the lower powers of the logarithms. The
latter coefficients are calculated in the leading order in N from the
Callan-Symanzik equation with results of the Lipatov method using as boundary
conditions. For calculating the self-energy at finite momentum, a modification
of the parquet approximation is used, that allows the calculations to be done
in an arbitrary finite logarithmic approximation but in the leading order in N.
It is shown that the phase transition point shifts in the complex plane,
thereby insuring regularity of the density of states for all energies. The
"spurious" pole is avoided in such a way that effective interaction remains
logarithmically weak.


title:
Some sum rules for the non-Fermi liquids: new applications taking into
  account the mass renormalization factor

abstract:
  Re-studying the non-fermi liquid one-particle Green function (NFLGF) we have
extended the work of A. Balatsky (Phil. Mag. Lett. 68, 251 (1993)) and L. Yin
and S. Chakravarty (Int. J. Mod. Phys. B 10, 805 (1996)), among others. We used
the moment approach of W. Nolting (Z. Phys. 255, 25 (1972)) to compute the
unknown parameters of the NFLGF's in the framework of the Hubbard model. The
zeroth order momentum requires that our one-particle Green function describe
fermionic degrees of freedom. In order to satisfy the first order sum rule a
renormalization, $\gamma\neq 1$, of the free electron mass is called for. The
second order sum rule or moment imposes a relation between the non-Fermi liquid
parameter, $\alpha$, the Coulomb interaction, U, and the frequency cutoff,
$\omega_c$. We have calculated the effect of the mass renormalization factor,
$\gamma$, on some physical quantities. As a new class of non-Fermi liquid
systems, in Appendix A, we have studied two inequivalent coupled Hubbard layers
for which we calculated the one-particle spectral functions on the layers and
perpendicular to them. We discuss the new features which appear due to the
shift in the two effective chemical potentials and proposed some experiments to
detect the features founded from our expressions.


title: Origin of the high piezoelectric response in PbZr(1-x)TixO3

abstract:
  High resolution x-ray powder diffraction measurements on poled PbZr(1-x)TixO3
(PZT) ceramic samples close to the rhombohedral-tetragonal phase boundary (the
so-called morphotropic phase boundary, MPB) have shown that for both
rhombohedral and tetragonal compositions, the piezoelectric elongation of the
unit cell does not occur along the polar directions but along those directions
associated with the monoclinic distortion. This work provides the first direct
evidence for the origin of the very high piezoelectricity in PZT.


title: Twist and writhe dynamics of stiff filaments

abstract:
  This letter considers the dynamics of a stiff filament, in particular the
coupling of twist and bend via writhe. The time dependence of the writhe of a
filament is $W_r^2\sim L t^{1/4}$ for a linear filament and $W_r^2\sim t^{1/2}
/ L $ for a curved filament. Simulations are used to study the relative
importance of crankshaft motion and tube like motion in twist dynamics.
Fuller's theorem, and its relation with the Berry phase, is reconsidered for
open filaments


title: Structure and Dynamics of Surface Adsorbed Clusters

abstract:
  Extensive numerical simulation are reported for the structure and dynamics of
large clusters on metal(100) surfaces. Different types of perimeter hopping
processes makes center-of-mass of the cluster to follow a a random walk
trajectory. Then, a {\it diffusion coefficient} $D$ can be defined as
$\lim\limits_{t\to \infty} D(t)$, with $D(t)=< d^2 >/(4t)$ and $d$ the
displacement of the center-of-mass. In the simulations, the dependence of the
diffusion coefficient on those perimeter hopping processes can be analyzed in
detail, since the relations between different rates for the processes are
explicitly considered as parameters.


title: Exactly solvable model illustrating far-from-equilibrium predictions

abstract:
  We describe an exactly solvable model which illustrates the Fluctuation
Theorem and other predictions for systems evolving far from equilibrium. Our
model describes a particle dragged by a spring through a thermal environment.
The rate at which the spring is pulled is arbitrary.


title: Metal-Insulator oscillations in a Two-dimensional Electron-Hole system

abstract:
  The electrical transport properties of a bipolar InAs/GaSb system have been
studied in magnetic field. The resistivity oscillates between insulating and
metallic behaviour while the quantum Hall effect shows a digital character
oscillating from 0 to 1 conducatance quantum e^2/h. The insulating behaviour is
attributed to the formation of a total energy gap in the system. A novel looped
edge state picture is proposed associated with the appearance of a voltage
between Hall probes which is symmetric on magnetic field reversal.


title:
Landau Zener method to study quantum phase interference of Fe8 molecular
  nanomagnets

abstract:
  We present details about an experimental method based on the Landau Zener
model which allows to measure very small tunnel splittings $\Delta$ in
molecular clusters Fe8. The measurements are performed with an array of
micro-SQUIDs. The observed oscillations of Delta as a function of the magnetic
field applied along the hard anisotropy axis are explained in terms of
topological quantum interference of two tunnel paths of opposite windings.
Transitions between M = -S and (S - n), with n even or odd, revealed a parity
(symmetry) effect which is analogous to the suppression of tunneling predicted
for half integer spins. This observation is the first direct evidence of the
topological part of the quantum spin phase (Berry phase) in a magnetic system.
The influence of intermolecular dipole interactions on the measured tunnel
splittings $\Delta$ are shown.


title: Impurity Scattering in f-wave Superconductor UPt_3

abstract:
  We study theoretically the effect of impurity scattering in f-wave (or
E_{2u}) superconductors. The quasi-particle density of states of f-wave
superconductor is very similar to the one for d-wave superconductor as in
hole-doped high T_c cuprates. Also in spite of anisotropy in Delta(k), both the
normalized superfluid density and the thermal conductivity is completely
isotropic.


title: A Fermi Surface study of Ba$_{1-x}$K$_{x}$BiO$_{3}$

abstract:
  We present all electron computations of the 3D Fermi surfaces (FS's) in
Ba$_{1-x}$K$_{x}$BiO$_{3}$ for a number of different compositions based on the
selfconsistent Korringa-Kohn-Rostoker coherent-potential-approximation
(KKR-CPA) approach for incorporating the effects of Ba/K substitution. By
assuming a simple cubic structure throughout the composition range, the
evolution of the nesting and other features of the FS of the underlying
pristine phase is correlated with the onset of various structural transitions
with K doping. A parameterized scheme for obtaining an accurate 3D map of the
FS in Ba$_{1-x}$K$_{x}$BiO$_{3}$ for an arbitrary doping level is developed. We
remark on the puzzling differences between the phase diagrams of
Ba$_{1-x}$K$_{x}$BiO$_{3}$ and BaPb$_{x}$Bi$_{1-x}$O$_{3}$ by comparing aspects
of their electronic structures and those of the end compounds BaBiO$_{3}$,
KBiO$_3$ and BaPbO$_3$. Our theoretically predicted FS's in the cubic phase are
relevant for analyzing high-resolution Compton scattering and
positron-annihilation experiments sensitive to the electron momentum density,
and are thus amenable to substantial experimental verification.


title: Suppression of the Kondo Effect in a Quantum Dot by Microwave Radiation

abstract:
  We have studied the influence of microwave radiation on the transport
properties of a semiconductor quantum dot in the Kondo regime. In the entire
frequency range tested (10--50 GHz), the Kondo resonance vanishes by increasing
the microwave power. This suppression of the Kondo resonance shows an
unexpected scaling behavior. No evidence for photon-sideband formation is
found. The comparison with temperature-dependence measurements indicates that
radiation-induced spin dephasing plays an important role in the suppression of
the Kondo effect.


title:
Optical nonlinearity due to intersubband transitions in semiconductor
  quantum wells

abstract:
  We have calculated the contribution of intersubband transitions to the third
order optical nonlinear susceptibility, $\chi^{(3)}(\omega,\omega,\omega)$ for
nonresonant as well as resonant third harmonic generation and
$\chi^{(3)}(\omega,-\omega,\omega)$ for nonlinear refraction and absorption. As
examples, we consider InAs/AlSb and GaAs/GaAlAs quantum wells. The effects of
finite barrier height, energy band nonparabolicity, and high carrier
concentrations are included. It is shown that quantum confinement, rather than
the band nonparabolicity, is responsible for high values of nonresonant
$\chi^{(3)}_{zzzz}$. Very high values of $\chi^{(3)}_{zzzz}$ are obtained for
third harmonic generation and two photon absorption for incident wavelength
near 10.6 $\mu$m. Intensity dependence of refractive index and of absorption
co-efficient is also discussed for intensity well above the saturation
intensity. Effective medium theory is used to incorporate the collective
effects.


title: Luttinger-liquid Parameter of Hubbard Chain and Hubbard Ladder

abstract:
  We study the Luttinger-liquid parameter $K_{\rho}$ of the Hubbard chain and
the Hubbard ladder models by the ordinary perturbation method combined with the
Luttinger-liquid relation. According to the Luttinger-liquid relation, the
critical exponent $K_{\rho}$ is related to the charge susceptibility $\chi_c$
and the Drude weight D by $ K\sb{\rho}={1/2}(\pi \chi_c D)^{1/2}$. By
calculating these quantities with the perturbation method, we obtain $K_{\rho}$
at the first-order analytically and up to the second-order numerically. We
compare these results with results of the Bethe ansatz for the Hubbard chain
and that of the numerical diagonalization for the Hubbard ladder. It shows that
the perturbation calculation of $K_{\rho}$ is reliable in the weak coupling
regime.


title:
Excitation spectrum and effective mass of the even-fraction quantum Hall
  liquid

abstract:
  To probe the nature of the even-fraction quantum Hall system, we have
investigated the low-lying excitation spectrum by means of exact
diagonalization for finite systems. We have found (i) a striking one-to-one
correspondence (i.e., a shell structure) between the spectrum and those for
free (composite) fermions, (ii) a surprisingly straight scaling plot for the
excitation energy that gives a zero gap (metal) in the thermodynamic limit,
(iii) the effective mass evaluated from the scaling becoming heavier for
filling factor = 1/2, 1/4, 1/6, but (iv) some deviations from the single-mode
or the Hartree-Fock composite fermion approximation as well.


title:
Defects as a reason of continuity of normal-incommensurate phase
  transitions

abstract:
  Almost all normal-incommensurate phase transitions observed experimentally
are continuous. We show that there is not any theoretical reason for this
general behaviour in perfect crystals. A normal-incommensurate phase transition
that is not too far from the mean-field tricritical point should be
discontinuous and it is highly improbable that so far reported
normal-incommensurate phase transitions lie very far from this point. To
understand this behaviour we study influence of defects on a hypothetical
first-order normal-incommensurate phase transition in a pure material. We have
found that this influence is strikingly different from that on other kinds of
first-order phase transitions. The change of the discontinuity of the order
parameter at the transition is negative and formally diverges within our
approximate theory. At the same time the diminishing of the phase transition
temperature remains finite. We interpret these results as an indication that at
least some of the observed seemingly second-order normal-incommensurate
transitions would be first-order transitions in defectless crystals.


title:
Incommensurate structures studied by a modified Density Matrix
  Renormalization Group Method

abstract:
  A modified density matrix renormalization group (DMRG) method is introduced
and applied to classical two-dimensional models: the anisotropic triangular
nearest- neighbor Ising (ATNNI) model and the anisotropic triangular
next-nearest-neighbor Ising (ANNNI) model. Phase diagrams of both models have
complex structures and exhibit incommensurate phases. It was found that the
incommensurate phase completely separates the disordered phase from one of the
commensurate phases, i. e. the non-existence of the Lifshitz point in phase
diagrams of both models was confirmed.


title:
Density of States near the Anderson Transition in a Space of
  Dimensionality d=4-epsilon

abstract:
  Asymptotically exact results are obtained for the average Green function and
the density of states in a Gaussian random potential for the space
dimensionality d=4-epsilon over the entire energy range, including the vicinity
of the mobility edge. For N\sim 1 (N is an order of the perturbation theory)
only the parquet terms corresponding to the highest powers of 1/epsilon are
retained. For large N all powers of 1/epsilon are taken into account with their
coefficients calculated in the leading asymptotics in N. This calculation is
performed by combining the condition of renormalizability of the theory with
the Lipatov asymptotics.

categories: cond-mat.dis-nn cond-mat.mes-hall hep-ph hep-th


title: Impurity Effects on the Flux Phase Quantum Critical Point Scenario

abstract:
  Impurity substitution of Zn in La-214 and (Y,Ca)-123 high-T_c superconductors
suppresses T_c but does not affect appreciably the onset of the pseudogap phase
in the underdoped region nor optimal doping or the position of the inferred
quantum critical point. Based on a 1/N expansion of the t-J model we explain
these findings as well as the similar dependence on a magnetic field in terms
of a quantum critical point scenario where a flux phase causes the pseudogap.


title:
Nodeless d-wave superconductivity in a quasi-one-dimensional organic
  superconductor under anion order

abstract:
  We propose a mechanism of nodeless d-wave superconductivity in a
quasi-one-dimensional organic superconductor under anion order. It is shown
that split of the Fermi surfaces due to the anion order eliminates the line
nodes of a d-wave gap function on the Fermi surfaces, and a small finite energy
gap appears in the quasi-particle excitations of superconductivity. We discuss
that temperature dependences of thermal conductivity and NMR relaxation rate
observed in a quasi-one-dimensional organic superconductor (TMTSF)2ClO4 can be
explained consistently by the small energy gap.


title: Dynamic scaling in vacancy-mediated disordering

abstract:
  We consider the disordering dynamics of an interacting binary alloy with a
small admixture of vacancies which mediate atom-atom exchanges. Starting from a
perfectly phase-segregated state, the system is rapidly heated to a temperature
in the disordered phase. A suitable disorder parameter, namely, the number of
broken bonds, is monitored as a function of time. Using Monte Carlo simulations
and a coarse-grained field theory, we show that the late stages of this process
exhibit dynamic scaling, characterized by a set of scaling functions and
exponents. We discuss the universality of these exponents and comment on some
subtleties in the early stages of the disordering process.


title:
Antiferromagnetism in a doped spin-Peierls model: classical and quantum
  behaviors

abstract:
  We address the problem of antiferromagnetism in a two dimensional model of
doped spin-Peierls system, at the classical and quantum levels. A Bethe-Peierls
solution is derived for the classical model, with an ordering temperature
proportional to the doping concentration. The quantum model is treated in a
cluster renormalization group showing a finite randomness behavior and an
antiferromagnetic susceptibility at low temperature.


title: Comparison of energy and phase relaxation in metallic wires

abstract:
  We have measured the phase coherence time, $\tau_{\phi},$ in long, narrow
wires of Au, Ag, and Cu, over the temperature range 40 mK-6 K. In the Cu and Au
wires, $\tau_{\phi}$ saturates at low temperature. In the Ag wire,
$\tau_{\phi}$ continues to increase down to the lowest temperatures measured;
moreover, its temperature dependence below about 1 K is consistent with
theoretical predictions of Altshuler, Aronov and Khmelnitskii published in
1982. These results cast doubt on recent assertions that saturation of
$\tau_{\phi}$ at low temperature is a universal phenomenon in metal wires. We
compare these results with those of recent experiments on energy relaxation in
similar metallic wires. The results of the two experiments are strongly
correlated, suggesting that a single (unknown) mechanism is the cause of the
enhanced phase and energy relaxation observed in some samples.


title: Energy redistribution between quasiparticles in mesoscopic silver wires

abstract:
  We have measured with a tunnel probe the energy distribution function of
quasiparticles in silver diffusive wires connected to two large pads
(``reservoirs''), between which a bias voltage was applied. From the dependence
in energy and bias voltage of the distribution function we have inferred the
energy exchange rate between quasiparticles. In contrast with previously
obtained results on copper and gold wires, these data on silver wires can be
well interpreted with the theory of diffusive conductors either solely, or
associated with another mechanism, possibly the coupling to two-level systems.


title:
Thermodynamical fluctuations and photo-thermal shot noise in
  gravitational wave antennae

abstract:
  Thermodynamical fluctuations of temperature in mirrors of gravitational wave
antennae are transformed through thermal expansion coefficient into additional
noise. This source of noise, which may also be interpreted as fluctuations due
to thermoelastic damping, may not be neglected and leads to the necessity to
reexamine the choice of materials for the mirrors. Additional source of noise
are fluctuations of the mirrors' surfaces caused by optical power absorbed in
dielectrical reflective layers.


title: Spin paramagnetism in d-wave superconductors

abstract:
  The Ginzburg-Landau equations are derived from the microscopic theory for
clean layered superconductors with $d_{x^2-y^2}$ pairing symmetry, including
the Pauli paramagnetism effect. The upper critical field $H_{c2}$ parallel to
the $c$ axis is calculated. A comparison with the experimental data for YBCO
suggests that, relative to the orbital effect, the Pauli paramagnetism
contribution to $H_{c2}$ is significant. The reversible magnetization $M$ in
high magnetic fields is also calculated, showing strong temperature dependence
of the slope $dM/dH$, as a consequence of the spin paramagnetism. A simple
expression for the high temperature spin susceptibility is derived, in a good
agreement with the Knight shift measurements on YBCO.


title: The sl_2 loop algebra symmetry of the six-vertex model at roots of unity

abstract:
  We demonstrate that the six vertex model (XXZ spin chain) with
$\Delta=(q+q^{-1})/2$ and $q^{2N}=1$ has an invariance under the loop algebra
of $sl_2$ which produces a special set of degenerate eigenvalues. For
$\Delta=0$ we compute the multiplicity of the degeneracies using Jordan Wigner
techniques


title:
Magnetic Field Dependence of the Paramagnetic to the High Temperature
  Magnetically Ordered Phase Transition in CeB6

abstract:
  We have measured the magnetic field dependence of the paramagnetic to high
temperature magnetically ordered phase transition TQ(H) in CeB6 from 2 to 30 T
using cantilever magnetometry. It is found that the phase separation
temperature continuously increases in field with an increasingly positive
slope. In addition, we find that measurements in strong magnetic field
gradients have no effect on the phase transition.


title:
Effective Sublattice Magnetization and Neel Temperature in Quantum
  Antiferromagnets

abstract:
  We present an analytic expression for the finite temperature effective
sublattice magnetization which would be detected by inelastic neutron
scattering experiments performed on a two-dimensional square-lattice quantum
Heisenberg antiferromagnets with short range N\'eel order. Our expression,
which has no adjustable parameters, is able to reproduce both the qualitative
behaviour of the phase diagram $M(T)xT$ and the experimental values of the
N\'eel temperature $T_{N}$ for either doped YBa$_{2}$Cu$_{3}$O$_{6.15}$ and
stoichiometric La$_{2}$CuO$_{4}$ compounds. Finally, we remark that by
incorporating frustration and 3D effects as perturbations is sufficient to
explain the deviation of the experimental data from our theoretical curves.


title: The Localization-Delocalization Transition in Quantum Dots

abstract:
  Single-electron capacitance spectroscopy precisely measures the energies
required to add individual electrons to a quantum dot. The spatial extent of
electronic wavefunctions is probed by investigating the dependence of these
energies on changes in the dot confining potential. For low electron densities,
electrons occupy distinct spatial sites localized within the dot. At higher
densities, the electrons become delocalized, and all wavefunctions are spread
over the full dot area. Near the delocalization transition, the last remaining
localized states exist at the perimeter of the dot. Unexpectedly, these
electrons appear to bind with electrons in the dot center.


title:
Stationary localized states due to nonlinear impurities described by the
  modified discrete nonlinear Schr\"odinger equation

abstract:
  The modified discrete nonlinear Schr\"odinger equation is used to study the
formation of stationary localized states in a one-dimensional lattice with a
single impurity and an asymmetric dimer impurity. A periodically modulated and
a perfectly nonlinear chain is also considered. Phase diagrams of localized
states for all systems are presented. From the mean square displacement
calculation, it is found that all states are not localized even though the
system comprises random nonlinear site energies. Stability of the states is
discussed.