title: Double photo-ionization of He near a polarizable surface

abstract:
  We calculate the differential cross-section of the direct double
photo-ionization of He physisorbed on a polarizable surface. By including the
influence of the surface potential in the correlated two-electron final state
wavefunction, we show that the differential cross-section carries detailed
information on the electronic correlations at the surface. In particular,
photo-emission along opposite directions, which is prohibited in the free
space, is allowed if the surface potential is long-ranged.


title: Overdamped sine-Gordon kink in a thermal bath

abstract:
  We study the sine-Gordon kink diffusion at finite temperature in the
overdamped limit. By means of a general perturbative approach, we calculate the
first- and second-order (in temperature) contributions to the diffusion
coefficient. We compare our analytical predictions with numerical simulations.
The good agreement allows us to conclude that, up to temperatures where
kink-antikink nucleation processes cannot be neglected, a diffusion constant
linear and quadratic in temperature gives a very accurate description of the
diffusive motion of the kink. The quadratic temperature dependence is shown to
stem from the interaction with the phonons. In addition, we calculate and
compute the average value $<\phi(x,t)>$ of the wave function as a function of
time and show that its width grows with $\sqrt{t}$. We discuss the
interpretation of this finding and show that it arises from the dispersion of
the kink center positions of individual realizations which all keep their
width.


title:
Normal State Magnetic Properties of Ni and Zn Substituted in
  YBa_{2}Cu_{3} O_{6+x}: Hole-Doping Dependence

abstract:
  We present SQUID susceptibility data on Zn and Ni substituted
YBa_{2}Cu_{3}O_{6+x}. Cross-checks with NMR yield an unprecedented accuracy in
the estimate of the magnetic susceptibility associated with the substituants,
from the underdoped to the lightly overdoped case. This allows us to determine
the Weiss temperature \theta for YBCO: its value is very small for all hole
dopings n_h. Since in conventional metals, the Kondo temperature,
$T_{K}<\theta$, magnetic screening effects would not be expected for $T\gg
\theta$; in contrast, increasing n_h produces a reduction of the small moment
induced by Zn^{2+} and a nearly constant effective moment for Ni^{2+}
corresponding to a spin 1/2 rather than to a spin 1.


title:
Calculation of ground states of four-dimensional +or- J Ising spin
  glasses

abstract:
  Ground states of four-dimensional (d=4) EA Ising spin glasses are calculated
for sizes up to 7x7x7x7 using a combination of a genetic algorithm and
cluster-exact approximation. The ground-state energy of the infinite system is
extrapolated as e_0=-2.095(1). The ground-state stiffness (or domain wall)
energy D is calculated. A D~L^{\Theta} behavior with \Theta=0.65(4) is found
which confirms that the d=4 model has an equilibrium spin-glass-paramagnet
transition for non-zero T_c.


title: On a trivial aspect of canonical specific heat scaling

abstract:
  We show that the canonical finite size scaling of the specific heat emerges
naturally - and in some sense trivially - from the assumption that the
microcanonical specific entropy exhibits no substantial system size dependence.


title:
Uniform susceptibility of classical antiferromagnets in one and two
  dimensions in a magnetic field

abstract:
  We simulated the field-dependent magnetization m(H,T) and the uniform
susceptibility \chi(H,T) of classical Heisenberg antiferromagnets in the chain
and square-lattice geometry using Monte Carlo methods. The results confirm the
singular behavior of \chi(H,T) at small T,H: \lim_{T \to 0}\lim_{H \to 0}
\chi(H,T)=1/(2J_0)(1-1/D) and \lim_{H \to 0}\lim_{T \to 0} \chi(H,T)=1/(2J_0),
where D=3 is the number of spin components, J_0=zJ, and z is the number of
nearest neighbors. A good agreement is achieved in a wide range of temperatures
T and magnetic fields H with the first-order 1/D expansion results [D. A.
Garanin, J. Stat. Phys. 83, 907 (1996)]


title: Phonon-Drag Thermopower at High Temperatures

abstract:
  The adiabatic cristal model is offered. It is shown that springy nuclei
oscillations relatively electronic envelops and waves of such oscillations
(inherent oscillations and waves) may exist in crystals. The analysis of
experimental temperature dependencies of resistivity in semiconductors with
electron-vibrational centres has shown that inherent oscillations effectively
interact with crystalline phonons as well as with electrons and holes, creating
powerful interaction of electrons and holes with phonons. The experimental
narrow peaks of phonon-drag thermoelectric power at Debye temperatures from 77K
to 700K confirm existence of inherent oscillations waves in crystals. Inherent
oscillations and waves gives rise to strong electron-phonon interaction and
probably can bring about superconductivity at temperatures as below so and well
above room temperature.


title:
Transmission and reflection studies of periodic and random systems with
  gain

abstract:
  The transmission (T) and reflection (R) coefficients are studied in periodic
systems and random systems with gain. For both the periodic electronic
tight-binding model and the periodic classical many-layered model, we obtain
numerically and theoretically the dependence of T and R. The critical length of
periodic system L[sub c][sup 0], above which T decreases with the size of the
system L while R approaches a constant value, is obtained to be inversely
proportional to the imaginary part cursive-epsilon (double-prime) of the
dielectric function cursive-epsilon . For the random system, T and R also show
a nonmonotonic behavior versus L. For short systems (L < Lc) with gain
(left-angle)ln T(right-angle) = (l[sub g][sup -1] - xi [sub 0][sup -1])L. For
large systems (L(very-much-greater-than)Lc) with gain (left-angle)ln
T(right-angle) = - (l[sub g][sup -1] + xi [sub 0][sup -1])L. Lc, lg, and xi 0
are the critical, gain, and localization lengths, respectively. The dependence
of the critical length Lc on cursive-epsilon (double-prime) and disorder
strength W are also given. Finally, the probability distribution of the
reflection R for random systems with gain is also examined. Some very
interesting behaviors are observed.


title: Probing the potential landscape inside a two-dimensional electron-gas

abstract:
  We report direct observations of the scattering potentials in a
two-dimensional electron-gas using electron-beam diffaction-experiments. The
diffracting objects are local density-fluctuations caused by the spatial and
charge-state distribution of the donors in the GaAs-(Al,Ga)As heterostructures.
The scatterers can be manipulated externally by sample illumination, or by
cooling the sample down under depleted conditions.


title:
Comment on ``Lyapunov Exponent of a Many Body System and Its Transport
  Coefficients''

abstract:
  In a recent Letter, Barnett, Tajima, Nishihara, Ueshima and Furukawa obtained
a theoretical expression for the maximum Lyapunov exponent $\lambda_1$ of a
dilute gas. They conclude that $\lambda_1$ is proportional to the cube root of
the self-diffusion coefficient $D$, independent of the range of the interaction
potential. They validate their conjecture with numerical data for a dense
one-component plasma, a system with long-range forces. We claim that their
result is highly non-generic. We show in the following that it does not apply
to a gas of hard spheres, neither in the dilute nor in the dense phase.

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


title:
Numerical Simulations of the 4D Edwards-Anderson Spin Glass with Binary
  Couplings

abstract:
  We present numerical results that allow a precise determination of the
transition point and of the critical exponents of the 4D Edwards-Anderson Spin
Glass with binary quenched random couplings. We show that the low T phase
undergoes Replica Symmetry Breaking. We obtain results on large lattices, up to
a volume $V=10^4$: we use finite size scaling to show the relevance of our
results in the infinite volume limit.


title: Energy Landscape of a Model Protein

abstract:
  The potential energy surface of an off-lattice model protein is characterized
in detail by constructing a disconnectivity graph and by examining the
organisation of pathways on the surface. The results clearly reveal the
frustration exhibited by this system and explain why it does not fold
efficiently to the global potential energy minimum. In contrast, when the
frustration is removed by constructing a `Go-type' model, the resulting graph
exhibits the characteristics expected for a folding funnel.


title:
Magnetic field effects on the density of states of orthorhombic
  superconductors

abstract:
  The quasiparticle density of states in a two-dimensional d-wave
superconductor depends on the orientation of the in-plane external magnetic
field H. This is because. in the region of the gap nodes, the Doppler shift due
to the circulating supercurrents around a vortex depend on the direction of H.
For a tetragonal system the induced pattern is four-fold symmetric and, at zero
energy, the density of states exhibits minima along the node directions. But
YBa_2C_3O_{6.95} is orthorhombic because of the chains and the pattern becomes
two-fold symmetric with the position of the minima occuring when H is oriented
along the Fermi velocity at a node on the Fermi surface. The effect of impurity
scattering in the Born and unitary limit is discussed.


title: Optimized local modes for lattice dynamical applications

abstract:
  We present a new scheme for the construction of highly localized lattice
Wannier functions. The approach is based on a heuristic criterion for
localization and takes the symmetry constraints into account from the start. We
compare the local modes thus obtained with those generated by other schemes and
find that they also provide a better description of the relevant vibrational
subspace.


title:
Prediction of Room Temperature High Thermoelectric Performance in n-type
  La(Ru,Rh)4Sb12

abstract:
  First principles calculations are used to investigate the band structure and
the transport related properties of unfilled and filled 4d skutterudite
antimonides. The calculations show that, while RhSb3 and p-type La(Rh,Ru)4Sb12
are unfavorable for thermoelectric application, n-type La(Rh,Ru)4Sb12 is very
likely a high figure of merit thermoelectric material in the important
temperature range 150-300 K.


title: On the thermodynamics of strongly correlated integrable electron systems

abstract:
  We reexamine the Yang-Yang-Takahashi method of deriving the thermodynamic
Bethe ansatz equations which describe strongly correlated electron systems of
fundamental physical interest, such as the Hubbard, $s-d$ exchange (Kondo) and
Anderson models. It is shown that these equations contain some additional terms
which may play an important role in the physics of the systems.


title: Symmetry between absorption and amplification in disordered media

abstract:
  We address the issue of whether amplification, like absorption, suppresses
wave transmission at large gain, as has been claimed in previous studies of
wave propagation in active random media. A closer examination reveals that the
paradoxical symmetry between absorption and amplification is an artifact of
unphysical solutions from the time-independent wave equation. Solutions from
the time-dependent equation demonstrate clearly that when gain is above the
threshold, the amplitude of both the transmitted and the reflected wave
actually increases with time, apparently without bound. The implications of the
current finding is discussed


title:
The Ground and Low Lying Excited States of the Two-Dimensional Hubbard
  Model

abstract:
  We have studied the ground state of the two-dimensional (2D) Hubbard model by
using a quantum monte method paying special attention to the shell structure
effect on finite size clusters. Our calculations show there is a gap for spin
excitations in the ground state and incommensurate peaks at $(\pi \pm \delta,
\pi)$ and $(\pi, \pi \pm \delta)$ in the spin correlation function for a low
lying excited state. In the ground state, the long range part of the d-wave
superconducting correlation function is enhanced and the momentum distribution
function at the Fermi level $(\pi,0)$ is rounded. The gap in spin excitations
and the momentum distribution function rounding is consistent with opening a
d-wave superconducting gap in the ground state.


title:
Mixed-spin systems: coexistence of Haldane gap and antiferromagnetic
  long range order

abstract:
  Recent experiments on the quasi-1D antiferromagnets $R_{2}BaNiO_{5}$ (R =
rare earth) have shown the existence of purely 1D Haldane gap excitations
propagating on the Ni chains. Below an ordering temperature, the gap
excitations survive and coexist with the conventional spin waves in the ordered
phase. We construct a model mixed-spin system in 2D for which the ground state
can be exactly specified. Using the Matrix Product Method, we show the
existence of Haldane gap excitations in the ordered phase. We consider
different cases of ordering to study the effect of ordering on the degeneracy
of the Haldane gap excitations.


title: Microforces and the Theory of Solute Transport

abstract:
  A generalized continuum framework for the theory of solute transport in
fluids is proposed and systematically developed. This framework rests on the
introduction of a generic force balance for the solute, a balance distinct from
the macroscopic momentum balance associated with the mixture. Special forms of
such a force balance have been proposed and used going back at least as far as
Nernst's 1888 theory of diffusion. Under certain circumstances, this force
balance yields a Fickian constitutive relation for the diffusive solute flux,
and, in conjunction with the solute mass balance, provides a generalized
Smoluchowski equation for the mass fraction. Our format furnishes a systematic
procedure for generalizing convection-diffusion models of solute transport,
allowing for constitutive nonlinearities, external forces acting on the
diffusing constituents, and coupling between convection and diffusion.


title: A model of evolution with interaction strength

abstract:
  Interaction strength is introduced in a model of evolution in d-dimension
space. It is realized by imposing a constraint concerning 2d differences of
fitnesses between that of any extremal site and those of its 2d nearest
neighbours at each time step in the evolution of the model. For any given
interaction strength between 0 and 1 the model can self-organize to a critical
state. Two exact equations found in Bak-Sneppen model still hold in our model
for different interaction strength. Simulations of one- and two-dimensional
models for ten different values of interaction are given. It is found that
self-organized threshold decreases with interaction strength increasing. It is
also shown that the critical exponent, and two basic exponents, avalanche
distribution, and avalanche dimension, are interaction strength dependent.


title: On the effects of irrelevant boundary scaling operators

abstract:
  We investigate consequences of adding irrelevant (or less relevant) boundary
operators to a (1+1)-dimensional field theory, using the Ising and the boundary
sine-Gordon model as examples. In the integrable case, irrelevant perturbations
are shown to multiply reflection matrices by CDD factors: the low-energy
behavior is not changed, while various high-energy behaviors are possible,
including ``roaming'' RG trajectories. In the non-integrable case, a Monte
Carlo study shows that the IR behavior is again generically unchanged, provided
scaling variables are appropriately renormalized.


title:
Impurity-induced spin polarization and NMR line broadening in underdoped
  cuprates

abstract:
  We present a theory of magnetic (S=1) Ni and nonmagnetic Zn impurities in
underdoped cuprates. Both types of impurities are shown to induce S=1/2 moments
on Cu sites in the proximity of the impurity, a process which is intimately
related to the spin gap phenomenon in cuprates. Below a characteristic Kondo
temperature, the Ni spin is partially screened by the Cu moments, resulting in
an effective impurity spin S=1/2. We further analyze the
Ruderman-Kittel-Kasiya-Yosida-type response of planar Cu spins to a
polarization of the effective impurity moments and derive expressions for the
corresponding ^{17}O NMR line broadening. The peculiar aspects of recent
experimental NMR data can be traced back to different spatial characteristics
of Ni and Zn moments as well as to an inherent temperature dependence of local
antiferromagnetic correlations.


title:
Orbital Dynamics: The Origin of Anomalous Magnon Softening in
  Ferromagnetic Manganites

abstract:
  We study the renormalization of magnons by charge and coupled orbital-lattice
fluctuations in colossal magnetoresistance compounds. The model considered is
an orbitally degenerate double-exchange system coupled to Jahn-Teller active
phonons. The modulation of ferromagnetic bonds by low-energy orbital
fluctuations is identified as the main origin of the unusual softening of the
zone-boundary magnons observed experimentally in manganites.


title: Orbital Polarons in the Metal-Insulator Transition of Manganites

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


title: Orbital order out of spin disorder: How to measure the orbital gap

abstract:
  The interplay between spin and orbital degrees of freedom in the Mott-Hubbard
insulator is studied by considering an orbitally degenerate superexchange
model. We argue that orbital order and the orbital excitation gap in this model
are generated through the order-from-disorder mechanism known previously from
frustrated spin models. We propose that the orbital gap should show up
indirectly in the dynamical spin structure factor; it can therefore be measured
using the conventional inelastic neutron scattering method.


title: From band insulator to Mott insulator in one dimension

abstract:
  We derive the phase diagram for the one-dimensional model of a ferroelectric
perovskite recently introduced by Egami, Ishihara and Tachiki [Science, {\bf
261}, 1307 (1993)]. We show that the interplay between covalency, ionicity and
strong correlations results in a spontaneously dimerized phase which separates
the weak-coupling band insulator from the strong-coupling Mott insulator. The
transition from the band insulator to the dimerized phase is identified as an
Ising critical point. The charge gap vanishes at this single point with the
optical conductivity diverging as $\sigma(\omega)\sim \omega^{-3/4}$. The spin
excitations are gapless above the second transition to the Mott insulator
phase.


title: Self-averaging of random and thermally disordered diluted Ising systems

abstract:
  Self-averaging of singular thermodynamic quantities at criticality for
randomly and thermally diluted three dimensional Ising systems has been studied
by the Monte Carlo approach. Substantially improved self-averaging is obtained
for critically clustered (critically thermally diluted) vacancy distributions
in comparison with the observed self-averaging for purely random diluted
distributions. Critically thermal dilution, leading to maximum relative
self-averaging, corresponds to the case when the characteristic vacancy
ordering temperature is made equal to the magnetic critical temperature for the
pure 3D Ising systems. For the case of a high ordering temperature, the
self-averaging obtained is comparable to that in a randomly diluted system.


title:
Comment on "Numerical Study on Aging Dynamics in the 3D Ising Spin Glass
  Model"

abstract:
  We show that the dynamical behavior of the 3D Ising spin glass with Gaussian
couplings is not compatible with a droplet dynamics. We show that this is
implied from the data of reference cond-mat/9904143, that, when analyzed in an
accurate way, give multiple evidences of this fact. Our study is based on the
analysis of the overlap-overlap correlation function, at different values of
the separation r and of the time t.


title:
The Carnot Cycle for Small Systems: Irreversibility and the Cost of
  Operations

abstract:
  We employ the recently developed framework of the energetics of stochastic
processes (called `stochastic energetics'), to re-analyze the Carnot cycle in
detail, taking account of fluctuations, without taking the thermodynamic limit.
  We find that both processes of connection to and disconnection from heat
baths and adiabatic processes that cause distortion of the energy distribution
are sources of inevitable irreversibility within the cycle. Also, the so-called
null-recurrence property of the cumulative efficiency of energy conversion over
many cycles and the irreversible property of isolated, purely mechanical
processes under external `macroscopic' operations are discussed in relation to
the impossibility of a perpetual machine, or Maxwell's demon.


title: Deceptive Apparent Nonadiabatic Magnetization Process

abstract:
  We discuss the effect of the thermal environment on the low-temperature
response of the magnetization of uniaxial magnets to a time-dependent applied
magnetic field. At sufficiently low temperatures the staircase magnetization
curves observed in molecular magnets such as Mn_{12} and Fe_8 display little
temperature dependence. However the changes of the magnetization at each step
do not seem to be directly related to the probability for a quantum mechanical
nonadiabatic transition. In order to explain this deceptive apparent
nonadiabatic behavior, we study the quantum dynamics of the system in a thermal
environment and propose a relation between the observed magnetization steps and
the quantum mechanical transition probability due to the nonadiabatic
transition.


title:
On the Relation between Interband Scattering and the "Metallic Phase" of
  Two Dimensional Holes in GaAs/AlGaAs

abstract:
  The "metallic" regime of holes in GaAs/AlGaAs heterostructures corresponds to
densities where two splitted heavy hole bands exist at a zero magnetic field.
Using Landau fan diagrams and weak field magnetoresistance curves we extract
the carrier density in each band and the interband scattering rates. The
measured inelastic rates depend Arrheniusly on temperature with an activation
energy similar to that characterizing the longitudinal resistance. The
"metallic" characteristics, namely, the resistance increase with temperature,
is hence traced to the activation of inelastic interband scattering. The data
are used to extract the bands dispersion relations as well as the two
particle-hole excitation continua. It is then argued that acoustic plasmon
mediated Coulomb scattering might be responsible for the Arrhenius dependence
on temperature. The absence of standard Coulomb scattering characterized by a
power law dependence upon temperature is pointed out.


title: Lee-Yang zeros and the Ising model on the Sierpinski Gasket

abstract:
  We study the distribution of the complex temperature zeros for the partition
function of the Ising model on a Sierpinski gasket using an exact recursive
relation. Although the zeros arrange on a curve pinching the real axis at T=0
in the thermodynamic limit, their density vanishes asymptotically along the
curve approaching the origin. This phenomenon explains the coincidence of the
low temperature regime on the Sierpinski gasket and on the linear chain.


title: Freezing by Heating in a Driven Mesoscopic System

abstract:
  We investigate a simple model corresponding to particles driven in opposite
directions and interacting via a repulsive potential. The particles move
off-lattice on a periodic strip and are subject to random forces as well. We
show that this model - which can be considered as a continuum version of some
driven diffusive systems - exhibits a paradoxial, new kind of transition called
here ``freezing by heating''. One interesting feature of this transition is
that a crystallized state with a higher total energy is obtained from a fluid
state by increasing the amount of fluctuations.


title: Optimal Self-Organization

abstract:
  We present computational and analytical results indicating that systems of
driven entities with repulsive interactions tend to reach an optimal state
associated with minimal interaction and minimal dissipation. Using concepts
from non-equilibrium thermodynamics and game theoretical ideas, we generalize
this finding to an even wider class of self-organizing systems which have the
ability to reach a state of maximal overall ``success''. This principle is
expected to be relevant for driven systems in physics like sheared granular
media, but it is also applicable to biological, social, and economic systems,
for which only a limited number of quantitative principles are available yet.


title: Andreev reflection in engineered Al/Si/InGaAs(001) junctions

abstract:
  Complete suppression of the native n-type Schottky barrier is demonstrated in
Al/InGaAs(001) junctions grown by molecular-beam-epitaxy. This result was
achieved by the insertion of Si bilayers at the metal-semiconductor interface
allowing the realization of truly Ohmic non-alloyed contacts in low-doped and
low-In content InGaAs/Si/Al junctions. It is shown that this technique is
ideally suited for the fabrication of high-transparency
superconductor-semiconductor junctions. To this end magnetotransport
characterization of Al/Si/InGaAs low-n-doped single junctions below the Al
critical temperature is presented. Our measurements show Andreev-reflection
dominated transport corresponding to junction transparency close to the
theoretical limit due to Fermi-velocity mismatch


title: Non-equilibrium dynamics of simple spherical spin models

abstract:
  We investigate the non-equilibrium dynamics of spherical spin models with
two-spin interactions. For the exactly solvable models of the d-dimensional
spherical ferromagnet and the spherical Sherrington-Kirkpatrick model the
asymptotic dynamics has for large times and for large waiting times the same
formal structure. In the limit of large waiting times we find in both models an
intermediate time scale, scaling as a power of the waiting time with an
exponent smaller than one, and thus separating the time-translation invariant
short-time dynamics from the aging regime. It is this time scale on which the
fluctuation-dissipation regime is violated. Aging in these models is similar to
that observed in spin glasses at the level of correlation functions, but
different at the level of response functions, and thus different at the level
of experimentally accessible quantities like the thermoremanent magnetization.


title: Excitonic ferromagnetism in the hexaborides

abstract:
  A ferromagnet with a small spontaneous moment but with a high Curie
temperature can be obtained by doping an excitonic insulator made from a spin
triplet exciton condensate. Such a condensate can occur in a semimetal with a
small overlap or a semiconductor with a small bandgap. We propose that it is
responsible for the unexpected ferromagnetism in the doped hexaboride material
Ca_{1-x}La_xB_6.


title: Magnetotransport in manganites and the role of quantal phases I: Theory

abstract:
  A microscopic picture of charge transport in manganites is developed, with
particular attention being paid to the neighborhood of the ferromagnet-to-
paramagnet phase transition. The basic transport mechanism invoked is
inelastically-assisted carrier hopping between states localized by magnetic
disorder. In the context of the anomalous Hall effect, central roles are played
by the Pancharatnam and spin-orbit quantal phases.


title:
Magnetotransport in manganites and the role of quantal phases II:
  Experiment

abstract:
  As in conventional ferromagnets, the Hall resistivity rho_xy of a
La_{2/3}(Ca,Pb)_{1/3}MnO_3 single crystal exhibits both ordinary and anomalous
contributions at low temperature. However, these contributions, unexpectedly,
have opposite signs. Near Tc, the ordinary contribution is no longer evident
and rho_xy is solely determined by the sample magnetization, reaching an
extremum at approximately 40% of the saturated magnetization. A new model for
the anomalous Hall effect, incorporating the quantal phase accumulated by
double-exchange, three-site hopping reproduces this result. Below Tc, rho_xy
reflects the competition between normal and anomalous Hall effects.


title:
Orientational Ordering in Sequence-Disordered Liquid Crystalline
  Polymers

abstract:
  Phase separation of sequence-disordered liquid crystalline polymers, a
promising class of technological and biological relevance, is studied by field
theory, and thermodynamic mechanisms responsible for orientational ordering
observed in experiments, are discussed. The theory developed predicts that
chemical disorder marginally affects the nematic/isotropic biphasic coexistence
width, but strongly impacts ordering; above a critical chemical disorder
threshold orientational ordering is precluded.


title:
Carrier-Carrier Correlations in an Optically Excited Single
  Semiconductor Quantum Dot

abstract:
  We applied low temperature diffraction limited confocal optical microscopy to
spatially resolve, and spectroscopically study photoluminescence from single
self-assembled semiconductor quantum dots. Using selective wavelength imaging
we unambiguously demonstrated that a single photoexcited quantum dot emits
light in a few very narrow spectral lines. The measured spectrum and its
dependence on the power of either cw or pulsed excitation are explained by
taking carrier correlations into account. We solve numerically a many body
Hamiltonian for a model quantum dot, and we show that the multi line emission
spectrum is due to optical transitions between confined exciton multiplexes. We
furthermore show that the electron-electron and hole-hole exchange interaction
is responsible for the typical appearance of pairs in the photoluminescence
spectra and for the appearance of red shifted new lines as the excitation power
increases. The fact that only a few spectral lines appear in the emission
spectrum strongly indicates fast thermalization. This means that a multiexciton
relaxes to its ground state much faster than its radiative lifetime.


title: Persistence and Quiescence of Seismicity on Fault Systems

abstract:
  We study the statistics of simulated earthquakes in a quasistatic model of
two parallel heterogeneous faults within a slowly driven elastic tectonic
plate. The probability that one fault remains dormant while the other is active
for a time Dt following the previous activity shift is proportional to the
inverse of Dt to the power 1+x, a result that is robust in the presence of
annealed noise and strength weakening. A mean field theory accounts for the
observed dependence of the persistence exponent x as a function of
heterogeneity and distance between faults. These results continue to hold if
the number of competing faults is increased. This is related to the persistence
phenomenon discovered in a large variety of systems, which specifies how long a
relaxing dynamical system remains in a neighborhood of its initial
configuration. Our persistence exponent is found to vary as a function of
heterogeneity and distance between faults, thus defining a novel universality
class.


title:
New approach to analysis of negative magnetoresistance. The statistics
  of the closed paths

abstract:
  It is shown that a new kind of information can be extracted from the Fourier
transform of negative magnetoresistance (MR) in 2D semiconductor structures.
The procedure proposed provides the information on the area distribution
function of the closed paths and on the area dependence of the mean length of
closed paths $\bar{L} (S)$ . Based on this line of attack the method of
analysis of the negative MR is suggested. The method is used for data treatment
of negative MR in 2D structures. In previous version (cond-mat/9902038) we
suppoused that the difference in area dependence of $\bar{L} (S)$ for
structures investigated came from the difference in scattering ansotropy. The
more detailed analysis shows that main reason for such difference is different
relations between momentum and phase relaxation times.


title:
Fast Dynamics of Glass-forming Liquids investigated by Dielectric
  Spectroscopy: a Comparison

abstract:
  We have performed dielectric spectroscopy on the molecular glass-formers
glycerol, propylene carbonate (PC) and the ionic melts CKN and CRN. The
frequency range covers almost 20 decades from some micro-Hz to some THz. The
meausrement results are compared to neutron and light scattering data and
analysed with mode coupling theory.


title: Coiling of Cylindrical Membrane Stacks with Anchored Polymers

abstract:
  We study experimentally a coiling instability of cylindrical multilamellar
stacks of phospholipid membranes, induced by polymers with hydrophobic anchors
grafted along their hydrophilic backbone. We interpret our experimental results
in terms of a model, in which local membrane curvature and polymer
concentration are coupled. The model predicts the occurence of maximally tight
coils above a threshold anchor occupancy. Indeed, only maximally tight coils
are observed experimentally. Our system is unique in that coils form in the
absence of twist.


title:
The transition from the adiabatic to the sudden limit in core level
  photoemission: A model study of a localized system

abstract:
  We consider core electron photoemission in a localized system, where there is
a charge transfer excitation. The system is modelled by three electron levels,
one core level and two outer levels. The model has a Coulomb interaction
between these levels and the continuum states into which the core electron is
emitted. The model is simple enough to allow an exact numerical solution, and
with a separable potential an analytic solution. We calculate the ratio
r(omega) between the weights of the satellite and the main peak as a function
of the photon energy omega. The transition from the adiabatic to the sudden
limit takes place for quite small photoelectron kinetic energies. For such
small energies, the variation of the dipole matrix element is substantial and
described by the energy scale Ed. Without the coupling to the photoelectron,
the corresponding ratio r0(omega) is determined by Ed and the satellite
excitation energy dE. When the interaction potential with the continuum states
is introduced, a new energy scale Es=1/(2Rs^2) enters, where Rs is a length
scale of the interaction potential. At threshold there is typically a (weak)
constructive interference between intrinsic and extrinsic contributions, and
the ratio r(omega)/r0(omega) is larger than its limiting value for large omega.
The interference becomes small or weakly destructive for photoelectron energies
of the order Es. For larger energies r(omega)/r0(omega) therefore typically has
a weak undershoot. If this undershoot is neglected, r(omega)/r0(omega) reaches
its limiting value on the energy scale Es.


title: Normal State Ettingshausen Effect in La_{2-x}Sr_{x}CuO_{4}

abstract:
  A method for measurements of small (metallic) Ettingshausen coefficient (P)
was developed. The influence of the dominating thermal effects, the Joule and
Thomson heats, was eliminated making use of the odd symmetry of the
Ettingshausen temperature gradient in respect with reversing of the direction
of the magnetic field and electrical current. The method was applied to
La_{2-x}Sr_{x}CuO_{4} ($x=0.03\div 0.35$) high-T_c superconductor in normal
state. We have found that in the whole composition range the Ettingshausen
coefficient is of the order of 10^{-7} m^{3} K/J which is characteristic of
typical metals. The coefficient changes sign from positive to negative near
$x\approx 0.07$. Weak variation of P is in contrast to the behavior of other
transport coefficients for La_{2-x}Sr_{x}CuO_{4}, as the thermoeletric power or
the Hall coefficient, which have been reported in literature to change their
values by more than two orders of magnitude with Sr doping.


title: Shot noise in ferromagnetic single electron tunneling devices

abstract:
  Frequency dependent current noise in ferromagnetic double junctions with
Coulomb blockade is studied theoretically in the limit of sequential tunneling.
Two different relaxation processes are found in the correlations between spin
polarized tunneling currents; low frequency spin fluctuations and high
frequency charge fluctuations. Spin accumulation in strongly asymmetric
junctions is shown to lead to a negative differential resistance. We also show
that large spin noise activated in the range of negative differential
resistance gives rise to a significant enhancement of the current noise.


title: Finite-connectivity systems as error-correcting codes

abstract:
  We investigate the performance of parity check codes using the mapping onto
Ising spin systems proposed by Sourlas. We study codes where each parity check
comprises products of K bits selected from the original digital message with
exactly C checks per message bit. We show, using the replica method, that these
codes saturate Shannon's coding bound for $K\to\infty$ when the code rate K/C
is finite. We then examine the finite temperature case to asses the use of
simulated annealing methods for decoding, study the performance of the finite K
case and extend the analysis to accommodate different types of noisy channels.
The connection between statistical physics and belief propagation decoders is
discussed and the dynamics of the decoding itself is analyzed. Further insight
into new approaches for improving the code performance is given.


title: Triangular Trimers on the Triangular Lattice: an Exact Solution

abstract:
  A model is presented consisting of triangular trimers on the triangular
lattice. In analogy to the dimer problem, these particles cover the lattice
completely without overlap. The model has a honeycomb structure of hexagonal
cells separated by rigid domain walls. The transfer matrix can be diagonalised
by a Bethe Ansatz with two types of particles. This leads two an exact
expression for the entropy on a two-dimensional subset of the parameter space.


title:
Paramagnetic reentrant effect in high purity mesoscopic AgNb proximity
  structures

abstract:
  We discuss the magnetic response of clean Ag coated Nb proximity cylinders in
the temperature range 150 \mu K < T < 9 K. In the mesoscopic temperature
regime, the normal metal-superconductor system shows the yet unexplained
paramagnetic reentrant effect, discovered some years ago [P. Visani, A. C.
Mota, and A. Pollini, Phys. Rev. Lett. 65, 1514 (1990)], superimposing on full
Meissner screening. The logarithmic slope of the reentrant paramagnetic
susceptibility chi_para(T) \propto \exp(-L/\xi_N) is limited by the condition
\xi_N=n L, with \xi_N=\hbar v_F/2 \pi k_B T, the thermal coherence length and
n=1,2,4. In wires with perimeters L=72 \mu m and L=130 \mu m, we observe
integer multiples n=1,2,4. At the lowest temperatures, \chi_para compensates
the diamagnetic susceptibility of the \textit{whole} AgNb structure.


title: Polaronic Excitons in ZnCdSe/ZnSe Quantum Wells

abstract:
  We present a detailed investigation of excitonic absorption in
$Zn_{0.69}Cd_{0.31}Se/ZnSe$ quantum wells under the application of a
perpendicular magnetic field. The large energy separation between heavy- and
light-hole excitons allows us to clearly resolve and identify magneto-excitonic
absorption resonant with the continuum edge of the 1S heavy-hole exciton.
Experimental values of the exciton binding energy are compared with results of
a theoretical model that includes the exciton-phonon interaction. The
remarkable agreemeent found unambiguously indicates the predominant polaronic
character of excitons in ZnSe-based heterostructures.


title: Elastic Constants and Charge Ordering in a'-NaV2O5

abstract:
  We present the temperature dependence of shear and longitudinal elastic
constants in a'-NaV2O5. For the longitudinal c22 and c33 modes we find
anomalies at Tc in contrast to the Spin Peierls substance CuGeO3 where only the
longitudinal mode along the chain shows a pronounced effect at TSP. The c66
shear mode (propagation along the chain in b-direction polarization in
a-direction) shows strong softening of 12%. Such a large effect is absent for
all shear modes in CuGeO3. We can interpret this softening with a coupling of
the exy symmetry strain to the charge fluctuation of B1g symmetry. We give the
possible low temperature charge distribution.


title:
Near field and far field scattering of surface plasmon polaritons by
  one-dimensional surface defects

abstract:
  A rigorous formulation for the scattering of surface plasmon polaritons (SPP)
from a one-dimensional surface defect of any shape that yields the
electromagnetic field in the vacuum half-space above the vacuum-metal interface
is developed by the use of an impedance boundary condition. The electric and
magnetic near fields, the angular distribution of the far-field radiation into
vacuum due to SPP-photon coupling, and the SPP reflection and transmission
coefficients are calculated by numerically solving the k-space integral
equation upon which the formulation is based. In particular, we consider
Gaussian-shaped defects and study the dependence of the above mentioned
physical quantities on their 1/e half-width a and height h. SPP reflection is
significant for narrow defects; maximum reflection (plasmon mirrors) is
achieved for a~lambda/10. For increasing defect widths, protuberances and
indentations behave differently. The former give rise to a monotonic increase
of radiation at the expense of SPP transmission for increasing defect
half-width. Indentations exhibit a significant increase of radiation (decrease
of SPP transmission) for half-widths of the order of or smaller than the
wavelength, but tend to total SPP transmission in an oscillatory manner upon
further increasing the half-width. Light-emitters might thus be associated with
either wide indentations, or protuberances with widths that are of the order of
or smaller than the wavelength.


title: Evidence for incommensurate spin fluctuations in Sr_2RuO_4

abstract:
  We report first inelastic neutron scattering measurements in the normal state
of Sr_2RuO_4 that reveal the existence of incommensurate magnetic spin
fluctuations located at ${\bf q}_0=(\pm 0.6\pi/a, \pm 0.6\pi/a, 0)$. This
finding confirms recent band structure calculations that have predicted
incommensurate magnetic responses related to dynamical nesting properties of
its Fermi surface.


title: Magnetism in heavy-fermion U(Pt,Pd)3 studied by mSR

abstract:
  We report mSR experiments carried out on a series of heavy-electron
pseudobinary compounds U(Pt1-xPdx)3 (x<=0.05). For x<=0.005 the zero-field muon
depolarisation is described by the Kubo-Toyabe function. However the
temperature variation of the Kubo-Toyabe relaxation rate does not show any sign
of the small-moment antiferromagnetic phase with TN~6 K (signalled by neutron
diffraction), in contrast to previous reports. The failure to detect the small
ordered moment suggests it has a fluctuating (> 10 MHz) nature, which is
consistent with the interpretation of NMR data. For 0.01<=x<=0.05 the muon
depolarisation in the ordered state is described by two terms of equal
amplitude: an exponentially damped spontaneous oscillation and a Lorentzian
Kubo-Toyabe function. These terms are associated with antiferromagnetic order
with substantial moments. The Knight-shift measured in a magnetic field of 0.6
T on single-crystalline U(Pt0.95Pd0.05)3 in the paramagnetic state shows two
signals for B perpendicular to c, while only one signal is observed for B||c.
The observation of two signals for B perpendicular to c, while there is only
one muon localisation site (0,0,0), points to the presence of two spatially
distinct regions of different magnetic response.


title:
Symmetry Analysis of Second Harmonic Generation at Surfaces of
  Antiferromagnets

abstract:
  Using group theory we classify the nonlinear magneto-optical response at
low-index surfaces of fcc antiferromagnets, such as NiO. Structures consisting
of one atomic layer are discussed in detail. We find that optical second
harmonic generation is sensitive to surface antiferromagnetism in many cases.
We discuss the influence of a second type of magnetic atoms, and also of a
possible oxygen sublattice distortion on the output signal. Finally, our
symmetry analysis yields the possibility of antiferromagnetic surface domain
imaging even in the presence of magnetic unit-cell doubling.


title:
Observation of Fluctuation-Dissipation-Theorem Violations in a
  Structural Glass

abstract:
  The fluctuation-dissipation theorem (FDT), connecting dielectric
susceptibility and polarization noise was studied in glycerol below its glass
transition temperature Tg. Weak FDT violations were observed after a quench
from just above to just below Tg, for frequencies above the alpha peak.
Violations persisted up to 10^5 times the thermal equilibration time of the
configurational degrees of freedom under study, but comparable to the average
relaxation time of the material. These results suggest that excess energy flows
from slower to faster relaxing modes.


title: Atoms in nanotubes: small dimensions and variable dimensionality

abstract:
  Newly discovered carbon nanotubes provide an environment in which small atoms
move relatively freely. An assembly of such atoms provides a realization of a
quasi-one dimensional system which is an ideal testing ground for concepts and
mathematics of statistical physics.


title:
Binary Mixtures of Bose-Einstein Condensates: Phase dynamics and Spatial
  Dynamics

abstract:
  We investigate the relative phase coherence properties and the occurrence of
demixing instabilities for two mutually interacting and time evolving
Bose-Einstein condensates in traps. Our treatment naturally includes the
additional decoherence effect due to fluctuations in the total number of
particles. Analytical results are presented for the breathe-together solution,
an extension of previously known scaling solution to the case of a binary
mixture of condensates. When the three coupling constants describing the
elastic interactions among the atoms in the two states are close to each other,
a dramatic increase of the phase coherence time is predicted. Numerical results
are presented for the parameters of the recent JILA experiments.


title: Thermally fluctuating superconductors in two dimensions

abstract:
  We describe the different regimes of finite temperature dynamics in the
vicinity of a zero temperature superconductor to insulator quantum phase
transition in two dimensions. New results are obtained for a low temperature
phase-only hydrodynamics, and for the intermediate temperature quantum-critical
region. In the latter case, we obtain a universal relationship between the
frequency-dependence of the conductivity and the value of the d.c. resistance.


title:
Quantum melting of magnetic long-range order near orbital degeneracy.
  Classical phases and Gaussian fluctuations

abstract:
  We study the effective spin-orbital model derived for the d9 ions in a
three-dimensional perovskite lattice, as in KCuF_3, where at each site the
doubly degenerate eg orbitals contain a single hole. The model describes the
superexchange interactions that depend on the pattern of orbitals occupied. We
present the ground state properties of this model, depending on the splitting
between the eg orbitals E_z, and the Hund's rule coupling in the excited d8
states, J_H. The classical phase diagram consists of six magnetic phases which
all have different orbital ordering: two antiferromagnetic (AF) phases with
G-AF order and either x2-y2 or 3z2-r2 orbitals occupied, two phases with mixed
orbital (MO) patterns and A-AF order, and two other MO phases with either C-AF
or G-AF order. All of them become degenerate at the multicritical point
M=(E_z,J_H)=(0,0). Using a generalization of linear spin-wave theory we study
both the transverse excitations which are spin-waves and
spin-and-orbital-waves, as well as the longitudinal (orbital) excitations. The
transverse modes couple to each other, and the spin-and-orbital-wave turns into
a soft mode near the M point. Therefore, quantum corrections to the
long-range-order parameter are drastically increased near the orbital
degeneracy, and classical order is suppressed in a crossover regime between the
G-AF and A-AF phases in the (E_z,J_H) plane. This behavior is reminiscent of
that found in frustrated spin models, and we conclude that orbital degeneracy
provides a new and physically realizable mechanism which stabilizes a spin
liquid ground state due to inherent frustration of magnetic interactions. We
also point out that such a disordered magnetic phase is likely to be realized
in LiNiO_2.


title: Finite-size effects in the self-organized critical forest-fire model

abstract:
  We study finite-size effects in the self-organized critical forest-fire model
by numerically evaluating the tree density and the fire size distribution. The
results show that this model does not display the finite-size scaling seen in
conventional critical systems. Rather, the system is composed of relatively
homogeneous patches of different tree densities, leading to two qualitatively
different types of fires: those that span an entire patch and those that don't.
As the system size becomes smaller, the system contains less patches, and
finally becomes homogeneous, with large density fluctuations in time.


title: Stripe order at low temperatures in La{2-x}Sr{x}NiO4 for 1/3 < x < 1/2

abstract:
  Stripe order in La{2-x}Sr{x}NiO4 beyond x = 1/3 was studied with neutron
scattering technique. At low temperatures, all the samples exhibit hole stripe
order. Incommensurability \epsilon of the stripe order is approximately linear
in the hole concentration n_h = x + 2\delta up to x = 1/2, where \delta denotes
the off-stoichiometry of oxygen atoms. The charge and spin ordering
temperatures exhibit maxima at n_h = 1/3, and both decrease beyond n_h > 1/3.
For 1/3 < n_h < 1/2, the stripe ordering consists of the mixture of the
\epsilon = 1/3 stripe order and the n_h = 1/2 charge/spin order.


title:
Metal-insulator transition in a multilayer system with a strong magnetic
  field

abstract:
  We study the Anderson localization in a weakly coupled multilayer system with
a strong magnetic field perpendicular to the layers. The phase diagram of 1/3
flux quanta per plaquette is obtained. The phase diagram shows that a
three-dimensional quantum Hall effect phase exists for a weak on-site disorder.
For intermediate disorder, the system has insulating and normal metallic phases
separated by a mobility edge. At an even larger disorder, all states are
localized and the system is an insulator. The critical exponent of the
localization length is found to be $\nu=1.57\pm0.10$.


title: Photon Assisted Tunneling in Quantum Dots

abstract:
  We review experiments on single electron transport through single quantum
dots in the presence of a microwave signal. In the case of a small dot with
well-resolved discrete energy states, the applied high-frequency signal allows
for inelastic tunnel events that involve the exchange of photons with the
microwave field. These photon assisted tunneling (PAT) processes give rise to
sideband resonances in addition to the main resonance. Photon absorption can
also lead to tunneling via excited states instead of tunneling via the ground
state of the quantum dot.


title:
Application of p-adic analysis to models of spontaneous breaking of the
  replica symmetry

abstract:
  Methods of p-adic analysis are applied to the investigation of the
spontaneous symmetry breaking in the models of spin glasses. A p-adic
expression for the replica matrix is given and moreover the replica matrix in
the models of spontaneous breaking of the replica symmetry in the simplest case
is expressed in the form of the Vladimirov operator of p-adic fractional
differentiation. Also the model of hierarchical diffusion (that was proposed to
describe relaxation of spin glasses) investigated using p-adic analysis.


title:
Percolation-type description of the metal-insulator transition in two
  dimensions

abstract:
  A simple non-interacting-electron model, combining local quantum tunneling
and global classical percolation (due to a finite dephasing time at low
temperatures), is introduced to describe a metal-insulator transition in two
dimensions. It is shown that many features of the experiments, such as the
exponential dependence of the resistance on temperature on the metallic side,
the linear dependence of the exponent on density, the $e^2/h$ scale of the
critical resistance, the quenching of the metallic phase by a parallel magnetic
field and the non-monotonic dependence of the critical density on a
perpendicular magnetic field, can be naturally explained by the model.


title:
Thermoelectric Transport Properties in Disordered Systems Near the
  Anderson Transition

abstract:
  We study the thermoelectric transport properties in the three-dimensional
Anderson model of localization near the metal-insulator transition [MIT]. In
particular, we investigate the dependence of the thermoelectric power S, the
thermal conductivity K, and the Lorenz number L_0 on temperature T. We first
calculate the T dependence of the chemical potential from the number density of
electrons at the MIT using averaged density of state obtained by
diagonalization. Without any additional approximation, we determine from the
chemical potential the behavior of S, K and L_0 at low T as the MIT is
approached. We find that the d.c. conductivity and K decrease to zero at the
MIT as T -> 0 and show that S does not diverge. Both S and L_0 become
temperature independent at the MIT and depend only on the critical behavior of
the conductivity.


title:
Orthogonality constraints and entropy in the SO(5)-Theory of
  HighT_c-Superconductivity

abstract:
  S.C. Zhang has put forward the idea that high-temperature-superconductors can
be described in the framework of an SO(5)-symmetric theory in which the three
components of the antiferromagnetic order-parameter and the two components of
the two-particle condensate form a five-component order-parameter with SO(5)
symmetry. Interactions small in comparison to this strong interaction introduce
anisotropies into the SO(5)-space and determine whether it is favorable for the
system to be superconducting or antiferromagnetic. Here the view is expressed
that Zhang's derivation of the effective interaction V_{eff} based on his
Hamiltonian H_a is not correct. However, the orthogonality constraints
introduced several pages after this 'derivation' give the key to an effective
interaction very similar to that given by Zhang. It is shown that the
orthogonality constraints are not rigorous constraints, but they maximize the
entropy at finite temperature. If the interaction drives the ground-state to
the largest possible eigenvalues of the operators under consideration
(antiferromagnetic ordering, superconducting condensate, etc.), then the
orthogonality constraints are obeyed by the ground-state, too.


title:
Analysis of the statistical behavior of genetic cluster-exact
  approximation

abstract:
  The genetic cluster-exact approximation algorithm is an efficient method to
calculate ground states of EA spin glasses. The method can be used to study
ground-state landscapes by calculating many independent ground states for each
realization of the disorder. The algorithm is analyzed with respect to the
statistics of the ground states and the valleys of the energy landscape.
Furthermore, the distribution inside each valley is evaluated. It is shown that
the algorithm does not lead to a true T=0 thermodynamic distribution, i.e. each
ground state has not the same frequency of occurrence when performing many
runs. An extension of the technique is outlined, which guarantees that each
ground states occurs with the same probability.


title: Transmission resonances on metallic gratings with very narrow slits

abstract:
  In this paper we show how transmission metallic gratings with very narrow and
deep enough slits can exhibit transmission resonances for wavelengths larger
than the period of the grating. By using a transfer matrix formalism and a
quasi-analytical model based on a modal expansion, we show that there are two
possible ways of transferring light from the upper surface to the lower one: by
the excitation of coupled surface plasmon polaritons on both surfaces of the
metallic grating or by the coupling of incident plane waves with waveguide
resonances located in the slits. Both mechanisms can lead to almost perfect
transmittance for those particular resonances.


title: Universality in self-organized critical slope models

abstract:
  The dynamics of critical slope self-organized critical models is studied,
using a previous mapping into a linear interface depinning model dragged at one
end. The model is solved obtaining the complete set of scaling exponents. Some
results are supported by previous RG developed for constant force linear
interface depinning models but others, like the linear dependency of the
susceptibility with system size, are intrinsic of this model which belongs to a
different universality class. The comparison of our results with numerical
simulations of ricepile and vortexpile models reported in the literature
reveals that, as in the constant force case, there are two universality classes
corresponding to random and periodic pinning.


title: Modelling of spatio-temporal patterns in bacterial colonies

abstract:
  A diffusion-reaction model for the growth of bacterial colonies is presented.
The often observed cooperative behavior developed by bacteria which increases
their motility in adverse growth conditions is here introduced as a nonlinear
diffusion term. The presence of this mechanism depends on a response which can
present hysteresis. By changing only the concentrations of agar and initial
nutrient, numerical integration of the proposed model reproduces the different
patterns shown by {\it Bacillus subtilis} OG-01.


title: Structure factor of thin films near continuous phase transitions

abstract:
  The two-point correlation function in thin films is studied near the critical
point of the corresponding bulk system. Based on fieldtheoretic renormalization
group theory the dependences of this correlation function on the lateral
momentum, the two distances normal to the free surfaces, temperature, and film
thickness are determined. The corresponding scattering cross section of X-rays
and neutrons under grazing incidence is calculated. It reveals the various
singularities of the two-point correlation function.


title:
Low-Temperature Spin Dynamics of Doped Manganites: roles of Mn-t2g and
  eg and O-2p states

abstract:
  The low-temperature spin dynamics of doped manganites have been analyzed
within a tight-binding model, the parameters of which are estimated by mapping
the results of ab initio density functional calculations onto the model. This
approach is found to provide a good description of the spin dynamics of the
doped manganites, observed earlier within the ab initio calculations. Our
analysis not only provides some insight into the roles of the eg and the t2g
states but also indicates that the oxygen p states play an important role in
the spin dynamics. This may cast doubt on the adaptability of the conventional
model Hamiltonian approaches to the analysis of spin dynamics of doped
manganites.


title: Geometrically constrained magnetic wall

abstract:
  The structure and properties of a geometrically constrained magnetic wall in
a constriction separating two wider regions are investigated theoretically.
They are shown to differconsiderably from those of an unconstrained wall, so
that the geometrically constrained magnetic wall truly constitutes a new kind
of magnetic wall, besides the well known Bloch and Neel walls. In particular,
the width of a constrained wall cann become very small if the characteristic
length of the constriction is small, as is actually the case in an atomic point
contact. This provides a simple, natural explanation for the large
magnetoresistance observed in ferromagnetic atomic point contacts.


title: Critical behaviour of a spin-tube model in a magnetic field

abstract:
  We show that the low-energy physics of the spin-tube model in presence of a
critical magnetic field can be described by a broken SU(3) spin chain. Using
the Lieb-Schultz-Mattis Theorem we characterize the possible magnetization
plateaus and study the critical behavior in the region of transition between
the plateaus m=1/2 and m=3/2 by means of renormalization group calculations
performed on the bosonized effective continuum field theory. We show that in
certain regions of the parameter space of the effective theory the system
remains gapless, and we compute the spin-spin correlation functions in these
regions. We also discuss the possibility of a plateau at m=1, and show that
although there exists in the continuum theory a term that might cause the
appearance of a plateau there, such term is unlikely to be relevant. This
conjecture is proved by DMRG techniques. The modifications of the three-leg
ladder Hamiltonian that might show plateaus at m =1,5/6,7/6 are discussed, and
we give the expected form of correlation functions on the m=1 plateau.


title: Magnon Damping by magnon-phonon coupling in Manganese Perovskites

abstract:
  Inelastic neutron scattering was used to systematically investigate the
spin-wave excitations (magnons) in ferromagnetic manganese perovskites. In
spite of the large differences in the Curie temperatures ($T_C$s) of different
manganites, their low-temperature spin waves were found to have very similar
dispersions with the zone boundary magnon softening. From the wavevector
dependence of the magnon lifetime effects and its correlation with the
dispersions of the optical phonon modes, we argue that a strong magneto-elastic
coupling is responsible for the observed low temperature anomalous spin
dynamical behavior of the manganites.


title:
Molecular weight effects on chain pull-out fracture of reinforced
  polymeric interfaces

abstract:
  Using Brownian dynamics, we simulate the fracture of polymer interfaces
reinforced by diblock connector chains. We find that for short chains the
interface fracture toughness depends linearly on the degree of polymerization
$N$ of the connector chains, while for longer chains the dependence becomes
$N^{3/2}$. Based on the geometry of initial chain configuration, we propose a
scaling argument that accounts for both short and long chain limits and
crossover between them.


title:
Electronic correlations, electron-phonon interaction, and isotope effect
  in high-Tc cuprates

abstract:
  Using a large-N expansion we present and solve the linearized equation for
the superconducting gap for a generalized t-J model which also contains phonons
within a Holstein model. The leading Tc has d-wave symmetry with phonons giving
a positive contribution to Tc. The corresponding isotope coefficient is very
small at optimal doping and increases towards the classical value 1/2 with
increasing dopings similar as in many cuprates.


title:
Superconductors of mixed order parameter symmetry in a Zeeman magnetic
  field

abstract:
  We study the effect of Zeeman magnetic field in the superconducting phase
with two component order parameter scenario, such as, $d_{x^2-y^2} +
e^{i\theta}{\alpha}$, where $\alpha = d_{xy}, s$. This scenario is equivalent
to applying magnetic field parrllel to Cu-O planes. In a weak magnetic field,
which does not cause much change to the predominant d-wave, supresses the minor
$\alpha$ component leading to pure d-wave phase. This observation is in
contrary to the effect of magnetic field applied in the c-direction to the Cu-O
planes which is believed to induce a minor component $\alpha$ to d-wave
superconductors.
  We also show that the response of such superconductors to a weak Zeeman
magnetic field can be quite different depending on the phase $\theta$ of the
minor component ($\alpha$).


title:
Exact ground state and kink-like excitations of a two dimensional
  Heisenberg antiferromagnet

abstract:
  A rare example of a two dimensional Heisenberg model with an exact dimerized
ground state is presented. This model, which can be regarded as a variation on
the kagome lattice, has several features of interest: it has a highly (but not
macroscopically) degenerate ground state; it is closely related to spin chains
studied by earlier authors; in particular, it is probably the first genuinely
two-dimensional quantum system to exhibit domain-wall-like ``kink'' excitations
normally found only in one-dimensional systems. In some limits it decouples
into non-interacting chains, purely dynamically and not because of weakening of
interchain couplings: indeed, paradoxically, this happens in the limit of
strong coupling of the chains.


title: An Analytical Approach to the Protein Designability Problem

abstract:
  We present an analytical method for determining the designability of protein
structures. We apply our method to the case of two-dimensional lattice
structures, and give a systematic solution for the spectrum of any structure.
Using this spectrum, the designability of a structure can be estimated. We
outline a heirarchy of structures, from most to least designable, and show that
this heirarchy depends on the potential that is used.


title:
On the Selfconsistent Theory of Josephson Effect in Ballistic
  Superconducting Microconstrictions

abstract:
  The microscopic theory of current carrying states in the ballistic
superconducting microchannel is presented. The effects of the contact length L
on the Josephson current are investigated. For the temperatures T close to the
critical temperature T_c the problem is treated selfconsistently, with taking
into account the distribution of the order parameter $\Delta (r)$ inside the
contact. The closed integral equation for $\Delta $ in strongly inhomogeneous
microcontact geometry ($L\lesssim \xi_{0}, \xi_{0}$ is the coherence length at
T=0) replaces the differential Ginzburg-Landau equation. The critical current
$I_{c}(L)$ is expressed in terms of solution of this integral equation. The
limiting cases of $L\ll \xi_{0}$ and $L\gg \xi_{0}$ are considered. With
increasing length L the critical current decreases, although the ballistic
Sharvin resistance of the contact remains the same as at L=0. For ultra short
channels with $L\lesssim a_{D}$ ($a_{D}\sim v_{F}/\omega_{D}, \omega_{D}$ is
the Debye frequency) the corrections to the value of critical current I_c(L=0)
are sensitive to the strong coupling effects.


title:
Quantum waveguides with a lateral semitransparent barrier: spectral and
  scattering properties

abstract:
  We consider a quantum particle in a waveguide which consists of an infinite
straight Dirichlet strip divided by a thin semitransparent barrier on a line
parallel to the walls which is modeled by a $\delta$ potential. We show that if
the coupling strength of the latter is modified locally, i.e. it reaches the
same asymptotic value in both directions along the line, there is always a
bound state below the bottom of the essential spectrum provided the effective
coupling function is attractive in the mean. The eigenvalues and
eigenfunctions, as well as the scattering matrix for energies above the
threshold, are found numerically by the mode-matching technique. In particular,
we discuss the rate at which the ground-state energy emerges from the continuum
and properties of the nodal lines. Finally, we investigate a system with a
modified geometry: an infinite cylindrical surface threaded by a homogeneous
magnetic field parallel to the cylinder axis. The motion on the cylinder is
again constrained by a semitransparent barrier imposed on a ``seam'' parallel
to the axis.

categories: cond-mat.mes-hall math-ph math.MP quant-ph


title: Functional Integration Approach to Hysteresis

abstract:
  A general formulation of scalar hysteresis is proposed. This formulation is
based on two steps. First, a generating function g(x) is associated with an
individual system, and a hysteresis evolution operator is defined by an
appropriate envelope construction applied to g(x), inspired by the overdamped
dynamics of systems evolving in multistable free energy landscapes. Second, the
average hysteresis response of an ensemble of such systems is expressed as a
functional integral over the space G of all admissible generating functions,
under the assumption that an appropriate measure m has been introduced in G.
The consequences of the formulation are analyzed in detail in the case where
the measure m is generated by a continuous, Markovian stochastic process. The
calculation of the hysteresis properties of the ensemble is reduced to the
solution of the level-crossing problem for the stochastic process. In
particular, it is shown that, when the process is translationally invariant
(homogeneous), the ensuing hysteresis properties can be exactly described by
the Preisach model of hysteresis, and the associated Preisach distribution is
expressed in closed analytic form in terms of the drift and diffusion
parameters of the Markovian process. Possible applications of the formulation
are suggested, concerning the interpretation of magnetic hysteresis due to
domain wall motion in quenched-in disorder, and the interpretation of critical
state models of superconducting hysteresis.


title: Spin-Gap Phases in Tomonaga-Luttinger Liquids

abstract:
  We give the details of the analysis for critical properties of spin-gap
phases in one-dimensional lattice electron models. In the Tomonaga-Luttinger
(TL) liquid theory, the spin-gap instability occurs when the backward
scattering changes from repulsive to attractive.
  This transition point is shown to be equivalent to that of the level-crossing
of the singlet and the triplet excitation spectra, using the c=1 conformal
field theory and the renormalization group. Based on this notion, the
transition point between the TL liquid and the spin-gap phases can be
determined with high-accuracy from the numerical data of finite-size clusters.
We also discuss the boundary conditions and discrete symmetries to extract
these excitation spectra. This technique is applied to the extended Hubbard
model, the t-J model, and the t-J-J' model, and their phase diagrams are
obtained. We also discuss the relation between our results and analytical
solutions in weak-coupling and low-density limits.


title:
Photon assisted tunneling in a superconducting SET transistor induced by
  the Josephson oscillations of a SQUID

abstract:
  Josephson oscillations generated by a SQUID were used to measure photon
assisted tunneling in a superconducting single electron tunneling (SET)
transistor. The SQUID was fabricated only a few microns from the SET
transistor. The close proximity of the generator to the SET transistor allowed
for a continuous sweep of the excitation frequency from 10 GHz to 190 GHz. The
amplitude of the Josephson oscillations were tuned by adjusting the flux
through the SQUID loop while the frequency was independently adjusted by
controlling the bias voltage across the SQUID. Current peaks which could be
attributed to photon assisted tunneling in the SET transistor were observed for
bias voltages less than 4*Delta/e.


title: Charge stripes seen with x-rays in La_(1.45)Nd_(0.4)Sr_(0.15)CuO_(4)

abstract:
  Superstructure reflections due to the ordering of holes into stripes in
La_(1.45)Nd_(0.4)Sr_(0.15)CuO_4 have been studied with high energy x-ray
diffraction. These reflections have been observed clearly for the first time in
a sample which is superconducting at low temperatures (T_c = 10 K). The stripe
peaks vanish above 62(5) K whereas the magnetic signal of the stripe ordering
which has been seen with neutrons before is already suppressed at 45 K. Our
results confirm that the ordering of spins and holes is driven by the charges
as it is found in the case of La_(1.6-x)Nd_(0.4)Sr_(x)CuO_(4) at the doping
level of x = 0.12.


title:
Spin-wave contributions to nuclear magnetic relaxation in magnetic
  metals

abstract:
  The longitudinal and transverse nuclear magnetic relaxation rates $1/T_1(T)$
and $1/T_2(T)$ are calculated for three- and two-dimensional (3D and 2D)
metallic ferro- and antiferromagnets (FM and AFM) with localized magnetic
moments in the spin-wave temperature region. The contribution of the one-magnon
decay processes is strongly enhanced in comparison with the standard $T$-linear
Korringa term, especially for the FM case. For the 3D AFM case this
contribution diverges logarithmically, the divergence being cut at the magnon
gap $\omega_0$ due to magnetic anisotropy, and for the 2D AFM case as
$\omega_0^{-1}$. The electron-magnon scattering processes yield $T^2\ln
(T/\omega_0)$ and $T^2/\omega_0^{1/2}$-terms in $1/T_1$ for the 3D AFM and 2D
FM cases, respectively. The two-magnon (``Raman'') contributions are
investigated and demonstrated to be large in the 2D FM case. Peculiarities of
the isotropic 2D limit (where the correlation length is very large) are
analyzed.


title:
Temperature-scaling behavior of the Hall conductivity for Hg-based
  superconducting thin films

abstract:
  The Hall conductivities of HgBa_{2}CaCu_{2}O_{6+\delta}and
HgBa_{2}Ca_{2}Cu_{3}O_{8+\delta} thin films are investigated for a magnetic
field parallel to the c axis. The mixed-state Hall conductivity for these
compounds is well described by \sigma_{xy}=C_{1}/H+C_{2}+C_{3}H. The prefactor
C_1 shows a temperature dependence of the form C_1 = A(1-t)^n near T_c, where
t=T/T_c is the reduced temperature. Contrary to the previous results, C_2 also
follows a temperature-scaling behavior similar to that of the coefficient C_1.
The observed value of n = 1.8 - 2.3 is comparable to the previously observed
values for YBa_{2}Cu_{3}O_{7-\delta} and La_{2-x}Sr_{x}CuO_{4}.


title: q-dependence of low-frequency Raman scattering in silica glass

abstract:
  Accurate measurements of the dependence of low-frequency Raman scattering on
the scattering angle were performed in two silica glasses. By a comparison of
spectra measured at a large scattering angle (close to back scattering) and a
small one (close to forward scattering), we for the first time observed a
q-dependence of the low-frequency light scattering in glasses. From the
magnitude of the effect, the vibration correlation length is estimated and
compared with results from the picosecond optical technique; a reasonable
agreement is found.


title: Modulated Phases in Spin-Peierls Systems

abstract:
  Lattice modulations in the high magnetic field phase and close to impurities
in spin-Peierls systems are considered and compared to experiment. Necessary
extensions of existing theories are proposed. The influence of zero-point
fluctuations on magnetic amplitudes is shown.


title: Exact Finite Size Study of the 2dOCP at Gamma=4 and Gamma=6

abstract:
  An exact numerical study is undertaken into the finite $N$ calculation of the
free energy and distribution functions for the two-dimensional one-component
plasma. Both disk and sphere geometries are considered, with the coupling
$\Gamma$ set equal to 4 and 6. Extrapolation of our data for the free energy is
consistent with the existence of a universal term ${\chi \over 12} \log N$,
where $\chi$ denotes the Euler characteristic of the surface, as predicted
theoretically. The exact finite $N$ density profile is shown to give poor
agreement with the contact theorem relating the density at contact and
potential drop to the pressure in the thermodynamic limit. This is understood
theoretically via a known finite $N$ version of the contact theorem.
Furthermore, the ideas behind the derivation of the latter result are extended
to give a sum rule for the second moment of the pair correlation in the finite
disk, which in the thermodynamic limit converges to the Stillinger-Lovett
result.


title:
Superdiffusion and Out-of-equilibrium Chaotic Dynamics with Many Degrees
  of Freedoms

abstract:
  We study the link between relaxation to the equilibrium and anomalous
superdiffusive motion in a classical N-body hamiltonian system with long-range
interaction showing a second-order phase-transition in the canonical ensemble.
Anomalous diffusion is observed only in a transient out-of-equilibrium regime
and for a small range of energy, below the critical one. Superdiffusion is due
to L\'evy walks of single particles and is checked independently through the
second moment of the distribution, power spectra, trapping and walking time
probabilities. Diffusion becomes normal at equilibrium, after a relaxation time
which diverges with N.

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


title: Quasiparticles in the superconducting state of Bi2Sr2CaCu2O8

abstract:
  Recent improvements in momentum resolution by a factor of 32 lead to
qualitatively new ARPES results on the spectra of Bi2Sr2CaCu2O8 (Bi2212) along
the (pi,pi) direction, where there is a node in the superconducting gap. With
improved resolution, we now see the intrinsic lineshape, which indicates the
presence of true quasiparticles at the Fermi momentum in the superconducting
state, and lack thereof in the normal state. The region of momentum space
probed here is relevant for charge transport, motivating a comparison of our
results to conductivity measurements by infrared reflectivity.


title: Effective potential for the massless KPZ equation

abstract:
  In previous work we have developed a general method for casting a classical
field theory subject to Gaussian noise (that is, a stochastic partial
differential equation--SPDE) into a functional integral formalism that exhibits
many of the properties more commonly associated with quantum field theories
(QFTs). In particular, we demonstrated how to derive the one-loop effective
potential. In this paper we apply the formalism to a specific field theory of
considerable interest, the massless KPZ equation (massless noisy vorticity-free
Burgers equation), and analyze its behaviour in the ultraviolet
(short-distance) regime. When this field theory is subject to white noise we
can calculate the one-loop effective potential and show that it is one-loop
ultraviolet renormalizable in 1, 2, and 3 space dimensions, and fails to be
ultraviolet renormalizable in higher dimensions. We show that the one-loop
effective potential for the massless KPZ equation is closely related to that
for lambda phi^4 QFT. In particular we prove that the massless KPZ equation
exhibits one-loop dynamical symmetry breaking (via an analog of the
Coleman-Weinberg mechanism) in 1 and 2 space dimensions, and that this
behaviour does not persist in 3 space dimensions.

categories: cond-mat.stat-mech chao-dyn hep-th nlin.CD


title:
Statistical mechanics of systems with heterogeneous agents: Minority
  Games

abstract:
  We study analytically a simple game theoretical model of heterogeneous
interacting agents. We show that the stationary state of the system is
described by the ground state of a disordered spin model which is exactly
solvable within the simple replica symmetric ansatz. Such a stationary state
differs from the Nash equilibrium where each agent maximizes her own utility.
The latter turns out to be characterized by a replica symmetry broken
structure. Numerical results fully agree with our analytic findings.


title: Meta-Percolation and Metal-Insulator Transition in Two Dimensions

abstract:
  According to the scaling theory of localization, all quantum electronic
states are localized in two-dimensional (2D) systems. One consequence of the
theory is that there is no quantum percolation transition in 2D. However, in a
real system at a finite temperature, electron phase coherent length is finite
and the system is between quantum and classical. We find, in such a 2D system,
a metal-insulator transition (MIT) caused by a novel type of percolation,
meta-percolation. The relevance to recently observed 2D MIT is also discussed.


title:
Mid-infrared absorption in YBa2Cu3O6: Failure of spin-wave theory in
  undoped cuprates?

abstract:
  The optical conductivity sigma (omega) of undoped YBa2Cu3O6 is studied in
detail in the mid-infrared range. Substitutions on all but the Ba site are used
to identify the prominent absorption processes at 2800 and 3800 cm^-1.
Experimental evidence for bimagnon-plus-phonon absorption is collected. A more
critical analysis of the lineshape and the spectral weight reveals the limits
of this approach. Although phonon-2-magnon multiple scattering seems to
reproduce the lineshape, the necessary coupling is unrealistically large. The
strong increase of high frequency spectral weight with increasing temperature
makes the failure of spin-wave theory even more evident.


title:
Two-Dimensional Electron-Hole Systems in a Strong Magnetic Field:
  Composite Fermion Picture for Multi-Component Plasmas

abstract:
  Electron-hole systems on a Haldane sphere are studied by exact numerical
diagonalization. Low lying states contain one or more types of bound charged
excitonic complexes Xk-, interacting through appropriate pseudopotentials.
Incompressible ground states of such multi-component plasmas are found. A
generalized multi-component Laughlin wavefunction and composite Fermion picture
are shown to predict the low lying states of an electron-hole gas at any value
of the magnetic field.


title: Dynamics of Phase Separation under Shear: A Soluble Model

abstract:
  The dynamics of phase separation for a binary fluid subjected to a uniform
shear are solved exactly for a model in which the order parameter is
generalized to an n-component vector and the large-n limit taken.
Characteristic length scales in directions parallel and perpendicular to the
flow increase as (t^5/\ln t)^{1/4} and (t/\ln t)^{1/4} respectively. The
structure factor in the shear-flow plane exhibits two parallel ridges as
observed in experiment.


title:
Finite temperature phase transition in the two-dimension Randomly
  Coupled Ferromagnet

abstract:
  We show using extensive simulation results and physical arguments that an
Ising system on a two dimensional square lattice, having interactions of random
sign between first neighbors and ferromagnetic interactions between second
neighbors, presents a phase transition at a non-zero temperature.


title: Thermodynamics and excitations of the one-dimensional Hubbard model

abstract:
  We review fundamental issues arising in the exact solution of the
one-dimensional Hubbard model. We perform a careful analysis of the Lieb-Wu
equations, paying particular attention to so-called `string solutions'. Two
kinds of string solutions occur: $\Lambda$ strings, related to spin degrees of
freedom and $k-\Lambda$ strings, describing spinless bound states of electrons.
Whereas $\Lambda$ strings were thoroughly studied in the literature, less is
known about $k-\Lambda$ strings. We carry out a thorough analytical and
numerical analysis of $k-\Lambda$ strings. We further review two different
approaches to the thermodynamics of the Hubbard model, the Yang-Yang approach
and the quantum transfer matrix approach, respectively. The Yang-Yang approach
is based on strings, the quantum transfer matrix approach is not. We compare
the results of both methods and show that they agree. Finally, we obtain the
dispersion curves of all elementary excitations at zero magnetic field for the
less than half-filled band by considering the zero temperature limit of the
Yang-Yang approach.


title:
Excitation of a Dipole Topological Mode in a Strongly Coupled
  Two-Component Bose-Einstein Condensate

abstract:
  Two internal hyperfine states of a Bose-Einstein condensate in a dilute
magnetically trapped gas of ${}^{87}$Rb atoms are strongly coupled by an
external field that drives Rabi oscillations between the internal states. Due
to their different magnetic moments and the force of gravity, the trapping
potentials for the two states are offset along the vertical axis, so that the
dynamics of the internal and external degrees of freedom are inseparable. The
rapid cycling between internal atomic states in the displaced traps results in
an adiabatic transfer of population from the condensate ground state to its
first antisymmetric topological mode. This has a pronounced effect on the
internal Rabi oscillations, modulating the fringe visibility in a manner
reminiscent of collapses and revivals. We present a detailed theoretical
description based on zero-temperature mean-field theory.


title:
Coupled-Cluster Approach to Electron Correlations in the Two-Dimensional
  Hubbard Model

abstract:
  We have studied electron correlations in the doped two-dimensional (2D)
Hubbard model by using the coupled-cluster method (CCM) to investigate whether
or not the method can be applied to correct the independent particle
approximations actually used in ab-initio band calculations. The double
excitation version of the CCM, implemented using the approximate coupled pair
(ACP) method, account for most of the correlation energies of the 2D Hubbard
model in the weak ($U/t \simeq 1$) and the intermediate $U/t$ regions ($U/t
\simeq 4$). The error is always less than 1% there. The ACP approximation gets
less accurate for large $U/t$ ($U/t \simeq 8$) and/or near half-filling.
Further incorporation of electron correlation effects is necessary in this
region. The accuracy does not depend on the system size and the gap between the
lowest unoccupied level and the highest occupied level due to the finite size
effect. Hence, the CCM may be favorably applied to ab-initio band calculations
on metals as well as semiconductors and insulators.


title: Second harmonics and compensation effect in ceramic superconductors

abstract:
  A three-dimensional lattice of the Josephson junctions with a finite
self-conductance is employed to model the ceramic superconductors. The
nonlinear ac susceptibility and the compensation effect are studied by Monte
Carlo simulations in this model. The compensation effect is shown to be due to
the existence of the chiral glass phase. We demonstrate, in agreement with
experiments, that this effect may be present in the ceramic superconductors
which show the paramagnetic Meissner effect.


title: Raman Scattering Study of Ba-doped C60 with t1g States

abstract:
  Raman spectra are reported for Ba doped fullerides, BaxC60(x=3,4,and 6). The
lowest frequency Hg modes split into five components for Ba4C60 and Ba6C60 even
at room temperature, allowing us a quantitative analysis based on the
electron-phonon couping theory. For the superconducting Ba4C60, the density of
states at the Fermi energy was derived as 7 eV-1, while the total value of
electron-phonon coupling \lambda was found to be 1.0, which is comparable to
that of K3C60. The tangential Ag(2) mode, which is known as a sensitive probe
for the degree of charge transfer on C60 molecule, shows a remarkable shift
depending on the Ba concentration, being roughly consistent with the full
charge transfer from Ba to C60. An effect of hybridization between Ba and C60
\pi orbitals is also discussed.


title: Anomalous Raman shift in the ternary fullerides with t1g states

abstract:
  Raman shift dependence of charge transfer is studied in the two fulleride
families of AxBa3C60 (x=0,3; A=K,Rb) and KxSm2.75C60 (x=0,3.25). It is found
that the Raman spectra are essentially identical to each other for all ternary
fullerides, The results show a crossover point at the boundary of t1u and t1g
bands for the Raman shift dependence of charge transfer. For the t1u
fullerides, the Ag(2) pinch mode follows the characteristic shift of ~6.3cm-1
per elementary charge widely observed in KxC60. The two Ag modes in t1g
fullerides show a complicated behavior. An anomalous Raman shift is observed in
the t1g fullerides. This anomalous Raman shift of the two Ag modes could not be
explained only by charge transfer.


title: Structure and properties of a novel fulleride Sm6C60

abstract:
  A novel fulleride Sm6C60 has been synthesized using high temperature solid
state reaction. The Rietveld refinement on high resolution synchrotron X-ray
powder diffraction data shows that Sm6C60 is isostructural with body-centered
cubic A6C60 (A=K, Ba). Raman spectrum of Sm6C60 is similar to that of Ba6C60,
and the frequencies of two Ag modes in Sm6C60 are nearly the same as that of
Ba6C60, suggesting that Sm is divalent and hybridization between C60 molecules
and the Sm atom could exist in Sm6C60. Resistivity measurement shows a weak
T-linear behavior above 180 K, the transport at low temperature is mainly
dominated by granular-metal theory.


title:
Model Calculation of Electron-Phonon Couplings in a Dimer with a
  Non-Degenerate Orbital

abstract:
  We evaluate all the electron-phonon couplings derived from the one-body
electronic interactions, in both the adiabatic and extreme non-adiabatic limit,
for a dimer with a non-degenerate orbital built from atomic wave functions of
Gaussian shape. We find largely different values of the coupling parameters in
the two cases, as well as different expressions of the corresponding terms in
the Hamiltonian.


title:
Exact free energy distribution function of a randomly forced directed
  polymer

abstract:
  We study the elastic (1+1)-dimensional string subject to a random gaussian
potential on scales smaller than the correlation radius of the disorder
potential (Larkin problem). We present an exact calculation of the probability
function ${\cal P} [F(u,L)]$ for the free energy $F$ of a string starting at
$(0,0)$ and ending at $(u,L)$. The function ${\cal P}(F)$ is strongly
asymmetric, with the left tail decaying exponentially ($\ln {\cal
P}(F\to-\infty)\propto F$) and the right tail vanishing as $\ln {\cal P}(F\to
+\infty)\propto -F^{3}$. Our analysis defines a strategy for future attacks on
this class of problems.


title: Anisotropic impurities in anisotropic superconductors

abstract:
  Physical properties of anisotropic superconductors like the critical
temperature and others depend sensitively on the electron mean free path. The
sensitivity to impurity scattering and the resulting anomalies are considered a
characteristic feature of strongly anisotropic pairing. These anomalies are
usually analyzed in terms of s-wave impurity scattering which leads to
universal pair breaking effects depending on only two scattering parameters,
the mean free path and the impurity cross section. We investigate here
corrections coming from anisotropies in the scattering cross section, and find
not only quantitative but also qualitative deviations from universal s-wave
isotropic pairbreaking. The properties we study are the transition temperature,
the density of states, quasiparticle bound states at impurities, and pinning of
flux lines by impurities.


title: High precision Monte Carlo study of the 3D XY-universality class

abstract:
  We present a Monte Carlo study of the two-component $\phi^4$ model on the
simple cubic lattice in three dimensions. By suitable tuning of the coupling
constant $\lambda$ we eliminate leading order corrections to scaling. High
statistics simulations using finite size scaling techniques yield
$\nu=0.6723(3)[8]$ and $\eta=0.0381(2)[2]$, where the statistical and
systematical errors are given in the first and second bracket, respectively.
These results are more precise than any previous theoretical estimate of the
critical exponents for the 3D XY universality class.


title: Collective oscillations of a classical gas confined in harmonic traps

abstract:
  Starting from the Boltzmann equation we calculate the frequency and the
damping of the monopole and quadrupole oscillations of a classical gas confined
in an harmonic potential. The collisional term is treated in the relaxation
time approximation and a gaussian ansatz is used for its evaluation. Our
approach provides an explicit description of the transition between the
hydrodynamic and collisionless regimes in both spherical and deformed traps.
The predictions are compared with the results of a numerical simulation.


title: Memory interference effects in spin glasses

abstract:
  When a spin glass is cooled down, a memory of the cooling process is
imprinted in the spin structure. This memory can be disclosed in a continuous
heating measurement of the ac-susceptibility. E.g., if a continuous cooling
process is intermittently halted during a certain aging time at one or two
intermediate temperatures, the trace of the previous stop(s) is recovered when
the sample is continuously re-heated [1]. However, heating the sample above the
aging temperature, but keeping it below Tg, erases the memory of the thermal
history at lower temperatures. We also show that a memory imprinted at a higher
temperature can be erased by waiting a long enough time at a lower temperature.
Predictions from two complementary spin glass descriptions, a hierarchical
phase space model and a real space droplet picture are contested with these
memory phenomena and interference effects.
  [1] K. Jonason, E. Vincent, J. Hammann, J. P. Bouchaud and P. Nordblad, Phys.
Rev. Lett. 31, 3243 (1998).


title: Electrical and Mechanical Properties of Twisted Carbon Nanotubes

abstract:
  We have evaluated the energies required to twist carbon nanotubes (NTs), and
investigated the effects of these distortions on their electronic structure and
electrical properties. The computed distortion energies are high, indicating
that it is unlikely that extensive twisting is the result of thermal
excitation. Twisting strongly affects the electronic structure of NTs. Normally
metallic armchair $(n,n)$ NTs develop a band-gap which initially scales
linearly with twisting angle and then reaches a constant value. This saturation
is associated with a structural transition to a flattened helical structure.
The values of the twisting energy and of the band-gap are strongly affected by
allowing structural relaxation in the twisted structures. Finally, we have used
the Landauer-B{\"u}ttiker formalism to calculate the electrical transport of
the metal-NT-metal system as a function of the NT distortion.


title:
Enhanced structural correlations accelerate diffusion in
  charge-stabilized colloidal suspensions

abstract:
  Theoretical calculations for colloidal charge-stabilized and hard sphere
suspensions show that hydrodynamic interactions yield a qualitatively different
particle concentration dependence of the short-time self-diffusion coefficient.
The effect, however, is numerically small and hardly accessible by conventional
light scattering experiments. Applying multiple-scattering decorrelation
equipment and a careful data analysis we show that the theoretical prediction
for charged particles is in agreement with our experimental results from
aqueous polystyrene latex suspensions.


title:
Small-scale properties of the KPZ equation and dynamical symmetry
  breaking

abstract:
  A functional integral technique is used to study the ultraviolet or short
distance properties of the Kardar-Parisi-Zhang (KPZ) equation with white
Gaussian noise. We apply this technique to calculate the one-loop effective
potential for the KPZ equation. The effective potential is (at least) one-loop
ultraviolet renormalizable in 1, 2, and 3 space dimensions, but
non-renormalizable in 4 or higher space dimensions. This potential is
intimately related to the probability distribution function (PDF) for the
spacetime averaged field. For the restricted class of field configurations
considered here, the KPZ equation exhibits dynamical symmetry breaking (DSB)
via an analog of the Coleman-Weinberg mechanism in 1 and 2 space dimensions,
but not in 3 space dimensions.


title:
Gauge Invariance and the Critical Properties of Quantum Hall Plateaux
  Transitions

abstract:
  A model consisting of a single massless scalar field with a topological
coupling to a pure gauge field is defined and studied. It possesses an SL(2,Z)
symmetry as a consequence of the gauge invariance. We propose that by adding
impurities the model can be used to describe transitions between Quantum Hall
plateaux. This leads to a correlation length exponent of 20/9, in excellent
agreement with the most recent experimental measurements.


title: Sliding Phases in XY-Models, Crystals, and Cationic Lipid-DNA Complexes

abstract:
  We predict the existence of a totally new class of phases in weakly coupled,
three-dimensional stacks of two-dimensional (2D) XY-models. These ``sliding
phases'' behave essentially like decoupled, independent 2D XY-models with
precisely zero free energy cost associated with rotating spins in one layer
relative to those in neighboring layers. As a result, the two-point spin
correlation function decays algebraically with in-plane separation. Our
results, which contradict past studies because we include higher-gradient
couplings between layers, also apply to crystals and may explain recently
observed behavior in cationic lipid-DNA complexes.


title: Pseudogap Formation in Models for Manganites

abstract:
  The density-of-states (DOS) and one-particle spectral function $\rm A({\bf
k}, \omega)$ of the one- and two-orbital models for manganites, the latter with
Jahn-Teller phonons, are evaluated using Monte Carlo techniques. Unexpectedly
robust pseudogap (PG) features were found at low- and
intermediate-temperatures, particularly at or near regimes where
phase-separation occurs as $\rm T$$\to$0. The PG follows the chemical potential
and it is caused by the formation of ferromagnetic metallic clusters in an
insulating background. It is argued that PG formation should be generic of
mixed-phase regimes. The results are in good agreement with recent
photoemission experiments for $\rm La_{1.2} Sr_{1.8} Mn_2 O_7$.