title: Electron--electron Interactions in Disordered Metals: Keldysh Formalism

abstract:
  We develop a field theory formalism for the disordered interacting electron
liquid in the dynamical Keldysh formulation. This formalism is an alternative
to the previously used replica technique. In addition it naturally allows for
the treatment of non-equilibrium effects. Employing the gauge invariance of the
theory and carefully choosing the saddle point in the $Q$-matrix manifold, we
separate purely phase effects of the fluctuating potential from the ones that
change quasi-particle dynamics. As a result, the cancellation of
super-divergent diagrams (double logarithms in d=2) is automatically build in
the formalism. As a byproduct we derive a non--perturbative expression for the
single particle density of states. The remaining low-energy sigma--model
describes the quantum fluctuations of the electron distribution function. Its
saddle point equation appears to be the quantum kinetic equation with an
appropriate collision integral along with collisionless terms. Altshuler-Aronov
corrections to conductivity are shown to arise from the one-loop quantum
fluctuation effects.


title:
Coupling of intrinsic Josephson oscillations in layered superconductors
  by charge fluctuations

abstract:
  The coupling of Josephson oscillations in layered superconductors is studied
with help of a tunneling Hamiltonian formalism. The general form of the current
density across the barriers between the superconducting layers is derived. The
induced charge fluctuations on the superconducting layers lead to a coupling of
the Josephson oscillations in different junctions. A simplified set of
equations is then used to study the non-linear dynamics of the system. In
particular the influence of the coupling on the current-voltage characteristics
is investigated and upper limits for the coupling strength are estimated from a
comparison with experiments on cuprate superconductors.


title: Kinetic growth of field-oriented chains in dipolar colloidal solutions

abstract:
  Experimental studies on the irreversible growth of field-induced chains of
dipolar particles suggest an asymptotic power-law behavior of several relevant
quantities. We introduce a Monte Carlo model of chain growth that explicitly
incorporates the anisotropic diffusion characteristic of a rod-like object.
Assuming a simple power-law form for the mean cluster size, $S(t) \sim t^z$,
the results of our model are in good agreement with the experimental
measurements of the dynamic exponent $z$. Nevertheless, an alternative
scenario, including logarithmic corrections to the standard power-law behavior,
provides a better and more insightful interpretation of the anomalous dynamic
exponent. In contrast to some experimental findings, we do not observe any
dependence of the exponents on the volume fraction of particles $\phi$.
Finite-size effects are also explored by simulating very long time evolutions
or highly concentrated systems. Two different behaviors are found, namely,
saturation and a crossover to a quasi one-dimensional regime.


title: Electron-energy-loss spectroscopy of the C60 molecule

abstract:
  In this work we present a theoretical study of EELS (electron-energy-loss
spectroscopy) experiments on the ${\rm C}_{60}$ molecule. Our treatment of the
problem is based on the simple two-fluid model originally proposed for the
description of plasma oscillations in graphite and fullerenes (see
Refs.\cite{14,15,16}). It is shown that in spite of the simplicity of the model
the calculated intensities of the EELS peaks are in good agreement with
experimental data which may indicate that the model can be used as a simple and
effective tool for the investigation of the collective behaviour of electrons
in fullerene systems.


title: Spin effects in single-electron tunneling in magnetic junctions

abstract:
  Spin dependent single electron tunneling in ferromagnetic double junctions is
analysed theoretically in the limit of sequential tunneling. The influence of
discrete energy spectrum of the central electrode (island)on the spin
accumulation, spin fluctuations and tunnel magnetoresistance is analysed
numerically in the case of a nonmagnetic island. It is shown that spin
fluctuations are significant in magnetic as well as in nonmagnetic junctions.


title: From the adiabatic piston to macroscopic motion induced by fluctuations

abstract:
  The controversial problem of an isolated system with an internal adiabatic
wall is investigated with the use of a simple microscopic model and the
Boltzmann equation. In the case of two infinite volume one-dimensional ideal
fluids separated by a piston whose mass is equal to the mass of the fluid
particles we obtain a rigorous explicit stationary non-equilibrium solution of
the Boltzmann equation. It is shown that at equal pressures on both sides of
the piston, the temperature difference induces a non-zero average velocity,
oriented toward the region of higher temperature. It thus turns out that
despite the absence of macroscopic forces the asymmetry of fluctuations results
in a systematic macroscopic motion. This remarkable effect is analogous to the
dynamics of stochastic ratchets, where fluctuations conspire with spatial
anisotropy to generate direct motion. However, a different mechanism is
involved here. The relevance of the discovered motion to the adiabatic piston
problem is discussed.


title:
Bose-Einstein condensation in a one-dimensional interacting system due
  to power-law trapping potentials

abstract:
  We examine the possibility of Bose-Einstein condensation in one-dimensional
interacting Bose gas subjected to confining potentials of the form $V_{\rm
ext}(x)=V_0(|x|/a)^\gamma$, in which $\gamma < 2$, by solving the
Gross-Pitaevskii equation within the semi-classical two-fluid model. The
condensate fraction, chemical potential, ground state energy, and specific heat
of the system are calculated for various values of interaction strengths. Our
results show that a significant fraction of the particles is in the lowest
energy state for finite number of particles at low temperature indicating a
phase transition for weakly interacting systems.


title: Dynamical regimes in DPD

abstract:
  We discuss theoretically the behavior of the velocity autocorrelation
function in the Dissipative Particle Dynamics model. Two dynamical regimes are
identified depending on the dimensionless model parameters. For low frictions a
mean field behavior is observed in which the kinetic theory for DPD provides
good predictions. For high frictions, collective hydrodynamic effects are
dominant. We have performed numerical simulations that validate the theory
presented.


title: Disorder and chain superconductivity in YBa_2Cu_3O_{7-\delta}

abstract:
  The effects of chain disorder on superconductivity in YBa_2Cu_3O_{7-\delta}
are discussed within the context of a proximity model. Chain disorder causes
both pair-breaking and localization. The hybridization of chain and plane
wavefunctions reduces the importance of localization, so that the transport
anisotropy remains large in the presence of a finite fraction $\delta$ of
oxygen vacancies. Penetration depth and specific heat measurements probe the
pair-breaking effects of chain disorder, and are discussed in detail at the
level of the self-consistent T-matrix approximation. Quantitative agreement
with these experiments is found when chain disorder is present.


title: Disorder and Localization in the Lowest Landau Level

abstract:
  We study the localization property of a two-dimensional noninteracting
electron gas in the presence of randomly distributed short-range scatterers. We
evaluate the participation number of the eigenstates obtained by exact
diagonalization technique. At low impurity concentrations we obtain
self-averaged values showing that all states, except those exactly at the
Landau level, are localized with finite localization length. We conclude that
there is no universal localization exponent and at least at low impurity
concentrations localization length does not diverge.


title: Roughening transition in a model for dimer adsorption and desorption

abstract:
  A solid-on-solid growth model for dimer adsorption and desorption is
introduced and studied numerically. The special property of the model is that
dimers can only desorb at the edges of terraces. It is shown that the model
exhibits a roughening transition from a smooth to a rough phase. In both phases
the interface remains pinned to the bottom layer and does not propagate. Close
to the transition certain critical properties are related to those of a
unidirectionally coupled hierarchy of parity-conserving branching-annihilating
random walks.


title:
Suppression of superconductivity in high-$T_c$ cuprates due to
  nonmagnetic impurities: Implications for the order parameter symmetry

abstract:
  We studied the effects of nonmagnetic impurities on high-temperature
superconductors by solving the Bogoliubov-de Gennes equations on a
two-dimensional lattice via exact diagonalization technique in a fully
self-consistent way. We found that s-wave order parameter is almost unaffected
by impurities at low concentrations while $d_{x^2-y^2}$-wave order parameter
exhibits a strong linear decrease with impurity concentration. We evaluated the
critical impurity concentration $n_i^c$ at which superconductivity ceases to be
0.1 which is in good agreement with experimental values. We also investigated
how the orthorhombic nature of the crystal structure affects the suppression of
superconductivity and found that anisotropy induces an additional s-wave
component. Our results support $d_{x^2-y^2}$-wave symmetry for tetragonal and
$s+d_{x^2-y^2}$-wave symmetry for orthorhombic structure.


title:
Simultaneous Observation of Fishtail Effect and Peak Effect in
  2H-Nbse$_2$

abstract:
  We report on the discovery of the simultaneous observation of Fishtail Effect
(FE) and Peak Effect (PE) via the study of angular dependence of dc
magnetization hysteresis loops in a clean crystal of 2H-NbSe$_2$. These result
clarify and establish the occurrence of the reentrant characteristics in order
to disorder transformation in an isothermal scan close to zero field
superconducting transition temperature of 2H-NbSe$_2$. The ubiquitous FE arises
from coalescence of two anomalous variations in current density J$_c$, one of
which is related to the collapse of the rigidity FLL close to H$_{c2}$ and
another one located at very low fields presumably corresponds to the pinning
induced order to disorder transformation. When the two effects get well
separated, an ordered state of FLL exists in the intermediate field region.


title:
Evidence of a first order like jump in equilibrium magnetization across
  the peak effect region in superconducting 2H-NbSe$_2$

abstract:
  We report magnetization hysteresis measurements in the peak effect region of
a very clean single crystal of superconducting 2H-NbSe$_2$ at temperatures very
close to T$_c$(0). Through measurement of minor magnetization curves, we infer
the equilibrium magnetization curve across the PE region. We observe a small
first order like change in equilibrium magnetization across the peak effect. We
relate this observation to the entropy change associated with the
order-disorder transformation. We further note that the first order like change
in the equilibrium magnetization observed in 2H-NbSe$_2$ is comparable to the
change(s)observed in the equilibrium magnetization at the FLL melting
transition in various high temperature superconductors.


title:
Spontaneous flux in a d-wave superconductor with
  time-reversal-symmetry-broken pairing state at {110} boundaries

abstract:
  The induction of an s-wave component in a d-wave superconductor is
considered. Near the {110}-oriented edges of such a sample, the induced s-wave
order parameter together with d-wave component forms a complex combination
d+e^{i\phi} s, which breaks the time reversal symmetry (BTRS) of the pairing
state. As a result, the spontaneous current is created. We numerically study
the current distribution and the formation of the spontaneous flux induced by
the current. We show that the spontaneous flux formed from a number of defect
lines with {110} orientation has a measurable strength. This result may provide
a unambiguous way to check the existence of BTRS pairing state at
{110}-oriented boundaries.


title:
Time-dependent Ginzburg-Landau equations for mixed d- and s-wave
  superconductors

abstract:
  A set of coupled time-dependent Ginzburg-Landau equations (TDGL) for
superconductors of mixed d- and s-wave symmetry are derived microscopically
from the Gor'kov equations by using the analytical continuation technique. The
scattering effects due to impurities with both nonmagnetic and magnetic
interactions are considered. We find that the d- and s-wave components of the
order parameter can have very different relaxation times in the presence of
nonmagnetic impurities. This result is contrary to a set of phenomenologically
proposed TDGL equations and thus may lead to new physics in the dynamics of
flux motion.


title: Magnetotransport in the doped Mott insulator

abstract:
  We investigate the Hall effect and the magnetoresistance of strongly
correlated electron systems using the dynamical mean-field theory. We treat the
low- and high-temperature limits analytically and explore some aspects of the
intermediate-temperature regime numerically. We observe that a
bipartite-lattice condition is responsible for the high-temperature result
$\sigma_{xy}\sim 1/T^2$ obtained by various authors, whereas the general
behavior is $\sigma_{xy}\sim 1/T$, as for the longitudinal conductivity. We
find that Kohler's rule is neither obeyed at high nor at intermediate
temperatures.


title:
Renormalized vs unrenormalized perturbation-theoretical approaches to
  the Mott transition

abstract:
  I employ an exactly soluble toy model to investigate why unrenormalized
perturbation theory works better than fully self-consistent approaches in
describing the correlation-driven metal-insulator transition.


title: Is the Glass Transition a Thermodynamic Phase Transition?

abstract:
  This paper has been withdrawn by the authors. It has been superceded by an
improved investigation: ``Thermodynamic Generalized Compressibility in a Glass
Forming Liquid,'' H. M. Carruzzo and C. C. Yu, cond-mat/0004320.


title: Temperature-dependent density profiles of trapped boson-fermion mixtures

abstract:
  We present a semiclassical three-fluid model for a Bose-condensed mixture of
interacting Bose and Fermi gases confined in harmonic traps at finite
temperature. The model is used to characterize the experimentally relevant
behaviour of the equilibrium density profile of the fermions with varying
composition and temperature across the onset of degeneracy, for coupling
strengths relevant to a mixture of
  $^{39}$K and $^{40}$K atoms.


title: Many-body spectral statistics of interacting Fermions

abstract:
  We have studied the appearance of chaos in the many-body spectrum of
interacting Fermions. The coupling of a single state to the Fermi sea is
considered. This state is coupled to a hierarchy of states corresponding to one
or several particle-hole excitations. We have considered various couplings
between two successive generations of this hierarchy and determined under which
conditions this coupling can lead to Wigner-Dyson correlations. We have found
that the cross-over from a Poisson to a Wigner distribution is characterized
not only by the ratio $V/\Delta_c$, but also by the ratio $\Delta_c/\delta$.
$V$ is the typical interaction matrix element, $\delta$ is the energy distance
between {\it many-body} states and $\Delta_c$ is the distance between many-body
states coupled by the interaction.


title: Glassy nature of stripe ordering in La(1.6-x)Nd(0.4)Sr(x)CuO(4)

abstract:
  We present the results of neutron-scattering studies on various aspects of
crystalline and magnetic structure in single crystals of
La(1.6-x)Nd(0.4)Sr(x)CuO(4) with x=0.12 and 0.15. In particular, we have
reexamined the degree of stripe order in an x=0.12 sample. Measurements of the
width for an elastic magnetic peak show that it saturates at a finite value
below 30 K, corresponding to a spin-spin correlation length of 200 A. A model
calculation indicates that the differing widths of magnetic and (previously
reported) charge-order peaks, together with the lack of commensurability, can
be consistently explained by disorder in the stripe spacing. Above 30 K, the
width of the nominally elastic signal begins to increase. Interpreting the
signal as critical scattering from slowly fluctuating spins, the temperature
dependence of the width is consistent with renormalized classical behavior of a
2-dimensional anisotropic Heisenberg antiferromagnet. Inelastic scattering
measurements show that incommensurate spin excitations survive at and above 50
K, where the elastic signal is neglible. We also report several results related
to the LTO-to-LTT transition.


title: The one-component plasma: a conceptual approach

abstract:
  The one-component plasma (OCP) represents the simplest statistical mechanical
model of a Coulomb system. For this reason, it has been extensively studied
over the last forty years. The advent of the integral equations has resulted in
a dramatic improvement in our ability to carry out numerical calculations, but
came at the expense of a physical insight gained in a simpler analytic theory.
In this paper we present an extension of the Debye-Hueckel (DH) theory to the
OCP. The theory allows for analytic calculations of all the thermodynamic
functions, as well as the structure factor. The theory explicitly satisfies the
Stillinger-Lovett and, for small couplings, the compressibility sum rules,
implying its internal self consistency.


title:
Comment on "Predominantly Superconducting Origin ...", by N. Miyakawa,
  et al

abstract:
  Recent tunneling studies in Bi2Sr2CaCu2O8 suggest that the pseudogap is
caused by superconducting fluctuations. By reanalyzing this data, we show that
instead there is a second non-superconducting component of the gap which
increases strongly with underdoping.


title:
Approximate ab initio calculation of vibrational properties of
  hydrogenated amorphous silicon with inner voids

abstract:
  We have performed an approximate ab initio calculation of vibrational
properties of hydrogenated amorphous silicon (a-Si:H) using a molecular
dynamics method. A 216 atom model for pure amorphous silicon (a-Si) has been
employed as a starting point for our a-Si:H models with voids that were made by
removing a cluster of silicon atoms out of the bulk and terminating the
resulting dangling bonds with hydrogens.
  Our calculation shows that the presence of voids leads to localized low
energy (30-50 cm^{-1}) states in the vibrational spectrum of the system. The
nature and localization properties of these states are analyzed by various
visualization techniques.


title: Excitation spectrum of the homogeneous spin liquid

abstract:
  We discuss the excitation spectrum of a disordered, isotropic and
translationally invariant spin state in the 2D Heisenberg antiferromagnet. The
starting point is the nearest-neighbor RVB state which plays the role of the
vacuum of the theory, in a similar sense as the Neel state is the vacuum for
antiferromagnetic spin wave theory. We discuss the elementary excitations of
this state and show that these are not Fermionic spin-1/2 `spinons' but spin-1
excited dimers which must be modeled by bond Bosons. We derive an effective
Hamiltonian describing the excited dimers which is formally analogous to spin
wave theory. Condensation of the bond-Bosons at zero temperature into the state
with momentum (pi,pi) is shown to be equivalent to antiferromagnetic ordering.
The latter is a key ingredient for a microscopic interpretation of Zhang's
SO(5) theory of cuprate superconductivity


title:
Semicircle: An exact relation in the Integer and Fractional Quantum Hall
  Effect

abstract:
  We present experimental results on the quantized Hall insulator in two
dimensions. This insulator, with vanishing conductivities, is characterized by
the quantization (within experimental accuracy) of the Hall resistance in units
of the quantum unit of resistance, h/e^2. The measurements were performed in a
two dimensional hole system, confined in a Ge/SiGe quantum well, when the
magnetic field is increased above the nu=1 quantum Hall state. This
quantization leads to a nearly perfect semi-circle relation for the diagonal
and Hall conductivities. Similar results are obtained with a higher mobility
n-type modulation doped GaAs/AlGaAs sample, when the magnetic field is
increased above the nu=1/3 fractional quantum Hall state.


title: Dislocations and Bragg glasses in two dimensions

abstract:
  We discuss the question of the generation of topological defects
(dislocations) by quenched disorder in two dimensional periodic systems. In a
previous study [Phys. Rev. B {\bf 52} 1242 (1995)] we found that, contrarily to
$d=3$, unpaired dislocations appear in $d=2$ above a length scale $\xi_D$,
which we estimated. We extend this description to include effects of freezing
and pinning of dislocations at low temperature. The resulting $\xi_D$ at low
temperature is found to be {\it larger} than our previous estimate, which is
recovered above a characteristic temperature. The dependence of $\xi_D$ in the
bare core energy of dislocations is a stretched exponential. We stress that for
all temperatures below melting $\xi_D$ becomes arbitrarily large at weak
disorder compared to the translational order length $R_a \gg a$. Thus there is
a wide region of length scales, temperature and disorder where Bragg glass like
behavior should be observable.


title: Coupled Ladders in a Magnetic Field

abstract:
  We investigate the phase transitions in two-leg ladders systems in the
incommensurate phase, for which the gap is destroyed by a magnetic field
($h_{c1}< h$) and the ladder is not yet totally saturated ($h < h_{c2}$). We
compute quantitatively the correlation functions as a function of the magnetic
field for an isolated strong coupling ladder $J_\perp \gg J_\parallel$ and use
it to study the phase transition occuring in a three dimensional array of
antiferromagnetically coupled ladders. The three dimensional ordering is in the
universality class of Bose condensation of hard core bosons. We compute the
critical temperature $T_c(h)$ as well as various physical quantities such as
the NMR relaxations rate. $T_c$ has an unusual camel-like shape with a local
minimum at $h=(h_{c1}+h_{c2})/2$ and behaves as $T_c \sim (h-h_{c1})^{2/3}$ for
$h\sim h_{c1}$. We discuss the experimental consequences for compounds such as
Cu_2(C_5H_{12}N_2)_2Cl_4


title: Stochastic Energetics of Non-uniform Temperature Systems

abstract:
  We propose an energetic interpretation ofstochastic processes described by
Langevin equations with non-uniform temperature. In order to avoid
It\^{o}-Stratonovich dilemma, we start with a Kramers equation, and derive a
Fokker-Plank equation by the renormalization group method. We give a proper
definition of heat for the system. Based on our formulations,we analyze two
examples, the Thomson effect and a Brownian motor which realizes the Carnot
efficiency.


title:
Quantum phase transitions in superconducting arrays under external
  magnetic fields

abstract:
  We study the zero-temperature phase transitions of two-dimensional
superconducting arrays with both the self- and the junction capacitances in the
presence of external magnetic fields. We consider two kinds of excitations from
the Mott insulating phase: charge-dipole excitations and single-charge
excitations, and apply the second-order perturbation theory to find their
energies. The resulting phase boundaries are found to depend strongly on the
magnetic frustration, which measures the commensurate-incommensurate effects.
Comparison of the obtained values with those in recent experiment suggests the
possibility that the superconductor-insulator transition observed in experiment
may not be of the Berezinskii-Kosterlitz-Thouless type. The system is also
transformed to a classical three-dimensional XY model with the magnetic field
in the time-direction; this allows the analogy to bulk superconductors,
revealing the nature of the phase transitions.


title: Mott-Hubbard transition in infinite dimensions

abstract:
  We calculate the zero-temperature gap and quasiparticle weight of the
half-filled Hubbard model with a random dispersion relation. After
extrapolation to the thermodynamic limit, we obtain reliable bounds on these
quantities for the Hubbard model in infinite dimensions. Our data indicate that
the Mott-Hubbard transition is continuous, i.e., that the quasiparticle weight
becomes zero at the same critical interaction strength at which the gap opens.


title:
Critical level statistics of a quantum Hall system with Dirichlet
  boundary conditions

abstract:
  We investigate numerically the influence of Dirichlet boundary conditions on
the nearest neighbor level spacing distribution $P(s)$ of a two-dimensional
disordered tight-binding model in the presence of a strong perpendicular
magnetic field. From the calculation of the second moment of $P(s)$ it is shown
that for Dirichlet boundary conditions, due to the presence of edge states, the
position of the critical energy shifts with increasing system size to the
location of the critical energy for periodic boundary conditions. An
extrapolation to infinite system size results in different critical (scale
independent) $P(s)$ distributions for periodic and Dirichlet boundary
conditions.


title:
Quantum Hall Effect induced by electron-electron interaction in
  disordered GaAs layers with 3D spectrum

abstract:
  It is shown that the observed Quantum Hall Effect in epitaxial layers of
heavily doped n-type GaAs with thickness (50-140 nm) larger the mean free path
of the conduction electrons (15-30 nm) and, therefore, with a three-dimensional
single-particle spectrum is induced by the electron-electron interaction. The
Hall resistance R_xy of the thinnest sample reveals a wide plateau at small
activation energy E_a=0.4 K found in the temperature dependence of the
transverse resistance R_xx. The different minima in the transverse conductance
G_xx of the different samples show a universal temperature dependence
(logarithmic in a large range of rescaled temperatures T/T_0) which is
reminiscent of electron-electron-interaction effects in coherent diffusive
transport.


title: Some spectral properties of the one-dimensional Dirac equation

abstract:
  We study spectral properties of a one-dimensional Dirac equation with various
disorder. We use replicas to calculate the exact density of state and typical
localization length of a Dirac particle in several cases. We show that they can
be calculated in any type of disorder obeying a Gaussian white noise
distribution. In particular, we study the random electric potential model, as
well as a mixed disorder case. We also clarify the supersymmetric alternative
derivation, even though it proves less efficient than the replica treatment for
such thermodynamic quantities. We show that the smallest dynamical algebra in
the Hamiltonian formalism is $u(1,1)$, preferably to $u(n,n)$ in the replica
derivation or $u(1,1|2)$ in the supersymmetric alternative. Finally, we discuss
symmetries in the disorder fields and show that there exists a non trivial
mapping between the electric potential disorder and the magnetic (or mass)
disorder.


title: Quantum critical behavior in the Kondo problem with a pseudogap

abstract:
  The variant of the single-impurity Kondo problem in which the conduction-band
density of states has a power-law pseudogap at the Fermi energy is known to
exhibit a zero-temperature phase transition at a finite exchange coupling. The
critical properties of this transition are studied both for N=2 and for N>>1,
where N is the spin degeneracy. The critical exponents are consistent with a
simple scaling form for the free energy. For any finite N, the temperature
exponent of the local spin susceptibility at the critical Kondo coupling varies
continuously with the power of the pseudogap. This raises the possibility that
a single-particle pseudogap is responsible for the anomalous behavior of
certain heavy-fermion metals close to a magnetic quantum phase transition.


title: Reply to "Comment on `Non-universal exponents in interface growth'"

abstract:
  We reply to the recent comment cond-mat/9809184 on our original paper
`Non-universal exponents in interface growth', Phys. Rev. Lett, 79, 2261
(1997).


title:
Dissipation effects on the superconductor-insulator transition in 2-D
  superconductors

abstract:
  Results on the superconductor to insulator transition in two-dimensional
films are analyzed in terms of coupling of the system to a dissipative bath.
Upon lowering the temperature the parameter that controls this coupling becomes
relevant and a wide range of metallic phase is recovered.


title:
Ballistic Electron Emission Microscopy on CoSi${}_2$/Si(111) interfaces:
  band structure induced atomic-scale resolution and role of localized surface
  states

abstract:
  Applying a Keldysh Green`s function method it is shown that hot electrons
injected from a STM-tip into a CoSi${}_2$/Si(111) system form a highly focused
beam due to the silicide band structure. This explains the atomic resolution
obtained in recent Ballistic Electron Emission Microscopy (BEEM) experiments.
Localized surface states in the $(2 \times 1)$-reconstruction are found to be
responsible for the also reported anticorrugation of the BEEM current. These
results clearly demonstrate the importance of bulk and surface band structure
effects for a detailed understanding of BEEM data.


title:
Molecular crystal approach for pi-conjugated polymers: from PPP
  Hamiltonian to Holstein model for polaron states

abstract:
  Starting from the $\pi$-electron Pariser-Parr-Pople (PPP) Hamiltonian which
includes both strong electron-phonon and electron-electron interactions, we
propose some strongly correlated wave functions of increasing quality for the
ground state of conjugated polymers. These wavefunctions are built by combining
different finite sets of local configurations extended at most over two
nearest-neighbour monomers. With this picture, the doped case with one
additional particle is expressed in terms of quasi-particle. Thus, the polaron
formation problem goes back to the study of a Holstein like model.


title:
Universality in Random Systems: the case of the 3-d Random Field Ising
  model

abstract:
  We study numerically the zero temperature Random Field Ising Model on cubic
lattices of various linear sizes $ 6 \le L \le 90 $ in three dimensions with
the purpose of verifying the validity of universality for disordered systems.
For each random field configuration we vary the ferromagnetic coupling strength
J and compute the ground state exactly.
  We examine the case of different random field probability distributions:
gaussian distribution, zero width bimodal distribution h_{i} = \pm 1, wide
bimodal distribution h_{i} = \pm 1 +\delta h (with a gaussian $\delta h$). We
also study the case of the randomly diluted antiferromagnet in a field,which is
thought to be in the same universality class.
  We find that in the infinite volume limit the magnetization is discontinuous
in J and we compute the relevant exponent, which, according to finite size
scaling, equals $ 1/ \nu $ . We find different values of $ \nu $ for the
different random field distributions, in disagreement with universality.


title:
Multiscale behaviour of volatility autocorrelations in a financial
  market

abstract:
  We perform a scaling analysis on NYSE daily returns. We show that volatility
correlations are power-laws on a time range from one day to one year and, more
important, that they exhibit a multiscale behaviour.


title: Solitons in Carbon Nanotubes

abstract:
  The symmetries of spontaneous lattice distortions in carbon nanotubes are
investigated. When the degeneracy of the ground states remains discrete, there
are solitons or domain walls connecting the different symmetry broken vaccua.
These solitons, similarly to the case of polyacetelene, are fractionally
charged states. In addition to the topological domain walls, there are polaron
states with discrete energies within the energy gap. The energies and shapes of
these localized mid-gap states should be accessible via STM spectroscopy.


title: Polymer Shape Anisotropy and the Depletion Interaction

abstract:
  We calculate the second and third virial coefficients of the effective
sphere-sphere interaction due to polymer depletion. By utilizing the anisotropy
of a typical polymer conformation, we can consider polymers that are roughly
the same size as the spherical inclusions. We argue that recent experiments can
confirm this anisotropy.


title: Self-assembled chains of graphitized carbon nanoparticles

abstract:
  We report a technique which allows self-assembly of conducting nanoparticles
into long continuous chains. Transport properties of such chains have been
studied at low temperatures. At low bias voltages, the charges are pinned and
the chain resistance is exponentially high. Above a certain threshold
($V_{T}$), the system enters a conducting state. The threshold voltage is much
bigger than the Coulomb gap voltage for a single particle and decreases
linearly with increasing temperature. A sharp threshold was observed up to
about 77 K. Such chains may be used as switchable links in Coulomb charge
memories.


title:
Quantum description of the orientational degrees of freedom in a biaxial
  nematic liquid

abstract:
  The quantum mechanical version of a classical model for studying the
orientational degrees of freedom corresponding to a nematic liquid composed of
biaxial molecules is presented. The effective degrees of freedom are described
by operators carrying an SU(3) representation, which allows the explicit
calculation of the partition function in the mean field approximation. The
algebraic consistency conditions are solved numerically and the equilibrium
phases of the system are determined. In particular, the entropy, the specific
heat and the order parameters are presented for different choices of the
constituent biaxial molecules. Our results reproduce the classical calculation
in the limit of high temperatures and high quantum numbers.


title: Mesoscopic fluctuations of tunneling through double quantum dots

abstract:
  We study fluctuations of conductance of two connected in series dots in the
Coulomb blockade regime. The pattern of the fluctuations turns out to be
extremely sensitive to a magnetic field. These conductance fluctuations also
provide information about the mesoscopic fluctuations of charge of a
partially-opened single quantum dot, which are hard to measure directly.


title: Quasiparticle density of states in dirty high-T_c superconductors

abstract:
  We study the density of quasiparticle states of dirty d-wave superconductors.
We show the existence of singular corrections to the density of states due to
quantum interference effects. We then argue that the density of states actually
vanishes in the localized phase as $|E|$ or $E^2$ depending on whether time
reversal is a good symmetry or not. We verify this result for systems without
time reversal symmetry in one dimension using supersymmetry techniques. This
simple, instructive calculation also provides the exact universal scaling
function for the density of states for the crossover from ballistic to
localized behaviour in one dimension. Above two dimensions, we argue that in
contrast to the conventional Anderson localization transition, the density of
states has critical singularities which we calculate in a $2+\epsilon$
expansion. We discuss consequences of our results for various experiments on
dirty high-$T_c$ materials.


title:
Universal Asymptotic Eigenvalue Distribution of Density Matrices and the
  Corner Transfer Matrices in the Thermodynamic Limit

abstract:
  We study the asymptotic behavior of the eigenvalue distribution of the
Baxter's corner transfer matrix (CTM) and the density matrix (DM) in the
White's density-matrix renormalization group (DMRG), for one-dimensional
quantum and two-dimensional classical statistical systems. We utilize the
relationship ${\rm DM}={\rm CTM}^4$ which holds for non-critical systems in the
thermodynamic limit. Using the known diagonal form of CTM, we derive exact
asymptotic form of the DM eigenvalue distribution for the integrable $S=1/2$
XXZ chain (and its related integrable models) in the massive regime. The result
is then recast into a ``universal'' form without model-specific quantities,
which leads to $\omega_{m}\sim \exp[-{\rm const.}(\log m)^2]$ for $m$-th DM
eigenvalue at larg $m$. We perform numerical renormalization group calculations
(using the corner-transfer-matrix RG and the product-wavefunction RG) for
non-integrable models, verifying the ``universal asymptotic form'' for them.
Our results strongly suggest the universality of the asymptotic eigenvalue
distribution of DM and CTM for a wide class of systems.


title: Transition Matrix Monte Carlo Method

abstract:
  We analyze a new Monte Carlo method which uses transition matrix in the space
of energy. This method gives an efficient reweighting technique. The associated
artificial dynamics is a constrained random walk in energy, producing the
result that correlation time is proportional to the specific heat.


title: Fixed Point of the Finite System DMRG

abstract:
  The density matrix renormalization group (DMRG) is a numerical method that
optimizes a variational state expressed by a tensor product. We show that the
ground state is not fully optimized as far as we use the standard finite system
algorithm, that uses the block structure B**B. This is because the tensors are
not improved directly. We overcome this problem by using the simpler block
structure B*B for the final several sweeps in the finite iteration process. It
is possible to increase the numerical precision of the finite system algorithm
without increasing the computational effort.


title: Upper critical field for electrons in two-dimensional lattice

abstract:
  We address a problem of the upper critical field in a lattice described by a
two-dimensional tight-binding model with the on-site pairing. We develop a
finite-system-approach which enables investigation of magnetic and
superconducting properties of electrons on clusters, consisting of a few
thousand sites. We discuss how the quasiparticle density of states changes with
the applied external magnetic field and present the temperature dependence of
the upper critical field. We also briefly discuss possible extension of the
model to account for the properties of high-temperature superconductors.


title: Finite size errors in quantum many-body simulations of extended systems

abstract:
  Further developments are introduced in the theory of finite size errors in
quantum many-body simulations of extended systems using periodic boundary
conditions. We show that our recently introduced Model Periodic Coulomb
interaction [A. J. Williamson et al., Phys. Rev. B 55, R4851 (1997)] can be
applied consistently to all Coulomb interactions in the system. The Model
Periodic Coulomb interaction greatly reduces the finite size errors in quantum
many-body simulations. We illustrate the practical application of our
techniques with Hartree-Fock and variational and diffusion quantum Monte Carlo
calculations for ground and excited state calculations. We demonstrate that the
finite size effects in electron promotion and electron addition/subtraction
excitation energy calculations are very similar.


title:
The effect of an external magnetic field on the gas-liquid transition in
  the Heisenberg spin fluid

abstract:
  We present the theoretical phase diagrams of the classical Heisenberg fluid
in an external magnetic field. A consistent account of correlations is carried
out by the integral equation method. A nonmonotoneous effect of fields on the
temperature of the gas-liquid critical point is found. Within the mean
spherical approximation this nonmonotoneous behavior disappears for short-range
enough spin-spin interactions.


title: Strong Tunneling in Double-Island Structures

abstract:
  We study the electron transport through a system of two low-capacitance metal
islands connected in series between two electrodes. The work is motivated in
part by experiments on semiconducting double-dots, which show intriguing
effects arising from coherent tunneling of electrons and mixing of the
single-electron states across tunneling barriers. In this article, we show how
coherent tunneling affects metallic systems and leads to a mixing of the
macroscopic charge states across the barriers. We apply a recently formulated
RG approach to examine the linear response of the system with high tunnel
conductances (up to 8e^2/h). In addition we calculate the (second order)
cotunneling contributions to the non-linear conductance. Our main results are
that the peaks in the linear and nonlinear conductance as a function of the
gate voltage are reduced and broadened in an asymmetric way, as well as shifted
in their positions. In the limit where the two islands are coupled weakly to
the electrodes, we compare to theoretical results obtained by Golden and
Halperin and Matveev et al. In the opposite case when the two islands are
coupled more strongly to the leads than to each other, the peaks are found to
shift, in qualitative agreement with the recent prediction of Andrei et al. for
a similar double-dot system which exhibits a phase transition.


title: Paramagnetic metal-insulator transition in the 2D Hubbard model

abstract:
  We study the transition from paramagnetic metal to paramagnetic insulator by
finite temperature Quantum Monte-Carlo simulations for the 2D Hubbard model at
half-filling. Working at the moderately high temperature T=0.33*t where the
spin correlation length has dropped to 1.5 lattice spacings, and scanning the
interaction strength U, we observe an unambiguous metal-insulator transition
with onset at U_c=4t. In the metallic phase there are no indications of any
`correlation narrowing' of the band width, nor for any decrease of spectral
weight at the Fermi energy. The Mott-Hubbard gap opens gradually with
increasing U and is accompanied by the formation of side bands near the Fermi
energy. In the first stages of gap formation the gap opens at (pi,0),
reminiscent of the `pseudogap' phenomenology in cuprate superconductors.


title:
Transport properties of quasiparticles with fractional exclusion
  statistics

abstract:
  We consider the ballistic transport of quasiparticles with exclusion
statistics through a 1D wire within the Landauer-Buttiker approach. We
demonstrate that quasiparticle transport coefficients (electrical and heat
conductance, as well as thermopower) are determined by the same general
formulae as for particles with normal statistics. By applying the developed
formalism to the ballistic transport of fractional charge it is shown that for
a wire in contact to quasiparticles reservoirs the transport coefficients
depend on the fractional charge. Specific features of resonant tunneling of
quasiparticles are discussed.


title: Raman Scattering Due to Disorder-Induced Polaritons

abstract:
  The selection rules for dipole and Raman activity can be relaxed due to local
distortion of a crystalline structure. In this situation a dipole-inactive mode
can become simultaneously active in Raman scattering and in dipole interaction
with the electromagnetic field. The later interaction results in
disorder-induced polaritons, which could be observed in first-order Raman
spectra. We calculate scattering cross-section in the case of a material with a
diamond-like average structure, and show that there exist a strong possibility
of observing the disorder induced polaritons.


title: Topological Twistons in Crystalline Polyethylene

abstract:
  We introduce an alternate model to describe twistons in crystalline
polyethylene. The model couples torsional and longitudinal degrees of freedom
and appears as an extension of a model that describes only the torsional
motion. We find exact solutions that describe stable topological twistons, in
good agreement with the torsional and longitudinal interactions in
polyethylene.


title:
Sharp k-space features in the order parameter within the interlayer
  pair-tunneling mechanism of high-T_c superconductivity

abstract:
  We study the k-dependence of the gap function of a bilayer superconductor,
using standard mean-field techniques applied to a 2D extended Hubbard model, in
the presence of coherent interlayer pair-tunneling and quenched coherent
single-particle tunneling. The intralayer pairing potential thus defined is
expandable in a finite number of basis functions for the irreducible
representations of the point-group of the perfectly square lattice, C_{4v}.
  This gives rise to a competition between s- and d-wave symmetry, as the
chemical potential is increased from the bottom to the top of a realistic band
for most cuprates. It allows for mixed-symmetry paired state at temperatures
below T_c, but never at T_c on a square lattice. Inclusion of the interlayer
pair-tunneling into the effective pairing potential leads to highly non-trivial
k-space structures, such as pronounced maxima along the Fermi line not seen in
the absence of interlayer pair-tunneling.
  We show how such a gap structure evolves with temperature and with band
filling, and how it affects various observables. In particular, a nonuniversal
value of the normalized jump in the specific heat at T_c will be evidenced, at
variance with the conventional universal BCS result.


title: Randomly Driven Granular Fluids: large scale structure

abstract:
  The nonequilibrium steady state of a granular fluid, driven by a random
external force, is demonstrated to exhibit long range correlations, which
behave as $\sim 1/r$ in three and $\sim \ln(L/r)$ in two dimensions. We
calculate the corresponding structure factors over the whole range of wave
numbers, and find good agreement with two-dimensional molecular dynamics
simulations. It is also shown by means of a mode coupling calculation, how the
mean field values for the steady state temperature and collision frequency, as
obtained from the Enskog-Boltzmann equation, are renormalized by long
wavelength hydrodynamic fluctuations.


title:
Shape of crossover between mean-field and asymptotic critical behavior
  in a three-dimensional Ising lattice

abstract:
  Recent numerical studies of the susceptibility of the three-dimensional Ising
model with various interaction ranges have been analyzed with a crossover model
based on renormalization-group matching theory. It is shown that the model
yields an accurate description of the crossover function for the
susceptibility.


title:
Superconductivity in the Attractive Hubbard Model: The Double Hubbard--I
  Approximation

abstract:
  Using the Dyson equation of motion for both the diagonal one-particle Green
function, $G(\vec{k},\omega)$ and off--diagonal Green function,
$F(\vec{k},\omega)$, at the level of the Hubbard--I decoupling scheme, we have
found that they have four poles symmetric in pairs, justifying a more
elaborated calculation done by the Z\"urich group by means of the $T$-Matrix
approach (Pedersen et al, Z. Physik B {\bf 103}, 21 (1997)) and the moment
approach of Nolting (Z. Physik {\bf 255}, 25 (1972)). We find that the energy
spectra and the weights of $G(\vec{k},\omega)$ and $F(\vec{k},\omega)$ have to
be calculated self-consistently. $G(\vec{k},\omega)$ satisfies the first two
moments while $F(\vec{k},\omega)$ the first sum rule. Our {\it order parameter}
$\alpha(T)$ is given by $1/N_s \sum_{{\vec{k}}}
\epsilon({\vec{k}})\Delta({\vec{k}})$. Due to the fact that we have a purely
local attractive interaction $\Delta(\vec{k})$ can be of {\it any} s--type
wave. However, for a {\it pure s--wave}, for which $\alpha(T) = 0$, we go back
to the mean--field $BCS$ results, with a renormalized chemical potential. In
this case, the off--diagonal Green function, $F(\vec{k},\omega)$, satisfies the
first two off--diagonal sum rules. We explicitly state the range of validity of
our approximation.


title: Optimally adapted multi-state neural networks trained with noise

abstract:
  The principle of adaptation in a noisy retrieval environment is extended here
to a diluted attractor neural network of Q-state neurons trained with noisy
data. The network is adapted to an appropriate noisy training overlap and
training activity which are determined self-consistently by the optimized
retrieval attractor overlap and activity. The optimized storage capacity and
the corresponding retriever overlap are considerably enhanced by an adequate
threshold in the states. Explicit results for improved optimal performance and
new retriever phase diagrams are obtained for Q=3 and Q=4, with coexisting
phases over a wide range of thresholds. Most of the interesting results are
stable to replica-symmetry-breaking fluctuations.


title: Noise Dressing of Financial Correlation Matrices

abstract:
  We show that results from the theory of random matrices are potentially of
great interest to understand the statistical structure of the empirical
correlation matrices appearing in the study of price fluctuations. The central
result of the present study is the remarkable agreement between the theoretical
prediction (based on the assumption that the correlation matrix is random) and
empirical data concerning the density of eigenvalues associated to the time
series of the different stocks of the S&P500 (or other major markets). In
particular the present study raises serious doubts on the blind use of
empirical correlation matrices for risk management.


title: Subtle interface magnetism of Fe/Au multilayers

abstract:
  By ab initio LMTO calculations in atomic-sphere approximation we have studied
the interlayer exchange coupling between Fe films separated by Au spacers in
infinite Fe/Au multilayers with (001) interface orientation. We also performed
detailed calculations of the magnetization and charge profiles across the
system. We find an enhancement of the Fe moments at the interface, which amount
to $\approx 2.8$ $\mu_B$ instead of the bulk value of 2.2 $\mu_B$, and we also
find a slight magnetic polarization of the order of 0.01 $\mu_B$ at the Au
interface layers. These results do not depend sensitively on the lattice
constants assumed in the calculation.
  When we try to optimize our results with respect to the ratio $R_{Fe}/R_{Au}$
of the Wigner-Seitz spheres for the mutual components, we often find the
optimum near charge neutrality of the interface monolayers: However, this is
not always the case, and usually the sign and the magnitude of the exchange
coupling depend sensitively on the optimal choice of the above-mentioned ratio.
In particular, for Fe$_5$/Au$_3$ we find an anomaly with a large dipole moment
at the interface and a related anomaly of the exchange coupling.


title:
A general methodology to price and hedge derivatives in incomplete
  markets

abstract:
  We introduce and discuss a general criterion for the derivative pricing in
the general situation of incomplete markets, we refer to it as the No Almost
Sure Arbitrage Principle. This approach is based on the theory of optimal
strategy in repeated multiplicative games originally introduced by Kelly. As
particular cases we obtain the Cox-Ross-Rubinstein and Black-Scholes in the
complete markets case and the Schweizer and Bouchaud-Sornette as a quadratic
approximation of our prescription. Technical and numerical aspects for the
practical option pricing, as large deviation theory approximation and Monte
Carlo computation are discussed in detail.


title:
A thermodynamically reversible generalization of Diffusion Limited
  Aggregation

abstract:
  We introduce a lattice gas model of cluster growth via the diffusive
aggregation of particles in a closed system obeying a local, deterministic,
microscopically reversible dynamics. This model roughly corresponds to placing
the irreversible Diffusion Limited Aggregation model (DLA) in contact with a
heat bath. Particles release latent heat when aggregating, while singly
connected cluster members can absorb heat and evaporate. The heat bath is
initially empty, hence we observe the flow of entropy from the aggregating gas
of particles into the heat bath, which is being populated by diffusing heat
tokens. Before the population of the heat bath stabilizes, the cluster
morphology (quantified by the fractal dimension) is similar to a standard DLA
cluster. The cluster then gradually anneals, becoming more tenuous, until
reaching configurational equilibrium when the cluster morphology resembles a
quenched branched random polymer. As the microscopic dynamics is invertible, we
can reverse the evolution, observe the inverse flow of heat and entropy, and
recover the initial condition. This simple system provides an explicit example
of how macroscopic dissipation and self-organization can result from an
underlying microscopically reversible dynamics.

categories: cond-mat.stat-mech comp-gas nlin.CG nlin.PS patt-sol


title: Novel massless phase of Haldane-gap antiferromagnets in magnetic field

abstract:
  The behavior of Haldane-gap antiferromagnets in strong magnetic field is not
universal. While the low-energy physics of the conventional 1D spin-1
Heisenberg model in its magnetized regime is described by one incommensurate
soft mode, other systems with somewhat perturbed coupling constants can possess
two characteristic soft modes in a certain range of the field strength. Such a
{\em two}-component Lutinger liquid phase is realised above the massive
Haldane-gap phase, and in general above any massive nonmagnetic phase, when the
ground state exhibits short range incommensurate fluctuations already in the
absence of the field.


title:
Interplay of disorder and spin fluctuations in the resistivity near a
  quantum critical point

abstract:
  The resistivity in metals near an antiferromagnetic quantum critical point
(QCP) is strongly affected by small amounts of disorder. In a quasi-classical
treatment, we show that an interplay of strongly anisotropic scattering due to
spin fluctuations and isotropic impurity scattering leads to a large regime
where the resistivity varies as T^alpha, with an anomalous exponent, alpha, 1
<= alpha <= 1.5, depending on the amount of disorder. I argue that this
mechanism explains in some detail the anomalous temperature dependence of the
resistivity observed in CePd_2Si_2, CeNi_2Ge_2 and CeIn_3 near the QCP.


title: Quantum Coherence in Fe8 Molecular Nanomagnets

abstract:
  We report observation of coherent quantum oscilations in spin-10 Fe8
molecular clusters. The powder of magnetically oriented Fe8 crystallites was
placed inside a resonator, in a dc magnetic field perpendicular to the
magnetization axis. The field dependence of the ac-susceptibility was measured
up to 5 T, at 680 MHz, down to 25 mK. Two peaks in the imaginary part of the
susceptibility have been detected, whose positions coincide, without any
fitting parameters, with the predicted two peaks corresponding to the quantum
splitting of the ground state in the magnetic field parallel and perpendicular
to the hard magnetization axis.


title: Dynamic Fracture in Single Crystal Silicon

abstract:
  We have measured the velocity of a running crack in brittle single crystal
silicon as a function of energy flow to the crack tip. The experiments are
designed to permit direct comparison with molecular dynamics simulations;
therefore the experiments provide an indirect but sensitive test of interatomic
potentials. Performing molecular dynamics simulations of brittle crack motion
at the atomic scale we find that experiments and simulations disagree showing
that interatomic potentials are not yet well understood.


title: Massive and Massless Behavior in Dimerized Spin Ladders

abstract:
  We investigate the conditions under which a gap vanishes in the spectrum of
dimerized coupled spin-1/2 chains by means of Abelian bosonization and Lanczos
diagonalization techniques. Although both interchain ($J'$) and dimerization
($\delta$) couplings favor a gapful phase, it is shown that a suitable choice
of these interactions yields massless spin excitations. We also discuss the
influence of different arrays of relative dimerization on the appearance of
non-trivial magnetization plateaus.


title: Binding of Oppositely Charged Membranes and Membrane Reorganization

abstract:
  We consider the electrostatic interaction between two rigid membranes, with
different surface charge densities of opposite sign, across an aqueous solution
without added salt. Exact solutions to the nonlinear Poisson-Boltzmann equation
are obtained and their physical meaning discussed. We also calculate the
electrostatic contribution to the free energy and discuss the renormalization
of the area per head group of the charged lipids arising from the Coulomb
interaction.


title:
Unconventional ferromagnetic and spin-glass states of the reentrant spin
  glass Fe0.7Al0.3

abstract:
  Spin excitations of single crystal Fe0.7Al0.3 were investigated over a wide
range in energy and reciprocal space with inelastic neutron scattering. In the
ferromagnetic phase, propagating spin wave modes become paramagnon-like
diffusive modes beyond a critical wave vector q0, indicating substantial
disorder in the long-range ordered state. In the spin glass phase, spin
dynamics is strongly q-dependent, suggesting remnant short-range spin
correlations. Quantitative model for S(energy,q) in the ``ferromagnetic'' phase
is determined.


title: Coarsening dynamics of adsorption processes with diffusional relaxation

abstract:
  We investigate the late coarsening stages of one dimensional adsorption
processes with diffusional relaxation. The nonequilibrium domain size
distribution is studied by means of the field theory associated to the
stochastic evolution. An exact asymptotic solution satisfying dynamical scaling
is given for cluster sizes smaller than the average domain length. Our results
are supported and compared with Monte Carlo simulations.


title: Occupation Numbers of a Half-Filled Landau Level

abstract:
  We demonstrate that a theory of the edge of a half-filled Landau level
recently proposed by Lee and Wen predicts results for the edge occupation
number similar to those of a variational trial wave function proposed
previously by us. We treat Lee and Wen's edge action of a half-filled Landau
level within the framework of bosonization theory, and show that the momentum
occupation numbers are determined by a product of two Green's functions, one
charged and one neutral. In the bulk region ($k<0$) we find a linear occupation
profile, $n_k \propto A+Bk$, while in the tail region ($k>0$) it is
exponentially decaying over the range $k\sim\Ln$, the momentum cutoff for
neutral mode. We find a good fit with the numerical results for occupation
numbers.


title:
Ferromagnetic ground state of an orbital degenerate electronic model for
  transition-metal oxides: exact solution and physical mechanism

abstract:
  We present an exact ground state solution of a one-dimensional electronic
model for transition-metal oxides in the strong coupling limit. The model
contains doubly degenerated orbit for itinerant electrons and the Hund coupling
between the itinerant electrons and localized spins. The ground state is proven
to be a full ferromagnet for any density of electrons. Our model provides a
rigorous example for metallic ferromagnetism in narrow band systems. The
physical mechanism for ferromagnetism and its relevance to high-dimensional
systems, like R$_{1-x}$X$_x$MnO$_3$, are discussed. Due to the orbital
degeneracy of itinerant electrons, the superexchange coupling can be
ferromagnetic rather than antiferromagnetic in the one-band case.


title: Impurity Expulsion From a Bose-Einstein Condensate

abstract:
  When a sizable, hard-sphere-like impurity is placed in a trapped
Bose-Einstein condensate, it will create a hole in the condensate wave
function. Since it costs more energy to drill a hole at the wave function's
maximum (near the trap center) than at its minimum (near the trap edge), the
impurity will be expelled from the condensate. This suggests that C-60
molecules freely falling through a condensate of atomic hydrogen may be
deflected by a radial acceleration comparable to g. The scattered fullerenes
would then form a characteristic banded density pattern outside of the trap.
The detection of these deflected molecules can be used to identify the onset of
Bose-Einstein condensation.


title:
Tests of Conformal Invariance in Randomness Induced Second-Order Phase
  Transitions

abstract:
  The conformal covariance of correlation functions is checked in the
second-order transition induced by random bonds in the two-dimensional 8-state
Potts model.
  The decay of correlations is obtained {\it via} transfer matrix calculations
in a cylinder geometry, and large-scale Monte Carlo simulations give access to
the correlations and the profiles inside a square with free or fixed boundary
conditions, respectively. In both geometries, conformal transformations
constrain the form of the spatial dependence, leading to accurate
determinations of the order parameter scaling index in good agreement with
previous independent determinations obtained through standard techniques. The
energy density exponent is also computed.


title: Four-dimensional gonihedric gauge spin system

abstract:
  We perform Monte Carlo simulations of a four-dimensional gauge invariant spin
system which describes random surfaces with gonihedric action. We develop the
analogy between the flat-crumpled phase transition of the lattice surface model
and the liquid-gas phase transition of non-ideal gases, and identify the
self-intersection coupling constant $k$ of the surface model with the pressure
$P$. As $k$ increases the system moves to a critical point in complete analogy
with the situation for non-ideal gases, where the liquid and the gas phases
approach each other with increasing $P$. We measure vacuum expectation values
of various operators and the corresponding critical indices.


title:
Soft modes, Gaps and Magnetization Plateaus in 1D Spin-1/2
  Antiferromagnetic Heisenberg Models

abstract:
  We study the one-dimensional spin-1/2 model with nearest and
next-to-nearest-neighbor couplings exposed to a homogeneous magnetic field
$h_{3}$ and a dimer field with period $q$ and strength $\delta$. The latter
generates a magnetization plateau at $M=(1-q/\pi)/2$, which evolves with
strength $\delta$ of the perturbation as $\delta^{\epsilon}$, where
$\epsilon=\epsilon(h_{3},\alpha)$ is related to the $\eta$-exponent which
describes the critical behavior of the dimer structure factor, if the
perturbation is switched of ($\delta=0$). We also discuss the appearance of
magnetization plateaus in ladder systems with $l$ legs.


title:
Path Integral for Composite Fermions in the Half-Filled Lowest Landau
  Level

abstract:
  We consider electrons in two dimensions in a strong magnetic field at half
filling of the lowest Landau level using the Chern-Simons approach. Starting
from a lattice Hamiltonian for the electrons, we derive a path integral (PI)
formulation for the composite fermions (CF) which respects the order of the
operators. We use a time lattice with intermediate times in order to have a PI
expressed in density fluctuations. This formulation reveals that there is no
infrared (IR) singularity in the grand-canonical potential in lowest order
perturbation theory.


title:
Computer simulations of the two-dimensional melting transition using
  hard disks

abstract:
  We present detailed Monte Carlo results for the two-dimensional melting
transition of various systems up to N=65536 hard disks. The simulations are
performed in the NVT ensemble, using a new updating scheme. In the isotropic
phase the bond orientational correlation length xi_6 and the susceptibility
chi_6 are measured and compared with the predictions of the
Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory. From the scaling
relation of xi_6 and chi_6 we calculate the critical exponent eta_6. In the
phase transition region we use finite-size scaling methods to locate the
disclination binding transition point and compare the results with the values
obtained from the behaviour in the isotropic phase. Additionally, we measure
the topological defect density, the pressure and the distribution of the second
moment of the local bond orientational order parameter. All results are in good
agreement with the KTHNY theory, while a first-order phase transition with
small correlation length and a one-stage continuous transition can be ruled
out.


title: Excitation spectrum of Andreev billiards with a mixed phase space

abstract:
  We present a semiclassical theory for the excitation spectrum of a ballistic
quantum dot weakly coupled to a superconductor, for the generic situation that
the classical motion gives rise to a phase space containing islands of
regularity in a chaotic sea. The density of low-energy excitations is
determined by quantum energy scales that are related in a simple way to the
morphology of the mixed phase space. An exact quantum mechanical computation
for the annular billiard shows good agreement with the semiclassical
predictions, in particular for the reduction of the excitation gap when the
coupling to the regular regions is maximal.

categories: cond-mat.mes-hall chao-dyn cond-mat.supr-con nlin.CD


title: Quantum suppression of shot noise in atom-size metallic contacts

abstract:
  The transmission of conductance modes in atom-size gold contacts is
investigated by simultaneously measuring conductance and shot noise. The
results give unambiguous evidence that the current in the smallest gold
contacts is mostly carried by nearly fully transmitted modes. In particular,
for a single-atom contact the contribution of additional modes is only a few
percent. In contrast, the trivalent metal aluminum does not show this property.


title:
Gas-Kinetic-Based Traffic Model Explaining Observed Hysteretic Phase
  Transition

abstract:
  Recently, hysteretic transitions to `synchronized traffic' with high values
of both density and traffic flow were observed on German freeways [B. S. Kerner
and H. Rehborn, Phys. Rev. Lett. 79, 4030 (1997)]. We propose a macroscopic
traffic model based on a gas-kinetic approach that can explain this phase
transition. The results suggest a general mechanism for the formation of
probably the most common form of congested traffic.


title:
Competition of Ferromagnetic and Antiferromagnetic Order in the Spin-1/2
  XXZ Chain at Finite Temperature

abstract:
  An analytical study is presented of the crossover in the gapless attractive
XXZ chain from antiferromagnetic to ferromagnetic behaviour at low to high
temperature, respectively. In particular, an analytic formula for the crossover
in the long distance asymptotics and explicit results for the nearest-neighbour
longitudinal correlation are obtained. We also provide results for the specific
heat and magnetic susceptibility for various anisotropies.


title: Self-organized Beating and Swimming of Internally Driven Filaments

abstract:
  We study a simple two-dimensional model for motion of an elastic filament
subject to internally generated stresses and show that wave-like propagating
shapes which can propel the filament can be induced by a self-organized
mechanism via a dynamic instability. The resulting patterns of motion do not
depend on the microscopic mechanism of the instability but only of the filament
rigidity and hydrodynamic friction. Our results suggest that simplified
systems, consisting only of molecular motors and filaments could be able to
show beating motion and self-propulsion.


title: Vanishing of phase coherence in underdoped Bi_2Sr_2CaCu_2O_8+d

abstract:
  Coherent time-domain spectroscopy is used to measure the screening and
dissipation of high-frequency electromagnetic fields in a set of underdoped
Bi_2Sr_2CaCu_2O_8+d thin films. The measurements provide direct evidence for a
phase-fluctuation driven transition from the superconductor to normal state,
with dynamics described well by the Berezinskii-Kosterlitz-Thouless theory of
vortex-pair unbinding.


title: Time reversal symmetry breaking superconductivity

abstract:
  We study time reversal symmetry breaking superconductivity with $\Delta_k =
\Delta_{x^2-y^2} (k) +e^{i\theta} \Delta_{\alpha}$ ($\alpha = s$ or $d_{xy}$)
symmetries. It is shown that the behavior of such superconductors could be {\em
qualitatively} different depending on the minor components ($\alpha$) and its
phase at lower temperatures. It is argued that such {\em qualitatively
different} behaviors in thermal as well as in angular dependencies could be a
{\em source} of consequences in transport and Josephson physics.
Orthorhombicity is found to be a strong mechanism for mixed phase (in case of
$\alpha = s$). We show that due to electron correlation the order parameter is
more like a pure $d_{x^2-y^2}$ symmetry near optimum doping.


title:
Schwinger-Keldysh Approach to Disordered and Interacting Electron
  Systems: Derivation of Finkelstein's Renormalization Group Equations

abstract:
  We develop a dynamical approach based on the Schwinger-Keldysh formalism to
derive a field-theoretic description of disordered and interacting electron
systems. We calculate within this formalism the perturbative RG equations for
interacting electrons expanded around a diffusive Fermi liquid fixed point, as
obtained originally by Finkelstein using replicas. The major simplifying
feature of this approach, as compared to Finkelstein's is that instead of $N
\to 0$ replicas, we only need to consider N=2 species. We compare the dynamical
Schwinger-Keldysh approach and the replica methods, and we present a simple and
pedagogical RG procedure to obtain Finkelstein's RG equations.


title: Coulomb Blockade Fluctuations in Strongly Coupled Quantum Dots

abstract:
  Quantum fluctuations of Coulomb blockade are investigated as a function of
the coupling to reservoirs in semiconductor quantum dots. We use fluctuations
in the distance between peaks $\Delta N$ apart to characterize both the
amplitude and correlation of peak motion. For strong coupling, peak motion is
greatly enhanced at low temperature, but does not show an increase in
peak-to-peak correlation. These effects can lead to anomalous temperature
dependence in the Coulomb valleys, similar to behavior ascribed to Kondo
physics.


title:
Bloch function description of a Bose-Einstein condensate in a finite
  optical lattice

abstract:
  We consider stationary and propagating solutions for a Bose-Einstein
condensate in a periodic optical potential with an additional confining optical
or magnetic potential. Using an effective mass approximation we express the
condensate wavefunction in terms of slowly-varying envelopes modulating the
Bloch modes of the optical lattice. In the limit of a weak nonlinearity, we
derive a nonlinear Schr\"{o}dinger equation for propagation of the envelope
function which does not contain the rapid oscillation of the lattice. We then
consider the ground state solutions in detail in the regime of weak, moderate
and strong nonlinear interactions. We describe the form of solution which is
appropriate in each regime, and place careful limits on the validity of each
type of solution.


title: Statistical Properties of Contact Maps

abstract:
  A contact map is a simple representation of the structure of proteins and
other chain-like macromolecules. This representation is quite amenable to
numerical studies of folding. We show that the number of contact maps
corresponding to the possible configurations of a polypeptide chain of N amino
acids, represented by (N-1)-step self avoiding walks on a lattice, grows
exponentially with N for all dimensions D>1. We carry out exact enumerations in
D=2 on the square and triangular lattices for walks of up to 20 steps and
investigate various statistical properties of contact maps corresponding to
such walks. We also study the exact statistics of contact maps generated by
walks on a ladder.


title:
Critical level statistics at the Anderson transition in four-dimensional
  disordered systems

abstract:
  The level spacing distribution is numerically calculated at the
disorder-induced metal--insulator transition for dimensionality d=4 by applying
the Lanczos diagonalisation. The critical level statistics are shown to deviate
stronger from the result of the random matrix theory compared to those of d=3
and to become closer to the Poisson limit of uncorrelated spectra. Using the
finite size scaling analysis for the probability distribution Q_n(E) of having
n levels in a given energy interval E we find the critical disorder W_c = 34.5
\pm 0.5, the correlation length exponent \nu = 1.1 \pm 0.2 and the critical
spectral compressibility k_c \approx 0.5.


title: Stacking Entropy of Hard Sphere Crystals

abstract:
  Classical hard spheres crystallize at equilibrium at high enough density.
Crystals made up of stackings of 2-dimensional hexagonal close-packed layers
(e.g. fcc, hcp, etc.) differ in entropy by only about $10^{-3}k_B$ per sphere
(all configurations are degenerate in energy). To readily resolve and study
these small entropy differences, we have implemented two different
multicanonical Monte Carlo algorithms that allow direct equilibration between
crystals with different stacking sequences. Recent work had demonstrated that
the fcc stacking has higher entropy than the hcp stacking. We have studied
other stackings to demonstrate that the fcc stacking does indeed have the
highest entropy of ALL possible stackings. The entropic interactions we could
detect involve three, four and (although with less statistical certainty) five
consecutive layers of spheres. These interlayer entropic interactions fall off
in strength with increasing distance, as expected; this fall-off appears to be
much slower near the melting density than at the maximum (close-packing)
density. At maximum density the entropy difference between fcc and hcp
stackings is $0.00115 +/- 0.00004 k_B$ per sphere, which is roughly 30% higher
than the same quantity measured near the melting transition.


title: Phase diagram of the metal-insulator transition in 2D electronic systems

abstract:
  We investigated the interdependence of the effects of disorder and carrier
correlations on the metal-insulator transition in two-dimensional electronic
systems. We present a quantitative metal-insulator phase diagram. Depending on
the carrier density we find two different types of metal-insulator transition -
a continuous localization for rs=<8 and a discontinuous transition at higher
rs. The critical level of disorder at the transition decreases with decreasing
carrier density. At very low carrier densities we find that the system is
always insulating. The value of the conductivity at the transition is
consistent with recent experimental measurements. The self-consistent method
which we have developed includes the effects of both disorder and correlations
on the transition, using a density relaxation theory with the Coulomb
correlations determined from numerical simulation data.


title: A Field Theory for Partially Polarized Quantum Hall States

abstract:
  We propose a new effective field theory for partially polarized quantum Hall
states. The density and polarization for the mean field ground states are
determined by couplings to two Chern-Simons gauge fields. In addition there is
a $\sigma$-model field, $\mh$, which is necessary both to preserve the
Chern-Simons gauge symmetry that determines the correlations in the ground
state, and the global SU(2) invariance related to spin rotations. For states
with non zero polarization, the low energy dynamics is that of a ferromagnet.
In addition to spin waves, the spectrum contains topological solitons, or
skyrmions, just as in the fully polarized case. The electric charge of the
skyrmions is given by $Q_{el}=\nu P Q_{top}$, where $\nu$ is the filling
fraction, $P$ the magnitude of the polarization, and $Q_{top}$ the topological
charge. For the special case of full polarization, the theory involves a single
scalar field and a single Chern-Simons field in addition to the $\sigma$-model
field, $\mh$. We also give a heuristic derivation of the model lagrangians for
both full and partial polarization, and show that in a mean field picture, the
field $\mh$ is necessary in order to take into account the Berry phases
originating from rotations of the electron spins.


title:
Microscopic membrane elasticity and interactions among membrane
  inclusions: Interplay between the shape, dilation, tilt and tilt-difference
  modes

abstract:
  A phenomenological Landau elasticity for the shape, dilation, and lipid-tilt
of bilayer membranes is developed. The shape mode couples with the sum of the
monolayers' tilt, while the dilation mode couples with the difference of the
monolayers' tilts. Interactions among membrane inclusions within regular arrays
are discussed. Inclusions modifying the membrane thickness and/or inducing a
tilt-difference due to their convex or concave shape yield a dilation-induced
attraction and a tilt-difference-induced repulsion. The resulting interaction
can stabilize 2D crystal phases, with the possible coexistence of different
lattice spacings when the dilation-tilt-difference coupling is large.
Inclusions favoring crystals are those with either a long-convex or a
short-concave hydrophobic core. Inclusions inducing a local membrane curvature
due to their conical shape repel one another. At short inclusions separations,
a tilt comparable with the inclusion's cone angle develops: it relaxes the
membrane curvature and reduces the repulsion. At large separations the tilt
vanishes, whatever the value of the shape-tilt coupling.


title:
SQUID nature of microwave absorption in a high-Tc superconducting
  Ho-Ba-Cu-0 single crystal

abstract:
  A high-Tc, superconducting Ho-Ba-Cu-O single crystal with Tc=86.8 K is
investigated on a modified ESR spectrometer at temperatures T>78 K. A signal is
observed, whose spectrum in the range of magnetic fields H<0.7 Oe has the form
of equivalent absorption lines with an interval \Delta = 0.009 Oe. Detailed
measurements of the dependence of the spectrum on the amplitude of the
microwave field, the temperature, and the orientation of the single crystal are
reported. It is established that the absorption spectrum exhibits a typical
behavior inherent in SQUIDs with a critical current that decays linearly with
increasing T. Three different techniques are used to determine the so-called
hysteresis parameter of the postulated SQUID: Li_c /\Phi_0 (L and i_c are the
inductance and critical current of the SQUID, and \Phi_0 is the quantum of
magnetic flux). All three techniques give the same result: Li_c (T=79
K)/\Phi_0= 3.1\pm 0.3, attesting to the existence of a SQUID-like structure in
the single crystal.


title: Extending the Broad Histogram Method for Continuous Systems

abstract:
  We propose a way of extending the Broad Histogram Monte Carlo method (BHMC)
to systems with continuous degrees of freedom, and we apply these ideas to
investigate the three-dimensional XY-model. Our method gives results in
excellent agreement with Metropolis and Histogram Monte Carlo simulations and
calculates for the whole temperature range 1.2<T<4.7 using only 2 times more
computer effort than the Histogram method for the range 2.1<T<2.2. Our way of
treatment is general, it can also be applied to other systems with continuous
degrees of freedom.


title:
Alignment transition in a nematic liquid crystal due to field-induced
  breaking of anchoring

abstract:
  We report on the alignment transition of a nematic liquid crystal from
initially homeotropic to quasi-planar due to field-induced anchoring breaking.
The initial homeotropic alignment is achieved by Langmuir-Blodgett monolayers.
In this geometry the anchoring strength can be evaluated by the Frederiks
transition technique. Applying an electric field above a certain threshold
provokes turbulent states denoted DSM1 and DSM2. While DSM1 does not affect the
anchoring, DSM2 breaks the coupling between the surface and the liquid crystal:
switching off the field from a DSM2 state does not immediately restore the
homeotropic alignment. Instead, we obtain a quasi-planar metastable alignment.
The cell thickness dependence for the transition is related to the


title:
Distributions of Time- and Distance-Headways in the Nagel-Schreckenberg
  Model of Vehicular Traffic: Effects of Hindrances

abstract:
  In the Nagel-Schreckenberg model of vehicular traffic on single-lane highways
vehicles are modelled as particles which hop forward from one site to another
on a one dimensional lattice and the inter-particle interactions mimic the
manner in which the real vehicles influence each other's motion. In this model
the number of empty lattice sites in front of a particle is taken to be a
measure of the corresponding distance-headway(DH). The time-headway(TH) is
defined as the time interval between the departures (or arrivals) of two
successive particles recorded by a detector placed at a fixed position on the
model highway. We investigate the effects of spatial inhomogeneities of the
highway (static hindrances) on the DH and TH distributions in the steady-state
of this model.


title:
Quantum computing and single-qubit measurements using the spin filter
  effect

abstract:
  Many things will have to go right for quantum computation to become a reality
in the lab. For any of the presently-proposed approaches involving spin states
in solids, an essential requirement is that these spins should be measured at
the single-Bohr-magneton level. Fortunately, quantum computing provides a
suggestion for a new approach to this seemingly almost impossible task: convert
the magnetization into a charge, and measure the charge. I show how this might
be done by exploiting the spin filter effect provided by ferromagnetic tunnel
barriers, used in conjunction with one-electron quantum dots.


title:
Surface structure of i-Al(68)Pd(23)Mn(9): An analysis based on the
  T*(2F) tiling decorated by Bergman polytopes

abstract:
  A Fibonacci-like terrace structure along a 5fold axis of i-Al(68)Pd(23)Mn(9)
monograins has been observed by T.M. Schaub et al. with scanning tunnelling
microscopy (STM). In the planes of the terraces they see patterns of dark
pentagonal holes. These holes are well oriented both within and among terraces.
In one of 11 planes Schaub et al. obtain the autocorrelation function of the
hole pattern. We interpret these experimental findings in terms of the
Katz-Gratias-de Boisseu-Elser model. Following the suggestion of Elser that the
Bergman clusters are the dominant motive of this model, we decorate the tiling
T*(2F) by the Bergman polytopes only. The tiling T*(2F) allows us to use the
powerful tools of the projection techniques. The Bergman polytopes can be
easily replaced by the Mackay polytopes as the decoration objects. We derive a
picture of ``geared'' layers of Bergman polytopes from the projection
techniques as well as from a huge patch. Under the assumption that no surface
reconstruction takes place, this picture explains the Fibonacci-sequence of the
step heights as well as the related structure in the terraces qualitatively and
to certain extent even quantitatively. Furthermore, this layer-picture requires
that the polytopes are cut in order to allow for the observed step heights. We
conclude that Bergman or Mackay clusters have to be considered as geometric
building blocks of the i-AlPdMn structure rather than as energetically stable
entities.


title: On loss of quantum coherence in an interacting Fermi gas

abstract:
  We clarify the path integral calculation, recently suggested by Golubev and
Zaikin, and show, contrary to their claim, that quasiparticles become fully
coherent quantum particles in the $T \to 0$ limit. The important physical point
is the inclusion of the recoil of the quasiparticle when interacting with
fluctuations in the rest of the Fermi gas.


title: Tests of Dynamical Scaling in 3-D Spinodal Decomposition

abstract:
  We simulate late-stage coarsening of a 3-D symmetric binary fluid. With
reduced units l,t (with scales set by viscosity, density and surface tension)
our data extends two decades in t beyond earlier work. Across at least four
decades, our own and others' individual datasets (< 1 decade each) show viscous
hydrodynamic scaling (l ~ a + b t), but b is not constant between runs as this
scaling demands. This betrays either the unexpected intrusion of a
discretization (or molecular) lengthscale, or an exceptionally slow crossover
between viscous and inertial regimes.


title:
Self-Assembled Triply Periodic Minimal Surfaces as moulds for Photonic
  Band Gap Materials

abstract:
  We propose systems with structures defined by self-assembled triply periodic
minimal surfaces (STPMS) as candidates for photonic bandgap materials. To
support our proposal we have calculated the photonic bands for different STPMS
and we have found that, at least, the double diamond and gyroid structures
present full photonic bandgaps. Given the great variety of systems which
crystalize in these structures, the diversity of possible materials that form
them and the range of lattice constants they present, the construction of
photonic bandgap materials with gaps in the visible range may be presently
within reach.


title: Persistence Length of Flexible Polyelectrolyte Chains

abstract:
  We calculate the dependence of the electrostatic persistence length, l_e, of
weakly charged flexible polyelectrolyte chains using a self-consistent
variational theory. The variation of l_e with \kappa, the inverse Debye
screening length, is controlled by the parameter l_0 l_B/A^2, where l_0 is the
bare persistence length, l_B is the Bjerrum length, and A is the mean distance
between charges along the chain. Several distinct regimes for the dependence of
l_e on \kappa emerge depending on the value of l_0 l_B/A^2. We show that when
l_0 l_B /A^2 << 1 we recover the classical result, l_e \propto \kappa^{-2}. For
intermediate values of l_0 l_B /A^2, l_e \propto \kappa^{-1}. In this regime
one can also get l_e \propto \kappa^{-y} with y < 1 depending on the strength
of the Coulomb interaction. Qualitative comparisons between our theory and
simulations as well as other theories are presented.


title:
Charge fluctuations close to phase separation in the two dimensional t-J
  model

abstract:
  We have studied the t-J model using the Green Function Monte Carlo technique.
We have obtained accurate energies well converged in the thermodynamic limit,
by performing simulations up to 242 lattice sites. By studying the energy as a
function of hole doping we conclude that there is no phase separation in the
physical region, relevant for HTc superconductors. This finding is further
supported by the hole-hole correlation function calculation. Remarkably, by
approaching the phase separation instability, for $J_c/t\sim 0.5$,this function
displays enhanced fluctuations at incommensurate wavevectors, scaling linearly
with the doping, in agreement with experimental findings.


title:
Existence of an omni-directional photonic band gap in one-dimensional
  periodic dielectric structures

abstract:
  It is shown that total reflection for all incident angles does not require a
two- or three-dimensional photonic crystal. We demonstrate that a
one-dimensional photonic crystal can exhibit total omni-directional reflection
for any incident wave within some frequency region. The formation of the
omni-directional gap is discussed and a wide range of realistic fabrication
parameters is proposed.


title:
Voltage fluctuations on a superconductor grain attached to a quantum
  wire

abstract:
  When a finite superconductor is in contact with a 1D normal conductor,
superconducting phase fluctuations lead to power-law response of the normal
subsystem. As a result, the charge fluctuations on the superconductor at zero
temperature have logarithmic correlator and a 1/\omega power spectrum
(1/f-noise). At higher temperatures 1/\omega is pushed to the high frequency
region, and 1/\omega^2 behavior prevails.

categories: cond-mat.supr-con cond-mat.mes-hall cond-mat.str-el quant-ph


title:
Anomalous Behavior of the Upper Critical Field in Extreme Type-II
  Superconductors at Low Temperatures

abstract:
  We present a detailed numerical calculation of the upper critical field
$H_{c2}(T)$ for a bulk extreme type-II superconductor. Particular emphasis is
placed on the high-field, low-temperature regime of the HT-phase diagram. In
this regime it is necessary to go beyond the standard semi-classical theory and
include the effects of Landau quantization of the electronic motion on the
superconducting state. The presence of Landau level quantization induces an
upward curvature in $H_{c2}(T)$ at $\sim 10%$ of $T_{c0}$ for those
superconducting systems in which the slope of $H_{c2}(T)$ at $T_{c0}$ is $\geq
$ 0.2 Tesla/Kelvin. We construct a simple analytical model that can account for
this behavior based on the renormalization of the BCS coupling constant by the
off-diagonal pairing of electrons on Landau levels.


title:
Generalized von Smoluchowski model of reaction rates, with reacting
  particles and a mobile trap

abstract:
  We study diffusion-limited coalescence, A+A<-->A, in one dimension, in the
presence of a diffusing trap. The system may be regarded as a generalization of
von Smoluchowski's model for reaction rates, in that: (a) it includes reactions
between the particles surrounding the trap, and (b) the trap is mobile -- both
considerations which render the model more physically relevant. As seen from
the trap's frame of reference, the motion of the particles is highly
correlated, because of the motion of the trap. An exact description of the long
-time asymptotic limit is found using the IPDF method, and exploiting a
"shielding" property of reversible coalescence that was discovered recently. In
the case where the trap also acts as a source -- giving birth to particles --
the shielding property breaks down, but we find an "equivalence principle":
Trapping and diffusion of the trap may be compensated by an appropriate rate of
birth, such that the steady state of the system is identical with the
equilibrium state in the absence of a trap.


title: Exact solution of a cellular automaton for traffic

abstract:
  We present an exact solution of a probabilistic cellular automaton for
traffic with open boundary conditions, e.g. cars can enter and leave a part of
a highway with certain probabilities. The model studied is the asymmetric
exclusion process (ASEP) with {\it simultaneous} updating of all sites. It is
equivalent to a special case ($v_{\rm max}=1$) of the Nagel-Schreckenberg model
for highway traffic, which has found many applications in real-time traffic
simulations. The simultaneous updating induces additional strong short range
correlations compared to other updating schemes. The stationary state is
written in terms of a matrix product solution. The corresponding algebra, which
expresses a system-size recursion relation for the weights of the
configurations, is quartic, in contrast to previous cases, in which the algebra
is quadratic. We derive the phase diagram and compute various properties such
as density profiles, two point functions and the fluctuations in the number of
particles (cars) in the system. The current and the density profiles can be
mapped onto the ASEP with other time discrete updating procedures. Through use
of this mapping, our results also give new results for these models.


title: Observation of Collective Excitations of the Dilute 2D Electron System

abstract:
  We report inelastic light scattering measurements of dispersive spin and
charge density excitations in dilute 2D electron systems reaching densities
less than 10^{10} cm^{-2}. In the quantum Hall state at nu=2, roton critical
points in the spin inter--Landau level mode show a pronounced softening as r_s
is increased. Instead of a soft mode instability predicted by Hartree--Fock
calculations for r_s ~ 3.3, we find evidence of multiple rotons in the
dispersion of the softening spin excitations. Extrapolation of the data
indicates the possibility of an instability for r_s >~ 11.


title: Computing Number Fluctuations

abstract:
  Here we try and delienate the properties of the function that corresponds to
fluctuations in the momentum distribution. The quantity denoted by $ N(k,k^{'})
$ is quite an interesting object. It satisfies various elegant sum rules and is
also quite singular in some respects. All these properties are brought out and
a formal connection is found between this object and the momentum distribution
of the interacting and non-interacting many-fermion systems. This exercise is
quite general in that it does not refer to any particular hamiltonian. It is
also quite useful since in an earlier preprint(cond-mat/9810043) we showed how
to compute the spectral function and single-particle lifetime of homogeneous
Fermi systems where the only undetermined quantity was this function $
N(k,k^{'}) $.


title: Equation of state for directed percolation

abstract:
  Using field-theoretic renormalization group methods we calculate the equation
of state for non-equilibrium systems belonging to the universality class of
directed percolation (Gribov process) to second order in epsilon = 4-d. By
introducing a parametric representation the result can be written to this order
in a very simple form. We use our result to obtain a universal amplitude ratio
to second order in epsilon.


title: Hubbard chain with a Kondo impurity

abstract:
  A Bethe Ansatz solution of a (modified) Hubbard chain with a Kondo impurity
of arbitrary spin S at a highly symmetric line of parameter space is proposed
and explored. Our results confirm the existence of a strong-coupling (line of)
fixed-point(s) with ferromagnetic Kondo coupling as first hypothetized by
Furusaki and Nagaosa on the basis of perturbative renormalization group
calculations. For on-site Hubbard repulsion and ferromagnetic Kondo exchange,
the ground state has spin S-1/2, i.e., is a singlet when S=1/2. The
contributions of the impurity to the magnetic susceptibility and
low-temperature specific heat are discussed. While the Wilson ratio is unity in
the half-filled band, it is found to be a function of density and interaction
away from half-filling.


title:
Renormalization of modular invariant Coulomb gas and Sine-Gordon
  theories, and quantum Hall flow diagram

abstract:
  Using the renormalisation group (RG) we study two dimensional electromagnetic
coulomb gas and extended Sine-Gordon theories invariant under the modular group
SL(2,Z). The flow diagram is established from the scaling equations, and we
derive the critical behaviour at the various transition points of the diagram.
Following proposal for a SL(2,Z) duality between different quantum Hall fluids,
we discuss the analogy between this flow and the global quantum Hall phase
diagram.


title: New solutions to the Reflection Equation and the projecting method

abstract:
  New integrable boundary conditions for integrable quantum systems can be
constructed by tuning of scattering phases due to reflection at a boundary and
an adjacent impurity and subsequent projection onto sub-spaces. We illustrate
this mechanism by considering a gl(m<n)-impurity attached to an open
gl(n)-invariant quantum chain and a Kondo spin S coupled to the supersymmetric
t-J model.


title:
Bloch Electrons in a Magnetic Field - Why Does Chaos Send Electrons the
  Hard Way?

abstract:
  We find that a 2D periodic potential with different modulation amplitudes in
x- and y-direction and a perpendicular magnetic field may lead to a transition
to electron transport along the direction of stronger modulation and to
localization in the direction of weaker modulation. In the experimentally
accessible regime we relate this new quantum transport phenomenon to avoided
band crossing due to classical chaos.


title: Magnetic Relaxation Phenomena in a CuMn Spin Glass

abstract:
  Experiments on the temperature and time dependence of the response function
and the field cooled magnetisation of a Cu(Mn) spin glass at temperatures below
the zero field spin glass temperature are used to explore the non-equilibrium
nature of the underlying spin configuration. The results imply that a certain
spin configuration is imprinted on the system as the temperature is decreased
at a constant cooling rate. The cooling rate governs the magnitude of the FC
magnetisation (M_{FC}(H,T)). Any intermittent halt at a constant temperature,
T_{i}, imprints an extended spin configuration, a process that is reflected
e.g. in a downward relaxation of M_{FC}. On continued cooling at the same rate,
the magnitude of M_{FC}(T) remains at a lower level than that of a continuous
cooling curve. These results are put into the context of the corresponding
behaviour of the response function as observed in measurements of the
relaxation of the zero field cooled magnetisation.


title: Transition from damage to fragmentation in collision of solids

abstract:
  We investigate fracture and fragmentation of solids due to impact at low
energies using a two-dimensional dynamical model of granular solids. Simulating
collisions of two solid discs we show that, depending on the initial energy,
the outcome of a collision process can be classified into two states: a damaged
and a fragmented state with a sharp transition in between. We give numerical
evidence that the transition point between the two states behaves as a critical
point, and we discuss the possible mechanism of the transition.


title: Fluids of rod-like particles near curved surfaces

abstract:
  We study fluids of hard rods in the vicinity of hard spherical and
cylindrical surfaces at densities below the isotropic-nematic transition. The
Onsager second virial approximation is applied, which is known to yield exact
results for the bulk properties in the limit of infinitely thin rods. This
approach requires the computation of the one-particle distribution function and
of the Mayer function which is greatly facilitated by an appropriate expansion
in terms of spherical harmonics. We determine density and orientational
profiles as well as the surface tension $\gamma$ as function of the surface
curvature radius $R$. Already in the low-density limit of non-interacting rods
$\gamma(R)$ turns out to be non-analytic at $1/R=0$, which prohibits the
application of the commonly used Helfrich expansion. The interparticle
interaction modifies the behavior of $\gamma(R)$ as compared to the low-density
limit quantitatively and qualitatively.