title: Ab Initio Calculation of Spin Gap Behavior in CaV4O9

abstract:
  Second neighbor dominated exchange coupling in CaV4O9 has been obtained from
ab initio density functional (DF) calculations. A DF-based self-consistent
atomic deformation model reveals that the nearest neighbor coupling is small
due to strong cancellation among the various superexchange processes. Exact
diagonalization of the predicted Heisenberg model yields spin-gap behavior in
good agreement with experiment. The model is refined by fitting to the
experimental susceptibility. The resulting model agrees very well with the
experimental susceptibility and triplet dispersion.


title: A Local Fluctuation Theorem

abstract:
  The Fluctuation Theorem (FT) gives an analytic expression for the
probability, in a nonequilibrium system of finite size observed for a finite
time, that the dissipative flux will flow in the reverse direction to that
required by the Second Law of Thermodynamics. In the present letter a Local
version of the Fluctuation Theorem (LFT), is derived. We find that in the case
of planar Poiseuille flow of a Newtonian fluid between thermostatted walls,
non-equilibrium molecular dynamics simulation results support LFT.


title:
Possible Mechanism for Superconductivity in Sulfur - Common Theme for
  Unconventional Superconductors?

abstract:
  Sulfur has recently been found to be a superconductor at high pressure. At
~93 GPa Tc is 10.1 K, and the sulfur is in a base-centered orthorhombic (b.c.o)
structure. At ~160 GPa Tc is 17 K and sulfur is in a rhobohedral (b-Po)
structure. The mechanism for superconductivity in sulfur is not known; in
particular, a band-structure calculation does not find superconductivity in
sulfur until 500 GPa. Following work by Anderson, in a 2D strongly interacting
non-fermi liquid system with some degree of disorder at T=0, the only known
conducting state is a superconductor. Following this idea it has been suggested
that both the HTc cuprates and the 2D electron gas systems are superconductors
with planar conducting planes. Similarly, here we suggest that the mechanism
for conductivity in sulfur are 2D conducting planes which emerge as the planare
rings in sulfur at lower pressure pucker at higher pressures (b.c.o. and b-Po).
As well, we note some other consequences for study of HTc materials of
Anderson's work.


title: The Fluctuation Theorem for Stochastic Systems

abstract:
  The Fluctuation Theorem describes the probability ratio of observing
trajectories that satisfy or violate the second law of thermodynamics. It has
been proved in a number of different ways for thermostatted deterministic
nonequilibrium systems. In the present paper we show that the Fluctuation
Theorem is also valid for a class of stochastic nonequilibrium systems. The
Theorem is therefore not reliant on the reversibility or the determinism of the
underlying dynamics. Numerical tests verify the theoretical result.


title:
Renormalization Group Study of the Intrinsic Finite Size Effect in 2D
  Superconductors

abstract:
  Vortices in a thin-film superconductor interact logarithmically out to a
distance on the order of the two-dimensional (2D) magnetic penetration depth
$\lambda_\perp$, at which point the interaction approaches a constant. Thus,
because of the finite $\lambda_\perp$, the system exhibits what amounts to an
{\it intrinsic} finite size effect. It is not described by the 2D Coulomb gas
but rather by the 2D Yukawa gas (2DYG). To study the critical behavior of the
2DYG, we map the 2DYG to the massive sine-Gordon model and then perform a
renormalization group study to derive the recursion relations and to verify
that $\lambda_\perp$ is a relevant parameter. We solve the recursion relations
to study important physical quantities for this system including the
renormalized stiffness constant and the correlation length. We also address the
effect of current on this system to explain why finite size effects are not
more prevalent in experiments given that the 2D magnetic penetration depth is a
relevant parameter.


title: Segregation of Granular Mixtures in a Rotating Drum

abstract:
  We formulate a phenomenological model for the segregation of binary mixtures
of rough and smooth granular materials in a rotating drum. Our model
successfully replicates a range of experimental behaviours, e.g., rapid radial
segregation; slow axial segregation; and nonuniform radial structure of axial
bands. We present some analytical results and detailed numerical results for
our model.


title:
Phase behavior of grafted chain molecules: Influence of head size and
  chain length

abstract:
  Constant pressure Monte Carlo simulations of a coarse grained off-lattice
model for monolayers of amphiphilic molecules at the air/water interface are
presented. Our study focusses on phase transitions within a monolayer rather
than on self aggregation. We thus model the molecules as stiff chains of
Lennard-Jones spheres with one slightly larger repulsive end bead (head)
grafted to a planar surface. Depending on the size of the head, the temperature
and the pressure, we find a variety of phases, which differ in tilt order
(including tilt direction), and in positional order. In particular, we observe
a modulated phase with a striped superstructure. The modulation results from
the competition between two length scales, the head size and the tail diameter.
As this mechanism is fairly general, it may conceivably also be relevant in
experimental monolayers. We argue that the superstructure would be very
difficult to detect in a scattering experiment, which perhaps accounts for the
fact that it has not been reported so far. Finally the effect of varying the
chain length on the phase diagram is discussed. Except at high pressures and
temperatures, the phase boundaries in systems with longer chains are shifted to
higher temperatures.


title:
Quantum lattice fluctuations in a frustrated Heisenberg spin-Peierls
  chain

abstract:
  As a simple model for spin-Peierls systems we study a frustrated Heisenberg
chain coupled to optical phonons. In view of the anorganic spin-Peierls
compound CuGeO3 we consider two different mechanisms of spin-phonon coupling.
Combining variational concepts in the adiabatic regime and perturbation theory
in the anti-adiabatic regime we derive effective spin Hamiltonians which cover
the dynamical effect of phonons in an approximate way. Ground-state phase
diagrams of these models are determined, and the effect of frustration is
discussed. Comparing the properties of the ground state and of low-lying
excitations with exact diagonalization data for the full quantum spin phonon
models, good agreement is found especially in the anti-adiabatic regime.


title:
Influence of Cu on spin-polaron tunneling in the ferromagnetic state of
  La(2/3)Ca(1/3)Mn(1-x)Cu(x)O(3) from the resistivity data

abstract:
  Nearly a 50% decrease of resistivity \rho (T,x) due to just 4% Cu doping on
the Mn site of La(2/3)Ca(1/3)Mn(1-x)Cu(x)O(3) is observed. Attributing the
observed phenomenon to the substitution induced decrease of the spin polaron
energy E_s(x) below the Curie point T_C(x)=T_C(0)(1-x), all data are found to
be well fitted by the nonthermal coherent tunneling expression \rho (T,x) =
\rho_0*exp(-\gamma M^2(T,x)) assuming
M(T,x)=M_R(x)+M_0(x)*tanh{\sqrt{[T_C(x)/T]^2-1}} for the magnetization in the
ferromagnetic state. The best fits through all the data points yield M_0(x)=
\sqrt(1-x)M_0(0), M_R(x)=\sqrt(x)M_0(0), and E_s(x)=E_s(0)(1-x)^4 for the Cu
induced modifications of the Mn spins dominated zero-temperature spontaneous
magnetization, the residual paramagnetic contribution, and spin-polaron energy,
respectively, with E_s(0)=0.12 eV.


title: Derivation of Hebb's rule

abstract:
  On the basis of the general form for the energy needed to adapt the
connection strengths of a network in which learning takes place, a local
learning rule is found for the changes of the weights. This biologically
realizable learning rule turns out to comply with Hebb's neuro-physiological
postulate, but is not of the form of any of the learning rules proposed in the
literature.
  It is shown that, if a finite set of the same patterns is presented over and
over again to the network, the weights of the synapses converge to finite
values.
  Furthermore, it is proved that the final values found in this biologically
realizable limit are the same as those found via a mathematical approach to the
problem of finding the weights of a partially connected neural network that can
store a collection of patterns. The mathematical solution is obtained via a
modified version of the so-called method of the pseudo-inverse, and has the
inverse of a reduced correlation matrix, rather than the usual correlation
matrix, as its basic ingredient. Thus, a biological network might realize the
final results of the mathematician by the energetically economic rule for the
adaption of the synapses found in this article.


title: Thermodynamics of quantum dissipative many-body systems

abstract:
  We consider quantum nonlinear many-body systems with dissipation described
within the Caldeira-Leggett model, i.e., by a nonlocal action in the path
integral for the density matrix. Approximate classical-like formulas for
thermodynamic quantities are derived for the case of many degrees of freedom,
with general kinetic and dissipative quadratic forms. The underlying scheme is
the pure-quantum self-consistent harmonic approximation (PQSCHA), equivalent to
the variational approach by the Feynman-Jensen inequality with a suitable
quadratic nonlocal trial action. A low-coupling approximation permits to get
manageable PQSCHA expressions for quantum thermal averages with a classical
Boltzmann factor involving an effective potential and an inner Gaussian average
that describes the fluctuations originating from the interplay of quanticity
and dissipation. The application of the PQSCHA to a quantum phi4-chain with
Drude-like dissipation shows nontrivial effects of dissipation, depending upon
its strength and bandwidth.


title:
Charge and Statistics of Quantum Hall Quasi-Particles. A numerical study
  of mean values and fluctuations

abstract:
  We present Monte Carlo studies of charge expectation values and charge
fluctuations for quasi-particles in the quantum Hall system. We have studied
the Laughlin wave functions for quasi-hole and quasi-electron, and also Jain's
definition of the quasi-electron wave function. The considered systems consist
of from 50 to 200 electrons, and the filling fraction is 1/3. For all
quasi-particles our calculations reproduce well the expected values of charge;
-1/3 times the electron charge for the quasi-hole, and 1/3 for the
quasi-electron. Regarding fluctuations in the charge, our results for the
quasi-hole and Jain quasi-electron are consistent with the expected value zero
in the bulk of the system, but for the Laughlin quasi-electron we find small,
but significant, deviations from zero throughout the whole electron droplet. We
also present Berry phase calculations of charge and statistics parameter for
the Jain quasi-electron, calculations which supplement earlier studies for the
Laughlin quasi-particles. We find that the statistics parameter is more well
behaved for the Jain quasi-electron than it is for the Laughlin quasi-electron.


title: Diffusion and rheology in a model of glassy materials

abstract:
  We study self-diffusion within a simple hopping model for glassy materials.
(The model is Bouchaud's model of glasses [J.-P. Bouchaud, J. Physique I 2,
1705 (1992)], as extended to describe rheological properties [P. Sollich, F.
Lequeux, P. Hebraud and M.E. Cates, Phys. Rev. Lett. 78, 2020 (1997)].) We
investigate the breakdown, near the glass transition, of the (generalized)
Stokes-Einstein relation between self-diffusion of a tracer particle and the
(frequency-dependent) viscosity of the system as a whole. This stems from the
presence of a broad distribution of relaxation times of which different moments
control diffusion and rheology. We also investigate the effect of flow
(oscillatory shear) on self-diffusion and show that this causes a finite
diffusivity in the temperature regime below the glass transition (where this
was previously zero). At higher temperatures the diffusivity is enhanced by a
power law frequency dependence that also characterises the rheological
response. The relevance of these findings to soft glassy materials (foams,
emulsions etc.) as well as to conventional glass-forming liquids is discussed.


title: Financial ``Anti-Bubbles'': Log-Periodicity in Gold and Nikkei collapses

abstract:
  We propose that imitation between traders and their herding behaviour not
only lead to speculative bubbles with accelerating over-valuations of financial
markets possibly followed by crashes, but also to ``anti-bubbles'' with
decelerating market devaluations following all-time highs. For this, we propose
a simple market dynamics model in which the demand decreases slowly with
barriers that progressively quench in, leading to a power law decay of the
market price decorated by decelerating log-periodic oscillations. We document
this behaviour on the Japanese Nikkei stock index from 1990 to present and on
the Gold future prices after 1980, both after their all-time highs. We perform
simultaneously a parametric and non-parametric analysis that are fully
consistent with each other. We extend the parametric approach to the next order
of perturbation, comparing the log-periodic fits with one, two and three
log-frequencies, the latter one providing a prediction for the general trend in
the coming years. The non-parametric power spectrum analysis shows the
existence of log-periodicity with high statistical significance, with a
prefered scale ratio of $\lambda \approx 3.5$ for the Nikkei index $\lambda
\approx 1.9$ for the Gold future prices, comparable to the values obtained for
speculative bubbles leading to crashes.


title: Anchoring of polymers by traps randomly placed on a line

abstract:
  We study dynamics of a Rouse polymer chain, which diffuses in a
three-dimensional space under the constraint that one of its ends, referred to
as the slip-link, may move only along a one-dimensional line containing
randomly placed, immobile, perfect traps. For such a model we compute exactly
the time evolution of the probability $P_{sl}(t)$ that the chain slip-link will
not encounter any of the traps until time $t$ and consequently, that until this
time the chain will remain mobile.


title: Charge Transport in Synthetic Metals

abstract:
  The phenomenology of charge transport in synthetic metals is reviewed. It is
argued that the conventional quasiparticle picture and Boltzmann transport
theory do not apply to these materials. The central ideas of Fermi liquid
theory are reviewed, and the significant corrections produced by quasiparticle
scattering from ferromagnetic spin fluctuations in liquid $^3$He are described.
It is shown that Sr$_2$RuO$_4$ does not display the symptoms of a
nearly-ferromagnetic Fermi liquid, so the source of its odd angular momentum
pairing remains to be understood. The solution of an assisted-tunneling model
of charge transport in quasi-one dimensional materials is described. This model
has a quantum critical point and gives a resistivity that is linear in
temperature or frequency, whichever is greater.


title:
Crossover to Potential Energy Landscape Dominated Dynamics in a Model
  Glass-forming Liquid

abstract:
  An equilibrated model glass-forming liquid is studied by mapping successive
configurations produced by molecular dynamics simulation onto a time series of
inherent structures (local minima in the potential energy). Using this
``inherent dynamics'' approach we find direct numerical evidence for the long
held view that below a crossover temperature, $T_x$, the liquid's dynamics can
be separated into (i) vibrations around inherent structures and (ii)
transitions between inherent structures (M. Goldstein, J. Chem. Phys. {\bf 51},
3728 (1969)), i.e., the dynamics become ``dominated'' by the potential energy
landscape. In agreement with previous proposals, we find that $T_x$ is within
the vicinity of the mode-coupling critical temperature $T_c$. We further find
that at the lowest temperature simulated (close to $T_x$), transitions between
inherent structures involve cooperative, string like rearrangements of groups
of particles moving distances substantially smaller than the average
interparticle distance.


title: Does Good Mutation Help You Live Longer?

abstract:
  We study the dynamics of an age-structured population in which the life
expectancy of an offspring may be mutated with respect to that of its parent.
When advantageous mutation is favored, the average fitness of the population
grows linearly with time $t$, while in the opposite case the average fitness is
constant. For no mutational bias, the average fitness grows as t^{2/3}. The
average age of the population remains finite in all cases and paradoxically is
a decreasing function of the overall population fitness.


title: The Josephson Plasma Resonance in Bi_2Sr_2CaCu_2O_8 in a Tilted Field

abstract:
  The dependence of the Josephson plasma frequency omega_p in Bi_2Sr_2CaCu_2O_8
on a tilted field H is reported. Measurements over a large range of B and tilt
angle theta allow a detailed comparison with a recent calculation by Koshelev.
With a slight modification of the model, close agreement is obtained. From the
fits, we find values for the in-plane correlation length and the zero-field
critical current density J_{c0} (4,600 A/cm^2 at 30 K). An analogy to Bragg
diffraction is described, as well as a picture for the fractional-exponent
behavior of omega_p versus H.


title:
Statistics of the Coulomb blockade peak spacings of a silicon quantum
  dot

abstract:
  We present an experimental study of the fluctuations of Coulomb blockade peak
positions of a quantum dot. The dot is defined by patterning the
two-dimensional electron gas of a silicon MOSFET structure using stacked gates.
This permits variation of the number of electrons on the quantum dot without
significant shape distortion. The ratio of charging energy to single particle
energy is considerably larger than in comparable GaAs/AlGaAs quantum dots. The
statistical distribution of the conductance peak spacings in the Coulomb
blockade regime was found to be unimodal and does not follow the Wigner
surmise. The fluctuations of the spacings are much larger than the typical
single particle level spacing and thus clearly contradict the expectation of
random matrix theory.


title: Extremal dynamics model on evolving networks

abstract:
  We investigate an extremal dynamics model of evolution with a variable number
of units. Due to addition and removal of the units, the topology of the network
evolves and the network splits into several clusters. The activity is mostly
concentrated in the largest cluster. The time dependence of the number of units
exhibits intermittent structure. The self-organized criticality is manifested
by a power-law distribution of forward avalanches, but two regimes with
distinct exponents tau = 1.98 +- 0.04 and tau^prime = 1.65 +- 0.05 are found.
The distribution of extinction sizes obeys a power law with exponent 2.32 +-
0.05.


title: Renormalization for Discrete Optimization

abstract:
  The renormalization group has proven to be a very powerful tool in physics
for treating systems with many length scales. Here we show how it can be
adapted to provide a new class of algorithms for discrete optimization. The
heart of our method uses renormalization and recursion, and these processes are
embedded in a genetic algorithm. The system is self-consistently optimized on
all scales, leading to a high probability of finding the ground state
configuration. To demonstrate the generality of such an approach, we perform
tests on traveling salesman and spin glass problems. The results show that our
``genetic renormalization algorithm'' is extremely powerful.


title:
A Path Integral Approach to Derivative Security Pricing: I. Formalism
  and Analytical Results

abstract:
  We use a path integral approach for solving the stochastic equations
underlying the financial markets, and we show the equivalence between the path
integral and the usual SDE and PDE methods. We analyze both the one-dimensional
and the multi-dimensional cases, with point dependent drift and volatility, and
describe a covariant formulation which allows general changes of variables.
Finally we apply the method to some economic models with analytical solutions.
In particular, we evaluate the expectation value of functionals which
correspond to quantities of financial interest.


title:
Resonant transmission of normal electrons through Andreev states in
  ferromagnets

abstract:
  Giant oscillations of the conductance of a superconductor - ferromagnet -
superconductor Andreev interferometer are predicted. The effect is due to the
resonant transmission of normal electrons through Andreev levels when the
voltage $V$ applied to the ferromagnet is close to $2h_0/e$ ($h_0$ is the
spin-dependant part of the electron energy). The effect of bias voltage and
phase difference between the superconductors on the current and the
differential conductance is presented. These efects allow a direct spectroscopy
of Andreev levels in the ferromagnet.


title:
A Path Integral Approach to Derivative Security Pricing: II. Numerical
  Methods

abstract:
  We discuss two numerical methods, based on a path integral approach described
in a previous paper (I), for solving the stochastic equations underlying the
financial markets: the Monte Carlo approach, and the Green function
deterministic numerical method. Then, we apply the latter to some specific
financial problems. In particular, we consider the pricing of a European
option, a zero-coupon bond, a caplet, an American option, and a Bermudan
swaption.


title:
Group-Theoretical Analysis of Second Harmonic Generation at (110) and
  (111) Surfaces of Antiferromagnets

abstract:
  Extending our previous work we classify the nonlinear magneto-optical
response at low index surfaces of fcc antiferromagnets, such as NiO.
Antiferromagnetic bilayers are discussed here as models for the termination of
bulk antiferromagnets.


title:
AC-Conductivity of Pinned Charge Density Wave Fluctuations in Quasi
  One-Dimensional Conductors

abstract:
  Quasi one-dimensional conductors which undergo a Peierls transition to a
charge density wave state at a temperature T_P show a region of one-dimensional
fluctuations above T_P. The Ginzburg-Landau-Langevin theory for the frequency
dependent collective conductivity from conductive fluctuations into the charge
density wave state is developed. By inclusion of a phase breaking term the
effect of local pinning due to random impurities is simulated. It is found that
the spectral weight of unpinned fluctuations is partly redistributed into a
pinned mode around a pinning frequency in the far infrared region. In addition,
selection rule breaking by the imputities makes the fluctuating amplitude mode
visible in the optical response.


title:
The Coexistence of Spinon Superconductivity and d-wave Superconductivity
  in Bi-2212 from Tunneling Measurements

abstract:
  We present electron-tunneling spectroscopy of slightly overdoped Bi2212
single crystals at low temperature using a break-junction technique. Below Tc,
we observe, at least, two different superconducting gaps. We show that (i) the
largest superconducting gap occurs due to pairing of spinons; (ii) the second
superconducting gap has the d-wave symmetry; (iii) the d-wave gap is smaller
than the spinon gap, however, predominant and occurs due to pairing of
fermionic excitations carrying charge; (iv) the spinon gap has either a s-wave
or (s+d) mixed symmetry with the maximum located in (p, p) direction on the
Fermi surface. It is possible that there exists the third superconducting gap
having a g-wave symmetry, which is approximately twice smaller in magnitude
than the d-wave gap. Thus, there are two different types of superconductivity
and two different types of carriers in Bi2212.


title: Fractionation of polydisperse systems: multi-phase coexistence

abstract:
  The width of the distribution of species in a polydisperse system is employed
in a small-variable expansion, to obtain a well-controlled and compact scheme
by which to calculate phase equilibria in multi-phase systems. General and
universal relations are derived, which determine the partitioning of the fluid
components among the phases. The analysis applies to mixtures of arbitrarily
many slightly-polydisperse components. An explicit solution is approximated for
hard spheres.


title:
Convective and Absolute Instabilities in the Subcritical Ginzburg-Landau
  Equation

abstract:
  We study the nature of the instability of the homogeneous steady states of
the subcritical Ginzburg-Landau equation in the presence of group velocity. The
shift of the absolute instability threshold of the trivial steady state,
induced by the destabilizing cubic nonlinearities, is confirmed by the
numerical analysis of the evolution of its perturbations. It is also shown that
the dynamics of these perturbations is such that finite size effects may
suppress the transition from convective to absolute instability. Finally, we
analyze the instability of the subcritical middle branch of steady states, and
show, analytically and numerically, that this branch may be convectively
unstable for sufficiently high values of the group velocity.


title: Probability distribution of the conductance at the mobility edge

abstract:
  Distribution of the conductance P(g) at the critical point of the
metal-insulator transition is presented for three and four dimensional
orthogonal systems. The form of the distribution is discussed. Dimension
dependence of P(g) is proven. The limiting cases $g\to\infty$ and $g\to 0$ are
discussed in detail and relation $P(g)\to 0$ in the limit $g\to 0$ is proven.


title:
Antiferromagnetism in doped anisotropic two-dimensional spin-Peierls
  systems

abstract:
  We study the formation of antiferromagnetic correlations induced by impurity
doping in anisotropic two-dimensional spin-Peierls systems. Using a mean-field
approximation to deal with the inter-chain magnetic coupling, the intra-chain
correlations are treated exactly by numerical techniques. The magnetic coupling
between impurities is computed for both adiabatic and dynamical lattices and is
shown to have an alternating sign as a function of the impurity-impurity
distance, hence suppressing magnetic frustration. An effective model based on
our numerical results supports the coexistence of antiferromagnetism and
dimerization in this system.


title:
Fluctuation of Conductance Peak Spacings in Large Semiconductor Quantum
  Dots

abstract:
  Fluctuation of Coulomb blockade peak spacings in large two-dimensional
semiconductor quantum dots are studied within a model based on the
electrostatics of several electron islands among which there are random
inductive and capacitive couplings. Each island can accommodate electrons on
quantum orbitals whose energies depend also on an external magnetic field. In
contrast with a single island quantum dot, where the spacing distribution is
close to Gaussian, here the distribution has a peak at small spacing value. The
fluctuations are mainly due to charging effects. The model can explain the
occasional occurrence of couples or even triples of closely spaced Coulomb
blockade peaks, as well as the qualitative behavior of peak positions with the
applied magnetic field.


title: Mesoscopic theory of the viscoelasticity of polymers

abstract:
  We have advanced our previous static theory of polymer entanglement involving
an extended Cahn-Hilliard functional, to include time-dependent dynamics. We go
beyond the Gaussian approximation, to the one-loop level, to compute the
frequency dependent storage and loss moduli of the system. The three parameters
in our theory are obtained by fitting to available experimental data on
polystyrene melts of various chain lengths. This provides a physical
representation of the parameters in terms of the chain length of the system. We
discuss the importance of the various terms in our energy functional with
respect to their contribution to the viscoelastic response of the polymeric
system.


title:
Phase separation and the segregation principle in the infinite-U
  spinless Falicov-Kimball model

abstract:
  The simplest statistical-mechanical model of crystalline formation (or alloy
formation) that includes electronic degrees of freedom is solved exactly in the
limit of large spatial dimensions and infinite interaction strength. The
solutions contain both second-order phase transitions and first-order phase
transitions (that involve phase-separation or segregation) which are likely to
illustrate the basic physics behind the static charge-stripe ordering in
cuprate systems. In addition, we find the spinodal-decomposition temperature
satisfies an approximate scaling law.


title:
Observation of the distribution of molecular spin states by resonant
  quantum tunneling of the magnetization

abstract:
  Below 360 mK, Fe magnetic molecular clusters are in the pure quantum
relaxation regime and we show that the predicted square-root time relaxation is
obeyed, allowing us to develop a new method for watching the evolution of the
distribution of molecular spin states in the sample. We measure as a function
of applied field H the statistical distribution P(\xi_H) of magnetic energy
bias \xi_H$ acting on the molecules. Tunneling initially causes rapid
transitions of molecules, thereby digging a hole in P(\xi_H) (around the
resonant condition \xi_H = 0). For small initial magnetization values, the hole
width shows an intrinsic broadening which may be due to nuclear spins.


title:
Patterned Geometries and Hydrodynamics at the Vortex Bose Glass
  Transition

abstract:
  Patterned irradiation of cuprate superconductors with columnar defects allows
a new generation of experiments which can probe the properties of vortex
liquids by confining them to controlled geometries. Here we show that an
analysis of such experiments that combines an inhomogeneous Bose glass scaling
theory with the hydrodynamic description of viscous flow of vortex liquids can
be used to infer the critical behavior near the Bose glass transition. The
shear viscosity is predicted to diverge as $|T-T_{BG}|^{-z}$ at the Bose glass
transition, with $z\simeq 6$ the dynamical critical exponent.


title: Heteropolymers in a Solvent at an Interface

abstract:
  Exact bounds are obtained for the quenched free energy of a polymer with
random hydrophobicities in the presence of an interface separating a polar from
a non polar solvent. The polymer may be ideal or have steric self-interactions.
The bounds allow to prove that a ``neutral'' random polymer is localized near
the interface at any temperature, whereas a ``non-neutral'' chain is shown to
undergo a delocalization transition at a finite temperature. These results are
valid for a quite general a priori probability distribution for both
independent and correlated hydrophobic charges. As a particular case we
consider random AB-copolymers and confirm recent numerical studies.


title: Collapse of Stiff Polyelectrolytes due to Counterion Fluctuations

abstract:
  The effective elasticity of highly charged stiff polyelectrolytes is studied
in the presence of counterions, with and without added salt. The rigid polymer
conformations may become unstable due to an effective attraction induced by
counterion density fluctuations. Instabilities at the longest, or intermediate
length scales may signal collapse to globule, or necklace states, respectively.
In the presence of added-salt, a generalized electrostatic persistence length
is obtained, which has a nontrivial dependence on the Debye screening length.


title:
Linear dependence of peak width in $\chi(\bq, \omega)$ vs T_c for YBCO
  superconductors

abstract:
  It is shown that the momentum space width of the peak in the spin
susceptibility, Im$\chi(q,\omega)$, is linearly proportional to the
superconducting $T_c$: $T_c = \hbar v^*\Delta q$ with $\hbar v^* \simeq 35
meV$\AA. This relation is similar to the linear relation between incommensurate
peak splitting and $T_c$ in LaSrCuO superconductors, as first proposed by
Yamada et al. (Phys. Rev. B 57, 6165, (1998)). The velocity $\hbar v^*$ is
smaller than Fermi velocity or the spin-wave velocity of the parent compound
and remains the same for a wide doping range. This result points towards strong
similarities in magnetic state of YBCO and LaSrCuO.


title:
Dynamic scaling for 2D superconductors, Josephson junction arrays and
  superfluids

abstract:
  The value of the dynamic critical exponent $z$ is studied for two-dimensional
superconducting, superfluid, and Josephson Junction array systems in zero
magnetic field via the Fisher-Fisher-Huse dynamic scaling. We find
$z\simeq5.6\pm0.3$, a relatively large value indicative of non-diffusive
dynamics. Universality of the scaling function is tested and confirmed for the
thinnest samples. We discuss the validity of the dynamic scaling analysis as
well as the previous studies of the Kosterlitz-Thouless-Berezinskii transition
in these systems, the results of which seem to be consistent with simple
diffusion ($z=2$). Further studies are discussed and encouraged.


title: Electromodulation of the bilayer $\nu$=2 quantum Hall phase diagram

abstract:
  We make a number of precise experimental predictions for observing the
various magnetic phases and the quantum phase transitions between them in the
$\nu$=2 bilayer quantum Ha ll system. In particular, we analyze the effect of
an external bias voltage on the quantum phase diagram, finding that a finite
bias should readily enable the experimental observation of the recently
predicted novel canted antiferromagnetic phase in transport and spin
polarization measurements.


title:
Pseudogaps and Extrinsic Losses in Photoemission Experiments on Poorly
  Conducting Solids

abstract:
  It is shown that a photoelectron, on being emitted from a conducting solid,
suffers a substantial energy change due to ohmic losses. Almost all of this
energy loss takes place after the electron leaves the solid. These losses may
be important in isotropic materials with relatively low conductivity, such as
certain colossal magnetoresistance manganates, and in very electrically
anisotropic materials such as high-T_c superconductors and 1-D conductors. In
these materials, the electric field of the photoelectron can penetrate the
system. These losses can drastically affect the observed lineshape on the meV
scale which is now observable due to improved resolution. In particular,
extrinsic losses of this type can mimic pseudogap effects and other peculiar
features of photoemission in cubic manganates. This general point is
illustrated with the particular case of La_{0.67}Ca_{0.33}MnO_3.


title: Statistics of Hartree-Fock Levels in Small Disordered Systems

abstract:
  We study the statistics of quasiparticle and quasihole levels in small
interacting disordered systems within the Hartree-Fock approximation. The
distribution of the inverse compressibility, given according to Koopmans'
theorem by the distance between the two levels across the Fermi energy, evolves
from a Wigner distribution in the non-interacting limit to a shifted Gaussian
for strong interactions. On the other hand the nature of the distribution of
spacings between neighboring levels on the same side of the Fermi energy
(corresponding to energy differences between excited states of the system with
one missing or one extra electron) is not affected by the interaction and
follows Wigner-Dyson statistics. These results are derived analytically by
isolating and solving the appropriate Hartree-Fock equations for the two
levels. They are substantiated by numerical simulations of the full set of
Hartree-Fock equations for a disordered quantum dot with Coulomb interactions.
We find enhanced fluctuations of the inverse compressibility compared to the
prediction of Random Matrix Theory, possibly due to localization of the
wavefunctions around the edge of the dot. The distribution of the inverse
compressibility calculated from the discrete second derivative with respect to
the number of particles of the Hartree-Fock ground state energy deviates from
the distribution of the level spacing across the Fermi energy. The two
distributions have similar shapes but are shifted with respect to each other.
The deviation increases with the strength of the interaction thus indicating
the breakdown of Koopmans' theorem in the strongly interacting limit.

categories: cond-mat.mes-hall chao-dyn nlin.CD nucl-th


title: Arrested Cracks in Nonlinear Lattice Models of Brittle Fracture

abstract:
  We generalize lattice models of brittle fracture to arbitrary nonlinear force
laws and study the existence of arrested semi-infinite cracks. Unlike what is
seen in the discontinuous case studied to date, the range in driving
displacement for which these arrested cracks exist is very small. Also, our
results indicate that small changes in the vicinity of the crack tip can have
an extremely large effect on arrested cracks. Finally, we briefly discuss the
possible relevance of our findings to recent experiments.


title:
Spin-Imbalance and Magnetoresistance in
  Ferromagnet/Superconductor/Ferromagnet Double Tunnel Junctions

abstract:
  We theoretically study the spin-dependent transport in a ferromagnet/super-
conductor/ferromagnet double tunnel junction. The tunneling current in the
antiferromagnetic alignment of the magnetizations gives rise to a spin
imbalance in the superconductor. The resulting nonequilibrium spin density
strongly suppresses the superconductivity with increase of bias voltage and
destroys it at a critical voltage Vc. The results provide a new method not only
for measuring the spin polarization of ferromagnets but also for controlling
superconductivity and tunnel magnetoresistance (TMR) by applying the bias
voltage.


title: Oscillator neural network model with distributed native frequencies

abstract:
  We study associative memory of an oscillator neural network with distributed
native frequencies. The model is based on the use of the Hebb learning rule
with random patterns ($\xi_i^{\mu}=\pm 1$), and the distribution function of
native frequencies is assumed to be symmetric with respect to its average.
Although the system with an extensive number of stored patterns is not allowed
to get entirely synchronized, long time behaviors of the macroscopic order
parameters describing partial synchronization phenomena can be obtained by
discarding the contribution from the desynchronized part of the system. The
oscillator network is shown to work as associative memory accompanied by
synchronized oscillations. A phase diagram representing properties of memory
retrieval is presented in terms of the parameters characterizing the native
frequency distribution. Our analytical calculations based on the
self-consistent signal-to-noise analysis are shown to be in excellent agreement
with numerical simulations, confirming the validity of our theoretical
treatment.


title:
Spectrum of elementary and collective excitations in the dimerized S=1/2
  Heisenberg chain with frustration

abstract:
  We have studied the low-energy excitation spectrum of a dimerized and
frustrated antiferromagnetic Heisenberg chain. We use an analytic approach,
based on a description of the excitations as triplets above a strong-coupling
singlet ground state. The quasiparticle spectrum is calculated by treating the
excitations as a dilute Bose gas with infinite on-site repulsion. Additional
singlet (S=0) and triplet (S=1) modes are found as two-particle bound states of
the elementary triplets. We have also calculated the contributions of the
elementary and collective excitations into the spin structure factor. Our
results are in excellent agreement with exact diagonalizations and dimer series
expansions data as long as the dimerization parameter $\delta$ is not too small
($\delta>0.1$), i.e. while the elementary triplets can be treated as localized
objects.


title: Femtosecond electron and spin dynamics probed by nonlinear optics

abstract:
  A theoretical calculation is performed for the ultrafast spin dynamics in
nickel using an exact diagonalization method. The present theory mainly focuses
on a situation where the intrinsic charge and spin dynamics is probed by the
nonlinear (magneto-)optical responses on the femtosecond time scale, i.e.
optical second harmonic generation (SHG) and the nonlinear magneto-optical Kerr
effect (NOLIMOKE). It is found that the ultrafast charge and spin dynamics are
observable on the time scale of 10 fs. The charge dynamics proceeds ahead of
the spin dynamics, which indicates the existence of a spin memory time. The
fast decay results from the loss of coherence in the initial excited state.
Both the material specific and experimental parameters affect the dynamics. We
find that the increase of exchange interaction mainly accelerates the spin
dynamics rather than the charge dynamics. A reduction of the hopping integrals,
such as present at interfaces, slows down the spin dynamics significantly.
Besides, it is found that a spectrally broad excitation yields the intrinsic
speed limit of the charge (SHG) and spin dynamics (NOLIMOKE) while a narrower
width prolongs the dynamics. This magnetic interface dynamics then should
become accessible to state of art time resolved nonlinear-optical experiments.


title:
Universal features of the off-equilibrium fragmentation with the
  Gaussian dissipation

abstract:
  We investigate universal features of the off-equilibrium sequential and
conservative fragmentation processes with the dissipative effects which are
simulated by the Gaussian random inactivation process. The relation between the
fragment multiplicity scaling law and the fragment size distribution is studied
and a dependence of scaling exponents on the parameters of fragmentation and
inactivation rate functions is established.


title: Effective action for Superconductors and BCS-Bose crossover

abstract:
  A standard perturbative expansion around the mean-field solution is used to
derive the low-energy effective action for superconductors at T=0. Taking into
account the density fluctuations at the outset we get the effective action
where the density $\rho$ is the conjugated momentum to the phase $\theta$ of
the order parameter. In the hydrodynamic regime, the dynamics of the
superconductor is described by a time dependent non-linear Schr\"odinger
equation (TDNLS) for the field $\Psi(x)=\sqrt{\rho/2} e^{i\theta}$. The
evolution of the density fluctuations in the crossover from weak-coupling (BCS)
to strong-coupling (Bose condensation of localized pairs) superconductivity is
discussed for the attractive Hubbard model. In the bosonic limit, the TDNLS
equation reduces to the the Gross-Pitaevskii equation for the order parameter,
as in the standard description of superfluidity. The conditions under which a
phase-only action can be derived in the presence of a long-range interaction to
describe the physics of the superconductivity of ``bad metals'' are discussed.


title: Reaction Diffusion Models in One Dimension with Disorder

abstract:
  We study a large class of 1D reaction diffusion models with quenched disorder
using a real space renormalization group method (RSRG) which yields exact
results at large time. Particles (e.g. of several species) undergo diffusion
with random local bias (Sinai model) and react upon meeting. We obtain the
large time decay of the density of each specie, their associated universal
amplitudes, and the spatial distribution of particles. We also derive the
spectrum of exponents which characterize the convergence towards the asymptotic
states. For reactions with several asymptotic states, we analyze the dynamical
phase diagram and obtain the critical exponents at the transitions. We also
study persistence properties for single particles and for patterns. We compute
the decay exponents for the probability of no crossing of a given point by,
respectively, the single particle trajectories ($\theta$) or the thermally
averaged packets ($\bar{\theta}$). The generalized persistence exponents
associated to n crossings are also obtained. Specifying to the process $A+A \to
\emptyset$ or A with probabilities $(r,1-r)$, we compute exactly the exponents
$\delta(r)$ and $\psi(r)$ characterizing the survival up to time t of a domain
without any merging or with mergings respectively, and $\delta_A(r)$ and
$\psi_A(r)$ characterizing the survival up to time t of a particle A without
any coalescence or with coalescences respectively. $\bar{\theta}, \psi, \delta$
obey hypergeometric equations and are numerically surprisingly close to pure
system exponents (though associated to a completely different diffusion
length). Additional disorder in the reaction rates, as well as some open
questions, are also discussed.


title: Global versus local billiard level dynamics: The limits of universality

abstract:
  Level dynamics measurements have been performed in a Sinai microwave billiard
as a function of a single length, as well as in rectangular billiards with
randomly distributed disks as a function of the position of one disk. In the
first case the field distribution is changed globally, and velocity
distributions and autocorrelation functions are well described by universal
functions derived by Simons and Altshuler. In the second case the field
distribution is changed locally. Here another type of universal correlations is
observed. It can be derived under the assumption that chaotic wave functions
may be described by a random superposition of plane waves.


title: Charging effects in niobium nanostructures

abstract:
  Three types of metallic nanostructures comprising niobium were investigated
experimentally; in all three types, electric transport at very low temperatures
was governed by Coulomb blockade effects.
  1. Thin film strips of niobium could be tuned into resistor strips by an
electrochemical anodisation process, using microfabricated masks and in situ
resistance monitoring. These resistors showed a transition from superconducting
to insulating behaviour with increasing sheet resistance, occurring at a value
approximately equal to the quantum resistance for Cooper pairs, h/(4e^2).
  2. Combining the anodisation technique with lateral size minimisation by
shadow evaporation, devices in a single electron transistor-like configuration
with two weak links and a small island between these were made. Direct evidence
for the Coulomb blockade in the anodisation thinned niobium films was found
when the transport characteristics could be modulated periodically by sweeping
the voltage applied to a gate electrode placed on top of the structure.
  3. Conventional single electron transistors with Al base electrodes, AlO_x
barriers formed in situ by oxidation, and Nb top electrodes were made by
angular evaporation. The output current noise of such a transistor was measured
as a function of bias voltage, gate voltage, and temperature. The low frequency
noise was found to be dominated by charge input noise. The dependence of the
noise on the bias voltage is consistent with self-heating of the transistor
activating the noise sources.


title: Spontaneous Symmetry Breaking in the Quantum Sine-Gordon Model

abstract:
  The spontaneous symmetry breaking in the quantum sine-Gordon model is studied
by a density matrix renormalization group. A phase diagram in the coupling
constant - system size plane is obtained.


title:
Patterns arising from the interaction between scalar and vectorial
  instabilities in two-photon resonant Kerr cavities

abstract:
  We study pattern formation associated with the polarization degree of freedom
of the electric field amplitude in a mean field model describing a nonlinear
Kerr medium close to a two-photon resonance, placed inside a ring cavity with
flat mirrors and driven by a coherent $\hat x$-polarized plane-wave field. In
the self-focusing case, for negative detunings the pattern arises naturally
from a codimension two bifurcation. For a critical value of the field intensity
there are two wave numbers that become unstable simultaneously, corresponding
to two Turing-like instabilities. Considered alone, one of the instabilities
would originate a linearly polarized hexagonal pattern whereas the other
instability is of pure vectorial origin and would give rise to an elliptically
polarized stripe pattern. We show that the competition between the two
wavenumbers can originate different structures, being the detuning a natural
selection parameter.


title: Numerical study of relaxation in electron glasses

abstract:
  We perform a numerical simulation of energy relaxation in three-dimensional
electron glasses in the strongly localized regime at finite temperatures. We
consider systems with no interactions, with long-range Coulomb interactions and
with short-range interactions, obtaining a power law relaxation with an
exponent of 0.15, which is independent of the parameters of the problem and of
the type of interaction. At very long times, we always find an exponential
regime whose characteristic time strongly depends on temperature, system size,
interaction type and localization radius. We extrapolate the longest relaxation
time to macroscopic sizes and, for interacting samples, obtain values much
larger than the measuring time. We finally study the number of electrons
participating in the relaxation processes of very low energy configurations.


title:
Phase separation and metal-insulator transitions in the spin-one-half
  Falicov-Kimball model

abstract:
  The ground-state phase diagram of the spin-one-half Falicov-Kimball model
(FKM) is studied in the one dimension using small-cluster exact-diagonalization
calculations. The resultant exact solutions are used to examine possibilities
for valence and metal-insulator transitions in this model. A number of
remarkable results are found. (i) The phase separation in the spin-one-half FKM
takes place for a wide range of $f$-electron concentrations $n_f$ and $d$-$f$
interactions $G$, including $G$ large. (ii) In the strong coupling limit
($G>4$) the model exhibits a pressure induced discontinuous insulator-metal
transition from an integer-valence state ($n_f=1$) into another integer-valence
state ($n_f=0$). (iii) For intermediate values of $G$ ($G\sim 2.5$) the FKM
undergoes a few discontinuous intermediate-valence transitions. There are
several discontinuous insulator-insulator transitions from $n_f=1$ to $n_f=1/2$
and a discontinuous insulator-metal transition from $n_f = 1/2$ to $n_f=0$.
(iv) In the weak coupling limit ($G<2$) the model undergoes a few consecutive
discontinuous and continuous intermediate-valence transitions as well as a
discontinuous metal-insulator transition.


title: Cluster transfer matrix method for the single electron box

abstract:
  With the newly developed cluster transfer matrix method, we calculate the
average electron number n vs nx (the polarization charge) for varying junction
conductance and its first derivative at nx=0 for finite temperatures, and
demonstrate that the new method is as powerful as the Monte Carlo and
renormalization group methods.


title: Essential finite-size effect in the 2D XY model

abstract:
  The thermodynamics of the 2D XY model is formulated by a transfer matrix
method and analyzed by a density matrix renormalization group. The finite-size
scaling and the beta function of the model are studied by the Roomany-Wyld
renormalization group theory. It is found that the 2D XY model has an essential
finite-size effect and the Berezinskii-Kosterlitz-Thouless transition with the
critical temperature TBKT = 0.892 appears in a finite system of 2000 - 3000
spins as a massless to massive transition with the effective critical
temperature Tc = 1.07 " 0.01.


title:
Chord distribution functions of three-dimensional random media:
  Approximate first-passage times of Gaussian processes

abstract:
  The main result of this paper is a semi-analytic approximation for the chord
distribution functions of three-dimensional models of microstructure derived
from Gaussian random fields. In the simplest case the chord functions are
equivalent to a standard first-passage time problem, i.e., the probability
density governing the time taken by a Gaussian random process to first exceed a
threshold. We obtain an approximation based on the assumption that successive
chords are independent. The result is a generalization of the independent
interval approximation recently used to determine the exponent of persistence
time decay in coarsening. The approximation is easily extended to more general
models based on the intersection and union sets of models generated from the
iso-surfaces of random fields. The chord distribution functions play an
important role in the characterization of random composite and porous
materials. Our results are compared with experimental data obtained from a
three-dimensional image of a porous Fontainebleau sandstone and a
two-dimensional image of a tungsten-silver composite alloy.


title:
The Dislocation Stress Functions From the Double Curl T(3)-Gauge
  Equation: Linearity and a Look Beyond

abstract:
  T(3)-gauge model of defects based on the gauge Lagrangian quadratic in the
gauge field strength is considered. The equilibrium equation of the medium is
fulfilled by the double curl Kroner's ansatz for stresses. The problem of
replication of the static edge dislocation along third axis is analysed under a
special, though conventional, choice of this ansatz. The translational gauge
equation is shown to constraint the functions parametrizing the ansatz (the
stress functions) so that the resulting stress component $\sigma_{3 3}$ is not
that of the edge defect. Another translational gauge equation with the double
curl differential operator is shown to reproduce both the stress functions, as
well as the stress tensors, of the standard edge and screw dislocations.
Non-linear extension of the newly proposed translational gauge equation is
given to correct the linear defect solutions in next orders. New gauge
Lagrangian is suggested in the Hilbert-Einstein form.


title: `Baryonic' bound-state instability in trapped fermionic atoms

abstract:
  We consider a homogeneous gas of spin-S fermionic atoms, as might occur near
the center of an optical trap. In the case where all scattering lengths are
negative and of the same magnitude we demonstrate the instability of the Fermi
sea to the condensation of bound `baryonic' composites containing 2S+1 atoms.
The gap in the excitation spectrum is calculated.

categories: cond-mat hep-ph physics.atom-ph quant-ph


title: Surface electronic scattering in d-wave superconductors

abstract:
  Theoretical model of a rough surface in a d-wave superconductor is studied
for the general case of arbitrary strength of electron scattering by an
impurity layer covering the surface. Boundary conditions for quasiclassical
Eilenberger equations are derived at the interface between the impurity layer
and the d-wave superconductor. The model is applied to the selfconsisrent
calculation of the surface density of states and the critical current in d-wave
Josephson junctions


title:
Direct simulation of ion beam induced stressing and amorphization of
  silicon

abstract:
  Using molecular dynamics (MD) simulation, we investigate the mechanical
response of silicon to high dose ion-irradiation. We employ a realistic and
efficient model to directly simulate ion beam induced amorphization. Structural
properties of the amorphized sample are compared with experimental data and
results of other simulation studies. We find the behavior of the irradiated
material is related to the rate at which it can relax. Depending upon the
ability to deform, we observe either the generation of a high compressive
stress and subsequent expansion of the material, or generation of tensile
stress and densification. We note that statistical material properties, such as
radial distribution functions are not sufficient to differentiate between
different densities of amorphous samples. For any reasonable deformation rate,
we observe an expansion of the target upon amorphization in agreement with
experimental observations. This is in contrast to simulations of quenching
which usually result in denser structures relative to crystalline Si. We
conclude that although there is substantial agreement between experimental
measurements and most simulation results, the amorphous structures being
investigated may have fundamental differences; the difference in density can be
attributed to local defects within the amorphous network. Finally we show that
annealing simulations of our amorphized samples can lead to a reduction of high
energy local defects without a large scale rearrangement of the amorphous
network. This supports the proposal that defects in amorphous silicon are
analogous to those in crystalline silicon.


title:
Elastic properties of a tungsten-silver composite by reconstruction and
  computation

abstract:
  We statistically reconstruct a three-dimensional model of a tungsten-silver
composite from an experimental two-dimensional image. The effective Young's
modulus ($E$) of the model is computed in the temperature range 25-1060^o C
using a finite element method. The results are in good agreement with
experimental data. As a test case, we have reconstructed the microstructure and
computed the moduli of the overlapping sphere model. The reconstructed and
overlapping sphere models are examples of bi-continuous (non-particulate)
media. The computed moduli of the models are not generally in good agreement
with the predictions of the self-consistent method. We have also evaluated
three-point variational bounds on the Young's moduli of the models using the
results of Beran, Molyneux, Milton and Phan-Thien. The measured data were close
to the upper bound if the properties of the two phases were similar ($1/6 < E_1
/E_2 < 6$).


title:
Entropic Elasticity, Cooperative Extensibility and Supercoiling Property
  of DNA: A Unified Viewpoint

abstract:
  A unified model is constructed to study the recently observed DNA entropic
elasticity, cooperative extensibility, and supercoiling property. With the
introduction of a new structural parameter (the folding angle $\phi$), bending
deformations of sugar-phosphate backbones, steric effects of nucleotide
basepairs, and short-range basestacking interactions are considered. The
comprehensive agreement of theoretical results with experimental observations
on both torsionally relaxed and negatively supercoiled DNAs strongly indicates
that, basestacking interactions, although short-ranged in nature, dominate the
elasticity of DNA and hence are of vital biological significance.

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


title:
Effect of the Fermi surface destruction on transport properties in
  underdoped cuprates

abstract:
  Motivated by recent experimental measurements on the Fermi surface(FS)
destruction in underdoped high-$T_{c}$ cuprates, we examine its effect on the
transport properties based on the Boltzmann equation approach. The effect is
modeled by simply taking the density of states for electrons in the gapped
regions to be zero. Within the nearly antiferromagnetic Fermi liquid model, we
calculate the temperature dependences of the dc resistivity, the inverse Hall
angle and the Hall coefficient. It is shown that the effect of the FS
destruction on transport properties is sensitive to the existance and the range
of the flat band near $(0,\pm\pi)$ in the dispersion of electrons, and the
anistropy of the relaxation rate along the Fermi surface. We find that the
experimental data are better described by the cold spot model, i.e., the
transports are determined mainly by the contribution of the electrons near the
Brillouin-zone diagonals.


title:
Spin accumulation induced resistance in mesoscopic ferromagnet/
  superconductor junctions

abstract:
  We present a description of spin-polarized transport in mesoscopic
ferromagnet-superconductor (F/S) systems, where the transport is diffusive, and
the interfaces are transparent. It is shown that the spin reversal associated
with Andreev reflection generates an excess spin density close to the F/S
interface, which leads to a spin contact resistance. Expressions for the
contact resistance are given for two terminal and four terminal geometries. In
the latter the sign depends on the relative magnetization of the ferromagnetic
electrodes.


title:
Impurity effects on the spin excitation spectra in a d-wave
  superconductor

abstract:
  The effects of nonmagnetic impurity on the spin excitation spectra in a
$d_{x^2-y^2}$-wave superconductor are examined, using the self-consistent
$t$-matrix approximation. It is shown that the impurity self-energy acts to
shift the position of the resonance peak to low frequencies and broaden the
peak. While the impurity vertex correction causes a broad spectral weight in
the spin gap at the impurity concentrations where no clear resonance peak is
observed. The gaplike feature still remains in low frequency region upon the
introduction of impurities. Incorporating these two effects, we find that the
result is in qualitative agreement with experiments on
YBa_{2}(Cu_{1-x}Zn_{x})_{3}O_{6+y}.


title:
Periodic Instanton and Phase Transition in Quantum Tunneling of Spin
  Systems

abstract:
  The quantum-classical transitions of the escape rates in a uniaxial spin
model relevant to the molecular magnet Mn$_{12}$Ac and a biaxial anisotropic
ferromagnetic particle are investigated by applying the periodic instanton
method. The effective free energies are expanded around the top of the
potential barrier in analogy to Landau theory of phase transitions. We show
that the first-order transitions occur below the critical external magnetic
field $h_x = 1/4$ for the uniaxial spin model and beyond the critical
anisotropy constant ratio $\lambda = 1/2$ for the biaxial ferromagnetic grains,
which are in good agreement with earlier works.


title:
Doped bilayer antiferromagnets: Hole dynamics on both sides of a
  magnetic ordering transition

abstract:
  The two-layer square lattice quantum antiferromagnet with spins 1/2 shows a
magnetic order-disorder transition at a critical ratio of the interplane to
intraplane couplings. We investigate the dynamics of a single hole in a bilayer
antiferromagnet described by a t-J Hamiltonian. To model the spin background we
propose a ground-state wave function for the undoped system which covers both
magnetic phases and includes transverse as well as longitudinal spin
fluctuations. The photoemission spectrum is calculated using the spin-polaron
picture for the whole range of the ratio of the magnetic couplings. This allows
for the study of the hole dynamics of both sides of the magnetic order-disorder
transition. For small interplane coupling we find a quasiparticle with
properties known from the single-layer antiferromagnet, e.g., the dispersion
minimum is at (pi/2,pi/2). For large interplane coupling the hole dispersion is
similar to that of a free fermion (with reduced bandwidth). The cross-over
between these two scenarios occurs inside the antiferromagnetic phase which
indicates that the hole dynamics is governed by the local environment of the
hole.


title: On the Short-Time Compositional Stability of Periodic Multilayers

abstract:
  The short-time stability of concentration profiles in coherent periodic
multilayers consisting of two components with large miscibility gap is
investigated by analysing stationary solutions of the Cahn-Hilliard diffusion
equation. The limits of the existence and stability of periodic concentration
profiles are discussed as a function of the average composition for given
multilayer period length. The minimal average composition and the corresponding
layer thickness below which artificially prepared layers dissolve at elevated
temperatures are calculated as a function of the multilayer period length for a
special model of the composition dependence of the Gibbs free energy. For
period lengths exceeding a critical value, layered structures can exist as
metastable states in a certain region of the average composition. The phase
composition in very thin individual layers, comparable with the interphase
boundary width, deviates from that of the corresponding bulk phase.


title: Conductance distribution between Hall plateaus

abstract:
  Mesoscopic fluctuations of two-port conductance and four-port resistance
between Hall plateaus are studied within a realistic model for a
two-dimensional electron gas in a perpendicular magnetic field and a smooth
disordered potential. The two-port conductance distribution $P(g)$ is concave
between $g=0$ and $g=1$ and is nearly flat between $g=0.2$ and $g=0.8$. These
characteristics are consistent with recent observations. The distribution is
found to be sharply peaked near the end-points $g=0$ and $g=1$. The
distribution functions for the three independent resistances in a four-port
Hall bar geometry are, on the other hand, characterized by a central peak and a
relatively large width.


title:
Nonlinear Magneto-Optics of freestanding Fe monolayers from first
  principles

abstract:
  The nonlinear magneto-optical Kerr-effect (NOLIMOKE) is determined for
freestanding Fe monolayers with several in-plane structures from first
principles. Based on the theory of nonlinear magneto-optics by H\"ubner and
Bennemann [Phys. Rev. B, {\bf 40}, 5973 (1989)] we calculate the nonlinear
susceptibilities of the monolayers using the ab initio FLAPW-method WIEN95 with
the additional implementation of spin-orbit coupling and the calculation of the
dipole transition matrix elements appropriate for freestanding monolayers. We
present results for the spectral dependence of the nonlinear susceptibility
tensor elements and the resulting intensities and Kerr angles. Special
emphasize is put on the effects of structural changes such as the variation of
the lattice constant and different surface orientations. The influence of
spin-orbit coupling on the tensor elements for different magnetization
directions is presented as well as the azimuthal dependence of the intensities
generated by several low index surfaces, showing the pronounced sensitivity of
second harmonic generation to lateral structural changes as well as magnetic
properties even in the monolayer range.


title: A Discrete Model for Nonequilibrium Growth Under Surface Diffusion Bias

abstract:
  A limited mobility nonequilibrium solid-on-solid dynamical model for kinetic
surface growth is introduced as a simple description for the morphological
evolution of a growing interface under random vapor deposition and surface
diffusion bias conditions. Simulations using a local coordination dependent
instantaneous relaxation of the deposited atoms produce complex surface mound
morphologies whose dynamical evolution is inconsistent with all the proposed
continuum surface growth equations. For any finite bias, mound coarsening is
found to be only an initial transient which vanishes asymptotically, with the
asymptotic growth exponent being 0.5 in both 1+1 and 2+1 dimensions. Possible
experimental implications of the proposed limited mobility nonequilibrium model
for real interface growth under a surface diffusion bias are critically
discussed.


title:
Microwave transport approach to the coherence of interchain hopping in
  (TMTSF)2PF6

abstract:
  We report a microwave study of the longitudinal and transverse transport
properties of the quasi-one-dimensional organic conductor (TMTSF)$_2$PF$_6$ in
its normal phase. The contactless technique have provided a direct measurement
of the temperature profile of the resistivity along the {\bf b'} direction and
in magnetic fields up to 14 T. A characteristic energy scale ($T_x \sim 40 K$)
has been observed which delimits a transient regime from an insulating to a
metallic behavior. This anomalous profile is discussed in terms of the onset of
coherent transport properties along the {\bf b'} direction below 40 K. This is
also supported by the observation of a finite longitudinal and transverse
magnetoresitances only below 40 K, indicative of a two-dimensional regime.
Below $T_x$, however, strong deviations with respect to a Fermi liquid behavior
are evidenced.


title: The addition spectrum of interacting electrons: Parametric dependence

abstract:
  The addition spectrum of a disordered stadium is studied for up to 120
electrons using the self consistent Hartree-Fock approximation for different
values of the dimensionless conductance and in the presence and absence of a
neutralizing background. In all cases a Gaussian distribution of the addition
spectrum is reached for $r_s \leq 1$. An empirical scaling for the distribution
width in the presence of a neutralizing background is tested and seems to
describe rather well its dependence on the dimensionless conductance.


title:
From Magnons to the Resonance Peak: Spin Dynamics in High-T_C
  Superconducting Cuprates by Inelastic Neutron Scattering

abstract:
  The spin dynamics of high temperature superconductors measured by inelastic
neutron scattering is reviewed. The spin susceptibility evolves a lot with
increasing doping from the undoped insulating state to the overdoped metallic
state. In the superconducting state, a strong magnetic resonance peak occurs in
the spin excitation spectrum whose energy is proportional to the
superconducting temperature. In underdoped and optimally doped regimes, normal
state antiferromagnetic fluctuations are observed, proving the existence of
strong electronic correlations in cuprates. They are characterized by a spin
pseudo-gap. In contrast, these dynamical spin correlations vanish in overdoped
samples.


title:
Predicted giant magnetic moment on non-{n0m} surfaces of d-wave
  superconductors --- Can it be observed and how?

abstract:
  It has been noted previously that the sizable areal density of midgap states
which must exist on any non-{n0m} surface of a d-wave superconductor can lead
to a giant magnetic moment. Here we show that this effect is observable, and
discuss two precise ways to observe it: (i) by directly measuring magnetic
moment in a system with a large density of internal {110} surfaces, or (ii) by
performing spin-polarized tunneling on a {110} surface. In both cases a
sufficiently large magnetic field should be applied in the [1$\bar 1$0]
direction.


title: Diffusion-reaction in thermal growth of silicon oxide films on Si

abstract:
  The thermal growth of silicon oxide films on Si in dry O2 is modelled as a
dynamical system, assuming that it is basically a diffusion-reaction
phenomenon. Relevant findings of the last decade are incorporated, as structure
and composition of the oxide/Si interface and O2 transport and reaction at
initial stages of growth. The present model departs from the well established
Deal and Grove framework (Deal, B.E. and Grove, A. S. General Relationship for
the Thermal Oxidation of Silicon, J. Appl. Phys. 36, 3770-3778 (1965))
indicating that its basic assumptions, steady-state regime and reaction between
O2 and Si at a sharp oxide/Si interface are only attained asymptotically.
Experimental growth kinetics by various authors, obtained for a wide range of
growth parameters are shown to collapse into one single curve when the scaling
properties of this model equations are explored.


title: A self-similar model for shear flows in dense granular materials

abstract:
  We propose a model to describe the quasistatic shearing of dry granular
materials, which notably captures the differences in velocity profiles recently
observed in 2 and 3-D Couette flow experiments. In our scheme, the steady-state
flow is due to the intermittent motion of particle clusters moving together
with the wall. The motion of a cluster is associated with the transient
formation of a fracture inside the sheared pack. The model is based on the
existence of a persistence length for the fractures, which imposes a
self-similar structure on the clusters. Through a probabilistic approach, we
can evaluate the rate of appearance of a cluster of a given size and obtain a
prediction for the average velocity profiles. We also predict the existence of
large stress fluctuations at the moving wall, which characteristics are in good
agreement with experimental data.


title:
Attractive Interactions Between Rod-like Polyelectrolytes: Polarization,
  Crystallization, and Packing

abstract:
  We study the attractive interactions between rod-like charged polymers in
solution that appear in the presence of multi-valence counterions. The
counterions condensed to the rods exhibit both a strong transversal
polarization and a longitudinal crystalline arrangement. At short distances
between the rods, the fraction of condensed counterions increases, and the
majority of these occupy the region between the rods, where they minimize their
repulsive interactions by arranging themselves into packing structures. The
attractive interaction is strongest for multivalent counterions. Our model
takes into account the hard-core volume of the condensed counterions and their
angular distribution around the rods. The hard core constraint strongly
suppresses longitudinal charge fluctuations.


title:
An Efficient Molecular Dynamics Scheme for Predicting Dopant Implant
  Profiles in Semiconductors

abstract:
  We present a highly efficient molecular dynamics scheme for calculating the
concentration profile of dopants implanted in group-IV alloy, and III-V zinc
blende structure materials. Our program incorporates methods for reducing
computational overhead, plus a rare event algorithm to give statistical
accuracy over several orders of magnitude change in the dopant concentration.
  The code uses a molecular dynamics (MD) model, instead of the binary
collision approximation (BCA) used in implant simulators such as TRIM and
Marlowe, to describe ion-target interactions. Atomic interactions are described
by a combination of `many-body' and screened Coulomb potentials. Inelastic
energy loss is accounted for using a Firsov model, and electronic stopping is
described by a Brandt-Kitagawa model which contains the single adjustable
parameter for the entire scheme. Thus, the program is easily extensible to new
ion-target combinations with the minimum of tuning, and is predictive over a
wide range of implant energies and angles.
  The scheme is especially suited for calculating profiles due to low energy,
large angle implants, and for situations where a predictive capability is
required with the minimum of experimental validation. We give examples of using
our code to calculate concentration profiles and 2D `point response' profiles
of dopants in crystalline silicon, silicon-germanium blends, and
gallium-arsenide. We can predict the experimental profile over five orders of
magnitude for <100> and <110> channeling and for non-channeling implants at
energies up to hundreds of keV.


title:
Field dependence of the vortex structure in d-wave and s-wave
  superconductors

abstract:
  We study the vortex structure and its field dependence within the framework
of the quasi-classical Eilenberger theory to find the difference between the
d_{x^2-y^2}- and s-wave pairings. We clarify the effect of the d_{x^2-y^2}-wave
nature and the vortex lattice effect on the vortex structure of the pair
potential, the internal field and the local density of states. The
d_{x^2-y^2}-wave pairing introduces a fourfold-symmetric structure around each
vortex core. With increasing field, their contribution becomes significant to
the whole structure of the vortex lattice state, depending on the vortex
lattice's configuration. It is reflected in the form factor of the internal
field, which may be detected by small angle neutron scattering, or the
resonance line shape of mu-SR and NMR experiments. We also study the induced s-
and d_{xy}-wave components around the vortex in d_{x^2-y^2}-wave
superconductors.


title: Theory of optical conductivity in doped manganites

abstract:
  The frequency and temperature dependence of the optical conductivity of
ferromagnetic manganites is explained within the framework of the bipolaron
theory. As these materials are cooled below the Curie temperature, the colossal
magnetoressitance (CMR) is accompanied by a massive transfer of the spectral
weight of the optical conductivity to lower frequencies. As with the CMR
itself, this change in the optical conductivity is explained by the
dissociation of bipolarons into small polarons by exchange interaction with the
localized Mn spins during the transition to the low temperature ferromagnetic
phase.


title:
Experimental determination of B-T phase diagram of YBa_2Cu_3O_7-d to
  150T for B perpendicular to c

abstract:
  The B-T phase diagram for thin film YBa_2Cu_3O_7-d with B parallel to the
superconducting layers has been constructed from GHz transport measurements to
150T. Evidence for a transition from a high T regime dominated by orbital
effects, to a low T regime where paramagnetic limiting drives the quenching of
superconductivity, is seen. Up to 110T the upper critical field is found to be
linear in T and in remarkable agreement with extrapolation of the longstanding
result of Welp et al arising from magnetisation measurements to 6T. Beyond this
a departure from linear behaviour occurs at T=74K, where a 3D-2D crossover is
expected to occur.


title:
Beyond Gross-Pitaevskii:local density vs. correlated basis approach for
  trapped bosons

abstract:
  We study the ground state of a system of Bose hard-spheres trapped in an
isotropic harmonic potential to investigate the effect of the interatomic
correlations and the accuracy of the Gross-Pitaevskii equation. We compare a
local density approximation, based on the energy functional derived from the
low density expansion of the energy of the uniform hard sphere gas, and a
correlated wave function approach which explicitly introduces the correlations
induced by the potential. Both higher order terms in the low density expansion,
beyond Gross-Pitaevskii, and explicit dynamical correlations have effects of
the order of percent when the number of trapped particles becomes similar to
that attained in recent experiments.


title: A Hartree-Fock Study of Persistent Currents in Disordered Rings

abstract:
  For a system of spinless fermions in a disordered mesoscopic ring,
interactions can give rise to an enhancement of the persistent current by
orders of magnitude. The increase in the current is associated with a charge
reorganization of the ground state. The interaction strength for which this
reorganization takes place is sample-dependent and the log-averages over the
ensemble are not representative. In this paper we demonstrate that the
Hartree-Fock method closely reproduces results obtained by exact
diagonalization. For spinless fermions subject to a short-range Coulomb
repulsion U we show that due to charge reorganization the derivative of the
persistent current is a discontinuous function of U. Having established that
the Hartree-Fock method works well in one dimension, we present corresponding
results for persistent currents in two coupled chains.


title:
Self-organization of vortices in type-II superconductors during magnetic
  relaxation

abstract:
  We revise the applicability of the theory of self-organized criticality (SOC)
to the process of magnetic relaxation in type-II superconductors. The driving
parameter of self-organization of vortices is the energy barrier for flux creep
and not the current density. The power spectrum of the magnetic noise due to
vortex avalanches is calculated and is predicted to vary with time during
relaxation.


title: Effect of disorder on the non-dissipative drag

abstract:
  In this paper we consider the effect of disorder on the non-dissipative
Coulomb drag between two mesoscopic metal rings at zero temperature. Ring 1 has
an Aharonov-Bohm flux present which creates a persistent current J_0. Ring 2
interacts with ring 1 via the Coulomb potential and a drag current, J_d is
produced. We show that this drag current persists with finite disorder in each
ring, and that for small disorder, J_d decreases with the square of the
disorder amplitude.


title: Kinetic theory of shot noise in non-degenerate diffusive conductors

abstract:
  We investigate current fluctuations in non-degenerate semiconductors, on
length scales intermediate between the elastic and inelastic mean free paths.
We present an exact solution of the non-linear kinetic equations in the regime
of space-charge limited conduction, without resorting to the drift
approximation of previous work. By including the effects of a finite voltage
and carrier density in the contact region, a quantitative agreement is obtained
with Monte Carlo simulations by Gonz{\'a}lez et al., for a model of an
energy-independent elastic scattering rate. The shot-noise power $P$ is
suppressed below the Poisson value $P_{Poisson}=2e\bar I$ (at mean current
$\bar I$) by the Coulomb repulsion of the carriers. The exact suppression
factor is close to 1/3 in a three-dimensional system, in agreement with the
simulations and with the drift approximation. Including an energy dependence of
the scattering rate has a small effect on the suppression factor for the case
of short-range scattering by uncharged impurities or quasi-elastic scattering
by acoustic phonons. Long-range scattering by charged impurities remains an
open problem.


title: Suppression of Giant Magnetoresistance by a superconducting contact

abstract:
  We predict that current perpendicular to the plane (CPP) giant
magnetoresistance (GMR) in a phase-coherent magnetic multilayer is suppressed
when one of the contacts is superconducting. This is a consequence of a
superconductivity-induced magneto-resistive (SMR) effect, whereby the
conductance of the ferromagnetically aligned state is drastically reduced by
superconductivity. To demonstrate this effect, we compute the GMR ratio of
clean (Cu/Co)_nCu and (Cu/Co)_nPb multilayers, described by an ab-initio spd
tight binding Hamiltonian. By analyzing a simpler model with two orbitals per
site, we also show that the suppression survives in the presence of elastic
scattering by impurities.


title: Can we Measure Superflow on Quenching He4?

abstract:
  Zurek has provided a simple picture for the onset of the lamda-transition in
He4, not currently supported by vortex density experiments. However, we argue
that the seemingly similar argument by Zurek that superflow in an annulus of
He4 at the quench will be measurable is still valid.


title:
Photoemission Studies on Bi2Sr2CaCuZnO - Electronic Structure Evolution
  and Temperature Dependence

abstract:
  An angle resolved photoelectron spectroscopy study was conducted on
BiSrCaCuZnO. A small amount of Zn substitution for Cu almost completely
suppresses the otherwise sharp spectral peak along the (0,0) to (pi,pi)
direction in BiSrCaCuZnO, while superconductivity with Tc as high as 83K
survives. This behavior contrasts markedly from that seen in cases where the
impurities are located off the CuO plane, as well as when the CuO planes are
underdoped. This effect is also accompanied by changes of low energy
excitations at (pi,0), near the anti-node position of the d-wave pairing state.
With Zn doping the size of the superconducting gap is significantly suppressed,
the width of the quasiparticle peak in the superconducting state becomes wider,
and the dip at higher binding energy is diminished. In addition, enhanced
temperature induced spectral changes also occur. We show intriguing systematic
lineshape changes with temperature that persist to a very high energy scale - a
result consistent with the idea that Zn enhances the local charge
inhomogeneity.


title:
Acoustic radiation controls friction: Evidence from a spring-block
  experiment

abstract:
  Brittle failures of materials and earthquakes generate acoustic/seismic waves
which lead to radiation damping feedbacks that should be introduced in the
dynamical equations of crack motion. We present direct experimental evidence of
the importance of this feedback on the acoustic noise spectrum of
well-controlled spring-block sliding experiments performed on a variety of
smooth surfaces. The full noise spectrum is quantitatively explained by a
simple noisy harmonic oscillator equation with a radiation damping force
proportional to the derivative of the acceleration, added to a standard viscous
term.


title: Nature's Way of Optimizing

abstract:
  We propose a general-purpose method for finding high-quality solutions to
hard optimization problems, inspired by self-organizing processes often found
in nature. The method, called Extremal Optimization, successively eliminates
extremely undesirable components of sub-optimal solutions. Drawing upon models
used to simulate far-from-equilibrium dynamics, it complements approximation
methods inspired by equilibrium statistical physics, such as Simulated
Annealing. With only one adjustable parameter, its performance proves
competitive with, and often superior to, more elaborate stochastic optimization
procedures. We demonstrate it here on two classic hard optimization problems:
graph partitioning and the traveling salesman problem.

categories: cond-mat.stat-mech adap-org math.OC nlin.AO


title:
The Entropy Production Fluctuation Theorem and the Nonequilibrium Work
  Relation for Free Energy Differences

abstract:
  There are only a very few known relations in statistical dynamics that are
valid for systems driven arbitrarily far-from-equilibrium. One of these is the
fluctuation theorem, which places conditions on the entropy production
probability distribution of nonequilibrium systems. Another recently discovered
far-from-equilibrium expression relates nonequilibrium measurements of the work
done on a system to equilibrium free energy differences. In this paper, we
derive a generalized version of the fluctuation theorem for stochastic,
microscopically reversible dynamics. Invoking this generalized theorem provides
a succinct proof of the nonequilibrium work relation.


title: Extremal Optimization of Graph Partitioning at the Percolation Threshold

abstract:
  The benefits of a recently proposed method to approximate hard optimization
problems are demonstrated on the graph partitioning problem. The performance of
this new method, called Extremal Optimization, is compared to Simulated
Annealing in extensive numerical simulations. While generally a complex
(NP-hard) problem, the optimization of the graph partitions is particularly
difficult for sparse graphs with average connectivities near the percolation
threshold. At this threshold, the relative error of Simulated Annealing for
large graphs is found to diverge relative to Extremal Optimization at equalized
runtime. On the other hand, Extremal Optimization, based on the extremal
dynamics of self-organized critical systems, reproduces known results about
optimal partitions at this critical point quite well.


title: Zn Impurities in BiSrCa(CuZn)O II - Temperature Induced Spectral Change

abstract: The contents of this paper were absorbed into cond-mat/9901349.


title:
The correlation length of the Heisenberg antiferromagnet with arbitrary
  spin S

abstract:
  Experiments and numerical data on the correlation length for large S disagree
strongly with the theoretical prediction based on the effective field theory
prescription of the magnon physics. The reason is that for large S, at any
accessible correlation lengths, the cut-off effects from the non-magnon scales
become large and can not be treated by an effective field theory. We study
these effects in a spin-wave expansion. The corrected prediction connects the
renormalized classical and the classical scaling regions smoothly and comes
close to the data.


title: Large Deviation Function of the Partially Asymmetric Exclusion Process

abstract:
  The large deviation function obtained recently by Derrida and Lebowitz for
the totally asymmetric exclusion process is generalized to the partially
asymmetric case in the scaling limit. The asymmetry parameter rescales the
scaling variable in a simple way. The finite-size corrections to the universal
scaling function and the universal cumulant ratio are also obtained to the
leading order.


title: Spin and chiral orderings of frustrated quantum spin chains

abstract:
  Ordering of frustrated S=1/2 and 1 XY and Heisenberg spin chains with the
competing nearest- and next-nearest-neighbor antiferromagnetic couplings is
studied by exact diagonalization and density-matrix renormalization-group
methods. It is found that the S=1 XY chain exhibits both gapless and gapped
`chiral' phases characterized by the spontaneous breaking of parity, in which
the long-range order parameter is a chirality, $\kappa_i =
S_i^xS_{i+1}^y-S_i^yS_{i+1}^x$, whereas the spin correlation decays either
algebraically or exponentially. Such chiral phases are not realized in the
S=1/2 XY chain nor in the Heisenberg chains.


title: The beta-relaxation dynamics of a simple liquid

abstract:
  We present a detailed analysis of the beta-relaxation dynamics of a simple
glass former, a Lennard-Jones system with a stochastic dynamics. By testing the
various predictions of mode-coupling theory, including the recently proposed
corrections to the asymptotic scaling laws, we come to the conclusion that in
this time regime the dynamics is described very well by this theory.


title: Residence Time Distribution for a Class of Gaussian Markov Processes

abstract:
  We study the distribution of residence time or equivalently that of ``mean
magnetization" for a family of Gaussian Markov processes indexed by a positive
parameter $\alpha$. The persistence exponent for these processes is simply
given by $\theta=\alpha$ but the residence time distribution is nontrivial. The
shape of this distribution undergoes a qualitative change as $\theta$
increases, indicating a sharp change in the ergodic properties of the process.
We develop two alternate methods to calculate exactly but recursively the
moments of the distribution for arbitrary $\alpha$. For some special values of
$\alpha$, we obtain closed form expressions of the distribution function.


title:
Size-dependent Correlation Effects in Ultrafast Optical Dynamics of
  Metal Nanoparticles

abstract:
  We study the role of collective surface excitations in the electron
relaxation in small metal particles. We show that the dynamically screened
electron-electron interaction in a nanoparticle contains a size-dependent
correction induced by the surface. This leads to new channels of quasiparticle
scattering accompanied by the emission of surface collective excitations. We
calculate the energy and temperature dependence of the corresponding rates,
which depend strongly on the nanoparticle size. We show that the
surface-plasmon-mediated scattering rate of a conduction electron increases
with energy, in contrast to that mediated by a bulk plasmon. In noble-metal
particles, we find that the dipole collective excitations (surface plasmons)
mediate a resonant scattering of d-holes to the conduction band. We study the
role of the latter effect in the ultrafast optical dynamics of small
nanoparticles and show that, with decreasing nanoparticle size, it leads to a
drastic change in the differential absorption lineshape and a strong frequency
dependence of the relaxation near the surface plasmon resonance. The
experimental implications of our results in ultrafast pump-probe spectroscopy
are also discussed.


title: Laser induced freezing of charge stabilized colloidal system

abstract:
  We present results from an extensive simulational study of the modulated
liquid $\longleftrightarrow$ crystal transition in a 2-d charge-stabilized
colloid subject to a 1-d laser field modulation commensurate with the
crystalline phase. {\em {Contrary to some earlier simulational and experimental
findings we do not find any reentrant liquid phase in our simulation.
Furthermore the transition remains first-order (albeit weak, with a fairly
large correlation length) even in the limit of infinite field, contrary to
mean-field predictions}}. In the modulated liquid phase, while the
translational order decays exponentially, the bond orientational order is
actually long ranged.


title:
Integrable Magnetic Model of Two Chains Coupled by Four-Body
  Interactions

abstract:
  An exact solution for an XXZ chain with four-body interactions is obtained
and its phase diagram is determined. The model can be reduced to two chains
coupled by four-body interactions, and it is shown that the ground state of the
two-chain model is magnetized in part. Furthermore, a twisted four-body
correlation function of the anti-ferromagnetic Heisenberg chain is obtained.


title:
Frequency Dependent Conductance of a Tunnel Junction in the
  Semiclassical Limit

abstract:
  The linear conductance of the a small metallic tunnel junction embedded in an
electromagnetic environment of arbitrary impedance is determined in the
semiclassical limit. Electron tunneling is treated beyond the orthodox theory
of Coulomb blockade phenomena by means of a nonperturbative path integral
approach. The frequency dependent conductance is obtained from Kubo's formula.
The theoretical predictions are valid for high temperatures and/or for large
tunneling conductance and are found to explain recent experimental data.


title: Electron-lattice kinetics of metals heated by ultrashort laser pulses

abstract:
  We propose a kinetic model of transient nonequilibrium phenomena in metals
exposed to ultrashort laser pulses when heated electrons affect the lattice
through direct electron-phonon interaction. This model describes the
destruction of a metal under intense laser pumping. We derive the system of
equations for the metal, which consists of hot electrons and a cold lattice.
Hot electrons are described with the help of the Boltzmann equation and
equation of thermoconductivity. We use the equations of motion for lattice
displacements with the electron force included. The lattice deformation is
estimated immediately after the laser pulse up to the time of electron
temperature relaxation. An estimate shows that the ablation regime can be
achieved.


title:
Ising pyrochlore magnets: Low temperature properties, ice rules and
  beyond

abstract:
  Pyrochlore magnets are candidates for spin-ice behavior. We present
theoretical simulations of relevance for the pyrochlore family R2Ti2O7 (R= rare
earth) supported by magnetothermal measurements on selected systems. By
considering long ranged dipole-dipole as well as short-ranged superexchange
interactions we get three distinct behaviours: (i) an ordered doubly degenerate
state, (ii) a highly disordered state with a broad transition to paramagnetism,
(iii) a partially ordered state with a sharp transition to paramagnetism. Thus
these competing interactions can induce behaviour very different from
conventional ``spin ice''. Closely corresponding behaviour is seen in the real
compounds---in particular Ho2Ti2O7 corresponds to case (iii) which has not been
discussed before, rather than (ii) as suggested earlier.


title: Cortical Potential Distributions and Cognitive Information Processing

abstract:
  The use of cortical field potentials rather than the details of spike trains
as the basis for cognitive information processing is proposed. This results in
a space of cognitive elements with natural metrics. Sets of spike trains may
also be considered to be points in a multidimensional metric space. The
closeness of sets of spike trains in such a space implies the closeness of
points in the resulting function space of potential distributions.

categories: cond-mat.dis-nn adap-org cond-mat.stat-mech cs.NE math-ph math.MP nlin.AO physics.bio-ph q-bio


title: Slave fermion theory of confinement in strongly anisotropic systems

abstract:
  We present a mean field treatment of a strongly correlated model of electrons
in a three-dimensional anisotropic system. The mass of the bare electrons is
larger in one spatial direction (the c-axis direction), than in the other two
(the ab-planes). We use a slave fermion decomposition of the electronic degrees
of freedom and show that there is a transition from a deconfined to a confined
phase in which there is no coherent band formation along the c-axis.


title:
Competition between zero bias anomaly and proximity effect in disordered
  systems

abstract:
  We investigate the suppression of the proximity effect in mesoscopic normal
metal/superconductor systems induced by Coulomb interaction. We identify and
elucidate the mechanism by which disorder leads to an amplification of this
effect. In particular, for strong enough disorder, the proximity effect is
shown to vanish. An expression for the tunneling density of states is obtained,
and experimental applications are discussed.


title:
The spectral conductance of a proximity superconductor and the
  re-entrance effect

abstract:
  In a mesoscopic metal in proximity with a superconductor, the electronic
conductance is enhanced in a very energy-sensitive way. In this paper, we
discuss the spectral conductance of a proximity superconductor from both the
theoretical and experimental point of view. The dependence of the spectral
conductance on the phase-breaking length, gap of the superconductor and
interface transparency is theoretically investigated. We present experimental
data on the re-entrance of the normal-state conductance at very low temperature
and bias voltage. A complete description of the experimental data needs taking
into account heating of the reservoirs by the bias current. In addition, we
show that the energy sensitivity of the proximity effect enables one to access
the energy distribution of the conduction electrons inside a mesoscopic sample.


title: Hubbard Fermi surface in the doped paramagnetic insulator

abstract:
  We study the electronic structure of the doped paramagnetic insulator by
finite temperature Quantum Monte-Carlo simulations for the 2D Hubbard model.
Throughout we use the moderately high temperature T=0.33t, where the spin
correlation length has dropped to < 1.5 lattice spacings, and study the
evolution of the band structure with hole doping. The effect of doping can be
best described as a rigid shift of the chemical potential into the lower
Hubbard band, accompanied by some transfer of spectral weight. For hole dopings
<20% the Luttinger theorem is violated, and the Fermi surface volume, inferred
from the Fermi level crossings of the `quasiparticle band', shows a similar
topology and doping dependence as predicted by the Hubbard I and related
approximations.