195 research outputs found
Mott insulators and correlated superfluids in ultracold Bose-Fermi mixtures
We study the effects of interaction between bosons and fermions in a
Bose-Fermi mixtures loaded in an optical lattice. We concentrate on the
destruction of a bosonic Mott phase driven by repulsive interaction between
bosons and fermions. Once the Mott phase is destroyed, the system enters a
superfluid phase where the movements of bosons and fermions are correlated. We
show that this phase has simultaneously correlations reminiscent of a
conventional superfluid and of a pseudo-spin density wave order
Drift without flux: Brownian walker with a space dependent diffusion coefficient
Space dependent diffusion of micrometer sized particles has been directly
observed using digital video microscopy. The particles were trapped between two
nearly parallel walls making their confinement position dependent.
Consequently, not only did we measure a diffusion coefficient which depended on
the particles' position, but also report and explain a new effect: a drift of
the particles' individual positions in the direction of the diffusion
coefficient gradient, in the absence of any external force or concentration
gradient.Comment: 4 pages, 4 ps figures, include
Interacting spin-1 bosons in a two-dimensional optical lattice
We study, using quantum Monte Carlo (QMC) simulations, the ground state
properties of spin-1 bosons trapped in a square optical lattice. The phase
diagram is characterized by the mobility of the particles (Mott insulating or
superfluid phase) and by their magnetic properties. For ferromagnetic on-site
interactions, the whole phase diagram is ferromagnetic and the Mott
insulators-superfluid phase transitions are second order. For antiferromagnetic
on-site interactions, spin nematic order is found in the odd Mott lobes and in
the superfluid phase. Furthermore, the superfluid-insulator phase transition is
first or second order depending on whether the density in the Mott is even or
odd. Inside the even Mott lobes, we observe a singlet-to-nematic transition for
certain values of the interactions. This transition appears to be first order
Tolerance and Sensitivity in the Fuse Network
We show that depending on the disorder, a small noise added to the threshold
distribution of the fuse network may or may not completely change the
subsequent breakdown process. When the threshold distribution has a lower
cutoff at a finite value and a power law dependence towards large thresholds
with an exponent which is less than , the network is not sensitive
to the added noise, otherwise it is. The transition between sensitivity or not
appears to be second order, and is related to a localization-delocalization
transition earlier observed in such systems.Comment: 12 pages, 3 figures available upon request, plain Te
Current Distribution in the Three-Dimensional Random Resistor Network at the Percolation Threshold
We study the multifractal properties of the current distribution of the
three-dimensional random resistor network at the percolation threshold. For
lattices ranging in size from to we measure the second, fourth and
sixth moments of the current distribution, finding {\it e.g.\/} that
where is the conductivity exponent and is the
correlation length exponent.Comment: 10 pages, latex, 8 figures in separate uuencoded fil
Attractive Hubbard Model on a Honeycomb Lattice
We study the attractive fermionic Hubbard model on a honeycomb lattice using
determinantal quantum Monte Carlo simulations. By increasing the interaction
strength U (relative to the hopping parameter t) at half-filling and zero
temperature, the system undergoes a quantum phase transition at 5.0 < U_c/t <
5.1 from a semi-metal to a phase displaying simultaneously superfluid behavior
and density order. Doping away from half-filling, and increasing the
interaction strength at finite but low temperature T, the system always appears
to be a superfluid exhibiting a crossover between a BCS and a molecular regime.
These different regimes are analyzed by studying the spectral function. The
formation of pairs and the emergence of phase coherence throughout the sample
are studied as U is increased and T is lowered
Self-Affinity in the Gradient Percolation Problem
We study the scaling properties of the solid-on-solid front of the infinite
cluster in two-dimensional gradient percolation. We show that such an object is
self affine with a Hurst exponent equal to 2/3 up to a cutoff-length
proportional to the gradient to the power (-4/7). Beyond this length scale, the
front position has the character of uncorrelated noise. Importantly, the
self-affine behavior is robust even after removing local jumps of the front.
The previously observed multi affinity, is due to the dominance of overhangs at
small distances in the structure function. This is a crossover effect.Comment: 4 pages, 4 figure
Ultracold Bosonic Atoms in Disordered Optical Superlattices
The influence of disorder on ultracold atomic Bose gases in quasiperiodic
optical lattices is discussed in the framework of the one-dimensional
Bose-Hubbard model. It is shown that simple periodic modulations of the well
depths generate a rich phase diagram consisting of superfluid, Mott insulator,
Bose-glass and Anderson localized phases. The detailed evolution of mean
occupation numbers and number fluctuations as function of modulation amplitude
and interaction strength is discussed. Finally, the signatures of the different
phases, especially of the Bose-glass phase, in matter-wave interference
experiments are investigated.Comment: 4 pages, 4 figures, using REVTEX
Bethe-Peierls Approximation for the 2D Random Ising Model
The partition function of the 2d Ising model with random nearest neighbor
coupling is expressed in the dual lattice made of square plaquettes. The dual
model is solved in the the mean field and in different types of Bethe-Peierls
approximations, using the replica method.Comment: Plane TeX file, 21 pages, 5 figures available under request to
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