204 research outputs found
Superfluid-Superfluid Phase Transitions in Two-Component Bose System
Depending on the Hamiltonian parameters, two-component bosons in an optical
lattice can form at least three different superfluid phases in which both
components participate in the superflow: a (strongly interacting) mixture of
two miscible superfluids (2SF), a paired superfluid vacuum (PSF), and (at a
commensurate total filling factor) the super-counter-fluid state (SCF). We
study universal properties of the 2SF-PSF and 2SF-SCF quantum phase transitions
and show that (i) they can be mapped onto each other, and (ii) their
universality class is identical to the (d+1)-dimensional normal-superfluid
transition in a single-component liquid. Finite-temperature 2SF-PSF(SCF)
transitions and the topological properties of 2SF-PSF(SCF) interfaces are also
discussed.Comment: 4pages, 2 figures, REVTe
The Stochastic Green Function (SGF) algorithm
We present the Stochastic Green Function (SGF) algorithm designed for bosons
on lattices. This new quantum Monte Carlo algorithm is independent of the
dimension of the system, works in continuous imaginary time, and is exact (no
error beyond statistical errors). Hamiltonians with several species of bosons
(and one-dimensional Bose-Fermi Hamiltonians) can be easily simulated. Some
important features of the algorithm are that it works in the canonical ensemble
and gives access to n-body Green functions.Comment: 12 pages, 5 figure
Revealing Superfluid--Mott-Insulator Transition in an Optical Lattice
We study (by an exact numerical scheme) the single-particle density matrix of
ultracold atoms in an optical lattice with a parabolic confining
potential. Our simulation is directly relevant to the interpretation and
further development of the recent pioneering experiment by Greiner et al. In
particular, we show that restructuring of the spatial distribution of the
superfluid component when a domain of Mott-insulator phase appears in the
system, results in a fine structure of the particle momentum distribution. This
feature may be used to locate the point of the superfluid--Mott-insulator
transition.Comment: 4 pages (12 figures), Latex. (A Latex macro is corrected
From Popov-Fedotov trick to universal fermionization
We show that Popov-Fedotov trick of mapping spin-1/2 lattice systems on
two-component fermions with imaginary chemical potential readily generalizes to
bosons with a fixed (but not limited) maximal site occupation number, as well
as to fermionic Hamiltonians with various constraints on the site Fock states.
In a general case, the mapping---fermionization---is on multi-component
fermions with many-body non-Hermitian interactions. Additionally, the
fermionization approach allows one to convert large many-body couplings into
single-particle energies, rendering the diagrammatic series free of large
expansion parameters; the latter is essential for the efficiency and
convergence of the diagrammatic Monte Carlo method.Comment: 4 pages, no figures (v2 contains some improvements; the most
important one is the generic complex chemical potential trick for
spins/bosons
Frustrated spin model as a hard-sphere liquid
We show that one-dimensional topological objects (kinks) are natural degrees
of freedom for an antiferromagnetic Ising model on a triangular lattice. Its
ground states and the coexistence of spin ordering with an extensive
zero-temperature entropy can be easily understood in terms of kinks forming a
hard-sphere liquid. Using this picture we explain effects of quantum spin
dynamics on that frustrated model, which we also study numerically.Comment: 5 pages, 3 figure
Absence of a Direct Superfluid to Mott Insulator Transition in Disordered Bose Systems
We prove the absence of a direct quantum phase transition between a
superfluid and a Mott insulator in a bosonic system with generic, bounded
disorder. We also prove compressibility of the system on the
superfluid--insulator critical line and in its neighborhood. These conclusions
follow from a general {\it theorem of inclusions} which states that for any
transition in a disordered system one can always find rare regions of the
competing phase on either side of the transition line. Quantum Monte Carlo
simulations for the disordered Bose-Hubbard model show an even stronger result,
important for the nature of the Mott insulator to Bose glass phase transition:
The critical disorder bound, , corresponding to the onset of
disorder-induced superfluidity, satisfies the relation , with the half-width of the Mott gap in the pure system.Comment: 4 pages, 3 figures; replaced with resubmitted versio
Vortex-Phonon Interaction in the Kosterlitz-Thouless Theory
The "canonical" variables of the Kosterlitz-Thouless theory--fields
and , generally believed to stand for vortices
and phonons (or their XY equivalents, like spin waves, etc.) turn out to be
neither vortices and phonons, nor, strictly speaking, {\it canonical}
variables. The latter fact explains paradoxes of (i) absence of interaction
between and , and (ii) non-physical contribution of small vortex
pairs to long-range phase correlations. We resolve the paradoxes by explicitly
relating and to canonical vortex-pair and phonon variables.Comment: 4 pages, RevTe
Berezinskii-Kosterlitz-Thouless transition in two-dimensional dipole systems
The superfluid to normal fluid transition of dipolar bosons in two dimensions
is studied throughout the whole density range using path integral Monte Carlo
simulations and summarized in the phase diagram. While at low densities, we
find good agreement with the universal results depending only on the scattering
length , at moderate and high densities, the transition temperature is
strongly affected by interactions and the elementary excitation spectrum. The
results are expected to be of relevance to dipolar atomic and molecular systems
and indirect excitons in quantum wells
Superfluid--Insulator Transition in Commensurate One-Dimensional Bosonic System with Off-Diagonal Disorder
We study the nature of the superfluid--insulator quantum phase transition in
a one-dimensional system of lattice bosons with off-diagonal disorder in the
limit of large integer filling factor. Monte Carlo simulations of two strongly
disordered models show that the universality class of the transition in
question is the same as that of the superfluid--Mott-insulator transition in a
pure system. This result can be explained by disorder self-averaging in the
superfluid phase and applicability of the standard quantum hydrodynamic action.
We also formulate the necessary conditions which should be satisfied by the
stong-randomness universality class, if one exists.Comment: 4 pages, 4 figures. Typo in figure 4 of ver. 3 is correcte
Superfluid--Insulator Transition in Commensurate Disordered Bosonic Systems:Large-Scale Worm-Algorithm Simulations
We report results of large-scale Monte Carlo simulations of
superfluid--insulator transitions in commensurate 2D bosonic systems. In the
case of off-diagonal disorder (quantum percolation), we find that the
transition is to a gapless incompressible insulator, and its dynamical critical
exponent is . In the case of diagonal disorder, we prove the
conjecture that rare statistical fluctuations are inseparable from critical
fluctuations on the largest scales and ultimately result in the crossover to
the generic universality class (apparently with ). However, even at strong
disorder, the universal behavior sets in only at very large space-time
distances. This explains why previous studies of smaller clusters mimicked a
direct superfluid--Mott-insulator transition.Comment: 6 pages, Latex, 7 figure
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