35 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
Supersolid phase of hardcore bosons on triangular lattice
We establish the nature of the supersolid phase observed for hardcore bosons
on the triangular lattice near half-integer filling factor, and study the phase
diagram of the system at finite temperature. We find that the solid order is
always of the (2m,-m',-m') with m changing discontinuously from positive to
negative values at half-filling, contrary to predictions of other phases, based
on an analogy with the properties of Ising spins in transverse magnetic field.
At finite temperature we find two intersecting second-order transition lines,
one in the 3-state Potts universality class and the other of the
Kosterlitz-Thouless type
Superglass Phase of Helium-four
We study different solid phases of Helium-four, by means of Path Integral
Monte Carlo simulations based on a recently developed "worm" algorithm. Our
study includes simulations that start off from a high-T gas phase, which is
then "quenched" down to T=0.2 K. The low-T properties of the system crucially
depend on the initial state. While an ideal hcp crystal is a clear-cut
insulator, the disordered system freezes into a "superglass", i.e., a
metastable amorphous solid featuring off-diagonal long-range order and
superfluidity
Commensurate Two-Component Bosons in Optical Lattice: Groundstate Phase Diagram
Two sorts of bosons in an optical lattice at commensurate filling factors can
form five stable superfluid and insulating groundstates with rich and
non-trivial phase diagram. The structure of the groundstate diagram is
established by mapping -dimensional quantum system onto a
-dimensional classical loop-current model and Monte Carlo simulations of
the latter. Surprisingly, the quantum phase diagram features, besides
second-order lines, a first-order transition and two multi-critical points. We
explain why first-order transitions are generic for models with paring
interactions using microscopic and mean-field arguments.Comment: 4 RevTex pages, 3 ps-figures; replaced with revised version accepted
by PRL: results of the MC simulations in 4D are briefly discusse
Worm Algorithm for Continuous-space Path Integral Monte Carlo Simulations
We present a new approach to path integral Monte Carlo (PIMC) simulations
based on the worm algorithm, originally developed for lattice models and
extended here to continuous-space many-body systems. The scheme allows for
efficient computation of thermodynamic properties, including winding numbers
and off-diagonal correlations, for systems of much greater size than that
accessible to conventional PIMC. As an illustrative application of the method,
we simulate the superfluid transition of Helium-four in two dimensions.Comment: Fig. 2 differs from that of published version (includes data for
larger system sizes
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
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
High Precision Measurement of the Thermal Exponent for the Three-Dimensional XY Universality Class
Simulations results are reported for critical point of the two-component
field theory. The correlation length exponent is measured to high
precision with the result . This value is in agreement with
recent simulation results [Campostrini \textit{et al}., Phys. Rev. B
\textbf{63}, 214503 (2001)], and marginally agrees with the most recent
space-based measurements of the superfluid transition in He [Lipa
\textit{et al}., Phys. Rev. B \textbf{68}, 174518 (2003)].Comment: a reference adde
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
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