6,718 research outputs found
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
Phase separation in the bosonic Hubbard model with ring exchange
We show that soft core bosons in two dimensions with a ring exchange term
exhibit a tendency for phase separation. This observation suggests that the
thermodynamic stability of normal bose liquid phases driven by ring exchange
should be carefully examined.Comment: 4 pages, 6 figure
Quantum phases of mixtures of atoms and molecules on optical lattices
We investigate the phase diagram of a two-species Bose-Hubbard model
including a conversion term, by which two particles from the first species can
be converted into one particle of the second species, and vice-versa. The model
can be related to ultra-cold atom experiments in which a Feshbach resonance
produces long-lived bound states viewed as diatomic molecules. The model is
solved exactly by means of Quantum Monte Carlo simulations. We show than an
"inversion of population" occurs, depending on the parameters, where the second
species becomes more numerous than the first species. The model also exhibits
an exotic incompressible "Super-Mott" phase where the particles from both
species can flow with signs of superfluidity, but without global supercurrent.
We present two phase diagrams, one in the (chemical potential, conversion)
plane, the other in the (chemical potential, detuning) plane.Comment: 7 pages, 10 figure
Ring Exchange and Phase Separation in the Two-dimensional Boson Hubbard model
We present Quantum Monte Carlo simulations of the soft-core bosonic Hubbard
model with a ring exchange term K. For values of K which exceed roughly half
the on-site repulsion U, the density is a non-monotonic function of the
chemical potential, indicating that the system has a tendency to phase
separate. This behavior is confirmed by an examination of the density-density
structure factor and real space images of the boson configurations. Adding a
near-neighbor repulsion can compete with phase separation, but still does not
give rise to a stable normal Bose liquid.Comment: 12 pages, 23 figure
Feshbach-Einstein condensates
We investigate the phase diagram of a two-species Bose-Hubbard model
describing atoms and molecules on a lattice, interacting via a Feshbach
resonance. We identify a region where the system exhibits an exotic super-Mott
phase and regions with phases characterized by atomic and/or molecular
condensates. Our approach is based on a recently developed exact quantum Monte
Carlo algorithm: the Stochastic Green Function algorithm with tunable
directionality. We confirm some of the results predicted by mean-field studies,
but we also find disagreement with these studies. In particular, we find a
phase with an atomic but no molecular condensate, which is missing in all
mean-field phase diagrams.Comment: 4 pages, 6 figure
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
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