4 research outputs found
Mott insulators in strong electric fields
Recent experiments on ultracold atomic gases in an optical lattice potential
have produced a Mott insulating state of Rb atoms. This state is stable to a
small applied potential gradient (an `electric' field), but a resonant response
was observed when the potential energy drop per lattice spacing (E), was close
to the repulsive interaction energy (U) between two atoms in the same lattice
potential well. We identify all states which are resonantly coupled to the Mott
insulator for E close to U via an infinitesimal tunneling amplitude between
neighboring potential wells. The strong correlation between these states is
described by an effective Hamiltonian for the resonant subspace. This
Hamiltonian exhibits quantum phase transitions associated with an Ising density
wave order, and with the appearance of superfluidity in the directions
transverse to the electric field. We suggest that the observed resonant
response is related to these transitions, and propose experiments to directly
detect the order parameters. The generalizations to electric fields applied in
different directions, and to a variety of lattices, should allow study of
numerous other correlated quantum phases.Comment: 17 pages, 14 figures; (v2) minor additions and new reference
Nature of the quantum phase transitions in the two-dimensional hardcore boson model
We use two Quantum Monte Carlo algorithms to map out the phase diagram of the
two-dimensional hardcore boson Hubbard model with near () and next near
() neighbor repulsion. At half filling we find three phases: Superfluid
(SF), checkerboard solid and striped solid depending on the relative values of
, and the kinetic energy. Doping away from half filling, the
checkerboard solid undergoes phase separation: The superfluid and solid phases
co-exist but not as a single thermodynamic phase. As a function of doping, the
transition from the checkerboard solid is therefore first order. In contrast,
doping the striped solid away from half filling instead produces a striped
supersolid phase: Co-existence of density order with superfluidity as a single
phase. One surprising result is that the entire line of transitions between the
SF and checkerboard solid phases at half filling appears to exhibit dynamical
O(3) symmetry restoration. The transitions appear to be in the same
universality class as the special Heisenberg point even though this symmetry is
explicitly broken by the interaction.Comment: 10 pages, 14 eps figures, include
Directed Loop Updates for Quantum Lattice Models
This article outlines how the quantum Monte Carlo directed loop update
recently introduced can be applied to a wide class of quantum lattice models.
Several models are considered: Spin-S XXZ models with longitudinal and
transverse magnetic fields, boson models with two-body interactions, and 1D
spinful fermion models. Expressions are given for the parameter regimes were
very efficient "no-bounce" quantum Monte Carlo algorithms can be found.Comment: 18 pages, 19 figure