20 research outputs found
Superfluid and supersolid phases of lattice bosons with ring-exchange interaction
We examine the superfluid phase of a hard-core boson model with
nearest-neighbor exchange J and four-particle ring-exchange K at half-filling
on the square lattice. At zero temperature we find that the superfluid in the
pure-J model is quickly destroyed by the inclusion of negative-K ring-exchange
interactions, favoring a state with a (pi,pi) ordering wavevector. Minimization
of the mean-field energy suggests that a supersolid state with coexisting
superfluidity, charge-density wave, and valence-bond-like order is formed. We
also study the behavior of the finite-T Kosterlitz-Thouless phase transition in
the superfluid phase, by forcing the Nelson-Kosterlitz universal jump condition
on the finite-T spin wave superfluid density. Away from the pure J point,
T_{KT} decreases rapidly for negative K, while for positive K, T_{KT} reaches a
maximum at some K \neq 0 in agreement with recent quantum Monte Carlo
simulations.Comment: 7 pages, 5 figure
Putting competing orders in their place near the Mott transition
We describe the localization transition of superfluids on two-dimensional
lattices into commensurate Mott insulators with average particle density p/q
(p, q relatively prime integers) per lattice site. For bosons on the square
lattice, we argue that the superfluid has at least q degenerate species of
vortices which transform under a projective representation of the square
lattice space group (a PSG). The formation of a single vortex condensate
produces the Mott insulator, which is required by the PSG to have density wave
order at wavelengths of q/n lattice sites (n integer) along the principle axes;
such a second-order transition is forbidden in the Landau-Ginzburg-Wilson
framework. We also discuss the superfluid-insulator transition in the direct
boson representation, and find that an interpretation of the quantum
criticality in terms of deconfined fractionalized bosons is only permitted at
special values of q for which a permutative representation of the PSG exists.
We argue (and demonstrate in detail in a companion paper: L. Balents et al.,
cond-mat/0409470) that our results apply essentially unchanged to electronic
systems with short-range pairing, with the PSG determined by the particle
density of Cooper pairs. We also describe the effect of static impurities in
the superfluid: the impurities locally break the degeneracy between the q
vortex species, and this induces density wave order near each vortex. We
suggest that such a theory offers an appealing rationale for the local density
of states modulations observed by Hoffman et al. (cond-mat/0201348) in STM
studies of the vortex lattice of BSCCO, and allows a unified description of the
nucleation of density wave order in zero and finite magnetic fields. We note
signatures of our theory that may be tested by future STM experiments.Comment: 35 pages, 16 figures; (v2) part II is cond-mat/0409470; (v3) added
new appendix and clarifying remarks; (v4) corrected typo
Collective transport in bilayer quantum Hall systems
Filling factor incompressible states in ideal bilayer quantum Hall
systems have spontaneous interlayer phase coherence and can be regarded either
as easy-plane pseudospin ferromagnets or as condensates of excitons formed from
electrons in one layer and holes in the other layer. In this paper we discuss
efforts to achieve an understanding of the two different types of transport
measurements (which we refer to as drag and tunneling experiments respectively)
that have been carried out in bilayer quantum Hall systems by the group of Jim
Eisenstein at the California Institute of Technology. In a drag experiment,
current is sent through one of the two-layers and the voltage drop is measured
in the other layer. We will argue that the finding of these experiments that
the voltage drop in the drag layer is different from that in the the drive
layer, is an experimental proof that these bilayers do not have
quasi-long-range excitonic order. The property that at the longitudinal
drag voltage increases from near zero when spontaneous coherence is initially
established, then falls back toward zero as it becomes well established, can be
explained as a competition between the broken symmetry and the gap to which it
gives rise. In the tunneling experiment, current is injected in one layer and
removed from the other layer. The absence of quasi-long-range order likely
explains the relatively small tunneling conductance per area found in the these
measurements.Comment: 6 pages, 3 figures, EP2DS-03 Conference Proceeding