378 research outputs found
Supersolids in the Bose-Hubbard Hamiltonian
We use a combination of numeric and analytic techniques to determine the
groun d state phase diagram of the Bose--Hubbard Hamiltonian with longer range
repulsi ve interactions. At half filling one finds superfluidity and an
insulating solid phase. Depending on the relative sizes of near--neighbor and
next near--neighbor interactions, this solid either follows a checkerboard or a
striped pattern. In neither case is there a coexistence with superfluidity.
However upon doping ``supersolid'' phases appear with simultaneous diagonal and
off--diagonal long range order.Comment: 11 pages, Revtex 3.0, 6 figures (upon request
Ordered states in the disordered Hubbard model
The Hubbard model is studied in which disorder is introduced by putting the
on-site interaction to zero on a fraction f of (impurity) sites of a square
lattice. Using Quantum Monte Carlo methods and Dynamical Mean Field theory we
find that antiferromagnetic long-range order is initially enhanced at
half-filling and stabilized off half-filling by the disorder. The Mott-Hubbard
charge gap of the pure system is broken up into two pieces by the disorder: one
incompressible state remains at average density n=1 and another can be seen
slightly below n=1+f. Qualitative explanations are provided.Comment: 17 pages, including 8 figures. Paper for Festschrift in honor of Hans
van Leeuwen's 65th birthda
Density of States and Critical Behavior of the Coulomb Glass
We present zero-temperature simulations for the single-particle density of
states of the Coulomb glass. Our results in three dimensions are consistent
with the Efros and Shklovskii prediction for the density of states.
Finite-temperature Monte Carlo simulations show no sign of a thermodynamic
glass transition down to low temperatures, in disagreement with mean-field
theory. Furthermore, the random-displacement formulation of the model undergoes
a transition into a distorted Wigner crystal for a surprisingly broad range of
the disorder strength.Comment: 4 pages, 2 figures, 1 tabl
Reversal-field memory in magnetic hysteresis
We report results demonstrating a singularity in the hysteresis of magnetic
materials, the reversal-field memory effect. This effect creates a
nonanalyticity in the magnetization curves at a particular point related to the
history of the sample. The microscopic origin of the effect is associated with
a local spin-reversal symmetry of the underlying Hamiltonian. We show that the
presence or absence of reversal-field memory distinguishes two widely studied
models of spin glasses (random magnets).Comment: 3 pages, 5 figures. Proceedings of "2002 MMM Conferece", Tampa, F
Phase Diagram of the Holstein-Hubbard Two-Leg Ladder
Using a functional renormalization group method, we obtain the phase diagram
of the two-leg ladder system within the Holstein-Hubbard model, which includes
both electron-electron and electron-phonon interactions. Our renormalization
group technique allows us to analyze the problem for both weak and strong
electron-phonon coupling. We show that, in contrast results from conventional
weak coupling studies, electron-phonon interactions can dominate
electron-electron interactions because of retardation effects.Comment: 4 page
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