arXiv subjects

R. T. Scalettar

Publications and source records attributed to R. T. Scalettar.

133 records · Page 8Linked to original sources

arXiv1995

Mean Field Theory of the Localization Transition

A mean field theory of the localization transition for bosonic systems is developed. Localization is shown to be sensitive to the distribution of the random site energies. It occurs in the presence of a triangular distribution, but not a uniform one. The inverse participation ratio, the single site Green's function, the superfluid order parameter and the corresponding susceptibility are calculated, and the appropriate exponents determined. All of these quantities indicate the presence of a new phase, which can be identified as the {\it Bose-glass}.

cond-mat↗

arXiv1994

Simultaneous Diagonal and Off Diagonal Order in the Bose--Hubbard Hamiltonian

The Bose-Hubbard model exhibits a rich phase diagram consisting both of insulating regimes where diagonal long range (solid) order dominates as well as conducting regimes where off diagonal long range order (superfluidity) is present. In this paper we describe the results of Quantum Monte Carlo calculations of the phase diagram, both for the hard and soft core cases, with a particular focus on the possibility of simultaneous superfluid and solid order. We also discuss the appearance of phase separation in the model. The simulations are compared with analytic calculations of the phase diagram and spin wave dispersion.

cond-mat↗

arXiv1994

Competition Between Antiferromagnetic Order and Spin-Liquid Behavior in the Two-Dimensional Periodic Anderson Model at Half-Filling

We study the two-dimensional periodic Anderson model at half-filling using quantum Monte Carlo (QMC) techniques. The ground state undergoes a magnetic order-disorder transition as a function of the effective exchange coupling between the conduction and localized bands. Low-lying spin and charge excitations are determined using the maximum entropy method to analytically continue the QMC data. At finite temperature we find a competition between the Kondo effect and antiferromagnetic order which develops in the localized band through Ruderman-Kittel-Kasuya-Yosida interactions.

cond-mat↗

arXiv1994

Competition between point and columnar disorder on the behavior of flux lines in 1+1 dimension

The behavior of flux lines in the presence of both columnar and point disorder in 1+1 dimension is investigated using Renormalization Group and world-line Quantum Monte Carlo techniques. In particular, we calculate the transverse wandering correlation function for a single boson and recover known results for point and columnar disorder separately. We then examine the existence of a localization transition of a flux line in the simultaneous presence of both types of disorder. We also perform RG for interacting flux lines in the presence of both disorder. RG indicates that the vortex glass is unstable with respect to arbitrary small amount of columnar disorder. Using QMC we find that the Bose glass transition temperature is reduced by the point disorder, in agreement with recent RG calculations. Further we find that the region posited to be an Anderson glass is completely destroyed.

cond-mat↗

arXiv1994

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.

cond-mat↗

arXiv1993

The Bose--Hubbard Model and Superfluid Staircases in $^{4}$He Films

The recent experimental observation of step-like structure in the superfluid density of $^{4}$He films on graphite is interpreted in terms of passage close to Mott insulating lobes in the Bose--Hubbard model. Plateaus develop in the superfluid density near the completion of each layer because of the He-He repulsion. We modify the Bose--Hubbard Hamiltonian, using a density-dependent He-He interaction term to model the He-substrate potential. Quantum Monte Carlo simulations yield results which agree well with the experimental data.

cond-mat↗

arXiv1993

Universal Conductivity in the Two dimensional Boson Hubbard Model

We use Quantum Monte Carlo to evaluate the conductivity $σ$ of the 2--dimensional disordered boson Hubbard model at the superfluid-bose glass phase boundary. At the critical point for particle density $ρ=0.5$, we find $σ_{c}=(0.45 \pm 0.07) σ_{Q}$, where $σ_{Q}= e_{*}^{2} / h$ from a finite size scaling analysis of the superfluid density. We obtain $σ_{c}=(0.47 \pm 0.08) σ_{Q}$ from a direct calculation of the current--current correlation function. Simulations at the critical points for other particle densities, $ρ=0.75$ and $1.0$, give similar values for $σ$. We discuss possible origins of the difference in this value from that recently obtained by other numerical approaches.

cond-mat↗