Green Function Monte Carlo with Stochastic Reconfiguration

A new method for the stabilization of the sign problem in the Green Function Monte Carlo technique is proposed. The method is devised for real lattice Hamiltonians and is based on an iterative ''stochastic reconfiguration'' scheme which introduces some bias but allows a stable simulation with constant sign. The systematic reduction of this bias is in principle possible. The method is applied to the frustrated J1-J2 Heisenberg model, and tested against exact diagonalization data. Evidence of a finite spin gap for J2/J1 >~ 0.4 is found in the thermodynamic limit.Comment: 13 pages, RevTeX + 3 encapsulated postscript figure

Quantum simulations of the superfluid-insulator transition for two-dimensional, disordered, hard-core bosons

We introduce two novel quantum Monte Carlo methods and employ them to study the superfluid-insulator transition in a two-dimensional system of hard-core bosons. One of the methods is appropriate for zero temperature and is based upon Green's function Monte Carlo; the other is a finite-temperature world-line cluster algorithm. In each case we find that the dynamical exponent is consistent with the theoretical prediction of $z=2$ by Fisher and co-workers.Comment: Revtex, 10 pages, 3 figures (postscript files attached at end, separated by %%%%%% Fig # %%%%%, where # is 1-3). LA-UR-94-270

The thermal conductivity reduction in HgTe/CdTe superlattices

The techniques used previously to calculate the three-fold thermal conductivity reduction due to phonon dispersion in GaAs/AlAs superlattices (SLs) are applied to HgTe/CdTe SLs. The reduction factor is approximately the same, indicating that this SL may be applicable both as a photodetector and a thermoelectric cooler.Comment: 5 pages, 2 figures; to be published in Journal of Applied Physic

A self-consistent perturbative evaluation of ground state energies: application to cohesive energies of spin lattices

The work presents a simple formalism which proposes an estimate of the ground state energy from a single reference function. It is based on a perturbative expansion but leads to non linear coupled equations. It can be viewed as well as a modified coupled cluster formulation. Applied to a series of spin lattices governed by model Hamiltonians the method leads to simple analytic solutions. The so-calculated cohesive energies are surprisingly accurate. Two examples illustrate its applicability to locate phase transition.Comment: Accepted by Phys. Rev.

ADHD Differences on the Stanford Binet Intelligence Scale, Fifth Edition

Attention-deficit/hyperactivity disorder (ADHD) is a common psychiatric diagnosis in childhood that requires a level of attention or hyperactivity that falls short of the expected developmental level. Past research shows cognitive discrepancies in ADHD populations with verbal deficiencies observed primarily in tasks that require a combined auditory and verbal component. Working memory has been a long acknowledged deficit in persons with ADHD. This research examines cognitive differences among children with ADHD on working memory and other components of the Stanford Binet, 5th edition (SB5). Stanford Binet verbal and nonverbal working memory was hypothesized to be different for the ADHD sample compared to controls and between ADHD subtypes. Participants were gathered from the Stanford Binet standardization sample that were diagnosed with ADHD and matched with a group of normal controls. Data was analyzed using ANOVA followed by a cluster analysis of discrepancies found at subtest and testlet levels. Due to matching and statistical control, results showed no differences in FSIQ, VIQ, or PIQ between normals and those with ADHD, but those with ADHD took an average of 20 minutes longer to complete the SB5, consistently showed greater response variability, and exhibited significant differential item functioning for Vocabulary, Object Series/Matrices, and the routing scales. Deficits in working memory appear to account for these differences

Radiation studies for GaAs in the ATLAS Inner Detector

We estimate the hardness factors and the equivalent 1 MeV neutron fluences for hadrons fluences expected at the GaAs positions wheels in the ATLAS Inner Detector. On this basis the degradation of the GaAs particle detectors made from different substrates as a function of years LHC operation is predicted.Comment: 11 pages, 6 Postscript figures, uses elsart.cls, submitted to Nucl. Inst. and Met

Center of mass and relative motion in time dependent density functional theory

It is shown that the exchange-correlation part of the action functional $A_{xc}[\rho (\vec r,t)]$ in time-dependent density functional theory , where $\rho (\vec r,t)$ is the time-dependent density, is invariant under the transformation to an accelerated frame of reference $\rho (\vec r,t) \to \rho ' (\vec r,t) = \rho (\vec r + \vec x (t),t)$, where $\vec x (t)$ is an arbitrary function of time. This invariance implies that the exchange-correlation potential in the Kohn-Sham equation transforms in the following manner: $V_{xc}[\rho '; \vec r, t] = V_{xc}[\rho; \vec r + \vec x (t),t]$. Some of the approximate formulas that have been proposed for $V_{xc}$ satisfy this exact transformation property, others do not. Those which transform in the correct manner automatically satisfy the ``harmonic potential theorem", i.e. the separation of the center of mass motion for a system of interacting particles in the presence of a harmonic external potential. A general method to generate functionals which possess the correct symmetry is proposed

Density-functionals not based on the electron gas: Local-density approximation for a Luttinger liquid

By shifting the reference system for the local-density approximation (LDA) from the electron gas to other model systems one obtains a new class of density functionals, which by design account for the correlations present in the chosen reference system. This strategy is illustrated by constructing an explicit LDA for the one-dimensional Hubbard model. While the traditional {\it ab initio} LDA is based on a Fermi liquid (the electron gas), this one is based on a Luttinger liquid. First applications to inhomogeneous Hubbard models, including one containing a localized impurity, are reported.Comment: 4 pages, 4 figures (final version, contains additional applications and discussion; accepted by Phys. Rev. Lett.