Density of states determined from Monte Carlo simulations

We describe method for calculating the density of states by combining several canonical monte carlo runs. We discuss how critical properties reveal themselves in $g(\epsilon)$ and demonstrate this by applying the method several different phase transitions. We also demonstrate how this can used to calculate the conformal charge, where the dominating numerical method has traditionally been transfer matrix.Comment: Major revision of paper, several references added throughout. Current version accepted for publication in Phys. Rev.

Band structure of SnTe studied by Photoemission Spectroscopy

We present an angle-resolved photoemission spectroscopy study of the electronic structure of SnTe, and compare the experimental results to ab initio band structure calculations as well as a simplified tight-binding model of the p-bands. Our study reveals the conjectured complex Fermi surface structure near the L-points showing topological changes in the bands from disconnected pockets, to open tubes, and then to cuboids as the binding energy increases, resolving lingering issues about the electronic structure. The chemical potential at the crystal surface is found to be 0.5eV below the gap, corresponding to a carrier density of p =1.14x10^{21} cm^{-3} or 7.2x10^{-2} holes per unit cell. At a temperature below the cubic-rhombohedral structural transition a small shift in spectral energy of the valance band is found, in agreement with model predictions.Comment: 4 figure

Pairing Correlations in the Two-Dimensional Hubbard Model

We present the results of a quantum Monte Carlo study of the extended $s$ and the $d_{x^2-y^2}$ pairing correlation functions for the two-dimensional Hubbard model, computed with the constrained-path method. For small lattice sizes and weak interactions, we find that the $d_{x^2-y^2}$ pairing correlations are stronger than the extended $s$ pairing correlations and are positive when the pair separation exceeds several lattice constants. As the system size or the interaction strength increases, the magnitude of the long-range part of both correlation functions vanishes.Comment: 4 pages, RevTex, 4 figures included; submitted to Phys. Rev. Let

NMR relaxation rates for the spin-1/2 Heisenberg chain

The spin-lattice relaxation rate $1/T_1$ and the spin echo decay rate $1/T_{2G}$ for the spin-$1\over 2$ antiferromagnetic Heisenberg chain are calculated using quantum Monte Carlo and maximum entropy analytic continuation. The results are compared with recent analytical calculations by Sachdev. If the nuclear hyperfine form factor $A_q$ is strongly peaked around $q=\pi$ the predicted low-temperature behavior [$1/T_1 \sim \ln{^{1/2}(1/T)}$, $1/T_{2G} \sim \ln{^{1/2}(1/T)}/\sqrt{T}$] extends up to temperatures as high as $T/J \approx 0.5$. If $A_q$ has significant weight for $q \approx 0$ there are large contributions from diffusive long-wavelength processes not taken into account in the theory, and very low temperatures are needed in order to observe the asymptotic $T \to 0$ forms.Comment: 9 pages, Revtex 3.0, 5 uuencoded ps figures To appear in Phys. Rev. B, Rapid Com

Unveiling Order behind Complexity: Coexistence of Ferromagnetism and Bose-Einstein Condensation

We present an algebraic framework for identifying the order parameter and the possible phases of quantum systems that is based on identifying the local dimension $N$ of the quantum operators and using the SU(N) group representing the generators of generalized spin-particle mappings. We illustrate this for $N$=3 by presenting for any spatial dimension the exact solution of the bilinear-biquadratic $S$=1 quantum Heisenberg model at a high symmetry point. Through this solution we rigorously show that itinerant ferromagnetism and Bose-Einstein condensation may coexist.Comment: 5 pages, 1 psfigur

Bose-Einstein Condensation at a Helium Surface

Path Integral Monte Carlo was used to calculate the Bose-Einstein condensate fraction at the surface of a helium film at $T=0.77 K$, as a function of density. Moving from the center of the slab to the surface, the condensate fraction was found to initially increase with decreasing density to a maximum value of 0.9 before decreasing. Long wavelength density correlations were observed in the static structure factor at the surface of the slab. Finally, a surface dispersion relation was calculated from imaginary-time density-density correlations.Comment: 8 pages, 5 figure

Conductance through Quantum Dots Studied by Finite Temperature DMRG

With the Finite temperature Density Matrix Renormalization Group method (FT-DMRG), we depeloped a method to calculate thermo-dynamical quantities and the conductance of a quantum dot system. Conductance is written by the local density of states on the dot. The density of states is calculated with the numerical analytic continuation from the thermal Green's function which is obtained directly from the FT-DMRG. Typical Kondo behaviors in the quantum dot system are observed conveniently by comparing the conductance with the magnetic and charge susceptibilities: Coulomb oscillation peaks and the unitarity limit. We discuss advantage of this method compared with others.Comment: 14 pages, 13 fiure

Evidence for the double degeneracy of the ground-state in the 3D ±J\pm J spin glass

A bivariate version of the multicanonical Monte Carlo method and its application to the simulation of the three-dimensional $\pm J$ Ising spin glass are described. We found the autocorrelation time associated with this particular multicanonical method was approximately proportional to the system volume, which is a great improvement over previous methods applied to spin-glass simulations. The principal advantage of this version of the multicanonical method, however, was its ability to access information predictive of low-temperature behavior. At low temperatures we found results on the three-dimensional $\pm J$ Ising spin glass consistent with a double degeneracy of the ground-state: the order-parameter distribution function $P(q)$ converged to two delta-function peaks and the Binder parameter approached unity as the system size was increased. With the same density of states used to compute these properties at low temperature, we found their behavior changing as the temperature is increased towards the spin glass transition temperature. Just below this temperature, the behavior is consistent with the standard mean-field picture that has an infinitely degenerate ground state. Using the concept of zero-energy droplets, we also discuss the structure of the ground-state degeneracy. The size distribution of the zero-energy droplets was found to produce the two delta-function peaks of $P(q)$.Comment: 33 pages with 31 eps figures include

Spin dynamics of SrCu2_2O3_3 and the Heisenberg ladder

The $S=1/2$ Heisenberg antiferromagnet in the ladder geometry is studied as a model for the spin degrees of freedom of SrCu$_2$O$_3$. The susceptibility and the spin echo decay rate are calculated using a quantum Monte Carlo technique, and the spin-lattice relaxation rate is obtained by maximum entropy analytic continuation of imaginary time correlation functions. All calculated quantities are in reasonable agreement with experimental results for SrCu$_2$O$_3$ if the exchange coupling $J \approx 850$K, i.e. significantly smaller than in high-T$_c$ cuprates.Comment: 11 pages (Revtex) + 3 uuencoded ps files. To appear in Phys. Rev. B, Rapid Com

One-dimensional models of disordered quantum wires: general formalism

In this work we describe, compile and generalize a set of tools that can be used to analyse the electronic properties (distribution of states, nature of states, ...) of one-dimensional disordered compositions of potentials. In particular, we derive an ensemble of universal functional equations which characterize the thermodynamic limit of all one-dimensional models and which only depend formally on the distributions that define the disorder. The equations are useful to obtain relevant quantities of the system such as density of states or localization length in the thermodynamic limit