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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 + 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 + 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.

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The Dzyaloshinski-Moriya (DM) anisotropic superexchange constant and the resulting electronparamagnetic-resonance (EPR) linewidth in Mn-based II-VI-compound diluted magnetic semiconductors {DMS) such as Cd 1 Mn Te are calculated quantitatively.An Anderson Hamiltonian, developed in a previous study of isotropic superexchange, describing correlated Mn 3d states hybridized with semiconducting sand p-derived levels, is generalized to include the anion spin-orbit coupling responsible for anisotropic superexchange. DM exchange is shown to be the dominant anisotropic interaction, with magnitude -Sgo of isotropic superexchange. The EPR line shape is calculated with use of a moment expansion of the magnetic response function to first order in inverse temperature together with a maximum-entropy ansatz. The calculated infinite-temperature linewidths are in good agreement with extrapolated experimental values. A novel fit of the theoretical temperature dependence to existing experimental linewidth data provides the first empirical value for the anisotropic exchange constant, in excellent agreement with the theoretical value. Calculated chemical trends for the exchange constants yield the experimentally expected linewidth trends.