Dynamical Behavior of the Dissipative Two-State System

We investigate the dynamical correlation function of a quantum-mechanical two-state system which is coupled to a bosonic heat bath, utilizing the equivalence between the spin-boson Hamiltonian and the 1/r^2 Ising model. The imaginary-time correlation function is calculated by Monte-Carlo simulations on the Ising system and then continued to real time by a Pade approximation. In the unbiased system, the transition from oscillatory to strongly damped behavior is found to occur at a coupling strength close to alpha = 1/2. The biased system favors coherent relaxation and displays a significantly larger crossover value alpha_c. We introduce the quasiparticle picture to describe the relevant behavior at intermediate time scales. Within this approximation, we map out phase diagrams for the unbiased and biased systems.Comment: RevTeX, 11 pages, 13 figures include

Multiparticle ring exchange in the Wigner glass and its possible relevance to strongly interacting two-dimensional electron systems in the presence of disorder

We consider a two-dimensional electron or hole system at zero temperature and low carrier densities, where the long-range Coulomb interactions dominate over the kinetic energy. In this limit the clean system will form a Wigner crystal. Non-trivial quantum mechanical corrections to the classical ground state lead to multiparticle exchange processes that can be expressed as an effective spin Hamiltonian involving competing interactions. Disorder will destroy the Wigner crystal on large length scales, and the resulting state is called a Wigner glass. The notion of multiparticle exchange processes is still applicable in the Wigner glass, but the exchange frequencies now follow a random distribution. We compute the exchange frequencies for a large number of relevant exchange processes in the Wigner crystal, and the frequency distributions for some important processes in the Wigner glass. The resulting effective low energy spin Hamiltonian should be the starting point of an analysis of the possible ground state phases and quantum phase transitions between them. We find that disorder plays a crucial role and speculate on a possible zero temperature phase diagram.Comment: 17 pages and 12 figure

Performance evaluation of ductless personalized ventilation in comparison with desk fans using numerical simulations

The performance of ductless personalized ventilation (DPV) was compared to the performance of a typical desk fan since they are both stand-alone systems that allow the users to personalize their indoor environment. The two systems were evaluated using a validated computational fluid dynamics (CFD) model of an office room occupied by two users. To investigate the impact of DPV and the fan on the inhaled air quality, two types of contamination sources were modelled in the domain: an active source and a passive source. Additionally, the influence of the compared systems on thermal comfort was assessed using the coupling of CFD with the comfort model developed by the University of California, Berkeley (UCB model). Results indicated that DPV performed generally better than the desk fan. It provided better thermal comfort and showed a superior performance in removing the exhaled contaminants. However, the desk fan performed better in removing the contaminants emitted from a passive source near the floor level. This indicates that the performance of DPV and desk fans depends highly on the location of the contamination source. Moreover, the simulations showed that both systems increased the spread of exhaled contamination when used by the source occupant

Screening of persistent currents in mesoscopic metal rings

The effect of the Coulomb-interaction on persistent currents in disordered mesoscopic metal rings threaded by a magnetic flux $\phi$ is studied numerically. We use the simplest form of ``self-consistent'' Hartree theory, where the spatial variations of the self-consistent Hartree potential are ignored. In this approximation the self-consistent Hartree energies are simply obtained by diagonalizing the non-interacting system via the Lanczos method and then calculating the (disorder-dependent) particle number on the ring self-consistently. In the diffusive regime we find that the variance of the total particle number is strongly reduced, in agreement with the prediction of the random-phase approximation. On the other hand, the variance of the number of energy levels in a small interval below the Fermi energy is not affected by the Coulomb interaction.Comment: RevTex, 5 figures, accepted for publication in Z. Phys.

Spectral correlations in disordered mesoscopic metals and their relevance for persistent currents

We use the Lanczos method to calculate the variance of the number of energy levels in an energy window of width E below the Fermi energy for non-interacting disordered electrons on a thin three-dimensional ring threaded by an Aharonov-Bohm flux . We find that for small E the flux-dependent part of the variance is well described by a well-known Feynman diagram involving two Cooperons. However, this result cannot be extrapolated to energies E where the energy-dependence of the average density of states becomes significant. We discuss consequences for persistent currents.Comment: minor modifications in the text, accepted for publication in Mod. Phys. Lett.