We investigate the phase structure of pure compact U(1) lattice gauge theory in 4 dimensions with the Wilson action supplemented by a monopole term. To overcome the suppression of transitions between the phases in the simulations we make the monopole coupling a dynamical variable. We determine the phase diagram and find that the strength of the first order transition decreases with increasing weight of the monopole term, the transition thus ultimately getting of second order. After outlining the appropriate topological characterization of networks of currents lines, we present an analysis of the occurring monopole currents which shows that the phases are related to topological properties.Comment: 22 pages (latex), 14 figures (available upon request), BU-HEP 94-

After developing an appropriate iteration procedure for the determination of the parameters, the method of simulated tempering has been successfully applied to the 2D Ising spin glass. The reduction of the slowing down is comparable to that of the multicanonical algorithm. Simulated tempering has, however, the advantages to allow full vectorization of the programs and to provide the canonical ensemble directly.

We report on a numerical reinvestigation of the Aoki phase in lattice QCD with two flavors of Wilson fermions where the parity-flavor symmetry is spontaneously broken. For this purpose an explicitly symmetry-breaking source term hψiγ5τ 3 ψ was added to the fermion action. The order parameter ψ iγ5τ 3 ψ was computed with the hybrid Monte Carlo algorithm at several values of (β, κ, h) on lattices of sizes 4 4 to 12 4 and extrapolated to h = 0. The existence of a parity-flavor breaking phase can be confirmed at β = 4.0 and 4.3, while we do not find parity-flavor breaking at β = 4.6 and 5.0.

We present a detailed study of the properties of the phase transition in the four-dimensional compact U(1) lattice gauge theory supplemented by a monopole term, for values of the monopole coupling λ such that the transition is of second order. By a finite size analysis we show that at λ = 0.9 the critical exponent is already characteristic of a second-order transition. Moreover, we find that this exponent is definitely different from the one of the Gaussian case. We further observe that the monopole density becomes approximately constant in the second-order region. Finally we reveal the unexpected phenomenon that the phase transition persists up to very large values of λ, where the transition moves to (large) negative β.

Resonance fluorescence of recoil-free y-rays from atomic nuclei dcywnd.s on the state of the chemical bonding of the atoms. The latter influences ihr position of' the energy level of the atomic nuclei, so that the energy required f ;~ exciiulion yf' resonance fluorescence is somewhat smaller or greater than the energy ofthe y-quanta r m i t t d by the radiation source. In order to obtain resonance fluorescence, therejorc, the radiation source and absorber have to be kept moving relative to each other. I f the inlensity yf thc rrsonance fluorescence is plotted akgainst this relative velocity, Miissbauer spectra are ohtainrd. The positions of the lines in these spectra make it possible to draw conclusions about the nature of the atom's bonds. The efficiency of this method is demonstrated by illustration with numerous iron compoundr. It was shown, jbr instance, that thc iron atoilis in "insoluble Prussian B k " have well-defined oxidation levels. "Turnbitll's Bhie'' shows ihe identical spectrum. A structure could be ascribed to lri-iron dodccacarhonyl that also cxplains the diama