Computer experiments have been performed to study the single-particle density of states, g (E), in the Coulomb gap. This gap occurs in disordered insulating systems as an effect of the long-range tail of the Coulomb interaction. In order to compare these numerical results with the analytical theory on a broader energy scale than previous authors, very large samples have been considered, up to 40000 and 125000 sites for dimensions d =2 and 3, respectively. Special algorithms have been developed for this aim. Our numerical results contradict the analytical theory from the literature in two main points: As E~p, the numerical values for g(E) are considerably smaller than the analytical predictions, and universality with respect to disorder is not present. It could not be decided finally whether or not g(E @} follows a power law, g (E)-~E -p~". Provided it does, the simulation results, v= 1.2+0. 1 and 2.6+0.2 for d =2 and 3, respectively, deviate from the analytical prediction, v=d -1. Moreover, as a first approach to the polaronic transport problem, the inhuence of relaxation down to stability with respect to all simultaneous two-electron hops is studied. In this case, g(E) is diminished, but not changed qualitatively. In particular, the exponential behavior analytically predicted is not observed.

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We study the metal-insulator transition in two sets of amorphous Si 1−x Ni x films. The sets were prepared by different, electron-beam-evaporation-based technologies: evaporation of the alloy, and gradient deposition from separate Ni and Si crucibles. The characterization included electron and scanning tunneling microscopy, glow discharge optical emission spectroscopy, energy dispersive X-ray analysis, and Rutherford back scattering. Investigating the logarithmic temperature derivative of the conductivity, w = d ln σ/d ln T , we observe that, for insulating samples, w(T ) shows a minimum, increasing at both low and high T . Both the minimum value of w and the corresponding temperature seem to tend to zero as the transition is approached. The analysis of this feature of w(T, x) leads to the conclusion that the transition in Si 1−x Ni x is very likely discontinuous at zero temperature in agreement with Mott's original views. 71.30.+h,71.23.Cq, Typeset using REVT E X

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An optimization algorithm is presented which consists of cyclically heating and quenching by Metropolis and local search procedures, respectively. It works particularly well when it is applied to an archive of samples instead of to a single one. We demonstrate for the traveling salesman problem that this algorithm is far more efficient than the usual simulated annealing; our implementation can compete concerning speed with recent, very fast genetic local search algorithms. [S0031-9007(97)