The zero-temperature universal conductivity of two-dimensional (2D) films at the superconductorinsulator transition is studied. The existence of a finite conductivity at T = 0 and the universality class for this transition is discussed. Neglecting disorder as a first approximation, so the transition is from a commensurate Mott-Hubbard insulator to a superconductor, we calculate analytically the universal conductivity for the 2D pure boson Hubbard model up to the first order in a large-N expansion and numerically by Monte Carlo simulation of the (2+1)-D XY model. From the Monte Carlo results we find the universal conductivity to be cr' = (0.285 6 0.02)o'q, where o'& = Rg = h/(2e) 6.45 kA. An analysis in one dimension suggests that in the presence of disorder, the universal conductivity in films might be somewhat smaller than this value. The possible existence of universal dissipation in He films is also discussed brieAy.

Universal properties of the zero temperature superconductor-insulator transition in two-dimensional amorphous films are studied by extensive Monte Carlo simulations of bosons in a disordered medium. We report results for both short-range and long-range Coulomb interactions for several different points in parameter space. In all cases we observe a transition from a superconducting phase to an insulating Bose glass phase. From finite-size scaling of our Monte Carlo data we determine the universal conductivity σ * and the critical exponents at the transition. The result σ * = (0.55 ± 0.06)(2e) 2 /h for bosons with long-range Coulomb interaction is roughly consistent with experiments reported so far. We also find σ * = (0.14± 0.03)(2e) 2 /h for bosons with short-range interactions.PACS numbers: 74.65.+n, 74.70.Mq, 74.75.+t Typeset using REVT E X From the work of Abrahams, Anderson, Licciardello, and Ramakrishnan 1 it is known that no true metallic behavior can be observed for non-interacting electrons at T = 0 in two dimensions, since all states will be localized by arbitrarily weak disorder. When repulsive interactions are turned on the situation is less clear but the general belief 2 is that a metallic phase still should be absent at T = 0 in the presence of disorder, although we know of no rigorous proof of this. However, in the presence of attractive interactions, a superconducting phase is expected 3 , both at T = 0 and finite T , even for a finite amount of disorder, because disorder is irrelevant 4 at the finite temperature transition, which is of the Kosterlitz-Thouless 5 type discussed below. The onset of superconductivity at T = 0 is presumed then, in d = 2, to be directly from the insulating phase with no intervening metallic phase. One should therefore in principle be able to observe a direct insulatorsuperconductor transition at zero temperature in two dimensions as a function of disorder and/or interaction strength. The main topic of this paper is to analyze such a transition and extract its universal features.Dimensionality and divergent length scales play an important role in continuous phase transitions. The diverging correlation length scale implies that many microscopic details are irrelevant. Furthermore physical quantities containing dimensions of length to some non-zero power typically diverge or vanish at the critical point. Two dimensions is special in that the conductivity contains no length scale units, i.e. the conductance per square is the same as the conductivity. Hence, right at the T = 0 quantum critical point, the conductivity is not only finite and nonzero but also universal 6,7 , even though it is zero in the insulating phase and infinite in the superconducting phase. This view differs from that of previous work 8 which parameterized the transition in terms of the normal state resistivity. The calculation of this universal conductivity is one of the main goals of the present paper. A short account on some of our results has already been published 9 .A schematic phase diagram is sh...

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The possibility of realizing a directed current for a quantum particle in a flashing asymmetric potential is investigated. It is found that quantum resonances, where the value of the effective Planck constant is equal to an integer or half-integer multiple of pi, give rise to a directed current. The effect should be readily observable in experiments.