The Upper Critical Field in Disordered Two-Dimensional Superconductors

We present calculations of the upper critical field in superconducting films as a function of increasing disorder (as measured by the normal state resistance per square). In contradiction to previous work, we find that there is no anomalous low-temperature positive curvature in the upper critical field as disorder is increased. We show that the previous prediction of this effect is due to an unjustified analytical approximation of sums occuring in the perturbative calculation. Our treatment includes both a careful analysis of first-order perturbation theory, and a non-perturbative resummation technique. No anomalous curvature is found in either case. We present our results in graphical form.Comment: 11 pages, 8 figure

Coulomb Zero-Bias Anomaly: A Semiclassical Calculation

Effective action is proposed for the problem of Coulomb blocking of tunneling. The approach is well suited to deal with the ``strong coupling'' situation near zero bias, where perturbation theory diverges. By a semiclassical treatment, we reduce the physics to that of electrodynamics in imaginary time, and express the anomaly through exact conductivity of the system $\sigma(\omega, q)$ and exact interaction. For the diffusive anomaly, we compare the result with the perturbation theory of Altshuler, Aronov, and Lee. For the metal-insulator transition we derive exact relation of the anomaly and critical exponent of conductivity.Comment: 9 pages, RevTeX 3.

Magnetic Flux Periodic Response of Nano-perforated Ultrathin Superconducting Films

We have patterned a hexagonal array of nano-scale holes into a series of ultrathin, superconducting Bi/Sb films with transition temperatures 2.65 K $<T_{co} <$5 K. These regular perforations give the films a phase-sensitive periodic response to an applied magnetic field. By measuring this response in their resistive transitions, $R(T)$, we are able to distinguish regimes in which fluctuations of the amplitude, both the amplitude and phase, and the phase of the superconducting order parameter dominate the transport. The portion of $R(T)$ dominated by amplitude fluctuations is larger in lower $T_{co}$ films and thus, grows with proximity to the superconductor to insulator transition.Comment: Revised title, abstract, text, figure

Driven diffusive system with non-local perturbations

We investigate the impact of non-local perturbations on driven diffusive systems. Two different problems are considered here. In one case, we introduce a non-local particle conservation along the direction of the drive and in another case, we incorporate a long-range temporal correlation in the noise present in the equation of motion. The effect of these perturbations on the anisotropy exponent or on the scaling of the two-point correlation function is studied using renormalization group analysis.Comment: 11 pages, 2 figure

Evolution of the Density of States Gap in a Disordered Superconductor

It has only recently been possible to study the superconducting state in the attractive Hubbard Hamiltonian via a direct observation of the formation of a gap in the density of states N(w). Here we determine the effect of random chemical potentials on N(w) and show that at weak coupling, disorder closes the gap concurrently with the destruction of superconductivity. At larger, but still intermediate coupling, a pseudo-gap in N(w) remains even well beyond the point at which off-diagonal long range order vanishes. This change in the elementary excitations of the insulating phase corresponds to a crossover between Fermi- and Bose-Insulators. These calculations represent the first computation of the density of states in a finite dimensional disordered fermion model via the Quantum Monte Carlo and maximum entropy methods.Comment: 4 pages, 4 figure

The Superconductor-Insulator Transition in a Tunable Dissipative Environment

We study the influence of a tunable dissipative environment on the dynamics of Josephson junction arrays near the superconductor-insulator transition. The experimental realization of the environment is a two dimensional electron gas coupled capacitively to the array. This setup allows for the well-controlled tuning of the dissipation by changing the resistance of the two dimensional electron gas. The capacitive coupling cuts off the dissipation at low frequencies. We determine the phase diagram and calculate the temperature and dissipation dependence of the array conductivity. We find good agreement with recent experimental results.Comment: 4 pages, 4 .eps figures, revte

Suppression of \bbox{T_c} in superconducting amorphous wires

The suppression of the mean field temperature of the superconducting transition, $T_c$, in homogeneous amorphous wires is studied. We develop a theory that gives $T_c$ in situations when the dynamically enhanced Coulomb repulsion competes with the contact attraction. The theory accurately describes recent experiments on $T_c$--suppression in superconducting wires, after a procedure that minimizes the role of nonuniversal mechanisms influencing $T_c$ is applied.Comment: RevTeX, 4 pages, 3 figure

Generation of polarization-entangled photon pairs in a Bragg reflection waveguide

We demonstrate experimentally that spontaneous parametric down-conversion in an AlGaAs semiconductor Bragg reflection waveguide can make for paired photons highly entangled in the polarization degree of freedom at the telecommunication wavelength of 1550 nm. The pairs of photons show visibility higher than 90% in several polarization bases and violate a Clauser-Horne-Shimony-Holt Bell-like inequality by more than 3 standard deviations. This represents a significant step toward the realization of efficient and versatile self pumped sources of entangled photon pairs on-chip.Comment: 9 pages, 4 figures, published versio

Energy Gap Induced by Impurity Scattering: New Phase Transition in Anisotropic Superconductors

It is shown that layered superconductors are subjected to a phase transition at zero temperature provided the order parameter (OP) reverses its sign on the Fermi-surface but its angular average is finite. The transition is regulated by an elastic impurity scattering rate $1/\tau$. The excitation energy spectrum, being gapless at the low level of scattering, develops a gap as soon as the scattering rate exceeds some critical value of $1/\tau_\star$.Comment: Revtex, 11 page

Infrared Studies of the Onset of Conductivity in Ultra-Thin Pb Films

In this paper we report the first experimental measurement of the infrared conductivity of ultra-thin quenched-condensed Pb films. For dc sheet resistances such that $\omega \tau \ll 1$ the ac conductance increases with frequency but is in disagreement with the predictions of weak localization. We attribute this behavior to the effects of an inhomogeneous granular structure of these films, which is manifested at the very small probing scale of infrared measurements. Our data are consistent with predictions of two-dimensional percolation theory.Comment: Submitted to Physical Review Letter