Superconductor-Insulator Transition in a Capacitively Coupled Dissipative Environment

We present results on disordered amorphous films which are expected to undergo a field-tuned Superconductor-Insulator Transition.The addition of a parallel ground plane in proximity to the film changes the character of the transition.Although the screening effects expected from "dirty-boson" theories are not evident,there is evidence that the ground plane couples a certain type of dissipation into the system,causing a dissipation-induced phase transition.The dissipation due to the phase transition couples similarly into quantum phase transition systems such as superconductor-insulator transitions and Josephson junction arrays.Comment: 4 pages, 4 figure

A Phase Glass is a Bose Metal: New Conducting State in 2D

In the quantum rotor model with random exchange interactions having a non-zero mean, three phases, a 1) phase (Bose) glass, 2) superfluid, and 3) Mott insulator, meet at a bi-critical point. We demonstrate that proximity to the bi-critical point and the coupling between the energy landscape and the dissipative degrees of freedom of the phase glass lead to a metallic state at T=0. Consequently, the phase glass is unique in that it represents a concrete example of a metallic state that is mediated by disorder, even in 2D. We propose that the experimentally observed metallic phase which intervenes between the insulator and the superconductor in a wide range of thin films is in actuality a phase glass.Comment: 4 pages, 1 .eps figure, final version to appear in Phys. Rev. Let

Can Short-Range Interactions Mediate a Bose Metal Phase in 2D?

We show here based on a 1-loop scaling analysis that short-range interactions are strongly irrelevant perturbations near the insulator-superconductor (IST) quantum critical point. The lack of any proof that short-range interactions mediate physics which is present only in strong coupling leads us to conclude that short-range interactions are strictly irrelevant near the IST quantum critical point. Hence, we argue that no new physics, such as the formation of a uniform Bose metal phase can arise from an interplay between on-site and nearest-neighbour interactions.Comment: 3 pages, 1 .eps file. SUbmitted to Phys. Rev.

The Field-Tuned Superconductor-Insulator Transition with and without Current Bias

The magnetic-field-tuned superconductor-insulator transition has been studied in ultrathin Beryllium films quench-condensed near 20 K. In the zero-current limit, a finite-size scaling analysis yields the scaling exponent product vz = 1.35 +/- 0.10 and a critical sheet resistance R_{c} of about 1.2R_{Q}, with R_{Q} = h/4e^{2}. However, in the presence of dc bias currents that are smaller than the zero-field critical currents, vz becomes 0.75 +/- 0.10. This new set of exponents suggests that the field-tuned transitions with and without dc bias currents belong to different universality classes.Comment: RevTex 4 pages, 4 figures, and 1 table minor change

Crossover and scaling in a two-dimensional field-tuned superconductor

Using an analysis similar to that of Imry and Wortis, it is shown that the apparent first order superconductor to metal transition, which has been claimed to exist at low values of the magnetic field in a two-dimensional field-tuned system at zero temperature,can be consistentlyinterpreted as a sharp crossover from a strong superconductor to an inhomogeneous state, which is a weak superconductor. The true zero-temperature superconductor to insulator transition within the inhomogenous state is conjectured to be that of randomly diluted XY model. An explaination of the observed finite temperature approximate scaling of resistivity close to the critical point is speculated within this model.Comment: 5 pages, 2 figures, corrected and modified according to referee Report

True Superconductivity in a 2D "Superconducting-Insulating" System

We present results on disordered amorphous films which are expected to undergo a field-tuned Superconductor-Insulator Transition. Based on low-field data and I-V characteristics, we find evidence of a low temperature Metal-to-Superconductor transition. This transition is characterized by hysteretic magnetoresistance and discontinuities in the I-V curves. The metallic phase just above the transition is different from the "Fermi Metal" before superconductivity sets in.Comment: 3 pages, 4 figure

On the possibility of a metallic phase in granular superconducting films

We investigate the possibility of finding a zero-temperature metallic phase in granular superconducting films. We are able to identify the breakdown of the conventional treatment of these systems as dissipative Bose systems. We do not find a metallic state at zero temperature. At finite temperatures, we find that the system exhibit crossover behaviour which may have implications for the analysis of experimental results. We also investigate the effect of vortex dissipation in these systems.Comment: 7 pages, ReVTeX3.0, 3 EPS figure

Fluctuation Conductivity in Insulator-Superconductor Transitions with Dissipation

We analyze here the fluctuation conductivity in the vicinity of the critical point in a 2D Josephson junction array shunted by an Ohmic resistor.We find that at the Gaussian level, the conductivity acquires a logarithmic dependence on $T/(T-T_c)$ when the dissipation is sufficiently small. In the renormalized classical regime, this logarithmic dependence gives rise to a leveling-off of the resistivity at low to intermediate temperatures when fluctuations are included. We show, however, that this trend does not persist to T=0 at which point the resistivity vanishes. The possible relationship of the leveling of the resistivity to the low temperature transport in granlar superconductors is discussed.Comment: 4 page

Absence of a Zero Temperature Vortex Solid Phase in Strongly Disordered Superconducting Bi Films

We present low temperature measurements of the resistance in magnetic field of superconducting ultrathin amorphous Bi films with normal state sheet resistances, $R_N$, near the resistance quantum, $R_Q={\hbar\over {e^2}}$. For $R_N<R_Q$, the tails of the resistive transitions show the thermally activated flux flow signature characteristic of defect motion in a vortex solid with a finite correlation length. When $R_N$ exceeds $R_Q$, the tails become non-activated. We conclude that in films where $R_N>R_Q$ there is no vortex solid and, hence, no zero resistance state in magnetic field. We describe how disorder induced quantum and/or mesoscopic fluctuations can eliminate the vortex solid and also discuss implications for the magnetic-field-tuned superconductor-insulator transition.Comment: REVTEX, 4 pages, 3 figure

The Bose Metal: gauge field fluctuations and scaling for field tuned quantum phase transitions

In this paper, we extend our previous discussion of the Bose metal to the field tuned case. We point out that the recent observation of the metallic state as an intermediate phase between the superconductor and the insulator in the field tuned experiments on MoGe films is in perfect consistency with the Bose metal scenario. We establish a connection between general dissipation models and gauge field fluctuations and apply this to a discussion of scaling across the quantum phase boundaries of the Bose metallic state. Interestingly, we find that the Bose metal scenario implies a possible {\em two} parameter scaling for resistivity across the Bose metal-insulator transition, which is remarkably consistent with the MoGe data. Scaling at the superconductor-metal transition is also proposed, and a phenomenolgical model for the metallic state is discussed. The effective action of the Bose metal state is described and its low energy excitation spectrum is found to be $\omega \propto k^{3}$.Comment: 15 pages, 1 figur