Kevin A. Parendo

Electrostatic tuning of the superconductor-insulator transition in two dimensions.

Superconductivity has been induced in insulating ultrathin films of amorphous bismuth using the electric field effect. The screening of the electron-electron interaction was found to increase with electron concentration in a manner correlated with the tendency towards superconductivity. This does not preclude an increase in the density of states being important in the development of superconductivity. The superconductor-insulator transition appears to belong to the universality class of the three dimensional XY model.

Electrostatic and parallel-magnetic-field tuned two-dimensional superconductor-insulator transitions

The 2D superconductor-insulator transition in disordered ultrathin amorphous bismuth films has been tuned both by electrostatic electron doping using the electric field effect and by the application of parallel magnetic fields. Electrostatic doping was carried out in both zero and nonzero magnetic fields, and magnetic tuning was conducted at multiple strengths of electrostatically induced superconductivity. The transitions were analyzed using finite size scaling with critical exponent products nu*z = 0.65-0.7. The parallel critical magnetic field increased with electron transfer as (dn_c-dn)^0.33, where dn is the electron transfer and dn_c is its critical value, and the critical resistance decreased linearly with dn. However at lower temperatures, in the insulating regime, the resistance became larger than expected from extrapolation of its temperature dependence at higher temperatures, and scaling failed. These observations imply that although the electrostatic- and parallel magnetic field- tuned superconductor-insulator transitions would appear to belong to the same universality class and to be delineated by a robust phase boundary that can be crossed either by tuning electron density or magnetic field, in the case of the field-tuned transition at the lowest temperatures, some different type of physical behavior turns on in the insulating regime.

Evidence of spatially inhomogeneous pairing on the insulating side of a disorder-tuned superconductor-insulator transition

Measurements of transport properties of amorphous insulating indium oxide thin films have been interpreted as evidence of the presence of superconducting islands on the insulating side of a disorder-tuned superconductor-insulator transition. Although the films are not granular, the behavior is similar to that observed in granular films. The results support theoretical models in which the destruction of superconductivity by disorder produces spatially inhomogenous pairing with a spectral gap.

Anomalous parallel-field negative magnetoresistance in ultrathin films near the superconductor-insulator transition

A parallel field negative magnetoresistance has been found in quench-condensed ultrathin films of amorphous bismuth in the immediate vicinity of the thickness-tuned superconductor-insulator transition. The effect appears to be a signature of quantum fluctuations of the order parameter associated with the quantum critical point.

Pendulum waves: A lesson in aliasing

A set of uncoupled pendula may be used to exhibit “pendulum waves,” patterns that alternately look like traveling waves, standing waves, and chaos. The pendulum patterns cycle spectacularly in a time that is large compared to the oscillation period of the individual pendula. In this article we derive a continuous function to explain the pendulum patterns using a simple extension to the equation for traveling waves in one dimension. We show that the cycling of the pendulum patterns arises from aliasing of this underlying continuous function, a function that does not cycle in time.

Tuning the 2D Superconductor‐Insulator Transition by Use of the Electric Field Effect

Some investigations of the superconductor‐insulator (SI) transitions in two dimensions have been hindered by aspects of the intrinsic disorder of the studied systems. As a solution to this problem, we have induced superconductivity in insulating, ultrathin films of amorphous bismuth by utilization of the electric field effect. This method of tuning the SI transition does not alter the intrinsic disorder. Analysis of the response to transferred charge density has revealed that screening and the density of states are both involved. This SI transition has been analyzed as a quantum phase transition using a finite size scaling analysis with electron concentration as a tuning parameter, yielding a critical exponent product vz = 0.7 ± 0.05. If z = 1 as expected, this product is consistent with the universality classes of the (2D+1) XY model and the 2D Boson Hubbard model in the absence of disorder.

Anomalous insulating state induced by parallel magnetic field in ultrathin bismuth films

Recent investigations of the magnetic‐field induced insulator in two‐dimensional superconducting systems suggest that unexpected physics occurs. However, direct comparisons with the intrinsic insulator have not been possible, nor have most experiments studied the effects of parallel magnetic fields. We have applied parallel magnetic fields to an insulating, amorphous Bi film, which exhibited Mott variable range hopping in the absence of field. We have also applied fields to an insulating film in which superconductivity was induced by electrostatic electron doping. Comparison of these magnetic‐field induced insulators with the intrinsic insulator reveals the appearance of an anomalous insulating phase at low temperatures.