A. M. Mack

Evidence of Vortices on the Insulating Side of the Superconductor-Insulator Transition

The magnetoresistance of ultrathin insulating films of Bi has been studied with magnetic fields applied parallel and perpendicular to the plane of the sample. Deep in the strongly localized regime, the magnetoresistance is negative and independent of field orientation. As film thicknesses increase, the magnetoresistance becomes positive, and a difference between values measured in perpendicular and parallel fields appears, which is a linear function of the magnetic field and is positive. This is not consistent with the quantum interference picture. We suggest that it is due to vortices present on the insulating side of the superconductor-insulator transition.

Anomalous Hopping Exponents of Ultrathin Films of Metals

and x are constantswhich depend on the disorder, the details of the inter-actions and the dimensionality of the system. Simpleactivated hopping over a constant barrier results in theArrhenius form with x = 1. For noninteracting electrons,when the average hopping distance depends on temper-ature due to the compromise between hopping to siteswhich are close in energy, but farther away, Mott variablerange hopping [1] is expected, with x = 1/(d+1), where dis the dimension. Efros and Shklovskii (ES) showed thatincluding Coulomb interactions between electrons resultsin a soft gap in the density of states at the Fermi energy,which changes the variable range hopping exponent tox = 1/2 in all dimensions [2].Hopping conduction has been investigated in a widevariety of materials, such as doped semiconductors [3,4],semiconducting heterostructures [5], amorphous metals[6–10], magnetic materials [11] and superconductors [12].Both the Mott and the ES forms of variable range hop-ping have been observed, as well as a crossover betweenthe two regimes [5,6]. It should be emphasized, how-ever, that it is often hard to distinguish between Mottand ES hopping, particularly in experiments in which theresistance changes only by one or two orders of magni-tude. The unambiguous identification of the Mott or EShopping can be further complicated by factors which areusually neglected, such as the granularity of the system,possible temperature dependence of R

Superconductor–Insulator Transitions in 2D: The Experimental Situation

Superconductor–insulator (SI) transitions in ultrathin films have attracted significant attention over the last decade because of the possibility that they are quantum phase transitions. Magnetic field, film thickness, or carrier concentration can be used as control parameters. The bosonic pictures of these transitions proposed some years ago are only in qualitative agreement with experiment. In particular, the critical resistance appears not to be universal, and there are variations in the values of critical exponents. It has been concluded that in real films fermionic degrees of freedom must be taken into account. There are also indications that the phase diagram may include a significant metallic phase separating the superconducting and insulating phases, and that the transition may have a significant percolative aspect. The experimental situation will be broadly reviewed with attention paid to issues relating to materials and measurements.

Amorphous Ge substrates: Active or passive participants in electrical transport in ultrathin metal films?☆

Abstract A comparison has been made between R(T) curves of Bi films grown on ceramic substrates held near helium temperatures with those first grown in this manner, and then overlaid with amorphous Ge ( a Ge) while held at helium temperatures. The dramatic changes in R(T) resulting from the overlay suggest that a Ge substrates play an active role in electrical transport. This result has implications for the interpretation of experiments on the superconductor-insulator transition.