G. Martinez-Arizala

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

Electric-field effect in ultrathin films near the superconductor-insulator transition

The effect of an electric field on the conductance of ultrathin films of metals deposited on substrates coated with a thin layer of amorphous Ge was investigated. A contribution to the conductance modulation symmetric with respect to the polarity of the applied electric field was found in regimes in which there was no sign of glassy behavior. For films with thicknesses that put them on the insulating side of the superconductor-insulator transition, the conductance increased with electric field, whereas for films that were becoming superconducting it decreased. Application of magnetic fields to the latter, which reduce the transition temperature and ultimately quench superconductivity, changed the sign of the reponse of the conductance to electric field back to that found for insulators. We propose that this symmetric response to capacitive charging is a consequence of changes in the conductance of the

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

a\ensuremath{-}\mathrm{Ge}

Nearly symmetric field effect in ultrathin metal films

layer, and is not a fundamental property of the physics of the superconductor-insulator transition as previously suggested.

Anomalous Field Effect in Ultrathin Films of Metals near the Superconductor-Insulator Transition

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.

Coulomb-glass-like behavior of ultrathin films of metals

Ultrathin films of Bi and Pb have been grown incrementally under ultra-high vacuum conditions on SrTiO3 substrates held below 10K. The substrates were provided with metal gates on their rear surfaces. For films with sheet resistance in excess of the quantum resistance for pairs, the conductance was found to increase with gate voltage of either sign. This response, which was temporally stable, was largest in the most resistive films where the conductance changed by the order of 50 percent with a gate bias of 20V. The effect decreased in magnitude with decreasing sheet resistance and increasing temperature. It vanished above a characteristic temperature, and was not present in superconducting films with sheet resistances (measured at 14K) less than the quantum resistance for pairs.