Z. Ovadyahu

Transition to zero dimensionality in granular aluminum superconducting films

Granular aluminum films prepared by evaporation in the presence of oxygen show two different behaviors when the oxygen content is increased. First, the average grain size (as measured by the dark-field electron microscopy technique) is reduced and the critical temperature is increased. This increase is neither due to a variation of the lattice parameter which, following our measurements, stays constant and equal to the value of pure Al, nor to a purely electronic effect linked to the small grain size, because in this first regime the grains are essentially in good electrical contact. In a second stage, the average grain size changes little but the mean free path becomes much smaller than the grain size, indicating a build up of insulating barriers between the grains. The critical temperature decreases and the transition becomes broader as the normal-state resistivity increases. We attribute this behavior to the appearance of large critical fluctuations, due to the fact that the decoupled grains tend to have a zero dimensionality.

Random perculation in metal-Ge mixtures

Abstract We have measured the electrical resistivity of InGe, PbGe and AlGe mixtures co-evaporated onto room temperature substrates, and studied their microstructure by transmission electron microscopy. In InGe and PbGe, where we have observed a random distribution of the constituents, the metal-insulator transition occurs at a metal concentration of about 15% vol., in agreement with random continuum percolation theory. In contrast, AlGe has a highly regular granular structure and a much higher critical concentration (∼ 55%), as do most metal-insulator thin film mixtures.

Some finite temperature aspects of the Anderson transition

The conductivity versus temperature, sigma (T) of disordered, three-dimensional indium oxide samples is analysed. It is demonstrated that the temperature dependence of sigma (T) as a function of disorder is in semi-quantitative agreement with current theories of the Anderson transition. In particular, it is shown that Imrys scale-dependent-diffusion regime exists over a well defined range of disorder. It is pointed out that at finite temperatures, insulating samples may appear to be conducting. This happens above a characteristic, disorder dependent temperature. It is also demonstrated that the situation is qualitatively different in a disordered two-dimensional system. A conjecture is then raised that the excess conductivity observed at high temperatures in three-dimensional systems reflects high-mobility states above a threshold energy. This conjecture is shown to be consistent with several hitherto unexplained experimental observations.

Disorder induced granularity in an amorphous superconductor

Abstract We present experimental evidence that shows that the transport mechanism on the insulating side of the superconductor to insulator transition may be modified by the presence of small “droplets” of non-percolating superconducting regions. From the conductivity point of view, the system behaves as a granular system despite the fact that the underlying structure is essentially amorphous.

Stress Aging in the Electron Glass

A new protocol for an aging experiment is studied in the electron-glass phase of indium-oxide films. In this protocol, the sample is exposed to a non-Ohmic electric field F for a waiting time t(w) during which the system attempts to reach a steady state (rather than relax towards equilibrium). The relaxation of the excess conductance Delta G after Ohmic conditions are restored exhibits simple aging as long as F is not too large.

Spin and quantum interference effects in hopping conductivity

Abstract We present results of high field magnetoconductance (MC) measurements on In 2 O 3−x films in the hopping regime. For relatively weak disorder, a purely positive MC is observed. With growing disorder, the MC acquires a negative component that tends to saturate at high fields. The data for the latter case are consistent with the co-existence of two mechanisms for MC; an isotropic, spin-alignment mechanism and an anisotropic, quantum-interference one.

Weak localization in indium oxide films

We have observed anomalous transport properties in films of conducting indium oxide in both three and two dimensions. It is found that the interplay between d, l and lin(T) (the films thickness, elastic and inelastic mean-free-paths, respectively) determines the systems dimensionality and the nature of the transport properties. In a previous study it was shown that experimental results on 2D samples of this material are adequately accounted for by modern localization theories. This conclusion is now being extended to the 3D range as well. At the same time we find strong evidence for the relevance of electron-electron interaction in both 2D and 3D samples below ~100K. In particular, the 3D → 2D crossover temperature as a function of d and the low-field negative magnetoresistance results agree quantitatively with a τin ~h/kBT law for the temperature variation of the inelastic relaxation time.

Transition to a microscopic diffusion regime and dimensional crossover in a disordered conductor

The low-temperature behaviour of the resistance of indium oxide samples is discussed within the framework of localisation theories. It is demonstrated that, in three dimensions, the interplay between the effective inelastic diffusion length lin and the correlation length xi determines the nature of the diffusion process. A crossover to two-dimensional behaviour is observed once lin becomes comparable with the sample thickness.

Suppression of inelastic electron-electron scattering in anderson insulators.

We report on measurements of absorption from applied ac fields in Anderson-localized indium-oxide films. The absorption shows a roll-off at a frequency that is much smaller than the electron-electron scattering rate measured at the same temperature in diffusive samples of this material. These results are interpreted as evidence for discreteness of the energy spectrum.

Compositional disorder and transport peculiarities in the amorphous indium oxides

We present results of the disorder-induced metal-insulator transition (MIT) in three-dimensional amorphous indium-oxide films. The amorphous version studied here differs from the one reported by Shahar and Ovadyahu [Phys. Rev. B 46, 10917 (1992)] in that it has a much lower carrier concentration. As a measure of the static disorder we use the dimensionless parameter kF � . Thermal annealing is employed as the experimental handle to tune the disorder. On the metallic side of the transition, the low temperature transport exhibits weak-localization and electron-electron correlation effects characteristic of disordered electronic systems. These include a fractional power-law conductivity versus temperature behavior anticipated to occur at the critical regime of the transition. The MIT occurs at a kF � ≈ 0.3 for both versions of the amorphous material. However, in contrast with the results obtained on the electron-rich version of this system, no sign of superconductivity is seen down to ≈0.3 K even