Charge transport in nanoparticle assemblies.

Quantum phase transitions and vortex dynamics in superconducting networks

We discuss the problem of eliminating the momentum variable in the phase space Langevin equations for a system of Brownian particles in two related situations: (i) position-dependent damping and (ii) existence of hydrodynamic interactions. We discuss the problems associated with the conventional elimination and we develop an alternative elimination procedure, in the Lagevin framework, which leads to the correct Smoluchowski equation. We give a heuristic argument on the basis of stochastic differential equations for the Smoluchowski limit and establish rigorously the limit for the general case of position-dependent friction and diffusion coefficents.

Adiabatic elimination for systems of Brownian particles with nonconstant damping coefficients

The electron transport properties of metallic glasses

Metal-cluster compounds and universal features of the hopping conductivity of solids

The electrical resistance has been measured on granular aluminum specimens with room-temperature resistivities between 1.5 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}3}$ and 1.3 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}1}$ \ensuremath{\Omega} cm, from 0.3 K to room temperature, in magnetic fields up to 9 T. The results show the importance of electron correlation effects on both sides of the metal-insulator transition. The Mott hopping law is not observed in any specimen over the whole temperature range. The results suggest the possibility of superconductivity in the insulating phase.

Conduction in granular aluminum near the metal-insulator transition

The temperature dependence of the electrical resistivity of granular aluminum films changes from one regime to another as their superconducting transition temperatures drop towards absolute zero. In nonsuperconducting specimens we observe the exponential behavior associated with strong localization. In the other regime the specimens show a weaker, logarithmic dependence.

Transition to Localization in Granular Aluminum Films

Localization and Electron-Interaction Effects in the Magnetoresistance of Granular Aluminum

Measurements have been made of the critical magnetic field of superconducting granular aluminum (dapprox.30 A) down to temperatures well below T/sub c/(t/sup 2/<0.1). The results show that it is possible to have the coupling between grains weak enough and the temperature low enough so that the critical field is that of the isolated grain while at the same time the large fluctuations which exist in isolated grains of this size are suppressed. The results for T/sub c/, together with those from other sources, indicate that the maximum T/sub c/ is a function of grain size only.

Coupling and isolation: Critical field and transition temperature of superconducting granular aluminum

We discuss the dependence of the normal‐state electrical resistivity on temperature and magnetic field for granular aluminum near the metal‐insulator transition. For specimens with sufficiently high room‐temperature resistivities the temperature dependence is similar to that of hoping conduction, suggesting that at absolute zero they may be insulators in the normal state. In specimens with a negative temperature coefficient of resistance there is an unexpectedly large negative magnetoresistance. We consider the evidence which the results provide for the possibility that superconductivity can exist in a system of localized electron states.

T. Chui

Papers

Conduction in granular aluminum near the metal-insulator transition

Transition to Localization in Granular Aluminum Films

Localization and Electron-Interaction Effects in the Magnetoresistance of Granular Aluminum

Coupling and isolation: Critical field and transition temperature of superconducting granular aluminum

The normal‐state conduction processes of granular superconductors