Low temperature transport in granular metals

TL;DR

This paper studies low-temperature electron transport in granular metals with high tunneling conductance, revealing a transition from coherent to localized behavior and proposing a modified metal-insulator transition point influenced by granular structure.

Contribution

It demonstrates that at low temperatures, coherent electron motion dominates in granular metals, and it derives a new criterion for the metal-insulator transition considering granular effects.

Findings

Conductivity decays with decreasing temperature similarly to homogeneous disordered metals.

The metal-insulator transition occurs at a tunnel conductance proportional to the logarithm of the ratio of charging energy to level spacing.

Granular structure influences conductivity even at low temperatures, affecting the transition point.

Abstract

We investigate transport in a granular metallic system at large tunneling conductance between the grains. We show that at low temperatures, $T\leq g_T\delta $, where $\delta$ is the single mean energy level spacing in a grain, the coherent electron motion at large distances dominates the physics, contrary to the high temperature ($T > g_T \delta $) behavior where conductivity is controlled by the scales of the order of the grain size. The conductivity of one and two dimensional granular metals, in the low temperature regime, decays with decreasing temperature in the same manner as that in homogeneous disordered metals, indicating thus an insulating behavior. However, even in this temperature regime the granular structure remains important and there is an additional contribution to conductivity coming from short distances. Due to this contribution the metal-insulator transition in three dimensions occurs at the value of tunnel conductance $g_T^C=(1/6\pi)\ln (E_C/\delta)$, where $E_C$ is the charging energy of an isolated grain, and not at the generally expected $g_T^C \propto 1$. Corrections to the density of states of granular metals due to the electron-electron interaction are calculated. Our results compare favorably with the logarithmic dependence of resistivity in the high-$T_c$ cuprate superconductors indicating that these materials may have a granular structure.