The nanostructural origin of the ac conductance in dielectric granular metals: the case study of Co_20(ZrO_2)_80

TL;DR

This paper investigates how the nanostructure of granular Co-ZrO2 thin films influences their ac conductance, revealing that a bimodal particle size distribution enables a combination of tunneling and capacitive conduction.

Contribution

It demonstrates that a bimodal size distribution of Co particles in granular films explains the universal ac response, supported by a quantitative model of a resistance-capacitor network.

Findings

Bimodal particle size distribution enables dual conduction mechanisms.

A simple cubic network model accurately describes experimental ac responses.

Microscopic parameters controlling ac conductance are identified.

Abstract

We show which is the nanostructure required in granular Co20(ZrO2)80 thin films to produce an ac response such as the one that is universally observed in a very wide variety of dielectric materials. A bimodal size distribution of Co particles yields randomly competing conductance channels which allow both thermally assisted tunneling through small particles and capacitive conductance among larger particles that are further apart. A model consisting on a simple cubic random resistance-capacitor network describes quantitatively the experimental results as functions of temperature and frequency, and enables the determination of the microscopic parameters controlling the ac response of the samples.