Universal size-dependent nonlinear charge transport in single crystals of the Mott insulator Ca$_2$RuO$_4$

TL;DR

This study investigates how the size of Ca$_2$RuO$_4$ single crystals affects the current needed to induce insulator-metal transition, revealing size-dependent nonlinear charge transport mechanisms beyond Joule heating effects.

Contribution

It demonstrates a four-order magnitude increase in required current density with decreasing crystal size and rules out Joule heating as the cause, highlighting inhomogeneous current distribution effects.

Findings

Current density increases with decreasing crystal size.

Size dependence is not due to Joule heating.

Inhomogeneous current distribution influences transition.

Abstract

The surprisingly low current density required for inducing the insulator to metal transition has made Ca$_2$RuO$_4$ an attractive candidate material for developing Mott-based electronics devices. The mechanism driving the resistive switching, however, remains a controversial topic in the field of strongly correlated electron systems. Here we probe an uncovered region of phase space by studying high-purity Ca$_2$RuO$_4$ single crystals, using the sample size as principal tuning parameter. Upon reducing the crystal size, we find a four orders of magnitude increase in the current density required for driving Ca$_2$RuO$_4$ out of the insulating state into a non-equilibrium (also called metastable) phase which is the precursor to the fully metallic phase. By integrating a microscopic platinum thermometer and performing thermal simulations, we gain insight into the local temperature during simultaneous application of current and establish that the size dependence is not a result of Joule heating. The findings suggest an inhomogeneous current distribution in the nominally homogeneous crystal. Our study calls for a reexamination of the interplay between sample size, charge current, and temperature in driving Ca$_2$RuO$_4$ towards the Mott insulator to metal transition.