Electronic transport mechanisms in a thin crystal of the Kitaev candidate $\alpha$-RuCl$_3$ probed through guarded high impedance measurements

TL;DR

This study probes electronic transport in thin $ ext{RuCl}_3$ crystals to explore the potential realization of Kitaev physics, revealing phase transitions, impurity effects, and deviations from expected transport mechanisms at low temperatures.

Contribution

It provides detailed measurements of transport mechanisms in thin $ ext{RuCl}_3$, highlighting impurity effects and deviations from conventional models near magnetic ordering.

Findings

Structural phase transition at ~160 K

Thermally activated transport above 30 K

Efros-Shklovskii VRH below 30 K

Abstract

$\alpha$-RuCl$_3$ is considered to be the top candidate material for the experimental realization of the celebrated Kitaev model. It is however known that additional interactions beyond the Kitaev model trigger in $\alpha$-RuCl$_3$, a long-range zigzag antiferromagnetic ground state. In this work, we investigate a nanoflake of $\alpha$-RuCl$_3$ through guarded high impedance measurements aimed at reaching through electronic transport, the regime where the system turns into a zigzag antiferromagnet. We investigated a variety of temperatures (\SI{1.45}{\kelvin} - \SI{175}{\kelvin}) and out-of-plane magnetic fields ranging up to \SI{11}{\tesla}. We found a clear signature of a structural phase transition at $\approx 160$\,K as reported for thin crystals of $\alpha$-RuCl$_3$, as well as a thermally activated behavior at temperatures above $\approx 30$\,K with a characteristic activation energy significantly smaller than the energy gap that we observe for $\alpha$-RuCl$_3$ bulk crystals through our Angle Resolved Photoemission Spectroscopy (ARPES) experiments. Additionally we found that below $\approx 30$\,K, transport is ruled by Efros-Shklovskii (ES) VRH. These observations point to the presence of Coulomb impurities in our thin crystals. Most importantly, our data shows that below the magnetic ordering transition known for bulk $\alpha$-RuCl$_3$ ($\approx 7$\,K), there is a clear deviation from VRH or thermal activation transport mechanisms. Our work demonstrates the possibility of reaching through specialized high impedance measurements, the thrilling ground states predicted for $\alpha$-RuCl$_3$ at low temperatures in the frame of the Kitaev model, and informs about the transport mechanisms in this material in a wide temperature range as well as on important characteristic quantities such as the localization length of the impurities in a thin $\alpha$-RuCl$_3$ crystal.