Temperature Dependent Zero-Field Splittings in Graphene

TL;DR

This study investigates how temperature affects zero-field spin splittings in graphene, revealing a decrease in splittings with rising temperature, which impacts understanding of its topological properties and potential quantum applications.

Contribution

It provides the first detailed experimental analysis of temperature dependence of zero-field splittings in graphene using sub-Terahertz ESR, highlighting mechanisms influencing spin-orbit interactions.

Findings

Spin splittings decrease as temperature increases.

Temperature dependence may be due to substrate expansion, electron-phonon interactions, or magnetic order.

Results enhance understanding of graphene's topological gap behavior.

Abstract

Graphene is a quantum spin Hall insulator with a 45 $\mu$eV wide non-trivial topological gap induced by the intrinsic spin-orbit coupling. Even though this zero-field spin splitting is weak, it makes graphene an attractive candidate for applications in quantum technologies, given the resulting long spin relaxation time. On the other side, the staggered sub-lattice potential, resulting from the coupling of graphene with its boron nitride substrate, compensates intrinsic spin-orbit coupling and decreases the non-trivial topological gap, which may lead to the phase transition into trivial band insulator state. In this work, we present extensive experimental studies of the zero-field splittings in monolayer and bilayer graphene in a temperature range 2K-12K by means of sub-Terahertz photoconductivity-based electron spin resonance technique. Surprisingly, we observe a decrease of the spin splittings with increasing temperature. We discuss the origin of this phenomenon by considering possible physical mechanisms likely to induce a temperature dependence of the spin-orbit coupling. These include the difference in the expansion coefficients between the graphene and the boron nitride substrate or the metal contacts, the electron-phonon interactions, and the presence of a magnetic order at low temperature. Our experimental observation expands knowledge about the non-trivial topological gap in graphene.