Existence of electron and hole pockets and partial gap opening in the correlated semimetal Ca3Ru2O7

TL;DR

This study reveals the coexistence of electron and hole pockets and a partial gap opening in Ca3Ru2O7, a correlated semimetal, through magneto-electrical and thermoelectric measurements, highlighting a transition from coherent to incoherent electronic behavior.

Contribution

It provides the first experimental evidence of a semimetallic Fermi surface with electron and hole pockets and a partial gap in Ca3Ru2O7 below its structural transition.

Findings

Presence of electron and hole pockets confirmed by thermopower signs.

Partial gap opening observed below the transition temperature T*.

Evidence of a Lifshitz transition at T*.

Abstract

The electronic band structure of correlated Ca3Ru2O7 featuring an antiferromagnetic as well as a structural transition has been determined theoretically at high temperatures, which has led to the understanding of the remarkable properties of Ca3Ru2O7 such as the bulk spin valve effects. However, its band structure and Fermi surface (FS) below the structural transition have not been resolved even though a FS consisting of electron pockets was found experimentally. Here we report magneto electrical transport and thermoelectric measurements with the electric current and temper- ature gradient directed along a and b axes of an untwined single crystal of Ca3Ru2O7 respectively. The thermopower obtained along the two crystal axes were found to show opposite signs at low temperatures, demonstrating the presence of both electron and hole pockets on the FS. In addition, how the FS evolves across T* = 30 K at which a distinct transition from coherent to incoherent behavior occurs was also inferred - the Hall and Nernst coefficient results suggest a temperature and momentum dependent partial gap opening in Ca3Ru2O7 below the structural transition, with a pos- sible Lifshitz transition occurring at T*. The experimental demonstration of a correlated semimetal ground state in Ca3Ru2O7 calls for further theoretical studies of this remarkable material.