A correlated insulator at the surface of the polar metal Ca$_3$Ru$_2$O$_7$

TL;DR

This study reveals that the surface of the polar metal Ca$_3$Ru$_2$O$_7$ becomes insulating due to structural relaxation and electronic correlations, contrasting with its metallic bulk, and highlights the delicate balance of phases in this material.

Contribution

It demonstrates the emergence of an insulating surface phase in a polar metal caused by surface relaxation and correlations, providing insight into phase control via structural distortions.

Findings

Surface hosts an insulating phase unlike the metallic bulk.

Structural relaxation influences electronic correlations leading to insulation.

Subtle distortions can switch the electronic phase of the material.

Abstract

We investigate the electronic structure at the surface of the correlated oxide Ca$_3$Ru$_2$O$_7$, a low-symmetry ruthenate oxide which hosts an unconventional polar-metal phase. From a combination of angle-resolved photoemission spectroscopy and scanning tunneling spectroscopy measurements, we demonstrate that the surface hosts an insulating phase, a distinct departure from metallicity within the bulk. Utilizing quantitative low-energy electron diffraction in conjunction with electronic structure calculations, we show how this results from a combined surface structure relaxation and the impact of marked electronic correlations in this system. Our findings highlight the proximity of Ca$_3$Ru$_2$O$_7$ to an insulating metallic state, and illustrate how subtle structural distortions can control its emergent electronic phases.