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

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.

6 pages main text + 4 pages supplementary, 4 figures in main text, 4 in supplementary, fixed missing ',' in author list in v2