Correlated Insulator at the Surface of the Polar Metal Ca_{3}Ru_{2}O_{7}.

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.