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Correlated Insulator at the Surface of the Polar Metal Ca3Ru2O7

Daniel Halliday1,2, Izidor Benedičič1,3, Andela Zivanovic1,3, Masahiro Naritsuka1, Brendan Edwards1, Tommaso Antonelli1, Naoki Kikugawa4, Dmitry A. Sokolov3, Craig Polley5 et al.

Andrew P. Mackenzie1,3, Georg Held2, Phil D. C. King1,*, and Peter Wahl1,6,†

  • *Contact author: pdk6@st-andrews.ac.uk
  • †Contact author: wahl@st-andrews.ac.uk
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Phys. Rev. Lett. 137, 096503 – Published 26 August, 2026

DOI: https://doi.org/10.1103/x9gh-xxwb

Abstract

We investigate the electronic structure at the surface of the correlated oxide Ca3Ru2O7, 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 Ca3Ru2O7 to an insulating metallic state, and illustrate how subtle structural distortions can control its emergent electronic phases.

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  50. We cannot exclude the possibility that the structural and electronic change at the surface may also modify its magnetic structure. However, a ferromagnetic (FM) configuration within each bilayer is very stable in the bulk [11, 12, 13, 14], and an in-plane reorientation of this moment direction opens only a partial gap within the bulk [36]. Therefore, we have considered only the AFMb magnetic phase within our calculations, but would expect qualitatively similar conclusions even if the magnetic structure is perturbed at the surface.

  51. http://www.cirrus.ac.uk.
  52. D. Halliday, I. Benedičič, A. Zivanovic, M. Naritsuka, B. Edwards, T. Antonelli, N. Kikugawa, D. A. Sokolov, C. Polley, A. P. Mackenzie, G. Held, P. D. King, and P. Wahl, Data for “Correlated insulator at the surface of the polar metal Ca3Ru2O7,” 10.17630/3c1f3113-2597-4bbb-9332-0713ce802b0a (2026).

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Phys. Rev. Lett. 137, 096503– Published 26 August, 2026
  • Received 23 March 2026
  • Revised 3 July 2026
  • Accepted 21 July 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

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