Spin-degenerate bulk bands and topological surface states associated with Dirac nodal lines in RuO2

TL;DR

This study uses micro-ARPES and first-principles calculations to reveal topological surface states in RuO2, showing the absence of altermagnetism and highlighting the importance of topological features for its spintronic and catalytic properties.

Contribution

It provides the first detailed experimental and theoretical analysis of topological surface states in RuO2, clarifying its magnetic nature and linking surface states to Dirac nodal lines.

Findings

Bulk bands match nonmagnetic calculations

Negligible spin polarization indicates absence of altermagnetism

Topological surface states originate from Dirac nodal lines

Abstract

Altermagnets are a novel platform to realize exotic electromagnetic properties distinct from those of conventional ferromagnets and antiferromagnets. We report results of micro-focused angle-resolved photoemission spectroscopy (ARPES) on RuO2, in which its altermagnetic nature has been under fierce debate in connection with crystal-orientation-dependent spintronic functionalities. By elucidating the band structure of the (100), (110) and (101) surfaces of a bulk single crystal using micro-ARPES, we found that, irrespective of the surface orientation, the experimental band structures show a good agreement with the bulk-band calculations for the nonmagnetic phase, but display a severe disagreement with those for the antiferromagnetic phase. Moreover, spin-resolved ARPES signifies a negligible spin polarization in the bulk bands, suggesting the absence of antiferromagnetism and altermagnetic spin splitting. In addition, we identified a nearly flat surface band and a dispersive one near the Fermi level at the (100)/(110) and (101) surfaces, respectively. Our first-principles calculations and analysis of Berry phase attribute these states to the topological surface bands emerging from the bulk Dirac nodal lines around the Fermi level. Our results indicate that such topological surface/interface states must be considered to understand the spintronic functionalities of RuO2, and may provide new insights into its catalytic characteristics.