Absence of altermagnetic spin splitting character in rutile oxide RuO$_2$

TL;DR

This study uses spin- and angle-resolved photoemission spectroscopy to investigate RuO$_2$, finding no evidence of the predicted altermagnetic spin splitting and challenging previous claims of its altermagnetic order.

Contribution

The paper provides the first direct spectroscopic evidence contradicting the altermagnetic spin splitting in RuO$_2$, clarifying its electronic and magnetic properties.

Findings

No observed altermagnetic spin splitting in RuO$_2$

Significant in-plane spin polarization antisymmetric about high-symmetry plane

Electronic structure aligns with non-magnetic predictions

Abstract

Rutile RuO$_2$ has been posited as a potential $d$-wave altermagnetism candidate, with a predicted significant spin splitting up to 1.4 eV. Despite accumulating theoretical predictions and transport measurements, direct spectroscopic observation of spin splitting has remained elusive. Here, we employ spin- and angle-resolved photoemission spectroscopy to investigate the band structures and spin polarization of thin-film and single-crystal RuO$_2$. Contrary to expectations of altermagnetism, our analysis indicates that RuO$_2$'s electronic structure aligns with those predicted under non-magnetic conditions, exhibiting no evidence of the hypothesized spin splitting. Additionally, we observe significant in-plane spin polarization of the low-lying bulk bands, which is antisymmetric about the high-symmetry plane and contrary to the $d$-wave spin texture due to time-reversal symmetry breaking in altermagnetism. These findings definitively challenge the altermagnetic order previously proposed for rutile RuO$_2$, prompting a reevaluation of its magnetic properties.