Bi-altermagnetism unveiled by sublattice-specific circular dichroism in resonant inelastic X-ray scattering

Ganesha Channagowdra^1*, A. Singh², H. Y. Huang¹, M. Furo³, Bin Gao^4,5, Pengcheng Dai^4,5, S-W. Cheong⁶, C. T. Chen¹, Jan Kune ̌s⁷, A. Fujimori^1,8,9, A. Hariki³, D. J. Huang^1,8,10

¹National Synchrotron Radiation Research Center, Hsinchu, Taiwan
²Department of Physics and Astrophysics, University of Delhi, Delhi, India
³Department of Physics and Electronics, Graduate School of Engineering, Osaka Metropolitan University, Osaka, Japan
⁴Rice Laboratory for Emergent Magnetic materials and Smalley-Curl Institute, Rice University, Texas, USA
⁵Department of Physics and Astronomy, Rice University, Texas, USA
⁶Keck Center for Quantum Magnetism and Department of Physics and Astronomy, Rutgers University, Piscataway, USA
⁷Department of Condensed Matter Physics, Faculty of Science, Masaryk University, Brno, Czech Republic
⁸Department of Physics, National Tsing Hua University, Hsinchu, Taiwan
⁹Department of Physics, University of Tokyo, Bunkyo-Ku, Tokyo, Japan
¹⁰Department of Electrophysics, National Yang Ming Chiao Tung University, Hsinchu, Taiwan

* Presenter:Ganesha Channagowdra, email:ganesha.c@nsrrc.org.tw

An altermagnet is a recently identified class of magnets that exhibit a zero net magnetic moment but break symmetry under the combined operations of parity and time reversal. It typically consists of two magnetic sites of opposite spins related by rotation within the unit cell. Here, we use circular dichroism (CD) in resonant inelastic X-ray scattering (RIXS) to identify a new form of altermagnetism, namely bi-altermagnetism, in the correlated insulator Fe₂Mo₃O₈, which comprises two altermagnetic sublattices: one with alternating quasi-octahedral Fe environments and the other with alternating tetrahedral Fe environments. We experimentally revealed the emergence of CD in the magnetic state, thereby probing mirror-symmetry breaking associated with altermagnetic order. Notably, the CD appeared at sublattice-specific excitations of the octahedral and tetrahedral sites, indicating symmetry breaking in both altermagnetic sublattices. Calculations based on a model with the bi-altermagnetic order along the c axis successfully reproduce the observed CD. Our findings provide compelling evidence for bi-altermagnetism in Fe₂Mo₃O₈, and showcase the use of RIXS-CD as a probe of magnetic sublattices in systems with zero net magnetization.

Keywords: Bi-altermagnetism, Non-relativistic limit, Spin-orbital excitations, Circular-dichroism, Resonant inelastic X-ray scattering