Altermagnetism and electric-field-driven transitions in Ca₂RuO₄

Giuseppe Cuono
Consiglio Nazionale delle Ricerche (CNR-SPIN), c/o Università degli Studi ”G. D’Annunzio”, 66100 Chieti, Italy

The Mott insulator Ca₂RuO₄ represents a benchmark system among correlated transition metal oxides, where the interplay between charge, spin, orbital, and lattice degrees of freedom gives rise to a rich landscape of competing quantum phases. Through first-principles calculations, we demonstrate that Ca₂RuO₄ hosts a non-relativistic, orbital-selective spin splitting in momentum space, establishing it as an altermagnetic system [1]. Our results reproduce the experimentally observed A-centered antiferromagnetic ground state, with the Néel vector aligned along the b-axis and spin canting along the a and c directions, but without weak ferromagnetism. Moreover, Ca₂RuO₄ exhibits relativistic spin-momentum locking, characterized by distinct even-parity wave orders for each spin component. The application of electric field or current triggers remarkable insulator-to-metal transitions, not via a uniform collapse of the Mott gap but through the emergence of in-gap states localized at structural domain boundaries [2]. Under nonequilibrium conditions, the system develops nonvolatile nanoscale stripe domains [3]. Under ferroelectric and antiferroelectric-like distortions, Rashba or Weyl-type spin-orbit coupling emerges, both preserving zero magnetization. The spin-momentum locking parallel to the electric field persists with Rashba coupling, while the other components show a p-wave character; in contrast, Weyl coupling removes nodal planes, leaving only nodal lines [4]. Altogether, Ca₂RuO₄ stands out as a model platform for correlation- and field-driven phase control, offering new opportunities to explore altermagnetism, orbital-selective Mott physics, and electrically tunable quantum functionalities in complex oxides.
[1] G. Cuono , R. M. Sattigeri, J. Skolimowski and C. Autieri, Journal of Magnetism and Magnetic Materials 586, 171163 (2023)
[2] D. Curcio, C. E. Sanders, A. Chikina, H. E. Lund, M. Bianchi, V. Granata , M. Cannavacciuolo , G. Cuono, C. Autieri, F. Forte, G. Avallone, A. Romano, M. Cuoco , P. Dudin, J. Avila, C. Polley, T. Balasubramanian , R. Fittipaldi , A. Vecchione and P. Hofmann, Phys. Rev. B 108, L161105 (2023)
[3] N. Gauquelin, F. Forte, D. Jannis, R. Fittipaldi, C. Autieri, G. Cuono, V- Granata, M- Lettieri, C. Noce, F. Miletto-Granozio, A. Vecchione, J. Verbeeck, and M. Cuoco, Nano Lett. 23, 7782−7789 (2023)
[4] A. Fakhredine, G. Cuono, J. Skolimowski, S. Picozzi and C. Autieri, in preparation