Quantum electrons at a band’s ‘saddle point’ become naturally magnetic

Electrons positioned at a specific “saddle point” within a material’s electronic bands can exhibit natural magnetism, according to a new theoretical framework proposed by Taisei Kitamura of Kyoto University and colleagues. The researchers detail a mechanism termed where a “divergent quantum metric” at this singular saddle point induces ferromagnetic correlation. This work proposes ferromagnetism occurring at a saddle point with band touching, ensuring ferromagnetism within Stoner theory, and is demonstrated within a two-dimensional model continuously connected to established flat-band ferromagnetism. The presence of a singular saddle point, a specific location where electronic bands touch within a material, directly induces ferromagnetic correlation via a mechanism termed quantum geometric ferromagnetism. This newly detailed process relies on the interplay between quantum geometry and electron correlation, establishing a theoretical framework for two-dimensional magnetism. The research demonstrates that a logarithmically divergent density of states, occurring when the Fermi energy aligns with this singular saddle point, favors the emergence of ferromagnetism, though this divergence alone is insufficient. The Stoner…