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FM: Fachverband Funktionsmaterialien

FM 9: Focus Session: Materials Discovery II – High throughput searches for functional magnetic materials (joint session FM/MA)

FM 9.5: Vortrag

Dienstag, 10. März 2026, 15:15–15:30, BEY/0138

Molecular orbital degeneracy lifting in NbSeI — •Keita Kojima¹, Hayato Takano¹, Youichi Yamakawa², Shunsuke Kitou³, Ryutaro Okuma¹, and Yoshihiko Okamoto¹ — ¹Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan — ²Department of Physics, Nagoya University, Nagoya 464-8602, Japan — ³Department of Advanced Materials Science, University of Tokyo, Kashiwa 277-8581, Japan

The breathing pyrochlore lattice, composed of alternating small and large tetrahedra, hosts molecular orbitals within the smaller tetrahedra that strongly influences its physical properties. Such molecular-orbital and frustration-driven effects have led to diverse electronic states in related compounds. We investigated NbSeI, a MoSBr-type material with a particularly large breathing distortion whose physical properties remain unexplored despite previous synthesis reports. We synthesized single crystals and conducted x-ray diffraction, physical property measurements, and first-principles calculations. While the calculations predict metallic flat bands originating from Nb 4d orbitals, magnetic and transport measurements reveal a nonmagnetic insulating state below 300 K. NbSeI also undergoes a structural transition at Ts = 106 K. Our structural studies demonstrate local atomic displacements above Ts and a trimer-like molecular transformation below Ts. Our results show that the combination of strong breathing distortion and flat-band*derived electronic structure stabilizes molecular-orbital degrees of freedom, advancing orbital physics beyond single-ion descriptions toward cluster-based electronic phenomena.

Keywords: Breathing pyrochlore; flat band instability; phase transition; orbital degeneracy lifting