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arXiv · 2605.30293

Spectroscopic evidence for a molecular orbital Kondo insulator

Ke-Jun Xu·Kuan H. Hsu·Nathan Giles-Donovan·Christopher T. Parzyck·Gi-Hyeok Lee·Wanli Yang·Jun Okamoto·Hsiao-Yu Huang·Di-Jing Huang·Joshua J. Kas·John Vinson·Zhi-Xun Shen·Dung-Hai Lee·Thomas P. Devereaux·Wei-Sheng Lee·Robert J. Birgeneau
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Abstract

A Kondo insulator (KI) is a prototypical example of a highly entangled phase of matter, where many-body interactions between local moments and delocalized electrons engender the non-magnetic insulating ground state. Conventionally, the local moments arise from atomic multiplet states with a narrow bandwidth, limiting Kondo coherence to low temperatures. Here, we realize a new paradigm for constructing the KI state with hybridized molecular orbitals in FeSb2. Resonant inelastic X-ray scattering (RIXS) at the Fe L-edge reveals distinct signatures of band-like continuum states and localized states. Comparisons with first-principles calculations establish a mixed-configuration ground state with hybridized Fe d-Sb p molecular orbitals as basis states. By systematically investigating the RIXS momentum, temperature, and doping dependences, we find propagating collective modes commensurate with many-body charge and spin excitations. Our results pave the way for understanding the emerging class of unconventional d electron insulators and engineering high temperature Kondo many-body states.

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Ke-Jun Xu, Kuan H. Hsu, Nathan Giles-Donovan, Christopher T. Parzyck, Gi-Hyeok Lee, Wanli Yang, Jun Okamoto, Hsiao-Yu Huang, Di-Jing Huang, Joshua J. Kas, John Vinson, Zhi-Xun Shen, Dung-Hai Lee, Thomas P. Devereaux, Wei-Sheng Lee, Robert J. Birgeneau. 2026-05-28. Spectroscopic evidence for a molecular orbital Kondo insulator. https://arxiv.org/abs/2605.30293

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