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arXiv · 2604.07805

Type-I and Type-II Saddle Points and a Topological Flat Band in a Bi-Pyrochlore Superconductor CsBi2

Yusei Morita·Yongkai Li·Yu-Hao Wei·Kosuke Nakayama·Zhiwei Wang·Hua-Yu Li·Takemi Kato·Seigo Souma·Kiyohisa Tanaka·Kenichi Ozawa·Jia-Xin Yin·Takashi Takahashi·Min-Quan Kuang·Yugui Yao·Takafumi Sato
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Abstract

The divergence of the electron density of states (DOS) plays an important role in enhancing many-body interactions and inducing various quantum phases in low-dimensional systems. However, such unique electronic structures remain experimentally elusive in three-dimensional (3D) systems, particularly those with strong spin-orbit coupling (SOC). Using angle-resolved photoemission spectroscopy and first-principles calculations for a Laves-phase superconductor CsBi$_2$, which features a Bi-pyrochlore 3D network with strong SOC, we identify two characteristic electronic structures with a large DOS. One is a dispersionless topological flat band with p-orbital character, locally formed around the U-K line, which enhances DOS near the Fermi level. The other involves type-I and type-II saddle points connected by a flat band, which cooperatively produce an enhancement in the DOS. Our findings suggest a novel mechanism for achieving a DOS enhancement and lay a foundation for exploring exotic phenomena driven by the interplay of multiple singularities with a large DOS, nontrivial topology, and strong SOC in 3D pyrochlores.

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Yusei Morita, Yongkai Li, Yu-Hao Wei, Kosuke Nakayama, Zhiwei Wang, Hua-Yu Li, Takemi Kato, Seigo Souma, Kiyohisa Tanaka, Kenichi Ozawa, Jia-Xin Yin, Takashi Takahashi, Min-Quan Kuang, Yugui Yao, Takafumi Sato. 2026-04-09. Type-I and Type-II Saddle Points and a Topological Flat Band in a Bi-Pyrochlore Superconductor CsBi2. https://arxiv.org/abs/2604.07805

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