Type-I and Type-II Saddle Points and a Topological Flat Band in a Bi-Pyrochlore Superconductor CsBi2
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, 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.
Keywords
Cite
@article{arxiv.2604.07805,
title = {Type-I and Type-II Saddle Points and a Topological Flat Band in a Bi-Pyrochlore Superconductor CsBi2},
author = {Yusei Morita and Yongkai Li and Yu-Hao Wei and Kosuke Nakayama and Zhiwei Wang and Hua-Yu Li and Takemi Kato and Seigo Souma and Kiyohisa Tanaka and Kenichi Ozawa and Jia-Xin Yin and Takashi Takahashi and Min-Quan Kuang and Yugui Yao and Takafumi Sato},
journal= {arXiv preprint arXiv:2604.07805},
year = {2026}
}
Comments
18 pages, 3 figures