超伝導を強める電子の「隠れた輪」を発見 ― 新しい超伝導材料設計への道筋 ―

2026-10-19 東北大学

図1.
(a) サドル点の模式図。電子バンドが水色の点線に沿って山（凸）、橙色の点線に沿って谷（凹）のようなエネルギー分散を示し、それらが交わる紫色の点がサドル点に対応する。
(b) サドルループの模式図。サドル点が紫色の輪に沿って連なった構造を持つ。

＜関連情報＞

• https://www.tohoku.ac.jp/japanese/2026/10/press20261019-03-singularity.html
• https://www.nature.com/articles/s41535-026-00905-4

サドルループ型ファン・ホーヴェ特異点の観測 Observation of a saddle-loop van Hove singularity

Y. Morita, K. Nakayama, N. Ito, T. Kato, T. Nakamura, H. Zhang, X. Tang, T. Lin, K. Yanagizawa, S. Souma, S. Masaki, T. Ikushima, Y. Moriyasu, K. Hagiwara, F. Matsui, K. Tanaka, K. Ozawa, D. Shiga, H. Kumigashira, M. Maeda, Y. Niimi, T. Kida, M. Hagiwara, T. K. Kim, … T. Sato
npj Quantum Materials  Published:25 June 2026
DOI:https://doi.org/10.1038/s41535-026-00905-4

Abstract

Exploiting electronic singularities to drive emergent quantum phenomena is a key basis in condensed matter physics, yet this powerful strategy has been largely limited to two dimensions, leaving the potential of three-dimensional materials often inaccessible. A “saddle-loop” singularity—a one-dimensional ring of saddle points—has been proposed as a way to extend the concept of van Hove singularities to three-dimensional electron systems, but has remained experimentally elusive. Here, we provide the direct experimental evidence for the saddle‑loop singularity in Pt(Bi_1−xSe_x)₂, using angle‑resolved photoemission spectroscopy. We also show that the saddle loop is positioned closer to the Fermi level at low x values where the superconducting critical temperature is higher. Furthermore, our advanced theoretical calculations reveal that the saddle loop originates from inter-orbital hybridization, establishing a generalizable design concept. Our findings demonstrate a viable mechanism for realizing singular electronic states in three-dimensional electron systems, providing a platform to investigate correlation-driven phenomena in three dimensions.