少数なのに強く結びつく高温超伝導電子を発見

2026-06-03 東京大学,東京理科大学,上智大学

４枚の超伝導面を有する銅酸化物高温超伝導体では、反強磁性秩序を背後とする少数キャリアが互いに強く結びつき、高い超伝導温度を実現する。

＜関連情報＞

• https://www.issp.u-tokyo.ac.jp/maincontents/news2.html?pid=30890
• https://www.nature.com/articles/s41467-026-73081-5

高温超伝導銅酸化物の小さなフェルミポケットによって引き起こされるBCS-BECクロスオーバー BCS-BEC crossover driven by small Fermi pockets of a high-Tc cuprate superconductor

Junhyeok Jeong,Yamato Enomoto,Yoshimitsu Kohama,Tomotaka Nakayama,Kotaro Ando,Kifu Kurokawa,Soonsang Huh,Zhuo Yang,Toshihiro Nomura,Matthew D. Watson,Timur K. Kim,Cephise Cacho,Chun Lin,Makoto Hashimoto,Donghui Lu,Shiro Sakai,Takami Tohyama,Kazuyasu Tokiwa & Takeshi Kondo
Nature Communications  Published:02 June 2026
DOI:https://doi.org/10.1038/s41467-026-73081-5

Abstract

Fermi arcs observed in underdoped cuprates have sparked debate over whether they represent segments of a large Fermi surface or small Fermi pockets. This ambiguity has long hindered their classification as either the conventional Bardeen-Cooper-Schrieffer (BCS) regime or the strongly coupled Bose-Einstein condensation (BEC) crossover limit. Here, using angle-resolved photoemission spectroscopy and quantum oscillations, we demonstrate the coexistence of a small Fermi pocket and a large superconducting gap in the clean inner CuO₂ layers of the four-layer cuprate Ba₂Ca₃Cu₄O₈(F,O)₂. This coexistence constitutes a hallmark of the BCS-BEC crossover and has remained elusive for decades. Despite the presence of antiferromagnetic (AF) order, the superconducting gap in the small pocket is remarkably large, yielding a gap-to-Fermi energy ratio (Δ_pocket/ε_F ~ 0.6) and a critical-to-Fermi temperature ratio (T_c/T_F ~ 0.13) that reach the theoretical upper bound for two-dimensional superconductivity. Unexpectedly, this BCS-BEC crossover emerges not as the carrier density decreases but as it increases, abruptly within a narrow doping range of less than 1%. These results provide a long-sought microscopic foundation for the d-wave pairing mechanism in doped AF-Mott insulators.