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
2026-06-02
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 CuO 2 layers of the four-layer cuprate Ba 2 Ca 3 Cu 4 O 8 (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.