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<p>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<sub>2</sub> layers of the four-layer cuprate Ba<sub>2</sub>Ca<sub>3</sub>Cu<sub>4</sub>O<sub>8</sub>(F,O)<sub>2</sub>. 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<sub>pocket</sub>/<i>ε</i><sub>F</sub>&#xa0;~&#xa0;0.6) and a critical-to-Fermi temperature ratio (<i>T</i><sub>c</sub>/<i>T</i><sub>F</sub>&#xa0;~&#xa0;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 <i>d</i>-wave pairing mechanism in doped AF-Mott insulators.</p>

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BCS-BEC crossover driven by small Fermi pockets of a high-Tc cuprate superconductor

  • 期刊论文

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  • 发表日期：

    2026年6月2日

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  • 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

摘要

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 (Δpocket/εF ~ 0.6) and a critical-to-Fermi temperature ratio (Tc/TF ~ 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.