BCS-BEC crossover driven by small Fermi pockets of a high-T<sub>c</sub> cuprate superconductor.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 42230570.
- Also identified by DOI 10.1038/s41467-026-73081-5.
- No licence information is recorded for this record.
- Because redistribution is not established, this page shows the abstract only. Follow the links below for the full text.
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 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>/ε<sub>F</sub> ~ 0.6) and a critical-to-Fermi temperature ratio (T<sub>c</sub>/T<sub>F</sub> ~ 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.