Junhyeok Jeong (a second-year doctoral student, Kondo Group) received the Best Poster Award at The Conference of Condensed Matter Physics 2026 (CCMP2026). This award is given to researchers who delivered an outstanding poster presentation at the conference.

The title of the award-winning presentation is “BCS-BEC crossover driven by small Fermi pockets of a high-Tc cuprate superconductor.”

In this study, the researchers focused on the four-layer cuprate high-temperature superconductor Ba₂Ca₃Cu₄O₈(F,O)₂, which has four CuO₂ planes (superconducting layers) within its unit cell. In this material, the inner planes are shielded from disorder by the outer planes, realizing exceptionally clean, ideal CuO₂ planes. Using angle-resolved photoemission spectroscopy (ARPES) and quantum oscillation (de Haas–van Alphen effect) measurements, the researchers confirmed that “small Fermi pockets,” formed against a background of antiferromagnetic order, exist in these clean inner planes. Small Fermi pockets had previously been reported in multilayer cuprates with five, six, or more layers, but this study is the first to observe them in a four-layer system. Remarkably, the researchers further discovered that a large superconducting gap exists on these Fermi pockets, which carry an extremely small number of carriers. This shows that a large superconducting gap can develop even under the coexistence of antiferromagnetic order, suggesting that antiferromagnetic order and superconductivity may not be in a simple competitive relationship. Moreover, the coexistence of small Fermi pockets and a large superconducting gap means that the superconducting Cooper pairs form an extremely strong coupling, suggesting the possibility of a BCS-BEC crossover in cuprates. Furthermore, the observation of a pairing gap (an energy gap appearing above the superconducting transition temperature) emerging with a slight increase in doping, together with the observation of a Bogoliubov flat band, strongly supports the BCS-BEC crossover scenario in cuprates.

This study was highly evaluated for presenting, for the first time, experimental evidence of a strongly coupled superconducting state—an intermediate state between BCS and BEC—in cuprates, and for opening a new research foundation on which experiment and theory can be discussed on the same footing, through the direct observation of ideal, clean CuO₂ planes.

Related Papers

• “BCS-BEC crossover driven by small Fermi pockets of a high-T_c cuprate superconductor” Nat. Commun. 17, 4810 (2026).

Related Links

• CCMP2026 (Award confirmation page)
• Kondo Group, ISSP, The University of Tokyo
• 2026.06.03Press ReleaseDiscovery of high-temperature superconducting electrons that bind strongly despite being few in number (in Japanese)
• 2023.07.14Press ReleaseConventional high-temperature superconductors found to have been disordered—Charge behavior dramatically altered by removing disorder (in Japanese)
• 2020.08.20Press ReleaseHigh-temperature superconducting electrons coexisting with antiferromagnetic electrons—Ending a 30-year-old unsolved problem in cuprate high-temperature superconductors (in Japanese)