Observation of Small Fermi Pockets in Multilayer Cuprates: The Doped-Mott State of Clean Inner CuO2 Planes

Understanding the intrinsic electronic properties of CuO2 planes at low carrier concentrations near the half-filled Mott state is essential for elucidating the microscopic mechanism of high-Tc superconductivity. The electronic phase diagram has been largely established through studies of single- and bilayer cuprates. In these systems, however, CuO2 planes lie directly next to dopant layers that inevitably introduce disorder. This disorder produces spatially inhomogeneous electronic states and obscures the intrinsic electronic structure, particularly in the heavily underdoped regime where screening is weak and correlations are strong. Consequently, disorder likely hinders the identification of the essential ingredients for high-Tc superconductivity. This limitation can be overcome by studying inner CuO2 planes of multilayer cuprates, which are spatially separated from dopant layers and provide a nearly ideal electronic environment [1].

In this talk, I present angle-resolved photoemission spectroscopy (ARPES) studies of multilayer cuprates [2-5]. The pristine inner CuO2 planes provide a unique opportunity to access the long-elusive intrinsic phase diagram of clean CuO2 planes. We observe small Fermi pockets centered at (π/2, π/2), revealing a lightly doped Mott-derived electronic state. A d-wave superconducting gap opens along these pockets, demonstrating the microscopic coexistence of superconductivity and antiferromagnetic order within a single CuO2 plane. Remarkably, the superconducting gap is comparable in magnitude to the small Fermi energy, placing the clean inner CuO2 plane in the BCS–BEC crossover regime [4]. Finally, we demonstrate that metallic screening by heavily over doped outer layers enhances superconducting coherence in the inner CuO2 plane [5], stabilizing it beyond the antiferromagnetic zone boundary up to the antinodal region. This result highlights electronic screening as a key parameter for high-Tc superconductivity.

References
[1] H. Mukuda et al., J. Phys. Soc. Jpn. 81, 011008 (2012).
[2] S. Kunisada et al., Science 369, 833 (2020).
[3] K. Kurokawa et al., Nat. Commun. 14, 4064 (2023).
[4] J. Jeong et al., Nat. Commun. 17, 4810 (2026).
[5] J. Jeong et al., arXiv 2507.23260, Nat. Commun., in press (2026).