Observation of small Fermi pockets protected by clean CuO2 sheets of a high-Tc superconductor

TL;DR

This study detects small Fermi pockets in a high-Tc cuprate superconductor using ARPES and quantum oscillations, revealing coexistence of superconductivity and antiferromagnetic order in clean CuO2 sheets, which advances understanding of Mott physics in high-Tc superconductors.

Contribution

First direct observation of small Fermi pockets in a high-Tc cuprate with pristine CuO2 planes, linking Fermi surface topology to superconductivity and antiferromagnetic order.

Findings

Small Fermi surface pockets observed around (pi/2, pi/2).

Superconducting gap opens along the pockets.

Superconductivity occurs without states near the antinodal region.

Abstract

The superconductivity of high transition temperature (Tc) occurs in copper oxides with carrier-doping to an antiferromagnetic (AF) Mott insulator. This discovery more than thirty years ago immediately led to a prediction about the formation of a small Fermi pocket. This structure, however, has not yet been detected, while it could be a key element in relating high-Tc superconductivity to Mott physics. To address this long-standing issue, we investigate the electronic structure of a five-layered Ba2Ca4Cu5O10(F,O)2 with inner CuO2 planes demonstrated to be cleanest ever in cuprates. Most surprisingly, we find small Fermi surface (FS) pockets closed around (pi/2,pi/2) consistently by angle-resolved photoemission spectroscopy (ARPES) and quantum oscillation measurements. The d-wave superconducting gap opens along the pocket, revealing the coexistence between the superconductivity and AF order in the same CuO2 sheet. Our data further indicate that the superconductivity can occur without contribution from the states near the antinodal region, which are shared by other competing excitations such as the charge density wave (CDW) and pseudogap states. This will have significant implications for understanding the superconductivity and puzzling Fermi arc phenomena in cuprates.