Observation of small Fermi pockets protected by clean CuO 2 sheets of a high- T c superconductor

Superconductivity in copper oxide materials emerges by doping a special kind of correlated state called the Mott insulator. However, studying what happens when a small concentration of charge carriers—holes or electrons—is added to a Mott insulator is experimentally challenging. It has been predicted that the so-called “Fermi pockets” should become visible during experimentation, but such pockets have not been unambiguously observed. Kunisada et al. studied the unusual cuprate Ba 2 Ca 4 Cu 5 O 10 (F,O) 2 , which has five copper oxide planes in a unit cell, whereas most cuprates have one or two (see the Perspective by Vishik). They observed two Fermi pockets in both photoemission and quantum oscillations data, with the innermost copper oxide planes playing a crucial role. Science , this issue p. 833 ; see also p. 775 Photoemission and quantum oscillations indicate Fermi pockets in a cuprate with five copper oxide planes per unit cell. In cuprate superconductors with high critical transition temperature ( T c ), light hole-doping to the parent compound, which is an antiferromagnetic Mott insulator, has been predicted to lead to the formation of small Fermi pockets. These pockets, however, have not been observed. Here, we investigate the electronic structure of the five-layered Ba 2 Ca 4 Cu 5 O 10 (F,O) 2 , which has inner copper oxide (CuO 2 ) planes with extremely low disorder, and find small Fermi pockets centered at (π/2, π/2) of the Brillouin zone by angle-resolved photoemission spectroscopy and quantum oscillation measurements. The d-wave superconducting gap opens along the pocket, revealing the coexistence between superconductivity and antiferromagnetic ordering in the same CuO 2 sheet. These data further indicate that superconductivity can occur without contribution from the antinodal region around (π, 0), which is shared by other competing excitations.