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Doped copper oxygen planes in a clean habitat

The precise route through which the antiferromagnetic Mott insulating phase evolves into the high-temperature superconducting phase still puzzles the copper-oxide high temperature superconductor research community. For example, upon exiting the antiferromagnetic phase, the sign and the magnitude of Hall number (nH) is consistent with the density of doped holes p (see Fig.1(a) adapted from[2]), not the expected value 1+p, namely the average number of holes per unit cell. The discrepancy can be caused by the Fermi surface reconstruction due to some kind of broken symmetry. Alternatively, in the absence of any symmetry breaking, this can signify the presence of topological order! The paper by Kunisada et al sheds considerable light on this important question. The key to their success is the growth of a Tc = 65K cuprate crystal that has five copper-oxygen planes per unit cell (the chemical formula is Ba2Ca4Cu5O10(F,O)2, see Fig.1(b))[1]. This is complemented by a high-resolution laser ARPES study, plus quantum oscillation, transport, and NMR measurements. The five CuO2 planes can be divided into three groups, two outer planes (OP), two middle-inner planes (IP1) and one inner-most planes (IP0). Because the dopant layers are in contact with the two outer planes OP only, it is claimed that both the doping level and the disorder decreases progressively from OP → IP1 → IP0. The Fermi surface image is shown in Fig.1(c) where the open Fermi surface segment (Fermi arc) is attributed to the outer planes, and the two closed Fermi surfaces are attributed to the middle-inner planes (larger Fermi pocket) and the inner-most planes (smaller Fermi pocket). Due to inter-layer hopping, one expects the slight splitting of the IP1 and OP bands. However due to the finite resolution such splitting can be hard to detect experimentally. The geometry and the location of IP0 and IP1 are consistent with the Fermi pockets due to antiferromagnetic order. This claim is supported by the NMR measurement of local magnetic moment[3]. By the measuring the Fermi pocket areas associated with IP0 and IP1, the estimated hole density are ∼ 0.04 and ∼ 0.09, respectively. This is consistent with their quantum oscillation result. The fact that the observed pockets are hole pockets is consistent with the Hall coefficient measurement. Both the OP and IP1 shows a superconducting gap but not IP0. Surprisingly the energy gap associated with IP1 is found to be larger than that associated with OP (see Fig.2(a) and