Particle-hole symmetry in the antiferromagnetic state of the cuprates

In the layered cuprate perovskites, the occurrence of high-temperature superconductivity seems deeply related to the unusual nature of the hole excitations. The limiting case of a very small number of holes diffusing in the antiferromagnetic (AF) background may provide important insights into this problem. We have investigated the transport properties in a series of crystals of YBa₂Cu₃O_y, and found that the temperature dependencies of the Hall coefficient R_H and thermopower S change abruptly as soon as the AF phase boundary is crossed. In the AF state at low temperatures T, both R_H and S are unexpectedly suppressed to nearly zero over a broad interval of T. We argue that this suppression arises from near-exact symmetry in the particle-hole currents. From the trends in R_H and S, we infer that the symmetry is increasingly robust as the hole density x becomes very small (x ≃ 0.01). We discuss implications for electronic properties both within the AF state and outside.