A two-plaquette polaron picture for the single-hole state of cuprates

The pseudogap phase in high T c superconductors has attracted intense research efforts in recent years. This phase is expected to be intimately linked with the state of an excess O hole doped in the CuO 2 plane. In the present paper, we investigate the nature of this single-hole state theoretically. Special attention is paid to the influences from the O sublattice. We argue that in the case of strong covalent bonding, the motions of O atoms can be very anisotropic: these atoms may find themselves in a single-well potential when moving perpendicular to the CuOCu bond directions, but in a double-well potential when moving along that direction. As a consequence, a two-plaquette localized polaron state can be realized for a doped hole in the CuO 2 plane. The energy of this state is evaluated via a variational method. It breaks several symmetries locally and these symmetries have been experimentally observed to be broken also in the pseudogap phase.