Bound Clusters and Pseudogap Transitions in Layered High-Tc Superconductors

We show that negatively charged dopant ions in hole-doped, layered, high-Tc superconductors induce at low doping bound clusters of four holes in CuO layers. This phenomenon requires double degeneracy of the hole band provided by the anti-Jahn-Teller effect, where dopant ions push apical oxygen ions into their symmetry positions and release holes from Copper atoms. Experimentally, that is seen as nanoscale inhomogeneity in the pseudogap region. The broad pseudogap transition contains two separate transitions. We connect the higher temperature transition to unbinding of clustered holes out of the spin-singlet state in the CuO plane. The lower temperature transition removes directional stripy order of these dipolar clusters, and the bosonic, spin-zero band disappears. We report results on energetics of the cluster formation and fit the density parameter r_s and dielectric constant to Hall measurements. Our model leads to the quantum critical point at doping 0.2 where bound clusters disappear at zero temperature.