Pseudogap and the superconducting energy in hole-doped high-temperature superconductors

Abstract

The pseudogap measured by different techniques in the high-temperature superconductors is interpreted as a two-particle screened Zhang-Rice singlet type state. The so-called (neutron) resonance energy measures the energy lowering of the superconducting state relative to the normal one and should be called the superconducting energy or the order parameter. The superconductivity in the high-temperature superconductors is in the crossover regime between a Bose-Einstein condensate and a BCS state, similar in physics to the situation for the fermionic cold atoms. The superconducting transition temperature is given as the product of the pseudogap energy $({E}_{\text{pg}})$ and the carrier density, leading to its parabolic dependence on the doping. The formation of the Zhang-Rice singlet and the accompanying creation of the pseudogap are instrumental for the occurrence of the high-temperature superconductivity, as also for the large magnetoresistance in the manganites. The lowest ionization state in transition-metal compounds therefore constitutes a promising tool for the engineering of materials.