Theory of high-temperature superconductivity in underdoped cuprates; the unusual electronic structure and the origin of the pseudogap

Abstract

By reviewing the first-principles studies of the many-electron electronic structures of underdoped and performed by Kamimura and co-workers, unusual electronic states are clarified. That is, the dopant holes move coherently by taking the Zhang-Rice spin singlet and Hund's coupling spin triplet alternately in the spin-correlated region of antiferromagnetic ordering due to the Cu localized spins, without destroying the antiferromagnetic order. This creates a metallic state which leads to superconductivity. The coexistence of (i) the antiferromagnetic spin ordering and (ii) the ordering in the appearance of the Zhang-Rice singlet and the Hund's coupling triplet results in the small Fermi surface for a carrier system and also leads to the decrease in the electronic entropy below a certain temperature at which the Fermi surface changes from a larger one to smaller ones. This is the microscopic origin of the pseudogap, the concept of which was originally proposed by Loram and co-workers in terms of the Fermi liquid picture.