Electronic structure and photoemission studies of late transition-metal oxides — Mott insulators and high-temperature superconductors

Abstract

Stimulated by the discovery of high-temperature superconductivity, the electronic structure of late 3d transition-metal oxides is presently one of the most extensively studied subjects in condensed matter physics. In this review, we hope to summarize the progress we have made and the problems we are facing. The emphasis of the review is on the latest angle-resolved photoemission studies that have provided much insight towards the understanding of these materials. This includes the recent experiments from transition-metal mono-oxides, normal state electronic structure and Fermi surface mapping of Bi 2Sr 2CaCu 2O 8+δ, Bi 2Sr 2CuO 6+δ, YBa 2Cu 3O 7− x , YBa 2Cu 4O 8, Nd 2− x Ce x CuO 4, and the superconducting gap of Bi 2Sr 2CaCu 2O 8+δ. For the transition-metal mono-oxides, we discuss the experimental manifestation of the four aspects of the electronic structure that make these Mott Hubbard insulators so interesting. This includes the large Coulomb interaction U (on the cation sites), the charge transfer as a result of strong hybridization, the energy dispersion in the crystal lattice, and the multiplet and magnetic splittings. For the high-temperature superconductors, we concentrate on the low energy excitations, the topology of the Fermi surface in the normal state, and the superconducting gap. Angle-resolved photoemission data show that the oxide superconductors have well defined Fermi surfaces. The volume of the Fermi surface in the high doping regime appears to be consistent with the results of band calculations. A striking feature of the low energy excitations is the presence of some very flat bands (due to a saddle point singularity in the band structure) which lie near the Fermi energy in p-type compounds near their optimal doping levels for superconductivity. The corresponding flat bands are well below the Fermi energy in n-type cuprates. The energy position of these flat bands is expected to have wide ranging effects on the physical properties of these materials, including the temperature dependence of the resistivity and the superconducting transition temperature. High-resolution photoemission has also been successfully applied to the study of the superconducting gap in Bi 2Sr 2CaCu 2O 8+δ. While the presently attainable energy resolution is poorer than that of many other spectroscopies, photoemission has the advantage that it is k-resolved as well as being very direct. This unique capability has enabled recent photoemission experiments to reveal the highly anisotropic nature of the superconducting gap in the a−b plane. This suggests the possibility of a detailed experimental determination of the superconducting order parameter.