Strongly Correlated Electrons

Abstract

Electron correlations are strong when the on-site electron-electron repulsions U are much larger than the energies associated with the hybridization of atomic orbitals belonging to different atoms (resonance energies). The latter are characterized in a solid by the width Wof the energy band under consideration. A large ratio U/W is expected in systems involving 4f or 5/ electrons, i.e., rare earth or actinide atoms. Yet systems with d electrons can be strongly correlated, too. A famous example is CoO. If we treated it within the independent electron approximation we would find that this substance is metallic, with an odd number of electrons per unit cell and a partially filled d band. In reality, however, CoO is an insulator. The same holds true for La₂CuO₄, a prototype for a class of materials with high superconducting transition temperatures (Sect. 14.2). CoO and La₂CuO₄ are not metallic because the strong electron correlations suppress the charge fluctuations required for a nonvanishing conductivity. Instead they are insulators.