Quasiparticles in strongly correlated electron systems in copper oxides

New experimental and theoretical results on the electronic structure and spectral properties of quasiparticles in copper oxides are reviewed. It is shown that the electronic structure transforms from antiferromagnetic insulators to optimally doped high-temperature superconductors as the doping level is varied. The experimental methods considered are primarily angular resolved photoelectron spectroscopy (ARPES), neutron scattering, and NMR. Two types of electronic structure calculations for data interpretation purposes are considered, namely, exact numerical methods for finite clusters (exact diagonalization and the quantum Monte Carlo method) and approximate schemes for an infinite lattice. As a result, a coherent unified picture emerges, in which magnetic polarons (which are carriers in a weakly doped antiferromagnetic lattice) transform into a system of Fermi quasiparticles dressed in short-range antiferromagnetic-type spin fluctuations. In the region of weakly doped metallic compositions, deviations from Fermi-liquid properties are seen, such as the failure of Luttinger's theorem, shadowy photoemission bands, and the spin pseudogap effect in spectral and thermodynamic measurements. The situation in the neighborhood of the insulator—metal concentration transition is noted to be least understood.