Pseudogap in high-temperature superconductors

This paper reviews the basic experimental facts and a number of theoretical models relevant to the understanding of the pseudogap state in high-temper ature superconductors. This state is observed in the region of less-than-op timal current carrier concentrations in the HTSC cuprate phase diagram and manifests itself by different anomalies in electronic properties, presumabl y due to fluctuations in the short-range antiferromagnetic order that occur as the antiferromagnetic region of the phase diagram is approached. The in teraction of current carriers with these fluctuations leads to the anisotro pic renormalization of the electronic spectrum and causes the system to beh ave as a non-Fermi liquid on certain portions of the Fermi surface. Simple theoretical models for describing the basic properties of the pseudogap sta te, in particular renormalization-induced anomalies in the superconducting state, are discussed.