Pseudogap in high-temperature superconductors

The paper reviews the basic experimental facts and a number of theoretical models relevant to the understanding of the pseudogap state in high-temperature superconductors. The state is observed in the region of less-than-optimal current-carrier concentrations in the HTSC cuprate phase diagram and manifests itself as various anomalies in the electronic properties, presumably due to the antiferromagnetic short-range-order fluctuations that occur as the antiferromagnetic region of the phase diagram is approached. The interaction of current carriers with these fluctuations leads to an anisotropic transformation of the electron spectrum and causes the system to behave as a non-Fermi liquid in certain regions of the Fermi surface. Simple theoretical models for describing the basic properties of the pseudogap state, in particular renormalization-induced anomalies in the superconducting state, are discussed.