Recent Progress in Physics of High-Temperature Superconductors

One hundred years after the discovery of superconductivity, we are now facing a new era that demands an increase in the superconducting transition temperatureTc. In addition to copper-based superconductors, iron-based superconductors that have been discovered recently have been considered high-temperature superconductors. The similarity and difference between the two high-Tcsystems are discussed on the basis of our recent theoretical and experimental understandings. While the pairing mechanism and non-Fermi liquid behaviors in transport properties may have a common origin between the two systems, the strengths of electron correlation are different: Cuprate is a doped Mott insulator, while iron pnictide is an itinerant system with a weak correlation. Pseudogap phenomena in hole-doped cuprates and their absence in electron-doped cuprate are regarded as a consequence of a strong correlation. Recent topics in cuprates about electron–hole asymmetry and pseudogap phenomenon are reviewed from a theoretical viewpoint. For iron pnictides, anisotropic behaviors in antiferromagnetic phases and new iron-selenide superconductors are discussed.