Nambu's theory and solid state physics. Focusing on superconductivity

The contribution of Nambu's theory to solid state physics is reviewed in focusing on superconductivity from the past to a prospect of the future. The correlation between BCS theory and Nambu's theory is discussed from the point of view of symmetry breakdown of a fermion system. In Association with symmetry breakdown, collective modes proportional to wave number in a dispersion relation are excited at a long wavelength limit. After the correlation between Meissner effect and gauge symmetry breakdown is referred, it is pointed out that Anderson-Higgs mechanism is built on Nambu-Goldstone theorem. Various superconductivity mechanisms were briefly introduced. For a newly discovered iron-based high Ts-superconductor, the Fe d-bands contribute an essential role for the origin of the superconductivity. The Fe-based superconductor falls to the Uemura classification scheme, which considers the correlation between the superconducting transition temperature and the effective Fermi temperature, in the same way of unconventional superconductors. (Y.K.)