Soft-plasmon mechanism for high-temperature superconductivity

Recently-discovered high-temperature superconductors border on a Mott metal - insulator transition; pure La2CuO4 undergoes a commensurate charge-density-wave transition and becomes an insulator. At other compositions, the incipient charge-density wave causes a drastic softening of those plasmon modes with wave vectors connecting points on the Fermi surface. These soft plasmons (typically tenths of an electron-volt) serve as the attraction which binds Cooper pairs, instead of the usual phonons. Consequently, transition temperatures are an order of magnitude larger than for phonon-induced superconductivity. One of the factors responsible for the strong electron - plasmon coupling is the influence of the empty f-orbitals of La or Ba. (author). 25 refs