抄録
A theory of the Meissner effect due to Cooper pairs between Bloch electrons is presented in the gauge-invariant form by including effects of strong electron correlation. The theory is applied to analyzing observed penetration depth λ(T) of YBa2Cu3O7 with Tc≈90 K. Observed λ(0)≈1400 Å can be explained, if the effective mass of quasiparticles is about ten times as large as the electron mass. By assuming that the superconducting gap is as large as 2εG⁄Tc≈8, as implied by many experiments, it is shown that anisotropic Cooper pairs between heavy electrons in the Mott-transition region can explain the observed λ(T) above 35 K. One of the possible scenarios to explain the data below 35 K is a successive superconducting transition at Tc2≈35 K, as implied by small but sharp anomalies in the transverse nuclear quadrupole relaxation rate. It is argued that the complete gap is open below Tc2≈35 K.
