Theory on Superconducting Transition from Pseudogap State

Abstract

The anomalous properties of High-$T_{{\\rm c}}$ cuprates are investigated both\nin the normal state and in the superconducting state. In particular, we pay\natte ntion to the pseudogap in the normal state and the phase transition from\nthe pse udogap state to the superconducting state.\n The pseudogap phenomena observed in cuprates are naturally understood as a\nprecursor of the strong coupling superconductivity. We have previously show n\nby using the self-consistent T-matrix calculation that the pseudogap is a resu\nlt of the strong superconducting fluctuations which are accompanied by the\nstron g coupling superconductivity in quasi-two dimensional systems [J. Phys.\nSoc. Jpn . {\\bf68} (1999) 2999.].\n We extend the scenario to the superconducting state. The close relation\nbetween the pseudogap state and the superconducting state is pointed out. Once\nt he superconducting phase transition occurs, the superconducting order\nparameter rapidly grows rather than the result of BCS theory. With the rapid\ngrowth of the order parameter, the gap structure becomes sharp, while it is\nremarkably broad in the pseudogap state. The characteristic energy scale of the\ngap does not chan ge. These results well explain the phase transition observed\nin the spectroscopi c measurements.\n Further, we calculate the magnetic and transport properties which show the\npseu dogap phenomena. The comprehensive understanding of the NMR, the neutron\nscatter ing, the optical conductivity and the London penetration depth is\nobtained both in the pseudogap state and in the superconducting state.\n