Theory of pseudogap phenomena in high-T-c cuprates and their magnetic field dependences

We point out that the pseudogap phenomena observed in under-doped high-T-c, cuprates are naturally understood as a precursor of the strong coupling su perconductivity and this idea does not contradict with their magnetic field dependences. The strong coupling superconductivity necessarily leads to th e strong fluctuations with a character of the pre-formed pairs. The pre-for med pairs have small dispersion and are thermally excited in the vicinity o f the critical point. The gap structures of the spectral weight and the den sity of states result from such strong fluctuations and give rise to the re duction of the NMR spin-lattice relaxation rate in the wide temperature reg ion. On the basis of this idea, the magnetic field dependence of the pseudo gap phenomena is naturally shown to be remarkably small in the strong coupl ing case. These features well explain the experiments. (C) 2000 Elsevier Sc ience B.V. All rights reserved.