Effects of high magnetic fields on the pseudogap and d-wave superconductivity in the underdoped and overdoped high-T-c superconductors

Based on measurements of the (CU)-C-63 Knight shift and the nuclear spin-la ttice relaxation rate under magnetic fields up to 28 T, we find that the ps eudogap in the slightly overdoped TISr2CaCu2O6.88 (T-c=68 K) is due to Coop er pair density fluctuations. Such fluctuations are also found in the under doped YBa2Cu4O8 (T-c=74 K), but are small and only survive up to T-sf simil ar or equal to T-c+15K=90 K, which is much lower than the held-independent pseudogap temperature T* similar or equal to 160 K. The implication on the phase diagram is discussed. At low temperatures in the superconducting stat e, a field-induced finite density of states was found in both compounds and is interpreted as due to the quasiparticles outside a vortex core in a d-w ave superconductor.