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-lattice relaxation rate under magnetic fields up to 28 T, we find that the pseudogap in the slightly overdoped TISr2CaCu2O6.88 (T-c=68 K) is due to Cooper pair density fluctuations. Such fluctuations are also found in the underdoped YBa2Cu4O8 (T-c=74 K), but are small and only survive up to T-sf similar 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 state, 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-wave superconductor.