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 ⁶³Cu 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 TlSr₂CaCu₂O_6.88 (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 ≃ T_c+15K=90 K, which is much lower than the field-independent pseudogap temperature T* ≃ 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.