Responses of the pseudogap and d-wave superconductivity to high magnetic fields in the underdoped high-T-c superconductor YBa2Cu4O8: An NMR study

The responses to the magnetic field (H) of the pseudogap and the supercondu ctivity in the underdoped high T-c cuprate YBa2Cu4O8 (T-c=74 K) are reporte d up to 23.2 T based on NMR measurements. Even though 23.2 T reduces T, by 26%, no effect is seen on the normal-state pseudogap, which persists in the temperature range where the superconductivity is destroyed by the field. F or temperatures T less than or equal to 15 K, the spin Knight shift increas es with increasing H and also the H and T variation of the Cu-63 nuclear sp in-lattice relaxation rate (1/T-1) is 1/T-1 proportional to TH. A possible explanation for this finding is offered as due to the H-induced quasipartic le states that extend from the d-wave vortex centers with ungapped spectrum . [S0163-1829(99)50438-4].