d-wave vortex and pseudogap in underdoped high-Tc, cuprate YBa2Cu4O8:: high-magnetic-field NMR

The responses to the magnetic field (H) of the superconductivity and the pseudogap in the underdoped high- T-c cuprate YBa2Cu4O8 (T-c = 74 K) are reported up to 23.2 T based on NMR measurements. For temperatures T less than or equal to 15 K, the Hand T-variation of the Cu-63 nuclear spin-lattice relaxation rate (1/T-1) is 1/T-1 proportional to TH and also the Knight shift increases with increasing H. We interpret our results as evidence for H-induced quasiparticle states that extend from the d-wave vortex centers with an ungapped spectrum. Even though H = 23.2 T reduces T-c by 19 K, no effect is seen on the normal-state pseudogap which persists in the temperature region where the superconductivity is destroyed by the field, suggesting that it is a consequence of strong electron correlations. (C) 2000 Elsevier Science B.V. All rights reserved.