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

The responses to the magnetic field (H) of the superconductivity and the ps eudogap in the underdoped high- T-c cuprate YBa2Cu4O8 (T-c = 74 K) are repo rted up to 23.2 T based on NMR measurements. For temperatures T less than o r equal to 15 K, the Hand T-variation of the Cu-63 nuclear spin-lattice rel axation 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-ind uced quasiparticle states that extend from the d-wave vortex centers with a n 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 reg ion where the superconductivity is destroyed by the field, suggesting that it is a consequence of strong electron correlations. (C) 2000 Elsevier Scie nce B.V. All rights reserved.