Pseudogap and superconductivity under high magnetic field in underdoped high-T-c cuprate

The responses to the magnetic held (H) of the superconductivity and the pse udogap in the underdoped high-T-c cuprate YBa2Cu4O8 (T-c=74 K) are reported up, to 23.2 T based on NMR measurements. Even though H=23.2 T reduces T-c by 19 K, no effect is seen on the normal-state pseudogap, which continues i n the temperature region where the superconductivity is destroyed by the fi eld. This suggests that the pseudogap, is a consequence of strong electron correlation effects. I;br temperatures T less than or equal to 15 It, the H - and 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 incre asing H. We interpret our results as evidence for H-induced quasiparticle s tates that extend from the d-wave vortex centers with an ungapped spectrum.