Pseudogap and superconductivity under high magnetic field in underdoped high-T_c cuprate

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. Even though H=23.2 T reduces T_c by 19 K, no effect is seen on the normal-state pseudogap, which continues in the temperature region where the superconductivity is destroyed by the field. This suggests that the pseudogap is a consequence of strong electron correlation effects. For temperatures T ≤ 15 K, the H-and T-variation of the ⁶³Cu nuclear spin-lattice relaxation rate (1/T₁) is 1/T₁ α 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.