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 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≤15 K, the H and T-variation of the ⁶³Cu nuclear spin-lattice relaxation rate (1/T₁) is 1/T₁∝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.