Responses of the pseudogap and d-wave superconductivity to high magnetic fields in the underdoped high-T_csuperconductor YBa₂Cu₄O₈: An NMR study

The responses to the magnetic field (H) of the pseudogap and the superconductivity in the underdoped high-T_ccuprate YBa₂Cu₄O₈(T_c=74K) are reported up to 23.2 T based on NMR measurements. Even though 23.2 T reduces T_cby 26%, no effect is seen on the normal-state pseudogap, which persists in the temperature range where the superconductivity is destroyed by the field. For temperatures T < ∼ 15K, the spin Knight shift increases with increasing H and also the H and T variation of the⁶³Cu nuclear spin-lattice relaxation rate (1/T₁) is 1/T₁∝ TH. A possible explanation for this finding is offered as due to the H-induced quasiparticle states that extend from the d-wave vortex centers with ungapped spectrum.