Responses of the pseudogap and d-wave superconductivity to high magnetic fields in the underdoped high-$T_c$ superconductor ${\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8$: An NMR study

The responses to the magnetic field (H) of the pseudogap and the superconductivity in the underdoped high-$T_c$ cuprate ${\mathrm{YBa}}_2{\mathrm{Cu}}_4{\mathrm{O}}_8\hspace{0.5em}{(T}_c=74\mathrm{}\hspace{0.5em}\mathrm{K})$ are reported up to 23.2 T based on NMR measurements. Even though 23.2 T reduces $T_c$ by 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 $T<~15\hspace{0.5em}\mathrm{K},$ the spin Knight shift increases with increasing H and also the H and T variation of the ${}^{63}\mathrm{Cu}$ nuclear spin-lattice relaxation rate ${(1/T}_1)$ is ${1/T}_1\propto \mathrm{TH}.\mathrm{}$ 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.