Superconductivity of LaFeAsO_1-xF_x under high pressure studied using NMR

Nuclear magnetic resonance (NMR) on an iron (Fe)-based superconductor LaFeAsO_1-xF_x (x = 0.08 and 0.14) was performed at ambient pressure and under pressure. The relaxation rate 1/T₁ for x = 0.14 shows T-linear behavior just above T_c, and pressure application enhances 1/T₁T similar to T_c. This fact implies that 1/T₁T = constant originates from Korringa relation and an increase in the density of state at the Fermi energy D(E_F) leads to the enhancement of T_c. In the case of x = 0.08, 1/T₁ measured at ambient pressure also shows T-linear behavior in a wide temperature range above T_c. The value of 1/T₁T is enhanced compared to the x = 0.14 doped sample, although their T_c values are in the same range. The enhancement of 1/T₁T for x = 0.08 is attributable to antiferromagnetic fluctuation rather than an increase in D(E_F).