Superconductivity of LaFeAsO1-xFx under high pressure studied using NMR

Nuclear magnetic resonance (NMR) on an iron (Fe)-based superconductor LaFeAsO1-xFx (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 Tc, and pressure application enhances 1/T1T similar to Tc. This fact implies that 1/T1T = constant originates from Korringa relation and an increase in the density of state at the Fermi energy D(EF) leads to the enhancement of Tc. In the case of x = 0.08, 1/T1 measured at ambient pressure also shows T-linear behavior in a wide temperature range above Tc. The value of 1/T1T is enhanced compared to the x = 0.14 doped sample, although their Tc 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(EF).