Enhancement of the critical temperature of the pnictide superconductor LaFeAsO_1-xF_x studied via⁷⁵As NMR under pressure

Nuclear-magnetic-resonance measurements of an iron (Fe)-based superconductor LaFeAsO_1-xF_x (x=0.08 and 0.14) were performed at ambient pressure and under pressure. The relaxation rate 1/ T ₁ for the overdoped samples (x=0.14) shows T -linear behavior just above T_c, and pressure application enhances 1/T₁ T similar to the behavior of T_c This implies that 1/T₁ T=constant originates from the Korringa relation, and an increase in the density of states at the Fermi energy D (E_F) leads to the enhancement of Tc. In the underdoped samples (x=0.08), 1/ T1 measured at ambient pressure also shows T -linear behavior in a wide temperature range above T_c. However, 1/ T₁ T shows Curie-Weiss-type T dependence at 3.0 GPa accompanied by a small increase in Tc, suggesting that predominant low-frequency antiferromagnetic fluctuation is not important for development of superconductivity or remarkable enhancement of T_c. The qualitatively different features between underdoped and overdoped samples are systematically explained by a band calculation with hole and electron pockets.