Origin of critical-temperature enhancement of an iron-based high-Tc superconductor, LaFeAsO1-xFx : NMR study under high pressure
Abstract
Nuclear magnetic resonance (NMR) measurements of an iron (Fe)-based superconductor LaFeAsO1-xFx (x = 0.08 and 0.14) were performed at ambient pressure and under pressure. The relaxation rate 1/T1 for the overdoped samples (x = 0.14) shows T-linear behavior just above Tc, and pressure application enhances 1/T1T similar to the behavior of Tc. This implies that 1/T1T = constant originates from the Korringa relation, and an increase in the density of states at the Fermi energy D(EF) leads to the enhancement of Tc. In the underdoped samples (x = 0.08), 1/T1T measured at ambient pressure also shows T-independent behavior in a wide temperature range above Tc. However, it shows Curie-Weiss-like T dependence at 3.0 GPa accompanied by a small increase in Tc, suggesting that predominant antiferromagnetic fluctuation suppresses development of superconductivity or remarkable enhancement of Tc. The qualitatively different features between underdoped and overdoped samples are systematically explained by a band calculation with hole and electron pockets.