Origin of critical-temperature enhancement of an iron-based high-T_c superconductor, LaFeAsO_{1-x}F_{x} : NMR study under high pressure

TL;DR

This study uses NMR measurements under high pressure to investigate the mechanisms behind the critical temperature enhancement in LaFeAsO_{1-x}F_{x} superconductors, revealing the roles of density of states and magnetic fluctuations.

Contribution

It provides new insights into how pressure influences electronic states and magnetic fluctuations, affecting T_c in iron-based superconductors, supported by experimental NMR data and band calculations.

Findings

Pressure increases density of states at Fermi level in overdoped samples.

Antiferromagnetic fluctuations suppress T_c in underdoped samples.

Different behaviors in underdoped and overdoped samples explained by band structure analysis.

Abstract

Nuclear magnetic resonance (NMR) measurements of an iron (Fe)-based superconductor LaFeAsO_{1-x}F_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/T_1T similar to the behavior of T_c. This implies that 1/T_1T = 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 T_c. In the underdoped samples (x = 0.08), 1/T_1T measured at ambient pressure also shows T-independent behavior in a wide temperature range above T_c. However, it shows Curie-Weiss-like T dependence at 3.0 GPa accompanied by a small increase in T_c, suggesting that predominant antiferromagnetic fluctuation suppresses 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.