Magnetic fluctuations and superconductivity inLaFeAsO1−xFxunder pressure as seen viaA75sNMR

Abstract

The relationship between antiferromagnetic (AF) fluctuation and superconductivity was investigated in the La1111 series, ${\text{LaFeAsO}}_{1\ensuremath{-}x}{\text{F}}_{x}$ ($x=0$, 0.05, 0.08, 0.10, and 0.14) by examining nuclear relaxation rates $(1/{T}_{1})$ at both ambient pressure and 3.0 GPa. The results show that the critical doping level at which low-frequency AF fluctuation vanishes is around the optimally doped regime $(x\ensuremath{\sim}0.10)$. Although the AF fluctuation is enhanced by applying pressure in the underdoped regime $(0.05\ensuremath{\le}xl0.10)$, the increase in critical transition temperature $({T}_{c})$ is small, whereas ${T}_{c}$ remarkably increases in the overdoped regime $(x=0.14)$, implying that the AF fluctuation is less important to the high-${T}_{c}$ mechanism than the density of states at the electron pocket. The $x$ dependence of ${T}_{c}$ at 3.0 GPa is similar to that of $R1111$ ($R=\text{Ce}$, Pr, Nd, etc.) with ${T}_{c}\ensuremath{\ge}40\text{ }\text{K}$ at ambient pressure. The relationship between ${T}_{c}$ and the pnictogen height or lattice constant indicates that pressure application is equivalent to full rare-earth substitution. This equivalence suggests that high ${T}_{c}$ above 40 K is realized when the AF fluctuation is absent.