Quantum critical behavior in heavily dopedLaFeAsO1−xHxpnictide superconductors analyzed using nuclear magnetic resonance

Abstract

We studied the quantum critical behavior of the second antiferromagnetic (AF) phase in the heavily electron-doped high-${T}_{c}$ pnictide, ${\mathrm{LaFeAsO}}_{1\ensuremath{-}x}{\mathrm{H}}_{x}$, by using a ${}^{75}\mathrm{As}$ and ${}^{1}\mathrm{H}$ nuclear magnetic resonance (NMR) technique. In the second AF phase, we observed a spatially modulated spin-density-wave-like state up to $x=0.6$ from the NMR spectral line shape and detected a low-energy excitation gap from the nuclear relaxation time ${T}_{1}$ of ${}^{75}\mathrm{As}$. The excitation gap closes at the AF quantum critical point (QCP) at $x\ensuremath{\approx}0.49$. The superconducting phase in a lower-doping regime contacts the second AF phase only at the AF QCP, and both phases are segregated from each other. The absence of AF critical fluctuations and the enhancement of the in-plane electric anisotropy are key factors for the development of superconductivity.