Quantum critical behavior in heavily doped LaFeAsO$_{1-x}$H$_x$ pnictide superconductors analyzed using nuclear magnetic resonance

TL;DR

This study investigates the quantum critical behavior in heavily doped LaFeAsO$_{1-x}$H$_x$ superconductors using NMR, revealing a quantum critical point where antiferromagnetic order vanishes and superconductivity emerges.

Contribution

It provides detailed NMR evidence of a quantum critical point in heavily doped LaFeAsO$_{1-x}$H$_x$, highlighting the relationship between antiferromagnetic order and superconductivity.

Findings

Detection of a spatially modulated spin-density-wave-like state up to x=0.6

Observation of a low-energy excitation gap closing at x≈0.49

Superconducting phase contacts the AF phase only at the QCP

Abstract

We studied the quantum critical behavior of the second antiferromagnetic (AF) phase in the heavily electron-doped high-$T_c$ pnictide, LaFeAsO$_{1-x}$H$_x$ by using $^{75}$As and $^{1}$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 lineshape and detected a low-energy excitation gap from the nuclear relaxation time $T_1$ of $^{75}$As. The excitation gap closes at the AF quantum critical point (QCP) at $x \approx 0.49$. The superconducting (SC) 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.