Quantum Phase transition under pressure in a heavily hydrogen-doped iron-based superconductor LaFeAsO

TL;DR

This study investigates how applying pressure affects the magnetic and superconducting phases in heavily hydrogen-doped LaFeAsO, revealing a quantum critical point and limited influence of AF fluctuations on optimal superconductivity.

Contribution

It demonstrates that pressure suppresses the second antiferromagnetic phase and reveals a quantum critical point in heavily H-doped LaFeAsO, providing new insights into its phase diagram.

Findings

Pressure suppresses the second AF phase at 3.0 GPa.

A quantum critical point emerges near high H-doping levels.

Optimal Tc remains unaffected by AF fluctuations.

Abstract

Hydrogen (H)-doped LaFeAsO is a prototypical iron-based superconductor. However, its phase diagram extends beyond the standard framework, where a superconducting (SC) phase follows an antiferromagnetic (AF) phase upon carrier doping; instead, the SC phase is sandwiched between two AF phases appearing in lightly and heavily H-doped regimes. We performed nuclear magnetic resonance (NMR) measurements under pressure, focusing on the second AF phase in the heavily H-doped regime. The second AF phase is strongly suppressed when a pressure of 3.0 GPa is applied, and apparently shifts to a highly H-doped regime, thereby a "bare" quantum critical point (QCP) emerges. A quantum critical regime emerges in a paramagnetic state near the QCP, however, the influence of the AF critical fluctuations to the SC phase is limited in the narrow doping regime near the QCP. The optimal SC condition ($T_c \sim$ 48 K) is unaffected by AF fluctuations.