# Pressure-induced quantum critical point in a heavily hydrogen-doped iron-based superconductor LaFeAsO

## TL;DR

This study reveals a pressure-induced quantum critical point in a heavily hydrogen-doped iron-based superconductor, where magnetic and electronic states undergo significant changes under pressure, highlighting the interplay between doping, pressure, and electronic structure.

## Contribution

It demonstrates the existence of a quantum critical point in LaFeAsO$_{1-x}$H$_x$ at high doping levels under pressure, linking magnetic phase transitions to orbital and Fermi surface changes.

## Key findings

- Pressure up to 2 GPa maintains a gapped spin-density-wave state.
- At 4.0 GPa, the gap closes indicating a quantum critical point.
- Coexistence of gapped and gapless magnetic fluctuations between 2 and 4 GPa.

## Abstract

An iron-based superconductor LaFeAsO$_{1-x}$H$_x$ (0 $\leq x \leq$ 0.6) undergoes two antiferromagnetic (AF) phases upon H doping. We investigated the second AF phase ($x$=0.6) using NMR techniques under pressure. At pressures up to 2 GPa, the ground state is a spin-density-wave state with a large gap; however, the gap closes at 4.0 GPa, suggesting a pressure-induced quantum critical point. Interestingly, the gapped excitation coexists with gapless magnetic fluctuations at pressures between 2 and 4 GPa. This coexistence is attributable to the lift up of the $d_{xy}$ orbital to the Fermi level, a Lifshitz transition under pressure.