# Quantum dynamics of hydrogen in iron-based superconductor LaFeAsO0.9D0.1 measured with inelastic neutron spectroscopy

## TL;DR

This study investigates hydrogen's quantum behavior in an iron-based superconductor using inelastic neutron scattering, revealing quantum rattling phenomena linked to potential minima and superconducting properties.

## Contribution

It provides the first experimental evidence of hydrogen quantum rattling in a superconductor, combining neutron scattering with first-principles calculations.

## Key findings

- Emergence of new excitations below Tc in the 5-15 meV range.
- Strong excitations at 14.5 meV and 11.1 meV broaden near 15-20 K.
- Quantum rattling of hydrogen linked to multiple potential minima.

## Abstract

Inelastic neutron scattering was performed for an iron-based superconductor LaFeAsO0.9D0.1, where most of D (deuterium) replaces oxygen, while a tiny amount goes into interstitial sites. By first-principle calculation, we characterize the interstitial sites for D (and for H slightly mixed) with four equivalent potential minima. Below the superconducting transition temperature Tc = 26 K,new excitations emerge in the range 5-15 meV, while they are absent in the reference system LaFeAsO0.9F0.1. The strong excitations at 14.5 meV and 11.1 meV broaden rapidly around 15 K and 20 K, respectively, where each energy becomes comparable to twice of the superconducting gap. The strong excitations are ascribed to a quantum rattling, or a band motion of hydrogen, which arises only if the number of potential minima is larger than two.