First-principles study of magnetism, lattice dynamics, and superconductivity in LaFeSiH$_x$

TL;DR

This study uses first-principles calculations to explore how hydrogen content affects the magnetic, vibrational, and superconducting properties of LaFeSiH$_x$, revealing potential for superconductivity and proton conduction.

Contribution

It provides a detailed first-principles analysis of LaFeSiH$_x$, highlighting the impact of hydrogen doping on magnetism, electron-phonon coupling, and superconductivity, which was not previously characterized.

Findings

Fully hydrogenated LaFeSiH is antiferromagnetic.

Superconducting $T_c$ is estimated to be 2-10 K for magnetic compositions.

LaFeSiH$_x$ may serve as a proton conductor with phase stability across various hydrogen levels.

Abstract

The structural, electronic, magnetic, and vibrational properties of LaFeSiH$_x$ for $x$ between 0 and 1 are investigated using density functional calculations. We find that the electronic and magnetic properties are strongly controlled by the hydrogen concentration $x$ in LaFeSiH$_x$. While fully hydrogenated LaFeSiH has a striped antiferromagnetic ground state, the underdoped LaFeSiH$_x$ for $x\leq0.75$ is not magnetic within the virtual crystal approximation or with explicit doping of supercells. The antiferromagnetic configuration breaks the symmetry of Fe $d$ orbitals and increases electron-phonon coupling up to $50\%$, especially for modes in the 20-50 meV range that are associated with Fe atomic movement. We find competing nearest and next-nearest neighbor exchange interactions and significant spin-phonon coupling, qualitatively similar but smaller in magnitude compared those found in LaOFeAs superconductors. The superconducting $T_c$ for antiferromagnetic LaFeSiH$_x$, assuming conventional superconductivity via McMillan's equation, therefore is computed to be 2-10 K, in contrast to $T_c\approx0$ for the nonmagnetic material. We also predict that the LaFeSiH$_x$ could be a good proton conductor due to phase stability with a wide range of hydrogen concentration $x < 1$.