Correlation Effects on Coupled Electronic and Structural Properties of Doped Rare-Earth Trihydrides

TL;DR

This study investigates how lattice distortions and electron correlation effects influence the electronic and structural properties of doped rare-earth trihydrides using advanced computational methods.

Contribution

It applies DFT+U and Quantum Monte Carlo methods to elucidate correlation effects and lattice distortions in $R$H$_3$, providing insights beyond standard DFT approaches.

Findings

Small Hubbard U values for $R_d$ and H$_s$/N$_p$ orbitals determined by LR-cDFT.

Pure FCC $R$H$_3$ remains metallic without a band gap in standard DFT.

Hydrogen distortions induce a small band gap, enhanced by DFT+U with self-consistent U.

Abstract

Rare-earth trihydrides ($R$H$_3$) exhibit intriguing coupled electronic and structural properties as a function of doping, hydrogen vacancies, and thermodynamic conditions. Theoretical studies of these materials typically rely on density functional theory (DFT), including the use of small supercells that may underestimate strong correlation effects and structural distortions which in turn may influence their metallicity. Here, we elucidate the roles of lattice distortions and correlation effects on the electronic properties of pristine and doped $R$H$_3$ by adopting DFT+U and Quantum Monte Carlo (QMC) methods. Linear-response constrained DFT (LR-cDFT) methods find Hubbard U $\approx 2$ eV for $R_d$ orbitals and U$\approx 6$ eV for H$_s$/N$_p$ orbitals. The small U on Lu$_d$ orbitals is consistent with QMC calculations on LuH$_3$ and LuH$_{2.875}$N$_{0.125}$. In pure face-centered-cubic (FCC) $R$H$_3$ ($R$=Lu,Y), neither DFT nor DFT+U with the self-consistently determined U is enough to create a band gap, however a supercell with hydrogen distortions creates a small gap whose magnitude increases when performing DFT+U with self-consistently determined U values. Correlation effects, in turn, have a moderate influence on the coupled structural and electronic properties of doped RH$_3$ compounds and may be important when considering the competition between structural distortions and superconductivity.