arXiv subjects

Russell J Hemley

Publications and source records attributed to Russell J Hemley.

3 recordsLinked to original sources

arXiv2024

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

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.

cond-mat.str-el↗

arXiv2024

Designing Quaternary Hydrides with Potential High T$_c$ Superconductivity

While hydrogen-rich materials have been demonstrated to exhibit high T$_c$ superconductivity at high pressures, there is an ongoing search for ternary and quaternary hydrides that achieve such high critical temperatures at much lower pressures. First-principles searches are impeded by the computational complexity of solving the Eliashberg equations for large, complex crystal structures. Here, we adopt a simplified approach using electronic indicators previously established to be correlated with superconductivity in hydrides. This is used to study complex hydride structures, which are predicted to exhibit promisingly high critical temperatures for superconductivity. In particular, we propose three classes of hydrides inspired by the FCC RH$_3$ structures that exhibit strong hydrogen network connectivity, as defined through the electron localization function. The first class [RH$_{11}$X$_3$Y] is based on a Pm$\overline{3}$m structure showing moderately high T$_c$, where the T$_c$ estimate from electronic properties is compared with direct Eliashberg calculations and found to be surprisingly accurate. The second class of structures [(RH$_{11}$)$_2$X$_6$YZ] improves on this with promisingly high density of states with dominant hydrogen character at the Fermi energy, typically enhancing T$_c$. The third class [(R$^1$H$_{11}$)(R$^2$H$_{11}$)X$_6$YZ] improves the strong hydrogen network connectivity by introducing anisotropy in the hydrogen network through a specific doping pattern. These model structures and the design principles provide the enough flexibility to optimize both T$_c$ and the structural stability of complex hydrides.

cond-mat.supr-con↗

arXiv2024

Quantum Stabilization and Flat Hydrogen-based Bands of Nitrogen-doped Lutetium Hydride

We explore electronic and structural properties of Fm$\overline{3}$m Lu-H-N structures with specific N,H ordering as plausible candidates for near-ambient superconductivity possibly originating from their remarkably narrow hydrogen-based bands at the Fermi level. Although LuH$_{2.875}$N$_{0.125}$ exhibits an instability persisting up to 17 GPa, it is anharmonically stable near ambient pressure when accounting for quantum nuclear effects. The presence of flat bands near $E_\text{F}$ is understood to arise from destructive\ quantum interference between N-p and surrounding H-s orbitals, with certain types of defects leaving the flat bands unaffected. The results suggest there is an optimal pressure near ambient where the superconducting $T_{\text{c}}$ is maximized in this structure by anharmonically-stabilized low-frequency and non-adiabatically coupled high-frequency hydrogen modes. Despite the metastability of this structure, its electronic properties and dynamical stability when calculated beyond a classical harmonic approach can explain the reported near-ambient superconductivity in Lu-H-N.

cond-mat.supr-con↗