[Paper Review] Novel Electronic Structure of Nitrogen-Doped Lutetium Hydrides

Adam Denchfield, Hyowon Park+1 more|arXiv (Cornell University)|May 29, 2023

Superconducting Materials and Applications4 citations

TL;DR

This study uses first-principles DFT and DFT+U calculations to identify novel electronic structures in nitrogen-doped lutetium hydrides, specifically Fm¯3m Lu8H23-xN superlattices, which exhibit flat bands, van Hove singularities, and intersecting Dirac cones near the Fermi level. These features—enhanced by electron correlation effects—suggest a strong mechanism for high-temperature superconductivity, with predicted Tc values significantly exceeding previous estimates.

ABSTRACT

First-principles density functional theory (DFT) calculations of Lu-H-N compounds reveal low-energy configurations of Fm$\overline{3}$m Lu$_{8}$H$_{23-x}$N structures that exhibit novel electronic properties such as flat bands, sharply peaked densities of states (van Hove singularities, vHs), and intersecting Dirac cones near the Fermi energy (E$_F$). These N-doped LuH$_3$-based structures also exhibit an interconnected metallic hydrogen network, which is a common feature of high-T$_c$ hydride superconductors. Electronic property systematics give estimates of T$_c$ for optimally ordered structures that are well above the critical temperatures predicted for structures considered previously. The vHs and flat bands near E$_F$ are enhanced in DFT+U calculations, implying strong correlation physics should also be considered for first-principles studies of these materials. These results provide a basis for understanding the novel electronic properties observed for nitrogen-doped lutetium hydride.

Motivation & Objective

• To identify stable, low-energy electronic structures in the Lu-H-N system consistent with experimental constraints from recent reports of near-ambient superconductivity.
• To investigate the role of nitrogen doping and hydrogen vacancies in stabilizing electronic features conducive to high-Tc superconductivity.
• To evaluate the impact of electron correlation effects (via DFT+U) on electronic structure features such as van Hove singularities and flat bands near the Fermi level.
• To provide a theoretical basis for the observed near-ambient superconductivity in nitrogen-doped lutetium hydride by linking electronic topology to electron-phonon coupling.
• To reconcile discrepancies between prior DFT studies that failed to find strong electron-phonon coupling and the experimental observation of superconductivity at 1 GPa.

Proposed method

• Employed density functional theory (DFT) with the PBE functional to compute electronic structures and total energies of various superlattice configurations in the Fm¯3m Lu8H23-xN system.
• Applied DFT+U with U = 8.2 eV on Lu d-orbitals to account for strong electron correlation effects, particularly near the Fermi level.
• Constructed 2x2x2 supercells from the parent Fm¯3m LuH3 structure, incorporating nitrogen in octahedral, tetrahedral, and chain-like configurations, and introduced octahedral hydrogen vacancies.
• Performed structural relaxation without symmetry constraints to determine optimized lattice parameters and atomic positions.
• Calculated and compared x-ray diffraction patterns from relaxed structures with experimental data to validate structural models.
• Used projected density of states (PDOS) and band structure analysis to identify van Hove singularities, flat bands, and Dirac cone features near the Fermi energy.

Experimental results

Research questions

• RQ1 What stable electronic structures emerge in nitrogen-doped lutetium hydrides that are consistent with experimental constraints and could support high-Tc superconductivity?
• RQ2 How do nitrogen doping and hydrogen vacancies influence the formation of flat bands, van Hove singularities, and Dirac cones near the Fermi level?
• RQ3 To what extent do electron correlation effects (via DFT+U) enhance or modify the electronic features critical for strong electron-phonon coupling?
• RQ4 How do the predicted electronic structures compare with experimental x-ray diffraction patterns, particularly regarding superlattice reflections?
• RQ5 What is the role of orbital hybridization (N p, Lu d, H s) in shaping the density of states and Fermi surface topology near the Fermi level?

Key findings

• The Fm¯3m Lu8H23-xN superlattice structures with nitrogen in octahedral sites (e.g., structure A) exhibit flat bands and a sharp van Hove singularity (vHs) near the Fermi level, indicating strong electron correlation effects.
• DFT+U calculations with U = 8.2 eV on Lu d-orbitals significantly enhance the vHs and flat bands, confirming the importance of strong correlation physics in these materials.
• Intersecting Dirac cones are observed at the K and W points in the Brillouin zone for structure A, suggesting topologically nontrivial electronic behavior.
• The vHs near E_F is primarily composed of N p, Lu d_{eg}, and octahedral H s orbitals, with minor contributions from other orbitals, and is sensitive to vacancy ordering and stoichiometry.
• Relaxed structures show a weakened but still prominent vHs, which shifts slightly away from E_F compared to unrelaxed configurations, indicating structural effects on electronic topology.
• Simulated x-ray diffraction patterns for relaxed structures match experimental data in the accessible 2θ range and reproduce key peak splittings and low-angle superlattice reflections, validating the structural models.