[Paper Review] Absence of electron-phonon-mediated superconductivity in hydrogen-intercalated nickelates

Simone Di Cataldo, Liang Si+2 more|arXiv (Cornell University)|Apr 7, 2023

Magnetic and transport properties of perovskites and related materialsMaterials Science3 citations

TL;DR

This study investigates whether electron-phonon coupling mediates superconductivity in hydrogen-intercalated nickelates, using density-functional theory and density-functional perturbation theory. Despite hydrogen's known role in enhancing electron-phonon coupling in hydrides, the calculations show negligible electron-phonon coupling strength, ruling out this mechanism as the origin of superconductivity in Nd0.75Sr0.25NiO2H0.25.

ABSTRACT

A recent experiment [X. Ding et al., Nature 615, 50 (2023)] indicates that superconductivity in nickelates is restricted to a narrow window of hydrogen concentration: 0.22 < x < 0.28 in Nd$_{0.8}$Sr$_{0.2}$NiO$_{2}$H$_{x}$. This reported necessity of hydrogen suggests that it plays a crucial role for superconductivity, as it does in the vast field of hydride superconductors. Using density-functional theory and its extensions, we explore the effect of topotactic hydrogen on the electronic structure and phonon-mediated superconductivity in nickelate superconductors. Our calculations show that the electron-phonon coupling in hydrogen-intercalated nickelates is not strong enough to drive the electron pairing, and thus cannot explain the reported superconductivity.

Motivation & Objective

• To assess whether electron-phonon coupling can explain superconductivity in hydrogen-intercalated nickelates.
• To investigate the role of topotactic hydrogen in enhancing electron-phonon coupling.
• To determine if the narrow superconducting dome in Nd0.8Sr0.2NiO2Hx is driven by enhanced electron-phonon interactions.
• To rule out conventional electron-phonon pairing as the mechanism behind the observed superconductivity.

Proposed method

• Performed density-functional theory (DFT) calculations using Quantum ESPRESSO with optimized norm-conserving Vanderbilt pseudopotentials.
• Conducted phonon calculations via density-functional perturbation theory (DFPT) on a 2×2×2 Γ-centered grid.
• Computed electron-phonon matrix elements on 16×16×16 and 24×24×24 grids with Gaussian smearing of 100 meV.
• Applied the McMillan formula to estimate Tc from λ (electron-phonon coupling strength) and ωlog (logarithmic average phonon frequency).
• Used a modified version of Quantum ESPRESSO to implement the rigid-band approximation for electron-phonon integration.
• Explored structural variations including La substitution and different hydrogen concentrations to test robustness of results.

Experimental results

Research questions

• RQ1 Can electron-phonon coupling explain the superconductivity observed in hydrogen-intercalated nickelates?
• RQ2 Is the narrow superconducting window (0.22 < x < 0.28) in Nd0.8Sr0.2NiO2Hx driven by enhanced electron-phonon coupling due to hydrogen?
• RQ3 Does hydrogen intercalation significantly increase the electron-phonon coupling strength λ or phonon frequencies ωlog in nickelates?
• RQ4 Is the electron-phonon mechanism viable for superconductivity in nickelates, given that it fails in non-hydrogenated systems?
• RQ5 Can optimal electron-phonon coupling be engineered via structural tuning (e.g., rare-earth substitution or varying H concentration)?

Key findings

• The electron-phonon coupling strength λ in Nd0.75Sr0.25NiO2H0.25 is found to be minimal, with no significant enhancement from hydrogen intercalation.
• The calculated Tc using the McMillan formula remains negligible, indicating no conventional electron-phonon superconductivity.
• Even when varying hydrogen concentration or substituting Nd with La, no finite Tc is obtained, suggesting no viable electron-phonon pathway.
• Phonon modes involving hydrogen do not yield high-energy, unscreened vibrations that would boost Tc.
• The absence of strong electron-phonon coupling rules out this mechanism as the origin of superconductivity in the reported system.
• The results imply that alternative pairing mechanisms—such as spin-fluctuation or orbital-driven pairing—must be responsible for the observed superconductivity.