[Paper Review] Absence of phonon-mediated superconductivity in La$_3$Ni$_2$O$_7$ under pressure

Zhenfeng Ouyang, Miao Gao+1 more|arXiv (Cornell University)|Mar 21, 2024

High-pressure geophysics and materialsEarth and Planetary Sciences3 citations

TL;DR

This study uses first-principles density functional theory and Wannier interpolation to investigate electron-phonon coupling in pressurized La₃Ni₂O₇, finding a weak electron-phonon coupling parameter (λ = 0.14) that cannot explain the observed ~80 K superconducting transition. The results indicate that superconductivity in this material is unconventional and likely driven by strong electronic correlations rather than phonon-mediated pairing.

ABSTRACT

A recent experimental study announced the emergence of superconductivity in La$_3$Ni$_2$O$_7$ under pressure, with the highest observed superconducting transition temperature ($T_c$) reaching approximately 80 K beyond 14 GPa. While extensive studies have been devoted to the electronic correlations and potential superconducting pairing mechanisms, there lack investigations into the phonon properties and electron phonon coupling. Using density functional theory in conjunction with Wannier interpolation techniques, we study the phonon properties and electron phonon interactions in La$_3$Ni$_2$O$_7$ under 29.5 GPa. Our findings reveal that the electron phonon coupling is insufficient to solely explain the observed high superconducting $T_c$ $\sim$ 80 K in La$_3$Ni$_2$O$_7$. And the calculated strong Fermi surface nesting may explain the experimental observed charge density wave transition in La$_3$Ni$_2$O$_7$. Our calculations substantiate La$_3$Ni$_2$O$_7$ is an unconventional superconductor.

Motivation & Objective

• To investigate the role of electron-phonon coupling in the high-Tc superconductivity observed in La₃Ni₂O₇ under pressure.
• To determine whether phonon-mediated pairing can account for the reported Tc ≈ 80 K in this material.
• To examine the influence of Fermi surface nesting on charge density wave (CDW) formation and electron-phonon coupling.
• To assess the dynamical stability and phonon spectrum of La₃Ni₂O₇ at 29.5 GPa using density functional perturbation theory.
• To clarify the mechanism behind unconventional superconductivity in nickelate perovskites by excluding phonon-mediated pairing.

Proposed method

• Employed density functional theory (DFT) with the generalized gradient approximation (GGA-PBE) for electronic structure calculations.
• Applied Wannier interpolation techniques to construct accurate tight-binding models for Fermi surface and electron-phonon coupling analysis.
• Used density functional perturbation theory (DFPT) to compute phonon dispersion and dynamical stability.
• Calculated the Eliashberg spectral function α²F(ω) and the electron-phonon coupling strength λ via the McMillan-Allen-Dynes formula.
• Evaluated Fermi surface nesting using the nesting function ξ(q) to identify momentum vectors with strong nesting character.
• Mapped the momentum-resolved electron-phonon coupling λq,v and correlated it with nesting vectors to assess enhancement mechanisms.

Experimental results

Research questions

• RQ1 Can electron-phonon coupling alone explain the high Tc ≈ 80 K superconductivity in pressurized La₃Ni₂O₇?
• RQ2 What is the strength and spatial distribution of electron-phonon coupling in La₃Ni₂O₇ under 29.5 GPa pressure?
• RQ3 To what extent does Fermi surface nesting contribute to the observed charge density wave (CDW) transition?
• RQ4 Is the electron-phonon coupling in La₃Ni₂O₇ sufficient to mediate BCS-type superconductivity?
• RQ5 How do the phonon modes and electronic structure interact to influence superconducting pairing in this system?

Key findings

• The electron-phonon coupling parameter λ is calculated to be 0.14, indicating a very weak coupling strength.
• Using the McMillan-Allen-Dynes formula with μ* = 0.1, the predicted Tc based on electron-phonon coupling is 0 K, ruling out phonon-mediated pairing.
• No imaginary phonon modes are found, confirming that La₃Ni₂O₇ is dynamically stable under 29.5 GPa pressure.
• Strong Fermi surface nesting is observed, particularly along the Γ–Z path and in-plane, which correlates with the locations of finite electron-phonon coupling.
• The nesting vectors ξ(q) match the q-points with non-zero λq,v, suggesting that nesting—not phonon softening—drives the weak coupling enhancement.
• The results support that superconductivity in La₃Ni₂O₇ is unconventional and likely driven by strong electronic correlations, not electron-phonon coupling.