Comment on "Superconductivity at low density near a ferroelectric quantum critical point: Doped SrTiO3"

Abstract

Wölfle and Balatsky Phys. Rev. B 98, 104505 (2018) have proposed a microscopic pairing mechanism for doped SrTiO${}_3$ (STO) based on the $\text{g}radient$ coupling of electronic density to the soft TO phonon mode. Since this coupling to TO phonons is usually weak, this conclusion is surprising, especially for a low density superconductor such as STO, where the density of states is small. A crucial step in the argument made by Wölfle and Balatsky is that the displacement vector of the TO mode is not strictly perpendicular to the momentum vector, making a deformation coupling possible. We show that they have made a mistake in computing the eigenvector and have grossly overestimated this lack of orthogonality. When corrected, the coupling is negligible. We also use transport data to put upper bounds on the coupling constant which are much smaller than the estimate by Wölfle and Balatsky. Finally, we also object to their use of the Eliashberg equation when the phonon frequency is larger than the Fermi energy.

References in corpus (3)

• Electron-Phonon Scattering in the Presence of Soft Modes and Electron Mobility in SrTiO${}_3$ Perovskite from First Principles
• Intrinsic Mobility Limiting Mechanisms in Lanthanum-doped Strontium Titanate
• Superconductivity at low density near a ferroelectric quantum critical point: doped SrTiO

Cited by in corpus (14)

• Superconductivity in dilute SrTiO${}_3$: a review
• Quasiparticle and Nonquasiparticle Transport in Doped Quantum Paraelectrics
• Anisotropic superconductivity mediated by ferroelectric fluctuations in cubic systems with spin-orbit coupling
• Superconductivity from energy fluctuations in dilute quantum critical polar metals
• Anharmonic phonon damping enhances the $T_c$ of BCS-type superconductors
• Effective Theory of Superconductivity in Strongly-Coupled Amorphous Materials
• Theory of superconductivity mediated by Rashba coupling in incipient ferroelectrics
• Generalized Rashba Electron-Phonon Coupling and Superconductivity in Strontium Titanate
• Multiband Quantum Criticality of Polar Metals
• A theory of criticality for quantum ferroelectric metals
• Synergetic ferroelectricity and superconductivity in zero-density Dirac semimetals near quantum criticality
• Reply to Comment on "Superconductivity at low density near a ferroelectric quantum critical point: doped SrTiO_3"
• Spin-dependent anisotropic electron-phonon coupling in KTaO${}_3$
• A large modulation of electron-phonon coupling and an emergent superconducting dome in doped strong ferroelectrics