Reply to Comment on "Superconductivity at low density near a ferroelectric quantum critical point: doped SrTiO_3"

TL;DR

This paper defends a microscopic theory of superconductivity in doped SrTiO3, addressing critiques about coupling mechanisms and energy cutoffs, and clarifies the roles of phonons and Coulomb interactions near a ferroelectric quantum critical point.

Contribution

The authors clarify the validity of their superconductivity model by countering critiques on coupling mechanisms and energy cutoff assumptions, reaffirming the role of Coulomb interactions.

Findings

The cutoff energy for Coulomb interaction is much larger than the Fermi energy.

Deformation potential coupling to soft modes cannot explain the isotope effect.

The energy range of phonon-mediated interaction exceeds the Fermi energy in SrTiO3.

Abstract

In our paper (W\"olfle and Balatsky, Phys. Rev. B 98, 104505 (2018)) we presented a microscopic theory of superconductivity for doped SrTiO$_{3}$ by proposing two pairing mechanisms acting simultaneously with relative strength depending on the closeness to the ferroelectric quantum critical point. The first mechanism rests on the dynamically screened Coulomb interaction, and the second assumed a coupling to the soft transverse optical phonon. In their comment Ruhman and Lee point out an error in our estimate of the deformation potential coupling to the soft mode. We agree that this type of coupling cannot explain the gigantic isotope effect observed experimentally, so that a different coupling mechanism needs to be found. As for the first pairing mechanism, Ruhman and Lee maintain the view expressed in their paper (Ruhman and Lee, Phys. Rev. B 94, 224515 (2016)) that the energy range over which the usual longitudinal optical phonon mediated interaction operates is limited by the Fermi energy. We object to this view and in this reply present evidence that the cutoff energy is much larger. In a weak coupling system such as SrTiO$_{3}$ the cutoff is given by the energy beyond which quasiparticles cease to be well defined.