A large modulation of electron-phonon coupling and an emergent superconducting dome in doped strong ferroelectrics

Jiaji Ma,Hanghui Chen,Ruihan Yang

doi:10.1038/s41467-021-22541-1

Jiaji Ma, Hanghui Chen + Show 1 more

Open Access

https://doi.org/10.1038/s41467-021-22541-1

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Abstract

We use first-principles methods to study doped strong ferroelectrics (taking BaTiO3 as a prototype). Here, we find a strong coupling between itinerant electrons and soft polar phonons in doped BaTiO3, contrary to Anderson/Blount’s weakly coupled electron mechanism for "ferroelectric-like metals”. As a consequence, across a polar-to-centrosymmetric phase transition in doped BaTiO3, the total electron-phonon coupling is increased to about 0.6 around the critical concentration, which is sufficient to induce phonon-mediated superconductivity of about 2 K. Lowering the crystal symmetry of doped BaTiO3 by imposing epitaxial strain can further increase the superconducting temperature via a sizable coupling between itinerant electrons and acoustic phonons. Our work demonstrates a viable approach to modulating electron-phonon coupling and inducing phonon-mediated superconductivity in doped strong ferroelectrics and potentially in polar metals. Our results also show that the weakly coupled electron mechanism for "ferroelectric-like metals” is not necessarily present in doped strong ferroelectrics.

Highlights

We use first-principles methods to study doped strong ferroelectrics
The key result from our calculation is that, contrary to Anderson/Blount’s argument for "ferroelectric-like metals”[32,33,34], we find that the phonon bands associated with the soft polar optical phonons are strongly coupled to itinerant electrons across the polar-tocentrosymmetric phase transition in doped BaTiO3
We test four different crystal structures of BaTiO3 with electron doping: the rhombohedral structure, the orthorhombic structure, the tetragonal structure and the cubic structure

Summary

Introduction

We use first-principles methods to study doped strong ferroelectrics (taking BaTiO3 as a prototype). BaTiO3, increasing the carrier density gradually reduces its polar distortions and induces a continuous polar-to-centrosymmetric phase transition[36,37]; and (2) the critical concentration for the phase transition is about 1021/cm[3], which is high enough so that the electron-phonon coupling can be directly calculated within the Migdal’s approximation (in contrast, in doped SrTiO3, superconductivity emerges at a much lower carrier concentration 1017–1020/cm[3] so that its Debye frequency is comparable to or even higher than the Fermi energy ħωD/εF ~ 1 − 10238, which invalidates the Migdal’s approximation and Eliashberg equation)[29]. The key result from our calculation is that, contrary to Anderson/Blount’s argument for "ferroelectric-like metals”[32,33,34], we find that the phonon bands associated with the soft polar optical phonons are strongly coupled to itinerant electrons across the polar-tocentrosymmetric phase transition in doped BaTiO3. We find that close to the critical concentration, lowering the crystal symmetry of doped BaTiO3 by imposing epitaxial strain further increases the superconducting temperature via a sizable coupling between itinerant electrons and acoustic phonon bands

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Results

Conclusion