Abstract
Electron-lattice interactions play a prominent role in quantum materials, making a deeper understanding of direct routes to phonon-mediated high-transition-temperature () superconductivity desirable. However, it has been known for decades that weak electron-phonon coupling gives rise to low values of , while strong electron-phonon coupling leads to lattice instability or formation of bipolarons, generally assumed to be detrimental to superconductivity. Thus, the route to high- materials from phonon-mediated mechanisms has heretofore appeared to be limited to raising the phonon frequency as in the hydrogen sulfides. Here we present a simple model for phonon-mediated high- superconductivity based on superfluidity of light bipolarons. In contrast to the widely studied Holstein model where lattice distortions modulate the electron’s potential energy, we investigate the situation where lattice distortions modulate the electron hopping. This physics gives rise to small-size, yet light bipolarons, which we study using an exact sign-problem-free quantum Monte Carlo approach demonstrating a new route to phonon-mediated high- superconductivity. We find that in our model generically and significantly exceeds typical upper bounds based on Migdal-Eliashberg theory or superfluidity of Holstein bipolarons. The key ingredient in this bipolaronic mechanism that gives rise to high is the combination of light mass and small size of bipolarons. Our work establishes principles for the design of high- superconductors via functional material engineering.
- Received 12 April 2022
- Revised 25 September 2022
- Accepted 17 November 2022
DOI:https://doi.org/10.1103/PhysRevX.13.011010
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
The search for superconductivity at relatively high temperatures is a major driver of research in physics. Superconductivity involves the binding of electrons into “Cooper pairs,” and in many materials the superconducting transition temperature is set by the binding strength. This suggests that increasing the pair binding may lead to superconductivity at higher temperatures, however, increasing the binding also leads to other effects that can reduce the transition temperature. Here, we use state-of-the-art quantum many-body methods to theoretically demonstrate a robust route to increasing the binding that circumvents these hurdles, thus providing a simple, concise mechanism for high-temperature superconductivity.
In standard theories of superconducting metals, the electron pair binding comes from interactions of electrons with the motion of atoms. Unfortunately, increasing the binding also vastly increases the mass of the pairs, leading to bound states that can move only extremely slowly and cannot superconduct at elevated temperatures. Further, because electrons are charged, any binding is opposed by Coulomb repulsion, and as the binding strength increases, the Coulomb interaction becomes more important.
Our key findings are that when the electron-atom coupling arises from modulation of the electron motion, the bound pairs retain a light mass, and the binding is minimally affected by the Coulomb repulsion, leading to superconductivity at temperatures higher than what was previously believed possible. This theory identifies simple physical principles that may serve as a basis for a quantum engineering approach to realizing stable and perhaps scalable room-temperature superconductors.
The analysis works best in a dilute limit where the pairs do not overlap significantly. Open challenges are to extend the theory to higher densities and to identify candidate materials.
