Unconventional superconducting pairing symmetry induced by phonons

TL;DR

This paper explores how phonons can induce unconventional superconducting pairing symmetries, including triplet states, in a model inspired by Sr$_2$RuO$_4$, challenging traditional views on phonon-mediated pairing.

Contribution

It demonstrates that combined electron-phonon and spin-fluctuation interactions can produce unconventional pairing symmetries, offering new insights into Sr$_2$RuO$_4$ superconductivity.

Findings

Phonons can induce non-s-wave pairing states.

Combined interactions lead to diverse singlet and triplet states.

Implications for understanding Sr$_2$RuO$_4$'s pairing mechanism.

Abstract

The possibility of non-s-wave superconductivity induced by phonons is investigated using a simple model that is inspired by Sr$_2$RuO$_4$. The model assumes a two-dimensional electronic structure, a two-dimensional spin-fluctuation spectrum, and three-dimensional electron-phonon coupling. Taken separately, each interaction favors formation of spin-singlet pairs (of s symmetry for the phonon interaction and d$_{x^2-y^2}$ symmetry for the spin interaction), but in combination, a variety of more unusual singlet and triplet states are found, depending on the interaction parameters. This may have important implications for Sr$_2$RuO$_4$, providing a plausible explanation of how the observed spin fluctuations, which clearly favor d$_{x^2-y^2}$ pairing, may still be instrumental in creating a superconducting state with a different (e.g., p-wave) symmetry. It also suggests an interpretation of the large isotope effect observed in Sr$_2$RuO$_4$. These results indicate that phonons could play a key role in establishing the order-parameter symmetry in Sr$_2$RuO$_4$, and possibly in other unconventional superconductors.