Phonon enhancement of electronic orders and negative isotope effect in the Hubbard-Holstein model on a square lattice

TL;DR

This study uses the functional renormalization group to explore how electron-phonon interactions influence electronic orders in the Hubbard-Holstein model, revealing a negative isotope effect and unconventional phase behavior.

Contribution

It provides new insights into the interplay of electron correlation and phonons, showing that lower-frequency phonons can enhance or be less destructive to electronic phases, leading to a negative isotope effect.

Findings

Spin-density-wave and charge-density-wave phases at half-filling.

Emergence of d-wave and s-wave superconductivity upon doping.

Lower-frequency phonons can promote or preserve electronic orders, causing a negative isotope effect.

Abstract

Looking for superconductors with higher transition temperature requires a guiding principle. In conventional superconductors, electrons pair up into Cooper pairs via the retarded attraction mediated by electron-phonon coupling. Higher-frequency phonon (or smaller atomic mass) leads to higher superconducting transition temperature, known as the isotope effect. Furthermore, superconductivity is the only instability channel of the metallic normal state. In correlated systems, the above simple scenario could be easily violated. The strong local interaction is poorly screened, and this conspires with a featured Fermi surface to promote various competing electronic orders, such as spin-density-wave, charge-density-wave and unconventional superconductivity. On top of the various phases, the effect of electron-phonon coupling is an intriguing issue. Using the functional renormalization group, here we investigated the interplay between the electron correlation and electron-phonon coupling in a prototype Hubbard-Holstein model on a square lattice. At half-filling, we found spin-density-wave and charge-density-wave phases and the transition between them, while no superconducting phase arises. Upon finite doping, d-wave/s-wave superconductivity emerges in proximity to spin-density-wave/charge-density-wave phases. Surprisingly, lower-frequency Holstein-phonons are either less destructive, or even beneficial, to the various phases, resulting in a negative isotope effect. We discuss the underlying mechanism behind and the implications of such anomalous effects.