Exploring superconductivity under strong coupling with the vacuum electromagnetic field

Strong light-matter interactions have generated considerable interest as a means to manipulate material properties. Here, we explore this possibility with the molecular superconductor Rb3C60 under vibrational strong coupling (VSC) to surface plasmon polaritons. By placing the superconductor-surface plasmon system in a SQUID magnetometer, we find that the superconducting transition temperature (Tc) increases from 30 to 45 K at normal pressures under VSC, displaying a well-defined Meissner effect. A simple theoretical framework is provided to understand these results based on an enhancement of the electron-phonon coupling. This proof-of-principle study opens a new tool box to not only modify superconducting materials but also to understand the mechanistic details of different superconductors.