Ghost Josephson plasmon in bilayer superconductors

This work provides a physical explanation for the existence of a ``ghost'' plasmon in bilayer superconductors. The authors show that one of the two Josephson plasmons is connected to counterflowing current fluctuations polarized perpendicularly to the planes. This effect makes it a transverse mode at small out-of-plane momenta, explaining why it is hidden in the density response. This work offers a new perspective on the understanding of collective excitations in systems with multiple intertwined degrees of freedom.