Hey so the "Surface Plasmon" wikipedia page says

Surface plasmons (SPs) are coherent delocalized electron oscillations that exist at the interface between any two materials where the real part of the dielectric function changes sign across the interface

What the fuck? Changes sign?

The real part of the complex dielectric function (i.e., complex permittivity) just says how charges displace in an electric field. "Reversing the sign" of it implies that charges go one way in one material and the other way in another material. This makes no fucking sense. Charges always go the same direction in an electric field.

Is the wikipedia page just wrong?