The spectacular changes of the $A_{1g}$-polarized Raman spectra of trilayer and four-layer high-$T_c$ cuprate superconductors below $T_c$ observed in recent experiments can be explained in terms of a new type of excitation that occurs in these systems, a Raman active $c$-axis plasmon. Its frequency, intensity, resonance properties, polarization properties, and coupling to phonon modes involving vibrations of the planar oxygens are estimated and shown to agree with those of the observed $A_{1g}$ superconductivity-induced electronic peak.

• Received 3 October 2002

DOI:https://doi.org/10.1103/PhysRevLett.90.077001