Strong superconductivity-induced phonon self-energy effects

We report strong superconductivity-induced phonon renormalization effects observed in the (Formula presented) superconductor (Formula presented) K). At the transition from the normal to the superconducting state, the (Formula presented) phonons at 240 and 390 (Formula presented), which correspond to vibrations of the plane oxygen atoms with some admixture of calcium vibrations, display an abrupt softening, and increase in linewidth within a rather narrow temperature interval. The changes of phonon self-energy are accompanied by a strong enhancement of the Raman intensity of the phonons in the superconducting state. The (Formula presented) Raman peak at 575 (Formula presented), related to the apex oxygen, and that at 487 (Formula presented) (tentatively attributed to excess oxygen) are not detectable in the normal state for incident light polarization in the (Formula presented) plane. They, however, show up in the superconducting state as coupled phonon-electron excitations. To our knowledge, such phonon self-energy effects are the strongest ones reported for the superconducting cuprates so far.