Strong superconductivity-induced phonon self-energy effects inHgBa2Ca3Cu4O10+δ

Abstract

We report strong superconductivity-induced phonon renormalization effects observed in the ${\mathrm{HgBa}}_{2}{\mathrm{Ca}}_{3}{\mathrm{Cu}}_{4}{\mathrm{O}}_{10+\ensuremath{\delta}}$ superconductor ${(T}_{c}=123$ K). At the transition from the normal to the superconducting state, the ${A}_{1g}$ phonons at 240 and 390 ${\mathrm{cm}}^{\ensuremath{-}1}$, 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 ${A}_{1g}$ Raman peak at 575 ${\mathrm{cm}}^{\ensuremath{-}1}$, related to the apex oxygen, and that at 487 ${\mathrm{cm}}^{\ensuremath{-}1}$ (tentatively attributed to excess oxygen) are not detectable in the normal state for incident light polarization in the $\mathrm{ab}$ 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.