Observation of a low-energy infrared anomoly in superconducting ${\mathrm{La}}_{1.85}$${\mathrm{Sr}}_{0.15}$${\mathrm{CuO}}_4$

We have studied the infrared reflectivity of normal and superconducting ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$ ${\mathrm{CuO}}_4$ up to 4000 ${\mathrm{cm}}^{\mathrm{-}1}$. For x=0.15 we observe an electronic excitation at about 60 meV (450 ${\mathrm{cm}}^{\mathrm{-}1}$), and, at essentially the same energy, an optic phonon. As we move away from the optimal high critical temperature ($T_c$) value of x=0.15, we find that these features become much weaker. We propose that this 60-meV electronic excitation and the phonon in resonance with it provide the mediating interaction which gives rise to superconductivity at such high temperatures in this class of materials.