Phonon anomalies and electron-phonon interaction in the ${\mathrm{RuSr}}_2{\mathrm{GdCu}}_2{\mathrm{O}}_8$ ferromagnetic superconductor:  Evidence from infrared conductivity

Critical behavior of the infrared reflectivity of ${\mathrm{RuSr}}_2{\mathrm{GdCu}}_2{\mathrm{O}}_8$ ceramics is observed near the superconducting $T_{\mathrm{SC}}=45\mathrm{}\hspace{0.5em}\mathrm{K}$ and magnetic $T_M=133\mathrm{}\hspace{0.5em}\mathrm{K}$ transition temperatures. The optical conductivity reveals the typical features of the c-axis optical conductivity of strongly underdoped multilayer superconducting cuprates. The transformation of the Cu-O bending mode at $288\hspace{0.5em}{\mathrm{cm}}^{-1}$ to a broad absorption peak at the temperatures between $T^{*}=90\mathrm{}\hspace{0.5em}\mathrm{K}$ and $T_{\mathrm{SC}}$ is clearly observed, and is accompanied by the suppression of spectral weight at low frequencies. The correlated shifts to lower frequencies of the Ru-related phonon mode at $190\hspace{0.5em}{\mathrm{cm}}^{-1}$ and the mid-IR band at $4800\hspace{0.5em}{\mathrm{cm}}^{-1}$ on decreasing temperature below $T_M$ are observed. It provides experimental evidence in favor of strong electron-phonon coupling of the charge carriers in the Ru-O layers which critically depends on the Ru core spin alignment. The underdoped character of the superconductor is explained by strong hole depletion of the ${\mathrm{CuO}}_2$ planes caused by the charge carrier self-trapping at the Ru moments.