Phonon anomalies and electron-phonon interaction in the RuSr2GdCu2O8 ferromagnetic superconductor:: Evidence from infrared conductivity -: art. no. 184505

Critical behavior of the infrared reflectivity of RuSr2GdCu2O8 ceramics is observed near the superconducting T-SC = 45 K and magnetic T-M = 133 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 cm(-1) to a broad absorption peak at the temperatures between T* = 90 K and T-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 cm(-1) and the mid-IR band at 4800 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 CuO2 planes caused by the charge carrier self-trapping at the Ru moments.