Phonon anomalies and electron-phonon interaction in the RuSr₂GdCu₂O₈ ferromagnetic superconductor: Evidence from infrared conductivity

Abstract

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.