Optical Detection of Polarons in High - Tc Cuprate

The optical conductivity σ (ω) of slightly e-doped single-crystals of (Nd,Gd)2CuO4-y shows local modes in the far-infrared as well as a broad infrared absorption centered at ∼ 0.1 eV (d-band). This latter shows a fine structure, in agreement with recent calculations of Alexandrov et al., which is made up by intense overtones of the local modes observed in the far-infrared. Similar polaronic structures are shown to exist in the normal metallic phase of Nd2-xCexCuO4-y and even in the σ (ω ) of YBCO crystals, measured by different authors. The present observations provide evidence for the existence of small polarons in all materials with a Cu-O plane