Fano effect in the a-b plane of Nd1.96Ce0.04CuO41y :

Reflectance measurements in different Nd1.96Ce0.04CuO41y samples with properly selected carrier concentrations provide firm evidence of Fano antiresonances for the four transverse optical Eu phonons in the a-b plane. An analysis of the Fano line shapes and of the dependence on temperature of the renormalized phonon frequencies allows us to determine the origin of the electronic continuum interacting with the phonons. Unlike in doped semiconductors, where the continuum is the free-carrier absorption band, here the continuum is provided by a polaron band at ;0.1 eV, present in most parent compounds of high-Tc superconductors. In the most doped sample, the polaron band softens as T decreases, thus indicating a delocalization of the polaronic carriers at low temperature.