Fano effect in the a-b plane of Nd_{1.96}Ce_{0.04}CuO_{4+y}: evidence of phonon interaction with a polaronic background

Reflectance measurements in properly selected samples of Nd$_{1.96}$Ce$_{0.04}$CuO$_{4+y}$ with different carrier concentrations provide a firm evidence of Fano antiresonances for the four transverse optical $E_u$ phonons in the $a-b$ plane. Fits performed without any previous subtraction of the electronic background, as well as the dependence on temperature of the renormalized phonon frequencies, show that the electronic continuum repsonsible for the Fano effect is the polaron band found in all parent compounds of high-T$_c$ superconductors.