FANO EFFECT IN THE A-B PLANE OF ND1.96CE0.04CUO4- EVIDENCE OF PHONON INTERACTION WITH A POLARONIC BACKGROUND(Y )

Reflectance measurements in different Nd1.96Ce0.04CuO4+y samples with properly selected carrier concentrations provide firm evidence of Fano antiresonances for the four transverse optical E-u phonons in the a-b plane. An analysis of the Fano line shapes and of the dependence on t emperature of the renormalized phonon frequencies allows us to determi ne the origin of the electronic continuum interacting with the phonons . Unlike in doped semiconductors. where the continuum is the free-carr ier absorption band, here the continuum is provided by a polaron band at similar to 0.1 eV, present in most parent compounds of high-T-c sup erconductors. In the most doped sample, the polaron band softens as T decreases, thus indicating a delocalization of the polaronic carriers at low temperature.