Abstract

The far- and mid-infrared reflectivity R(ω) of e-doped single crystals belonging to the family M2−xCexCuO4−y(M = Pr, Nd, Gd; 0<x<0.15; 0<y<0.04) has been studied between 300 and 20 K. In addition to the phonons predicted for the T' structure, R(ω) shows local modes in the far infrared as well as a broad infrared absorption centered at about 0.1 eV (d or J band). These features depend strongly on both T and y. We have resolved the d band at low T, in samples doped by oxygen vacancies. We demonstrate its polaronic origin by showing that it is made up of intense overtones of the local modes observed in the far infrared. We also find that Ce-doped superconductors (x> 0.12,y = 0.03) have the same polaronic structure as the semiconducting ones, partially superimposed on a weak Drude term.

Introduction

Since the discovery of high-Tc superconductors (HTSC), infrared reflectivity measurements have been largely employed to investigate both electronic and transport properties of these cuprates [1]. Early spectra already showed several intriguing features common to all HTSC families, which were then attributed to the peculiar properties of the Cu–O plane. Those features are well reproducible and could be studied in greater detail as soon as large single crystals became available. However, their interpretation is still being debated. In the insulating parent compounds of HTSC, one observes phonon modes in the far infrared, and a broad band at high frequencies (from about 1.5 to 2.5 eV). This latter has been unanimously assigned to charge-transfer (CT) transitions between O 2p and Cu 3d orbitals.