Polaronic optical absorption in electron-doped and hole-doped cuprates
P. Calvani, M. Capizzi, S. Lupi, P. Maselli, A. Paolone, P. Roy
Abstract
Polaronic features similar to those previously observed in the photoinduced spectra of cuprates have been detected in the reflectivity spectra of chemically doped parent compounds of high-critical-temperature superconductors, both
n
-type and
p
-type. In Nd
2
CuO
4−y
these features, whose intensities depend both on doping and temperature, include local vibrational modes in the far infrared and a broad band centered at
∼
1000 cm
−1
. The latter band is produced by the overtones of two (or three) local modes and is well described in terms of a small-polaron model, with a binding energy of about 500 cm
−1
. Most of the above infrared features are shown to survive in the metallic phase of Nd
2−x
Ce
x
Cu0
4−y
, Bi
2
Sr
2
CuO
6
, and YBa
2
Cu
3
O
7−y
, where they appear as extra-Drude peaks. The occurrence of polarons is attributed to local modes strongly coupled to carriers, as shown by a comparison with tunneling results.