Polaronic optical absorption in electron-doped and hole-doped cuprates

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 $\sim$ 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.

File latex, 31 p., submitted to Physical Review B. Figures may be faxed upon request