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

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 ${\mathrm{Nd}}_2$${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$ these features, whose intensities depend both on doping and temperature, include local vibrational modes in the far infrared and a broad band centered at ∼ 1000 ${\mathrm{cm}}^{\mathrm{-}1}$. The latter band is produced by the overtones of two (or three) local modes and is well described in terms of a small-polaron model, with a binding energy of about 500 ${\mathrm{cm}}^{\mathrm{-}1}$. Most of the above infrared features are shown to survive in the metallic phase of ${\mathrm{Nd}}_{2\mathrm{-}\mathit{x}}$${\mathrm{Ce}}_{\mathit{x}}$${\mathrm{CuO}}_{4\mathrm{-}\mathit{y}}$, ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CuO}}_6$, and ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{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. © 1996 The American Physical Society.