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 {ital n} type and {ital p} type. In Nd{sub 2}CuO{sub 4{minus}{ital 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 {approximately} 1000 cm{sup {minus}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{sup {minus}1}. Most of the above infrared features are shown to survive in the metallic phase of Nd{sub 2{minus}{ital x}}Ce{sub {ital x}}CuO{sub 4{minus}{ital y}}, Bi{sub 2}Sr{sub 2}CuO{sub 6}, and YBa{sub 2}Cu{sub 3}O{sub 7{minus}{ital 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. {copyright} {ital 1996 The American Physical Society.}