Abstract
Photoinduced infrared absorption measurements of YBa 2Cu 3O 7−δ ( δ=0.75) are reported. We observed photoinduced infrared active vibrational modes, associated phonon bleachings, and an electronic transition from a self-localized gap state. The results indicate a local structural distortion in the CuO plane around the photogenerated charge carriers. Remarkable similarities in photoinduced activity between conducting polymers and high T c superconductors were found. The photoinduced structural distortion and the associated self-localized gap state demonstrate that the photoexcitations are relatively long-lived polarons or bipolarons. The dynamic mass associated with the distortion is smaller in YBa 2Cu 3O 7−δ than in La 2CuO 4, which is comparable to those in conducting polymers. Since these characteristic features are not observed in the isostructural compound, La 2NiO 4, we conclude that polaron (or bipolaron) formation plays an important role in the high temperature superconductivity.
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