Photoinduced self-localized structural distortions inYBa2Cu3O7−δ

Photoinduced absorption measurements of $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ ($\delta =0.75\mathrm{}$) from 150 to 4000 ${\mathrm{cm}}^{-1\mathrm{}}$ 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 tetragonal to orthorhombic distortion around the charge carriers. The dynamic mass associated with the distortion is smaller in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ than in ${\mathrm{La}}_2$Cu${\mathrm{O}}_4$. The photoinduced structural distortion and the associated self-localized gap state demonstrate that the photoexcitations are polarons or bipolarons. Since these photoinduced absorption features are not observed in the isostructural compound ${\mathrm{La}}_2$Ni${\mathrm{O}}_4$ we conclude that polaron (or bipolaron) formation may play an important role in the high-temperature superconductivity.