Light-Induced Metastable State in Charge-Ordered YBa$_2$Cu$_3$O$_{6+x}$
We report temporal changes in Raman-scattering spectra of detwinned YBa$_2$Cu$_3$O$_{6+x}$ single crystals under exposure to red laser light polarized along the crystallographic $a$-axis. A recent publication by Bakr et al. (Phys. Rev. B 88 214517 (2013)) identified new Raman-active modes that appear upon cooling below $T \sim 200$ K, and attributed these modes to charge ordering phenomena observed in x-ray scattering and nuclear magnetic resonance experiments on the same materials. Here we report that the intensities of these modes depend not only on temperature and oxygen content, but also on the cumulative photon dose absorbed by the YBa$_2$Cu$_3$O$_{6+x}$ samples. The light-induced changes in the Raman spectra exhibit a stretched-exponential time dependence, with a characteristic time that increases strongly upon cooling. They also depend strongly on doping and are most pronounced for $x \sim 0.6$. In fully illuminated samples, the mode intensities exhibit a monotonic temperature dependence indicative of a second-order phase transition. The findings indicate a metastable state generated by light-induced rearrangement of the oxygen dopants. We hypothesize that the new Raman phonons are associated with a three-dimensional charge-ordered state induced by light illumination, analogous to a state that was recently observed by resonant x-ray scattering in oxygen-disordered YBa$_2$Cu$_3$O$_{6+x}$ films.