Optically induced metastability in untwinned single-domain ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

Raman spectra of untwinned ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ with ${\mathit{T}}_{\mathit{c}}$=91 K indicate a 2.2-eV resonance for incident light polarized in the CuO chain direction. The resonance is characterized by the appearance of nominally forbidden phonon modes at 232 and 597 ${\mathrm{cm}}^{\mathrm{-}1}$. Below 100 K the forbidden modes can be optically bleached if the photon energy exceeds 2.2 eV. The resulting optically induced low-temperature state is metastable; upon warming the crystal to T>200 K the forbidden modes reappear as before.