Anisotropic ultraviolet Raman resonance in underdoped Y Ba 2 Cu 3 O 6.7

We report an inverse Raman-bleaching anomaly of oxygen-vacancy-activated phonons in underdoped $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.7}$ excited in resonance at $4.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. These modes are closely related to modes that photobleach under visible excitation. The resonance and selection rules match the prediction from the $ab$ in-plane anisotropy of the dielectric function, as does their growth of intensity under illumination. These properties support the oxygen-vacancy-reordering model for photoinduced changes in underdoped $\mathrm{Y}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$. We suggest that a similar coupling to anisotropic electronic resonances is responsible for the recently discovered $ab$ anisotropies in the phonon dispersion.