Superconductivity-induced effect on “Raman-forbidden” modes in underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ single crystals

A remarkable resonance-Raman effect has been observed for some “defect oxygen” modes in underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ single crystals. In resonant conditions, we have studied the temperature dependence of frequencies for these originally Raman-forbidden modes as well as for the Raman-active ones. It has been found that, similar to the $B_{1g}$-like mode, the $\mathrm{yy}$-polarized “Raman-forbidden” modes soften below $T_c,$ while the $A_{1g}$-like modes dominating in the $\mathrm{zz}$-polarized spectra do not. This difference in the phonon behaviors indicates a peculiarity of the interaction between the phonons and quasi-two-dimensional electron system in high-$T_c$ superconductors. Above $T_c,$ no clear phonon anomaly relevant to a pseudogap has been observed.