Anisotropy of low-energy electronic Raman scattering in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$

Low-energy electronic Raman scattering was measured in superconducting c-axis- and a-axis-oriented thin films of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ for light polarized in the Cu-${\mathrm{O}}_2$ planes and perpendicular to these planes. The superconducting gap feature in the spectrum below the phase-transition temperature was clearly observed for in-plane polarization, while it was absent for normal-to-plane polarization.