Raman detection of the superconducting gap in Ba-Y-Cu-O superconductors

We have utilized an iodine absorption cell to enable detailed Raman studies at low frequency (down to 15 ${\mathrm{cm}}^{-1\mathrm{}}$) in a thin film of the high-temperature superconductor ${\mathrm{Ba}}_2$Y${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$. Subtraction of spectra just above and well into the superconducting phase reveals a weak broad feature (maximum near 400 ${\mathrm{cm}}^{-1\mathrm{}}$, half width at half maximum approximately 250 ${\mathrm{cm}}^{-1\mathrm{}}$), which we tentatively interpret as due to scattering near the superconducting gap. The large width of the feature may be partially related to smearing of the transition, or to gap anisotropy. Its onset near 200 ${\mathrm{cm}}^{-1\mathrm{}}$ suggests an energy gap $2\Delta$ near 25 meV. Assuming that the surface region probed in the scattering has the same $T_c$ as that measured for the overall film (onset 85 K), we obtain the ratio $r=\frac{2\Delta }{k_B{T}_c}=3.4\mathrm{}\pm 1.5$, consistent with the BCS weak-coupling value of 3.5.