Raman detection of superconducting gap in Ba-Y-Cu-O superconductor.

We have utilized an iodine absorption cell to enable detailed Raman studies at low frequency (down to 15 cm sup (-1)) in a thin film of the high temperature superconductor Ba sub 2 YCu sub 3 O sub 7. Subtraction of spectra just above and well into the superconducting phase reveals a weak broad feature (maximum near 400 cm sup (-1), HWHM ~250 cm sup (-1)) 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 cm sup (-1) suggests an energy gap 2DELTA near 25 meV. Assuming that the surface region probed in the scattering has the same T sub c as that measured for the overall film (onset 85 K), we obtain the ratio 2DELTA/k sub B T sub c =3. 4+-1.5, consistent with the BCS weak-coupling value of 3.5. Despite the large estimated uncertainty in r, the results clearly rule out the large r values reported recently on the basis of tunneling measurements.