Electronic Raman scattering of Tl-2201 and Tl-2223 and the symmetry of the energy gap

The high temperature superconductors Tl2Ba2CuO6+δ (Tl-2201) and Tl2Ba2Cu3O10 (Tl-2223) with one and three CuO2 planes were investigated by Raman scattering in order to see whether the electronic Raman scattering cross-section depends on the number of CuO2 planes, i.e. sheets of the Fermi surface. We present measurements of polarized Raman spectra in different scattering geometries on two single crystals of Tl-2201 (Tc = 90 K, 80 K), and one single crystal of Tl-2223 (Tc = 118 K), for temperatures below and above Tc. In B1g-symmetry the peak in the electronic Raman scattering is found at 460 cm−1 (Tc = 90 K), or 430 cm−1 (Tc = 80 K) for Tl-2201 and at 610 cm−1 for Tl-2223. This corresponds to 2Δ/kBTc = 7.4 ± 0.4 for both compounds. The A1g- and B2g symmetry components show smaller values for the peak position and a decrease linear in ω of the scattering intensity towards lower frequencies. The B1g-component shows a decrease cubic in ω of the scattering intensity towards lower frequencies.
The spectral features of Tl-2201 and Tl-2223 are found to be independent of the number of CuO2-planes, i.e. of specific details of the Fermi surface. These observations are similar to the measurements on over- and underdoped Bi2Sr2CaCu2O8+δ single crystals. They are consistent with a dx2−y2-wave pairing mechanism, based on the theoretical model of Devereaux and Einzel.