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

Abstract:
The high temperature superconductors Tl₂Ba₂CuO_6+d (Tl-2201) and Tl₂Ba₂Ca₂Cu₃O₁₀ (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 ( T_c = 90 K, 80 K), and one single crystal of (T_c = 118 K), for temperatures below and above T_c. In B_1g- symmetry the peak in the electronic Raman scattering is found at 460 cm^-1 (T_c = 90 K), or 430 cm^-1 (T_c = 80 K) for Tl-2201 and at 610 cm^-1 for  Tl-2223. This corresponds to 2D/k_BT_c = 7.4±0.4 for both compounds. The A_1g- and B_2g-symmetry components show smaller values for the peak position and a decrease linear in w of the scattering intensity towards lower frequencies. The B_1g-component shows a decrease cubic  in w 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 Bi₂Sr₂CaCu₂O_8+d single crystals. They are consistent with a d_x2_-y2-wave pairing mechanism, based on the theoretical model of Devereaux and Einzel.