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

Abstract

The high temperature superconductors Tl 2Ba 2CuO 6+δ (Tl-2201) and Tl 2Ba 2Cu 3O 10 (Tl-2223) with one and three CuO 2 planes were investigated by Raman scattering in order to see whether the electronic Raman scattering cross-section depends on the number of CuO 2 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 Tl-2223 ( 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 2 Δ/ k B T 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 ω of the scattering intensity towards lower frequencies. The B 1g-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 CuO 2-planes, i.e. of specific details of the Fermi surface. These observations are similar to the measurements on over- and underdoped Bi 2Sr 2CaCu 2O 8+δ single crystals. They are consistent with a d x 2− y 2 -wave pairing mechanism, based on the theoretical model of Devereaux and Einzel.