Electronic Raman scattering in Tl₂Ba₂CuO_6+d : Symmetry of the order parameter, oxygen doping effects, and normal-state scattering.
 

 Abstract:
Single crystals of the optimally doped, moderately and strongly overdoped high-temperature  superconductor  Tl₂Ba₂CuO_6+d (Tl-2201) with T_c= 80, 56, and 30 K, respectively, have been investigated by polarized Raman scattering. By taking the peak position of the B_1g component of electronic Raman scattering as 2D₀ we found that the reduced gap value (2D₀/k_BT_c  ) strongly decreases with increasing  doping. The behavior of the low-frequency scattering for the  B_1g  and B_2g scattering components is similar for optimally doped and overdoped crystals and can be described by a w³ and w law, respectively, which  is consistent  with a d-wave symmetry of the order parameter. In  contrast to the optimally doped Tl-2201 in both moderately and strongly overdoped Tl-2201, the relative (compared to the B_1g) intensity of the A_1g scattering component is suppressed. We suggest that the van Hove singularity is responsible for the observed changes of Raman intensity and reduced gap value with doping. Electronic Raman scattering in the normal state is discussed in the context of the scattering from impurities and compared to the existing infrared data.  The scattering rate evaluated from the Raman measurements is smaller for the overdoped samples, compared to the moderately overdoped samples.