Superconductivity-induced Resonance Raman Scattering in Multi-layer   High-Tc Superconductors
  Resonant Raman scattering below Tc has been discovered in several Bi-, Hg-, Tl-based high-Tc superconductors with three or four CuO2-layers. For Bi2Si2Ca2Cu3O10+d, we found an unexpected crossover of the pair-breaking peak in the A1g-spectrum from a broad bump at hw = 6kBTc for Eexc = 2.54eV to a sharp peak at hw = 8kBTc for Eexc = 2.18eV, together with a strong enhancement of the Ca-phonons. Under resonant conditions, the relative positions of the pair breaking peaks in A1g, B1g, and B2g channels are 2Delta(A1g) = 2Delta(B1g) > 2Delta(B2g). This relation implies that the A1g Raman channel is free from the Coulomb screening effect, just as predicted theoretically for a d-wave multi-layer superconductor but have never been observed experimentally thus far. The observed resonance effect is the evidence that the electronic state in the inner CuO2-planes is different from that of the outer CuO2-planes. 
Mikhail Limonov, Sergey Lee, Setsuko Tajima, Akio Yamanaka