Raman Spectroscopy in YBa 2 Cu 3 O 6+x and Bi 2 Sr 2 (Ca x Y 1—x )Cu 2 O 8+δ : Pseudogap and Superconducting Gap

We present results of Raman‐scattering experiments in differently doped YBa 2 Cu 3 O 6+ x (Y‐123) and Bi 2 Sr 2 (Ca x Y 1— x )Cu 2 O 8+δ (Bi‐2212) single crystals. When reducing the doping level x the scattering intensity in B 1g symmetry increases while it decreases in the B 2g channel. This anisotropic behavior can be attributed to a qualitative change of the Fermi surface topology. At B 2g symmetry we find evidence of a pseudogap Δ* in the normal state below a characteristic temperature T * ≈ 180 K. In the superconducting state, the spectra obtained from overdoped samples show the well‐known pair‐breaking (2Δ) features in both B 1g and B 2g symmetries while in the underdoped regime pair‐breaking maxima can only be observed in the B 2g channel. The magnitude of the gap can be estimated to be approximately 8 kT c and is, from our experiments, different from Δ*.