Pseusogap in Cuprates by Electronic Raman Scattering

We present Raman experiments on underdoped and overdoped Bi2Sr2CaCu2O8+delta (Bi-2212) single crystals. We reveal the pseudogap in the electronic Raman spectra in the B-1g and B-2g geometries. In these geometries we probe respectively, the antinodal (AN) and nodal (N) regions corresponding to the principal axes and the diagonal of the Brillouin zone. The pseudogap appears in underdoped regime and manifests itself in the B-1g spectra by a strong depletion of the low energy electronic continuum as the temperature decreases. We define a temperature T* below which the depletion appears and the pseudogap energy, omega(PG) the energy at which the depeletion closes. The pseudogap is also present in the B-2g spectra but the depletion opens at higher energy than in the B-1g spectra. We observe the creation of new electronic states inside the depletion as we enter the superconducting phase. This leads us to conclude (as proposed by S. Sakai et al. [1]) that the pseudogap has a different structure than the superconducting gap and competes with it. We show that the nodal quasiparticle dynamic is very robust and almost insensitive to the pseudogap phase contrary to the antinodal quasiparticle dynamic. We finally reveal, in contrast to what it is usually admitted, an increase of the nodal quasiparticle spectral weight with underdoping. We interpret this result as the consequence of a possible Fermi surface disturbances in the doping range p = 0.1 - 0.2.