Electronic Raman scattering in copper oxide superconductors: Understanding the phase diagram

Electronic Raman scattering measurements have been performed on hole doped copper oxide (cuprate) superconductors as a function of temperature and doping level. In the superconducting state, coherent Bogoliubov quasiparticles develop preferentially over the nodal region in the underdoped regime. We can then define the fraction of coherent Fermi surface, f(c) around the nodes for which quasiparticles are well defined and superconductivity sets in. We find that f(c) is doping dependent and leads to the emergence of two energy scales. We then establish in a single gap scenario, that the critical temperature T-c is proportional to f(c)Delta(max) where Delta(max) is the maximum amplitude of the d-wave superconducting gap. In the normal state, the loss of antinodal quasiparticles spectral weight detected in the superconducting state persists and the spectral weight is only restored above the pseudogap temperature T*. Such a dichotomy in the quasiparticles dynamics of underdoped cuprates is responsible for the emergence of the two energy scales in the superconducting state and the appearance of the pseudogap in the normal state. We propose a 3D phase diagram where both the temperature and the energy phase diagrams have been plotted together. This 3D diagram advocates in favor of a low temperature phase transition inside the superconducting dome. We anticipate that the development of coherent excitations only on a restricted part of the Fermi surface is a general feature in high T-c cuprate superconductors on approaching the Mott insulating side. (c) 2011 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.