Electronic Raman Scattering and Phonon Self-Energy Effects in the R-123 System: Signatures of Gap and Pseudogap

Abstract

Using Raman spectroscopy and transport measurements we investigate thin epitaxial films of Y{1-x}(Pr,Ca)xBa2Cu3O{6+y}. We explore the electronic Raman responses obtained after subtraction of phononic excitations, and the 2Delta peaks that form out of the electronic background below Tc. We find that the energy of the B1g 2Delta peak increases monotonically with decreasing doping until the peaks become unresolvable. In contrast, the peaks in A1g symmetry follow Tc being resolvable in the Pr-doped films. The B2g responses are weak and a 2Delta peak is only detected at the highest doping level. As a consequence of strong electron-phonon coupling, the B1g phonon at 340 cm^-1 exhibits a pronounced Fano-type line shape. We use a phenomenological model to describe the line shape that takes into account real and imaginary part of the electronic response. It allows us to obtain the self-energy corrections and the mass- enhancement factor lambda as a measure of the coupling. In the normal state we find lambda=0.015 around optimal doping and decreasing values with decreasing doping. The electron-phonon coupling increases strongly below T_c in overdoped samples in which the B1g 2Delta peaks appear in the vicinity of the phonon. Self-energy effects observed in the superconducting state can only partly be assigned to the redistributing electronic response. Anomalies with respect to frequency, linewidth, and intensity remain. They appear at increasing tempera- tures with decreasing doping and we provide evidence that they are connected to the presence of the pseudogap. We supplement our study by a comparison with single crystal data and investigate the influence of site-substitutional disorder on the electronic response and the electron-phonon interaction.