Electronic Raman scattering and phonon self‐energy effects in the R‐123 system: Signatures of gap and pseudogap

Abstract

AbstractUsing Raman spectroscopy and transport measurements we investigate thin epitaxial films of Y1—x(Pr, Ca)xBa2Cu3O6+y. We explore the electronic Raman responses in A1g, B1g, and B2g symmetry obtained after subtraction of phononic excitations, and especially, the 2Δ peaks that form out of the electronic background below Tc. We find that the energy of the B1g 2Δ peak, which is a measure of the gap value, increases monotonically with decreasing doping until the peak becomes unresolvable. In contrast, the peak in A1g symmetry follows Tc being still resolvable in the Pr‐doped films. The B2g response is weak and a 2Δ 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 λ as a measure of the coupling. In the normal state we find λ = 0.015 around optimal doping and decreasing values with decreasing doping. The electron‐phonon coupling increases strongly below Tc in overdoped samples in which the B1g 2Δ 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 temperatures 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.