Analysis of laser angle-resolved photoemission spectra of Ba2Sr2CaCu2O8+δ in the superconducting state: Angle-resolved self-energy and the fluctuation spectrum

We analyze the ultrahigh-resolution laser angle-resolved photoemission spectroscopy intensity from the slightly underdoped Bi2Sr2CaCu2O8+delta in the superconductive (SC) state. The momentum distribution curves were fitted at each energy omega employing the SC Green's function along several cuts perpendicular to the Fermi surface with the tilt angle theta with respect to the nodal cut. The clear observation of particle-hole mixing was utilized such that the complex self-energy as a function of omega is directly obtained from the fitting. The obtained angle-resolved self-energy is then used to deduce the Eliashberg function alpha F-2((+))(theta,omega) in the diagonal channel by inverting the d-wave Eliashberg equation using the maximum entropy method. Besides a broad featureless spectrum up to the cutoff energy omega(c), the deduced alpha F-2 exhibits two peaks around 0.05 and 0.015 eV. The former and the broad feature are already present in the normal state, while the latter emerges only below T-c. Both peaks become enhanced as T is lowered or the angle theta moves away from the nodal direction. The implication of these findings is discussed.