Measuring spin polarization vectors in angle-resolved photoemission spectroscopy

The quantitative analysis of spin-polarized photoemission data is discussed. An angle-resolving photoelectron spectrometer equipped with a three-dimensional (3D) spin polarimeter produces complete data sets consisting of photoemission intensities as well as spin asymmetry curves for three orthogonal vector components. In a two-step fitting routine, the photoemission spectrum is first dissected into individual peaks and background. Assigning trial spin polarization vectors to each of them, the asymmetry curves can be modeled until the best fit is reached. This procedure is crucial when analyzing strongly overlapping peaks or weak signals sitting on a large unpolarized background, especially in the presence of non-collinear spins. It is robust against strong intensity variations due to matrix element effects because it references the spin polarization contribution of each band to the measured peak intensity. The method is applied to 2D systems where spin-orbit effects lead to spin splittings and complex momentum-dependent spin structures. Presented case studies include surface alloys of Bi and Pb on Ag(111) that show a giant Rashba effect.