On the origin of circular dichroism in angular resolved photoemission from graphene, graphite, and WSe₂ family of materials

The Kubo formula for quantum transport, often used to determine topological properties of solids, is an integral of the Berry curvature over the Brillouin zone (BZ), and contains interband velocities, for determination of which one needs spin and orbital characters, as well as phases of Bloch wave functions. Circular dichroism in angle-resolved photoemission (CD-ARPES) is one of the promising techniques for obtaining experimental insight into orbital angular momentum (OAM) in dispersive bands. Therefore, it is important to understand how non-vanishing CD-ARPES signal arises in graphene, a material where Dirac bands are made from C 2p_z orbitals that carry zero OAM, spin-orbit-coupling (SOC) can be neglected, and Berry curvature effectively vanishes. On this example, within the dipole approximation, we discuss various contributions to the CD-ARPES [1] which include phase shifts of the participating partial waves in the atomic photoionization [2], the finite inelastic-mean-free-path induced CD [3], the interatomic phase shifts in the far field [4], and the CD due to multiple scattering of the excited electron [5]. Using tabulated phase shifts and radial integrals [6] we predict photon energies at which CD signal might exhibit sign changes, and compare the prediction to experimental results. Further, we perform similar analysis for WSe₂, a material where orbital characters are relatively well-defined, however, varying over BZ, with different contributions at K, K', and Γ points. This can be translated into understanding of CD-ARPES from topological states, including Dirac cones in strong topological insulators and Fermi arcs in Weyl semimetals. Finally, we briefly discuss how other components needed to derive the Berry curvature, the spin characters [4], and the phases of the wave functions, can be accessed through angle-resolved photoemission and its spin-polarized variant.

[1] Plucinski, arXiv:2309.02187 (2023), [2] Dubs et al., Phys. Rev. B 32, 8389 (1985) [3] Moser, J. Electron Spectrosc. Relat. Phenom. 214, 29 (2017) [4] Heider, et al., Phys. Rev. Lett. 130, 146401 (2023) [5] Daimon et al., Jpn. J. of Appl. Phys. 32, L1480 (1993) [6] Goldberg et al., J. Electron Spectrosc. Relat. Phenom. 21, 285 (1981)