On the origin of circular dichroism in angular resolved photoemission from graphene, graphite, and WSe$_2$ family of materials

TL;DR

This paper explains the origin of circular dichroism in angle-resolved photoemission from materials like graphene and WSe$_2$, showing that it arises mainly from orbital effects rather than topological properties.

Contribution

It derives the CD-ARPES signal from $p_z$ orbitals using simple algebraic formulas, clarifying its origin in non-topological materials like graphene.

Findings

Significant CD-ARPES signals are predicted across a wide photon energy range.

The process from $p_z$ orbitals explains observed dichroism in graphene.

Electron inelastic mean free path also contributes to CD-ARPES signals.

Abstract

Circular dichroism in angle-resolved photoemission (CD-ARPES) is one of the promising techniques for obtaining experimental insight into topological properties of novel materials, in particular to the orbital angular momentum (OAM) in dispersive bands, which might be related, albeit certainly in a non-trivial way, to the momentum resolved Berry curvature of the 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 $|2p_z\rangle$ orbitals that carry zero OAM, spin-orbit-coupling (SOC) can be neglected, and Berry curvature effectively vanishes. Dubs et al., Phys. Rev. B 32, 8389 (1985) have demonstrated non-vanishing cricular dichroism in angular distribution (CDAD) from an oriented $p_z$ orbital, and this process can be responsible for the experimentally observed CD-ARPES in graphene. In this paper, we derive the CD-ARPES from $p_z$ orbitals by elementary means, using only simple algebraic formulas and tabulated numerical values, and show that it leads to significant CD-ARPES signal over the entire vacuum ultraviolet and soft x-ray energy range, with an exception of the photon energy region near $h\nu \approx 40$ eV. We also demonstrate that another process, emerging from the finite electron inelastic mean free path, also leads to CD-ARPES of the potentially similar order of magnitude, as previously discussed by Moser, J. Electron Spectrosc. Relat. Phenom. 214, 29 (2017). We present calculated CDAD maps for selected orbitals and briefly discuss the consequences of the findings for CD-ARPES, focusing on graphene, graphite and WSe$_2$.