Reexamining circular dichroism in photoemission from a topological insulator

The orbital angular momentum (OAM) of electron states is an important ingredient for topological and quantum geometric quantities in solids. For example, Dirac surface states with helical spin- and orbital- angular momenta are a hallmark of a 3D topological insulator. Angle-resolved photoemission spectroscopy (ARPES) with variable circular light polarization, known as circular dichroism (CD), has been widely employed as a direct probe of OAM, and by proxy, of the Berry curvature of electronic bands in energy- and momentum- space. Indeed, topological surface states have been shown to exhibit angle-dependent CD, and more broadly, CD is interpreted as evidence of spin-orbit coupling.

Meanwhile, it is well-known that CD originates from the photoemission matrix element, which can have extrinsic contributions related to the experimental geometry. Therefore, it is important to broadly examine CD-ARPES to determine the scenarios in which it provides a robust probe of intrinsic material physics. We performed CD-ARPES on the canonical topological insulator Bi₂Se₃ over a wide range of incident photon energies. Not only do we observe angle-dependent CD in the surface states, as expected, but we find CD of a similar magnitude in virtually all bulk bands as well. Since OAM is forbidden by inversion symmetry in the bulk, we conclude this originates from symmetry-breaking in the photoemission process. Comparison with theoretical calculations supports this view and suggests that “hidden” OAM—localized to atomic sites within each unit cell—is a substantial contribution. We will comment on the general implications for interpreting CD in ARPES.