Identification of kz -Dependent Orbital Characters for Electronic States in Bi2Se3

Abstract

The topological insulator bismuth selenide (Bi2Se3) offers unique functional advantages in both photocatalysis and lithium-ion battery anode applications. The orbital nature of the bands near the Fermi level is crucial for understanding and manipulating the physical and chemical properties of catalytic and energy materials. While standard density functional theory calculations provide orbital projections on atomic sites and k-dependent band structures, the detailed spatial orbital distribution and k-resolved orbital mixing at the Fermi level—particularly in distinguishing bulk vs. surface contributions in topological insulators like Bi2Se3—remain an important consideration that requires careful interpretation. Here, we employ photon energy-dependent multidimensional photoelectron spectroscopy with first-principles calculations to probe the orbital properties of both surface and bulk states in the topological insulator Bi2Se3. The photoelectron constant energy contour (CEC) measurements reveal distinct shapes for the bulk conduction and valence bands, in contrast to the invariant CECs of the surface state. The observed variations in the bulk states contributed by the pz orbital arise from the significant differences in crystal symmetry sampled at different kz positions within a three-dimensional periodic potential field from two adjacent quintuple layers (QLs). These results advance fundamental understanding of the electronic structure in catalytic and energy materials and lay a framework for the rational design of multifunctional devices.