Reavealing spatial orbital nature of Bi2Se3

The spatial orbital nature of the bands near the Fermi level are crucial for understanding and manipulating the physical and chemical properties of quantum and energy materials. While the state-of-the-art theoretical studies, including calculations with or without spin-orbit coupling, offer insights into orbital distribution, which only provide homogenized representations. Experimental investigations spaning spatial scales from angstroms to nanometers remains scarce. Here, we employ photon energy-dependent multidimensional photoelectron spectroscopy to probe the spatial orbital properties of the both surface and bulk states in topological insulator Bi ₂ Se ₃ . The photoelectron constant energy contours (CECs) measurements reveal distinct shapes for the bulk conduction and valence bands, in contrast to the invariant CECs of the surface state. Notably, we demonstrate that p _z orbital from two adjacent unit cells contribute to the bulk states, wheres the surface states are predominately governed by the p _z orbital from a single unit cell. These findings provide deeper insights into the electronic structure of quantum and energy materials, offering a pathway towards the rational design of multifunctional devices.