In solid state systems, electrons form electronic band structures in the energy-momentum space. These band structures are the basis for macroscopic electronic and optical properties such as electrical transport, heat transport, and certain types of magnetism. Angle-resolved photoemission spectroscopy (ARPES) is a powerful tool to directly measure the electronic band structure. The principle of ARPES is based on the photoelectric effect, which was first explained by Albert Einstein. A light beam with a photon energy higher than the material's work function is incident on the sample in an ultrahigh vacuum environment (<1e-10 Torr). Photoelectrons are generated and collected by a hemispherical analyzer. The analyzer resolves the emission angle and the kinetic energy of the photoelectrons. Based on the conservation laws of energy and momentum, one can re-construct the electronic binding energy as a function of crystal momentum, which is precisely the electronic band structure.
More precisely, ARPES measures the "single-particle spectral function," which encodes not only the band structure but also the electronic interactions. Therefore, using ARPES we understand the fundamental electronic interactions in a solid, and can relate to collective quantum phenomena such as superconductivity, charge order, and topological phases. In our lab, we connect MBE with an in situ ARPES setup, which enables a positive feedback loop for scientific investigation: MBE produces novel materials for ARPES studies; ARPES provides instant understanding which is crucial for MBE synthesis.