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© Wikimedia Commons, Public Domain. Author: Saiht, 15 June 2009.

Angle-resolved photoemission spectroscopy (ARPES), also known as ARUPS (angle-resolved ultraviolet photoemission spectroscopy), is a direct experimental technique to observe the distribution of the electrons (more precisely, the density of single-particle electronic excitations) in the reciprocal space of solids. ARPES is one of the most direct methods of studying the electronic structure of the surface of solids. ARPES gives information on the direction, speed and scattering process of valence electrons in the sample being studied (usually a solid). This means that information can be gained on both the energy and momentum of an electron, resulting in detailed information on band dispersion and the Fermi surface. This technique is a refinement of ordinary photoemission spectroscopy. It has proved particularly valuable in helping unravel some of the many puzzling aspects of the behavior of high-temperature superconductors, as we shall see in Section 7. (Unit: 8)