Hard X-ray angle-resolved photoemission (HARPES)
Angle-resolved photoemission spectroscopy (ARPES) has developed into a powerful technique for studying the electronic structure of crystals. Several classes of materials such as high-temperature superconductors, grapheme, and topological insulators have been successfully investigated with this technique, which makes use of the photoelectric effect, in which an electron is emitted from a solid when it is excited by a photon. The relationship between the kinetic energy of the photoelectron, Ekin, and the frequency, ν, of the photon, Ekin = hν − Eb, where h is Planck's constant and Eb is the electron binding energy, was postulated by Albert Einstein and became of of the milestones in the development of quantum mechanics. For the case of electrons photoemitted from crystalline solids here exists a conservation relationship between the photoelectron momentum and the initial state momentum of the electron in the solid. Therefore, by measuring the momentum, energy, and possibly also the spin of photoelectrons, one can deduce the properties of the initial electronic states in the solid.
Only those electrons which escape the solid without an additional loss of energy and change of momentum carry the direct information related to the initial state. The inelastic mean-free path (IMFP) of the electron depends on its kinetic energy inside the solid. For valence band electrons in the typical ARPES experiment with photon energies of 10–100 eV the IMFP is only about 5–10 Å, therefore the information depth for initial states is limited to the surface region. This situation is not desirable in case bulk bands or buried interfaces are of interest, and one way around this problem is to use photons with higher energy in the X-ray regime.
We have performed ARPES measurements at hν = 870 eV  and at 1.25 keV  for which the IMFPs are increased to 10–20 Å, thus partially overcoming the limitation of surface sensitivity. Clear valence band dispersions were observed in these experiments, and temperature dependence of their contract allowed us to estimate the feasibility of the high-energy ARPES (HARPES) at hard X-ray photon energies via the simple Debye-Waller model.
We have followed up with reporting the first hard X-ray ARPES (HARPES) measurements at 3–6 keV on tungsten and GaAs single crystals, thus enhancing the probing depth to 30–60 Å, deeper into the bulk . See more info on this research here:
Subsequently we have applied the HARPES method to the GaMnAs ferromagnetic semiconductor , and provided an insight into the origin of ferromagnetism. See more information on this research here:
 L. Plucinski, J. Minar, B.C. Sell, J. Braun, H. Ebert, C.M. Schneider, and C.S. Fadley, “Band mapping in higher-energy x-ray photoemission: Phonon effects and comparison to one-step theory”, Phys. Rev. B 78, 035108 (2008).
 C. Papp, L. Plucinski, J. Minar, J. Braun, H. Ebert, and C. S. Fadley, “Band mapping in x-ray photoelectron spectroscopy: an experimental and theoretical study of W(110) with 1.25 keV excitation”, Phys. Rev. B 85, 045433 (2011).
 A. X Gray, C. Papp, S. Ueda, B. Balke, Y. Yamashita, L. Plucinski, J. Minar, J. Braun, E. R. Ylvisaker, C. M. Schneider, W. E. Pickett, H. Ebert, K. Kobayashi, and C. S. Fadley, “Probing Bulk Electronic Structure with Hard-X-Ray Angle-Resolved Photoemission: W and GaAs”, Nature Materials 10, 759 (2011).
 A. X. Gray, J. Minar, S. Ueda, P. R. Stone, Y. Yamashita, J. Fujii, J. Braun, L. Plucinski, C. M. Schneider, G. Panaccione, H. Ebert, O. D. Dubon, K. Kobayashi, and C. S. Fadley, “Bulk Electronic Structure of the Dilute Magnetic Semiconductor Ga(1-x)Mn(x)As via Hard X-Ray Angle-Resolved Photoemission”, Nature Materials (2012), DOI: 10.1038/NMAT3450.