M. Weinert , Brookhaven

Angle-Resolved Photoemission Spectroscopy (ARPES) is one of the important, if not the most important, techniques for investigating the electronic structure of materials [1]. The energy versus momentum distributions of the electronic states determine in a real sense the properties of materials. The availablity of insertation devices at light sources and recent advances [2] in detectors and instrumentation, allow energy and angular resolutions on the order of <5 meV and <0.2^\circ, resolutions that were unavailable just a few years ago.

These higher resolutions present a challenge to theory on several fronts. First, effects that could safely be ignored in comparison between theory and experiment, are now readily observable in the data [3,4]. Second, the types of problems that are being addressed have increased in complexity [5,6]. For example, photoemission is being used to study [7-10] the high-T_c materials around the transition temperatures. The high resolution allows the observation of changes in the states around the gap.

The studies of these types of ``correlated'' systems raise fundamental questions concerning the interpretation of the experiments, including whether or not a quasiparticle discription makes sense. Understanding these issues will involve advances in both the theoretical treatment of correlated systems, and then the modeling of the photoemission spectra itself [11-13].

For more traditional systems in which the one-electron states of conventional electronic structure (band) theory are reasonable approximations to the quasiparticles, there are both needs and opportunities for theory. The interpretation and understanding of the experimental data are significantly enhanced when there is a close coupling of theory and experiment. For many problems, ground state calculations are sufficient for extracting the important microscopic aspects and for understanding such properties as surface states and bonding mechanicisms. In other cases, calculations including matrix elements and final state effects are necessary. These final state effects and loss processes can manifest themselves as satellite structures and/or distortions of the spectra. In addition, beyond electron-electron interactions, the interactions of electrons with other quasiparticles (e.g., phonons, magnons) may affect the spectra, giving rise to temperature-dependent variations in the intensity.

To tackle the various problems related to ARPES requires advances in both fundamental theory and development of new computational approaches. The list of topics above is by no means comprehendsive. Simplified models are useful for understanding, but detailed comparisons with experiment require accurate first-principles approaches, and a close coupling between experiment and theory.

References:
[1] P. D. Johnson, Rep. Prog. Phys. {\bf 60}, 1217 (1997).
[2] N. Martensson, et al., J. Elec. Spectroscopy and Related Phenomena {\bf 70}, 117 (1994). [3] T. J. Kreutz, T. Greber, P. Aebi, and J. Osterwalder, Phys. Rev. B {\bf 58}, 1300 (1998).
[4] A. Goldeni, C. Cepak, E. Magnano, A. D. Laine, S. Vandre, and M. Sancrotti, Phys. Rev. B {\bf 58}, 2228 (1998).
[5] A. Arko, et al., Phys. Rev. B {\bf 56}, R7041 (1997).
[6] M. Garnier, D. Purdie, K. Breuer, M. Hengsberger, and Y. Baer, Phys. Rev. B {\bf 56}, R11399 (1997).
[7] T. Saitoh, et al., Phys. Rev. B {\bf 56}, 8836 (1997).
[8] A. G. Loeser, et al., Phys. Rev. B {\bf 56}, 14185 (1997).
[9] M. C. Schabel, et al., Phys. Rev. B {\bf 57}, 6090, 6107 (1998).
[10] H. C. Schmelz, et al., Phys. Rev. B {\bf 57}, 10936 (1998).
[11] C.-O. Almbladh, Physica Scripta {\bf 32}, 341 (1985); Phys. Rev. B {\bf 34}, 3798 (1986).
[12] M. R. Norman, M. Randeria, H. Ding, and J. C. Camuzano, Phys. Rev. B {\bf 57}, R11093 (1998).
[13] L. Hedin, J. Michiels, and J. E. Inglesfield, Phys. Rev. B (to appear).