Photoemission as a Probe of the Collective Excitations in Condensed Matter Systems

摘要

New developments in instrumentation have recently allowed photoemission measurements to be performed with very high energy and momentum resolution.(1) This has allowed detailed studies of the self-energy corrections to the lifetime and mass renormalization of excitations in the vicinity of the Fermi level. These developments come at an opportune time. Indeed the discovery of high temperature superconductivity in the cuprates and related systems is presenting a range of challenges for condensed matter physics.(2) Does the mechanism of high T(sub c) superconductivity represent new physics. Do we need to go beyond Landau's concept of the Fermi liquid.(3) What, if any, is the evidence for the presence or absence of quasiparticles in the excitation spectra of these complex oxides. The energy resolution of the new instruments is comparable to or better than the energy or temperature scale of superconductivity and the energy of many collective excitations. As such, photoemission has again become recognized as an important probe of condensed matter. Studies of the high T(sub c) superconductors and related materials are aided by the observation that they are two dimensional. To understand this, we note that the photoemission process results in both an excited photoelectron and a photohole in the final state. Thus the experimentally measured photoemission peak is broadened to a width reflecting contributions from both the finite lifetime of the photohole and the momentum broadening of the outgoing photoelectron.