Spin-Polarized Photoelectron Spectroscopy
Besides mass and charge, electrons also possess a spin. By detecting the direction of the spin of photoemitted electrons one can gain important information about the photoexcitation process and the properties of the sample under investigation. There are t
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10.1 General Description Besides mass and charge, electrons also possess a spin. By detecting the direction of the spin of photoemitted electrons one can gain important information about the photo excitation process and the properties of the sample under investigation. There are two modes in which a PE experiment can produce a spin polarization of the photo emitted electron. One can use unpolarized light to excite polarized electrons in a sample, which is a good way to investigate, e.g., magnetic materials, and one can employ circularly polarized light to excite transitions between states that are split by spin-orbit interactions, thereby obtaining spin-polarized electrons in the final state. This second method is useful for producing beams of polarized electrons, which can then be used for other experiments. Unfortunately, the measurement of spin polarization is a non-trivial problem and there are relatively few laboratories that can employ this very elegant method. Therefore, we shall simply outline the principles of such experiments whilst referring the reader to the excellent reviews [10.1-10.6] for further details. The quantity that is measured is the polarization of a beam of electrons with respect to a direction in space, where the polarization P is defined as P = nj -nl
(10.1) nj +nl with n j and n 1 being the number of up- and down-polarized electrons. The measurement of the polarization is often performed by using the spindependent scattering of electrons at high energies (Mott detector) [10.7]. The spin-dependent diffraction of low energy electrons by a solid can also be used (SPLEED detector) [10.8]. The most promising detector seems to be one which uses the spin-dependent absorption of electrons in a solid [10.9,10.10].
10.2 Examples of Spin-Polarized Photoelectron Spectroscopy The application of spin-polarized photoemission is of particular interest for magnetic materials. Figure 10.1 shows the schematic density of states of ferS. Hüfner, Photoelectron Spectroscopy © Springer-Verlag Berlin Heidelberg 2003
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10. Spin-Polarized Photoelectron Spectroscopy
romagnetic Ni. The spin-up and spin-down bands are shifted with respect to one another by the exchange splitting L1Eex' which has a theoretical value of L1Eex = 0.6eV. Experimentally, however, one observes L1Eex = 0.3eV as determined by photoemission. Note that Fig. 10.1 is an oversimplification (rigid band splitting); the exchange splitting is actually energy and wave-vector dependent, but the essence of the band magnetism is contained in the figure. A critical test of this picture can be made by measuring the spin polarization of electrons photoemitted from Ni near the photothreshold. The results of such a measurement are shown in Fig. 10.2. As expected from Fig. 10.1, the electrons indeed show a large negative spin polarization at the photothreshold, which rapidly changes sign with increasing photon energy, as likewise expected from the schematic density of states, since with increasing photon energy the number of photoexcited majority-spin e
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