A New Spin on Photoemission Spectroscopy Page: 94 of 259
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by circularly polarized light, and thermal diffusion of these charge carriers becomes a dom-
inant transport mechanism. A spin-lifetime, which is the average time before a spin-flip
scattering event occurs, can be introduced. It can be shown that if the spin-lifetime is much
shorter than the regular photoelectron lifetime, depolarization of these carriers can occur.85
For magnetic materials, spin-dependent scattering in step two can lead to interesting
effects. It is generally found that the low kinetic energy secondary electrons (large in-
creasing tail of measured spectrum in Figure 2.4) can have considerable polarization, even
greater than that of the valence band.87 This is generally explained qualitatively with a
basic phase space arguments. 22 For a photoelectron to flip its spin by scattering via the
Coulomb interaction (total spin-conserving), an electron from an occupied state must also
flip its spin and scatter to an available state above EF. The ferromagnetism of the sample
typically means there is a larger number of minority spin states above EF than majority
states. So for spin flip scatterings, it is easier for an occupied majority electron to find an
available minority state above EF, making it more likely for a spin minority photoelectron
to flip to majority than vice versa. This leads to an interesting situation where the high-
est spin polarization of the photoemitted spectrum is at the kinetic energies where there
is maximum intensity (note the height of the secondary peak in Figure 2.4). This effect
has been taken advantage of in making polarized secondary electron scanning microscopes
(or scanning electron microscope with polarization analysis, SEMPA) for high resolution
magnetic domain microscopy.88
Despite the various possibilities for loss of spin-information in transport and transmis-
sion through the surface, it has been experimentally found in many cases that spin polariza-
tion of primary photoelectrons is not significantly altered in the emission process, allowing
straightforward interpretation of the experiment. In other cases things can be less clear,
although even in these cases, the loss of spin-polarization can be used for understanding
the energy and spin dependent scattering of excited electrons.89
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Jozwiak, Chris. A New Spin on Photoemission Spectroscopy, thesis or dissertation, December 1, 2008; United States. (https://digital.library.unt.edu/ark:/67531/metadc1014237/m1/94/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.