A New Spin on Photoemission Spectroscopy Page: 97 of 259
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This example well illustrates that the spin-orbit splitting of the valence band is critical
for the effect. If there were no spin-orbit splitting, the P1/2 levels at I7 would also be able
to participate in overlapping transitions. A similar analysis to above shows that with right
circularly polarized light, the additional mj = -1/2 -> m* = +1/2 transition would occur
at a rate of 2 relative to the others. This would result in a total excited spin ratio of 3 to
3; in other words, no spin polarization. Equally critical is the use of circularly polarized
light. Inspection can show that linearly polarized light, with its different selection rules,
will not result in spin polarization (this is perhaps easiest to see by thinking of the linear
polarized light as a superposition of both left and right circular light). This is actually a firm
rule for bulk transitions: excitation by linear polarization will not create spin polarization,
but circular/elliptical may and reversal of the helicity reverses the polarization effect.85 It
should be noted that this example and rule explicitly involves only photoemission normal
to the surface; in general, at emission angles away from normal, spin-orbit splitting can
lead to polarized emission even with linear or unpolarized light.95 We will see an example
of this in section 6.2.3.
The present effect with circularly polarized light is sometimes referred to as "optical spin
orientation" and is more thoroughly described in ref. 89. More exhaustive group theoretical
approaches to the symmetry of the bands, including away from the F point, can describe
the effect more completely. By measuring the spin polarization of photoemission from
samples with strong spin-orbit coupling, one can thus work backwards to experimentally
map the symmetries of the probed bandstructure. Examples of this, in particular the
resulting symmetry-resolved bandmapping of Pt, are well reviewed in ref. 96. Other good
examples of using this technique to investigate spin-orbit splitting in the valence bands of
non-magnetic 3d transition metals can be found in ref. 97.
An extremely interesting, although difficult to interpret, spin-resolved photoemission
experiment has also been performed on the HTC superconducting cuprates.21 These HTC's
are of course not ferromagnetic - in fact they have an antiferromagnetic background. Av-
eraged over the many unit cells within a reasonable illumination spot, there will be no spin
polarization in the initial states. However, again through the use of circularly polarized light
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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/m1/97/: accessed April 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.