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Angle-integrated photoelectron spectroscopy can be accomplished with this instrument by defining a small acceptance angle producing the familiar spectra of conventional analyzers.
In addition, we can selectively obtain angle-resolved spectra from a specific emission angle to produce angle-resolved UPS curves. When the analyzer is fixed to a selected binding energy, we can obtain an image which gives the intensity of photoemission as a function of emission angle. This angular distribution contains all of the dependence of the transition matrix element on the incident photon polarization.
This photon polarization dependence allows one to unambiguously determine the symmetry of the state. This is particularly useful in studies of magnetic dichroism (MLD and MCD). With a comparison to predicted distributions from our theory effort, one can establish clear links between the electronic structures that are observed and models for material behavior.
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The great advantage of this instrument is the ability to simulatneously image, in a 64 degree cone, a large number of emission directions from a given binding energy. This "parallel" data acquisition allows us to measure a very large amount of k-space in a short period of time. By "stacking" images obtained with different photon energies from a specific initial energy, we are able to map band structure through the Brillouin zone. Unlike a conventional ARPES spectrum, which gives the location of a band as a single point in the BZ, our images give a locus of points describing the intersection of an initial state energy with a slice through the BZ. A single image contains much more information than an ARPES spectrum.
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This method of band mapping, essentially looking at horizontal slices through the E(k) is ideal for mapping constant energy surfaces like the Fermi surface. Here, the intersection of a free-electron sphere with the Fermi surface determines the directions of emission. By changing the photon energy, we change the dimension of the final-state sphere and therefore the location in the BZ that we sample. A series of images obtained at a range of photon energies can be stacked to generate an experimental mapping of the Fermi surface.
There are several advantages of this method over magnetic resonance measurements. First, impure samples can be readily mapped and the measurements can be conducted at elevated temperatures where interesting things like phase transitions can change the Fermi surface. Secondly, we obtain information on the structure of the FS throughout the zone including regions of low symmetry, and not just on extremal cross-sections. Finally, atomically-thin films can be studied, providing information on materials of technological importance such as magnetic multilayers.
We have constructed an ellipsoidal-mirror analyzer for photoelectron spectroscopy. This instrument allows one to image the angular distribution of photoelectrons emitted from surfaces. It is an energy-resolving analyzer where the electrons pass through the instrument with a constant pass energy, giving a constant energy resolution. We typically operate it with a resolution of ~100meV with the limitation on resolution being thought to be due to inhomogeneities in the work functions of grids and mirror.