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Facilities for the Performance of Fano Effect
Measurements as a Probe of Electron Correlation
J.G. Tobin'*, S.W. Yul, T. Komesu2, B.W. Chung',
S.A. Morton' '2 and G.D. Waddill2
1 Lawrence Livermore National Laboratory, Livermore, CA, USA
2 University of Missouri- Rolla, Rolla, MO, USA
Fano Effect measurements are the key to direct observation of the Kondo or spin
shielding intrinsic to models of electron correlation. The Fano Effect is the observation
of spin polarized photoelectron emission from NONMAGNETIC materials, under
chirally selective excitation, such as circularly polarized photons. Below are described
three spectrometers, with which Fano Effects measurements have been made.
The key measurements are based upon spin-resolving and photon-dichroic photoelectron
spectroscopy. True spin-resolution is achieved by the use of a Mini-Mott detection
scheme. The photon-dichroic measurements include the variants x-ray magnetic circular
and linear dichroism angular distributions (XMCDAD and XMLDAD). Both a multi-
channel, energy dispersive collection scheme as well as the spin-detecting Mini-Mott
apparatus are used in data collection.
Device 1. Spin Spectrometer
The "Spin Spectrometer" was previously based at the Spectromicroscopy Facility
(Beamline 7) at the Advanced Light Source at Lawrence Berkley National Laboratory
(LBNL, Berkeley, CA, USA) and is now located at Beamline 4 EPU (Elliptically
Polarized Undulator) at the Advanced Photon Source at Argonne National Laboratory
near Chicago, IL, USA. The high angular and energy resolution with high throughput is
achieved via the use of an 11-inch mean diameter hemispherical analyzer supplied by
Physical Electronics. Included in this package is an electron collection lens stack with an
adjustable aperture, permitting selection of various angular and sample spot sizes. The
novel aspect of our PHI analyzer is that the multi-channel detector has a hole in the
center, permitting the direct passage of energy analyzed electrons into the electron optics,
without resorting to an electron switchyard. The presence of the hole does cause some
problems when the multi-channel (non-spin) detection is being used: an increase in dark
and background counts. Dark counts are defined as non-zero electron counting that
occurs when the multi-channel detection is "on" but no excitation is striking the sample.
Background counts are the counts underlying the elastic photoelectron peaks, e.g., a core-
MRS Symp Proc, Tobin Page 1 11/15/06
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Tobin, J G; Yu, S W; Komesu, T; Chung, B W; Morton, S A & Waddill, G D. Facilities for the Performance of Fano Effect Measurements as a Probe of Electron Correlation, article, November 8, 2006; Livermore, California. (digital.library.unt.edu/ark:/67531/metadc885199/m1/3/: accessed October 20, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.