Facilities for the Performance of Fano Effect Measurements as a Probe of Electron Correlation
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Facilities for the Performance of Fano Effect Measurements as a Probe of Electron Correlation J. G. Tobin1, S. W. Yu1, T. Komesu2, B W Chung1, S. A. Morton1, and G. D. Waddill2 1 LLNL, Livermore, CA, 94550 2 U. Missouri-Rolla, Rolla, MO, 65401 Abstract 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. Introduction 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 multichannel, 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-
level, when actual collection is underway. Regardless, under many conditions, these problems are inconsequential. Spin resolution is achieved by directing the electrons through the optics and into the Mini-Mott detector. (Figure 1) In this case, the high voltages on the channel plates are turned off and the channel plates and anode assembly become part of the first lens stack, directing the electrons into the 90° spherical sector. The 90°sector is run at a relatively high pass energy: energy resolution is provided solely by the hemisphere and the photon monochromator. Because the multi-channel detection
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