1992    
1993    
1994    
1995    
1996    
1997    
1998    
1999    
2000    
2001    
2002    
2003    
2004    
2005    
2006    
2007    
2008    
2009    
2010    
2011    
2012    
2013    
2014    
2015    
2016    
Schumann, F. O., Winkler, C., Kerherve, G., Kirschner, J.

Mapping the electron correlation in two-electron photoemission
Physical Review B 73, (4),pp 041404/1-4 (2006)
Electronic correlations are manifested in many-body effects like superconductivity and magnetism. Established theoretical concepts show that the Coulomb and exchange interaction result in a tendency of two electrons to avoid each other, leading to an exchange-correlation (xc) hole. We will report on double photoemission (DPE) experiments using a time-of-flight setup consisting of a small central collector surrounded by a resistive anode. The first allows detection only within a narrow solid angle, therefore fixing the momentum. The resistive anode covers a solid angle of  ∼ 1 sr, the determination of the impact position results in momentum resolution. As a pulsed light source we used synchrotron radiation and we studied a NaCl(100) surface upon excitation with 34 eV photons. The very existence of coincidences is already a manifestation of the correlation. The onset of pair emission occurs when energy conservation allows the ejection of two electrons from the highest occupied level. We have made two key observations. If E1 and E2 are fixed such that a pair emission from the top of the valence band is possible, a zone of reduced intensity with a diameter of  ∼ 1.1 Å –1 is visible. Recent calculations on DPE from a Cu(100) surface display exactly such a feature due to the xc hole. Hence we prove experimentally the very existence of the xc hole in double photoemission. The zone of reduced intensity disappears whenever emission below the top of the valence band becomes possible, indicating the sensitivity of the xc hole to inelastic scattering.