Speaker
Varghese Anto Chirayath
(Department of Physics, University of Texas at Arlington, 76019)
Description
We discuss our recent report on the direct observation of a low-energy Auger electron emission process initiated by the creation of a deep valence hole in single layer graphene through positron annihilation. Here, an electron from a higher energy level in the valence band fills the valence hole created by positron-electron annihilation. The energy released because of this relaxation is transferred to a third electron, which may have enough energy to escape into the vacuum by overcoming the surface barrier. Emission of electrons into the vacuum through this kind of Auger relaxation of valence hole is possible in single layer graphene because of its wide valence band (~ 20 eV). The time of flight (TOF)-PAES data from single layer graphene showed the presence of a low energy peak at ~ 4 eV as a result of this Auger transition. An empirical model of the Auger process developed with inputs from ab-initio calculations reproduced the low energy Auger line shape. Comparison of experimental and theoretically generated Auger spectrum revealed that more than 80% of the deep valence holes relaxed via an Auger relaxation process. PAES was able to observe this low energy Auger peak because of its ability to measure the Auger spectrum from surfaces without any background from processes unrelated to the Auger relaxation and due to the sensitivity of the technique to the top mono-layer of the sample. The top layer sensitivity comes from the fact that the low energy positrons (< 1 eV) which are deposited on the surface get trapped in an image potential well in the vacuum side of the sample and as a result selectively annihilate with core or valence electrons of the top mono-layer. We will use the present results to motivate how the top layer sensitivity of PAES, when combined with a spin polarized positron beam, can be used to probe the spin density of states in 2D materials and thus, possibly shed light on some of the exotic properties on the surfaces of novel 2 D materials.
Primary author
Varghese Anto Chirayath
(Department of Physics, University of Texas at Arlington, 76019)
Co-authors
Dr
Alex Weiss
(Department of Physics, University of Texas at Arlington, 76019)
Mr
Alexander Fairchild
(Department of Physics, University of Texas at Arlington 76019)
Dr
Ali Koymen
(Department of Physics, University of Texas at Arlington, 76019)
Dr
Bart Partoens
(Department of Physics, Universiteit Antwerpen, Antwerpen 2020, Belgium)
Dr
Bernardo Barbiellini-Amidei
(Department of Physics, Northeastern University, Boston, Massachusetts 02115)
Mr
Randall Gladen
(Department of Physics, University of Texas at Arlington, 76019)
Dr
Rolando Saniz
(Department of Physics, Universiteit Antwerpen, Antwerpen 2020, Belgium)
Mr
Vincent Callewaert
(Department of Physics, Universiteit Antwerpen, Antwerpen 2020, Belgium)
