The University of Texas at Arlington physicists have created a brand-new method for measuring the properties of the uppermost atomic layer of materials without taking into account data from the deeper levels.
The auger-mediated positron sticking (AMPS) technique was used by researchers from the Positron Lab at the University of Texas at Arlington to create a unique spectroscopic instrument for selectively measuring the electronic structure of materials’ surfaces.
A new article, “Photoemission spectroscopy using virtual photons emitted by positron sticking: A complementary probe for top-layer surface electronic structures,” published in the journal Physical Review Letters (PRL), details the new technique.
Additionally, a Viewpoint piece explaining the significance of the study to the field was published in the online journal Physics with the title “Spectroscopy That Doesn’t Scratch the Surface.” PRL editors commission viewpoint articles for papers they think will generate significant attention.
Alex Fairchild, postdoctoral scholar in the Positron Lab, is the study’s lead author. Co-authors include Varghese Chirayath, assistant professor of research; Randall Gladen, postdoctoral researcher; Ali Koymen, professor of physics; and Alex Weiss, professor, and chair of the UTA Department of Physics. Bernardo Barbiellini, professor of physics at LUT University in Finland, also contributed to the project.
Our AMPS results showed how virtual photons emitted following positron-sticking interact preferably with electrons that extend further into the vacuum than with electrons that were more localized to the atomic site. Our results are thus essential to understand how positrons interact with surface electrons and are extremely important to understand other similarly surface-selective, positron-based techniques.
Varghese Chirayath
The AMPS process, in which positrons (the antimatter of electrons) adhere to surfaces directly and then emit electrons, was discovered and first described by Weiss, Weiss’s collaborators, and graduate student Saurabh Mukherjee in 2010 at UTA. Those results were published in a paper in PRL.
“Alex (Fairchild) and Varghese figured out how to use this phenomenon that we discovered in 2010 to measure the top layer and get information about the electronic structure and the behavior of the electrons in the top layer,” Weiss said. “That will determine a material’s many properties, including conductivity, and can have important implications for building devices.”
Fairchild said the AMPS process is unique because it uses virtual photons to measure the topmost atomic layer.
“This is different from typical techniques like photoemission spectroscopy, where a photon penetrates multiple layers into the bulk of a material and therefore contains the combined information of the surface and subsurface layers,” Fairchild said.
“Our AMPS results showed how virtual photons emitted following positron-sticking interact preferably with electrons that extend further into the vacuum than with electrons that were more localized to the atomic site,” Chirayath said.
“Our results are thus essential to understand how positrons interact with surface electrons and are extremely important to understand other similarly surface-selective, positron-based techniques.”
Weiss stated that due to the characteristics of its positron beam, the UTA Positron Lab is now the only location where this approach could have been developed.
“At present, UTA probably has the only lab in the world that has a positron beam that can get down to the low energies needed to observe this phenomenon,” Weiss said.
The study was supported by funding from the Welch Foundation and the National Science Foundation.
