Physicists at the University of Texas at Arlington have developed a new technique that can measure the properties of the topmost atomic layer of materials without including infor­mation from the underlying layers. The researchers from the Positron Lab utilized auger-mediated positron sticking (AMPS) to develop the novel spectro­scopic tool to measure the electronic structure of the surface of materials selectively. 

The AMPS process, in which positrons stick directly to surfaces followed by electron emission, was first observed and described by Saurabh Mukherjee, a graduate student, along with Alex Weiss, chair of the Department of Physics, and other colleagues, in 2010. “Alex Fairchild and Varghese Chirayath 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 noted that the Positron Lab is currently the only place this technique could have been developed, due to the capa­bilities of its positron beam. “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. (Source: UTA)

Reference: A. J. Fairchild et al.: Photoemission Spectroscopy Using Virtual Photons Emitted by Positron Sticking: A Complementary Probe for Top-Layer Surface Electronic Structures, Phys. Rev. Lett. 129, 106801 (2022); DOI: 10.1103/PhysRevLett.129.106801

Link: Dept. of Physics, University of Texas at Arlington, Arlington, USA