Using a process called auger-mediated positron sticking (AMPS), scientists at the Positron Lab in the UTA Department of Physics have developed a new technique that can measure the properties of the top atomic layer of materials.
This new spectroscopic tool uses virtual photons to selectively measure the electronic structure of the top atomic layer. When incoming positrons change from vacuum states to bound surface states on the sample surface, they produce virtual photons with the energy to excite electrons in the vacuum.
The short interaction range of the virtual photons limits the penetration depth to approximately the Thomas-Fermi shield length. Measurements and analysis of the kinetic energies of the emitted electrons on a single layer of graphene on copper and the clean copper substrate show that the ejected electrons originate exclusively from the top atomic layer.
Alex Weiss, professor and chair of the UTA Department of Physics, said: “We have discovered how we can use this phenomenon discovered in 2010 to measure the top layer and obtain information about the electronic structure and behavior of the electrons in the top layer. That determines the many properties of a material, including conductivity, and can have important implications for building devices.”
Alex Fairchild, a postdoctoral scientist in the Positron Lab, the study’s lead author, said: “The AMPS process is unique in that it uses virtual photons to measure the top atomic layer.”
“This differs from typical techniques such as photoemission spectroscopy, where a photon penetrates multiple layers in most of a material and therefore contains the combined information of the surface and subsurface layers.”
Varghese Chirayath, assistant research professor, said: “Our AMPS results showed how virtual photons emitted after positron sticking preferentially interact with electrons extending further into the vacuum than with more localized electrons to the atomic site. Thus, our results are essential for understanding how positrons interact with surface electrons and are extremely important to understand other similar surface-selective positron-based techniques.”
Weiss noted that the UTA Positron Lab is currently the only place where this technique could have been developed due to the capabilities of the positron beam.
“UTA probably has the only lab in the world with a positron beam that can reach the low energies needed to observe this phenomenon.”
- Alexander J. Fairchild et al. Photoemission spectroscopy using virtual photons emitted from Positron Sticking: a complementary probe for upper layer electronic surface structures. Physical assessment letters. DOI: 10.1103/PhysRevLett.129.106801