Scanning Tunneling and Atomic Force Microscopy and Interfacial Chemistry Measurements at the Niobium Surface

• Darren Veit (University of Chicago)

Room: Meeting Room Superconducting radio frequency (SRF) technology is the key enabler for current and future high-energy and high-beam-power accelerators. Performance of SRF cavities for accelerators is characterized by their quality factor, Q0, a measure of their efficiency of operation, and the maximum accelerating field, Eacc, that they can sustain before their quality factor degrades. Chemical and structural defects at the surface and in the nearsurface region of niobium SRF cavities can negatively or positively affect cavity performance. In this talk I discuss our investigations of the surface structure, chemistry, and oxidative states of single-crystal niobium samples as well as technical grade polycrystalline SRF cavity samples using scanning tunneling microscopy (STM), atomic force microscopy (AFM), Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS). These tools allow us to obtain atomically resolved top-down images of the niobium surface, to take surface sensitive chemical composition and oxidative state measurements, and to profile the concentrations of chemical contaminants and dopants as a function of depth into the niobium surface. Inspired by recent discoveries at the Fermi National Accelerator Laboratory that involve the effects of nitrogen-doping that enhance the operating characteristics of accelerator cavities, I discuss our efforts to explore the possible relationship between nitrogen-doping and nanoscale niobium hydride surface precipitation. Ordered hydride phases are known to form on cavity surfaces during cooldown to operational temperatures. They are non-superconducting and correlate strongly with diminished cavity performance. It is possible that nitrogen-doping serves as a hydrogen “trap” preventing hydrides from forming at the niobium surface during cooldown. I also present initial results on depth-profiling experiments performed on nitrogen-doped cavity samples provided by Fermi National Accelerator Laboratory.