We report new studies that illuminate the physics of the superconducting (S) to normal (N) transition associated with the ion bombardment of MgB₂. The investigation is motivated by our recent reports of successful Josephson Junction (JJ) arrays with exceptional uniformity, where weak link (N-state) barrier line is formed by ion bombardment using the Rutgers He⁺ Ion Microscope (HeIM) (30 keV, <0.5nm diameter beam spot size), in a patterned, epitaxial MgB₂ film [1].
To investigate the mechanism of the ion beam induced suppression of the superconducting properties, we measured the fluence dependence of 30keV He⁺ irradiation on the material properties of superconducting MgB₂ thin films. Using hard x-ray photoelectron spectroscopy changes in valence band structure (density of states at the Fermi level, DoS(E_f)) were determined. The disorder is shown to correlate with the change in the DoS(E_f) and hence the superconductivity. The physical picture that emerges is consistent with that described by Testardi and Matheiss [2] where the short mean free paths and associated lifetimes suppress the critical DoS(E_f).

[1] L.Kasaei et al. IEEE Transactions on Applied Superconductivity, 29 (2019);
[2] Testardi L and Mattheiss L Physical Review Letters 41 1612
Mattheiss L and Testardi L 1979 B 20 2196