Effects of Ageing on the Microwave Surface Resistance of MgB2 Superconductor Films Stored in Low Vacuum

Nb is a very popular material in superconductive electronic devices such as superconducting single-photon detectors and radio-frequency (RF) cavities for particle accelerators. Before MgB 2 can be used as a substitute material, the stability of its properties over time must be confirmed. We studied how ageing over a period of 514 weeks affected the microwave surface resistance ( R S ) and the normal-state resistivity (ρ) of epitaxially-grown MgB 2 films. MgB 2 films with thicknesses of 1.0 μm and 1.7 μm were prepared using the hybrid physical CVD method, and exhibited a critical temperature ( T C ) of 39 K. They were stored in low-vacuum to prevent reactions with humidity, except when the films were taken out of storage for measurement purposes. Variation in R S with the ageing period appeared to be significant at very low temperatures. The R S values of 1.0 μm-thick and 1.7 μm-thick MgB 2 films increased by ~ 8 times and ~ 3 times, respectively, over ~ 10 years. The aged MgB 2 films appeared to have reduced π-band gap energy values. The pristine MgB 2 films value of 2.3 meV was reduced to 1.3–1.7 meV over ~ 10 years. The T C of the MgB 2 films remained almost the same regardless of the ageing time. The enhanced R S and ρ in the aged MgB 2 films were attributed to increased intraband scattering within the π-band and the σ-band. Graphical Abstract