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Magnesium diboride (MgB$_2$) is a superconductor with the highest critical temperature among conventional (electron-phonon) BCS-type superconductors at ambient pressure, making it a promising candidate for increasing the operating temperature of superconducting single-photon detectors [1]. The suitability of a material for such devices can be judged [2] via the maximal velocity of Abrikosov vortices in the film, inferred from the last voltage point before the abrupt transition to the highly resistive state associated with flux-flow instability (FFI). However, the high electron–electron scattering rate in MgB$_2$ suppresses nonequilibrium phenomena near the critical temperature, making this assessment challenging.
In this work, we compare four gold-capped single-crystal MgB$_2$ films with thicknesses between 12 and 45 nm. For all films we determine the pinning efficiency and critical current, both of which increase with film thickness. Measurements of the current–voltage characteristics for thinner films reveal multiple voltage jumps instead of a single FFI transition. We attribute this behaviour to the formation of regions with preferential vortex motion that evolve into normal domains [3]. This behaviour is supported by time-dependent Ginzburg-Landau modelling. For the 30 nm and 45 nm films, we observe single FFI transitions and extract vortex velocities up to 10–20 km/s. These values are the highest deduced for MgB2 films so far and are comparable with the fastest vortex velocities recently found for other superconducting materials [4,5].
[1] J. Nagamatsu et al, Nature 410 (2001) 36.
[2] S.-Z. Lin et al, Phys. Rev. B 87 (2013) 184507.
[3] C. Schmid, A. Pokusinskyi, M. Gruber, C. Pfaff, T. Courtois, A. Kasatkin, K. Dumesnil, S. Mangin, T. Hauet and O. Dobrovolskiy, Crystal structure effects on vortex dynamics in superconducting MgB$_2$ thin films, arXiv:2604.14022 (2026).
[4] L. Embon et al, Nat. Commun. 8 (2017) 85.
[5] O. Dobrovolskiy et al, Nat. Commun. 11 (2020) 3291.