Annealing-induced grain coarsening and voltage kinks in superconducting NbRe films

TL;DR

This study explores how annealing affects grain size, vortex dynamics, and voltage behavior in NbRe superconducting films, revealing voltage kinks linked to normal domain formation and potential applications in sensing.

Contribution

It demonstrates that annealing enlarges grain size in NbRe films, leading to vortex-channeling and voltage kinks, advancing understanding of grain effects on superconductivity and vortex behavior.

Findings

Annealing increases grain size from 2 nm to 8 nm.

Annealed films show voltage kinks in I-V curves.

Voltage kinks are linked to normal domain nucleation and growth.

Abstract

NbRe, a non-centrosymmetric superconductor with a transition temperature $T_\mathrm{c}$ up to 9\,K, attracts interest for its strong antisymmetric spin-orbit coupling and suitability for single-photon detection. While bulk and thin-film polycrystalline NbRe are well studied, how superconductivity and vortex dynamics evolve with increasing grain size in thin films is largely unknown. Here, we investigate as-grown and annealed 20\,nm-thick NbRe films, where annealing increases the average crystallite size from approximately $2$\,nm to $8$\,nm, and study vortex dynamics via current-voltage ($I$-$V$) measurements over a broad temperature and magnetic field range. In contrast to as-grown films, where the low-resistive state breaks down due to flux-flow instability, annealed films exhibit multiple voltage kinks in the $I$-$V$ curves. We attribute these kinks to the nucleation and growth of normal domains, as further suggested by time-dependent Ginzburg-Landau simulations. Overall, the annealed films form superconducting networks with vortex-channeling paths along the grain boundaries, while localized heating and voltage kinks could be harnessed for discrete-resistance switching and sensing.