Onset of the transitional flux-avalanche regime in bulk NbTi controlled by the thermal boundary conductance
Thermomagnetic avalanches in type-II superconductors occur in two qualitatively different regimes, electromagnetically controlled in thin films and thermally limited in bulk samples, distinguished by the sign of the temperature derivative of the threshold field $H_\text{th}(T)$. The two regimes are separated by a critical thermal boundary conductance $h_c$, and a non-monotonic $H_\text{th}(T)$ has been predicted in the transitional region where the interface conductance $h$ approaches $h_c$. We approach this region in a bulk NbTi disk by raising the interface coupling above the pure-nonadecane baseline with a silver-filled interface layer. Whereas the pure interface gives a monotonically decreasing $H_\text{th}(T)$, the silver-filled interface produces a non-monotonic dependence not previously realized in a bulk superconductor: a temperature interval of positive slope, $dH_\text{th}/dT > 0$, terminating in a maximum at $T^* \approx 6.1$--$6.4$~K. The effect is reproduced for two independent silver-filled compositions; in a third, with the highest loading, the low-temperature decrease is absent altogether and $H_\text{th}(T)$ is flat up to the same $T^*$, the evolution expected for stronger coupling. The position of the maximum is set by the intrinsic properties of NbTi, independent of the silver content. The onset of the positive-slope interval coincides in temperature with a change of the avalanche morphology from narrow channeled fingers to broad fronts. The reversal of the sign of $dH_\text{th}/dT$ is a direct experimental signature of the onset of the transitional regime, in which heat removal during the instability becomes dynamically relevant.