We studied the high driving force regime of the current-voltage transport response in the mixed state of amorphous molybdenum-germanium superconducting films to the point where the flux flow becomes unstable. The observed nonlinear response conforms with the classic Larkin-Ovchinikov picture with a quasiparticle-energy-relaxation rate dominated by the quasiparticle recombination process. The measured energy relaxation rate was found to have a magnitude and temperature dependence in agreement with theory.
Vortex dynamics in molybdenum-germanium superconducting films were found to well approximate the unpinned free limit even at low driving forces. This provided an opportunity to empirically establish the intrinsic character of free flux flow and to test in detail the validity of theories for this regime beyond the Bardeen-Stephen approximation. Our observations are in good agreement with the mean-field result of timedependent Ginzburg-Landau theory.
We report magneto-transport measurements on a superconducting molybdenum-germanium (MoGe) film of thickness d=50 nm in parallel magnetic fields and show evidence of a transition from a Meissner state to a resistive state of spontaneous perpendicular vortices generated by thermal fluctuations above a certain temperature T > Tv (B). Here Tv appears to match the vortex core explosion condition d ≈ 4.4ξ(Tv), where ξ is the coherence length. For T > Tv, we observed that a nonlinear current-voltage (IV ) response (Ohmic at low currents and the power law V ∝ I β at higher I) is exponentially dependent on B 2 . We propose a model in which the resistive state at T > Tv is due to thermally-activated hopping of spontaneous perpendicular vortices tuned by the pairbreaking effect of the parallel B.
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