Flux-flow instabilities in amorphousNb0.7Ge0.3microbridges

Abstract

We report measurements of the electric field vs current density ͓E(J)͔ characteristics in the mixed state of amorphous Nb 0.7 Ge 0.3 microbridges. Close to the transition temperature T c the Larkin-Ovchinnikov theory of nonlinear flux flow and the related instability describes the data quantitatively up to ϳ70% of the upper critical magnetic field B c2 and over a wide electric-field range. At lower temperatures the nonlinearities of E(J) can be described by electron heating which reduces B c2 and leads to a second type of flux flow instability, as shown by a scaling analysis of the high-dissipation data.

Strongly nonequilibrium flux flow in the presence of perforating submicron holes

“…This is the regime of the strong nonequilibrium (SN), the physics of which is discussed in Ref. [3]. We shall return to this regime later, but at this point we note that, at all three characteristic temperatures, for sufficiently high J the holes invariably enhance the dissipation -in contrast to regime I, and this is the central topic of this article.…”

“…[3] and [4], which are required for addressing the SN vortex transport in the presence of perforating holes.…”

“…The physics underlying these nonlinearities is either vortex-core shrinking close to T c , proposed theoretically by Larkin and Ovchinnikov (LO) [5], or vortex-core expansion due to electron heating above the bath temperature [6], depending on the experimental conditions. The above effect has been identified experimentally in a number of superconductors ranging from simple metals (Al, Sn, In) [7,8] to high-T c materials [9], including amorphous Nb 1−x Ge x [2,3,10]. To our knowledge, the modification of the dissipation in this regime by perforating holes has so far not been investigated, presumably because both the nature of the strongly nonequilibrium state and the effects of holes on vortex transport in the weak nonequilibrium were until recently not well understood.…”

“…Our recent investigation of vortex transport in amorphous Nb 0.7 Ge 0.3 microbridges [2,3,4] has utilised the above-outlined advantageous properties of this material. We have obtained a quantitative understanding of several mechanisms related to vortex dynamics in general, which has been used to deal with some aspects of the role of small (of the order of the magnetic-field penetration depth λ) perforating holes on the vortex-motion dissipation.…”

“…Having these topics analysed in Refs. [3] and [4], respectively, we now have a basis for turning to the present subject.…”