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Register a new userEvidence of a new low field cross-over in the vortex critical velocity of type-II superconducting thin films
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We measure current-voltage characteristics as function of magnetic field and temperature in Nb strips of different thickness and width. The instability voltage of the flux flow state related to the vortex critical velocity v* is studied and compared with the Larkin-Ovchinnikov theory. Beside the usual power-law dependence v* ~ B^-1/2, in the low field range a new cross-over field, Bcr1, is observed below which v* decreases by further lowering the external magnetic field B. We ascribe this unexpected cross-over to vortex channeling due to a fan-like penetration of the applied magnetic field as confirmed by magneto-optic imaging. The observation of Bcr1 becomes a direct evidence of a general feature in type-II superconducting films at low fields, that is a channel-like vortex motion induced by the inhomogeneous magnetic state caused by the relatively strong pinning.
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The current distribution across the thickness of a current-carrying rectangular film in the Meissner state was established long ago by the London brothers. The distribution across the width is more complicated but was later shown to be highly non-uniform, diverging at the edges. Accordingly, the standard view for type II superconductors is that vortices enter at the edges and, with increasing current, are driven inwards until they self-annihilate at the centre, causing dissipation. This condition is presumed to define the critical current. However we have shown that, under self-field (no external field), the transport critical current is a London surface current where the surface current density equals the critical field divided by {lambda}, across the entire width. The critical current distribution must therefore be uniform. Here we report studies of the current and field distribution across commercial YBa2Cu3O7 conductors and confirm the accepted non-uniform distribution at low current but demonstrate a radical crossover to a uniform distribution at critical current. This crossover ends discontinuously at a singularity and calculations quantitatively confirm these results in detail. The onset of self-field dissipation is, unexpectedly, thermodynamic in character and the implied vortex-free critical state seems to require new physics.
In a class of type II superconductor films, the critical current is determined by the Bean-Livingston barrier posed by the film surfaces to vortex penetration into the sample. A bulk property thus depends sensitively on the surface or interface to an adjacent material. We theoretically investigate the dependence of vortex barrier and critical current in such films on the Rashba spin-orbit coupling at their interfaces with adjacent materials. Considering an interface with a magnetic insulator, we find the spontaneous supercurrent resulting from the Zeeman field and interfacial spin-orbit coupling to substantially modify the vortex surface barrier. Thus, we show that the critical currents in superconductor-magnet heterostructures can be controlled, and even enhanced, via the interfacial spin-orbit coupling. Since the latter can be controlled via a gate voltage, our analysis predicts a class of heterostructures amenable to gate-voltage modulation of superconducting critical currents. It also sheds light on the recently observed gate-voltage enhancement of critical current in NbN superconducting films.
Interactions between vortices in thin superconducting films are investigated in the crossover (intertype) regime between superconductivity types I and II. We consider two main factors responsible for this crossover: a) changes in the material characteristics of the film and b) variations of the film thickness controlling the effect of the stray magnetic fields outside superconducting sample. The analysis is done within the formalism that combines the perturbation expansion of the microscopic equations to one order beyond the Ginzburg-Landau theory with the leading contribution of the stray fields. It is shown that the latter gives rise to qualitatively different spatial profile and temperature dependence of the vortex interaction potential, as compared to bulk vortex interactions. The resulting interaction is long-range repulsive while exhibiting complex competition of attraction and repulsion at small and intermediate separations of vortices. This explains the appearance of vortex chains reported earlier for superconducting films.
Vortex trapping is investigated in thin-film strips of superconducting material. We present a model for the critical field above which vortex trapping occurs in these strips. This model includes the pairing energy of vortex-antivortex pairs in addition to the energy of single vortices. Experimental verification of the model with a scanning SQUID microscope shows very good agreement between the model and experiments on YBa2Cu3O7-delta and Nb strips. Statistical analysis of the vortex distribution in the strips above the critical field has been performed and a comparison has been made between Nb and YBa2Cu3O7-delta for the distributions in the lateral and longitudinal directions.
Vortex dynamics in superconductors have received a great deal of attention from both fundamental and applied researchers over the past few decades. Because of its critical role in the energy relaxation process of type-II superconductors, vortex dynamics have been deemed a key contributor to the response rate of the emerging superconducting single photon detector (SSPD). With the support of electrical transport measurements under external magnetic fields, vortex dynamics in superconducting a-MoSi thin films are investigated in this work. It is ascertained that the vortex state changes from pinned to flux flow under the influence of the Lorentz force. The critical vortex velocity v* and quasi-particle inelastic scattering time {tau}* under different magnetic fields are derived from the Larkin-Ovchinnikov model. Under high magnetic fields, the v* power law dependence (v*~B-1/2) collapses, i.e., v* tends to zero, which is attributed to the obstruction of flux flow by the intrinsic defects, while the {tau}* increases with the increasing magnetic field strength. In addition, the degree of vortex rearrangement is found to be enhanced by photon-induced reduction in potential barrier, which mitigates the adverse effect of film inhomogeneity on superconductivity in the a-MoSi thin films. The thorough understanding of the vortex dynamics in a-MoSi thin films under the effect of external stimuli is of paramount importance for both further fundamental research in this area and optimization of SSPD design.
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