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Register a new userInfluence of surface irregularities on barriers for vortex entry in type-II superconductors
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A theoretical description of the influence of surface irregularities (such as wedge-like cracks) on the Bean-Livingston energy barrier is presented. A careful quantitative estimate of the field of the first vortex entry H* into a homogeneous bulk superconductor with crack-like defects is obtained. This estimate is based on an exact analytical solution for the Meissner current distribution in a bulk superconductor with a single two-dimensional infinitely thin crack. This thin crack is characterized by the lowest H* value, and, thus, appears to be the best gate for vortex entry.
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The knowledge of vortex nucleation barriers is crucial for applications of superconductors, such as single-photon detectors and superconductor-based qubits. Contrarily to the problem of finding energy minima and critical fields, there are no controllable methods to explore the energy landscape, identify saddle points, and compute associated barriers. Similar problems exist in high-energy physics where the saddle-point configurations are called sphalerons. Here, we present a generalization of the string method to gauge field theories, which allows the calculation of energy barriers in superconductors. We solve the problem of vortex nucleation, assessing the effects of the nonlinearity of the model, complicated geometry, surface roughness, and pinning.
A review is given on the theory of vortex-glass phases in impure type-II superconductors in an external field. We begin with a brief discussion of the effects of thermal fluctuations on the spontaneously broken U(1) and translation symmetries, on the global phase diagram and on the critical behaviour. Introducing disorder we restrict ourselves to the experimentally most relevant case of weak uncorrelated randomness which is known to destroy the long-ranged translational order of the Abrikosov lattice in three dimensions. Elucidating possible residual glassy ordered phases, we distinguish betwee positional and phase-coherent vortex glasses. The discussion of elastic vortex glasses, in two and three dimensions occupy the main part of our review. In particular, in three dimensions there exists an elastic vortex-glass phase which still shows quasi-long-range translational order: the `Bragg glass. It is shown that this phase is stable with respect to the formation of dislocations for intermediate fields. Preliminary results suggest that the Bragg-glass phase may not show phase-coherent vortex-glass order. The latter is expected to occur in systems with weak disorder only in higher dimensions. We further demonstrate that the linear resistivity vanishes in the vortex-glass phase. The vortex-glass transition is studied in detail for a superconducting film in a parallel field. Finally, we review some recent developments concerning driven vortex-line lattices moving in a random environment.
In order to characterize flux flow through disordered type-II superconductors, we investigate the effects of columnar and point defects on the vortex velocity / voltage power spectrum in the driven non-equilibrium steady state. We employ three-dimensional Metropolis Monte Carlo simulations to measure relevant physical observables including the force-velocity / current-voltage (I-V) characteristics, vortex spatial arrangement and structure factor, and mean flux line radius of gyration. Our simulation results compare well to earlier findings and physical intuition. We focus specifically on the voltage noise power spectra in conjunction with the vortex structure factor in the presence of weak columnar and point pinning centers. We investigate the vortex washboard noise peak and associated higher harmonics, and show that the intensity ratios of the washboard harmonics are determined by the strength of the material defects rather than the type of pins present. Through varying columnar defect lengths and pinning strengths as well as magnetic flux density we further explore the effect of the material defects on vortex transport. It is demonstrated that the radius of gyration displays quantitatively unique features that depend characteristically on the type of material defects present in the sample.
Dynamics of vortices in strongly type-II superconductors with strong disorder is investigated within the frustrated three-dimensional XY model. For two typical models in [Phys. Rev. Lett. {bf 91}, 077002 (2003)] and [Phys. Rev. B {bf 68}, 220502(R) (2003)], a strong evidence for the finite temperature vortex glass transition in the unscreened limit is provided by performing large-scale dynamical simulations. The obtained correlation length exponents and the dynamic exponents in both models are different from each other and from those in the three-dimensional gauge glass model. In addition, a genuine continuous depinning transition is observed at zero temperature for both models. A scaling analysis for the thermal rounding of the depinning transition shows a non-Arrhenius type creep motion in the vortex glass phase, contrarily to the recent studies..
We study effects of pinning on the dynamics of a vortex lattice in a type II superconductor in the strong-pinning situation and determine the force--velocity (or current--voltage) characteristic combining analytical and numerical methods. Our analysis deals with a small density $n_p$ of defects that act with a large force $f_p$ on the vortices, thereby inducing bistable configurations that are a characteristic feature of strong pinning theory. We determine the velocity-dependent average pinning-force density $langle F_p(v)rangle$ and find that it changes on the velocity scale $v_p sim f_p/eta a_0^3$, where $eta$ is the viscosity of vortex motion and $a_0$ the distance between vortices. In the small pin-density limit, this velocity is much larger than the typical flow velocity $v_c sim F_c/eta$ of the free vortex system at drives near the critical force-density $F_c = langle F_p(v=0)rangle propto n_p f_p$. As a result, we find a generic excess-force characteristic, a nearly linear force--velocity characteristic shifted by the critical force-density $F_c$; the linear flux-flow regime is approached only at large drives. Our analysis provides a derivation of Coulombs law of dry friction for the case of strong vortex pinning.
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