Evidence for Cooper pair diffraction on the vortex lattice of superconducting niobium
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Abstract:
We investigated the Abrikosov vortex lattice (VL) of a pure niobium single crystal with the muon spin rotation ($\ensuremath{\mu}$SR) technique. Analysis of the $\ensuremath{\mu}$SR data in the framework of the BCS-Gor'kov theory allowed us to determine microscopic parameters and the limitations of the theory. With decreasing temperature the field variation around the vortex cores deviates substantially from the predictions of the Ginzburg-Landau theory and adopts a pronounced conical shape. This is evidence of partial diffraction of Cooper pairs on the VL predicted by Delrieu for clean superconductors.Keywords:
Cooper pair
Lattice (music)
Type-II superconductor
Muon spin spectroscopy
A method for the registration of Abrikosov vortices in type-II superconductors has been employed for studying magnetic-flux propagation through a superconducting film. This method is based on measurements of the vortex-induced magnetoresistance in normal-metal microprobes placed in the immediate vicinity of a superconductor surface. The sensitivity of such probes provides a registration of individual vortex movements in a submicrometer area of the superconductor. With use of this technique, it is demonstrated that both the penetration of magnetic flux into the superconductor and its withdrawal are accomplished not by individual vortex movements but by the hopping of vortex bundles. The size of the hopping bundles appears to be much larger than the correlation volume in the vortex lattice. A description of flux penetration based on the critical-state model, which assumes a smooth gradient of the vortex concentration across the sample, has been found to break down on the microscopic scale.
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Magnetic flux quantum
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Quantum vortex
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We report on Bitter decoration studies of the magnetic flux line lattice in the type II superconductor ${\mathrm{NbSe}}_{2}$ during flux creep experiments over barriers, a surface step. We find that these steps can act as ``vortex diodes.'' By imaging the real space structures of the vortex lattices as they flow over these steps and measuring the change in vortex density, we are able to measure the energetic height of the barrier as well as the elastic correlation length of the vortices along their length. At low fields, we find that the vortices are correlated over distances of roughly $5\ensuremath{\mu}\mathrm{m}$.
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Adding defect on type II superconductor can influence to vortex dynamic. It declares vortex movement, so speed of vortex is more stable. In this research, we applied lattice defect. In case to study dynamics of the vortex in type II superconductor material, this study was based on numerical solution of the Time-Dependent Ginzburg Landau (TDGL) equations by means of finite difference method with Forward Time Centred Space (FTCS) scheme. Applied external current density Je and external magnetic field He to superconductor material generate a vortex flow from higher magnetic field to lower magnetic field. Furthermore, applied external current density Je without external magnetic field He generates vortex flow between lattice defects.
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Ginzburg–Landau theory
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Type-II superconductor
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T-symmetry
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London approximation is used to solve the problem of magnetic vortex destroying order parameter in a superconducting layer of arbitrary thickness embedded between bulk superconducting screens. Like the well-known 2D-vortices in layered superconductors, the vortices under consideration interact logarithmically at large distances and carry magnetic flux θ<θ0, the unit flux quantum. The stability of long vortices in a layered superconductor is discussed with respect to the decay of shorter vortices.
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In superconductors, the interaction between vortices as a function of the distance between them can be either monotonic or nonmonotonic mainly due to the special characteristic parameter lengths, i.e., the penetration depth and coherence length. In traditional type-II superconductors with purely repulsive vortex-vortex interactions, a triangular vortex lattice is formed, which is also known as the Abrikosov vortex lattice. In superconductors with competing vortex-vortex interactions, such as type-I, low-κ and type-1.5 superconductors, much more complex vortex patterns can be formed. Because of the analogy to other systems with modulated phases, the study of vortex matter has attracted a lot of interest. In this chapter, we present recent progress in this field concerning direct visualization of these vortex patterns with scanning Hall probe microscopy.
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Recent experimental studies of the flux line lattices in type-II superconductors are reviewed. All the experiments were performed using the Bitter decoration technique which provides means of direct observation of static vortex structures at the surface of the superconductor. Various aspects of vortex behaviour that can be studied in decoration experiments are considered, in particular vortex lattice ordering and the effects of pinning and anisotropy on the equilibrium vortex arrangement. New vortex phases, such as vortex chains and oval vortices, which were recently discovered in high-temperature superconductors, are also discussed.
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The vortex motion in a type II superconductor narrows the inhomogeneous static NMR lineshape. This method gives a new way to study the flux flow of vortices in type II superconductors. In principle in a single crystal it can give a measure of the value and direction of the vortex speed. The NMR lineshape is discussed in the nonlinear flux creep region.
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Tourbillon
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The quantization of magnetic flux in superconductors is usually seen as vortices penetrating the sample.While vortices are unstable in bulk type I superconductors, restricting the superconductor causes a variety of vortex structures to appear.We present a systematic study of giant vortex states in type I superconductors obtained by numerically solving the Ginzburg-Landau equations.The size of the vortices is seen to increase with decreasing film thickness.In type I superconductors, giant vortices appear at intermediate thicknesses but they do not form a well-defined vortex lattice.In the thinnest type I films, singly quantized vortices seem to be stabilized by the geometry of the sample instead of an increase in the effective Ginzburg-Landau parameter.
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