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Pinning of Vortices in Type II Superconductors Martin Buchacek 25 - - PowerPoint PPT Presentation
Pinning of Vortices in Type II Superconductors Martin Buchacek 25 - - PowerPoint PPT Presentation
Pinning of Vortices in Type II Superconductors Martin Buchacek 25 November 2015 Type II superconductors 1935: vortex phase observed experimentally in 1935 by Lev Shubnikov 1957 theoretical model by Alexei Aborikosov (Nobel prize in 2003
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Vortex phase
◮ The most stable configuration is the triangular lattice a△
- j
- B
r |ψ(r)| ξ λ ≫ ξ B(r) ◮ a△ ≃ 1.075
- φ0
B
1/2
◮ ξ is comparable to the size of single unit cell (ξ ≃ 0.4nm in
YBa2Cu3O7−x)
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Onset of resistance
◮ Ideal type II superconductor where vortices are free to move
develops resistance.
H B Hc1 Hc2 R ∼ Rn B
Hc2
- B
- jext,
E
- f,
v
◮ dissipation ∼ j · E ◮ If vortices are pinned, they do not move until we reach a
critical current jc.
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Pinning
◮ Real materials are not perfect superconductors ⇒ vortices of
magnetic field are attracted to the local pinning centres
YBa2Cu3O7−x point vacancies 3D pinning centres line defects (screw dis- clocations) twin planes separating regions of different unit cell orientation
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Flux creep
Veff = V − fu
◮ jump rate ∝ e−∆F0/T ◮ small currents: vortices are ‘locked’ in the valleys and jumps
are rare
◮ large enough currents give rise to the critical force: the
potential landscape becomes tilted and jumps are more frequent ⇒ sudden increase in resistance
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Resistance in the presence of pinning
a b c H = 0 T H = 8 T T = Tm
Figure 1: Resistive transition in YBa2Cu3O7−x crystal. Various magnetic fields applied are parallel to c-axis2
- 2W. Kwok et al., Phys. Rev. Lett. 69, 3370 (1992)
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Phases of the vortex lattice
◮ competition between elasticity, disorder and thermal
fluctuations
◮ glass phase: barriers diverge as j → 0 and inhibit vortex
motion ⇒ recover ‘genuine’ superconductor with ρ → 0
◮ disorder in the vortex glass phase is so strong that it destroys
the vortex lattice structure
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Pinning mechanism: weak vs. strong pinning
◮ Discussion above is related to the collective action of many
‘weak’ pins
unpinned pinned weak pinning strong pinning x x e(x) e(x) x unpinned pinned ◮ Strong pins act individually on the vortices. At certain regions,
there are bistable solutions with pinned and unpinned vortices.
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Research tasks in the strong pinning theory
◮ Thermal excitations in strong pinning
theory
◮ Collective action of strong pins.
T
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Thank you for your attention.
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Resources
- G. Blatter, D. Geshenkbein: Vortex Matter. In