A new understanding of flux pinning       in defect-engineered superconductors       Stuart Wimbush, Nick Long       Super...
Introduction — Flux pinning in superconductors        • Flux pinning is determined by the microstructure of the sample.   ...
Introduction — Critical current anisotropy        •                                                                     1....
The vortex path model        • We consider mathematically the statistical population of pinned          vortex paths throu...
The vortex path model        • We consider mathematically the statistical population of pinned          vortex paths throu...
The vortex path model        • We consider mathematically the statistical population of pinned          vortex paths throu...
The vortex path model — Summary        •N. Long Supercond. Sci. Technol. 21 (2008) 025007.www.irl.cri.nz
The vortex path model — Features        • Shape of the angular peak functions:                                            ...
Pulsed laser deposited YBCO thin films      The broad ab-peak is                                                   The abs...
YBCO thin films with Ba2YNbO6 additions                                                                               c-ax...
YBCO thin films with Ba2YNbO6 additions        • Here, the strong c-axis pinning initially dominates until the field is   ...
YBCO films with Gd3TaO7 + Ba2YNbO6 additions        • (Unexpectedly) forms c-axis Ba2R(Nb,Ta)O6 segmented nanorods (7     ...
YBCO films with Gd3TaO7 + Ba2YNbO6 additions        • Intensely dominating c-axis peak drops out by 3 T matching field.   ...
Conclusion        • We have identified multiple deficiencies in the mass anisotropy          approach currently taken to a...
YBCO thin films with Gd3TaO7 additions        • Gd2TaO7 forms highly linear, through-thickness nanorods (5 nm          dia...
YBCO thin films with Gd3TaO7 additions        • High rate deposition doesn’t allow nanorods to form, but they can         ...
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14.40 o8 s wimbush

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Research 6: S Wimbush

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14.40 o8 s wimbush

  1. 1. A new understanding of flux pinning in defect-engineered superconductors Stuart Wimbush, Nick Long Superconductivity & Energy Group Image: ORNLNZIP Conference, Wellington, New Zealand 17–19 October 2011
  2. 2. Introduction — Flux pinning in superconductors • Flux pinning is determined by the microstructure of the sample. • It manifests itself in the measured critical current density, Jc. H Φ0 H = nΦ0 5×1014 Φ0/m2 = 1T { 500 in every µm2 } J F FLorentz = J × B Jc× B = Fpinning {B = Φ0/A} Sources of pinning: • Flux line (vortex-vortex) interactions. • Non-superconducting regions of the sample (defects). • Exotic sources (magnetic interactions).www.irl.cri.nz
  3. 3. Introduction — Critical current anisotropy • 1.0 H||ab H||c H||ab H Critical current density Jc (MA/cm ) 0.9 θ=0° 2 0.8 J 0.7 0.6 0.5 0.4 0.1 T 0.3 0.2 0.1 Nb isotropic γ=1 0.0 -120 -90 -60 -30 0 30 60 90 120 Applied field angle  (°)L. Civale et al. Appl. Phys. Lett. 84 (2004) 2121.www.irl.cri.nz
  4. 4. The vortex path model • We consider mathematically the statistical population of pinned vortex paths through the sample. Surface pinning (open and substrate interface) J θ H Intrinsic pinning due to planar structure F SUBSTRATE Jc θN. Long Supercond. Sci. Technol. 21 (2008) 025007.www.irl.cri.nz
  5. 5. The vortex path model • We consider mathematically the statistical population of pinned vortex paths through the sample. Surface pinning (open and substrate interface) J θ H F Intrinsic pinning due to planar structure F Random pinning (nanoparticles, point defects) SUBSTRATE Jc • A population of defects providing pinning in the direction orthogonal to the primary pinning defects broadens the Jc peak. θN. Long Supercond. Sci. Technol. 21 (2008) 025007.www.irl.cri.nz
  6. 6. The vortex path model • We consider mathematically the statistical population of pinned vortex paths through the sample. Surface pinning (open and substrate interface) J Intrinsic pinning due to planar structure F Random pinning (nanoparticles, point defects) ab-plane pinning c-axis pinning (platelets) (grain boundaries, twin plane intersections, threading dislocations) SUBSTRATE Jc θ H • We sum the multiplicity of possible vortex paths through the sample for a given field direction. θN. Long Supercond. Sci. Technol. 21 (2008) 025007.www.irl.cri.nz
  7. 7. The vortex path model — Summary •N. Long Supercond. Sci. Technol. 21 (2008) 025007.www.irl.cri.nz
  8. 8. The vortex path model — Features • Shape of the angular peak functions: Γ = 0.2 Γ = 0.3 Γ = 0.5 Angular Lorentzian Γ = 1.0 Angular Gaussian Γ = 1 uniform distribution 0 30 60 90 120 150 180 0 30 60 90 120 150 180  (°)  (°) • The vortex path model is a maximum entropy formulation: Entropy (natural units) 7 Uniform Uniform No preferred direction Increasing Lorentzian Preferred direction entropy 6 Gaussian Preferred direction and defined angular spread 5 Lorentzian Gaussian 4 0 1 2 3 Scale factor E. T. Jaynes, Phys. Rev. 106 (1957) 620; 108 (1957) 171.www.irl.cri.nz
  9. 9. Pulsed laser deposited YBCO thin films The broad ab-peak is The absence of a c-axis commonly mistaken as peak is often mistakenly a signature of mass taken as evidence of a anisotropy. lack of c-axis pinning. • Three components: – Narrow ab-peak: Intrinsic pinning broadened by short-scale interactions with surface roughness. – Broad ab-peak: Intrinsic pinning broadened by large-scale interactions with through-thickness defects (grain boundaries, twin plane intersections, threading dislocations). – Uniform component: Indication of the existence of strong c-axis and ab-plane pinning able to combine to effectively pin at all angles.www.irl.cri.nz
  10. 10. YBCO thin films with Ba2YNbO6 additions c-axis • Ba2YNbO6 forms nanorods (15 nm diameter, 100 nm long, 40 nm spacing) oriented along the c-direction in YBCO. • Additionally, many randomly-positioned nanoparticle inclusions are seen.G. Ercolano et al. Supercond. Sci. Technol. 23 (2010) 022003.www.irl.cri.nz
  11. 11. YBCO thin films with Ba2YNbO6 additions • Here, the strong c-axis pinning initially dominates until the field is increased beyond the matching field of the nanorods (~1.5 T). Then the broad ab-plane pinning peak reappears. • We predict that increasing the field still further will cause the broad ab-peak to dominate further while the c-axis peak drops out completely, as for the pure YBCO films.G. Ercolano et al. Supercond. Sci. Technol. 24 (2011) 095012.www.irl.cri.nz
  12. 12. YBCO films with Gd3TaO7 + Ba2YNbO6 additions • (Unexpectedly) forms c-axis Ba2R(Nb,Ta)O6 segmented nanorods (7 nm diameter, 30 nm long, 30 nm spacing), together with ab-plane R2O3 platelets (25-30 nm long), and R248 nanoparticles. • Unsurprisingly, this dense defect structure results in a complex behaviour.G. Ercolano et al. Supercond. Sci. Technol. 24 (2011) 095012.www.irl.cri.nz
  13. 13. YBCO films with Gd3TaO7 + Ba2YNbO6 additions • Intensely dominating c-axis peak drops out by 3 T matching field. • Other components at low field are all related to this strongly dominant pinning interacting with the other sources. • Beyond 3 T, the interactions with ab-pinning sources become comparable and then dominate to higher fields.G. Ercolano et al. Supercond. Sci. Technol. 24 (2011) 095012.www.irl.cri.nz
  14. 14. Conclusion • We have identified multiple deficiencies in the mass anisotropy approach currently taken to analyse angular Jc data of superconductors: – At best, it describes the data in terms of generally meaningless parameters, unrelated to any physical property. – It cannot explain the data because it offers no link between the observed Jc and the underlying microstructure responsible for it. – All features of the data resulting from this approach are also present in isotropic superconductors, where it does not apply. • We have proposed an alternative statistical model of vortex paths in the superconductor that directly links the angular Jc data to the underlying microstructure responsible for pinning. • We have shown that the model robustly describes the behaviour of many different classes of sample, succinctly explaining features of the data for which an explanation is otherwise lacking.www.irl.cri.nz
  15. 15. YBCO thin films with Gd3TaO7 additions • Gd2TaO7 forms highly linear, through-thickness nanorods (5 nm diameter, 10-20 nm spacing). • If deposited too quickly, the rods do not have time to form and nanoparticles are formed instead.S. A. Harrington et al. Supercond. Sci. Technol. 22 (2009) 022001.www.irl.cri.nz
  16. 16. YBCO thin films with Gd3TaO7 additions • High rate deposition doesn’t allow nanorods to form, but they can propagate through thin (single) layers.S. A. Harrington et al. Nanotechnology 21 (2010) 095604.www.irl.cri.nz

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