14.40 o8 s wimbush
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14.40 o8 s wimbush

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Research 6: S Wimbush

Research 6: S Wimbush

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14.40 o8 s wimbush 14.40 o8 s wimbush Presentation Transcript

  • 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
  • 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
  • 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 View slide
  • 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 View slide
  • 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
  • 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
  • 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: 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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
  • 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