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Small-Angle Neutron Scattering & T he Superconducting Vortex Lattice

Small-Angle Neutron Scattering & T he Superconducting Vortex Lattice. Superconductors: What & Why. Discovered in 1911 By H. Kammerlingh-Onnes , who observed at complete loss of resistance in mercury below 4.2 K.

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Small-Angle Neutron Scattering & T he Superconducting Vortex Lattice

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  1. Small-Angle Neutron Scattering&The Superconducting Vortex Lattice

  2. Superconductors: What & Why • Discovered in 1911 By H. Kammerlingh-Onnes, who observed at complete loss of resistance in mercury below 4.2 K. • Displays an intriguing response to applied magnetic fields (Meissner effect, mixed state). • Many aspects still not understood on microscopic level. • Immense potential for practical applications. 36.5 MW ship propulsion motor (American Superconductor) Loss free energy transport (physicsweb.org) Magnetic levitation (Railway Technical Research Institute, Japan)

  3. Magnetic properties • Superconductors “allergic” to magnetic fields. • At low fields: Complete flux expulsion (Meissner effect). • Superconducting screening currents will produce opposing field cancelling applied field.

  4. Superconducting vortices • For type-II superconductors in the mixed state, the applied magnetic field penetrates in vortices or flux lines. • Each vortex carries one flux quantum of magnetic flux: • The vortices forms an ordered array - the vortex-lattice (ignoring pinning, melting, etc….). University of Oslo, Superconductivity lab.

  5. Small-angle neutron scattering SANS-I beam line at Paul Scherrer Institute, Villigen (Switzerland). • Neutrons scattered by periodic magnetic field distribution, allowing imaging of the vortex lattice (VL). • Typical values: l = 10 Å d = 1000 Å

  6. Sample environment • The diffraction pattern is directly measured on 2D detector. • Cryomagnet cool sample and contain magnets. Must rotate around two axes to satisfy Bragg condition for VL.

  7. LuNi2B2C • Member of RNi2B2C family of SC’s (R = Y, Dy, Ho, Er, Tm, Lu). • Tc = 16. 6 K, Hc2(2 K) = 7.3 T. Relatively well understood, good case study. • Intriguing in-plane anisotropy: a) FS anisotropy + non-local electrodynamics → VL symmetry transitions. b) Anisotropic s-wave (s+g?) gap symmetry (nodes along 100). K. Maki, P. Thalmeier, H. Won, Phys. Rev. B 65, 140502(R) (2002). V. G. Kogan et al., Phys. Rev. B 55, R8693 (1997). N. Nakai et al., Phys. Rev. Lett. 89, 237004 (2002).

  8. VL reflectivity and form factor • Absolute VL reflectivity → vortex form factor. • Form factor can be measured continuously as function of scattering vector, q :

  9. VL field reconstruction LuNi2B2C J. M. Densmoreet al., Phys. Rev. B 79, 174522 (2009)

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