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Ted Forgan

Birmingham Condensed Matter Physics Group. Pauli-limiting effects in the flux lattice of CeCoIn 5. Ted Forgan. School of Physics and Astronomy, University of Birmingham, U.K. and. =>. =>. Superconductivity at high B – what determines B c2 ?. ‘Heavy fermion’ materials have

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Ted Forgan

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  1. Birmingham Condensed Matter Physics Group Pauli-limiting effects in the flux lattice of CeCoIn5 Ted Forgan School of Physics and Astronomy, University of Birmingham, U.K. Birmingham Condensed Matter Website Ted Forgan

  2. and => => Superconductivity at high B – what determines Bc2? ‘Heavy fermion’ materials have low Tc (~ 1 K) but high Bc2 (~ 10 T) Birmingham Condensed Matter Website Ted Forgan

  3. => Compare => => Superconductivity and magnetic fields – effect of electron spin Pauli paramagnetism favours normal state - because paired antiparallel electrons cannot line up with B What energy balance decides if paramagnetism is important? Birmingham Condensed Matter Website Ted Forgan

  4. paired i.e. Bz  but 2mB k k Pairing controlled by spin energy e D ~ D0 exp (iqz) with q = ( k + k ) eF This was predicted by Fulde & Ferrell in 1964 k Carries a supercurrent // B ! Maybe D ~ cos (qz) – Larkin & Ovchinnikov (same time) “LOFF / FFLO” Flux lines modulated along z Birmingham Condensed Matter Website Ted Forgan

  5. proposed shape of flux lines Speed of sound Phase Diagram Maybe FFLO in CeCoIn5 i.e. flux lines turn into “strings of sausages” Birmingham Condensed Matter Website Ted Forgan

  6. NMR on CeCoIn5 versus B and T Kumagai et al. cond-mat/0605394 Birmingham Condensed Matter Website Ted Forgan

  7. Flux Lattice inCeCoIn5, B // c Hexagonal => Rhombic => Square => Rhombic => Hexagonal d-wave superconductivity + paramagnetic effects?? Birmingham Condensed Matter Website Ted Forgan

  8. d-wave heavy-fermion CeCoIn5 Note that we could go very close to Hc2 – just 50 mT below ! B – T Phase Diagram Birmingham Condensed Matter Website Ted Forgan

  9. Note: intensity falls rapidly to zero near Bc2 in a conventional material Results in niobium near Bc2 Diffracted Intensity vs. T at B = 0.2 T log scale linear scale Birmingham Condensed Matter Website Ted Forgan

  10. d-wave heavy-fermion CeCoIn5 flux lines show stronger field contrast at high field – even up to Bc2 ! |F|2 Electron spins lining up parallel in the de-paired flux line cores... Published in ‘Science’ (Bianchi et al) Jan ‘08 Is there a sign of FFLO –for B // c ?? B Birmingham Condensed Matter Website Ted Forgan

  11. Birmingham Condensed Matter Website Ted Forgan

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