1 / 39

Analysis of proximity effects in S/N/F and F/S/F junctions

Analysis of proximity effects in S/N/F and F/S/F junctions. Han-Yong Choi Na-Young Lee / SKKU Hyeonjin Doh / Toronto Kookrin Char / SNU KIAS workshop 2005. 10. 25 ~ 10. 29. Superconductivity (S) vs. Ferromagnetism (F). N. S. F. Proximity effect. Plan.

ilya
Télécharger la présentation

Analysis of proximity effects in S/N/F and F/S/F junctions

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Analysis of proximity effects in S/N/F and F/S/F junctions Han-Yong Choi Na-Young Lee / SKKU Hyeonjin Doh / Toronto Kookrin Char / SNU KIAS workshop 2005. 10. 25 ~ 10. 29.

  2. Superconductivity (S) vs. Ferromagnetism (F) SKKU condensed-matter theory group

  3. N S F Proximity effect SKKU condensed-matter theory group

  4. Plan I. Introduction to proximity effect. S/N, S/F. II. S/N/F. Issues of SNU data. III. Usadel equation. Odd triplet pairing. Results. IV. F/S/F. V. Summary and outlook. SKKU condensed-matter theory group

  5. dPb (nm) Tc/Tc,S S N Y dCu (nm) I. Introduction S/N bilayers: 1960’s. [de Gennes, Rev. Mod. Phys. (’64)] xCu ~ 40 nm [Werthamer, Phys.Rev. (’63)] For SKKU condensed-matter theory group

  6. F S 0-state p-state Min Tc vs. dF S/F bilayers: 1980’s & 90’s SKKU condensed-matter theory group

  7. S F S F h x 2h K U Origin of oscillations dirty limit (oscillation suppressed). SKKU condensed-matter theory group

  8. S N F SN Y Tc SNF 0 dN II. S/N/F trilayers • Expectations: only one length scale in N. • Experiments: “surprises” two more length scales. SKKU condensed-matter theory group

  9. 1. Short length SKKU condensed-matter theory group

  10. 2. Intermediate length SKKU condensed-matter theory group

  11. Au & Cu SKKU condensed-matter theory group

  12. dF = 10 nm dN = 3 nm dS = 23 nm Another way of looking atthe short length superconductor • Which has the highestTc? normal metal ferromagnetic metal dF = 10 nm dF = 10 nm dS = 23 nm dS = 26 nm SKKU condensed-matter theory group

  13. How to understand? • 1. Obvious/mundane explanation. Bad interfaces. higher interface resistance higher Tc. But, interface resistance bet metals are similar. Oscillations in Tc vs. dF. • 2. More exotic explanation. From new physics like triplet pairing? Inhomogeneous exchange fields are predicted to induce enhanced superconductivity by spin triplet excitations. [Rusanov et al, PRL (2004), Bergeret et al, PRL (2001), …]. SKKU condensed-matter theory group

  14. Nb/Au/Co60Fe40 SKKU condensed-matter theory group

  15. Two options to understand the short length scale (~ 2 nm) SKKU condensed-matter theory group

  16. Triplet? SKKU condensed-matter theory group

  17. z x O S N F III. Usadel formalism Usadel equation SKKU condensed-matter theory group

  18. z x O S N F Boundary conditions Self-consistency relation • Boundary modeled by • Boundary conditions. SKKU condensed-matter theory group

  19. Odd triplet pairing? • Antisymmetry requirement (at t1=t2): F changes sign under For Odd frequency triplet pairing. SKKU condensed-matter theory group

  20. Solution: by extending the Green’s function method of Fominov et al, PRB 2002. SKKU condensed-matter theory group

  21. Solution • The basic idea is to solve the homogeneous equations with appropriate boundary conditions to obtain a single equation for the singlet pairing component , • and the boundary conditions in terms of and within the S region. • The obtained differential equation is then solved by constructing Green’s function following standard procedure, say, in Arfken. SKKU condensed-matter theory group

  22. Triplet pairing in S/N/F • S = conventional s-wave singlet superconductor. Tc determined by the singlet pairing component. • Triplet pairing components are induced in addition to the singlet component (via spin-flip scatterings). • Triplet components are s-wave (even in k), and odd in frequency. Long length scale. • Triplet components change Tc indirectly by changing singlet component via boundary conditions. SKKU condensed-matter theory group

  23. Procedures for understandingTc vs. dN of Nb/Au/CoFe. Parameters of Usadel equation: (for i = S, N, F), Tc0. hex,(interface) 1. Fit S/F (Nb/CoFe): hex, Tc0. 2. Fit S/N (Nb/Au): 3. Fit S/N/F (Nb/Au/CoFe) to determine SKKU condensed-matter theory group

  24. Nb/CoFe From S/F, SKKU condensed-matter theory group

  25. Nb/Au From S/N, SKKU condensed-matter theory group

  26. Quantitative analysis S/N/F From S/N/F, No need to introduce SKKU condensed-matter theory group

  27. Usadel calculations. • By solving the Usadel equation, because S/N/F still has two interfaces (mathematically) in the limit dN 0. • Short length scale of ~ 2-3 nm: • The length scale over which electrons feel the interface. • Not the physical material length. SKKU condensed-matter theory group

  28. Pairing amplitudes F N S SKKU condensed-matter theory group

  29. Triplet components F N S SKKU condensed-matter theory group

  30. 2. Intermediate length • Could never match the experimental observations of more than one length scales. • Intermediate length not understood. SKKU condensed-matter theory group

  31. Yamazaki et al.: Nb/Au/Fe (MBE) Length scale of 2.1 nm. SKKU condensed-matter theory group

  32. Nb/Au/Co60Fe40 SKKU condensed-matter theory group

  33. Results for S/N/F • It seems that it is the interface resistance that caused the Tc jump (short length scale) on Tc vs. dN for Nb/Au/CoFe. • S/F : • S/N/F : for continuity. • Intermediate length of ~ 20 nm not understood. • Oscillations in Tc vs. dF not understood. SKKU condensed-matter theory group

  34. F S F F S F IV. F/S/F • Parallel & antiparallel because the F effect is canceled in antiparallel junctions. Proximity switch device. SKKU condensed-matter theory group

  35. Gu et al., PRL 2002 You et al., PRB 2004 • is much smaller in experiment compared with theoretical calculation. • Why? SKKU condensed-matter theory group

  36. Why? • Two F’s are not identical. • Triplet components (induced by spin flip scatterings at S/F interfaces). SKKU condensed-matter theory group

  37. F F S Triplet pairing components. • Tunneling conductance for FSF. • Effects of triplet pairing components. SKKU condensed-matter theory group

  38. S S M M F F M M Nb/SrRuO3 SKKU condensed-matter theory group

  39. V. Summary & Outlook • No need for triplet pairing components for Nb/Au/CoFe. • It is the interface resistance that caused the Tc jump. Short length scale of ~ 2 nm: the length scale over which electrons feel the interface. Not the physical material length. • Not understood: intermediate length of ~ 20 nm, Tc vs. dF of S/N/F. • Tc difference between parallel and antiparallel F’s of F/S/F is reduced by triplet components. • Search for the odd-frequency triplet pairing in artificial junctions of S, N, and F. SKKU condensed-matter theory group

More Related