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Josephson supercurrent through a topological insulator surface state Nb/Bi 2 Te 3 /Nb junctions

Josephson supercurrent through a topological insulator surface state Nb/Bi 2 Te 3 /Nb junctions. Marieke Snelder MESO 2012 - Chernogolovka 17 June 2012. Acknowledgements. MESA+ Institute for Nanotechnology, University of Twente , The Netherlands M. Veldhorst (first author) M. Hoek

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Josephson supercurrent through a topological insulator surface state Nb/Bi 2 Te 3 /Nb junctions

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  1. Josephson supercurrent through a topological insulator surface state Nb/Bi2Te3/Nb junctions • Marieke Snelder • MESO 2012 - Chernogolovka • 17 June 2012

  2. Acknowledgements MESA+ Institute for Nanotechnology, University of Twente, The Netherlands M. Veldhorst (first author) M. Hoek T. Gang W. van der Wiel A.A. Golubov H. Hilgenkamp (also from Leiden University, The Netherlands) A. Brinkman High Field Magnet Laboratory, Nijmegen, The Netherlands U. ZeitlerV. K. Guduru University of Wollongong, Australia X. L. Wang

  3. Motivation S-wave Superconductor TI surface states

  4. Motivation P-wave Superconductor Natural place to look for Majorana fermions (single zero-energy modes)

  5. Motivation Essential: supercurrent must couple to surface states Characterize junction V+ V- I+ I- TI S-wave SC

  6. Content Part 1 Bi2Te3 Part 2 S/TI/S junctions Part 3 Josephson supercurrent through the surface states

  7. Content Part 1 Bi2Te3 Part 2 S/TI/S junctions Part 3 Josephsonsupercurrentthrough the surfacestates

  8. Bi2Te3

  9. Fabrication Mechanical cleavage Photolithography contacts

  10. Hall measurements μB>>1 So no quantum oscillations expected n=8.3 x 1019 cm-3 μ=250 cm2/Vs (bulk is conductive) lmfp=22 nm

  11. Shubnikov-de-Haas oscillations B =Bcos(θ) T Left graph: Quantum oscillations @T=4.2K Right graph: Oscillations from 2D channel

  12. Shubnikov-de-Haas oscillations Left graph: Dingle temperature 1.65 K, μ=8300 cm2/Vs Right graph: Effective mass 0.16m0

  13. Shubnikov-de-Haas oscillations B =Bcos(θ) T I.M. Lifshitz and A.M. Kosevich, Sov. Phys. JETP 2, 636 (1956)

  14. Lifshitz-Kosevich formalism ;Former is 0.01, later is 10 So in normal case nth minima=0 through 1/B=0

  15. Lifshitz-Kosevich formalism • Surface states present • @ 1/B=0, n=-1/2 (Berry phase of π) •  linear dispersion relation • lmfp=105 nm Minima Maxima Non-trivial surface states present

  16. Content Part 1 Bi2Te3 Part 2 S/TI/S junctions Part 3 Josephsonsupercurrentthrough the surfacestates

  17. S/TI/S junctions Length Nb Nb Bi2Te3 Bi2Te3 Nb: blue Bi2Te3: red Width=500 nm 50 100 150 200 250 300 nm Junction lengths

  18. Josephson supercurrent Hallmarksfor a Josephsonjunction: • Shapiro steps • Modulation Ic versus B-field

  19. First hallmark – Shapiro steps • Microwave frequency ω • @2eV/ħ=nωShapiro steps • (Energy Cooper pairs resonantto energy microwave) ω = 10 GHz V= n x 20.7 μVT=1.6 K

  20. First hallmark – Shapiro steps

  21. Second hallmark – Ic-B modulation Modulation Ic DC SQUID oscillations B I Josephson supercurrent present M. Veldhorst, C. G. Molenaar et al. Appl. Phys. Lett. 100, 072602 (2012)

  22. Second hallmark – Ic-B modulation

  23. Second hallmark – Ic-B modulation B I • Area uncertain: • Penetrationdepth • Flux focussing • Sincfunctiononlyvalidfor large L and W ratio

  24. Content Part 1 Bi2Te3 Part 2 S/TI/S junctions Part 3 Josephsonsupercurrentthrough the surfacestates

  25. Link supercurrent and surface states Surface states Supercurrent We have junctions between 50 and 250 nm and: lmfp=22 nm (bulk states) → diffusive transport lmfp=105 nm (surface states) → ballistic transport Do we have a supercurrent through the surface states?

  26. Link supercurrent and surface states Dirty or clean? Definitely clean

  27. Josephson supercurrent has been realized through the surface states of Bi2Te3 Provides prospects for Majorana devices……but What is the best topological insulator for this purpose? (stability, insulating in the bulk, Dirac cone in the gap) What is the smoking gun experiment with 3D topological insulators to observe Majorana fermions? M. Veldhorst, M. Snelder et al. Nature Materials, 11, 417–421 (May 2012)

  28. Lifshitz-Kosevich formalism Extrapolating till 1/B=0 → nth minima in resistivity is zero at 1/B=0 in ‘normal case’ (Berry phase=0) E Maxima resistivity Minima resistivity k

  29. Lifshitz-Kosevich formalism Extrapolating till 1/B=0 → Berry phase is π Minima Maxima

  30. AFM Bi2Te3 flakes Step are unit cells

  31. First hallmark – Shapiro steps Fitting Shapiro steps with Bessel functions Only can be done if IcRn product is larger than the position of the steps (20.7 μV in these measurements)

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