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STM/S Imaging Studies in the Vortex State

STM/S Imaging Studies in the Vortex State. Anjan K. Gupta Physics Department, IIT, Kanpur (Tutorial, IVW10 at TIFR). Tunneling Current. For typical Metals. So,. z=d. I increases ~ 10 times if d decreases by 1. Ref.: J. G. Simmons, J. Appl. Phys. 34 , 1793 (1963). Quantum Tunneling.

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STM/S Imaging Studies in the Vortex State

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  1. STM/S Imaging Studies in the Vortex State Anjan K. Gupta Physics Department, IIT, Kanpur (Tutorial, IVW10 at TIFR)

  2. Tunneling Current For typical Metals So, z=d Iincreases ~ 10 times if d decreases by 1 Ref.: J. G. Simmons, J. Appl. Phys. 34, 1793 (1963) Quantum Tunneling

  3. Scanning Tunneling Microscope A Fresh Cleaved HOPG (Graphite)

  4. STM Schematics sample 5 mm < 0.5mm Sample holder Fine XYZ positioner The coarse Approach ~0.25mm step 0.1 • Fine XYZ positioner • The coarse Approach • Sample & Tip holder • Vibration isolation • Electronics & Software

  5. The First STM 5 cm Ref.: Binnig, Rohrer, Gerber, and Weibel, APL50, 178 (1982)

  6. STM at IITK IITK HOPG in ambience

  7. Tunneling Current Tip d Sample Sample (+ve bias) E e empty eV Nsam V Ntip kBT filled Tip

  8. Tunneling Current Topography : k ~ 1 Å-1 Sample (+ve bias) E e empty eV Spectroscopy : Nsam Nt w~kT kBT filled Tip ~ Nsam (eV) (T ~ 0) Ref.: J. Bardeen, Phys. Rev. Lett. 6, 57 (1961) Tersoff & Hamann, Phys. Rev. B 31, 805 (1985) eV

  9. Tunneling Spectrum Tip d Sample Example: BCS gap in a Superconductor Superconductor Vs. insulator Normal Metal STS STM Junction Sandwich Junction Features in electronic DOS within DE<< EF(~ f) can be resolved extremely well with an energy resolution ~ kT Energy Resolution ~ 29 meV

  10. Scanning Tunneling Spectroscopy 1) Measure I(V) at each point and differentiate Lock-In Impractical: 128x128 image takes >2hrs. and >16MB memory (0.5s/spec. & 1kB for 512 of 2 bytes points) 2) ac-Modulation: Z-Feedback v0 sin wt Band-widths: Amp Tip Scanning speed: Sampling time > tlock-in > 1/w + Sample ~80 s./im (w = 2kHz, tlock-in = 3msec, S.T. = 5msec., 128x128 )

  11. STS: Vortex Imaging 2H-NbSe2 at T = 1.8K and 1 Tesla, dI/dV at 1.3mV Tc=7.2K, D=1meV, x||=8nm, k||=30 200G, 350nm Ref.: H. F. Hess et.al., Phys. Rev. Lett. 62, 214–216 (1989)

  12. LDOS in Vortex Core B-dG eqns. in vortex core: Bound (E<D0) & Scattering (E>D0) States LDOS [N(E)] (Flat near EF) For lowest E Ref.: F. Gygi and M. Schluter, PRB41, 822 (1990); ibid, PRB43, 7609 (1991)

  13. STS vs. Theory Anisotropy 200G, 350nm STS Th H=500G, 1.3K 0mV 150nm Theory Th 0.5mV Anisotropy of periodic potential and magnetic field F. Gygi and M. Schluter, PRB43, 7609 (1991)

  14. Vortex Imaging (low Tc) MgB2 0.05T 0.2T 0.5T 0.5T 680 nm V0/VD Mo2.7Ge 4.2K YNi2B2C 700nm (4.2 K) Ref.:M. R. Eskildsen, PRL89, 187003 (2002) No bound states ! Au covered for passivation Ref.: G. J. C. van Baarle, APL82, 1081 (2003) LuNi2B2C (4.2 K) 1.5 T 0.5 T || c 290nm 150nm Ref.: H. Sakata, PRL84, 1583 (2000) Ref.: Y. De Wilde PRL78, 4273 (1997)

  15. Vortex Imaging (high Tc) YBCO123 Ref.: I. Maggio-Aprile, PRL75, 2754 (1995) 4.2K, 6T (field cooled) Bi2Sr2CaCu2O8 Ref.: S. H. Pan, PRL85, 1536 (2000) 0T, 4.2K, 7mV 7.25 T, 4.2K, 7mV

  16. Vortex Pinning Ion irradiated (Au24+) NbSe2 at 4.2K Topography & tunneling conductance images at various fields STS, 0.5T,0.5mV Topography STS, 0.1T,0.5mV STS, 4mT,0.5mV Ref.: S. Behler, PRL72, 1750 (1994)

  17. Vortex Pinning NbSe2, 0.6T, 4.2K, pristine NbSe2, 0.6T, 4.2K, ion (6GeV Pb) irradiated Ref.: A. M. Troyanovski, Nature399, 665, 1999

  18. Magnetic Vortex: SP-STM Fe island on W (110) Cr coated W tip, 10K Ref.: A. Wachowiak, Science 298, 577. Xth International Vortex State Studies Workshop STM/S Imaging Studies in the Vortex State

  19. Thank You !

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