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SpiDME meeting, Nijmegen , May 2007

SpiDME meeting, Nijmegen , May 2007. First principles STM simulations. Stefano Sanvito and Nadjib Baadji. Computational Spintronics Group School of Physics and CRANN, Trinity College. Project summary. People. Dr. Nadjib Baadji (Uni. Strasbourg), April ‘07

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SpiDME meeting, Nijmegen , May 2007

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  1. SpiDME meeting, Nijmegen, May 2007 First principles STM simulations Stefano Sanvito and Nadjib Baadji Computational Spintronics Group School of Physics and CRANN, Trinity College

  2. Project summary People Dr. Nadjib Baadji (Uni. Strasbourg), April ‘07 Mr. Sankar Kesanakurthi (U. Hiderabad), April ‘07 Visits Sanvito to Hamburg (Feb. 2007)

  3. Outline • A simple model for transport • Ab initio transport theory Smeagol • SP-STM for molecules • Salen on Cu • Outlook

  4. A simple model

  5. GL GR e mL mR V  0

  6. mL GR V e mR GL V ≠ 0 Out of equilibrium In equilibrium 2

  7. mL GR V e mR GL V ≠ 0 2|EF-|

  8. mL GR V e mR R L L R GL V ≠ 0 Charging the molecule ( ( ) ) U N f - e = e + - e m 0 0

  9. E +eV/2 F E -eV/2 E F T(E)

  10. Ab initio Transport Theory

  11. The problem I

  12. HLM HRM H1 H1 H1 H1 H0 H0 H0 H0 H0 H0   L R H= HM+H0 +H0 +H0 +…. HM (n) HM+SL (E)+SR(E)

  13. Lead’s Self-energy A. R. Rocha and S. Sanvito, PRB 70, 094406 (2004) Molecule Green function Density Matrix Current

  14. KS-DFT Hamiltonian We implemented NEGF in Siesta • Localized multiple-z Pseudo-atomic orbitals (non-orthogonal) • Optimized Pseudopotential • Super-cells with up to 2,000 atoms D. Sánchez-Portal, P. Ordejón, E. Artacho, and J.M. Soler, Int. J. Quant. Chem. 65, 453 (1997)

  15. http://www.smeagol.tcd.ie/ Mailing list http://lists.tchpc.tcd.ie/listinfo/smeagol-discuss A. R. Rocha et al., Phys. Rev. B 73, 085414 (2006); Nature Materials 4, 335 (2005)

  16. Some examples Problems with molecular transport C. Toher et al., PRL 95, 146402 (2005) Ni point contacts A.R.Rocha et al., cond-mat/0701512 Fe/MgO TMR junction I. Rungger et al Spin Torque M. Stamenova et al., in preparation DNA transport A.R.Rocha et al., in preparation Molecular Spin valves Nature Mat. 4, 335 (2005)

  17. SP-STM for molecules

  18. Topographic Images 80nA Au on Au V=250mV d=0.4nm 40nA 0nA 100nA Ni on Ni V=250mV d=0.4nm 50nA 0nA

  19. I I I +I Polarization plots 20% 10% 10% -10% I to tip 0% -30%   500 mV 250 mV P=   -40% -30% -55% -45% I from tip -70% -60% -250 mV -500 mV

  20. I I I I +I I I AP AP P Does the GMR mirror the polarization ? 10%   P= -10%   -30% 250 mV -10% R= -15% -20%

  21. 20% 500 mV 10% 250 mV -45% -250 mV -50% -500 mV

  22. How does it work ? V=0

  23. TIP M+S V=0

  24. S   S tip  V tip

  25. TIP M+S V=0

  26. TIP M+S V=400mV Current to the tip

  27. TIP M+S V=0

  28. TIP M+S V=-400mV Current to the S+M

  29. Conclusion SP-STM for molecules • Direct calculations of the tunneling currents are possible and include: • Some prospects of investigating the bonding of molecules on magnetic surfaces • Electronic Structure of the tip • Tip to sample interaction • Charging of the moleculae • Accurate determination of the spin-polarization • Non-collinear spin • Spin-orbit

  30. Salen on Cu (001) & (111)

  31. Molecule C2 C3 C2 C3 C1 C1 N,N'-BIS(SALICYLIDENE)ETHYLENEDIAMINO-TM Where TM could be : Cu, Zn, Ni or Co

  32. Big DOS (arb. units) Small E (eV) Comparison between the DOS of the Salen molecule and the hypothetical small molecule

  33. Big Small E (eV)

  34. Molecule on Cu surfaces (un-relaxed) DOS (arb. units) Cu-salen on Cu(001) E (eV) Cu-salen on Cu(111)

  35. Relaxation on Cu(001) surface Unrelaxed structure Relaxed structure

  36. DOS for different TM-salen Cu 4s13d10 DOS (arb. units) E (eV)

  37. Zn 4s23d10 DOS (arb. units) E (eV)

  38. Co 4s23d7 DOS (arb. units) E (eV)

  39. Ni 4s23d8 DOS (arb. units) E (eV)

  40. Simulation STM images Free Cu-Salen EF < E < EF +0.2eV EF-0.2eV < E < EF I molecule to tip I tip to molecule

  41. Constant current STM images Cu-Salen un-relaxed EF < E < EF +0.2eV EF-0.2eV < E < EF I molecule to tip I tip to molecule

  42. Cu (a) (b) Zn (c) (d) EF < E < EF +0.2eV EF-0.2eV < E < EF I molecule to tip I tip to molecule

  43. STM for salen on Cu • This is very much work in progress • First find the right atomic configuration • Then simulate the current • Compare the images for different TM • Hopefully they will compare with experiments

  44. integral of the DOS near Ef (pos. & neg. bias L-resolved DOS for Cu atom in the small molecule Cu DOS in free mole. and in mole. on Cu (001) L-resolved DOS for Zn atom in the small molecule

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