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Stefano Sanvito

CCTN’05, Göteborg 2005. Smeagol. the ultimate tool for electronic transport at the nanoscale. Stefano Sanvito. Computational Spintronics Group Physics Department, Trinity College Dublin. Aknowledgements. Alexandre Reily Rocha. The Smeagol partners. J. Ferrer (Oviedo)

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Stefano Sanvito

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  1. CCTN’05, Göteborg 2005 Smeagol the ultimate tool for electronic transport at the nanoscale Stefano Sanvito Computational Spintronics Group Physics Department, Trinity College Dublin

  2. Aknowledgements • Alexandre Reily Rocha The Smeagol partners • J. Ferrer (Oviedo) • V. M. Garcia Suarez (Oviedo) • C.J. Lambert (Lancaster) • S. Bailey (Lancaster) • Science Foundation of Ireland • Enterprise Ireland

  3. http://www.smeagol.tcd.ie

  4. The idea Smeagol implements the NEGF method in DFT In particular we use SIESTA • Localized multiple-z Pseudo-atomic orbitals • Optimized Pseudopotential • Ceperley-Alder Exchange correlation • Large k-point sampling • Super-cells with up to 1,000-10,000 atoms D. Sánchez-Portal, P. Ordejón, E. Artacho, and J.M. Soler, Int. J. Quant. Chem. 65, 453 (1997)

  5. H4 H4 H3 H3 SL H1 H1 SR H2 H2 Non-equilibrium Green Function Scattering Region H0 H0 H0 H0 H0 H0 H0 H0 H0 Hs[n] Right Lead Left Lead f(mL) f(mR) mL=eF+V/2 mR=eF -V/2

  6. Hermitian problem for an open infinite system H= Hs+H0 +H0 +H0 +…. Non-hermitian problem for finite system Hs+SL +SR

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

  8. Total Hamiltonian Green Function Analytic form from Green equation and continuity Surface GF Use of boundary conditions Surface Green Functions Schrödinger equation S. Sanvito et al., Phys. Rev. B 59, 11936 (1999)

  9. Designed for magnetic materials d orbitals fairly localized: no coupling between unit cells H1 is close to singular Unitary Transformation • Highlights uncoupled states between principal layers • States are eliminated and couplings are renormalized • Numerically Stable We can solve for very complicated leads and the size of the system might be drastically downfolded!!! A. R. Rocha et al., in preparation

  10. Why Smeagol is better then everything money can buy? • It converges !! It can use leads with complicated electronic structure • Both finite and periodic systems • Parallel in real space, k and E • It is spin-polarized • It is non-collinear • Now LDA+SIC and LDA+U

  11. What has Smeagol done? H2 molecular junction V. Garcia-Suarez, PRB in press Fe/MgO TMR junction In preparation Molecular Spin valves Nature Materials 4, 335 (2005) Pt point contacts V. Garcia-Suarez, PRL in press DNA transport In preparation

  12. Molecular Spin Valves Experimental Results by Petta et al., PRL, 93, 136601 (2004) A. R. Rocha et al, Nature Materials 4, 335 (2005)

  13. I I I P AP AP R=

  14. Conclusions • Smeagol is a powerful tool to study spin-transport at the nanoscale. • In Ni/Molecule/Ni the I-V presents a non-trivial behaviour and the GMR is strongly BIAS dependent • For pure tunneling GMR given by DOS at the contact • Prospects for large GMR devices

  15. Outlook • LDA+SIC and LDA+U: now • Spin-orbit coupling: end 2005 • Current induced forces: end 2005 • Multi-terminal: mid 2006 • Inelastic effects (phonons): mid 2006 • Free distribution: September 2005 http://www.smeagol.tcd.ie

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