CCTN’ 05, Göteborg 2005 Stefano Sanvito Computational Spintronics Group Physics Department, Trinity College Dublin
Aknowledgements Ø Alexandre Reily Rocha The Smeagol partners Ø J. Ferrer (Oviedo) Ø V. M. Garcia Suarez (Oviedo) Ø C. J. Lambert (Lancaster) Ø S. Bailey (Lancaster) Ø Science Foundation of Ireland Ø Enterprise Ireland
http: //www. smeagol. tcd. ie
Smeagol implements the NEGF method in DFT In particular we use SIESTA Ø Localized multiple-z Pseudo-atomic orbitals Ø Optimized Pseudopotential Ø Ceperley-Alder Exchange correlation Ø Large k-point sampling Ø 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)
Scattering Region H 0 H 0 H 40 1 2 3 H SL Left Lead f(m. L) H 0 H 0 H 1 H 0 4 3 2 Hs[n] SR Right Lead f(m. R) m. L=e. F+V/2 m. R=e. F -V/2
Hermitian problem for an open infinite system H= Hs+H 0 +…. Non-hermitian problem for finite system Hs+SL +SR
Lead’s Self-energy A. R. Rocha, and S. Sanvito, PRB 70, 094406 (2004) Scatterer’s Green function Density Matrix Current
Total Hamiltonian Schrödinger equation Green Function Analytic form from Green equation and continuity Surface GF Use of boundary conditions S. Sanvito et al. , Phys. Rev. B 59, 11936 (1999)
d orbitals fairly localized: no coupling between unit cells H 1 is close to singular Unitary Transformation Ø Highlights uncoupled states between principal layers Ø States are eliminated and couplings are renormalized Ø 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
It converges !! It can use leads with complicated electronic structure Ø Ø Both finite and periodic systems Ø Parallel in real space, k and E Ø It is spin-polarized Ø It is non-collinear Ø Now LDA+SIC and LDA+U
Fe/Mg. O TMR junction In preparation H 2 molecular junction V. Garcia-Suarez, PRB in press Molecular Spin valves Nature Materials 4, 335 (2005) DNA transport Pt point contacts In preparation V. Garcia-Suarez, PRL in press
Experimental Results by Petta et al. , PRL, 93, 136601 (2004) A. R. Rocha et al, Nature Materials 4, 335 (2005)
I P I AP R= I AP
Ø Smeagol is a powerful tool to study spin-transport at the nanoscale. Ø In Ni/Molecule/Ni the I-V presents a non-trivial behaviour and the GMR is strongly BIAS dependent Ø For pure tunneling GMR given by DOS at the contact Ø Prospects for large GMR devices
Ø LDA+SIC and LDA+U: now Ø Spin-orbit coupling: end 2005 Ø Current induced forces: end 2005 Ø Ø Ø Multi-terminal: mid 2006 Inelastic effects (phonons): mid 2006 Free distribution: September 2005 http: //www. smeagol. tcd. ie
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