Stefano SanvitoComputational Spintronics Group
Physics Department, Trinity College Dublin
CCTN’05, Göteborg 2005
Aknowledgements
J. Ferrer (Oviedo)
V. M. Garcia Suarez (Oviedo)
C.J. Lambert (Lancaster)
S. Bailey (Lancaster)
Science Foundation of Ireland
Enterprise Ireland
Alexandre Reily Rocha
The Smeagol partners
http://www.smeagol.tcd.ie
Smeagol implements the NEGF method in DFT
Localized multiple- 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)
In particular we use SIESTA
Scattering Region
Right LeadLeft Lead
L=F+V/2
R=F -V/2
Hs[n]
f(L) f(R)
H0 H0 H0 H0H0H0H0H0 H0 H1H1 H2 H2H3H3 H4H4
LR
H= Hs+H0 +H0 +H0 +….
Hermitian problem for an open infinite system
Non-hermitian problem for finite system
Hs+L +R
Lead’s Self-energy
Density Matrix
Current
Scatterer’s Green function
A. R. Rocha, and S. Sanvito, PRB 70, 094406 (2004)
1H 1H 1H 1H 1H 1H0H 0H 0H 0H 0H
z
Total Hamiltonian
Schrödinger equation
Green Function
Analytic form from Green equation and continuity
Surface GFUse of boundary conditions
S. Sanvito et al., Phys. Rev. B 59, 11936 (1999)
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
It converges !! It can use leads with complicated electronic structure
It is spin-polarized
Both finite and periodic systems
Parallel in real space, k and E
It is non-collinear
Now LDA+SIC and LDA+U
Molecular Spin valves
Nature Materials 4, 335 (2005)
Fe/MgO TMR junction
In preparation
DNA transport
In preparation
H2 molecular junction
V. Garcia-Suarez, PRB in press
Pt point contacts
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)
R=IP IAP
IAP
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
http://www.smeagol.tcd.ie
LDA+SIC and LDA+U: now
Current induced forces: end 2005
Inelastic effects (phonons): mid 2006 Free distribution: September 2005
Spin-orbit coupling: end 2005
Multi-terminal: mid 2006
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