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

L R

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