Stefano Sanvito

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CCTN05, Göteborg 2005
Smeagol
the ultimate tool for electronic transport at the
nanoscale
Stefano Sanvito
Computational Spintronics Group Physics
Department, Trinity College Dublin
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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

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http//www.smeagol.tcd.ie
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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)
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Non-equilibrium Green Function
Scattering Region
H0
H0
H0
H0
H0
H0
H0
H0
H0
Hsn
Right Lead
Left Lead
f(mL)
f(mR)
mLeFV/2
mReF -V/2
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Hermitian problem for an open infinite system
H HsH0 H0 H0 .
Non-hermitian problem for finite system
HsSL SR
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Leads Self-energy
A. R. Rocha, and S. Sanvito, PRB 70, 094406
(2004)
Scatterers Green function
Density Matrix
Current
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Surface Green Functions
Schrödinger equation
S. Sanvito et al., Phys. Rev. B 59, 11936 (1999)
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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
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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 LDASIC and LDAU

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

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Outlook

• LDASIC and LDAU 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