quantum transport beyond the independent-electron approximation
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Quantum transport beyond the
independent-electron approximation
Rex Godby
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 2
Outline• Introduction to the quantum transport
problem• Ab initio quantum conductance in the
presence of e-e interaction– TDDFT– MBPT
• Stroboscopic wavepacket approach for calculating and interpreting quantum transport
+
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 3
Bothersome aspects of quantum transport
Ab-initio modelNon-equilibrium
Quantum Mechanics
Many-body problem
Conductance
G=I/U
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 4
Experiments & modelling
X. D. Cui et al. Science, Vol 294, 571-574 (2001)
Experiments:
Octanedithiol/Au: R ≈ 900 MΩ [X. D. Cui et al., Science (2001).]Benzene-di-amin/Au: R ≈ 2 MΩ [Quek, Nano Lett. (2007).]Benzene-di-thiol/Au: R ≈ 18±12 MΩ [M. A. Reed et al. Science (1997).]H2/Pt: G ≈ 0.95 G0 (≈ 1/(13 kΩ)) [R.H.M.Smit et al. Nature (2002).]
Theory – Density Functional Theory + NEGF:
for G ≈ G0 generally goodfor G << G0 poor e.g. G ≈ 0.046 G0 for Benzene-di-amin/Au [Quek, Nano Lett. (2007).]
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 5
The standard modelMads Brandbyge et al. PRB (2002).
1. Ab Initio Quantum Conductance with e-e
Interaction
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 7
Quantum Transport Theories
• Conductance in 1-electron or mean-field theory given by Landauer formula
• Drawbacks of usual approach:– Can be orders of magnitude wrong– Difficult to generalise to many-body case– Calculation of T not readily compatible with periodic
bcs
F
2
ETheG
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 8
Our Approach• “Beyond ground-state DFT” description of
quantum transport still troublesome • Formulate the linear-response theory of
conductance for rigorous ab-initio modelling within a supercell technique:– well defined conductance – converged basis set – realistic e-e interaction
4-point conductance
Plane-wave basis
GW method
P. Bokes, J. Jung and RWG, PRB 2007
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 9
The 4-point conductance
?
P. Bokes, J. Jung and RWG, PRB 2007
VIG P /4 /2 IG P
For constrictions the correction term goes
to 1
G2P
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 10
Integrals - real space formulation
irreducible polarization:
Correction factor:
“2P” conductance:
irr z , z ' n zV to t z '
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 11
Real-space integrals for G
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 12
Long time limit - periodic box
Low- behaviour limited by system size
Order of limit essential: First L then 0
Lv /2 Fmin
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 13
Relation to electronic structure
Model nanojunction: wire with a gapNumber of atoms in the lead:
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 14
Gold contacts
Perpendicular k-point sampling improves extrapolation
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 15
Calculating including interactions
• Time-dependent density-functional theory – e.g. J. Jung, P. Bokes, and RWG, PRL 2007– but the form of the XC kernel is delicate
• Na Sai, PRL(2005), Koentopp PRB(2006), Toher PRL(2005), Burke PRL(2005)
• or, better: Many-body perturbation theory
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 16
Hedin’s Equations
• With /G relation, exact closed equations for G, etc.
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 17
Electronic Excitations: G1 and G2
| N,0 | N+1,s | N–1,s | N,s
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 18
The GW Approximation
')';',()';',(2
);',( '
deGWi ixxxxxx
where ),( rx is space+spin
In the time domain
)';',()';',()';',( ttGttiWtt xxxxxx
W is the dynamically screened Coulomb interaction between electrons
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 19
GW(ST) method
Au (6s), FCC
• Corrects band-gaps and band alignment • Introduces finite lifetime• GW and quantum transport:
– Thygesen JCP (2007), Darancet PRB (2007), Neaton PRL (2007), ....
• Our implementation: real-space / imaginary-time:– Rojas, RWG, Needs PRL (1995)
• Finite temperatures (metals): Verstraete (2008)• Effect on conductance… in progress
2. Stroboscopic Wavepacket Approach
for Calculating and Interpreting Quantum
Transport
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 21
Motivation• Non-linear transport?• Time-dependent transport?• How about physical insight?
P. Bokes, F. Corsetti and RWG, PRL (July 2008)
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 22
Stroboscopic Wavepacket Basis I.
1. Each basis function to be localised in space
Example: free space in 1D
2. Occupying subset of the basis we recover some many-electron eigenstate3. Basis functions generated by time propagation
Density Hie ˆ
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 23
Stroboscopic Wavepacket Basis II.
Reference Hamiltonian: infinite systemcontinuous spectrum translational symmetry (locality)
Choice of normalisation:
The initial set:
n – energy-band index Arbitrary unitary rotation
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 24
Stroboscopic Wavepacket Basis III.
Propagation of the initial set
The time step n=2/n guarantees orthogonality
... and completeness
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 25
Steady-state transport, Landauer
Reference H
Different H', non-translationally-invariant
Scattering WP
Right-going WP basis
Left-going WP basis
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 26
Stroboscopic propagation
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 27
Compactness of basisIllustrated for continuous-time propagation
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 28
Propagation through barrier
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 29
Time-dependence: switching-on
td-NEGF from G. Stefanucci and C. O. Almbladh PRB (2004).
Wavepackets give physical picture for the period of the non-linear oscillations
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 30
Edge-induced spin Hall effect I.
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 31
Edge-induced spin Hall effect II.
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 32
Edge-induced spin Hall effect III.
Acknowledgements and Summary
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 34
Collaborators• Peter Bokes• Matthieu Verstraete• Jeil Jung• Fabiano Corsetti
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 35
Summary• 4-point conductance PRB 2007
– well defined for interacting systems– numerically feasible in supercell geometry– GW calculations in progress
• Stroboscopic wavepacket basis PRL 2008
– particularly suitable for transport problems– applications for TD transport and spin Hall
effect
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 36
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 37
Numerical convergence of wavepacket basis
Quantum transport beyond the indep.-electron approx. - Towler Institute, 30 July 2008 38
Jellium wire vs. Na wire
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