Beyond DFT-Landauer Quantum Transport: Ab Initio GW-NEGF
in Nano- and Molecular ElectronicsPierre Darancet, Tonatiuh Rangel, Andrea Ferretti,
Paolo Emilio Trevisanutto, Didier Mayou,
Gian-Marco Rignanese, Valerio OlevanoInstitut Néel, CNRS Grenoble
European Theoretical Spectroscopy Facility
Outline:1. Quantum-Transport problem2. DFT-Landauer Formalism3. Non-Equilibrium Green's Functions (NEGF) Theory4. e-e scattering effects and GW5. Results:
• Au monoatomic chain• BDA / BDT@gold
6. Conclusions
Molecular and NanoElectronicsQuantum Transport
Both an Experimental, a Technologicaland a Theoretical Challenge!
Nanoscale Electronics Devices
Real Theoretical challenge: predict ab initio the I/V Characteristics
Quantum Transport problem
LeftContact
L
V
V=L−R
RightContact
R
lead leadconductor
Nanoscale Conductor:finite number of states,out of equilibrium,dissipative effects Mesoscopic Leads:
large but finite number of states,partial equilibrium,ballistic
Macroscopic Reservoirs:continuum of states,thermodynamic equilibrium
We need:
• a First Principle description of the Electronic Structure
• for Finite Voltage: Open System and Out-of-Equilibrium description.
Landauer Theory
C =2e²hM T
[eV ]
Landauer Formula
S. Datta, (1995)R. Landauer, IBMJ. Res. Dev. (1957)
LeftContact
µL
RightContact
µR
lead leadconductor
T
state-of-the-art
Landauer Theory C =2e²hM T
Landauer FormulaT Transmittance
1) Calculated via a standard Quantum Mechanics approach:
H=
T R Transmission and Reflection coefficients
Example: potential barrier
2) Or via a Green's function approach
−H G r ,r ' ,=r ,r '
probability that an electron of energy εinjected in r be transmitted in r' r r'
Landauer Formula in Green's functions
Gc=−H c− l−r −1
conductor-lead coupling
conductorGreen's functions
l=H lc† glH lc
r=H crgrH cr†
gl , r=−H l , r −1
l , r=i[l ,rr−l , r
a]
G l Glc G lcr
Gcl Gc Gcr
Grcl Grc Gr=−Hl −H lc 0
−H lc† −Hc −H cr
0 −Hcr†
−H r −1
lead self-energies
leads bulk Green's functions
T=tr [lGcrrGc
a] Fisher-Lee relation
conductor
leads
c rl
Landauer on top of DFT
H=H l H lc 0
H lc† H c Hcr
0 Hcr† Hr
What to take for the hamiltonian?
the DFT Kohn-Sham hamiltonian!
conductor
leads
c rl
(represented on a real space basis like atomic orbitals, Wannier functions, etc.)
PtH2
K.S. Thygesen K.W. Jakobsen,Chem. Phys. (2005)
Depletion ofPt d states
EXP (break junctions): C(0) ~ 1 [G0=2e2/h]
R.H.M. Smit et al, Nature 419, 906 (2002)
Example of Landauer conductance
Landauer approach
Correctly describes:
✔ Contact Resistance
✔ Scattering on Defects, Impurities
✔ Non-commensurability patterns
DFT-Landauer drawbacks
✗ The DFT Kohn-Sham electronic structure is in principle unphysical.
✗ DFT, no Open Systems, no Out-of-Equilibrium Theory:
✗ only linear response, small bias.
✗ Non interacting quasiparticles:
✗ only coherent part of transport.
Need to go Beyond!
Beyond LB-DFT:2 Possibilities
• TDDFT for Quantum Transport✔promising possibility
✗ need a suitable approx for xc
✗ cannot deal with open systems
• NEGF (NonEquilibrium Green's Function theory)✔ full access to all observables
✔ it has maybe a more intuitive physical meaning
Non-Equilibrium Green's Function Theory (NEGF)
(improperly called Keldysh theory)
Much more complete framework, allows to deal with:
• Many-Body description of incoherent transport (electron-electron interaction, electronic correlations and also electron-phonon);
• Out-of-Equilibrium situation;
• Access to Transient response (beyond Steady-State);
• Reduces to Landauer for coherent transport.
The theory is due to the works of Schwinger, Baym, Kadanoff and Keldysh
Many-Body Finite-Temperatureformalism
H= T V W
H = e−H
tr [e− H ]statistical weight
observable
hamiltonianmany-body
o=∑i e−E i ⟨ i∣o∣i ⟩
∑i e−Ei
=tr [ H o]
Valerio Olevano, CNRS Grenoble
NEGF formalism
H t = H U t = T V W U t
ot =tr [ H oH t ] tt0
H = e−H
tr [e− H ]
statistical weight referred tothe unperturbed Hamiltonian andthe equilibrium situation before t
0
observable
hamiltonian
many-body + time-dependence
Valerio Olevano, CNRS Grenoble
Time Contour
ot=tr [ s t0−i , t0 s t0 ,t o t s t ,t 0 ]
tr [ s t0−i , t0 ]
evolution operator
Heisenberg representationoH t = st0 ,t ot st , t0
st ,t0=T {exp −i∫t 0t dt ' H t ' }
st0−i , t0=e− H
ot =tr [TC [exp−i∫Cdt ' H t ' o t ]]
tr [T C[exp−i∫C dt ' H t ' ]]
trick to put the equilibrium weightinto the evolution
Valerio Olevano, CNRS Grenoble
Contour and Perturbation Theory
• To recover perturbation theory (Wick's theorem, Feynman diagrams, etc.) you have to declare the Green's function and all the quantities on the Closed Contour.
Gco x1, x2=−iT c {H x1H† x2}
contour ordered Green's function
Gcox1, x2=
Gt ox1, x2 t1,t2∈C
Gatox1, x2 t1,t2∈C∧
G x1 , x2 t1∈C , t2∈C∧
G x1 , x2 t1∈C∧ ,t2∈C{
Valerio Olevano, CNRS Grenoble
Keldysh Formulation
GkoG=G11 G12
G21 G22
G11x1,x2=Gt ox1,x2
G12x1,x2=Gx1,x2
G21x1,x2=G x1,x2
G22x1,x2=Gatox1,x2
G '= 0 Ga
Gr Gk Gk=G
G
G ''=Gr Gk
0 Ga
Keldysh formulation
Larkin-Ovchinnikovformulation
Keldysh Green's function
Schwinger-Keldyshcontour
Valerio Olevano, CNRS Grenoble
Green's and Correlation FunctionsGr
G
G
−iG = f FD A
iG =[1− f FD ]A
Gt o=Gr
G
A=iGr−Ga
Out of Equilibriumwe need to introduceat least three unrelated“Green's” functions.
At Equilibrium the Correlationfunctions are related to the Green's function through the Fermi-Dirac distribution.
Spectral Function
Once we know the Green's and the Correlation functions,the problem is solved!They contain all the physics!
hole and electrondistribution functions
Valerio Olevano, CNRS Grenoble
NEGF Fundamental KineticEquations
Gr=[−H c−
r]−1
G =Gr
Ga
G =Gr
Ga
Caveat!: in case we want to consider also the transient,then we should add another term to these equations:
G =Gr
Ga
1GrrG0
1aGa Keldysh equation
Valerio Olevano, CNRS Grenoble
Self-Energy and Scattering Functions
r
−i = f FD
i =[1− f FD ]
=ir−a
Self-Energy
At Equilibrium
In-scattering function (represent the rate at which the electrons come in)
Out-scattering function
Decay Rate
Valerio Olevano, CNRS Grenoble
Quantum Transport:composition of the Self-energy
r
=∑ppr
e−phr
e−er
interactionwith the leads
electron-phononinteraction
→ SCBA (Frederiksen et al. PRL 2004)
electron-electroninteraction
-> ?
Critical point : • Choice of relevant approximations for the Self-Energy and the in/out scattering functions
Why GW?Direct and Exchange terms:
Band Structure Renormalization
Collisional Term:Band structure renormalization for Electronic Correlations +
e-e Scattering -> Conductance Degrading Mechanisms, Resistance, non-coherent transport
G0W0
Self-consistent Hartree-Fock
NEGF Quantum Transportresolution scheme
So far applied to model Hamiltonians (Anderson, Kondo) But very few applications for real systems
Iterating the Kinetic Equations(G and Σ have to be recalculated at each iteration):
Highly Time-consuming
Our Scheme• Approximations:
1) G0W0 Non Self-Consistently
2) Equilibrium (linear response, small bias)
3) Neglect Transient (Steady-State)
• We take into account:1) Many-Body Correlations (Renormalization of the
electronic structure)
2) e-e Scattering (appearance of Resistance and Loss-of-Coherence)
3) Finite Lifetime and Dynamical effects (beyond Plasmon-Pole GW)
Flow diagram
DFT
MLWFs GW
Real space Ham.
Transport
http://www.abinit.org
http://www.wannier-transport.orghttp://www.wannier.org
Monoatomic Gold chain:GW vs DFT bandplot
P. Darancet, A. Ferretti, D. Mayou, et V. Olevano, PRB 75, 075102 (2007).
PPM Plasmon-Pole ModelAC Analytic ContinuationCD Contour Deformation(more or less the same)
GW vs DFTLandauer Conductance
• non-negligible rearrangement of the conductance channels• Still ballistic conductor (flat plateaus)
Self-Energy and Spectral Function
• The Analytic Continuation smooths the more accurate Contour Deformation
e-e scattering only in the Conductor
Broadening of the peaks:➔ QP lifetime
Loss of Conductance:➔ Appearance of Resistance
e-e scattering in conductor and leads
➔No contact resistance (small increase in the central part)➔Appearance of Satellite Conductance Channels
sate
ll ite
C / V characteristics: GW vs EXP
e-e
e-ph
}EXPERIMENT
P. Darancet, A. Ferretti, D. Mayou, and V. Olevano, PRB 75, 075102 (2007).
N. Agrait , PRL 88, 216803 (2002) Frederiksen et al., PRL 93, 256601 (2004)
Experiment vs DFT on BDA
BDA
No mol.
Break Au junctions:L. Venkataraman et al., Nano Letters 6, 458 (2006)
zero-bias conductance [G0]
EXP 0.007DFT 0.018
DFT overestimate of the conductance!
Is it due to e-e many-body effects?
G = 0.007
First essays: 1PM-IC model
• The , simplified to a HOMO-LUMO shissor operator, are adjusted on the experiment or on another model: the Image-Charge (IC) model
1PM=∑mm∣m
mol ⟩ ⟨mmol∣
m
Neaton et al. PRL 97, 216405 (2006)Quek et al. Nano Letters 7, 3472 (2007)Mowbray et al. JCP 128, 11103 (2008)
• The self-energy is modeled by a simple one-projector (on molecular orbitals) model (1PM):
1PM vs DFT vs Experiment
• The 1PM model reduces the conductance
zero-bias conductance [G0]
DFT 0.018EXP 0.0071PM 0.004
Our GW calculation on BDA@gold
• Cost of 1 iteration in BDA:~ 250 bands 250 x 250 matrix elements→
96 irreducible k-points, 11463 plane waves
~ 1 month on 32 processors.
• Approximations:✗ neglect of the non-hermitian component
real quasiparticles, no lifetimes→
✗ neglect of the dynamic behavior
• Fully accounted:✔non diagonal elements
Ab Initio GW conductance
• Ab initio GW reduces the conductance
• GW is still half-way →further SC iterations?
• 1PM is beyond
zero-bias conductance [G0]
DFT 0.018GW 0.013EXP 0.0071PM 0.004
Conductance reduction mechanism
LDOS difference: GW - DFT
Eigenchannel difference: GW - 1PM
Important changes both on the leads and on the molecule
Important differences between the 1PM model and the ab initio GW
→ the 1PM model reduces the conductance but does not reproduce the correct physics. Red: + Blue: -
3PM model3PM=∑m
mmol∣m
mol ⟩ ⟨mmol∣∑l
lAu−sp∣l
Au−sp ⟩ ⟨lAu−sp∣∑l
lAu−d∣l
Au−d ⟩ ⟨lAu−d∣
BDA BDT-H BDT-h BDT-p
DFT 0.018 0.034 0.28 0.37
GW 0.013 In progress
3PM 0.011 0.010 0.25 0.36
1PM 0.004 In progress NA NA
EXP 0.007 0.010
3PM model:
• More physical
• Promising results
BDA BDT
Conclusions
• GW e-e correlations in quantum transport:– Static real part of self-energy (GW-LB)
➔ modification of the conductance profile.➔ reduction of the 0-bias conductance
– Non-hermitean part of the self-energy (NEGF-GW)➔ loss-of-coherence, dissipative effects.
– Full dynamical self-energy (NEGF-GW)➔ appearance of satellites
• GW in good agreement with the Experiment both on:
– the smooth drop in the Au-chain conductance
– the absolute 0-bias conductance in BDA@gold