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Adiabatic Pumping through a Quantum Dotwith Coulomb Interaction
Jurgen Konig
Institut fur Theoretische Physik III
Ruhr-Universitat Bochum
– p.1
Collaborators & Publications
Collaborators:
Janine Splettstoesser (Bochum, Pisa)
Michele Governale (Bochum)
Rosario Fazio (Trieste, Pisa)
Fabio Taddei (Pisa)
Nina Winkler (Bochum)
Publications:
Splettstoesser, Governale, JK, Fazio, PRL 95, 246803 (2005)
Splettstoesser, Governale, JK, Fazio, PRB 74, 085305 (2006)
Splettstoesser, Governale, JK, Taddei, Fazio, cm/0612257
Winkler, Governale, JK, in preparation– p.2
Outline
Introduction
adiabatic pumping and Coulomb interaction
Nonequilibrium Green’s function approach
(Splettstoesser, Governale, JK, Fazio, PRL ’05)
relate charge to instantaneous Green’s functions
Diagrammatic transport theory
(Splettstoesser, Governale, JK, Fazio, PRB ’06)
systematic perturbation theory in tunnel coupling
Pumping through a metallic island
(Winkler, Governale, JK, in preparation)
Pumping in proximity of a superconducting lead
(Splettstoesser, Governale, JK, Taddei, Fazio, preprint ’06)
– p.3
Introduction: Mesoscopic Classical Pump
pumping through double-island system
n1 n2
U1
C1 C2
C C C
U2
U1
U2
Coulomb blockade is dominant
Q = 1 electron/cycle
H. Pothier et al., EPL 17,249 (1992)
– p.4
Introduction: Quantum Pump
Thouless, PRB 27, 6083 (1983)
µµ conductor
Mesoscopic
X
X
2
1
X
X
2
1
phase-coherent mesoscopic conductor
periodic variation of some properties X1(t), X2(t)
adiabatic pumping: Ω = pumping frequency ≪ 1lifetime
pumped charge Q = Ipump/Ω is geometric
weak, sinusoidal pumping:
δX1(t) = δX1 sin(Ωt), δX2(t) = δX2 sin(Ωt − ϕ)
Q ∝ δX1δX2 sin ϕ
– p.5
Introduction: Scattering Formalism
scattering matrix S(X1(t),X2(t))
b1
b2
= S
a1
a2
Scattering
region
1a
1b
2a
2b
emissivity: charge emitted by lead m in response to X
dnm
dX=
1
2π
∑
α∈m,β
Im
∂Sα,β
∂XS∗
α,β
Buttiker, Thomas, Pretre, Z. Phys. B 94, 133, (1994)
Brouwer’s formula
Qm =e
π
∫
AdX1dX2
∑
α∈m,β
Im
∂S∗α,β
∂X1
∂Sα,β
∂X2
Brouwer, PRB 58, R10135 (1998) – p.6
Introduction: Scattering Formalism
Brouwer’s formula
Qm =e
π
∫
AdX1dX2
∑
α∈m,β
Im
∂S∗α,β
∂X1
∂Sα,β
∂X2
Brouwer, PRB 58, R10135 (1998)
applicable when scattering matrix is known
(non-interacting systems)
what to do in presence of (strong) interaction?
– p.6
Introduction: Quantum Dot
ε
LV VR
ε + U
E = 0 ǫ ǫ 2ǫ + U
model parameters:
single level with energy ǫ + ∆ǫ(t)
charging energy U
tunnel-coupling strengths ΓL,R(t) = 2πρL,R|VL,R(t)|2
temperature T
pumping parameters: ∆ǫ(t),ΓL(t),ΓR(t)
– p.7
Outline
Introduction
adiabatic pumping and Coulomb interaction
Nonequilibrium Green’s function approach
(Splettstoesser, Governale, JK, Fazio, PRL ’05)
relate charge to instantaneous Green’s functions
Diagrammatic transport theory
(Splettstoesser, Governale, JK, Fazio, PRB ’06)
systematic perturbation theory in tunnel coupling
Pumping through a metallic island
(Winkler, Governale, JK, in preparation)
Pumping in proximity of a superconducting lead
(Splettstoesser, Governale, JK, Taddei, Fazio, preprint ’06)
– p.8
Nonequilibrium Green’s Function Approach
start: current formula for full time dependence
method: adiabatic expansion of Green’s functions
goal: current in terms of instantaneous Green functions
– p.9
Nonequilibrium Green’s Function Approach
start: current formula for full time dependence
Jauho, Wingreen, Meir, Phys. Rev. B, 50, 5528 (1994)
JL = −et∫
−∞dt′
∫
dωπ
Im
e−iω(t′−t)ΓL (ω, t′, t) [G< (t, t′) + f (ω) Gr (t, t′)]
with G<,r (t, t′): time-dependent dot Green’s functions
method: adiabatic expansion of Green’s functions
goal: current in terms of instantaneous Green functions
– p.9
Nonequilibrium Green’s Function Approach
start: current formula for full time dependence
method: adiabatic expansion of Green’s functions
Dyson equation:
G (t, t′) = g (t, t′) +∫
dt1dt2G (t, t1) Σ (t1, t2)g (t2, t′)
self energy: Σ (t1, t2)= Σ(t1, t2, H(τ)τ∈[t1,t2])
linearize time dependence:
H(τ) → H (t0) + (τ − t0) H (t0)
average-time approximation: τ → t1+t22
adiabatic expansion:
Σ(
t1, t2, H(τ)τ∈[t1,t2]
)
→
Σ (t1, t2, H(t0)) +(
t1+t22 − t0
) ∂Σ(t1,t2,H(t0))∂t0
goal: current in terms of instantaneous Green functions
– p.9
Nonequilibrium Green’s Function Approach
start: current formula for full time dependence
method: adiabatic expansion of Green’s functions
goal: current in terms of instantaneous Green functions
Splettstoesser, Governale, JK, Fazio, PRL 95, 246803 (2005)
Sela and Oreg, PRL 96, 166802 (2006)
JL (t) = − eπ
∫
dω(
− ∂f∂ω
)
Re
ddt
[ΓL (t) Gr0 (ω, t)] (Gr
0 (ω, t))−1 Ga0 (ω, t)
+ vertex-correction terms
Gr,a0 (ω, t): instantaneous Green’s functions
average-time approximation exact if
U = 0
U → ∞ and linear order in Γ
T = 0 (U arbitrary)– p.9
Results for Single-Level Quantum Dot
Splettstoesser, Governale, JK, Fazio, PRL 95, 246803 (2005)
pumped charge per period: Q =∫ T0 JL (t) dt
pumping parameters: ΓL,R (t) = Γ2 + ∆ΓL,R (t)
weak pumping: Q ∝ η =∫ T0
˙∆ΓL(t)∆ΓR(t)dt
Q = −eηΓ
πΓLΓR
∫
dω
(
−∂f
∂ω
)
∂δ(ω)
∂ΓT (ω)
phase of Green’s function Gr0(ω) = |Gr
0(ω)| exp[iδ(ω)]
transmission probability T (ω) = 2ΓLΓR/Γ · Im[Gr0(ω)]
– p.10
Results for Single-Level Quantum Dot
Splettstoesser, Governale, JK, Fazio, PRL 95, 246803 (2005)
high-temperature regime T > TK
use equations-of-motions method for Gr0(ω)
-10 0 10
ε/Γ
0
0.01Q
.Γ2 /e
η
U infinite, (∆ΓL, ∆Γ
R )
U=0, (∆ΓL, ∆Γ
R)
interaction changes pumping characteristics
– p.10
Results for Single-Level Quantum Dot
Splettstoesser, Governale, JK, Fazio, PRL 95, 246803 (2005)
high-temperature regime T > TK
use equations-of-motions method for Gr0(ω)
-10 0 10
ε/Γ
0
0.01
0.02
0.03
0.04
0.05Q
.Γ2 /e
η
U infinite, (∆ΓL, ∆Γ
R )
U=0, (∆ΓL, ∆Γ
R)
U=0, (∆ΓL, ∆ε)
pumping by time-dependent level renormalization
– p.10
Results for Single-Level Quantum Dot
Splettstoesser, Governale, JK, Fazio, PRL 95, 246803 (2005)
low-temperature regime T ≪ TK
T = 0 result (exact): Q = −4eη
Γ
∂ ¯〈n〉
∂Γsin2
(
π ¯〈n〉)
use slave-boson mean-field method for ¯〈n〉
-4 -3 -2 -1 0ε/Γ
0
0.5
1
Q . Γ
2 /eη
– p.10
Outline
Introduction
adiabatic pumping and Coulomb interaction
Nonequilibrium Green’s function approach
(Splettstoesser, Governale, JK, Fazio, PRL ’05)
relate charge to instantaneous Green’s functions
Diagrammatic transport theory
(Splettstoesser, Governale, JK, Fazio, PRB ’06)
systematic perturbation theory in tunnel coupling
Pumping through a metallic island
(Winkler, Governale, JK, in preparation)
Pumping in proximity of a superconducting lead
(Splettstoesser, Governale, JK, Taddei, Fazio, preprint ’06)
– p.11
Diagrammatic Transport Theory
J.K., Schoeller, Schon, PRL ’96; J.K., Schmid, Schoeller, Schon, PRB ’96
ingredients:
– quantum dot with strong interaction
– tunnel coupling to noninteracting leads
– nonequilibrium due to finite bias voltage
– finite temperature
Hamiltonian:
H = Hleads + Hdot + HT ≡ H0 + HT
general idea:
– integrate out leads −→ reduced density matrix for dot
– expand in tunnel coupling
– treat interaction exactly
– work on Keldysh contour– p.12
Derivation of Diagrams
a) goal: calculate expectation values
〈A(t)〉 = tr[Aρ(t)] = tr[eiHtAe−iHtρ0]
ρ0
A
exp[−iHt]
exp[iHt]
time
– p.13
Derivation of Diagrams
b) go to interaction picture H = H0 + HT
〈A(t)〉 = tr[T ei∫
dt′HT(t′)ATe−i∫
dt′HT(t′)ρ0]
ρ0
A
T
exp[ −i dt’ H (t’) ]T
exp[ i dt’ H (t’) ]
– p.13
Derivation of Diagrams
c) expand in tunneling
〈A(t)〉 = tr[∑
n(−i)n∫
dtiTKHT(t1) . . . HT(tn)Aρ0]
H T H T H T H T H T H T H T
H TH TH TH TH TH TH TH T
TH
ρ0
A
– p.13
Derivation of Diagrams
d) integrate out leads (noninteracting)
Wick’s theorem → contract vertices in pairs
ρ0
A
0 00
0
0
0 000
L R L R L
L R
L
reduced density matrix for quantum dot
charging energy treated exactly
– p.13
Kinetic Equation for Probabilities
= p(t )0
t
+ p(t )1
0p (t) = p(t ) ....χ
χ
Σ Σ Σ Σ
tΣ
χ
Master equation: pχ(t) =∑
χ′
t∫
t0
dt′Σχχ′(t, t′)pχ′(t′)
transition rate: Σχχ′ = Σχ χ
χχ’
’χ
= sum of irreducible diagrams
– p.14
Calculation of Current
IL(t) = e∑
s
∑
χχ′
t∫
t0
dt′sΣsLχχ′(t, t′)pχ′(t′)
pχ′ : probability for state χ′ on dot
Σχχ′ : tunnel rate from χ′ to χ
ΣsLχχ′ : rate from χ′ to χ with s electrons entering from left
– p.15
Adiabatic Expansion of Diagrams
Splettstoesser, Governale, JK, Fazio, PRB 74, 085305 (2006)
starting point:d
dtp (t) =
∫ t
−∞dt′ W
(
t, t′)
p(
t′)
expand X(τ) → X(t) + (τ − t) ddτ
X(τ)|τ=t about final time
adiabatic expansion (Ω ≪ Γ) for
kernels: W(t, t′) → W(i)t (t − t′) + W
(a)t (t − t′)
probabilities: p(t) → p(i)t + p
(a)t
perturbation expansion in tunnel coupling Γ
instantaneous kernel: W(i)t → W
(i,1)t + W
(i,2)t + . . .
adiabatic correction: W(a)t → W
(a,1)t + . . .
instantaneous probability: p(i)t → p
(i,0)t + p
(i,1)t + . . .
adiabatic correction: p(a)t → p
(a,−1)t + p
(a,0)t + . . .
– p.16
Results: Current in Low-Order Tunneling
Splettstoesser, Governale, JK, Fazio, PRB 74, 085305 (2006)
current in zeroth-order in Γ
I(0)L (t) = −e
ΓL
Γ
d
dt〈n〉(i,0)
ε
nonzero only if level position is time dependent
current in first order in Γ
I(1)L (t) = −e
d
dt
(
〈n〉(i,broad,L))
+ΓL
Γ
d
dt〈n〉(i,ren)
only level renormalization contributes to pumping
– p.17
Results: Pumping with Level & One Barrier
Splettstoesser, Governale, JK, Fazio, PRB 74, 085305 (2006)
lowest-order contribution
Q(0)ΓL,ǫ = e
ΓR
Γ2η1
d
dǫ¯〈n〉
(i,0)
η1 =∫ T0
∂∆ΓL
∂t∆ǫdt: area enclosed in parameter space
-10 0 10
ε/Γ0
0.025
0.1
Q Γ L
,ε[e
η 1/Γ2 ]
U=0U=4ΓU=8ΓU=50Γ
– p.18
Results: Pumping with Two Barriers
Splettstoesser, Governale, JK, Fazio, PRB 74, 085305 (2006)
lowest-order contribution due to level renormalization
ǫ → ǫ + σ (ǫ,Γ, U) with σ ∝ Γ ln UmaxkBT,ǫ
Q(1)ΓL,ΓR
= eη2
Γ2
d
dǫ
(
¯〈n〉(i,0)
)
σ(
ǫ, Γ, U)
η2 =∫ T0
∂∆ΓL
∂t∆ΓRdt: area enclosed in parameter space
-40 -20 0ε/Γ
-0.05
-0.03
0
0.03
0.05
QΓ L
,ΓR[e
η 2/Γ2 ]
U=0.1ΓU=4ΓU=20ΓU=30Γ pure interaction effect
access to level renormalization
different sign for the two peaks
– p.19
Outline
Introduction
adiabatic pumping and Coulomb interaction
Nonequilibrium Green’s function approach
(Splettstoesser, Governale, JK, Fazio, PRL ’05)
relate charge to instantaneous Green’s functions
Diagrammatic transport theory
(Splettstoesser, Governale, JK, Fazio, PRB ’06)
systematic perturbation theory in tunnel coupling
Pumping through a metallic island
(Winkler, Governale, JK, in preparation)
Pumping in proximity of a superconducting lead
(Splettstoesser, Governale, JK, Taddei, Fazio, preprint ’06)
– p.20
Results: Current in Low-Order Tunneling
Winkler, Governale, JK, in preparation
current in zeroth-order in α0
I(0)L (t) = −e
αL0
α0
d
dt〈n〉(i,0)
α 0L
∆
α 0R
nonzero only if charging gap ∆ is time dependent
current in first order in α0
I(1)L (t) = −e
d
dt
(
〈n〉(i,broad,L))
+αL
0
α0
d
dt〈n〉(i,ren)
only gap renormalization contributes to pumping
– p.21
Results: Pumping with Two Barriers
Winkler, Governale, JK, in preparation
lowest-order contribution due to gap renormalization
∆ → ∆ + σ (∆,Γ, EC) with σ ∝ −∆ln EC
maxkBT,∆
Q(1)
αL
0,αR
0
= eη2
α20
d
d∆
(
¯〈n〉(i,0)
)
σ(
∆, Γ, EC
)
η2 =∫ T0
∂∆αL
0
∂t∆αR
0 dt: area enclosed in parameter space
pure interaction effect
access to gap renormalization
– p.22
Outline
Introduction
adiabatic pumping and Coulomb interaction
Nonequilibrium Green’s function approach
(Splettstoesser, Governale, JK, Fazio, PRL ’05)
relate charge to instantaneous Green’s functions
Diagrammatic transport theory
(Splettstoesser, Governale, JK, Fazio, PRB ’06)
systematic perturbation theory in tunnel coupling
Pumping through a metallic island
(Winkler, Governale, JK, in preparation)
Pumping in proximity of a superconducting lead
(Splettstoesser, Governale, JK, Taddei, Fazio, preprint ’06)
– p.23
Pumping in Proximity of Superconductor
Splettstoesser, Governale, JK, Taddei, Fazio, cm/0612257
Γ ΓSN
dotN N/S
transport takes place via Andreev reflection
compare N-dot-N to N-dot-S system
study U = 0 and U = ∞ limit
– p.24
Nonequilibrium Green’s Function Approach
current in terms of instantaneous Green’s functions:
JL (t) = −e
2π
∫
dω
(
−∂f
∂ω
)
Re
Tr
[
τ3d
dt
[
ΓL (t) Gr0 (ω, t)
] (
Gr0 (ω, t)
)−1Ga
0 (ω, t)
]
average-time approximation neglects
JcorrL =
e
2π
∫
dω
∫
dω′
πRe
Tr[
Ga0(ω
′, t)τ3ΓL (ω, t) Gr0(ω
′, t)
Σcorr,< (ω′, t) + f (ω′)(
Σcorr,r (ω′, t) − Σcorr,a (ω′, t))
ω′ − ω − i0+
– p.25
Pumping through Noninteracting Dot
Wang, Wei, Wang, Guo, APL 79, 3977 (2001)
Blaauboer, PRB 65, 235318 (2002)
Splettstoesser, Governale, JK, Taddei, Fazio, cm/0612257
ratio QS/QN for T = 0
0 0.5 1 1.5 2 2.5 3
ΓS/Γ
N
0
1
2
3
4
5Q
S/Q
N
for all choices of pumping parameters X,Y ∈ ΓN,ΓS, ǫ– p.26
Pumping through Noninteracting Dot
Splettstoesser, Governale, JK, Taddei, Fazio, cm/0612257
ratio QS/QN for T ∼ ΓN
0 1 2 3 4
ΓS/Γ
N
0
5
10
15
20
QS/Q
N
ΓN
,ΓS
ΓS,ε
ΓN
, ε
depends on choice of pumping parameters
– p.27
Outline
Introduction
adiabatic pumping and Coulomb interaction
Nonequilibrium Green’s function approach
(Splettstoesser, Governale, JK, Fazio, PRL ’05)
relate charge to instantaneous Green’s functions
Diagrammatic transport theory
(Splettstoesser, Governale, JK, Fazio, PRB ’06)
systematic perturbation theory in tunnel coupling
Pumping through a metallic island
(Winkler, Governale, JK, in preparation)
Pumping in proximity of a superconducting lead
(Splettstoesser, Governale, JK, Taddei, Fazio, preprint ’06)
– p.28