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Charge and Statistics of Dilute Laughlin Quasiparticles
Transcript of Charge and Statistics of Dilute Laughlin Quasiparticles
Charge and Statistics of Dilute Laughlin Quasiparticles
C. L. Kane University of Pennsylvania
I. Introduction
II. Transmission of Dilute Quasiparticlesthrough a point contact
• Laughlin Quasiparticle and Shot Noise
• Corbino Geometry (Moty Heiblum et al. ’03)• Telegraph Noise and fractional statistics• Luttinger Liquid Theory
• Three Terminal Geometry for preparing“dilute quasiparticle beam”
(Moty Heiblum et al. ’02)• Shot Noise Puzzle• Luttinger Liquid Theory
PRL 90, 226802 (03)
III. Proposed measurement of fractional statistics: Telegraph Noise
PRB 67, 45307 (03) w/ Matthew Fisher (ITP)
Laughlin Quasiparticle for ν = 1/m
• Energy Gap• Quantized Hall Conductance
Fractionally Charged Excitations
xy xydQ dE ddt dt
σ σ Φ= ⋅ =∫
( / ) / *xyQ h e e m eσ= = ≡
ν = 1/m
σxx = 0σxy = (1/m)e2/h
Fractional Statistics
• Phase Θ = π/m under interchange:
• Phase when one circles another:
e*
e*
Halperin 1984;Arovas, Schrieffer, Wilczek 1984
ie ΘΨ → Ψ
2ie ΘΨ → Ψ
• Signature of topological orderThouless 1989;Wen, Niu 1990
m-fold topologicaldegeneracy on torus
Measurement of Fractional Charge1. Capacitive Measurement
Resonant Tunneling Through Anti-dotGoldman & Su (Science, 1995)
∆VBG = e*/C
2. Shot Noise MeasurementBackscattering at Quantum Point Contact
Kane, Fisher (PRL 1994);De Picciotto et al. (Nature 1997); Glattli et al. (PRL 1997)
ItV
0<I2>ω~0 = e* Ib<I2>ω~0 = e It
Ibe
-e-e*
e* 0
V
Strong Pinch Off Weak Pinch Off
Electron Tunneling Quasiparticle Tunneling
Three Terminal ConfigurationComforti et al. (Nature 2002)
• When QPC2 is open, a “Dilute Beam” of quasiparticles is isolated in lead 3.
• Transport of Dilute beam through QPC2 probestransmission of individual quasiparticles.
Transmission of Dilute QuasiparticlesThrough a Point Contact
Dilute : q ~ .45 e
Not Dilute : q ~ e
Can Dilute Charge e/3 Quasiparticles Traverse a Nearly Opaque Barrier ?
q
t1
Comforti et al. (Nature 2002)
Luttinger Liquid Model : ν = 1/m
[ ]3
2
1
1 1 2 2 2 3
( )4
v cos( ) v cos( )
Fi x i i
i
mvH dx x
Vt
φπ
φ φ φ φ=
= ∂ +
− − + −
∑∫
Analysis:
QPC1:• Perturbative for small v1 (Dilute Limit)
QPC2:• Perturbative for small v2
• Perturbative for large v2 (small t2)• Exact Solution for m=2 via fermionization
Perturbative Analysis1. Small t2 (T<<V)
2. Small v2 (T<<V)
2 2 2 2/ 31 2( ) v t m mI V c V + −=
2 2 2 2/ 31 2( ) v t m mS V ec V + −=
2 21 2
1 21 2/ 2 2 /
v v( , ) 1m mI V T c cV T− −
⎡ ⎤= −⎢ ⎥
⎣ ⎦2 21 2
1 31 2/ 2 2 /
v v( , ) * 1m mS V T e c cV T− −
⎡ ⎤= −⎢ ⎥
⎣ ⎦
22
2-2/m
v*T
Q e c= +
Q e=
Perturbation theory diverges for T=0 even for fixed finite V.
Exact Solution
1. Zero Temperature:
For m=2 map problem to free fermions
2
in 42
2( ) ( ) 1 ( )16vVI V I V K
π⎛ ⎞
= − −⎜ ⎟⎝ ⎠
( ) ( )S V eI V=
Transmission through QPC2
V/v22
Q = e , independent of v2, even when transmission through QPC2 is nearly perfect.
2. Finite Temperature:
Limits T 0 and v2 0 do not commute:
Q(T 0,v2=0) = e/m
Q(T=0,v2 0) = e
Interpretation: Andreev ReflectionThree possible scattering products
for an incident quasiparticle
1. Transmission
Probability T
2. Reflection
Probability R
3. Andreev Reflection
Probability A
Theory predicts T=0 at zero temperature• Strong Pinch-off: R~1, 0<A<<1• Weak Pinch-off: R=1-1/m, A=1/m
Observe Andreev processes by measuring correlations between transmitted and reflected currents.
What about the experiments?
0.0 0.2 0.4 0.6 0.8 1.00.0
0.2
0.4
0.6
0.8
1.0
23mK 20% Dilution23mK 10% Dilution40mK 20% Dilution74mK 20% Dilution
Effe
ctiv
eC
harg
e, q
2/e
Transmission of QPC2, t2
Theoryfor T 0
The measured charge does approach e for small t2 and low temperature, but “high” temperaturedata remains unexplained.
• Role of irrelevant operators, eg. (dφ/dx)3
• Role of smooth edges near point contact.
Fractional Statistics
“h/e”
• Phase Θ = π/m under interchange:
• Phase when one circles another:
e*
e*
e*
• Model: Bosons + Statistical Flux
Halperin 1984;Arovas, Schrieffer, Wilczek 1984
*2 ( / )
2 /
e h e
mπ
Θ =
=
ie ΘΨ → Ψ
2ie ΘΨ → Ψ
Measurement of Fractional StatisticsQuantum Interference Necessary To
Measure Statistical Phase
Two Point Contact InterferometerChamon et al. (1997)
Φ
I = I0 + ∆I cos[e* Φ /h + 2Θ Nqpenc]
Aharonov Bohm Phase Statistical Phase
I
• Net phase changes by 2Θ = 2π/m when qp tunnels from outside ring to inside of ring.
• Equilibrium h/e* oscillations eliminated by qp tunneling.
Y. Ji, Y. Chung, D. Sprinzak, M. Heiblum, D. Mahalu and H. Shtrikman, Nature (03)
A “Mach-Zehnder Interferometer”
An Ohmic contact in the middle of a ring
D1
S
BS1 M1
M2 BS2
D2
airbridge
QPC1
MG1
SD1
QPC2
mesa
MG2
D2
0 50 100 150 2000
5
10
15
-9.0 -7.5 -6.0 -4.5 -3.0
Time (minute) ~ Magnetic Field
Modulation Gate Voltage, VMG (mV)
Curr
ent
(a.u
.)
ν = 1
Telegraph Noise in ring with inner contact:A Direct Signature of Fractional Statistics
• Net phase changes by 2Θ = 2π/m when qp passes from lead 1 or 2 to lead 3.
• m-state telegraph noise for n = 1/m
• Related to m-fold “topological degeneracy”
• <I2>ω~0 = e* ∆I2/I3
Chiral Luttinger Liquid Model
( * ) ( * )*v vi e V t i e V tU e eα α α αφ φα α α α ακ κ∆ − − ∆ −+ −= +
Tunneling at QPCα :
Klein Factors
( )( )
232
2 2~03 3
coth * / 2*2 1 sin / sinh * / 2
e V Te III e V Tω
∆=+ Θ
Low Frequency Noise :
[ ]3 3
2
1 1( )
4F
i x i ii
mvH dx x Uαα
φπ = =
= ∂ +∑ ∑∫
2
3
*~2
e II∆
2 2
23
*~4 sin
e IG T
∆Θ
e*V3 >> kT
e*V3 << kT
Conclusion• Fractionally charged quasiparticles can not traverse
a nearly opaque barrier. At T=0 they can’t even getpast a nearly perfectly transmitting barrier!
• Telegraph Noise is predicted for a ring with an inner contact. It is a direct consequence of a fractional statistical phase, and has a unique signature in the low frequency noise.
Questions• Origin of observed suppression of transmitted
charge for moderately weak tunneling and moderately low temperature?
Smooth edges?Irrelevant operators?
• Observation of Andreev processes?
• Exact Solution for n=1/3?
• Telegraph Noise for Hierarchical Quantum Hall States?
• Effect of dephasing on edge state transport?