Post on 25-Feb-2016
description
Josephson supercurrent through a topological insulator surface stateNb/Bi2Te3/Nb junctions
Marieke SnelderMESO 2012 - Chernogolovka
17 June 2012
AcknowledgementsMESA+ Institute for Nanotechnology, University of Twente, The NetherlandsM. Veldhorst (first author)M. HoekT. GangW. van der WielA.A. GolubovH. Hilgenkamp (also from Leiden University, The Netherlands)A. Brinkman
High Field Magnet Laboratory, Nijmegen, The NetherlandsU. ZeitlerV. K. Guduru
University of Wollongong, AustraliaX. L. Wang
2
Motivation
S-wave SuperconductorTI surface states
3
Motivation
P-wave Superconductor
Natural place to look for Majorana fermions(single zero-energy modes)
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Essential: supercurrent must couple to surface statesCharacterize junction
Motivation
S-wave SC TI
I+ I-V+ V-
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Part 1 Bi2Te3
Part 2 S/TI/S junctionsPart 3 Josephson supercurrent through the
surface states
Content
Part 1 Bi2Te3
Part 2 S/TI/S junctionsPart 3 Josephson supercurrent through the
surface states
Content
Bi2Te3
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Fabrication
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Mechanical cleavagePhotolithography contacts
0 2 4 6 8 10
-4
-2
0
Hal
l Res
ista
nce
()
Magnetic field (B)
n=8.3 x 1019 cm-3
μ=250 cm2/Vs(bulk is conductive)
lmfp=22 nm
Hall measurements
μB>>1So no quantum oscillations expected
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Shubnikov-de-Haas oscillations
Left graph: Quantum oscillations @T=4.2KRight graph: Oscillations from 2D channel
B =Bcos(θ)T
0 10 20 30 40 50 60 70 80 900.0
0.5
1.0
1.5
2.0
2.5
3.0
1/B
(nor
mal
ized
)Angle ()
Landau level n=1-6 2D
3D
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0 5 10 15 20 25 30 3534
36
38
40
42
44
46
48
60o50o
40o30o20o10o
Res
ista
nce()
Magnetic field (T)
0o
0 5 10 15 20 25 30 3534
36
38
40
42
44
46
48
60o50o
40o30o20o10o
Res
ista
nce()
Magnetic field (T)
0o
Shubnikov-de-Haas oscillations
Left graph: Dingle temperature 1.65 K, μ=8300 cm2/VsRight graph: Effective mass 0.16m0
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0 1 2 3 4 5 60
1
2
3
4
5
6
7
R (m
)
Magnetic field (T)0 2 4 6 8 10 12 14 16 18 20 22
0.0
0.2
0.4
0.6
0.8
1.0
Res
ista
nce
(R0K
)Temperature (K)
0 5 10 15 20 25 30 3534
36
38
40
42
44
46
48
60o50o
40o30o20o10o
Res
ista
nce()
Magnetic field (T)
0o
Shubnikov-de-Haas oscillations
B =Bcos(θ)T
I.M. Lifshitz and A.M. Kosevich, Sov. Phys. JETP 2, 636 (1956) 13
Lifshitz-Kosevich formalism
14
;Former is 0.01, later is 10
So in normal case nth minima=0 through 1/B=0
-1 0 1 2 3 4 5 6 7 80.000.020.040.060.080.100.120.140.16
1/B
(T-1)
nth minima xx
Lifshitz-Kosevich formalism
MinimaMaxima
• Surface states present• @ 1/B=0, n=-1/2 (Berry phase of π) linear dispersionrelation• lmfp=105 nmNon-trivial surface
states present
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Part 1 Bi2Te3
Part 2 S/TI/S junctionsPart 3 Josephson supercurrent through the
surface states
Content
S/TI/S junctions
Nb NbBi2Te3Bi2Te3
50 100 150 200 250 300 nmJunction lengths
Length
Nb: blueBi2Te3: redWidth=500 nm
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Hallmarks for a Josephson junction:1)Shapiro steps2)Modulation Ic versus B-field
Josephson supercurrent
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First hallmark – Shapiro steps
• Microwave frequency ω• @2eV/ħ=nω Shapiro
steps• (Energy Cooper pairs
resonant to energy microwave)
ω = 10 GHzV= n x 20.7 μV
T=1.6 K
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-50 0 50-300
-200
-100
0
100
Cur
rent
(A
)
Voltage (V)
Increasing powershifted for clarity
First hallmark – Shapiro steps
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Second hallmark – Ic-B modulation
B
I
Modulation Ic
DC SQUID oscillations
Josephson supercurrent present
-40
-30
-20
-10
0
10
20
30
40
-7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7
-12 -9 -6 -3 0 3 6 9 12
Crit
ical
cur
rent
(A
)
L=5
1.3=0.2
Flux(/0)
Applied field (T)
M. Veldhorst, C. G. Molenaar et al. Appl. Phys. Lett. 100, 072602 (2012)21
-30 -20 -10 0 10 20 300
5
10
15
20
25
30
Crit
ical
cur
rent
(A
)
Magnetic field (mT)
LxW 50nmx500nm
-30 -20 -10 0 10 20 300
5
10
15
20
25
30
35
Crit
ical
cur
rent
(A
)Magnetic field (mT)
LxW 100nmx500nm
-30 -20 -10 0 10 20 300
5
10
15
20
25
30
Crit
ical
cur
rent
(A
)
Magnetic field (mT)
LxW 50nmx500nm
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Second hallmark – Ic-B modulation
-30 -20 -10 0 10 20 300
5
10
15
20
25
30
Crit
ical
cur
rent
(A
)
Magnetic field (mT)
LxW 50nmx500nmB
I
Area uncertain:• Penetration depth• Flux focussingSinc function only valid for large L and W ratio
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Second hallmark – Ic-B modulation
Part 1 Bi2Te3
Part 2 S/TI/S junctionsPart 3 Josephson supercurrent through the
surface states
Content
Link supercurrent and surface states
Surface states
Supercurrent
We have junctions between 50 and 250 nm and:lmfp=22 nm (bulk states) → diffusive transport
lmfp=105 nm (surface states) → ballistic transport
Do we have a supercurrent through the surface states?
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0 50 100 150 200 250 300
1
10
Measurements
Crit
ical
cur
rent
(A
)
Junction length (nm)
0 1 2 3 4 5 60
5
10
15
20
25
30
35 Dirty limit,
Usadel Clean limit,
Eilenberger Measurements
Crit
ical
cur
rent
(A
)Temperature(K)
0 50 100 150 200 250 300
1
10
Measurements
Crit
ical
cur
rent
(A
)
Junction length (nm)
Dirty or clean?
0 50 100 150 200 250 300
1
10
Dirty limit, Usadel
Clean limit, Eilenberger
Measurements
Crit
ical
cur
rent
(A
)
Junction length (nm)
0 1 2 3 4 5 60
5
10
15
20
25
30
35 Dirty limit,
Usadel Clean limit,
Eilenberger Measurements
Crit
ical
cur
rent
(A
)Temperature(K)
Link supercurrent and surface states
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Definitely clean
0 1 2 3 4 5 60
5
10
15
20
25
30
35 Dirty limit,
Usadel Clean limit,
Eilenberger Measurements
Crit
ical
cur
rent
(A
)Temperature(K)
Josephson supercurrent has been realized through the surface states of Bi2Te3
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Provides prospects for Majorana devices……but
What is the best topological insulator for this purpose? (stability, insulating in the bulk, Dirac cone in the gap)What is the smoking gun experiment with 3D topological insulators to observe Majorana fermions?
M. Veldhorst, M. Snelder et al. Nature Materials, 11, 417–421 (May 2012)
Lifshitz-Kosevich formalism
E
Maxima resistivity Minima resistivity
k
Extrapolating till 1/B=0→nth minima in resistivity is zero at 1/B=0 in ‘normal case’ (Berry phase=0)
Lifshitz-Kosevich formalism
-1 0 1 2 3 4 5 6 7 80.000.020.040.060.080.100.120.140.16
1/B
(T-1)
nth minima xx
MinimaMaxima
Extrapolating till 1/B=0→Berry phase is π
AFM Bi2Te3 flakes
Step are unit cells
First hallmark – Shapiro steps
Fitting Shapiro steps with Bessel functions
Only can be done if IcRn product is larger than the position of the steps (20.7 μV in these measurements)
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