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)

4

Essential: supercurrent must couple to surface statesCharacterize junction

Motivation

S-wave SC TI

I+ I-V+ V-

5

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

8

Fabrication

9

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

10

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

19

-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?

25

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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