Quantum transport phenomena in

3D topological insulator thin films

Ryutaro Yoshimi A,

A. Tsukazaki B, M. Mogi A, K. Yasuda A,

Y. Kozuka A, J. Falson A, J. G. Checkelsky D, K. S. Takahashi C,

N. Nagaosa A, C, M. Kawasaki A, C, and Y. Tokura A, C

Univ. of Tokyo A, Tohoku Univ. B, RIKEN-CEMS C, MIT D

1June 3rd 2015 Workshop on

“New Perspectives in Spintronic and Mesoscopic Physics”

Introduction to Topological Insulator

Bulk Band

Bulk Band

Surface

State

E

Topological Insulator

Conventional Insulator Topological Insulator

(e.g. Bi2Te3, Sb2Te3)

Bulk Band

Bulk Band

Metal

Bulk Band

EF

k

EF EF

・Bulk (inside the sample) is insulating

・Surface is conductive

conductive insulating

Characteristics of topological surface state 3

Two-dimensional surface gapless band

- massless Dirac dispersion

- spin momentum locking

- gapless under

time reversal symmetry

Dissipationless spin-current

on the surface

Future application to spintronics device or quantum computation

Momentum space

up

down

Spin

Real space

D. Hsieh et al.,

Nature (2009)

Bulk VB

Bulk CBsurface state

Experiment on bulk single crystal 4

σ3D

G2D

thickness: d

Bi2Se3, Bi2Te3, Sb2Te3, …..

・Small band gap (~ 250 meV)

・Bulk residual carrier by crystal defects

Y. Ando, Review in JPSJ (2013)

Reduction of bulk conduction

・Exfoliation (reduction in d)

・Find insulating compound

(reduction in σ3D)

D. Hsieh et al.,

Nature (2009)

Bi2Se3

Gtotal = G2D + σ3D x d

bulk

conduction

surface

Complete suppression of bulk transport in thin film 5

EF

・Thin film device enable us to access the ideal

Dirac states of TI without bulk state

・Surface/interface degrees of freedom shows up

Surface/interface degrees of freedom

TI

Substrate

surface

interface

Thin film + FET device

・Surface dominant transport

・EF tuning in the Dirac state

・Controlling surface/interface

(lift the degeneracy)

by layered structureSubstrate

TITI

Quantum transport in surface/interface states of TI 6

Quantum Hall Effect Quantum Anomalous

Hall Effect

+1

0

-1

J. G. Checkelsky, R. Yoshimi et.al.

Nature Physics, 10, 731 (2014)

Bulk insulating thin film

FET device for EF tuning

R. Yoshimi, et. al

Nature Commun. 6,6627 (2015)

Thin film

device fabricationTunneling

Spectroscopy

Detection of

interface Dirac state

R. Yoshimi et al.,

Nature Materials13 253 (2014)

Quantization at zero field

(Bi1-ySby)2Te3

Crx(Bi1-ySby)2-xTe3

InP substrate

QHE in semi-

magnetic TI bilayer

R. Yoshimi et al., submitted.

QHE at higher temperature

EF

mag

non-mag

6

5

4

3

2

1

0

Rxx (

k

)

3000

T (K)

300 300 300 300 300 300

x = 0 x = 0.4 x = 0.6 x = 0.8 x = 0.9 x = 0.94 x = 1

x 0.5

(Bi1-xSbx)2Te3 (BST) thin film 7

EF

EF

-300

-200

-100

0

100

200

300R

yx (

)

90

B (T)

9 9 9 9 9 9

x 0.1

x 0.1x 0.2

T = 2 K

Bi2Te3 Sb2Te3

EF

・Substrate: InP (lattice matches)

・Carrier density widely controlled

with Sb content

T = 2 K

20 nm500 nm

0nm 8.6nm

4.4nm

8.6nm

AFM

Molecular Beam Epitaxy (MBE)

RHEED

Surface/Interface Dirac states in topological insulator 8

Non TI (ν = 0)

TI (ν = 1)

”Surface”

”Interface”

D. Hsieh et al., Nature (2009)

ARPES

Bi2Se3

T. Hanaguri et al.,

PRB (2010)

STM

- Surface Dirac states are verified by ARPES and STM

- Dirac state at interface is not yet clearly observed

Edge states at the boundary of TI

vacuum (ν = 0; Non TI)

Surface Dirac state

Film

Substrate

Interface Dirac states and its Landau level

formation can be spectroscopically detected

Detecting the interface Dirac state of TI 9

- Thin depletion layer acts as the tunneling barrier

- Tunneling conductance is proportional to the interface density of states

Tunneling spectroscopy on TI/non-TI p-n heterojunction

Dirac state

Non TI TI

-40

-20

0

20

40

I (m

A)

-0.3 -0.2 -0.1 0.0 0.1 0.2 0.3

V (V)

T = 10 KB = 0 T

x = 1 (Sb2Te3)n - InP p-(Bi1-xSbx)2Te3

FilmSubstrate

-Typical Esaki-diode character in I-V curve

R. Yoshimi et al., Nature Materials 13 253 (2014)

dI/d

V (

a.u

.)

0.200.150.100.050.00

V (V)

x = 1

dI/dV in (Bi1-xSbx)2Te3/InP junctions 10

A

B

C

Dirac state

A B CInP Sb2Te3

Beyond “B”

Electrons flow

through Dirac state

d

I/d

V (

B)

(a.u

.)

0.250.200.150.100.050.00

V (V)

1 T

14 T

dI/d

V (

a.u

.)

x = 1 (Sb2Te3)

B = 0 T

14 T

T = 10 K

Landau level formation observed in tunneling spectra11

- Oscillatory feature in ΔdI/dV

- Landau levels (n = 0, ±1) due to interface Dirac dispersion (∝ B1/2)

Dirac point

(g ~ 11)

+1

0

-1

ΔdI/dV (B)= dI/dV(B) - dI/dV(0 T)

Tunneling conductance dI/dVR. Yoshimi et al.,

Nature Materials

13 253 (2014)

Ambipolar transport in FET device (x = 0.84) 12

30

20

10

0

-10

Rxx, R

yx (

k

)

-4 -2 0 2 4

VG - VCNP (V)

T = 40 mKB = 3 T

Ryx

Rxx

x = 0.84

・Ambipolar conduction by gate tuning

EF is modulated across Dirac point

・mobility μ ~ 1,500 cm2/Vs, n ~ 4 x 1011 cm-2

EF

electronhole

R. Yoshimi, et. al

Nature Commun.

6,6627 (2015)

Charge neutral

point

・AlOx capping before patterning

・Low temperature process (< 120 Co)

・Gate dielectric by ALD at R.T.

Transport measurement in dilution fridgeFET device

Quantum Hall Effect in BST 13

ν = -1

ν = 0

・Quantum Hall plateau σxy= ±e2/h (B = 14 T)

・2D nature of Dirac states

・At charge neutral point, σxy = 0 (ν = 0) QHE

ν = +1

ν = 0

R. Yoshimi, et. al

Nature Commun. 6,6627 (2015)

Temperature dependence 14

ν = ±1 ： Rxx is metallic

ν = 0 ： Rxx is insulating

R. Yoshimi, et. al

Nature Commun.

6,6627 (2015)

ν = 0 state by LL at top/botom Dirac state 15

・weak B dependence in σxy plateau

・Different environment of top and

bottom state→ δDP~ 50 meV

σxy = 0

Etop bottom

Landau levels at top and bottom surfacePhase diagram of QH states

n = 0

n = 1

Potential difference

δDP~ 50 meV

EF

InP sub.

(Bi1-xSbx)2Te3

top

bottom

Stabilization of ν = 0 state in layered structure 16

InP sub.

Sb rich

InP sub.

ν = 0

(Bi1-xSbx)2Te3

(Bi1-xSbx)2Te3

ν = 1ν = 0

B = 14 T

ν = 1ν = -1

Structure Conductivity mapping

J. G. Checkelsky,

R. Yoshimi et al.,

Nature Phys. (2014)

Observation of Quantum Anomalus Hall Effect

InP substrate

Crx(Bi1-ySby)2-xTe3

Ryx = RyxA = h/e2

Rxx = 0

M

Top Bottom

B = 0 T EF

Crx(Bi1-ySby)2-xTe3

C-. Z. Chang et al., Science 340 167 (2013).

Residual Rxx

Rxx

Ryx

・Spontaneous gap opening by magnetism

・Quantization of Anomalous Hall term

QAHE

・QAHE with help of magnetic field

(Rxx at 0 T ~ 0.5 h/e2)

QHE in magnetic/non-magnetic TI bilayer 18

Ryx = R0B + RyxA

Ordinary

Hall termAnomalous

Hall term

BST CBST

-20

-10

0

10

20

Ryx (

k

)

-2 -1 0 1 2B (T)

R0B

Ryx

AT = 0.5 K

VG = -1.3 V

-15 0 15B (T)

-1

0

1

Ryx (h

/e2)

-20

0

20

Ryx (

k

T = 0.5 K

VG = -7.0 V -4.0 V -1.3 V 0.2 V 1.1 V 2.1 V 5.0 V

B = 0 T

EF

Top Bottom

・Ryx reaches h/e2 by adding up OHE and AHE

・Band structure

gapped CBST surface

gapless BST surface

Transport Phenomena at 0T

3

2

1

0

xx (

e2/h

)

-1

0

1

xy (

e2/h

)

-4 0 4

VG (V)

120

80

40

0

Rxx (

k

)

-20

0

20

Ryx (

k

)

-4 0 4

VG (V)

-1

0

1R

yx (h

/e2)

T = 0.5 K

B = 0 T

EF

Top Bottom

gap of CBST layer

Magnetic field dependence

120

80

40

0

Rxx (

k

)

-20

0

20

Ryx (

k

)

-4 0 4

VG (V)

-1

0

1R

yx (h

/e2)

B = 0 T

T = 0.5 K

2 T

6 T8 T

10 T

14 T12 T

4 T

3

2

1

0

xx (

e2/h

)

-1

0

1

xy (

e2/h

)

-4 0 4

VG (V)

EF

Top Bottom

ν = 0

ν = +1

ν = 0

ν = +1

・ν = +1, 0 states due to LL formation are observed

Summary 21

QHE in BSTTunneling

Spectroscopy

Detection of

interface

Dirac state

R. Yoshimi et al.,

Nature Materials13 253

(2014)

FET device of

BST thin film

QHE with ν = ±1, 0

R. Yoshimi, et. al

Nature Commun.

6,6627 (2015)

QAHE in CBST

J. G. Checkelsky,

R. Yoshimi et al.,

Nature Phys. (2014)

Cr-doped BST

Quantization

at B = 0 T

QHE in bilayer

R. Yoshimi et al.,

submitted.

QHE at higher

temperature