Transport properties of topological insulator heterostructures and ultrathin

films

International Center for Quantum Materials, School of Physics,

Peking University

Jian Wang (王健 )

2013 Sanya

Demonstration of surface transport in a hybrid Bi2Se3/Bi2Te3 heterostructure

Crossover between weak antilocalization and weak localization of bulk states in ultrathin Bi2Se3 films

Molecular beam epitaxial growth of Bi2Te3 and Sb2Te3 topological insulators on GaAs (111) substrates: a potential route to fabricate topological insulator p-n junction

arXiv: 1308.5311

1. Demonstration of Surface Transport in a Hybrid Bi2Se3/Bi2Te3 Heterostructure

Yanfei Zhao

Prof. Qi-Kun Xue, Prof. Xucun Ma, Prof. Ke He,Dr. Cuizu Chang

Prof. Yong Wang

Motivation: heterostructure is important

Topological insulator-superconductor: Majorana fermionsTopological insulator-ferromagnet: QAHE, magnetic monopole etcTopological insulator-trival insulator: topological surface stateTopological insulator-topological insulator: nothing happened?

Experiment: MBE+ARPES

Substrate: • Graphene-terminated SiC (0001) (for ARPES)• Sapphire (0001) (for transport measurements)

ARPES: • He-Iα (21.21 eV)

MBE: • Bi, Se, Te co-evaporation

From Xue’s group

Experiment: TEM

The work was done at department of Materials Science and Engineering, Zhejiang university

(b) (c)

Te

Bi2Se3

Bi2Te3

Bi2Te3

Sapphire

(a)

TEM Images of 1 QL Bi2Se3 / 19 QL Bi2Te3 film

Experiment: TransportPhysical Property Measurement System (PPMS)

Temperature: 1.9K~400K (Dilution: 50mK)

Magnetic Field: 16 Tesla

PhD Student: Yanfei Zhao

Topological InsulatorBi2Se3 thin film

Y. Zhang, Nature Physics (2009)

Y. L. Chen, Science (2009)

Bi2Te3 thin film

Question 1: Is it a 3D topological insulator?

Growing only 1 QL Bi2Se3 film on Bi2Te3 thick films

------ Heterostructure What will happen？

Question 2: If it is a topological insulator,

it behaves like Bi2Se3 or Bi2Te3?

ARPES for Bi2Se3, Bi2Te3 and Heterostructure

Question 1: Is it a 3D topological insulator?

Yes, it is !

Question 2: If it is a topological insulator, the surface state behaves more like Bi2Se3 or Bi2Te3?

Bi2Se3

Resistance -Temperature Behavior

With decreasing temperature (T), Rsq displays metallic behavior at high T region and becomes weakly insulating at low T regime Resistance upturn is due to the coexistence of weak antilocalization and electron-electron interaction PHYSICAL REVIEW B 83, 245438 (2011)

Magneto-resistance Behavior

Nonlinear MR

The behaviors were confirmed by many samples grown by MBE

1 QL Bi2Se3 on the top of heterostructure plays a significant role in the transport property

Linear and non-saturated MRLinear and non-saturating MR

Weak Antilocalization Effect

Weak antilocalization effect in the 1 QL Bi2Se3 / 19 QLs Bi2Te3 film is similar to 20 QL Bi2Se3

Good fit using the HLN formula

2D Weak Antilocalization Effect

2

2 2 2

0

1ln2 4 2 4

B

eBel Bel

After subtracting bulk weak anti-localization effect in the 1 QL Bi2Se3 /

19 QLs Bi2Te3 film, it also behaves similar to 20 QL Bi2Se3

Question 2: If it is a 3D topological insulator,

it behaves more like Bi2Se3 or Bi2Te3?

Transport Measurement:

Answer: Bi2Se3 (Linear MR & WAL) Moreover, 1 QL Bi2Se3 on the top of heterostructure

plays a significant role in the transport property

Solid lines are the results of a combined WAL and EEI theory. Considering the g-factor of 1 QL Bi2Se3 / 19 QLs Bi2Te3 film is not so sure, we fixed g-factor to be 30 and 20. The fitting curves are plotted by blue and orange respectively.

The fitting combined weak antilocalization with e-e interaction theory of 1 QL Bi2Se3 / 19 QLs Bi2Te3 film in low field at T=4K

l F

g-factor

30 -0.32 194nm 0.6

20 -0.25 193nm 0.93

PHYSICAL REVIEW B 83, 245438 (2011)

Summary ARPES experiments provide the direct evidence that the surface state of 1 QL Bi2Se3 / 19 QL Bi2Te3 heterostructure exhibits similar surface state of 20 QL Bi2Se3

Both linear MR and WAL effect have unambiguously shown that the hetero-structure behaves more like Bi2Se3 even though there is only 1 QL Bi2Se3 film grown on 19 QL Bi2Te3 film in the heterostructure

Studying on this TI-TI heterostructure may provide a platform to artificially modulate the bulk and surface electronic structures of TIs respectively and pave a way to design new TI devices

arXiv: 1308.5311

2. Crossover between Weak Antilocalization and Weak Localization of Bulk States in Ultrathin Bi2Se3 FilmsJian Wang

X. C. Xie, H. C. Wang, H. W. Liu, Y. F. Zhao, Y. Sun Q. K. Xue, K. He, X. C. Ma, C. Z. Chang,Z. C. Xia, H. K. Zuo,

Collaborators:

arXiv:1310.5194

Huichao Wang

Motivation

• Bulk states cannot be neglected in present 3D topological insulators

• The study of magnetoresistance in parallel field of topological insulator films is not much

• Pulsed magnetic field (up to 100 T) is an effective way to study topological insulators

Nature Physics 6, 960 (2010): 60 T

Research Status of Transport Properties of Topological

Insulator(TI)

Crossover between weak antilocalization and weak localization of

bulk states in ultrathin Bi2Se3 films Linear magneto-resistance (MR) in perpendicular field

Negative MR phenomena of TI films in parallel magnetic field

Theoretical explanation

Summary

Outline

New quantum materials---Topological Insulator

Topological insulators are a type of materials protected by time-reversal symmetry with special surface states crossing the bulk gap. The surface states has a linear energy dispersion, revealing a spin-polarized Dirac cone. The spin-momentum locked surface states always show weak antilocalization (WAL) effect. X. L. Qi and S. C. Zhang, Phys. Today 63(1), 33 (2010).

M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).

ARPES is a direct way to detect surface state.

Transport properties of topological insulators

PRL 103, 246601 (2009)

Science 329, 821 (2010)

PRB 83, 245438 (2011)Nat. Mater. 2609, 225 (2010)

Universal conductance fluctuation (UCF), weak antilocalization (WAL), A-B effect, SdH oscillations, e-e interaction…

PRB 83, 165440 (2011)

Bulk states of the existing 3D topological insulatorsThe existing three-dimensional TIs are not ideally insulating even at low temperatures.

(molecular beam epitaxy) MBE-grown 45 QLs (quintuple layers) Bi2Se3 films

PHYSICAL REVIEW B 83, 245438 (2011)

Science 329, 821 (2010)

By selective cleaving from Bi2Te3 crystals

TI bulk states with strong spin-orbit coupling(SOC) cannot be neglected

Research Status of Transport Properties of Topological

Insulator(TI)

Crossover between weak antilocalization and weak localization of

bulk states in ultrathin Bi2Se3 films Linear magneto-resistance (MR) in perpendicular field

Negative MR phenomena of TI films in parallel magnetic field

Theoretical explanation

Summary

Outline

Samples----grown by MBE

An ultra-high vacuum MBE-ARPES-STM combined system

for 5QLs Bi2Se3 ultrathin film

Prof. Qi-Kun Xue, Prof. Xucun Ma, Prof. Ke He, Dr. Cuizu Chang

Transport Measurements

• Pulsed High Magnetic Field (PHMF) 15ms rise time and 135 ms descend time. magnetic and current directions are reversed.

• Physical Property Measurement System (PPMS)Scan field mode: linear, driven;Stable magnetic field

Au wires/ In balls as leads

Sapphire

5QL Bi2Se3

Se capping layer

I-V+ V-

I+

R-T Characteristic of Sample 1 (5QLs Bi2Se3 ultrathin film)

PHYSICAL REVIEW B 83, 245438 (2011)

Hall Resistance of Sample 1 (5QL Bi2Se3 thin film)

4.2 K

The sheet carrier density ns~1.89×1013 cm-2

Mobility μ~316.6 cm2/Vs

Linear MR In Perpendicular Magnetic Field

WAL

Linear MR

“it is likely that this linear MR is intrinsically tied to the 2D Dirac electrons occupying the surface state.”

Bi2Se3 nanoribbons by V-L-S mechanism

Xuan P. A. Gao GroupACS NANO 5(9),7510-7516 (2011)

Linear MR is likely due to linear energy dispersion of the gapless topological surface states of quantum origin (Abrikosov’s quantum linear MR model).

Research Status of Transport Properties of Topological

Insulator(TI)

Crossover between weak antilocalization and weak localization of

bulk states in ultrathin Bi2Se3 films Linear magneto-resistance (MR) in perpendicular field

Negative MR phenomena of TI films in parallel magnetic field

Theoretical explanation

Summary

Outline

MR In Parallel Magnetic Field of Sample 1 (5QL Bi2Se3 thin film): crossover

The results by ac measurement and dc measurement in pulsed magnetic field are identical.0.8% decrease from ~25T to 50T at 4.2K;0.6% decrease from ~11T to 50T at 77K

Interestingly, MR of the films are closely related to the relative orientation of the parallel magnetic field and the excitation current.

[110]

[-110]

[001]

V+

I+ I-V-

B[110]

[-110]

[001]

V+

I+ I-V-

B

Sample 2 shows similar properties as sample 1

Weaker negative MR effect.Similar angular dependence on the angle between the parallel field and current.

Control experiments with sample 1 — different crystal orientation

The MR behavior is closely related to the angle between the in-plane magnetic field and the current.

Angular dependence of MR in parallel field

Negative MR is independent of the current intensity and the current direction.

Control experiments with sample 1

Research Status of Transport Properties of Topological

Insulator(TI)

Crossover between weak antilocalization and weak localization of

bulk states in ultrathin Bi2Se3 films Linear magneto-resistance (MR) in perpendicular field

Negative MR phenomena of TI films in parallel magnetic field

Theoretical explanation

Summary

Outline

Quantum interferences in system with strong SOCTwo main parameters dominant for the quantum interference: the phase coherence time and the spin-orbit scattering time SO

3. Smaller SO than 2.

Smaller SO than 1.

SO

G. Bergmann, Phys. Rep. 107, 1 (1984).

WL

WAL(—WL?)

WAL—WL

Crossover from WAL to WL in TI bulk states

222

2 2 20 || ||

1 3N ln 1 ln 12 22

SO

i

LLe RRR L L

[1] B. L. Altshuler, A. G. Aronov, D. E. Khmelnitskii, and A. I. Larkin, Quantum Theory of Solids, edited by I. M. Lifshitz (Mir, Moscow, 1982), P. 130.[2] V. K. Dugaev and D. E. Khmelnitskii, Sov. Phys. JETP 59, 1038 (1984).

2|| 4 BL L l d d

||B I

140L nm 30nmSOL at 4.2 K20nmSOL at 77 K

|| || B I

140L nm 18nmSOL at 4.2 K

40L nm 14nmSOL at 77 K40L nm

BLeB

/ 22Fel k l nm

Explanation for the MR anisotropy

The WAL effect can be suppressed more largely by the perpendicular field than by the parallel field (relative to the motion path).

B//I

Be

Be

I

main trend of the motion paths in the configuration;

Be----the effective magnetic field of electrons due to the carrying momentum;

I----the excitation current in the film plane;

B//----the external magnetic field in the plane;

Compared with the B////I case, the aforementioned differences lead to weaker effective SOC effect and larger effective so in the case of B//┴I .

In SOC mechanism, the moving electron experiences an effective magnetic field Be.

Research Status of Transport Properties of Topological

Insulator(TI)

Crossover between weak antilocalization and weak localization of

bulk states in ultrathin Bi2Se3 films Linear magneto-resistance (MR) in perpendicular field

Negative MR phenomena of TI films in parallel magnetic field

Theoretical explanation

Summary

Outline

I. The crossover from WAL to WL in TI bulk states is demonstrated directly by the parallel field MR behaviors of the 5 QLs Bi2Se3 films, which is quite significant for a better understanding of the existing 3D TI materials.II. Besides, we find novel MR anisotropy under different relative orientations of the parallel magnetic field and current, which can be explained qualitatively in the SOC mechanism. More detailed analysis needs further theoretical investigations.

Summary

arXiv:1310.5194

3. Molecular beam epitaxial growth of Bi2Te3 and Sb2Te3 topological insulators on GaAs (111) substrates: a potential route to fabricate topological insulator p-n junction

AIP Advances 3, 072112 (2013); doi: 10.1063/1.4815972

Dr. Zhaoquan ZengUniversity of Arkansas

AIP Advances 3, 072112 (2013); doi: 10.1063/1.4815972

Encourage experts in applications:Topological insulator (TI) PN junctiondevices on GaAs or TI-GaAs PN junctions1.2D excitons?2.Optical electrics3.Thermoelectrics

“中美科学家在光滑基座上种出拓扑绝缘体”（新华网、人民网、科技日报、中国科学院等）

Eurek Alert

Conclusions

Demonstration of surface transport in a hybrid Bi2Se3/Bi2Te3 heterostructure

Jian Wang Group, SCIENTIFIC REPORTS 3, 3060 (2013); DOI: 10.1038/srep03060 1 (arXiv: 1308.5311 )

Crossover between weak antilocalization and weak localization of bulk states in ultrathin Bi2Se3 films

arXiv:1310.5194 (2013)

Molecular beam epitaxial growth of Bi2Te3 and Sb2Te3 topological insulators on GaAs (111) substrates: a potential route to fabricate topological insulator p-n junction

Jian Wang Group, AIP Advances 3, 072112 (2013); doi: 10.1063/1.4815972 (arXiv:1301.0362)

Thanks !

Huichao Wang Ying XingYanfei Zhao Yangwei

Zhang

My postdoc and PhD candidates:

Dr. Yi Sun

Welcome to Jian Wang’s Group:

http://www.phy.pku.edu.cn/icqmjianwanggroup/index.html

International Center for Quantum Materials, School of Physics,

Peking University

Yi Liu