Transport properties of topological insulator
heterostructures and ultrathin films
Jian Wang (王健)
2013 Sanya
International Center for Quantum Materials, School of Physics,
Peking University
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
1. Demonstration of Surface Transport in a Hybrid
Bi2Se3/Bi2Te3 Heterostructure
arXiv: 1308.5311
Prof. Qi-Kun Xue, Prof. Xucun Ma, Prof. Ke He,
Dr. Cuizu Chang
Prof. Yong Wang
Yanfei Zhao
Motivation: heterostructure
is important
Topological insulator-superconductor: Majorana fermions
Topological insulator-ferromagnet: QAHE, magnetic monopole etc
Topological insulator-trival insulator: topological surface state
Topological insulator-topological insulator: nothing happened?
Experiment: MBE+ARPES
ARPES:
• He-Iα (21.21 eV)
MBE:
• Bi, Se, Te co-evaporation
Substrate:
• Graphene-terminated SiC (0001)
(for ARPES)
• Sapphire (0001)
(for transport measurements)
From Xue’s group
Experiment: TEM
TEM Images of 1 QL Bi2Se3 / 19 QL Bi2Te3 film
(a)
(b)
(c)
Te
Bi2Te3
Bi2Se3
Bi2Te3
Sapphire
The work was done at department of Materials Science and
Engineering, Zhejiang university
Experiment: Transport
Physical Property Measurement System (PPMS)
Temperature: 1.9K~400K (Dilution: 50mK)
Magnetic Field: 16 Tesla
PhD Student: Yanfei Zhao
Topological Insulator
Bi2Se3 thin film
Y. Zhang, Nature Physics (2009)
Bi2Te3 thin film
Y. L. Chen, Science (2009)
Growing only 1 QL Bi2Se3 film on
Bi2Te3 thick films
------ Heterostructure
What will happen？
Question 1:
Is it a 3D topological insulator?
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 electronPHYSICAL REVIEW B 83, 245438 (2011)
electron interaction
Magneto-resistance Behavior
Linear and non-saturating MR
Nonlinear MR
Linear and non-saturated 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
Weak Antilocalization Effect
Good fit using the HLN formula
Weak antilocalization effect in
the 1 QL Bi2Se3 / 19 QLs Bi2Te3
film is similar to 20 QL Bi2Se3
2D Weak Antilocalization Effect
    B     0 
 e2
 2
2
 

1

  
ln 

2 
2 



4
Bel
2
4
Bel
 
 
 


After subtracting bulk weak antilocalization effect in the 1 QL Bi2Se3 /
19 QLs Bi2Te3 film, it also behaves
similar to 20 QL Bi2Se3
Transport Measurement:
Question 2:
If it is a 3D topological insulator,
it behaves more like Bi2Se3 or Bi2Te3?
Answer: Bi2Se3 (Linear MR & WAL)
Moreover, 1 QL Bi2Se3 on the top of heterostructure
plays a significant role in the transport property
The fitting combined weak antilocalization with e-e interaction theory of 1 QL Bi2Se3 /
19 QLs Bi2Te3 film in low field at T=4K
PHYSICAL REVIEW B 83, 245438 (2011)
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.
g-factor

l
30
-0.32
194nm
0.6
20
-0.25
193nm
0.93
F
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 heterostructure 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 Films
arXiv:1310.5194
Jian Wang
Collaborators:
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,
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
Outline
 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
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
ARPES is a direct way to detect surface state.
(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).
Transport properties of topological insulators
Universal conductance fluctuation (UCF), weak antilocalization (WAL), A-B
effect, SdH oscillations, e-e interaction…
PRL 103, 246601 (2009)
Nat. Mater. 2609, 225 (2010)
PRB 83, 245438 (2011)
Science 329, 821 (2010)
PRB 83, 165440 (2011)
Bulk states of the existing 3D topological insulators
The existing three-dimensional TIs are not ideally insulating even at low temperatures.
By selective cleaving from Bi2Te3
crystals
Science 329, 821 (2010)
(molecular beam epitaxy) MBE-grown 45 QLs
(quintuple layers) Bi2Se3 films
PHYSICAL REVIEW B 83, 245438 (2011)
TI bulk states with strong spin-orbit coupling(SOC) cannot be
neglected
Outline
 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
Samples----grown by MBE
for 5QLs Bi2Se3
ultrathin film
An ultra-high vacuum MBEARPES-STM combined
system
Prof. Qi-Kun Xue, Prof. Xucun Ma,
Prof. Ke He, Dr. Cuizu Chang
Transport Measurements
• Physical Property Measurement System (PPMS)
Scan field mode: linear, driven;
Stable magnetic field
• Pulsed High Magnetic Field (PHMF)
 15ms rise time and 135 ms descend time.
 magnetic and current directions are reversed.
I+
V+
V-
Se capping layer
5QL Bi2Se3
Sapphire
Au wires/ In balls as leads
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
Linear MR
WAL
Linear MR is likely due to linear energy dispersion of the gapless
topological surface states of quantum origin (Abrikosov’s quantum linear MR
model).
Bi2Se3 nanoribbons
by V-L-S mechanism
“it is likely that this linear MR is intrinsically tied to the 2D Dirac electrons occupying the
surface state.”
Xuan P. A. Gao Group
ACS NANO 5(9),7510-7516 (2011)
Outline
 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
MR In Parallel Magnetic Field of Sample 1 (5QL Bi2Se3 thin film): crossover
Interestingly, MR of the films are closely related to the relative orientation of the
parallel magnetic field and the excitation current.
[-110]
[001]
[-110]
[110]
I+
IV+
[001]
B
V-
I+
IV+
0.8% decrease from ~25T to 50T at 4.2K;
0.6% decrease from ~11T to 50T at 77K
[110]
B
V-
The results by ac measurement and dc
measurement in pulsed magnetic field
are identical.
Sample 2 shows similar properties as sample 1
Control experiments with sample 1
— different crystal orientation
Weaker negative MR effect.
Similar angular dependence on the angle between the parallel
field and current.
Angular dependence of MR in parallel field
The MR behavior is closely related to the angle between the in-plane
magnetic field and the current.
Control experiments with sample 1
Negative MR is independent of the current intensity and the current
direction.
Outline
 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
Quantum interferences in system with strong SOC
Two main parameters dominant for the quantum interference: the
phase coherence time tj and the spin-orbit scattering time tSO
3. Smaller
t
SO
than 2.
WAL(—WL?)
t than 1.
WAL—WL
2. Smaller
1. tj < t
SO
SO
WL
G. Bergmann, Phys. Rep. 107, 1 (1984).
Crossover from WAL to WL in TI bulk states
B||  I
B|| || I
Lj  140nm LSO  30nm at 4.2 K
Lj  140nm LSO  18nm at 4.2 K
Lj  40nm LSO  20nm at 77 K
Lj  40nm LSO  14nm at 77 K
e2 R
R
 Ni
R0
2 2
1 
L2SO  
Lj2  3 
 ln 1  2   ln 1  2  
L||  2 
L||  
 2 
L||  4 L2B l d d
LB 
eB
  el / k F  l  22nm
[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).
Explanation for the MR anisotropy
In SOC mechanism, the moving electron experiences an effective magnetic field Be.
The WAL effect can be suppressed more largely by the perpendicular field than by the
parallel field (relative to the motion path).
I
Be
I
B//
Be
Compared with the B////I case, the aforementioned differences lead to weaker effective SOC
effect and larger effective tso in the case of B//┴I .
I----the excitation current in the film plane;
B//----the external magnetic field in the plane;
Be----the effective magnetic field of electrons due to the carrying momentum;
main trend of the motion paths in the configuration;
Outline
 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
Summary
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.
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
Dr. Zhaoquan Zeng
University of Arkansas
AIP Advances 3, 072112 (2013);
doi: 10.1063/1.4815972
AIP Advances 3, 072112 (2013);
doi: 10.1063/1.4815972
Encourage experts in applications:
Topological insulator (TI) PN junction
devices on GaAs or TI-GaAs PN junctions
1.2D excitons?
2.Optical electrics
3.Thermoelectrics
Eurek Alert
“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）
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)
International Center for Quantum Materials, School of Physics,
Peking University
My postdoc and PhD candidates:
Dr. Yi Sun
Yanfei Zhao
Huichao
Wang
Ying Xing
Yangwei
Zhang
Welcome to Jian Wang’s Group:
http://www.phy.pku.edu.cn/icqmjianwanggroup/index.html
Thanks !
Yi Liu