薛其坤
清华大学物理系
拓扑绝缘体：一种新的量子材料MBE Growth and STM/ARPES Study
OUTLINE 1.拓扑绝缘体简介
Info highway for chips in the future
Conductor Insulator
材料的分类: 能带理论
(固体物理的能带论)
g=1g=2g=3g=0
Valence Band
Conduction Band
Valence Band
Conduction Band
Spin up
Spin down
K
2“band twisting”
Strong spin ‐orbit
coupling
Conductor
Insulator
Topological Insulator
Insulating (bulk)
conducting (surface)
Spin-Orbital Coupling
材料的分类(新): 拓扑能带理论
g=1g=2g=3
g=0
E∝K
pc
E =Massless Dirac Fermions
Effective speed of light v F ~ c /300.
k x
k y
E g
Paradox
without mass ，，•psudo ‐spin
•Klein Paradox •Linear n~E ，Linear σ~E ，Linear m~E •Localization ？•Universal σ？
Spin=1/2
+
−
狭义相对论预期了“自旋轨道耦合”
Helical Spin Structure
Fermions
Four seasons in a day
One night in a year
k
E
Momentum Space
Info
in the future Spintronics?
前沿科学研究
量子反常霍尔效应/自旋霍尔效应磁单极
Majorana 费米子
分数量子统计(Anyon)
拓扑磁性绝缘体Axion 研究……
Dark matter on your desktop?
Wilczek, Nature 458, 129 (2009)
物质≠反物质(CP 不对称)
暗物质(轴子)
标准模型
磁单极(磁荷)
±
e iθ(anyon)
电荷＋磁荷＝任意子(anyon)
Majorana费米子
量子计算：
满足非阿贝尔统计的拓扑准粒子进行位置交换操作
拓扑绝缘体
Zhang et al., Nat. Phys. 5, 438 (2009)
Strong 3D Topological Insulators
Xia et al., Nat. Phys. 5, 398 (2009)
Sb 2Te 3
Bi 2Te 3
Bi 2Se 3
Bi 2Se 3
Δ=0.36eV
Bi2Te3
Fisher (Stanford)
Dirac
Cone
Nat. Phys. 2009
Bulk Insulating Material difficult Thin Films by MBE and MOCVD?
Si, GaAs, Sapphire…
OUTLINE
2.拓扑绝缘体薄膜的分子束外延生长(MBE)及电子结构(Bi
Te3/Bi2Se3/Sb2Te3)
2
扫描隧道显微镜：
由瑞士科学家Binnig和Rohrer博士于1981年发明
人类首次：
9“看到”单个原子、分子
9“操纵”单个原子、分子1986年诺贝尔物理学奖
Fe
tip
sample
A
Ga
As4
Se4
MBE
STM
cryostat
20ML Pb Thin Films
•STM/STS: 4K •ARPES: 1meV •RHEED •5x10‐11Torr
MBE ‐STM ‐角分辨光电子能谱System
STM
MBE
ARPES
Photon energy: 21.2eV(HeI) Energy resolution: 10 meV Angular resolution: 0.2°T=77 K
Experimental parameters
Si wafer
Real ‐Time Electron Diffraction
强度振荡
T Bi >>T Si >T Se(Te)
Se (Te )‐rich
(Se 2/Bi>20)
反射式高能电子衍射
E
E F
0.0
0.1
0.2
Bi Te “intrinsic”
Conduction Band
Valence Band
E F
23 200 nm x 200 nm
Bi2Se3on graphene on SiC
‐120mV 50 QL
OUTLINE
3.扫描隧道显微镜(STM) 研究
拓扑绝缘体的基本性质
TI Vaccum Normal insulator
Boundary
Band Cutting
Topological insulator
p c H mc p c H G
G G G •=+•=σβ
α2
(m=0)
•Massless 2D Dirac Equation
•Boundary /Surface •Time Reversal Symmetry
Moore, Nature 2010
Conductor
Insulator
Topological Insulator
Insulating (bulk)
conducting (surface)
Spin-Orbital Coupling
材料的分类(新): 拓扑能带理论
n-doped Geometry of Ag defects
V=400mV V=150mV V=100mV V=50mV
V=200mV
V=300mV Surface nature of topological states!
电子受到Ag 杂质散射导致的表面驻波
FFT
V=400mV
V=150mV
V=100mV
V=50mV
V=200mV
V=300mV
dI/dV mappings
M
K
SBZ
Г
400 mV
100 mV 50 mV
150 mV 300 mV 200 mV
M
K
K ‐space
Only Γ‐Μdirection
Γ
入射电子
反射电子
Zhang et al., PRL 103, 266803 (2009)
Info highway for chips in the future。