Transcript Slide 1
Observations of Metal-insulator Transition and Strong Surface States in Bi
2-x
Sb
x
Se
3
Thin Films Cheng Zhang, Xiang Yuan, Weiyi Wang, Yanwen Liu and Faxian Xiu*
Department of Physics and State Key Laboratory of Surface Physics Fudan University, Shanghai, 200433, China
Introduction
Topological Insulators (TIs)
Spin-orbit coupling
Massless Dirac fermion
Linear dispersion
Time reversal symmetry protected surface/edge state (a)
Metal-insulator transition
(b)
x=1.6
x=1.4
x=1.3
x=1.2
x=0.8
x=0
Y. Chen et al. Science 325, 2009
Challenges in TIs
Origin of topological nontrivial state:
Band inversion driven by strong spin-orbit interaction (SOI) (Z 2 topological insulator)
x=1.3
x=1.4
x=1.6
At low doping regime, the metallic behavior of R s vs temperature comes from the degenerate semiconducting bulk. At high doping regime, the insulator behavior indicates Fermi level lies in the gap.
(a) (b) (d)
Mainly happen in narrow gap semiconductors (Bi 2 Se 3 , Bi 2 Te 3 , and Sb 2 Te 3 )
Ioffe-Regel criterion
(c) (e)
Bulk conduction vs surface conduction, not a real insulator in the bulk
Motivation
Band engineering by doping Bi 2 Se 3 with Sb
Achieving high surface conduction for further TI based spintronic device
Topological phase transition by decreasing SOI strength
Results
(a)
x=0 x=0.8
x=1.2
x=1.3
x=1.4
x=1.6
Quantum oscillation in High Doping Sample
(b)
5.00 nm
(a)
3 2.5
x= 0
1.5
2 1 0.5
0 0 3 2.5
0.5
1 X[ 祄 ] 1.5
2
x= 1.2
1.5
1 2 0.5
0 0 5 0.5
1 X[ 祄 ] 1.5
2 4 2.5
3 0.00 nm 6.00 nm 2.5
3 0.00 nm 12.00 nm
x= 1.4
2 3 1 0 0 1 2 3 4 5 0.00 nm
(b) A B
Se and Bi-Se bonds merges together, resulting in the observed shift of peak B.
(c) (d) (e)
0 ° 30 ° 45 ° 60 ° 90 ° Characterizations of Bi 0.6
Sb 1.4
Se 3 thin films, indicating a orthorhombic lattice structure. Shubnikov-de Hass oscillations analysis of the surface states of Bi 0.6
Sb 1.4
Se 3 , evident proof for the existing of Dirac fermion.
Conclusion
● ● ●
Increasing the Sb doping induces a metal-insulator transition; Close to the critical point of metal-insulator transition, strong SdH oscillations are observed, suggesting the presence of topological surface state; Our results demonstrate a feasible approach to suppress the bulk influence and manipulate the band structure while maintaining topological nontrivial surface.