Recent progress on topological insulator nanowires----Hefei Institutes of Physical Science, The Chinese Academy of Sciences

Topological insulator (TI) is a new state of matter in condensed matter physics predicted theoretically and verified by experiments recently. Exploration or manipulation of the surface states of the TI is a focus because of its relevant for applications on improved spintronic devices and potentially useful for quantum computing. However, direct probing the topological nature by electrical transport is still a major challenge. This is because the electrical transport measurement is a bulk sensitive experiment, even small imperfections in the as-grown near-stoichiometric crystals will result in a metallic behavior in bulk and thus overwhelm or mask the surface contribution. Prof. TIAN Mingliang’s group in China High Magnetic Field Laboratory (CHMFL), Hefei Institute for Physical Science of Chinese Academy of Sciences has made a series of progresses in probing the surface states of quasi-one dimensional (Q1D) TI nanowires by the electrical transport technique.
Compared with three dimensional bulk TIs, Q1D TI nanowires show a number of advantages in probing the surface state due to its unique curved cylindrical geometry and extreme large surface to volume ratio. Dr Wei Ning in Tian’s group recently carefully investigated the angle-dependent magnetoconductance of single-crystal Bi2Te3 nanowire with various diameters. It was found that the magnetoconductance of these nanowires in low magnetic field regime can be well described by 1D weak antilocalization (WAL) model due to the size confinement effect which is in contrast to the 2D nature in 3D bulk system, where the dephasing length of the electrons follows T-1/3 dependence. Meanwhile, such a 1D surface WAL was found to be enhanced significantly with the decrease of the wire diameter.
More interestingly, in the high magnetic field regime, Tian et al. detected both the predicted anomalous Aharonov-Bohm (AB) quantum oscillations in parallel field and the 1/2-shifted Shubnikov de Haas (SdH) oscillations in perpendicular field in an individual 50 nm diameter wire. These data provide an unambiguous evidence of coexistence of the nontrivial topological Dirac state and trivial electron state on the surface of Tl nanowires , where the topological surface state inTI nanowires is robust regardless of the surface oxidations.
These discoveries open a new avenue to understand the evolutions of surface states in 1D scale and to explore the possible Majorana fermions in heterostructures of TI nanowire and superconductor. The related works were published on Scientific Reports respectively as indicated below..
Any correspondence and requests for materials should be addressed to M.T. or W.N ( tianml@hmfl.ac.cn or ningwei@hmfl.ac.cn)

Appendix:
Dual evidence of surface Dirac states in thin cylindrical topological insulator Bi2Te3 nanowires
(http://www.nature.com/srep/2013/130206/srep01212/full/srep01212.html)
One-dimensional weak antilocalization in single-crystal Bi2Te3 nanowires
（http://www.nature.com/srep/2013/130328/srep01564/full/srep01564.html）

Figure 1. (top left )SEM and TEM images of Bi2Te3 nanowires with diameter of 50 nm; (top right) SdH quantum oscillations in perpendicular magnetic field; (bottom right) the Landau index number versus reciprocal magnetic field, indicating two kinds of SdH oscillations in surface states. (bottom left) AB quantum oscillations indicating the surface nature of the conduction.

Figure 2 The angle-dependent magnetoconductance of Bi2Te3 nanowires with diameter of 300 nm, 100 nm and 50 nm. The solid red lines are the fit with one-dimensional localization theory. All the traces were normalized to a single curve in low field range, indicating the surface nature of the magnetoconductance.