Electrical Transport Studies Of Topological Insulator Bi2te3 Nanotubes
Open Access
- Author:
- Du, Renzhong
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- November 19, 2015
- Committee Members:
- Qi Li, Dissertation Advisor/Co-Advisor
Qi Li, Committee Chair/Co-Chair
Moses Hung Wai Chan, Committee Member
Nitin Samarth, Committee Member
Joan Marie Redwing, Committee Member - Keywords:
- Topological Insulator
Quantum Transport
Nanotubes - Abstract:
- Topological insulators (TIs) are bulk insulators with unusual gapless metallic surface states protected by time reversal symmetry. The topological surface states are expected to yield unique phenomena, such as spin-momentum locking and the suppression of non-magnetic backscattering. The surface states with a Dirac cone like dispersion relation have been observed directly using Angle-resolved photoemission spectroscopy (ARPES). Their signatures in electrical transport properties have also been reported although the transport behaviors are complicated by the fact that the samples often have dominating conduction from bulk channels due to high carriers density from impurity states. It is theoretically predicted that the surface states are robust against strong disorders, but it has not been tested experimentally. Bi2Te3, a member of the Bi2Se3 family, has been demonstrated to be a promising candidate of 3D topological insulators. In this work, various transport studies have been carried out on Bi2Te3 nanotubes with strong disorders. As essentially narrow gap semiconductors with band gaps of 100 to 300 meV, the Bi2Se3 family often have conducting bulk due to impurity doping and lattice imperfection, such as vacancies, dislocations, and defects. Several approaches have been applied to reduce bulk conductivity. The first one is to compensate bulk carriers by chemical doping or field effect, which drags Fermi level back to bang gap. Another is to fabricate nano-scale samples with enhanced surface-to-volume ratio, and effectively reduce the weight of bulk conduction. In our work we have prepared Bi2Te3 nanotubes by solution phase method utilizing Kirkendall effect. Due to the nature of this process, polycrystalline Bi2Te3 nanotubes form with extremely strong disorders. In this way, bulk carriers are strongly localized and the samples are brought from a heavily doped band semiconductor to the regime of Anderson insulator. We have demonstrated that the bulk conductance of the nanotubes is truly insulating and characterized by Mott Variable Range Hopping (VRH) mechanism. Despite the strongly disordered bulk channel, Aharonov-Bohm (AB) like oscillations in magnetoconductance have been observed on the nanotubes at low temperatures. The conductance oscillations differ from those on either ballistic or diffusive normal metals. With the combination of theoretical analysis and simulations, we show that these oscillations originate from the outer surface states of the nanotubes. Their behaviors are consistent with the prediction of the “anomalous AB effect” of topological surface states. In comparison with published work on clean TIs, we provide a direct demonstration of the fundamental aspect of topological surface states, namely their robustness to time-reversal invariant disorder. The interplay between a TI and a superconductor (SC) is another interesting topic to explore the exotic behaviors of topological surface states. For example, proximity induced superconductivity on topological insulators leads to the new phase of topological superconductivity, which serves a host of Majorana fermions. On the other hand, proximity effect can be suppressed on topological surface states, with the presence of an anomalous resistance increase. We report a systematical study of the anomalous resistance increase on Bi2Te3 nanotubes with superconducting Nb electrodes. With various measurement strategies, we show that this resistance increase behavior occurs on the nanotubes rather than from SC/TI interface effects. It is induced from the contact with superconducting Nb without requiring current flowing through the Nb. The characteristic length of the anomalous resistance increase reaches as far as 450 nm at low temperature. Gate experiments confirm that the surface states of the nanotubes are responsible for this effect. Our results indicate that a new charge correlation may establish on the surface states of Bi2Te3 nanotubes under the proximity of superconductor, suggesting further studies on this subject to explore the underlying physical origins, both theoretically and experimentally.