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Electrical Transport Properties of Topological Insulators and Graphene

  • Author(s): Wang, Zhiyong
  • Advisor(s): Shi, Jing
  • et al.
Abstract

This dissertation summarizes my work on the study of topological insulators, graphene, and transition metal dichalcogenides, especially on the electrical transport studies. There are mainly two parts in this dissertation. The first part is the study on topological insulators. Bi2Se3 and Bi2Te2Se single crystals are synthesized and characterized. Calcium dopants are introduced to Bi2Se3 to compensate for the excess electrons generated by selenium vacancies in the as-grown single crystals. An n- to p-type transition is then realized. The bulk insulating state is achieved. In Bi2Te2Se bulk samples, extremely high low-temperature resistivity (> 2 Ohm cm) is achieved. Nanodevices of Bi2Se3 and Bi2Te2Se are then fabricated. A lithography-free technique is developed for device fabrication in order to well maintain the pristine state of bulk samples. Electron beam irradiation is performed to manually adjust the Fermi levels in the devices. Further control of the Fermi level is realized with the application of gate voltages. The insulating temperature behavior is achieved in devices upon electron beam irradiation. And the gate modulation grows as the electron beam irradiation dosage increases. The field-effect mobility is greatly enhanced and a ten-fold increase is obtained.

The second part is focused on graphene and transition metal dichalcogenides. Graphene is expected to exhibit ferromagnetism induced by the magnetic proximity effect when it is placed on a magnetic material. With enhanced spin-orbit coupling, the anomalous Hall effect can be realized in graphene. Meanwhile, a topological gap is opened at the Dirac point, making it possible to realize the quantized anomalous Hall effect when the Fermi level is in the gap. In this dissertation, graphene devices are transferred to yttrium iron garnet thin films, a ferrimagnetic material. At low temperatures, anomalous Hall effect is observed. Further studies on the temperature dependence and gate dependence of the anomalous Hall effect is performed.

For single-layer MoS2, at the valence band maxima, the band is split by 160 meV due to strong spin-orbit coupling. Spin-up and spin-down electrons reside in different bands due to the broken inversion symmetry. Valley and spin degrees of freedom of the valence bands are inherently coupled in single-layer MoS2. It is an ideal material to study the valley Hall effect.

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