Magneto-infrared spectroscopy of emerging topological materials
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Topological insulators (TIs) have recently attracted much attentions due to their robust edge/surface states against disorders and external disturbance under the protection of symmetries. Such robust states hold great promise for application in spintronics and quantum computing. Therefore, intense explorations and characterizations of possible TIs have been carried out in the scientific community. In this thesis, we use magneto-infrared spectroscopy to study the electronic band structures of two TI candidates. The first candidate is InAs/GaSb double quantum wells. We showed that when its band structure crosses the boundary from the normal state to the inverted state, multiple absorption modes emerge. This normal-inverted state transition can be described semi-quantitatively with an eight-band k. p model. We further demonstrate that the transition is widely adjustable with the effects of strain, magnetic field and quantum well widths, which paves the way for band engineering of optimal InAs/GaSb structures for realizing novel topological states as well as for device applications. Another candidate studied in this thesis is zirconium pentatelluride (ZrTe5). The transmission spectra measured at zero magnetic field is suggestive of a quasi-2D nature in bulk ZrTe5 similar to that in graphene. The Landau level transitions clearly follow a square-root magnetic field dependence, which is a signature of Dirac bands, only with a small energy gap of about 9.4 meV. A four-fold splitting in low lying interband transitions are resolved under high magnetic fields, while circular polarization resolved measurements help identify a significant contribution from electron-hole asymmetry. We employ a model based on the Bernevig-Hughes-Zhang effective Hamiltonian to determine the values of the Fermi velocity, Dirac mass (or gap), electron-hole asymmetry, and electron and hole g-factors in ZrTe5. Our results support the topological Dirac semimetal picture with a small energy gap.