Oxide materials as building blocks for artificial topological insulators

The need for more energy efficient consumer electronics is growing. Topological insulators, a relatively new class of materials, could possibly function as a new building block for the next generation electronics. When a topological insulator is physically connected to a trivial insulator, conducting surface states form at the interface while the bulk of both materials remains insulating.

Current experimentally verified topological insulators possess relatively small bulk band gaps that do not exceed the thermal excitation energy at room temperature. Therefore, applications of the desired surface states properties are hindered by the contribution from bulk carriers. Using oxide materials for artificially designed topological insulators potentially enlarges the size of the band gap and provides the possibility of tuning of the material properties by various design principles.

In this dissertation, the materials systems BaBiO3 and perovskite Y-Bi-O and fabricated as thin films using pulsed laser deposition. Theoretical calculations of the electronic band structures show the presence of a band inversion, indicating nontrivial topology. However, both compounds suffer from major degradation effects, preventing the use of conventional techniques. Various in situ techniques are, therefore, used such as angle-resolved photoemission spectroscopy, scanning tunneling microscopy and x-ray photoelectron diffraction.

Furthermore, it is shown that BaBiO3 accommodates for the 12% lattice mismatch with the SrTiO3 substrate by the formation of a rocksalt structure. Although, BaBiO3 thin films are fabricated with a high quality, accessing the topological insulating state remains difficult. By using the BaBiO3 film as a buffer layer, the energetically unfavorable perovskite phase is stabilized in Y-Bi-O. On this system, linear dispersing states are observed, hinting to the presence of a Dirac cone and thus a topological insulating phase. The use of complex oxides opens up many routes towards the realization of the first oxide topological insulator and applications at room temperature could become feasible.