Agnieszka Wolos
University of Warsaw, Poland
Title: Three-dimensional topological insulators - a new phase of quantum matter; growth issues and the properties of bismuth chalcogenides
Biography
Biography: Agnieszka Wolos
Abstract
It has been recently discovered that bismuth chalcogenides, previously recognized for its thermoelectric properties, hide a surprise in the band structure - topologically protected surface states. They are so-called three-dimensional topological insulators constituting a new state of quantum matter. They possess a band gap in the bulk and metallic surface states resulting from the change of the topological invariant at the interface with other material (could be the vacuum). The crystal structure of bismuth chalcogenides hosts considerable amounts of lattice defects, which results in highly conducting bulk covering the surface electric transport. In order to investigate properties of both the bulk and the surface states, Bi2Te3, Bi2Se3, and Bi2Te2Se were grown by the vertical Bridgman method at the Institute of Electronic Materials Technology, Warsaw. Undoped Bi2Te3 is p-type due to bismuth anti-sites while Bi2Se3 is n-type due to selenium vacancies. The structure of Bi2Te2Se prevents formation of both types of defects, resulting in the lowest conductivity of all the three materials. The conductivity of Bi2Se3 was reduced by varying the stoichiometry and applying calcium acceptor doping. The limits for the reduction of the bulk concentration will be discussed. Bulk and surface states were investigated using the contactless microwave spectroscopy (with the use of a standard electron paramagnetic resonance spectrometer). Properties of the bulk conduction electron spin resonance will be presented, while the presence of the surface states is manifested in the cyclotron resonance and weak anti-localization phenomena.
