Electronic structure, magneto-oscillations and optical properties of bismuth selenide single crystals

We present a comprehensive experimental and theoretical study of the electronic structure, magneto-transport, and optical properties of bismuth selenide Bi₂Se₃ single crystals grown by the Bridgman-Stockbarger method. Despite being a prototypical topological insulator, the studied crystals exhibit metallic conductivity in the bulk due to a high electron concentration (∼10¹⁹ cm⁻³). Magneto-transport measurements reveal Shubnikov-de Haas oscillations with a frequency of 230 T. Analysis of the oscillation amplitude, phase, and their angular dependence in tilted magnetic fields demonstrates that these quantum oscillations originate from a three-dimensional Fermi surface with a trivial Berry phase, indicative of bulk charge carriers. We determine key electronic parameters, including the effective cyclotron mass, Dingle temperature, and quantum mobility. Intriguingly, a negative magnetoresistance is observed at high tilt angles, the origin of which is discussed. Optical measurements in the infrared to ultraviolet range further reveal a suppression of the Drude response and dominant interband transitions, consistent with low carrier concentration despite metallic transport and the calculated theoretical band structure, which indicates a low density of states at the Fermi energy. In the calculations, its value was adjusted to take into account the presence of bulk charge carriers in samples, this made it possible to plot the Fermi surface. The band inversion between Bi-6p and Se1-4p electronic states in Bi₂Se₃ is also calculated, as well as the Dirac cone structure in the surface modeling, which are characteristic of topological insulators. These results provide a complete picture of the electronic properties of bulk-conducting Bi₂Se₃, distinguishing the contributions from its trivial bulk states.