Despite intensive investigations of Bi2Se3 in past few years, the size and nature of the bulk energy band gap of this well-known 3D topological insulator still remain unclear. Here we report on a combined magneto-transport, photoluminescence and infrared transmission study of Bi2Se3, which unambiguously shows that the energy band gap of this material is direct and reaches E g = (220 ± 5) meV at low temperatures.

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ABSTRACT:X-band microwave spectroscopy is applied to study the cyclotron resonance in Bi 2 Te 3 exposed to ambient conditions. With its help, intraband transitions between Landau levels of relativistic fermions are observed. The Fermi velocity equals to 3260 m/s, which is much lower than has been reported in the literature for samples cleaved in vacuum. Simultaneous observation of bulk Shubnikov Keywords: topological insulators, cyclotron resonance, relativistic fermions.In the last five years a new class of topological quantum states has emerged, referred to as topological insulators. 1,2,3,4,5,6,7,8,9,10 Topological properties of the family of bismuth compound crystals, i.e. Bi 2 Te 3 , 5 Bi 2 Se 3 , 8 and Bi 2 Te 2 Se, 9 have been confirmed in spectacular angle resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy and spectroscopy 11 experiments. Other experiments, including electric transport or optical measurements, are difficult to perform because of high bulk conductance due to native crystal lattice defects. 6,8,12 The defects responsible for a high bulk carrier concentration are most probably Bi antisites in Bi 2 Te 3 leading to typically highly metallic ptype, and Se vacancies in Bi 2 Se 3 giving n-type conductivity. 6,13,14 In order to suppress the contribution of the conducting bulk states and expose the surface states in the experiment, very thin samples with 2 the thickness of a few quintet layers are typically studied. 15,16 An alternative approach is to use surface sensitive experimental techniques or techniques disregarding the metallic bulk. One of them is the already mentioned ARPES, which probes surface regions from a few up to tens of nm. In this communication we describe the results of microwave spectroscopy measurements performed in an Xband (9.5 GHz) resonator. The microwave penetration depth in the applied experimental conditions ranges from 4 μm to 40 μm. The technique can actually probe independently both bulk and surface properties, being partly insensitive to the detrimental influence of the metallic bulk conductivity.Similar approach has been recently applied to study surface states in Bi 2 Se 3 topological insulator, where 2D cyclotron resonance has been revealed thanks to the application of the microwave cavity transmission technique. 17 The Bi 2 Te 3 crystal was grown by the vertical Bridgman method, with an excess of tellurium in the melt in order to suppress the formation of Bi antisite defects. Growth details have been described in Ref. 18. Segregation effects, characteristic of the Bridgman method, caused the gradient of stoichiometry along the crystal growth direction. For a given temperature, the equilibrium composition of the liquid phase is typically different than of the solid phase. In the case of Bi 2 Te 3 , this causes enrichment of the liquid phase in tellurium during the growth process, and formation of the concentration gradient of defects related to tellurium excess. The crystal, highly p-type at the crystal seed due to large concentration ...

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