Nuclear magnetic resonance (NMR) and transport measurements have been performed at high magnetic fields and low temperatures in a series of n-type Bi2Se3 crystals. In low density samples, a complete spin polarization of the electronic system is achieved, as observed from the saturation of the isotropic component of the 209 Bi NMR shift above a certain magnetic field. The corresponding spin splitting, defined in the phenomenological approach of a 3D electron gas with a large (spinorbit-induced) effective g-factor, scales as expected with the Fermi energy independently determined by simultaneous transport measurements. Both the effective electronic g-factor and the "contact" hyperfine coupling constant are precisely determined. The magnitude of this latter reveals a non negligible s-character of the electronic wave function at the bottom of the conduction band. Our results show that the bulk electronic spin polarization can be directly probed via NMR and pave the way for future NMR investigations of the electronic states in Bi-based topological insulators. Bismuth selenide, Bi 2 Se 3 , known for years as a narrow gap semiconductor, has recently appeared as one of the first examples of "3D topological insulators"[1-3]. As such unique state of matter, it is characterized by the coexistence of 2-dimensional conducting surface states with an insulating bulk material. The charge carriers at the surface behave as massless relativistic particles (Dirac fermions) with a spin locked to their translational momentum. These so-called "helical Dirac fermions", which promise applications in the field of spintronic [4] and quantum computation [5], have recently raised a considerable interest (see Ref. 6 for a review). As a matter of fact, the existence of gapless states at the boundary of the material is related to a well-defined change in the bulk band structure. In Bi 2 Se 3 , this originates from a parity inversion of the valence and conduction band in the presence of a large spin-orbit coupling [1].In an effort to deepen our understanding of the spin properties of topological insulators, a characterization of the coupling between the charge carriers and the nuclei in the Bi 2 Se 3 matrix is of high importance. Indeed, nuclear spins can inherently couple to the topologically protected electronic states and limit their coherence time. On the other hand, this hyperfine coupling can be efficiently exploited to probe the electronic system via NMR techniques. In particular, an electronic system bearing nonzero spin polarization acts as an effective local magnetic field which modifies the nuclei resonance frequency. This so-called "Knight shift" has previously been extensively studied to probe the electronic spin polarisation [7] as well as the spatial symmetry of the wave functions [8, 9] in some semiconductor-based bulk or low dimensional systems. A couple of recent works have investigated the NMR properties of Bi 2 Se 3 samples and revealed signatures of the bulk electronic states [10, 11]. These measurements were however l...

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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The Faraday effect is a representative magneto-optical phenomenon, resulting from the transfer of angular momentum between interacting light and matter in which time-reversal symmetry has been broken by an externally applied magnetic field. Here we report on the Faraday rotation induced in the prominent 3D topological insulator Bi2Se3 due to bulk interband excitations. The origin of this non-resonant effect, extraordinarily strong among other non-magnetic materials, is traced back to the specific Dirac-type Hamiltonian for Bi2Se3, which implies that electrons and holes in this material closely resemble relativistic particles with a non-zero rest mass.

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