Three-dimensional topological insulators are fascinating materials with insulating bulk yet metallic surfaces that host highly mobile charge carriers with locked spin and momentum. Remarkably, surface currents with tunable direction and magnitude can be launched with tailored light beams. To better understand the underlying mechanisms, the current dynamics need to be resolved on the timescale of elementary scattering events (∼10 fs). Here, we excite and measure photocurrents in the model topological insulator Bi2Se3 with a time resolution of 20 fs by sampling the concomitantly emitted broadband terahertz (THz) electromagnetic field from 0.3 to 40 THz. Strikingly, the surface current response is dominated by an ultrafast charge transfer along the Se–Bi bonds. In contrast, photon-helicity-dependent photocurrents are found to be orders of magnitude smaller than expected from generation scenarios based on asymmetric depopulation of the Dirac cone. Our findings are of direct relevance for broadband optoelectronic devices based on topological-insulator surface currents.

Topological insulators are a class of solids in which the non-trivial inverted bulk band structure gives rise to metallic surface states that are robust against impurity scattering. In three-dimensional (3D) topological insulators, however, the surface Dirac fermions intermix with the conducting bulk, thereby complicating access to the low-energy (Dirac point) charge transport or magnetic response. Here we use differential magnetometry to probe spin rotation in the 3D topological material family (Bi2Se3, Bi2Te3 and Sb2Te3). We report a paramagnetic singularity in the magnetic susceptibility at low magnetic fields that persists up to room temperature, and which we demonstrate to arise from the surfaces of the samples. The singularity is universal to the entire family, largely independent of the bulk carrier density, and consistent with the existence of electronic states near the spin-degenerate Dirac point of the 2D helical metal. The exceptional thermal stability of the signal points to an intrinsic surface cooling process, probably of thermoelectric origin, and establishes a sustainable platform for the singular field-tunable Dirac spin response.

Topological insulators are a promising class of materials for applications in the field of spintronics. New perspectives in this field can arise from interfacing metal-organic molecules with the topological insulator spin-momentum locked surface states, which can be perturbed enhancing or suppressing spintronics-relevant properties such as spin coherence. Here we show results from an angle-resolved photemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM) study of the prototypical cobalt phthalocyanine (CoPc)/Bi2Se3 interface. We demonstrate that that the hybrid interface can act on the topological protection of the surface and bury the Dirac cone below the first quintuple layer.

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.