Three dimensional topological insulator bismuth selenide (Bi2Se3) is expected to possess strong spin-orbit coupling and spin-textured topological surface states, and thus exhibit a high charge to spin current conversion efficiency. We evaluate spin-orbit torques in Bi2Se3/Co40Fe40B20 devices at different temperatures by spin torque ferromagnetic resonance measurements. As temperature decreases, the spin-orbit torque ratio increases from ~ 0.047 at 300 K to ~ 0.42 below 50 K. Moreover, we observe a significant out-of-plane torque at low temperatures. Detailed analysis indicates that the origin of the observed spin-orbit torques is topological surface states in Bi2Se3. Our results suggest that topological insulators with strong spin-orbit coupling could be promising candidates as highly efficient spin current sources for exploring next generation of spintronic applications.

) is a topological insulator exhibiting helical spin polarization and strong spin-orbit coupling. The spin-orbit coupling links the charge current to spin current via the spin Hall effect (SHE). We demonstrate a Bi 2 Se 3 spin detector by injecting the pure spin current from a magnetic permalloy layer to a Bi 2 Se 3 thin film and detect the inverse SHE in Bi 2 Se 3 . The spin Hall angle of Bi 2 Se 3 is found to be 0.0093 ± 0.0013 and the spin diffusion length in Bi 2 Se 3 to be 6.2 ± 0.15 nm at room temperature. Our results suggest that topological insulators with strong spin-orbit coupling can be used in functional spintronic devices.

robust under electrolyte gating compared with other materials such as manganites and vanadium oxides.Graphene, a single atomic layer of carbon, has attracted huge interest in a wide range of studies. Graphene has been recognized for excellent mechanical strength, chemical stability, and the highest electrical mobility of carriers due to the unique conical band structure. [ 19,20 ] These characteristics enable the application of graphene in modulators in both the visible [ 21,22 ] and THz ranges. [ 10,11,17,23,24 ] The modulation in the visible range is enabled by interband transitions limiting the absorption to only 2.3%. [ 25,26 ] On the other hand, in the THz range, the intraband transitions of graphene dominate, and the electric-fi eld amplitude modulation is much more signifi cant. A total (100%) electric-fi eld modulation of a graphene-based THz modulator was predicted, but the modulation depth was demonstrated to be 15% experimentally. [ 10 ] Furthermore, it was theoretically demonstrated that graphene-based THz modulators could have very low insertion losses by optimizing the substrates. [ 10 ] Nevertheless, further improvements in the modulation depth are required for practical applications.In this work, we experimentally demonstrate and numerically support the excellent performance of THz modulators based on graphene/ionic liquid/graphene sandwich structures. The modulation covers a broadband frequency range from 0.1 to 2.5 THz with a modulation depth of up to 99% by applying a small gate voltage of 3 V. To our knowledge, this is the highest modulation ratio from graphene-based THz devices to date. The outstanding performance of graphene-based device benefi ts from two key components: i) the linear conical band structure of the graphene and ii) the powerful gating effect of the ionic liquid. First, due to the linear band structure of graphene, the Fermi level in the vicinity of the Dirac point can be linearly controlled by tuning the gate voltage, which subsequently changes the THz transmittance. Second, the strong gating effect of the ionic liquid derives from the fact that charges accumulate within several nanometers in proximity to the graphene/ionic liquid interfaces. [ 27,28 ] Consequently, the magnitude of the electric fi eld on graphene is very large, which results in effective tuning of the graphene's Fermi level. For the massless charge carriers in graphene, there is a power-law dependence | E F | ∝ | n | 1/2 , where E F is the Fermi level and n is the carrier concentration. [ 29 ] Thus the gating is essentially tuning the carrier concentration in graphene (or electrical conductivity). [ 30 ] According to the Drude model, the graphene optical conductivity is equivalent to its electrical conductivity at the THz regime in our studies. [ 10,31 ] As a result, the electrical gating is capable of modulating THz waves. Moreover, our sandwich structures make use of the high mobility of both electrons and holes in graphene, [ 20 ] unlike the previous semiconductor-based modulators, [ 12,13 ] where the ho...

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Absence of backscattering and occurrence of weak anti-localization are two characteristic features of topological insulators. We find that the introduction of defects results in the appearance of a negative contribution to magnetoresistance in the topological insulator BiSbTeSe 2 , at temperatures below 50 K. Our analysis shows that the negative magnetoresistance originates from an increase in the density of defect states created by introduction of disorder, which leaves the surface states unaffected. We find a decrease in the magnitude of the negative magnetoresistance contribution with increasing temperature and a robustness of the topological surface states to external disorder.2 Three-dimensional (3D) topological insulators (TIs) are a newly discovered state of matter with insulating bulk and conducting surfaces which are protected from backscattering by time reversal symmetry. [1][2][3][4][5][6][7] The band structure of a 3D TI consists of linearly dispersing surface states within the bulk band gap and is in the form of a spin-momentum locked Dirac cone which is a consequence of band inversion due to large spin-orbit coupling. The most commonly studied TI material to date is Bi 2 Se 3 due to its relatively large bulk gap of 0.3 eV and favorable band structure, which allows experimentalists to access the topological surface states easily. 7 However, transport experiments using Bi 2 Se 3 are plagued by large bulk conductivity due to the rapid formation of Se vacancies, which make it extremely challenging to access the physics of the surface states, since transport is overwhelmed by the bulk. [8][9][10][11] One of the possible ways to address this issue is to dope the material in order to decrease the concentration of Se vacancies and increase the insulating character of the bulk. 8,[12][13][14][15][16] Magneto-transport measurements in TIs typically show weak anti-localization (WAL) behavior due to the large spin-orbit coupling and the linearly dispersing surface states. [17][18][19][20] The spin-momentum locking of the metallic surface states causes a suppression of backscattering which results in a π Berry phase acquired by the electrons executing time reversed paths. This results in destructive interference and observation of the WAL which is a quantum correction to the conductivity in an externally applied magnetic field. However, a few reports have previously demonstrated a signature of weak localization (WL) in TIs in the case of ultra-thin films in which the hybridization between the top and bottom surface wave-functions results in a gap opening in the surface states. [21][22][23] It has been also reported that magnetically doped TIs can exhibit WL. 24,25 Given the large spin-orbit coupling and the absence of backscattering in the metallic surface states, the observation of WL in TIs is an interesting phenomenon, which is not fully understood.3 Furthermore, the robustness of the surface states against external disorder has not been experimentally investigated.In this work, we devise an experiment to int...

A Bi 2 Se 3 topological insulator field effect transistor is investigated by using ionic liquid as an electric double layer gating material, leading to a conductance modulation of 365% at room temperature. We discuss the role of charged impurities on the transport properties. The conductance modulation with gate bias is due to a change in the carrier concentration, whereas the temperature dependent conductance change is originated from a change in mobility. Large conductance modulation at room temperature along with the transparent optical properties makes topological insulators as an interesting (opto)electronic material. a)

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Topological insulators (TIs) form a new class of materials with insulating bulk and surface conduction ensured by topologically protected surface states (TPSS). We investigate the impact of the helicity of a normally incident laser beam on the photovoltaic effect in the TI Bi2Se3. The observation of a helicity dependent photovoltaic effect for normally incident light indicates the presence of out-of-plane spin components for some TPSSs due to the hexagonal warping. In addition, fluctuations in the electrostatic potential at the surface locally break the rotational symmetry of the film allowing the helicity dependent photovoltaic effect. Our result suggests that engineering local electrostatic potentials in Bi2Se3 would allow the control of optically generated spin currents, which may be useful for applications in spin-optoelectronics.