The newly-discovered three-dimensional strong topological insulators (STIs) exhibit topologically-protected Dirac surface states 1,2 . While the STI surface state has been studied spectroscopically by e.g. photoemission 3-5 and scanned probes 6-10 , transport experiments 11-17 have failed to demonstrate the most fundamental signature of the STI: ambipolar metallic electronic transport in the topological surface of an insulating bulk. Here we show that the surfaces of thin (<10 nm), low-doped Bi 2 Se 3 (≈10 17 /cm 3 ) crystals are strongly electrostatically coupled, and a gate electrode can completely remove bulk charge carriers and bring both surfaces through the Dirac point simultaneously. We observe clear surface band conduction with linear Hall resistivity and well-defined ambipolar field effect, as well as a charge-inhomogeneous minimum conductivity region 18-20 . A theory of charge disorder in a Dirac band 19-21 explains well both the magnitude and the variation with disorder strength of the minimum conductivity (2 to 5 e 2 /h per surface) and the residual (puddle) carrier density (0. 4 x 10 12 cm -2 to 4 x 10 12 cm -2 ). From the measured carrier mobilities 320 cm 2 /Vs to 1,500 cm 2 /Vs, the charged impurity densities 0.5 x 10 13 cm -2 to 2.3

First we discuss whether R nl arises due to charge current or spin current flowing between F3 and F4. Ideally, charge current would flow only between F3 and F2, eliminating contributions to the R nl from magnetoresistance of the ferromagnetic electrodes (anisotropic magnetoresistance), the channel, or the electrode-channel interface. However, because R nl is ~3 orders of magnitude smaller than the device resistance, it is possible that some charge current flows through a tortuous path from F3to F4 and F5. We investigate this by measuring the gate voltage and temperature dependence of R nl . Figure 3a shows the gate voltage dependence of R nl in the parallel and antiparallel state, R nl,p and R nl,ap , as well as their average value. Figure 3b shows the non-local spinvalve signal ΔR. R avg , R nl,p and R nl,ap all show a peak near the CNP (10 V < V g < 30 V), while ΔR is near zero in this region. Well outside this region (V g < -20 or V g > 40 V), R nl,p and R nl,ap have nearly equal magnitude and opposite sign (R avg is near zero) and ΔR is larger and shows quasi-periodic oscillations with V g . The peak in R avg (V g ) near the CNP suggests that charge current does flow in the region between F3 and F4 for these gate voltages. However, R avg (V g ) is not simply proportional to ρ(V g ) but rather drops to near zero at large V g while ρ(V g ) remains finite. Thus the finite R avg (V g ) near the CNP is likely due to the inhomogenous nature of graphene near the CNP 21,22 ; here percolating electron and hole regions may cause a tortuous current path.Away from the CNP, R avg (V g ) drops to near zero, indicating small charge current.Yet R nl,p and R nl,ap remain finite, with near equal magnitude and opposite sign. This is as 4 expected for a pure spin current flowing from F3 to F4, and cannot be explained by a magnetoresistive signal arising from any charge current between F4 and F5. The Hall effect is another possible source of V nl , however, the Hall voltage would be expected to grow large and switch sign near the CNP, rather than showing a peak. Figure 4 shows the temperature dependence of R avg and ΔR for V g = 0. Here R avg is finite similar to Figure 3, but somewhat larger for this electrode configuration. The spin-valve signal ΔR is seen to drop with temperature approximately as ΔR ∝ T -1 , while R avg is much more weakly temperature dependent; again indicating a different origin for ΔR and R avg . The inset shows a measurement at 300 K performed at higher current; the spin-valve signal can still be observed, confirming expectations of reduced spin scattering in graphene even to high temperature.We now discuss the magnitude of the spin-valve signal ΔR. For an Ohmicallycontacted spin-valve device, the non-local signal may be estimated using Eqn. 22 of reference [17]; we estimate in this case the signal should be on order 10 -5 Ω. However, we observe finite contact resistance of order 10 kΩ per electrode as estimated from the difference between two-probe and four-probe resistance measurements. In the limit of...

Ultrathin (approximately three quintuple layer) field-effect transistors (FETs) of topological insulator Bi(2)Se(3) are prepared by mechanical exfoliation on 300 nm SiO(2)/Si susbtrates. Temperature- and gate-voltage-dependent conductance measurements show that ultrathin Bi(2)Se(3) FETs are n-type and have a clear OFF state at negative gate voltage, with activated temperature-dependent conductance and energy barriers up to 250 meV.

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