Observation of Dirac Holes and Electrons in a Topological Insulator

Institute of Scientific and Industrial Research, Osaka University, Ibaraki, Japan.
Physical Review Letters (Impact Factor: 7.51). 07/2011; 107(1):016801. DOI: 10.1103/PhysRevLett.107.016801
Source: PubMed


We show that in the new topological-insulator compound Bi(1.5)Sb(0.5)Te(1.7)Se(1.3) one can achieve a surfaced-dominated transport where the surface channel contributes up to 70% of the total conductance. Furthermore, it was found that in this material the transport properties sharply reflect the time dependence of the surface chemical potential, presenting a sign change in the Hall coefficient with time. We demonstrate that such an evolution makes us observe both Dirac holes and electrons on the surface, which allows us to reconstruct the surface band dispersion across the Dirac point.

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    • "In BiSbTeSe 2 , the Dirac point nearly coincides with E F , it thus may serve as a benchmark for the bulk carrier dynamics at very low carrier concentrations. For a sample thickness d 10 µm, the bulk conductance of BiSbTeSe 2 is low enough at low temperatures to be out-weighted by the surface conductance [23] [27] [28]. This allows to observe a hallmark of topological transport , the half-integer quantum Hall effect, at temperatures up to 35 K [27]. "
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    ABSTRACT: Three-dimensional topological insulators harbour metallic surface states with exotic properties. In transport or optics, these properties are typically masked by defect-induced bulk carriers. Compensation of donors and acceptors reduces the carrier density, but the bulk resistivity remains disappointingly small. We show that measurements of the optical conductivity in BiSbTeSe$_2$ pinpoint the presence of electron-hole puddles in the bulk at low temperatures, which is essential for understanding DC bulk transport. The puddles arise from large fluctuations of the Coulomb potential of donors and acceptors, even in the case of full compensation. Surprisingly, the number of carriers appearing within puddles drops rapidly with increasing temperature and almost vanishes around 40 K. Monte Carlo simulations show that a highly non-linear screening effect arising from thermally activated carriers destroys the puddles at a temperature scale set by the Coulomb interaction between neighbouring dopants, explaining the experimental observation semi-quantitatively. This mechanism remains valid if donors and acceptors do not compensate perfectly.
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    • "Nevertheless, the technique is very sensitive to relatively weak vibrations, and careful design is typically needed to ensure vibration isolation [2]. One of the most important STM applications is imaging in strong magnetic fields, which is crucial for the study of high-temperature superconductors [3] [4] [5] [6], the Dirac nature of the surface states of topological insulators [7] [8] [9], the quantum Hall effect in low-dimensional materials [10] [11], and the vortex formation in quantum dots [12]. To this end, the microscope is routinely housed in a superconducting magnet, which has the advantage of tranquility [13] [14] [15] [16]. "
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    ABSTRACT: We report the achievement of the first atomically resolved scanning tunneling microscope (STM) imaging in a water-cooled magnet (WM), where the extremely harsh vibrations and noises have been the major challenge. This homebuilt WM-STM features an ultra-rigid and compact scan head in which the coarse approach is driven by our new design of the TunaDrive piezoelectric motor. A three-level spring hanging system is exploited for vibration isolation. Room-temperature raw-data images of graphite with quality atomic resolution were obtained in very high magnetic fields up to 27 T in a 32 mm bore WM whose absolute maximum field is 27.5 T at the power rating of 10 MW. This record of 27 T has exceeded the maximum field strength of the conventional superconducting magnets. Besides, our WM-STM has also paved the way to the STM imaging in the 45 T, 32 mm bore hybrid magnet, which is the world's flagship magnet and can produces the highest steady magnetic field at present.
    Nano Research 05/2015; DOI:10.1007/s12274-015-0889-5 · 7.01 Impact Factor
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    • "The single crystals of Bi1.5Sb0.5Te1.7Se1.3, Bi2Se3, and Sn-doped (0.4%) Bi2Te2Se were grown by a Bridgman method in evacuated quartz tubes [5] [6] [7]. 20-nm-thick Ni81Fe19 thin films were deposited in a high vacuum by electron-beam evaporation on cleaved surfaces of TIs. "
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    ABSTRACT: Detection and manipulation of electrons' spins are key prerequisites for spin-based electronics or spintronics. This is usually achieved by contacting ferromagnets with metals or semiconductors, in which the relaxation of spins due to spin-orbit coupling limits both the efficiency and the length scale. In topological insulator materials, on the contrary, the spin-orbit coupling is so strong that the spin direction uniquely determines the current direction, which allows us to conceive a whole new scheme for spin detection and manipulation. Nevertheless, even the most basic process, the spin injection into a topological insulator from a ferromagnet, has not yet been demonstrated. Here we report successful spin injection into the surface states of topological insulators by using a spin pumping technique. By measuring the voltage that shows up across the samples as a result of spin pumping, we demonstrate that a spin-electricity conversion effect takes place in the surface states of bulk-insulating topological insulators Bi1.5Sb0.5Te1.7Se1.3 and Sn-doped Bi2Te2Se. In this process, due to the two-dimensional nature of the surface state, there is no spin current along the perpendicular direction. Hence, the mechanism of this phenomenon is different from the inverse spin Hall effect and even predicts perfect conversion between spin and electricity at room temperature. The present results reveal a great advantage of topological insulators as inborn spintronics devices.
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