Combining the ability to prepare high-quality, intrinsic Bi2Te3 topological insulator thin films of low carrier density with in-situ protective capping, we demonstrate a pronounced, gate-tunable change in transport properties of Bi2Te3 thin films. Using a back-gate, the carrier density is tuned by a factor of ∼ 7 in Al2O3 capped Bi2Te3 sample and by a factor of ∼ 2 in Te capped Bi2Te3 films. We achieve full depletion of bulk carriers, which allows us to access the topological transport regime dominated by surface state conduction. When the Fermi level is placed in the bulk band gap, we observe the presence of two coherent conduction channels associated with the two decoupled surfaces. Our magnetotransport results show that the combination of capping layers and electrostatic tuning of the Fermi level provide a technological platform to investigate the topological properties of surface states in transport experiments and pave the way towards the implementation of a variety of topological quantum devices. PACS numbers: 73.50.-h, 73.20.-r, 75.47.-m arXiv:1604.03722v1 [cond-mat.mes-hall]

Thin layers of topological insulator materials are quasi‐2D systems featuring a complex interplay between quantum confinement and topological band structure. To understand the role of the spatial distribution of carriers in electrical transport, the Josephson effect, magnetotransport, and weak anti‐localization are studied in bottom‐gated thin Bi2Te3 topological insulator films. The experimental carrier densities are compared to a model based on the solutions of the self‐consistent Schrödinger–Poisson equations and they are in excellent agreement. The modeling allows for a quantitative interpretation of the weak antilocalization correction to the conduction and of the critical current of Josephson junctions with weak links made from such films without any ad hoc assumptions.

Using high-Tc Josephson junctions made of YBa2Cu3O7−δ deposited across MgO bicrystal boundary, at terahertz (THz) frequency band, we demonstrated both fundamental and harmonic mixing. Radiation from a far-infrared laser was coupled to the junction, which was integrated with a planar bow-tie antenna, via an extended hyperhemispherical silicon lens. Fundamental mixing manifested itself in the junction’s dc current–voltage (I–V) curve as a third Shapiro step in addition to those two induced by the THz laser lines from a slightly misaligned resonator. In harmonic mixing between a THz laser line and a microwave local oscillator, the highest harmonic number we could get was 490 with a signal-to-noise ratio of 9 dB at the intermediate frequency.

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The design and operation of a CO:! laser based on a glow discharge in a metal tube is described. Up to 15 W power was obtained. The operating voltage is low compared with a similar discharge in an insulating tube.