A prominent feature of topological insulators (TIs) is the surface states comprising of spin-nondegenerate massless Dirac fermions. Recent technical advances have made it possible to address the surface transport properties of TI thin films by tuning the Fermi levels of both top and bottom surfaces. Here we report our discovery of a novel planar Hall effect (PHE) from the TI surface, which results from a hitherto-unknown resistivity anisotropy induced by an in-plane magnetic field. This effect is observed in dual-gated devices of bulk-insulating Bi2−xSbxTe3 thin films, where the field-induced anisotropy presents a strong dependence on the gate voltage with a characteristic two-peak structure near the Dirac point. The origin of PHE is the peculiar time-reversal-breaking effect of an in-plane magnetic field, which anisotropically lifts the protection of surface Dirac fermions from backscattering. The observed PHE provides a useful tool to analyze and manipulate the topological protection of the TI surface.

The charge-current-induced spin polarization is a key property of topological insulators for their applications in spintronics. However, topological surface states are expected to give rise to only one type of spin polarization for a given current direction, which has been a limiting factor for spin manipulations. Here we report that in devices based on the bulk-insulating topological insulator BiSbTeSe2, an unexpected switching of spin polarization was observed upon changing the chemical potential. The spin polarization expected from the topological surface states was detected in a heavily electron-doped device, whereas the opposite polarization was reproducibly observed in devices with low carrier densities. We propose that the latter type of spin polarization stems from topologically-trivial two-dimensional states with a large Rashba spin splitting, which are caused by a strong band bending at the surface of BiSbTeSe2 beneath the ferromagnetic electrode used as a spin detector. This finding paves the way for realizing the "spin transistor" operation in future topological spintronic devices. arXiv:1605.04149v2 [cond-mat.mes-hall]

Devices to generate on-demand non-local spin entangled electron pairs have potential application as solid-state analogues of the entangled photon sources used in quantum optics. Recently, Andreev entanglers that use two quantum dots as filters to adiabatically split and separate the quasi-particles of Cooper pairs have shown efficient splitting through measurements of the transport charge but the spin entanglement has not been directly confirmed. Here we report measurements on parallel quantum dot Josephson junction devices allowing a Josephson current to flow due to the adiabatic splitting and recombination of the Cooper pair between the dots. The evidence for this non-local transport is confirmed through study of the non-dissipative supercurrent while tuning independently the dots with local electrical gates. As the Josephson current arises only from processes that maintain the coherence, we can confirm that a current flows from the spatially separated entangled pair.

Electrical control over electron spin is a prerequisite for spintronics spin-based quantum information processing. In particular, control over the interaction between the orbital motion and the spin state of electrons would be valuable, because this interaction influences spin relaxation and dephasing. Electric fields have been used to tune the strength of the spin-orbit interaction in two-dimensional electron gases, but not, so far, in quantum dots. Here, we demonstrate that electrical gating can be used to vary the energy of the spin-orbit interaction in the range 50-150 µeV while maintaining the electron occupation of a single self-assembled InAs quantum dot. We determine the spin-orbit interaction energy by observing the splitting of Kondo effect features at high magnetic fields.

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Graphene has strong potential for electrical biosensing owing to its two-dimensional nature and high carrier mobility which transduce the direct contact of a detection target with a graphene channel to a large conductivity change in a graphene field-effect transistor (G-FET). However, the measurable range from the graphene surface is highly restricted by Debye screening, whose characteristic length is less than 1 nm at physiological ionic strength. Here, we demonstrated electrical biosensing utilizing the enzymatic products of the target. We achieved quantitative measurements of a target based on the site-binding model and real-time measurement of the enzyme kinetics in femtoliter microdroplets. The combination of a G-FET and microfluidics, named a “lab-on-a-graphene-FET”, detected the enzyme urease with high sensitivity in the zeptomole range in 100 mM sodium phosphate buffer. Also, the lab-on-a-graphene-FET detected the gastric cancer pathogen Helicobacter pylori captured at a distance greater than the Debye screening length from the G-FET.

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We measure the nondissipative supercurrent in a single InAs self-assembled quantum dot ͑QD͒ coupled to superconducting leads. The QD occupation is both tuned by a back-gate electrode and lateral side gate. The geometry of the side gate allows tuning of the QD-lead tunnel coupling in a region of constant electron number with an appropriate orbital state. Using the side-gate effect we study the interplay between Kondo correlations and superconducting pairing on the QD, observing a decrease in the supercurrent when the Kondo temperature is reduced below the superconducting energy gap in qualitative agreement with theoretical predictions.