The topological state that emerges at the surface of a topological insulator (TI) and at the TI-substrate interface are studied in metal-hBN-Bi2Se3 capacitors. By measuring the RF admittance of the ...capacitors versus gate voltage, we extract the compressibility of the Dirac state located at a gated TI surface. We show that even in the presence of an ungated surface that hosts a trivial electron accumulation layer, the other gated surface always exhibits an ambipolar effect in the quantum capacitance. We succeed in determining the velocity of surface Dirac fermions in two devices, one with a passivated surface and the other with a free surface that hosts trivial states. Our results demonstrate the potential of RF quantum capacitance techniques to probe surface states of systems in the presence of a parasitic density-of-states.
It is well established that topological insulators sustain Dirac fermion surface states as a consequence of band inversion in the bulk. These states have a helical spin polarization and a linear ...dispersion with large Fermi velocity. We report on a set of experimental observations supporting the existence of additional massive surface states. These states are also confined by the band inversion at a topological-trivial semiconductor heterojunction. While first introduced by Volkov and Pankratov (VP) before the understanding of the topological nature of such a junction, they were not experimentally identified. Here we identify their signatures on transport properties at high electric field. By monitoring the ac admittance of HgTe topological-insulator field-effect capacitors, we access the compressibility and conductivity of surface states in a broad range of energies and electric fields. The Dirac states are characterized by a compressibility minimum, a linear energy dependence, and a high mobility persisting up to energies much larger than the transport band gap of the bulk. At higher energies, we observe multiple anomalous behaviors in conductance, charge metastability, and Hall resistance that point towards the contribution of massive surface states in transport scattering and charge transfer to the bulk. The spectrum of these anomalies agrees with predictions of a phenomenological model of VP states in a smooth topological heterojunction. The model accounts for the finite interface depth, the effect of electric fields including Dirac screening, and predicts the energy of the first VP state. The massive surface states are a hallmark of topological heterojunctions, whose understanding is crucial for transport studies and applications.
High-mobility hexagonal boron nitride (hBN)/graphene/hBN heterostructures are able to reach intrinsic limits of transport. Here, we investigate optoelectronic mixing, which is a demanding function ...combining efficient photodetection and fast carrier dynamics. Using such a heterostructure embedded in a coplanar waveguide, we obtain a record conversion efficiency of about −40 dB for frequencies up to 65 GHz. This performance is obtained at high doping in the photobolometric regime. We provide a microscopic model of the photodetection, which accurately describes the experimental observations, allows the assessment of the intrinsic limits of our device, and paves the way for device optimization by revealing the different mechanisms at play.
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•FIB allows production of commercially viable nanopore-based devices.•FIB technique applicable to free-standing and atomically-thin sheets.•FIB applicable for biosensing, energy, and ...ultrafiltration device fabrication.•FIB patterned h-BN flakes are very promising for biosensing applications.
Fabrication of nanopores and nanomasks has recently emerged as an area of considerable interest for research applications ranging from optics, to electronics and to biophysics. In this work we evaluate and compare the fabrication of nanopores, using a finely focused gallium beam, in free-standing membranes/films made of Si, SiN, and SiO2 (having thicknesses of a few tens of nanometers) and also in graphene and hexagonal boron nitride (h-BN) atomically thin suspended sheets. Mechanical resistance, charging effects and patterning performances are evaluated and compared. In spite of the very different properties of the membranes we report that reproducible nanopore fabrication in the sub-10nm range can be achieved in both amorphous and atomically thin sheets using Ga+ focused ion beams (FIB).
Abstract
The topological state that emerges at the surface of a topological insulator (TI) and at the TI-substrate interface are studied in metal–hBN–Bi
2
Se
3
capacitors. By measuring the RF ...admittance of the capacitors versus gate voltage, we extract the compressibility of the Dirac state located at a gated TI surface. We show that even in the presence of an ungated surface that hosts a trivial electron accumulation layer, the other gated surface always exhibits an ambipolar effect in the quantum capacitance. We succeed in determining the velocity of surface Dirac fermions in two devices, one with a passivated surface and the other with a free surface that hosts trivial states. Our results demonstrate the potential of RF quantum capacitance techniques to probe surface states of systems in the presence of a parasitic density-of-states.
Two‐particle interferometry in quantum Hall edge channels Marguerite, A.; Bocquillon, E.; Berroir, J.‐M. ...
Physica Status Solidi. B: Basic Solid State Physics,
March 2017, 2017-03-00, 20170301, 2017-03, Volume:
254, Issue:
3
Journal Article
Peer reviewed
Open access
Since the pioneering works of Hanbury‐Brown and Twiss, intensity–intensity correlations have been widely used in astronomical systems, for example, to detect binary stars. They reveal statistics ...effects and two‐particle interference, and offer a decoherence‐free probe of the coherence properties of light sources. In the quantum Hall edge channels, the concept of quantum optics can be transposed to electrons, and an analogous two‐particle interferometry can be developed, in order to characterize single‐electron states. We review in this article the recent experimental and theoretical progress on this topic.
Two‐photon interferometry has been widely used in astronomical systems since the late 1950s, for example to detect binary stars. In quantum Hall edge channels, this concept can be transposed to electrons propagating in electron quantum optics devices. Two‐particle interferometry then reveals statistics effects and provides means to characterize an electronic quantum state. In this Feature Article, the recent experimental and theoretical progress in the field is reviewed.
We describe here the realization of a single electron source similar to single photon sources in optics. On-demand single electron injection is obtained using a quantum dot connected to the conductor ...via a tunnel barrier of variable transmission (quantum point contact). Electron emission is triggered by a sudden change of the dot potential which brings a single energy level above the Fermi energy in the conductor. A single charge is emitted on an average time ranging from 100 ps to 10 ns ultimately determined by the barrier transparency and the dot charging energy. The average single electron emission process is recorded with a 0.5 ns time resolution using a real-time fast acquisition card. Single electron signals are compared to simulation based on scattering theory approach adapted for finite excitation energies.
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