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Register a new userEdgeless and Purely Gate-Defined Nanostructures in InAs Quantum Wells
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Nanostructures in InAs quantum wells have so far remained outside of the scope of traditional microfabrication techniques based on etching. This is due to parasitic parallel conduction arising from charge carrier accumulation at the physical edges of samples. Here we present a technique which enables the realization of quantum point contacts and quantum dots in two-dimensional electron gases of InAs purely by electrostatic gating. Multiple layers of top gates separated by dielectric layers are employed. Full quantum point contact pinch-off and measurements of Coulomb-blockade diamonds of quantum dots are demonstrated.
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We present low temperature transport measurements on double quantum dots in InAs nanowires grown by metal-organic vapor phase epitaxy. Two dots in series are created by lithographically defined top-gates with a procedure involving no extra insulating layer. We demonstrate the full tunability from strong to weak coupling between the dots. The quantum mechanical nature of the coupling leads to the formation of a molecular state extending over both dots. The excitation spectra of the individual dots are observable by their signatures in the nonlinear transport.
Semiconducting nanowires (NWs) are a versatile, highly tunable material platform at the heart of many new developments in nanoscale and quantum physics. Here, we demonstrate charge pumping, i.e., the controlled transport of individual electrons through an InAs NW quantum dot (QD) device at frequencies up to $1.3,$GHz. The QD is induced electrostatically in the NW by a series of local bottom gates in a state of the art device geometry. A periodic modulation of a single gate is enough to obtain a dc current proportional to the frequency of the modulation. The dc bias, the modulation amplitude and the gate voltages on the local gates can be used to control the number of charges conveyed per cycle. Charge pumping in InAs NWs is relevant not only in metrology as a current standard, but also opens up the opportunity to investigate a variety of exotic states of matter, e.g. Majorana modes, by single electron spectroscopy and correlation experiments.
Spin-orbit coupling in solids describes an interaction between an electrons spin, an internal quantum-mechanical degree of freedom, with its linear momentum, an external property. Spin-orbit interaction, due to its relativistic nature, is typically small in solids, and is often taken into account perturbatively. It has been recently realized, however, that materials with strong spin-orbit coupling can lead to novel states of matter such as topological insulators and superconductors. This exciting development might lead to a number of useful applications ranging from spintronics to quantum computing. In particular, theory predicts that narrow band gap semiconductors with strong spin-obit coupling are a suitable platform for the realization of Majorana zero-energy modes, predicted to obey exotic non-Abelian braiding statistics. The pursuit for realizing Majorana modes in condensed matter systems and investigating their exotic properties has been a subject of intensive experimental research recently. Here, we demonstrate the first realization of gate-defined wires where one-dimensional confinement is created using electrostatic potentials, on large area InAs two dimensional electron systems (2DESs). The electronic properties of the parent 2DES are fully characterized in the region that wires are formed. The strength of the spin-orbit interaction has been measured and tuned while the high mobility of the 2DES is maintained in the wire. We show that this scheme could provide new prospective solutions for scalable and complex wire networks.
We consider electrostatically coupled quantum dots in topological insulators, otherwise confined and gapped by a magnetic texture. By numerically solving the (2+1) Dirac equation for the wave packet dynamics, we extract the energy spectrum of the coupled dots as a function of bias-controlled coupling and an external perpendicular magnetic field. We show that the tunneling energy can be controlled to a large extent by the electrostatic barrier potential. Particularly interesting is the coupling via Klein tunneling through a resonant valence state of the barrier. The effective three-level system nicely maps to a model Hamiltonian, from which we extract the Klein coupling between the confined conduction and valence dots levels. For large enough magnetic fields Klein tunneling can be completely blocked due to the enhanced localization of the degenerate Landau levels formed in the quantum dots.
We report transport measurements of strained InAs/In$_{x}$Ga$_{1-x}$Sb composite quantum wells (CQWs) in the quantum spin Hall phase, focusing on the control of the energy gap through structural parameters and an external electric field. For highly strained CQWs with $x = 0.4$, we obtain a gap of 35 meV, an order of magnitude larger than that reported for binary InAs/GaSb CQWs. Using a dual-gate configuration, we demonstrate an electrical-field-driven topological phase transition, which manifests itself as a re-entrant behavior of the energy gap. The sizeable energy gap and high bulk resistivity obtained in both the topological and normal phases of a single device open the possibility of electrical switching of the edge transport.
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