Browsing by Author "Knez, Ivan"

Now showing 1 - 6 of 6
  • Thumbnail Image
    Item
    Andreev Reflection of Helical Edge Modes in InAs=GaSb Quantum Spin Hall Insulator
    (American Physical Society, 2012) Knez, Ivan; Du, Rui-Rui; Sullivan, Gerard
    We present an experimental study of S−N−S junctions, with N being a quantum spin Hall insulator made of InAs/GaSb. A front gate is used to vary the Fermi level into the minigap, where helical edge modes exist [Phys. Rev. Lett. 107, 136603 (2011)]. In this regime we observe a ∼2e2/h Andreev conductance peak, consistent with a perfect Andreev reflection on the helical edge modes predicted by theories. The peak diminishes under a small applied magnetic field due to the breaking of time-reversal symmetry. This work thus demonstrates the helical property of the edge modes in a quantum spin Hall insulator.
  • Thumbnail Image
    Item
    Enhancement of the v=5/2 Fractional Quantum Hall State in a Small In-Plane Magnetic Field
    (American Physical Society, 2012) Liu, Guangtong; Zhang, Chi; Tsui, D.C.; Knez, Ivan; Levine, Aaron; Du, R.R.; Pfeiffer, L.N.; West, K.W.
    Using a 50-nm-width ultraclean GaAs=AlGaAs quantum well, we have studied the Landau level filling factor v=5/2 fractional quantum Hall effect in a perpendicular magnetic field B 1:7 Tand determined its dependence on tilted magnetic fields. Contrary to all previous results, the 5=2 resistance minimum and the Hall plateau are found to strengthen continuously under an increasing tilt angle 0< <25 (corresponding to an in-plane magnetic field 060 , and the composite fermion series [2 þ p=ð2p 1Þ, p ¼ 1; 2] can be identified. Based on our results, we discuss the relevance of a Skyrmion spin texture at v= 5/2 associated with small Zeeman energy in wide quantum wells, as proposed by Wo´js et al
  • Thumbnail Image
    Item
    Observation of Edge Transport in the Disordered Regime of Topologically Insulating InAs/GaSb Quantum Wells
    (American Physical Society, 2014-01) Knez, Ivan; Rettner, Charles T.; Yang, See-Hun; Parkin, Stuart S.P.; Du, Lingjie; Du, Rui-Rui; Sullivan, Gerard
    We observe edge transport in the topologically insulating InAs=GaSb system in the disordered regime. Using asymmetric current paths we show that conduction occurs exclusively along the device edge, exhibiting a large Hall signal at zero magnetic fields, while for symmetric current paths, the conductance between the two mesoscopicly separated probes is quantized to 2e2=h. Both quantized and self-averaged transport show resilience to magnetic fields, and are temperature independent for temperatures between 20 mK and 1 K.
  • Thumbnail Image
    Item
    Quantum transport in inverted indium arsenide/gallium antimonide composite quantum wells
    (2010) Knez, Ivan; Du, Rui-Rui
    We present a comprehensive study of low temperature quantum transport in double gated InAs/GaSb composite quantum wells. Recently, it has been proposed that this system in inverted regime should exhibit the topologically insulating (TI) phase, characterized by an energy gap in the bulk and gapless edge modes, protected from backscattering by time reversal symmetry. We sweep the Fermi level through the bulk mini-gap, observing resistance peaks and finding strong evidence for the existence of the mini-gap; however, the mini-gap does not show insulating behavior, with a residual bulk conductivity which is a few times larger then the expected contribution from the edge. Our data indicate, that bulk conductivity is not an issue of "dirt", which can be improved by simply reducing the amount of disorder, but a fundamental property of strongly coupled electron-hole systems in realistic materials, which must be considered in studies of proposed TI edge modes.
  • Thumbnail Image
    Item
    Robust Helical Edge Transport in Gated InAs/GaSb Bilayers
    (American Physical Society, 2015) Du, Lingjie; Knez, Ivan; Sullivan, Gerard; Du, Rui-Rui
    We have engineered electron-hole bilayers of inverted InAs/GaSb quantum wells, using dilute silicon impurity doping to suppress residual bulk conductance. We have observed robust helical edge states with wide conductance plateaus precisely quantized to 2e2/h in mesoscopic Hall samples. On the other hand, in larger samples the edge conductance is found to be inversely proportional to the edge length. These characteristics persist in a wide temperature range and show essentially no temperature dependence. The quantized plateaus persist to a 12ﾠT applied in-plane field; the conductance increases from 2e2/h in strong perpendicular fields manifesting chiral edge transport. Our study presents a compelling case for exotic properties of a one-dimensional helical liquid on the edge of InAs/GaSb bilayers.
  • Thumbnail Image
    Item
    Transport Properties of Topological Phases in Broken Gap Indium Arsenide/Gallium Antimonide Based Quantum Wells
    (2012) Knez, Ivan; Du, Rui-Rui
    The quantum Spin Hall Insulator (QSHI) is a two-dimensional variant of a novel class of materials characterized by topological order, whose unique properties have recently triggered much interest and excitement in the condensed matter community. Most notably, the topological properties of these systems hold great promise in mitigating the difficult problem of decoherence in implementations of quantum computers. Although QSHI has been theoretically predicted in a few different materials, prior to the work presented in this thesis, only the HgTe/CdTe semiconductor system has shown direct evidence for the existence of this phase. Ideally insulating in the bulk, QSHI is characterized by one-dimensional channels at the sample perimeter, which have a helical property, with carrier spin tied to the carrier direction of motion, and protected from elastic back-scattering by time-reversal symmetry. In this thesis we present low temperature transport measurements, showing strong evidence for the existence of proposed helical edge channels in InAs/CaSb quantum wells, which thus emerge as an important alternate to HgTe/CdTe quantum wells in studies of two-dimensional topological insulators and superconductors. Surprisingly, edge modes persist in spite of comparable bulk conduction of non-trivial origin and show only weak dependence on magnetic field in mesoscopic devices. We elucidate that the seeming independence of edge on bulk transport comes due to the disparity in Fermi wave-vectors between the bulk and the edge, leading to a total internal reflection of the edge modes. Furthermore, low Schottky barrier of this material system and good interface to superconductors allows us to probe topological properties of helical channels in Andreev reflection measurements, opening a promising route towards the realization of topologically superconducting phases hosting exotic Majorana modes.