Finite Conductivity in Mesoscopic Hall Bars of Inverted InAs/GaSb Quantum Wells

We have studied experimentally the low temperature conductivity of mesoscopic size InAs/GaSb quantum well Hall bar devices in the inverted regime. Using a pair of electrostatic gates we were able to move the Fermi level into the electron-hole hybridization state, and observe a mini gap. Temperature dependence of the conductivity in the gap shows residual conductivity, which can be consistently explained by the contributions from the free as well as the hybridized carriers in the presence of impurity scattering, as proposed by Naveh and Laikhtman [Euro. Phys. Lett., 55, 545-551 (2001)]. Experimental implications for the stability of proposed helical edge states will be discussed.Comment: 5 pages, 4 figure

Evidence for Helical Edge Modes in Inverted InAs/GaSb Quantum Wells

We present an experimental study of low temperature electronic transport in the hybridization gap of inverted InAs/GaSb composite quantum wells. Electrostatic gate is used to push the Fermi level into the gap regime, where the conductance as a function of sample length and width is measured. Our analysis shows strong evidence for the existence of helical edge modes proposed by Liu et al [Phys. Rev. Lett., 100, 236601 (2008)]. Edge modes persist inspite of sizable bulk conduction and show only a weak magnetic field dependence - a direct consequence of gap opening away from zone center.Comment: 4 pages, 4 figure

Perfect Andreev Reflection of Helical Edge Modes in InAs/GaSb Quantum Wells

We present an experimental study of inverted InAs/GaSb composite quantum wells in the hybridization regime and contacted by superconducting electrodes. A front gate is used to vary the Fermi level into the mini-gap, where recent experiments indicate existence of helical edge modes [arXiv:1105.0137]. Zero bias dips in differential resistance are observed across the mini-gap, suggesting transport dominated by Andreev reflection processes. Evolution of the mini-gap differential resistance with applied bias as well as measured mini-gap excess current of 150 nA are in good agreement with the prediction of perfect Andreev reflection of the helical edge modes, which is necessitated by the absence of back-scattering channels. The perfect Andreev reflection occurs in spite of a finite barrier at the interface and shows strong sensitivity to time-reversal breaking - hallmarks of the helical nature of quantum spin Hall edges

Andreev Reflection of Helical Edge Modes in InAs=GaSb Quantum Spin Hall Insulator

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

Enhancement of the ν=5/2\nu = 5/2 Fractional Quantum Hall State in a Small In-Plane Magnetic Field

Using a 50-nm width, ultra-clean GaAs/AlGaAs quantum well, we have studied the Landau level filling factor $\nu = 5/2$ fractional quantum Hall effect in a perpendicular magnetic field $B \sim$ 1.7 T and 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 $0 < \theta < 25^\circ$ (corresponding to an in-plane magnetic field 0 $<$ $B_\parallel$ $< 0.8$ T). In the same range of $\theta$ the activation gaps of both the 7/3 and the 8/3 states are found to increase with tilt. The 5/2 state transforms into a compressible Fermi liquid upon tilt angle $\theta > 60^\circ$, and the composite fermion series [2+$p/(2p\pm1)$], $p =$ 1, 2 can be identified. Based on our results, we discuss the relevance of a Skyrmion spin texture at $\nu = 5/2$ associated with small Zeeman energy in wide quantum wells, as proposed by W$\acute{\text o}$js $et$ $al$., Phys. Rev. Lett. 104, 086801 (2010).Comment: 5+ pages, 3 figures, accepted for by Phy. Rev. Let

Quantum transport in inverted indium arsenide/gallium antimonide composite quantum wells

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

Modeling the electrocoagulation treatment of orthophosphate with aluminium electrodes

Provedeno je istraživanje učinka elektrokoagulacijskog procesa s aluminijevim (Al) elektrodama na smanjenje masene koncentracije fosfata u vodi. Primijenjene su gustoće struje od 30, 75 i 150 A m-2 te inicijalna koncentracija fosfata od 27 mg/L (PO4-P). Postignuta je učinkovitost uklanjanja od 86% nakon 90 min rada reaktora s ugrađenim Al elektrodama pri gustoći struje od 75 A m-2 i uz inicijalnu koncentraciju NaCl od 2 g dm-3. Osim koncentracije fosfata (PO4-P), u komori elektrokoagulacijskog laboratorijskog pilot uređaja praćene su i promjene koncentracije otopljenog kisika (DO), te promjene temperature vode i pH. 3D numerički model primijenjen je za simulaciju polja strujanja i koncentracije fosfata u komori elektrokoagulacijskog uređaja. Modelom pronosa fosfata obuhvaćeni su procesi generiranja i taloženja flokula, te adsorpcije fosfata na flokule. Modelska parametrizacija temelji se na rezultatima mjerenja.The efficiency of the electrocoagulation (EC) process for the removal of phosphate (PO4-P) is analyzed using a batch pilot device with aluminum electrodes situated within the reactor. The applied current density at the electrodes (30, 75 and 150 A/m2) was the focus of the research. The initial PO4-P concentration is the same in all experiments (27 mg PO4-P/L). The concentrations of PO4-P and dissolved oxygen (DO), as well as the water temperature and pH, were measured in the batch chamber. The initial PO4-P concentration is reduced by 86 % after 90 min of reactor operation when the initial NaCl concentration is 2 g/L, while the current density is maintained at 75 A/m2. Additionally, a three-dimensional (3D) numerical model for PO4-P removal via the EC process is developed. By using the 3D numerical model, spatially continuous velocity fields and the PO4-P concentration were determined. Numerical model of PO4-P transport includes the processes of phosphate adsorption on flocculent, along with coagulated particles settling. Numerical model Parametrisation relies on the measurement results from the experiments made on batch pilot device

Modeling the electrocoagulation treatment of orthophosphate with aluminium electrodes

Provedeno je istraživanje učinka elektrokoagulacijskog procesa s aluminijevim (Al) elektrodama na smanjenje masene koncentracije fosfata u vodi. Primijenjene su gustoće struje od 30, 75 i 150 A m-2 te inicijalna koncentracija fosfata od 27 mg/L (PO4-P). Postignuta je učinkovitost uklanjanja od 86% nakon 90 min rada reaktora s ugrađenim Al elektrodama pri gustoći struje od 75 A m-2 i uz inicijalnu koncentraciju NaCl od 2 g dm-3. Osim koncentracije fosfata (PO4-P), u komori elektrokoagulacijskog laboratorijskog pilot uređaja praćene su i promjene koncentracije otopljenog kisika (DO), te promjene temperature vode i pH. 3D numerički model primijenjen je za simulaciju polja strujanja i koncentracije fosfata u komori elektrokoagulacijskog uređaja. Modelom pronosa fosfata obuhvaćeni su procesi generiranja i taloženja flokula, te adsorpcije fosfata na flokule. Modelska parametrizacija temelji se na rezultatima mjerenja.The efficiency of the electrocoagulation (EC) process for the removal of phosphate (PO4-P) is analyzed using a batch pilot device with aluminum electrodes situated within the reactor. The applied current density at the electrodes (30, 75 and 150 A/m2) was the focus of the research. The initial PO4-P concentration is the same in all experiments (27 mg PO4-P/L). The concentrations of PO4-P and dissolved oxygen (DO), as well as the water temperature and pH, were measured in the batch chamber. The initial PO4-P concentration is reduced by 86 % after 90 min of reactor operation when the initial NaCl concentration is 2 g/L, while the current density is maintained at 75 A/m2. Additionally, a three-dimensional (3D) numerical model for PO4-P removal via the EC process is developed. By using the 3D numerical model, spatially continuous velocity fields and the PO4-P concentration were determined. Numerical model of PO4-P transport includes the processes of phosphate adsorption on flocculent, along with coagulated particles settling. Numerical model Parametrisation relies on the measurement results from the experiments made on batch pilot device

Transport Properties of Topological Phases in Broken Gap Indium Arsenide/Gallium Antimonide Based Quantum Wells

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