Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session F13: Quantum Dots, Wires And Wells: Electronic Phenomena |
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Sponsoring Units: DCMP Chair: Paul Simmonds, Boise State University Room: 007D |
Tuesday, March 3, 2015 8:00AM - 8:12AM |
F13.00001: Using Dissipation to Stabilize a Quantum Critical Point in Two Quantum Dots Gu Zhang, Eduardo Novais, Harold Baranger We show how dissipation could be used to stabilize the two-impurity Kondo critical point in a double dot system. In the absence of dissipation, this intermediate coupling fixed point cannot be reached because charge transfer from the source to drain lead, always present in a realistic system, is a relevant perturbation. By using dissipative leads, as recently introduced in single dot experiments, this charge transfer can be suppressed, thus allowing the intermediate-coupling non-Fermi-liquid quantum critical point to be reached. We expect that when dissipation exceeds a critical value, zero conductance will be observed except at this critical point, which then is its experimental signature. We also studied the effect of dissipation on the two critical points at which the interdot exchange flows either to zero or infinity. [Preview Abstract] |
Tuesday, March 3, 2015 8:12AM - 8:24AM |
F13.00002: Stabilizing lateral strained-Si/SiGe material quantum dots Sergei Studenikin, G. Poulin-Lamarre, A. Sachrajda, T. Lu, N. Bishop, T. Pluym, P. Kotula, M. Lilly, M. Carroll In enhancement-mode SiGe quantum dot structures [2], 2DEG electrons are generated via the application of a positive global gate. The carrier mobility in such structures is limited by disorder potential at the oxide/silicon interface. Recently it was shown that the deteriorating effect of charge fluctuations can be substantially mitigated by incorporating a shielding electron layer at the surface -- a thin Si quantum well cap layer [1,2]. This cap layer can, however, cause instabilities, drifts and hysteretic behaviour. In this work we study the stability of a tunable quantum dot defined by lateral gates in a Si/SiGe structure with a thin silicon cap layer and Si3N4 dielectric layer between the global gate and the structure [2]. Different ``stabilization'' procedures are explored to stabilise the device using both dc transport and charge sensing measurements at 300 mK.~ \\[4pt] [1] T. M. Lu, et al., Applied Physics Letters \textbf{99} (2011)\\[0pt] [2] C.-T. Huang, J.-Y. Li, K. S. Chou, and J. C. Sturm, Applied Physics Letters \textbf{104} (2014) [Preview Abstract] |
Tuesday, March 3, 2015 8:24AM - 8:36AM |
F13.00003: Beyond hydrostatic strain in empirical pseudopotentials for the electronic structure of InGaAs quantum dots Ceyhun Bulutay, Asli Cakan Self-assembled quantum dots (SAQDs) are among the prime candidates for realizing semiconductor qubits. Even though much progress has been achieved toward understanding their electronic structure, more efforts are needed to reach the desired quantitative level for a precise control of the carrier and nuclear spin degrees of freedom. In this respect, the empirical pseudopotential method has been highly successful for structures involving more than hundred thousand atoms. However, due to lack of self-consistency, their use in strained environments, as in SAQDS, requires vital improvement. The main contribution of this work is to develop empirical pseudopotentials valid for inhomogeneous strain environments caused by cation alloying in InGaAs SAQDs. In our presentation, we first validate our approach with the ab initio density functional theory results based on Projector Augmented-Wave technique. This is followed by a comparison of the electronic structure results with and without strain-dependent pseudopotentials for InGaAs SAQDs having an alloy composition of 20-30\% indium, which is typically the case in the current samples. [Preview Abstract] |
Tuesday, March 3, 2015 8:36AM - 8:48AM |
F13.00004: Gigahertz quantized charge pumping through bottom gate defined InAs nanowire quantum dots Andreas Baumgartner, Minkyung Jung, Samuel d'Hollosy, Vitaly Guzenko, Morten Madsen, Jesper Nygard, Christian Sch\"onenberger We demonstrate charge pumping of individual electrons through a highly tunable InAs nanowire quantum dot at frequencies up to 1.2 GHz. The dot is induced electrostatically in the nanowire by a series of local bottom gates. A periodic modulation of a single gate is sufficient to obtain a dc current proportional to the applied frequency. We show how the dc bias, the modulation amplitude and the voltages on the local gates can be used to control the number of charges conveyed per cycle, which leads to characteristic current plateaus as a function of the respective parameter. In addition, we discuss the accuracy of the current plateaus and the impact of non-adiabatic electron distributions at large frequencies. Quantized charge pumping in InAs nanowires is relevant for current standards in metrology due to the typically large level spacing, and opens up the opportunity to investigate a variety of exotic states and transport processes using single electron spectroscopy and single electron correlation experiments, for example in Majorana bound states or in a Cooper pair splitter. [Preview Abstract] |
Tuesday, March 3, 2015 8:48AM - 9:00AM |
F13.00005: Majorana fermions in hybrid superconductor-semiconductor nanowire devices V. Mourik, K. Zuo, D.J. van Woerkom, F.R. de Vries, O. Gul, H. Zhang, M.A.W. de Moor, D. Car, E.P.A.M. Bakkers, L.P. Kouwenhoven Our experiment carried out in hybrid superconductor-semiconductor nanowire devices gave the first experimental indications for the existence of Majorana fermions [1], but many open questions need to be answered. Majorana fermions have to come in pairs, before we were only capable of probing one Majorana fermion. Majorana fermions should be fully gate controllable, which could not be demonstrated convincingly. Upon bringing Majorana fermions closer together, an energy splitting between the two is expected, giving rise to a pair of split peaks instead of a single zero bias peak (ZBP). We are performing new experiments in similar but improved three terminal normal-superconductor-normal InSb nanowire devices. This enables the possibility to probe Majorana fermions occurring at the ends of the superconducting contact by using tunneling spectroscopy. Furthermore, the devices have an improved gate design enabling more efficient gating under the superconducting contact and they have improved contact interfaces resulting in less undesired resonant states. We have observed ZBP's in a large magnetic field range, an oscillatory behavior from ZBP to split peak and back, and tunability of ZBP's by gates underneath the superconducting contact. [1] V. Mourik, K. Zuo et al., Science 2012 [Preview Abstract] |
Tuesday, March 3, 2015 9:00AM - 9:12AM |
F13.00006: Spin-orbit coupling in semiconductor nanowires: Physical limits for Majorana states Tiago de Campos, Guilherme Sipahi, Paulo Eduardo de Faria Junior, Carlos Bastos, Igor Zutic, Martin Gmitra, Jaroslav Fabian Proximity induced superconductivity in semiconductor nanowires with spin-orbit coupling (SOC) provides a promising realization of Majorana fermions [1,2]. While SOC is typically included within only the first conduction band, such simplified models lack a more detailed understanding of Majorana fermions and their implementation. We perform systematic and comprehensive numerical investigations of SOC in zinc-blende and wurtzite semiconductor cylindrical nanowires. We employ the k.p method, with input parameters fitted to first-principles calculations, to determine realistic values of the Rashba and Dresselhaus spin-orbit fields in nanowires of varying diameters. Specifically, we use a state of the art 14 band k.p formalism together with the envelope function approach [1] to obtain the electronic band structure for various compounds, and analyze the effect of the quantum confinement on the effective masses and spin-orbit splitting of the subbands. We also make specific suggestions towards the optimal orientation and geometry, evaluating the prospects of the nanowires as platforms to observe Majorana states. [1] J. Alicea, Rep. Prog. Phys. 75, 076501 (2012). [2] A. Yu. Kitaev, Phys-Usp. 44, 131 (2001). [3] P. E. Faria Junior and G. M. Sipahi, J. Appl. Phys. 10, 103716 (2012). [Preview Abstract] |
Tuesday, March 3, 2015 9:12AM - 9:24AM |
F13.00007: Current phase relation in nanowire based Josephson junctions Daniel Szombati, Stevan Nadj-Perge, Attila Geresdi, Vincent Mourik, Kun Zuo, David Woerkom, Diana Car, Erik Bakkers, Leo Kouwenhoven Junctions based on small band-gap nanowires are convenient platform for studying Josephson effect in the presence of strong spin-orbit coupling. As predicted by theory, due to the interplay between strong Zeeman interaction and large spin orbing coupling in these nanowires, the critical current and in particular current phase relation exhibits rich set of features in the presence of external magnetic field and electrostatic gating. We study supercurrent transport through Indium Antimonide nanowires contacted using Niobium-Titanium-Nitride leads using both current and phase bias measurements. Our results provide useful insights into superconductor/semiconductor hybrid systems capable of hosting Majorana fermions, potential building blocks for topological quantum computing. [Preview Abstract] |
Tuesday, March 3, 2015 9:24AM - 9:36AM |
F13.00008: The Quantum Pinch Effect in Semiconducting Quantum Wires M.S. Kushwaha We investigate a two-component, cylindrical, quasi-one-dimensional quantum plasma subjected to a {\em radial} confining harmonic potential and an applied magnetic field in the symmetric gauge. It is demonstrated that such a system as can be realized in semiconducting quantum wires offers an excellent medium for observing the quantum pinch effect at low temperatures. An exact analytical solution of the problem allows us to make significant observations: surprisingly, in contrast to the classical pinch effect, the particle density as well as the current density display a {\em determinable} maximum before attaining a minimum at the surface of the quantum wire. The effect will persist as long as the equilibrium pair density is sustained. Therefore, the technological promise that emerges is the route to the precise electronic devices that will control the particle beams at the nanoscale. [Preview Abstract] |
Tuesday, March 3, 2015 9:36AM - 9:48AM |
F13.00009: Electrical and thermal transport in inhomogeneous quantum wires Wade DeGottardi, K.A. Matveev We present a theoretical study of the transport properties of long quantum wires. Our theory accounts for long-range disorder and electron interactions of arbitrary strength, both of which are expected to be crucial to an understanding of experimental results. Our approach involves extending the usual bosonization scheme to account for the finite lifetime of the excitations. We cast our results in terms of the thermal conductivity and bulk viscosity of the electron liquid. At sufficiently high temperatures, we show that our results reduce to those expected from classical hydrodynamics. [Preview Abstract] |
Tuesday, March 3, 2015 9:48AM - 10:00AM |
F13.00010: Characterization of NiSi nanowires as field emitters and limitations of Fowler-Nordheim model at the nanoscale Amina B. Belkadi, E. Gale, A.F. Isakovic Nanoscale field emitters are of technological interest because of the anticipated faster turn-on time, better sustainability and compactness. This report focuses on NiSi nanowires as field emitters for two reasons: (a) possible enhancement of field emission in nanoscale field emitters over bulk, and (b) achieving the same field emission properties as in bulk, but at a lower energy cost. To this end, we have grown, fabricated and characterized NiSi nanowires as field emitters. Depending on the geometry of the NiSi nanowires (aspect ratio, shape etc.), the relevant major field emission parameters, such as (1) the turn-on field, (2) the work function, and (3) the field enhancement factor, can be comparable or even superior to other recently explored nanoscale field emitters, such as CdS and ZnO. We also report on a comparative performance of various nanoscale field emitters and on the difficulties in the performance comparison in the light of relatively poor applicability of the standard Folwer-Nordheim model for field emission analysis for the case of the nanoscale field emitters. Proposed modifications are discussed. [Preview Abstract] |
Tuesday, March 3, 2015 10:00AM - 10:12AM |
F13.00011: Ultra-low noise atomically patterned nanostructures in Si Saquib Shamim, Bent Weber, Michelle Y. Simmons, Arindam Ghosh Advancement in scanning tunnelling microscopy (STM) based lithography has made it possible to achieve low resistivity atomic scale wires and single donor quantum dot devices in silicon. Due to extreme sensitivity of these devices to any disorder or charge traps, it is of paramount importance to explore the noise magnitude in these systems. Here we investigate low frequency noise measurements in two STM patterned atomic scale wires of phosphorous dopants in Si of diameters $4.5$~nm and $1.5$~nm. The variation of noise with gate voltage indicates that the noise arises due to trapping-detrapping of electrons between the wire and charged traps. The Hooge parameter for these wires is $10^{-4}$ to $10^{-6}$ (for different gate voltages), which is one of the lowest reported for any one-dimensional system. The reason for such low noise magnitude can be two-fold. First, a complete monolithic fabrication procedure avoids any direct metallic contact to the one-dimensional system and hence prevents any Schottky barrier. Second possibility is that the Coulomb repulsion between the charges on traps doesn't allow many traps to be activated simultaneously. Aimed at being the backbone of silicon quantum computation scheme, a reduced noise in these devices is technologically crucial. [Preview Abstract] |
Tuesday, March 3, 2015 10:12AM - 10:24AM |
F13.00012: Understanding electronic band-edge properties of GaSbAs/GaAs nanostructures through k.p theory simulations Christina Jones, Emmanouil Kioupakis Gallium antimonide (GaSb) nanostructures embedded in gallium arsenide (GaAs) have been predicted by theory and have been found experimentally to demonstrate both type-I and type-II band alignment. The ability of GaSb nanostructures to exhibit two band alignment types makes them versatile in applications such as LEDs, photodetectors, and charge-based memory elements. We present a systematic study of the mechanisms behind the band alignment type in order to understand the underlying physics behind the alignment transition and allow for prediction and optimization of electronic properties. We employ the eight-band k.p method through a commercially available software package (nextnano) to calculate the band structure by self-consistently solving the Schroedinger and Poisson equations and including strain and polarization charges. Results obtained for GaSbAs/GaAs quantum wells show both type-I and type-II band alignment depending on strain and composition. Calculated band-edges are compared to published experimental results. [Preview Abstract] |
Tuesday, March 3, 2015 10:24AM - 10:36AM |
F13.00013: Resonant Subband Landau Level Coupling in GaAs/AlGaAs/GaAs Coupled Quantum Wells Li-Chun Tung, Dmitry Smirnov Subband energies and intersubband couplings of symmetric GaAs/AlGaAs/GaAs coupled quantum wells have been investigated by magneto-infrared spectroscopy at 4K up to 35T. Most of the proposed quantum well infrared photodetectors consist of coupled quantum wells, and the detection of infrared photons is accomplished via exciting bound electrons of lower subbands to a continuous state via intersubband transitions. With the presence of a small in-plane magnetic field, subbands due to the mixing of wave functions of individual quantum wells are studied by the anti-level crossing resonance of the Landau levels belonging to different subbands. Intersubband coupling between the first subband of the coupled quantum wells (Symmetric mixing of the lowest subbands of the individual quantum wells) to the third subband is observed, while the others are forbidden. The symmetry selection rule for the intersubband transitions of symmetric coupled quantum wells will be discussed in the presentation. [Preview Abstract] |
Tuesday, March 3, 2015 10:36AM - 10:48AM |
F13.00014: Mott transition in the coupled quantum wells with the external periodic potential Oleg Berman, Roman Kezerashvili, Yurii Lozovik, Klaus Ziegler We study a system of spatially separated electrons and holes in two coupled quantum wells within a temperature-dependent mean-field approach. A periodic potential is applied to the quantum wells which allows us to modify the spectral properties of the electrons and the holes. This system exhibits a rich phase diagram, consisting of a BCS phase with electron-hole pairs, an electron-hole plasma and a bosonic Mott phase of tightly bound electron-hole pairs. The latter have no phase coherence in contrast to the pairs in the BCS phase. We discuss the transitions between the different phases in terms of temperature, density and interaction strength. [Preview Abstract] |
Tuesday, March 3, 2015 10:48AM - 11:00AM |
F13.00015: Terahertz spectroscopy of two-dimensional electron-hole pairs: probing Mott physics of magneto-excitons Qi Zhang, Weilu Gao, John Watson, Michael Manfra, Junichiro Kono Density-dependent Coulomb interactions can drive electron-hole ($e-h)$ pairs in semiconductors through an excitonic Mott transition from an excitonic gas into an $e-h$ plasma. Theoretical studies suggest that these interactions can be strongly modified by an external magnetic field, including the absence of inter-exciton interactions in the high magnetic field limit in two dimensions, due to an $e-h$ charge symmetry, which results in ultrastable magneto-excitons. Here, we present a systematic experimental study of $e-h$ pairs in photo-excited undoped GaAs quantum wells in magnetic fields with ultrafast terahertz spectroscopy. We simultaneously monitored the dynamics of the intraexcitonic 1$s$-2$p$ transition (which splits into 1$s$-2$p_{\mathrm{+}}$ and 1$s$-2$p_{\mathrm{-}}$ transitions in a magnetic field) and the cyclotron resonance of unbound electrons and holes up to 10 Tesla. We found that the 1$s$-2$p_{\mathrm{-}}$ absorption feature is robust at high magnetic fields even under high excitation fluences, indicating magnetically enhanced stability of excitons. We will discuss the Mott physics of magneto-excitons as a function of temperature, $e-h$ pair density, optical pump delay time, as well as magnetic field, and also compare two-dimensional excitons in GaAs quantum wells with three-dimensional excitons in bulk GaAs. [Preview Abstract] |
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