Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session A44: Focus Session: Nanoscale Transport - Wires, Dots, Point Contacts |
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Sponsoring Units: DMP Chair: Charlie Marcus, Harvard University Room: Colorado Convention Center 507 |
Monday, March 5, 2007 8:00AM - 8:12AM |
A44.00001: Time Resolved Characterization of Tunneling in a Quantum Dot Kenneth MacLean, Sami Amasha, Iuliana Radu, Dominik Zumbuhl, Marc Kastner, Micah Hanson, Arthur Gossard Measurements are presented of the rates for tunneling on and off a laterally defined GaAs quantum dot as a function of drain source bias, plunger gate voltage, and magnetic field. The measurements are obtained using a quantum point contact as a real-time charge sensor, and utilizing pulsed gate techniques. In zero magnetic field, we find evidence that the tunneling is elastic, and that the observed exponential dependences of the tunneling rates on drain-source bias and plunger gate voltage agree quantitatively with a model that takes into account changes in the electron energy relative to the top of the tunnel barrier. In a magnetic field applied parallel to the two dimensional electron gas, we resolve contributions to the tunneling from the two Zeeman sublevels, and discuss how the magnetic field modifies the tunneling rates. This work has been supported by the ARO (W911NF-05-1-0062), the NSF (DMR-0353209), and in part by the NSEC Program of the NSF (PHY-0117795). [Preview Abstract] |
Monday, March 5, 2007 8:12AM - 8:24AM |
A44.00002: Charge Transitions in a Quantum Dot Induced by an Adjacent Quantum Point Contact S. Amasha, K. MacLean, D. M. Zumb\"{u}hl, I. P. Radu, M. A. Kastner, M. P. Hanson, A. C. Gossard Quantum point contact (QPC) charge sensors have become an important tool for measuring the occupation of laterally gated quantum dots in AlGaAs/GaAs heterostructures. However, electrical fluctuations across the QPC have been shown to induce changes in the dot occupation. Using real-time charge detection techniques, we observe this effect in the increased rates at which electrons tunnel on and off the dot with increasing bias applied across the adjacent QPC. Applying an in-plane magnetic field splits the orbital states by the Zeeman energy. We present measurements of the probability of being in the excited spin state after a large bias pulse is applied across the QPC. We propose that changes in dot occupation can qualitatively account for an observed enhancement in the probability of being in the excited spin state. This work is supported by the ARO (W911NF-05-1-0062), the NSF (DMR-0353209) and in part by the NSEC Program of the NSF (PHY-0117795). [Preview Abstract] |
Monday, March 5, 2007 8:24AM - 8:36AM |
A44.00003: Interpretation of Fano lineshape reversal in quantum waveguides H.R. Sadeghpour, N. Moiseyev, S. Klaiman Fano lineshape parameter (q) reversal is a proxy for interaction beyond the usual interference of indistinguishable quantum pathways. Reversal of the Fano parameter has been observed recently in quantum dots (QD). We show that such a profile reversal may come about from the interaction of interlopping over-the-top states (shape resonances) in the ``non-resonant'' channel with the QD bound states, interacting with the continuum channel (Feshbach resonances). Using this mechanism we show that with minimal modifications of the QD parameters, we can affect the presence or absence of interlopping resonances and hence lineshape profile reversal, as a way of coherence engineering. [Preview Abstract] |
Monday, March 5, 2007 8:36AM - 9:12AM |
A44.00004: Spontaneous Spin Polarization in Quantum Point Contacts Invited Speaker: Mesoscopic systems exhibit a range of non-trivial spin-related phenomena in the low density regime, where inter-particle Coulomb interactions become comparable to their kinetic energy. In zero-dimensional systems spontaneous polarization of a few-electron quantum dot leads to a spin blockade, a remarkable effect where mismatch of a single spin blocks macroscopic current flow. In two-dimensional hole gases there is an experimental evidence of a finite spin polarization even in the absence of a magnetic field. In one-dimensional systems – quantum wires and quantum point contacts - a puzzling so-called ``0.7 structure'' has been observed below the first quantization plateau. Experiments suggest that an extra plateau in the conductance vs gate voltage characteristic at $0.7\times 2e^2/h$ is spin related, however, the origin of the phenomenon is not yet understood and is highly debated. We report direct measurements of finite polarization of holes in a quantum point contact (QPC) at conductances $G < 2e^2/h$ [1]. We incorporated QPC into a magnetic focusing device so that polarization can be measured directly using a recently developed spatial spin separation technique [2]. Devices are fabricated from p-type GaAs/AlGaAs heterostructures. A finite polarization is measured in low-density regime, when conductance of a point contact is tuned to $< 2e^2/h$. We found that polarization is stronger in samples with well defined ``0.7 structure''. \newline\newline [1] L.P. Rokhinson, L.N. Pfeiffer and K.W. West,``Spontaneous spin polarization in quantum point contacts,'' Physical Review Letters {\bf 96}, 156602 (2006) \newline [2] L.P. Rokhinson, V. Larkina, Y.B. Lyanda-Geller, L.N. Pfeiffer and K.W. West, ``Spin separation in cyclotron motion,'' Physicsl Review Letters {\bf 93}, 146601 (2004) [Preview Abstract] |
Monday, March 5, 2007 9:12AM - 9:24AM |
A44.00005: ``0.7'' Conductance Anomaly in quantum point contacts J. Shabani, R.N. Bhatt We demonstrate that an anomaly close to 0.7(2e$^{2}$/h) [rather than 0.5(2e$^{2}$/h) as in a Kondo-type model$^{1}$] in the conductance plot of quantum point contacts$^{2}$ arises naturally in a model with a quasi-bound state \textit{at the Fermi level} within an Anderson impurity model framework. The same model yields good agreement with the observed dependence$^{3}$ of conductance with gate voltage, magnetic field, temperature and also with the observed zero bias anomaly. Further implications within this model are explored and contrasted with other proposed explanations of the anomaly$^{1}$. \newline 1. Y. Meir, K. Hirose and N. S. Wingreen, Phys. Rev. Lett. \textbf{89,} 196802 (2002). \newline 2. K. J. Thomas \textit{et al.}, Phys. Rev. Lett. \textbf{77}, 135 (1996). \newline 3. S. M. Cronenwett \textit{et al.}, Phys. Rev. Lett.\textbf{ 88}, 226805 (2002). [Preview Abstract] |
Monday, March 5, 2007 9:24AM - 9:36AM |
A44.00006: Spin-orbit induced spin-density wave in a quantum wire Suhas Gangadharaiah, Jianmin Sun, Oleg Starykh We consider an interacting quantum wire in the presence of a magnetic field and spin-orbit interaction. We show that under a subtle interplay of magnetic and spin-orbit terms, new scattering channels open up when the magnetic field and the spin-orbit axes are orthogonal: two electrons with opposite momentum and in the same spin-subband scatter into a different spin-subband while conserving momentum. This scattering process is relevant and results in a spin-density wave (SDW) state. We next analyze charge transport property in a scenario when the SDW state survives the presence of a single weak impurity. We find that the single particle back-scattering off a non-magnetic impurity becomes irrelevant. The sensitivity of the SDW state, and hence the charge transport, to the mutual orientation and magnitude of the magnetic and spin-orbit terms can be used for the experimental verification of this novel field and spin- orbit induced state. [Preview Abstract] |
Monday, March 5, 2007 9:36AM - 9:48AM |
A44.00007: Suppression of Landau level spin splitting in quantum point contacts Iuliana Radu, J.B. Miller, E. Levenson-Falk, S. Amasha, D.M. Zumb\"{u}hl, M.A. Kastner, C.M. Marcus, L.N. Pffeifer, K.W. West We investigate low temperature transport properties of split-gate devices lithographically patterned on a GaAs/AlGaAs heterostructure containing a 2D electron gas with mobility 2000 m$^{2}$/Vs in a perpendicular magnetic field. By using quantum point contacts (QPCs) with different lithographic widths and varying the voltage applied on the gates for each QPC, we can control the width of the conduction channel continuously from $\sim$3000 to $\sim$100nm. The width of the channel is estimated from the low-field magnetic field dependence of the conductance through the QPC. We find that the spin-splitting of the Landau levels is suppressed in the QPCs compared to the bulk, and we measure the filling factor $\nu$$_{max}$ above which spin splitting can no longer be observed. Surprisingly, we find that $\nu$$_{max}$ is approximately half the number of quantum channels in the QPC for all widths less than ~1200 nm. This work was partially supported by ARO (W911NF-05-1-0062), by the NSEC program of NSF (PHY-0117795), by NSF (DMR-0353209) and by Project Q of Microsoft. [Preview Abstract] |
Monday, March 5, 2007 9:48AM - 10:00AM |
A44.00008: Effect of a strong spin-charge separation on tunneling into a 1D wire with impurity Andrew Meyertholen, Lingfeng Zhang, Michael Fogler We analyze the tunneling of electrons into a 1D nanowire with a large difference in velocities of spin and charge excitations: charges are ``fast,'' spins are ``slow.'' This system is modeled as a Wigner crystal of charges whose spins are ordered as in a antiferromagnetic Heisenberg spin chain. If the wire contains an impurity, electron tunneling in its vicinity causes a novel type of the orthogonality catastrophe. The tunneling electron shifts the charge distribution of a Wigner-crystal, which causes a shake-up processes in the spin sector. The corresponding suppression of the tunneling has a novel temperature dependence, which can be used for an experiemental validation of the spin-charge separation in low-density nanowires. [Preview Abstract] |
Monday, March 5, 2007 10:00AM - 10:12AM |
A44.00009: Quantum Dots on Silicon Nanowires Hyuk Ju Ryu, Jeremy Higgins, Pinray Huang, Jeremy Streifer, Robert Hamers, Song Jin, Mark Eriksson Silicon nanowires have single-crystal structure, well-controlled doping, and can be integrated into devices using either directed assembly and dielectrophoresis or electron-beam lithography and lift-off. Such nanowires, with nanometer size in two dimensions, provide advantages for the fabrication of ultra-small silicon quantum dots with potentially long spin coherence times. We present methods for the fabrication of silicon nanowire-based single electron transistors, and we show results of both room temperature and low temperature transport measurements. The metal electrode structure and annealing process have been intensively investigated to obtain the necessary contact properties. Either metal/nanowire contacts or electrostatically depleted regions have been used for tunneling barriers for quantum dots. Coulomb blockade has been demonstrated successfully, showing 1.3 aF and 1.1 meV for the gate capacitance and the charging energy respectively. Studies of double quantum dots and spin-dependent effects are ongoing. [Preview Abstract] |
Monday, March 5, 2007 10:12AM - 10:24AM |
A44.00010: Ballistic hole transport and spin-orbit effects in GaAs quantum wires Alex Hamilton, R. Danneau, O. Klochan, W.R. Clarke, A.P. Micolich, L.H. Ho, M.Y. Simmons, M. Pepper, D.A. Ritchie, K. Muraki, Y. Hirayama, U. Zuelicke Studying the spin degree of freedom of charge carriers in semiconductors has become an area of significant current interest. Although spin-orbit coupling is extremely strong in p-type semiconductors such as GaAs, to date there have been only a limited number of experiments on holes in p-GaAs nanostructures. Here we present results from extremely high quality 1D hole quantum wires that show up to 10 clean and stable quantized conductance plateaus at B=0 [1.2]. The strong spin-orbit coupling leads to an extreme anisotropy of the Zeeman spin splitting of the 1D hole levels depending on whether the magnetic field is parallel or perpendicular to the quantum wire. Our results show that confining holes to a 1D system fundamentally alters their spin properties, and that it is possible to tune these properties by electrostatically changing the width of the 1D system [3]. [1] O. Klochan, \textit{et al}, Appl. Phys. Lett. \textbf{89}, 092105 (2006). [2] R. Danneau, \textit{et al}, Appl. Phys. Lett. \textbf{88}, 012107 (2006) [3] R. Danneau, \textit{et al}, Phys. Rev. Lett., \textbf{97} 026403 (2006). [Preview Abstract] |
Monday, March 5, 2007 10:24AM - 10:36AM |
A44.00011: Quantum dots in graphene Peter Silvestrov, K.B. Efetov We suggest a way of confining quasiparticles by an external potential in a small region of a graphene strip. Transversal electron motion plays a crucial role in this confinement. Properties of thus obtained graphene quantum dots are investigated theoretically for different types of the boundary conditions at the edges of the strip. The (quasi)bound states exist in all systems considered. At the same time, the dependence of the conductance on the gate voltage carries an information about the shape of the edges. [Preview Abstract] |
Monday, March 5, 2007 10:36AM - 10:48AM |
A44.00012: Microwave Conductivity of Silicon Nanowire Arrays Mark Lee, C. Highstrete, A.L. Vallett, S.M. Dilts, J.M. Redwing, T.S. Mayer We have measured the microwave conductivity spectra of silicon nanowire (SiNW) parallel arrays from room temperature to 4K. ~Doped (n-type and p-type) and nominally undoped SiNWs were synthesized by vapor-liquid-solid growth and assembled by AC dielectrophoresis into parallel arrays spanning the electrodes of coplanar waveguides (CPWs). ~The CPW complex reflection and transmission coefficients were measured from 0.1 to 50 GHz. ~Measurements of identical bare CPWs were utilized to calculate the frequency dependent complex conductivity and power dissipation of the SiNW arrays and provide estimates of these quantities for individual SiNWs in this configuration. The conductivity of the undoped SiNWs is purely imaginary, indicating a bound charge response. The doped SiNWs have a real component that, upon preliminary analysis, increases with frequency consistent with free charge disorder effects. No loss is measured for the undoped SiNWs, but loss due to the doped SiNWs is consistently measured and increases with frequency. *Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Monday, March 5, 2007 10:48AM - 11:00AM |
A44.00013: Persistent mobility edges and anomalous quantum diffusion in order-disorder separated quantum films Jianxin Zhong, G. Malcolm Stocks A novel concept of order-disorder separated quantum films is proposed for the design of ultra-thin quantum films of a few atomic layers thick with unconventional transport properties. The concept is demonstrated through studying an atomic bilayer comprised of an ordered layer and a disordered layer. Without the disordered layer or the ordered layer, the system is a conducting two-dimensional (2D) crystal or an insulating disordered 2D electron system. Without the order-disorder phase separation, a disordered bilayer is insulating under large disorder. In an order-disorder separated atomic bilayer, however, we show that the system behaves remarkably different from the conventional ordered or disordered electron systems, exhibiting metal-insulator transitions with persistent mobility edges and super-diffusive anomalous quantum diffusion. Application of the model to double-layer graphene systems will be discussed. [Preview Abstract] |
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