Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session L24: Focus Session: Transport in Nanostructures IV: 2DES, Dots, and QPCs |
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Sponsoring Units: DMP Chair: Misha Fogler, University of California, San Diego Room: Morial Convention Center 216 |
Tuesday, March 11, 2008 2:30PM - 2:42PM |
L24.00001: Coulomb and Spin Blockade transport through molecules and quantum dots Bhaskaran Muralidharan, Owen Miller, Avik Ghosh, Supriyo Datta In this talk we address some common theoretical grounds between molecular electronics and quantum-dot transport. Here, we focus on how theoretical models based on Coulomb Blockade (CB) theory can be successfully applied in order to theoretically interpret various notable transport experiments in both molecular electronics and quantum dot transport. We first show that, a majority of low-temperature molecular experiments can be explained easily, using a simplified CB theory. In the later part, we focus on how the many-body excitation spectrum of the molecule/quantum dot plays a significant role, in many other experiments. This includes, not-so-commonly observed transport effects such as Negative Differential Resistance (NDR) and bi-stability, resulting from asymmetry within the molecule or within the quantum dot array. [Preview Abstract] |
Tuesday, March 11, 2008 2:42PM - 2:54PM |
L24.00002: Influence of coherent level mixing on the resonant currents at anti-crossings between two single particle levels of a vertical quantum dot C. Payette, D.G. Austing, G. Yu, J.A. Gupta, S.V. Nair, B. Partoens, S. Amaha, Y. Tokura, S. Tarucha We study single electron resonant tunneling through weakly coupled vertical quantum dot molecules. Using the ground state of one of the constituent dots as an energy filter, we can probe the single particle energy spectrum of the other dot. Overall, the spectra are well modeled by assuming the dots are either circular or elliptical and parabolic, except in the regions where two or more single particle states approach each other. In these regions, we observe pronounced level mixing behavior. Here, we focus on the numerous two level anti-crossings, examining the conditions which lead to either simple transfer of the resonant current strengths between the two branches or concurrent enhancement and suppression of the resonant current in the two branches. We show that both types of behavior can be understood using a simple coherent level mixing model. [Preview Abstract] |
Tuesday, March 11, 2008 2:54PM - 3:06PM |
L24.00003: Electron-Phonon Kinetics and Transport in 2D Structures of Reduced Electron Concentrations Andrei Sergeev, Michael Reizer, Vladimir Mitin Usually, screening of the electron-phonon (e-ph) interaction is considered in linear approximation. In this case in 2D systems, the Debye screening radius $r_D$ is independent on the electron concentration, $n$. The linear approximation ignores the discreteness of the electron charge and it is not applicable for diluted systems. Here we show that the screening radius for e-ph interaction is in fact $\max (r_D, n^{-1/2}) $. For this reason, e-ph interaction is drastically enhanced in the diluted systems. In particular, a value of the deformation potential is increased by a factor of $n^{1/2}/r_D \approx R_s/a_0 = r_s$. The suggested approach explains puzzling data [1], which demonstrate that the deformation potential between holes and phonons in dilute 2D GaAs is twenty times stronger than expected from the theory. Strong coupling increases all e-ph phenomena. Using the Keldysh diagrammatic technique, we calculate kinetic and transport characteristics for diluted 2D systems. [1] X.P.A. Gao et al, Phys. Rev Lett. 94, 086402 (2005). [Preview Abstract] |
Tuesday, March 11, 2008 3:06PM - 3:42PM |
L24.00004: Novel measurement techniques for probing quantum point contacts Invited Speaker: Conductance measurements of quantum point contacts (QPCs) reveal an anomalous plateau at roughly 0.7 x 2e$^{2}$/h, when the mode occupation is just short of making a fully transmitting 1D channel available. Past experiments have built a consensus that this so-called ``0.7 structure'' is related to electron spin and electron-electron interaction, but the detailed description remains controversial. ~We have performed measurements on two new kinds of devices which give new insight into the interactions of electrons in these clean quasi-one dimensional systems. One device allows us to measure the compressibility of the electrons in a QPC for the first several conduction modes. ~Comparison with density functional calculations give new information about the relative importance of interactions (including exchange) as the density in the QPC is depleted.~ The second device allows us to measure the local density of states (DOS) in the QPC as we tunnel directly into the constriction.~ Deviations from the 1D DOS would help to develop a more complete picture of the transport through a QPC. We acknowledge support from the ONR Young Investigator Program, Award No. N00014-01-1-0569 and a Research Corporation Research Innovation Award, No. RI1260. [Preview Abstract] |
Tuesday, March 11, 2008 3:42PM - 3:54PM |
L24.00005: Ballistic hole transport and the 0.7 anomaly in p-type GaAs quantum wires A.R. Hamilton, R. Danneau, O. Klochan, W.R. Clarke, L.H. Ho, A.P. Micolich, M.Y. Simmons, M. Pepper, D.A. Ritchie Studying the spin degree of freedom of charge carriers in semiconductors is 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. We have fabricated extremely high quality 1D hole quantum wires that show up to 10 extremely clean and stable quantized conductance plateaus at B=0 [1]. In contrast to 1D electrons, we observe an extreme anisotropy of the Zeeman spin splitting of the 1D energy levels depending on whether the magnetic field is parallel or perpendicular to the quantum wire [2]. We use this anisotropy to show that the 0.7 feature and zero bias anomaly are both spin related in hole quantum wires [3]. [1] O. Klochan \emph{et al}, APL 89, 092105 (2006); R. Danneau \emph{et al}, ibid 88, 012107 (2006). [2] R. Danneau\emph{et al}, PRL 97, 026403 (2006). [3]R. Danneau, et al, PRL (in press). [Preview Abstract] |
Tuesday, March 11, 2008 3:54PM - 4:06PM |
L24.00006: Elementary Events of Electron Transfer in a Voltage-Driven Quantum Point Contact Mihajlo Vanevic, Yuli Nazarov, Wolfgang Belzig We find that the statistics of electron transfer in a coherent quantum point contact driven by an arbitrary time-dependent voltage is composed of elementary events of two kinds: unidirectional one-electron transfers determining the average current and bidirectional two-electron processes contributing to the noise only. This result pertains at vanishing temperature while the extended Keldysh-Green's function formalism in use also enables the systematic calculation of the higher-order current correlators at finite temperatures. [Preview Abstract] |
Tuesday, March 11, 2008 4:06PM - 4:18PM |
L24.00007: Zeeman splitting and subband spacing in ballistic Ga$_{0.25}$In$_{0.75}$As/InP quantum point contacts Theodore Martin, A. Szorkovszky, C.A. Marlow, L. Samuelson, H. Linke, R.P. Taylor, A.P. Micolich, A.R. Hamilton Spin-resolved transport in low-dimensional, solid state systems is a leading area of research at the nanoscale, due to potential device applications that combine quantization with the spin degree of freedom. The realization of such devices requires both a well-resolved energy level spectrum and a large splitting of the spin-degeneracy. Here we investigate the transport properties of a ballistic quantum point contact (QPC) etched into a high indium content strained GaInAs/InP heterostructure, a system with strong spin-orbit coupling and large 1D subband spacings. We have measured the 1D subband spacing using two independent methods, and find it to be $\sim $10 meV, with a very steep confining potential. We also present data studying the Zeeman splitting of the 1D subbands for different magnetic field orientations. [Preview Abstract] |
Tuesday, March 11, 2008 4:18PM - 4:30PM |
L24.00008: Magnetoconductance of interacting electrons in quantum wires in the integer quantum Hall regime. Igor Zozoulenko, Siarhei Ihnatsenka We present systematic quantitative description of the magnetoconductance of the split-gate quantum wires. Accounting for the exchange and correlation interactions within the spin density function theory (DFT) leads to the lifting of the spin degeneracy and formation of the spin-resolved plateaus at odd values of $e^{2}/h$. We show that the width of the odd conductance steps in the spin DFT calculations is equal to the width of the transition intervals between the conductance steps for the spinless Hartree electrons. A detailed analysis of the structure of compressible/incompressible strips and the evolution of the Hartree and the spin-DFT subband structure provides an explanation of this finding. Our spin-DFT calculations reproduce not only qualitatively, but rather quantitatively all the features in the magnetoconductance observed in the experiment [1] including the unexpected effect of the collapse of the odd conductance plateaus at lower fields. \\ $[1]$ I. P. Radu, J. B. Miller, S. Amasha, E. Levenson-Falk, D. M. Zumbuhl, M. A. Kastner, C. M. Marcus, L. N. Pfeiffer, and K. W. West, unpublished. [Preview Abstract] |
Tuesday, March 11, 2008 4:30PM - 4:42PM |
L24.00009: The Virtual Scanning Tunneling Microscope: A Novel Probe Technique for Imaging Two-Dimensional Electron Systems Adam Sciambi, David Goldhaber-Gordon, Seth Bank, Arthur Gossard We propose a novel probe technique, the virtual scanning tunneling microscope (VSTM), which will provide both spatial and spectroscopic information about two-dimensional electron systems (2DESs) in semiconductor heterostructures. The VSTM's innovation is the addition of a second 'probe' 2DES separated by a low barrier from the sample 2DES below. Simulations show that a positively-biased tip held above the sample surface can greatly diminish the interlayer barrier and induce tunable tunneling between the two 2DESs. If the tip is scanned, the tunneling region will follow below, acting as a virtual tip while screening the true tip from the sample 2DES. This probe technique is motivated by interesting local 2DES physics that can only be studied indirectly because of the depth of 2DESs; we describe a range of predicted spatially-organized phases of 2D electrons which could be accessed with this new probe. We follow with experimental results showing induced tunneling in a GaAs/AlGaAs bilayer 2DES sample, which we characterize thoroughly and use to tunnel into a quantum Hall liquid. [Preview Abstract] |
Tuesday, March 11, 2008 4:42PM - 4:54PM |
L24.00010: Virtual Scanning Tunneling Microscope: Modeling Interlayer Tunneling Between Two-Dimensional Electron Systems in the Ballistic Regime Katherine Luna, Eun-Ah Kim, Paul Oreto, Steven Kivelson We study a theoretical model for the virtual scanning tunneling microscope (VSTM), which is a proposal to use interlayer tunneling in a bi-layer system as a way to probe two-dimensional electron systems (2DES) in semiconductor heterostructures. We model the bi-layer in the presence of weak tunneling between the layers using an analog of the spin-boson model. Previously, such a system was modeled in the diffusive regime by Levitov and Shytov [1], and they predicted a zero-bias anomaly, where the tunneling conductance vanishes singularly near zero-bias as a result of Coulomb blocking. Motivated by the availability of high mobility samples and the goal of using VSTM to probe the physics of clean 2DES dominated by interactions, we focus on tunneling in the ballistic regime. We find the absence of a zero-bias anomaly due to extremely efficient screening in the ballistic regime. We discuss the implications of our results on ongoing experimental efforts. [1] S. Levitov and A.V. Shytov. JETP Lett. \textbf{66}, 214 (1997). [Preview Abstract] |
Tuesday, March 11, 2008 4:54PM - 5:06PM |
L24.00011: Imaging electron local density of states inside mesoscopic quantum rings B. Hackens, V. Bayot, Marco Pala, X. Wallart, S. Bollaert, A. Cappy, F. Martins, T. Ouisse, H. Sellier, J. Chevrier, S. Huant We combine scanning gate microscopy (SGM) experiments and simulations to demonstrate imaging of the electron local density of states within open quantum rings (QRs). SGM is based on a weak electrostatic perturbation of the electron system by a charged tip, which alters the transmission of electrons through the system. When the QRs are in the ballistic and coherent regime of transport, conductance fringes are observed in SGM images when the tip scans over the QR area as well as in its vicinity. Comparing our results to quantum mechanical simulations of transport in realistic QRs, we demonstrate that the fringes observed over the QR area are directly connected to the local density of states inside the QR [1]. Moreover, the magnetic field dependence of the fringes observed in the vicinity of the QRs indicates that they originate from the electrostatic Aharonov-Bohm effect, and correspond to iso-phase lines for electrons [2]. From these results, one can expect to design new kinds of quantum nanodevices based on a precise spatial control of electron interferences and trajectories. [1]~F. Martins et al., PRL 99,136807 (2007). [2]~ B. Hackens et al., Nat. Phys. 2, 826 (2006). [Preview Abstract] |
Tuesday, March 11, 2008 5:06PM - 5:18PM |
L24.00012: Origin of nonlinear current-voltage characteristics in nanowires Francois Leonard, A. Alec Talin, B. S. Swartzentruber, Xin Wang, Stephen D. Hersee The current-voltage characteristics of nanowires are often observed to be nonlinear, and this behavior has been ascribed to Schottky barriers at the contacts. We present electronic transport measurements on GaN nanowires and demonstrate that the nonlinear behavior originates instead from space-charge limited current. Analysis of published experimental data in several nanowire materials shows that this behavior is common, and should be expected whenever diffusive transport dominates and the effective carrier concentration is low. A theory of space-charge limited current in nanowires is presented, and correctly predicts the scaling of the current with the nanowire aspect ratio. [Preview Abstract] |
Tuesday, March 11, 2008 5:18PM - 5:30PM |
L24.00013: ABSTRACT WITHDRAWN |
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