Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session B18: Quantum Dots, Wires, and QH Edge States |
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Sponsoring Units: DCMP Chair: Woowon Kang, University of Chicago Room: LACC 406A |
Monday, March 21, 2005 11:15AM - 11:27AM |
B18.00001: Spatially resolved manipulation of single electrons in quantum dots using a scanned probe Alessandro Pioda, Slavo Kicin, Thomas Ihn, Martin Sigrist, Andreas Fuhrer, Klaus Ensslin, Andreas Weichselbaum, Sergio Ulloa, Matthias Reinwald, Werner Wegscheider Single electrons in a quantum dot have been manipulated with a scanning force microscope. The scanning metallic tip was coupled capacitively to the electrons confined in a lithographically defined gate-tunable quantum dot at a temperature of 300 mK. Single electrons were made to hop on or off the dot by moving the tip or by changing the tip bias voltage owing to the Coulomb-blockade effect. Spatial images of conductance resonances map the interaction potential between the tip and individual electronic quantum dot states. Under certain conditions this interaction is found to contain a tip-voltage induced and a tip-voltage independent contribution. Alessandro Pioda, Slavo Kicin, Thomas Ihn, Martin Sigrist, Andreas Fuhrer, Klaus Ensslin, Andreas Weichselbaum, Sergio E. Ulloa, Matthias Reinwald and Werner Wegscheider, Phys. Rev. Lett. 93, 216801 (2004), cond-mat/0411264 [Preview Abstract] |
Monday, March 21, 2005 11:27AM - 11:39AM |
B18.00002: Direct observation of edge state transport in a QPC via scanning gate microscopy Nobuyuki Aoki, Carlo DaCunha, David Ferry Direct imaging of edge state transport through a quantum point contact (QPC) has been achieved using scanning gate microscopy (SGM). With a metallic tip floating about 40nm above the surface, it was possible to scatter electrons in such a way that their trajectories do not hit the QPC and produce a significant change in conductance. This could be achieved at high magnetic fields and small constrictions. Images indicate clear differences from images taken at no magnetic field. The images change depending on the gate bias, which moves them on the conductance plateaus in the transmission curve at constant magnetic field, indicating the formation of edge channels crossing the constriction near its potential walls. Furthermore, some features present below the first plateau could suggest a direct imaging of composite fermions. [Preview Abstract] |
Monday, March 21, 2005 11:39AM - 11:51AM |
B18.00003: Imaging of resonant transport through single electron localized states in the quantum Hall effect Gary Steele, Ray Ashoori, Loren Pfeiffer, Ken West We measure transport properties of incompressible states in the quantum Hall effect (QHE) with a novel scanning probe method. Using scanning charge accumulation (SCA) imaging, a metal STM tip probes charging of a very high mobility 2D electron gas buried 100 nm below the surface of a GaAs heterostructure. Working at a magnetic field near integer Landau level filling and applying DC voltage on our tip, we induce a bubble of charge under the tip in an adjacent Landau level. A ring of incompressible quantum Hall liquid separates this bubble from the bulk. Measuring charging of the bubble reveals direct information about transport through the incompressible ring. The resistance of the ring is seen to be $>$100 G$\Omega$ but drops abruptly by more than four orders of magnitude as the tip is scanned. The images suggest that this drop is caused by potential fluctuations that create small islands in the strip. Resonant tunneling through such islands, mediated by Coulomb Blockade, produces striking filamentary patterns. The results provide direct measurements of microscopic transport mechanisms in the QHE. [Preview Abstract] |
Monday, March 21, 2005 11:51AM - 12:27PM |
B18.00004: The 0.7 structure in Cleaved Edge Overgrowth Wires Invited Speaker: Rafi de Picciotto We study transport in a clean one-dimensional wire fabricated by cleaved edge overgrowth in molecular beam epitaxy. At low electron densities, we observe a feature in the differential conductance similar to the so-called ``0.7 structure'', found in quantum point contact devices. Using a simple model, we show that the 0.7 structure is observed in the non-linear differential conductance when the ratio between the bias and the Fermi energy exceeds four. Alternatively, this feature occurs in the linear conductance when the temperature surpasses the Fermi energy and the electrons in the wire undergo a transition from a degenerate into a non-degenerate liquid. [Preview Abstract] |
Monday, March 21, 2005 12:27PM - 12:39PM |
B18.00005: The 0.7 structure in undoped quantum wires J. A. Seamons, E. Bielejec, J. L. Reno, M. P. Lilly We report transport measurements for quantum wires formed in undoped GaAs/AlGaAs heterostructures. The density of the 2D electrons, induced with an external gate, can be varied from $2 \times 10^{10}$ to over $3 \times 10^{11}$ cm$^{-2}$. Quantum wires are patterned using wet or dry etching that creates split gates and a central top gate. When a single 1D subband is occupied, some wires exhibit dramatic oscillations and multiple structures that strongly depend on the 2D density. Other wires show a more typical 0.7 structure and no additional oscillatory features. Measurements of the 1D density using magnetic depopulation find a weak change in 1D density as the 2D density is changed by an order of magnitude. We will compare the conductance value of the 0.7 structure, the presence of additional oscillations and the values of the main conductance plateaus for wires with widths varying from 400 to 800 nm. The role of 1D subband spacing and disorder will be discussed. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000. [Preview Abstract] |
Monday, March 21, 2005 12:39PM - 12:51PM |
B18.00006: Influence of Magnetic Moment Formation on the Conductance of Coupled Quantum Wires V.I. Puller, L.G. Mourokh, J.P. Bird, Y. Ochiai We develop a model for the resonant interaction between a pair of coupled quantum wires, under conditions where self-consistent effects lead to the formation of a local magnetic moment in one of the wires. Our analysis is motivated by the experimental results of Morimoto et al. [Appl. Phys. Lett. 82, 3952 (2003)], who showed that the conductance of one of the quantum wires exhibits a resonant peak at low temperatures, whenever the other wire is swept into the regime where local-moment formation is expected. In order to account for these observations, we develop a theoretical model for the inter-wire interaction that calculated the transmission properties of one (the fixed) wire when the device potential is modified by the presence of an extra scattering term, arising from the presence of the local moment in the swept wire. Our analysis clearly shows that the observation of a resonant peak in the conductance of the fixed wire is correlated to the appearance of additional structure (near $0.75\cdot$ or $0.25\cdot 2e^2/h$) in the conductance of the swept wire, in agreement with the experimental results of Morimoto et al. [Preview Abstract] |
Monday, March 21, 2005 12:51PM - 1:03PM |
B18.00007: Tunneling and nonlinear transport in a vertically coupled GaAs/AlGaAs double quantum wire system Edward Bielejec, S. Lyo, J. Seamons, J. Reno, M. Lilly We report low-dimensional tunneling and nonlinear transport in a vertically coupled quantum wire system. The nanostructure is fabricated in a high quality parallel double quantum well heterostructure. Using a novel flip chip technique developed at Sandia to align top and bottom split gates to form low- dimensional constrictions in each of the independently contacted quantum wells we explicitly control the subband occupation of the individual wires. This configuration allows for the study of 2D-2D, 2D-1D and 1D-1D behaviors. In addition to the expected 2D-2D tunneling results, we have found additional tunneling features that are related to the 1D quantum wires. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000. [Preview Abstract] |
Monday, March 21, 2005 1:03PM - 1:15PM |
B18.00008: Nonlinear Vertical Resonant Tunneling in Double Quantum Wells in an In-Plane Magnetic Field S.K. Lyo, E. Bielejec, J.A. Seamons, J.L. Reno, M.P. Lilly, Yun-pil Shim We present a theory and data for the tunneling current between two electron-gas layers separated by a wide barrier in an in-plane magnetic field $B$. The electron gases are separately in equilibrium with their chemical potentials displaced by the bias energy $V$. The observed tunneling current and the differential conductance show interesting domain structure on the $B-V$ plane. Our theory, based on incoherent tunneling and the $B$-induced displacement of the energy dispersion curves, generates a similar domain structure, yielding excellent agreement with the data from GaAs/Al$_x$Ga$_{1-x}$As double quantum wells. An extension of the current result to 2D-1D tunneling will be discussed. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the U.S. DOE under contract DE-AC04-94AL85000. [Preview Abstract] |
Monday, March 21, 2005 1:15PM - 1:27PM |
B18.00009: Anomalous Tunneling Characteristics of InGaAs/\\InAlAs Double-well Structures Y. Lin, J. Nitta, A.K.M. Newaz, W. Song, E.E. Mendez We report tunneling phenomena in double quantum-well structures that are at odds with the conventional parallel-momentum- conserving picture of tunneling between two-dimensional systems. The thickness of the In$_{0.53}$Ga$_{0.47}$As wells was the same in all the structures, 53{\AA} and 82{\AA}, but that of the three In$_{0.52}$Al$_{0.48}$As barriers varied among the structures: either all 82{\AA}, or all 100{\AA}, or 100{\AA} with 53{\AA} in the middle. We have found that the tunneling current is mostly determined by the alignment between the emitter and a quantum state in the adjacent well, not by that between states in both wells. With magnetic fields parallel to the tunneling current, we have also found the field- dependent features before the peak voltage of the main resonance that correspond to tunneling channels into the Landau levels of the well near the emitter. These results provide evidence of the violation of in-plane momentum conservation in two-dimensional systems. [Preview Abstract] |
Monday, March 21, 2005 1:27PM - 1:39PM |
B18.00010: Tunneling between Two Counterpropagating Quantum Hall Edge States through a Narrow Aperture in a Line Junction P. Jiang, C. Chien, W. Kang, L.N. Pfeiffer, K.W. Baldwin, K.W. West We report on the tunneling characteristics of a quantum Hall line junction with a small aperture in an otherwise thin, nearly ideal tunnel barrier (width = 8.8 nm). The conductance through this particular junction reduces to that of a single mode quantum point contact under magnetic field and we can discern the tunneling property of two strongly coupled quantum Hall droplets. Near filling factors $\nu = 2$, 1, and 2/3, we detect a sharp resonance in the zero-bias conductance at low temperatures. For fractional fillings near $\nu = 1/2$ and $\nu = 1/3$, the zero-bias conductance is strongly suppressed, and the peak evolves into a sharp minimum. We interpret the observed zero-bias features in terms of models of tunneling proposed for a line junction in the integer[1] and the fractional[2] quantum Hall regimes.\\ ~[1] E. Kim and E. Fradkin, Phys. Rev. B {\bf 67}, 45317 (2003).\\ ~[2] E. Papa and A.H. MacDonald, Phys. Rev. Lett. {\bf 93}, 126801 (2004).\\ [Preview Abstract] |
Monday, March 21, 2005 1:39PM - 1:51PM |
B18.00011: The role of Coulomb interactions in quantum Hall inter-edge tunneling Akakii Melikidze, Kun Yang We study theoretically the tunneling between quantum Hall edge states across a line junction as a model for the experiment of I. Yang et al, Phys. Rev. Lett. 92, 056802 (2004). We specifically concentrate on the role of Coulomb interactions in the cascade of phase transitions that has been observed experimentally as the magnetic field was varied. We conclude that despite the strong interactions, the zero-temperature phase transitions should be in the same universality class as one would expect for non-interacting edge excitations. This is in disagreement with the behavior inferred in the experiment. We attribute this to the fact that the true low-temperature universality regime had not been attained. The relevant crossover temperature emerges naturally in our model. This work was supported by NSF grant No. DMR-0225698. [Preview Abstract] |
Monday, March 21, 2005 1:51PM - 2:03PM |
B18.00012: Universality of the Edge Tunneling Exponent of Fractional Quantum Hall Liquids Xin Wan, F. Evers, E. H. Rezayi In a microscopic model of fractional quantum Hall liquids with electron-electron interactions and confinement, we calculate the edge Green's function via exact diagonalization. Our results for $\nu=1/3$ and 2/3 suggest that in the presence of Coulomb interaction, the sharpness of the edge and the strength of the edge confining potential, which can lead to edge reconstruction, are the parameters that are relevant to the universality of the electron tunneling I-V exponent. In particular, for $\nu=1/3$ with Coulomb interactions and a hard edge, the tunneling exponent is non-universal as found previously. However, the universal value is recovered if the hard-edge confinement is relaxed or a neutralizing background charge is placed a distance $d < d_c$ from the 2-D layer. At the critical distance $d_c$ the edge reconstructs and we find two distinct power law regimes over two different length scales. On the longer length scale the system recovers its universal exponent. [Preview Abstract] |
Monday, March 21, 2005 2:03PM - 2:15PM |
B18.00013: Observation of spin-charge separation and localization in a one-dimensional quantum wire Ophir M. Auslaender, Hadar Steinberg, Amir Yacoby, Yaroslav Tserkovnyak, Bertrand I. Halperin, Kirk W. Baldwin, Loren N. Pfeiffer, Kenneth W. West We report on measurements of quantum many-body modes in ballistic wires and their dependence on Coulomb interactions. To this end we measure tunneling between two parallel wires in an AlGaAs/GaAs heterostructure as a function of electron density. When the density is high we clearly observe two spin modes and one charge mode of the coupled wire system. Mapping the dispersion velocities of these modes as a function of decreasing density, we find good agreement between the data and theoretical calculations of the charge velocity, although theory also predicts an additional charge mode that is not observed. We also find that, within experimental precision, the measured spin velocity is smaller than predicted theoretically. Decreasing density further, we observe spontaneous localization at a critical density. A simultaneous measurement of the two terminal conductance, which exhibits typical conductance steps, shows localization transitions concurrent with each conductance drop. [Preview Abstract] |
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B18.00014: Quantum Hall Line Junctions as Tunable One-Dimensional Electron Systems Emiliano Papa, Allan MacDonald Incompressible quantum Hall states can be partitioned when a sufficiently large voltage is applied to a narrow metallic gate that crosses the sample, creating a line junction with counterpropogating edge states. This work is motivated by experiments of Roddaro et al. which demonstrate the occurrance of low-bias low-temperature edge state transport anomalies in line junction systems, even in the case of $\nu=1$ bulk quantum Hall states. The sense of the transport anomalies changes from enhanced tunneling to suppressed transmission through the line junction as the gate voltage is increased. We [PRL 04] have previously emphasized that interactions across line junctions are always important and that transport properties are not universal. Here we show that the presence of metallic gates strongly influences these interactions. We find that the interaction strength between edges on oposite sides of the gate changes sign from repulsive at small distances (lower $V_g$) to attractive at larger distances (higher $V_g$), in agreement with experiment. The magnitude of the attraction implied by this estimate appears to be weaker than suggested by experiment, however, although stronger than the effect of phonons studied in earlier work. We propose a number of alternate experimental possibilities for detecting the presence of attractive effective interactions, including real-time detection of partial Andreev scattering and studies of the edge magnetoplasmon excitation spectrum of the partitioned system. [Preview Abstract] |
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