Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session U37: Focus Session: Nanowire and Nanodot Quantum Devices |
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Sponsoring Units: DMP Chair: Alexey Bezryadin, University of Illinois, Urbana-Champaign Room: Baltimore Convention Center 340 |
Thursday, March 16, 2006 8:00AM - 8:12AM |
U37.00001: Quantum Coherence and Time Dependent Conductance Fluctuations in Dilute Magnetic Semiconductors Sungbae Lee, Aaron Trionfi, Timo Schallenberg, Hiro Munekata, Doug Natelson Low temperature electrical properties of ferromagnetic nanowires are influenced by the interplay between disorder, quantum coherence, and magnetic correlations. Quantum coherence corrections to the conductance are of particular interest, and can be difficult to characterize experimentally. Time dependent universal conductance fluctuation (TD-UCF) at low temperatures provides a means of assessing these effects. Samples were fabricated by standard electron beam lithography and ion etching technique using In$_{1-x}$Mn$_{x}$As quantum well samples grown by off-equilibrium molecular beam epitaxy. Initial measurements of temperature and field-dependent TD-UCF in these devices are presented and compared with permalloy nanowire data. [Preview Abstract] |
Thursday, March 16, 2006 8:12AM - 8:24AM |
U37.00002: Microwave-enhanced decoherance in one-dimensional metal wires Jian Wei, Sergei Pereverzev, Michael Gershenson We report the effect of monochromatic microwave (MW) radiation on the weak localization corrections to the conductivity of quasi-one-dimensional silver wires. Due to the improved electron cooling in the wires, the MW-induced dephasing was observed without a concomitant overheating of electrons over wide ranges of the MW power $P_{MW}$ and frequency $f$. The observed dependences of the conductivity and MW-induced dephasing rate on $P_{MW}$ and $f$ are in agreement with the theory by Altshuler, Aronov, and Khmelnitsky.\footnote[1]{B.L. Altshuler, A.G. Aronov, and D.E. Khmelnitsky, Sol. State Communi. \textbf{39}, 619 (1981).} Our results suggest that the saturation of dephasing time, often observed at $T \leq$ 0.1 K, may be caused by an insufficient screening of the sample from the external microwave noise. [Preview Abstract] |
Thursday, March 16, 2006 8:24AM - 8:36AM |
U37.00003: Toward Single-Walled Nanotube Aharonov-Bohm interferometers Jinseong Heo, Marc Bockrath Single-walled carbon nanotubes (SWNTs), which have micron-scale phase coherent lengths at low temperatures, are grown so that they cross themselves, producing a loop that may act as an Aharonov-Bohm interferometer. In order to determine electron pathways at the junction, we performed scanned gate microscopy (SGM) using an Atomic Force Microscopy(AFM) tip as a local gate. If a SGM signal is observed when the tip is over any particular segment it indicates current flow in that segment. Surprisingly, one semiconducting tube showed that most electrons tunnel into the other segment at the junction without flowing through the loop. For other samples, however, current flow was observed in the loop. Taken together, this suggests the possibility of controlling tunneling probabilities at the junction. Moreover, for metallic or small bandgap nanotubes, we reproducibly observe an unusual conductance peak near zero gate voltage, obtaining nearly identical behavior from devices made from the same ~100 micron long nanotube. The results and interpretations of our ongoing experiments will be discussed. [Preview Abstract] |
Thursday, March 16, 2006 8:36AM - 9:12AM |
U37.00004: Quantum Transport of Semiconductor Nanowires Coupled to Superconductors Invited Speaker: We report the electronic transport properties of the first superconducting transistors based on semiconductor nanowires (ref. Y.-J. Doh et al., Science 309, 272 (2005)). These nanoscale superconductor-semiconductor devices enable the fabrication of new nanoscale superconducting electronic circuits and at the same time provide new opportunities for the study of fundamental quantum transport phenomena. Indium arsenide (InAs) semiconductor nanowires individually contacted by two aluminum-based superconductor electrodes yield surprisingly low contact resistances. Below 1 kelvin, the high transparency of the contact gives rise to proximity-induced supercondcutivity and a resistance-free current (supercurrent) can flow through the nanowire from one superconducting contact to another. The supercurrent can be switched on and off by a gate voltage acting on the electron density in the nanowire. A variation in gate voltage induces universal fluctuations in the normal-state conductance, which are clearly correlated to critical current fluctuations. The alternating-current Josephson effect gives rise to Shapiro steps in the voltage- current characteristic under microwave irradiation. For indium phosphide (InP) nanowire devices, however, Coulomb blockade effect dominates the electrical transport, which can be modeled as a quantum dot weakly coupled to superconductors. As a result of BCS (Bardeen-Cooper-Schriffer) singularity of density of states, a negative differential conductance is observed in the superconducting state. By applying high magnetic field, Zeeman splitting is observed and g-factor of InP nanowire is estimated to be 1.5. [Preview Abstract] |
Thursday, March 16, 2006 9:12AM - 9:24AM |
U37.00005: Four-probe measurements of individual metallic and superconducting nanowires Sheng-Yong Xu, Mingliang Tian, Nitesh Kumar, Moses H. W. Chan We have performed 4-probe measurements for a number of individual Au, Ag, Sn and Zn nanowires. The resistivity of these nanowires is always larger than their bulk values, and varies with the crystallinity of the nanowires. Single crystalline Sn nanowires have a resistivity similar to that of bulk Sn, while polycrystalline Au nanowires shows a resistivity twenty to thirty times higher than that of bulk Au. For Sn nanowires with diameter from 280 nm to 70 nm, the superconducting transition temperature $T_{c}$ remains the same as, or slightly lower than the $T_{c}$ of bulk Sn, 3.7 K. The critical current density of these Sn nanowires is measured to be in the order of 105 A/cm2 at low temperature. The critical fields of the nanowires, as expected, are much higher than that of Sn films. A finite resistance, which increases with increasing excitation current, is observed in all the samples. The origins of this finite resistance will be discussed. We have also obtained evidence that the four metallic leads assembled on each single nanowire may have remarkable effects on the transport properties of the nanowire. [Preview Abstract] |
Thursday, March 16, 2006 9:24AM - 9:36AM |
U37.00006: Modifying Mesoscopic 1/f Noise Via Surface Chemistry. A. Trionfi, J.W. Ciszek, J.M. Tour, D. Natelson Attempts to extrinsically control the 1/f noise related to Universal Conductance Fluctuation Theory in quasi-one dimensional Au wires were made using self-assembled monolayer (SAM) molecules. Measurements before and after the deposition of the SAM molecule comparing the noise power amplitude and the phase coherence of the devices via the weak localization magnetoresistance and noise power amplitude versus magnetic field were performed. The resulting data were used to determine if the 1/f noise is approaching the so-called saturated limit as the system is lowered from 14 to 2 K. The results may also shed light on the microscopic details of the two level systems responsible for time-dependent conductance fluctuations in normal metals. Preliminary results will be reported. [Preview Abstract] |
Thursday, March 16, 2006 9:36AM - 9:48AM |
U37.00007: Coulomb Blockade Imaging of Few-Electron Quantum Dots in a Magnetic Field P. Fallahi, R. Westervelt, M. Stopa, M. Hanson, A.C. Gossard One-electron quantum dots are important candidates for quantum information processing. We have developed a technique to image electrons inside a quantum dot in the Coulomb blockade regime, using a scanning probe microscope (SPM) at liquid He temperatures (1). We have used this technique to image the last electron in the dot in a strong perpendicular magnetic field.dots are formed in a two-dimensional electron gas in a GaAs/AlGaAs heterostructure by surface gates. Images are obtained by recording the dot conductance while scanning the SPM tip above the dot. SPM Images show a ring of increased conductance about the center of the dot, corresponding to a Coulomb blockade peak in the dot conductance.observe changes in the shape and the size of the conductance rings with magnetic field. This is due to a combination of energy shifts and orbital changes of the electrons in the quantum dot. \newline (1) P. Fallahi \textit{et al,} Nano Letters 5, 223 (2005) [Preview Abstract] |
Thursday, March 16, 2006 9:48AM - 10:00AM |
U37.00008: Kondo and Superconducting Proximity Effect in Semiconductor Nanostructures Andy Vidan, Michael Stopa, Robert Westervelt, Micah Hanson, A.C. Gossard, Jie Xiang, Charles M. Lieber We have fabricated a unique device containing three quantum dots in a GaAs/AlGaAs heterostructure containing a two-dimensional electron gas using lithographically patterned gates and an etched trench in the center of the ring. By only energizing certain gates, this device allows us to study electron transport through a single dot, a double dot, or a triple dot ring. We can determine the absolute number of electrons in a quantum dot using a nearby charge sensor and find that we are able to tune a single dot to the one and two electron regime. We find several sharp peaks in the differential conductance, occurring at both zero and finite source-drain bias, for the one and two electron quantum dot. At zero source-drain bias, the temperature and magnetic field dependence of the conductance is consistent with a standard Kondo resonance. We attribute the peaks at finite-bias to a Kondo effect through excited states of the quantum dot. We also present recent observations of supercurrents in Ge/Si 1D nanowire heterostructures. [Preview Abstract] |
Thursday, March 16, 2006 10:00AM - 10:12AM |
U37.00009: Transition to Scarred States Probed by a Single Electron Spectrometer Robert E. Parrott, Dominik M. Zumbuhl, Eric J. Heller, Charles M. Marcus, Micah P. Hanson, Arthur C. Gossard We examine the use of a novel mesoscopic spectrometer to image electron dynamics in a large, open lateral quantum dot in a perpendicular magnetic field $B_{z} $. The spectrometer is comprised of a small single-electron quantum dot weakly coupled to the large dot via a tunnel junction; the current through the dots is measured at finite bias. By varying the energy level of the small dot, the local density of states, $g(r_{o},E)$ is measured at the tunnel junction. An unexpected interference effect persists at energies 1 meV below $E_{F}$. Using semiclassical and quantum approaches, we show these interference bands in $(E, B_{z})$ are due to a dominant, diamond-shaped periodic orbit in the large dot. As $B_{z}$ is varied, the orbit changes to unstable: the dominant state becomes a scar. This transition is marked by an abrupt change in the area of the orbit, and hence in the spacing $\Delta B_{z}$ of the bands. In a billiard model of the system, this coincides with a pitchfork bifurcation of the orbit. The existence of the bands depends strongly on whether the orbit has appreciable magnitude at the tunnel junction, belying the local nature of the measurement. In this way the tunnel junction, coupled to the small dot, acts like a fixed STM tip embedded in the 2DEG. [Preview Abstract] |
Thursday, March 16, 2006 10:12AM - 10:24AM |
U37.00010: Magneto-transport studies in mesoscopic InAs 2DEG devices Ernesto Marinero, Thomas Boone, Liesl Folks, Bruce Gurney, Jordan Katine, Sergio Nicoletti We report on magneto-transport studies in lithographically patterned InAs 2DEG devices. Electron transport between adjacent current injection and extraction channels was studied as a function of temperature and magnetic field. The mean free path in the quantum well at RT and 5 K in our devices is approximately 280 nm and 980 nm respectively. The spacing between the current tabs in our devices range from 1000 nm to 300 nm and typical tab widths range from 300 nm to 100 nm. Low temperature measurements reveal contributions of ballistic transport in our devices; fluctuations in magneto-resistance are observed at distinct values of the perpendicularly applied magnetic field. Said fluctuations will be discussed in the context of ballistic electron focusing trajectories and possible contributions from the quantum Hall effect. [Preview Abstract] |
Thursday, March 16, 2006 10:24AM - 10:36AM |
U37.00011: Asymmetry of Nonlinear Transport and Electron Interactions in Quantum Dots D. M. Zumb\"uhl, C. M. Marcus, M. P. Hanson, A. C. Gossard The magnetic field symmetry of conductance beyond the linear source-drain bias regime in open chaotic GaAs quantum dots is experimentally investigated using gate-controlled shape distortion to gather ensemble statistics. We measure a conductance component $g_{B-}$ antisymmetric in perpendicular magnetic field $B$ which is of the form $\tilde g=\alpha V B$ for source-drain bias voltages $V$ smaller than the quantum dot level spacing $\Delta$ and for $B$ smaller than a flux quantum through the device area. Interestingly, according to recent theories, $\tilde g$ of this form vanishes in absence of electron interactions and $\alpha$ is proportional to the electron interaction strength. $g_{B-}$ shows mesoscopic fluctuations with shape gate and with $B$ and $V$ on a scale of the flux quantum through the dot area and quantum dot level spacing $\Delta$, respectively. As anticipated by theories, the average coefficient $\alpha$ measured over an ensemble of dot shapes vanishes. The standard deviation of $\alpha$ is used to characterize the strength of electron interactions. We discuss the dependence of the typical $\alpha$ on the number of modes in the quantum-point-contact leads, compare our experiment with theories and discuss related issues of electron equilibration, decoherence and thermal smearing in the quantum dot. This work was partially supported by DARPA QuIST, ARO/ARDA and by the NSEC program of the NSF. [Preview Abstract] |
Thursday, March 16, 2006 10:36AM - 10:48AM |
U37.00012: Two Electron Singlet Triplet Spectroscopy Iuliana Radu, D.M. Zumb\"uhl, C.R. Dillard, G. Granger, M.A. Kastner, M.P. Hanson, A.C. Gossard We present measurements of few electron quantum dots formed by lateral depletion of a GaAs/AlGaAs 2D electron gas by surface gates. The two electron regime, on which we focus here, is characterized by singlet and triplet states which are relevant for quantum computation proposals. These two states are revealed in electronic transport through the dot in various ways: sequential tunneling, inelastic cotunneling as well as by an additional mode of transport we ascribe to sequential tunneling activated by inelastic cotunneling. These various signatures provide independent ways to measure the singlet-triplet energy splitting J over large ranges of gate voltages. We present the temperature, magnetic field and tunnel-coupling dependence of these transport features, which are in good agreement with recent theory. Further, we observe signatures of spin-blockade that becomes visible for source-drain voltages exceeding the triplet energy. This work was partially supported by the ARO (W911NF-05-1-0062), by the NSEC program of the NSF (PHY-0117795) and by NSF (DMR-0353209). [Preview Abstract] |
Thursday, March 16, 2006 10:48AM - 11:00AM |
U37.00013: Superpoissonian noise with positive current correlations Yuanzhen Chen, Richard Webb We report shot noise cross correlation measurements in a beam splitter configuration. We fabricated our devices in a two dimensional electron gas in a GaAs/AlGaAs heterostructure using a splitting gate technology. Electrons tunneling across tunnel barriers are incident on a beam splitter and are scattered into two different channels. Shot noise cross correlation between the two electrical currents is measured as a function of both the transmission coefficient of the beam splitter and the Fano factor of the tunnel barriers. Due to the Fermi statistics of electrons, such a measurement usually yields a negative correlation. However, in some barriers under certain circumstances, a positive correlation has also been observed. A correspondence between the Fano factor of the tunnel barriers and the cross correlation has been established. For example, positive cross correlation is always associated with barriers exhibiting superpoissonian shot noise (with a Fano factor greater than one). Studies on the frequency dependence of shot noise suggest that the observed positive cross correlation can be related to the dynamics of localized states in the tunnel barriers. [Preview Abstract] |
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