Science of Nanoscale Systems and their Device Applications
Science of Nanoscale Systems and their Device Applications
Poster Abstracts

Group A

Title: Fabrication and Electrical Characterization of Nanowire Heterostructure Double Quantum Dots
Authors: L.E. Fröberg, A. Fuhrer, M.T. Björk, and L. Samuelson
Institution: Lund University
Presenter: Linus E. Fröberg
Heterostructure nanowires are interesting for electron transport studies in low dimensions as well as for device applications. We have focused on InAs/InP nanowires and have previously shown few electron single quantum dots displaying the well-defined shell structure, related to their hexagonal cross section. In this study we have grown double quantum dots defined by three InP barriers in InAs nanowires and studied the electron transport (at 4.2 K). In the linear response we obtain irregularly spaced Coulomb peaks indicating level alignment in the two dots. Moving towards less positive Vg empties the dots of electrons and for certain Vsd, levels in the two dots align and strong resonances appear. The dots are grown asymmetric and when the last electron leaves the small dot and the stability diagram opens up the large dot still contains electrons. The charging of the large dot induces jagged features on the borderlines in a stability diagram and different spacing between the charging events displays the energy level spacing in the large dot. Thus, we can perform single dot spectroscopy of the shell structure of the large dot using the small dot as an emitter.

Title: Nanowire Standing Wave Oscillator
Authors: X. Li, W. Andress, and D. Ham
Institution: Harvard University
Presenter: Xiaofeng Li
In this poster we present the general idea of Standing Wave Oscillator (SWO), our 10 GHz wave-adapted tapered line SWO fabricated by standard IC, and our novel design of quantum wire (nanowire/nanotube) standing wave oscillator. Our quantum wire SWO truly utilizes the 1-D nature of electronic transport in nanowire/nanotube, and can potentially achieve miniature size, low cost and high speed.

Title: Quantum Dots in Nanowires
Authors: J.W.W. van Tilburg, M. van Kouwen, J.A. van Dam, M. Scheffler, F. Rijkers,
E. Minot, V. Zwiller, and L. Kouwenhoven
Institution: Delft University of Technology; Kavli Institute of Nanoscience
Presenters: Juriaan van Tilburg and Maarten van Kouwen
Fabrication of semiconducting nanowires is relatively simple and during growth a huge versatility in materials can be implemented without suffering from defects caused by lattice mismatches. Because of this, interesting devices can be made, combining different materials along the axis of the wire or perpendicular to it. Nanowires offer a combination of single-electron and single-photon control that is ideal for quantum information processing. Electron spins in nanowire quantum dots have long decoherence times and are a promising choice for storing and manipulating quantum information. Photons are ideal for transmitting and measuring quantum information in the form of polarization states. Here we present some of the present investigations into nanowires that are done at the Kavli Institute of Nanoscience in Delft. Interests specifically are in the electrical and optical properties of III-V and Si semiconducting nanowires. By confining a region in the wire a quasi one-dimensional, atom-like quantum dot can be defined. Gate manipulation allows single- or few-electron energy states to be accurately controlled, providing an excellent playground for the study of quantum phenomena.

Title: Control of Valley-Splitting in Silicon-on-Insulator MOSFETs
Authors: K. Takashina, Y. Ono, A. Fujiwara, H. Inokawa, Y. Takahashi, and Y. Hirayama
Institution: NTT Basic Research Laboratories, SORST/Japan Science and Technology Agency
Presenter: Kei Takashina
The valley splitting, which lifts the degeneracy of the lowest two valley states in a SiO2/(100)Si/SiO2 quantum well is examined through transport measurements. We demonstrate that the valley splitting can be continuously enhanced to 10 s of meV, and show that it can be directly observed as a step in the conductance well above liquid helium temperature even at zero magnetic field, defining a boundary between valley-polarized and unpolarized regions.

Title: Active Optical Antennas
Authors: E. Cubukcu, E.A. Kort, and K.B. Crozier
Institution: Harvard University
Presenter: Ertugrul Cubukcu
We have modeled and implemented nanoscale optical antennas on the facet of a commercial near-infrared diode laser, and have directly observed the highly localized enhancement of the laser field using an apertureless near-field scanning optical microscope.

Title: Quantum State Transfer from Photon to Electron Spin for Quantum Repeater
Authors: H. Kosaka, T. Kutsuwa, K. Arai, Y. Rikitake, K. Ono, H. Imamura T. Takagahara,
Y. Mitsumori, and K. Edamatsu
Institution: Tohuko University
Presenter: Hideo Kosaka
We present quantum-state-transfer (QST) from photon polarization qubit to electron spin qubit for quantum repeaters. We have investigated GaAs/AlGaAs quantum structure with zero electron g-factor, which showed nonlinear B-field dependence of spin precession, suggesting photon-to-nuclei polarization transfer through electron spin polarization. 2DEG with quantum-point-contact on InGaAs/InP showed telecom-wavelength-sensitive quantum transport. Efficient/high-fidelity QST conditions were also theoretically found.

Title: Tunable Infrared Surface Plasmon Absorption in Semimetallic Nanoparticle/Semiconductor Heterostructures
Authors: M. Hanson, E.R. Brown, and A.C. Gossard
Institution: University of California, Santa Barbara
Presenter: Micah Hanson
We report strong infrared attenuation attributed to surface plasmon resonances on semimetallic ErAs and ErSb nanoparticles embedded in semiconductor matrices grown by molecular beam epitaxy. The position of these resonances can be shifted by growth conditions and matrix material from 1.2 to 4 µm. The intensity of the attenuation can be increased independently of the resonance position by increasing the density of particles. Maximum absorption coefficients exceeding 104/cm have been measured for the composite GaAs/ErAs materials at 1.55 µm and in GaSb/ErSb at 4 µm. The resonance is also observed to shift depending on the polarization of the incident radiation due to the asymmetry of the nanoparticles.

Title: Charge Detection Measurement of a Double Quantum Dot
Authors: T. Hayashi, T. Fujisawa, R. Tomita, and Y. Hirayama
Institution: NTT Basic Research Laboratories, SORST/Japan Science and Technology
Presenter: Toshiaki Hayashi
Quantum point contact (QPC) has attracted a lot of interest as a fast single-charge detector. We performed real-time observation of charge states in a double quantum dot (QD) by monitoring the current passing through an adjacent QPC. We also investigated the energy relaxation time of the double QD by applying rectangular voltage pulses to the double QD.

Title: Measurement of Large Valley Splitting in a Si/SiGe 2DEG Point Contact
Authors: L.M. McGuire, K.A. Slinker, S. Goswami, J.O. Chu, and M.A. Eriksson
Institution: University of Wisconsin
Presenter: Lisa McGuire
Silicon two-dimensional electron gases (2DEGs) have a near degeneracy due to two low-lying valley states. If this result from open 2DEGs is applicable to quantum dots, then valleys in silicon could be expected to lead to two closely spaced orbital states in dots — states that in principle could compete with spin states in qubit applications. Here, we measure the valley splitting as a function of magnetic field in Si/SiGe 2DEG point contacts defined by metal top-gates. Our results indicate that the valley splitting in confined structures is substantially larger than in open 2DEGs. Using a pair of point contacts on a quantum dot, we apply a small ac bias to the source-drain and measure the differential current as we pinch off the channel by applying a negative voltage to the top gates. As the voltage on the top gates is varied in zero magnetic field, we observe the conventional steps at conductance values of multiples of 4e2/h. By applying a perpendicular magnetic field, we lift both the spin and valley degeneracies, and we see corresponding steps in conductance at every e2/h. By fitting the conductance as a function of a magnetic field, we can extract both the subband spacing and the valley splitting energy. Further, by fitting the conductance steps at different temperatures an estimate of the electron temperature can be obtained. At high magnetic fields, step-like features appear at noninteger conductance values in addition to the integer steps.

Title: Shot Noise in a Quantum Point Contact
Authors: L. DiCarlo, D.T. McClure, D.J. Reilly, C.M. Marcus, L.N. Pfeiffer, and K.W. West
Institution: Harvard University
Presenter: Yiming Zhang
We report detailed simultaneous measurements of shot noise and dc transport in a quantum point contact (QPC) as a function of source-drain bias, gate voltage and in-plane magnetic field. The magnetic field evolution of the 0.7 structure in both conductance and noise is clearly visible and is compared to a simple model, giving good quantitative agreement.

Title: Cubic Dresselhaus Spin Orbit Coupling in Small Quantum Dots
Authors: J.J. Krich and B.I. Halperin
Institution: Harvard University
Presenter: Jacob Krich
Due to the suppression of linear spin-orbit effects in small quantum dots in two-dimensional electron systems, the cubic Dresselhaus spin-orbit coupling can play a significant role in such phenomena as the variance of conductance through a dot. We characterize the different spin-orbit coupling terms by the strength of the anti-crossings they induce in the eigenstates of a closed quantum dot as an in-plane magnetic field is increased, and we perform numerical simulations in a chaotic billiard model to estimate the RMS anti-crossing energy. We investigate the conditions under which the cubic Dresselhaus effects may be measurable and significant for realizable dot configurations.

Title: Spin Hall Effect in n-doped GaAs
Authors: H.A. Engel, B.I. Halperin, and E.I. Rashba
Institution: Harvard University
Presenter: Hans-Andreas Engel
In the spin Hall effect, an electric current in a system with spin-orbit coupling induces a transverse spin current which leads to nonequilibrium spin accumulation near sample boundaries. Generating and manipulating nonequilibrium spin magnetization by electric fields is one of the most desirable goals of semiconductor spintronics, because electric fields have potentialities for accessing individual spins at nanometer scales. In this talk, I review the different spin-orbit coupling mechanisms in direct gap semiconductors and the implications of these mechanisms for the spin Hall effect. In particular, we recently developed a theory that accounts for spin-orbit coupling at charged impurities. This coupling leads to extrinsic spin currents that contain skew scattering and side jump contribution [1]. Applying our theory to bulk n-GaAs, without any free parameters, we find spin currents that are in reasonable agreement with recent experiments by Kato et al. [2].
[1] H.-A. Engel, B.I. Halperin, and E.I. Rashba, Phys. Rev. Lett. 95, 166605 (2005).
[2] Y.K. Kato, R.C. Myers, A.C. Gossard, and D.D. Awschalom, Science 306, 1910 (2004).

Title: Designer Hamiltonians: Two Channel Kondo Effect in Single Electron Transistor
Authors: David Goldhaber-Gordon, Ronald Potok, and Ileana Rau
Institution: Stanford University
Presenter: David Goldhaber-Gordon
In this poster, I will present transport measurements on a novel semiconductor nanostructure designed to display many-body phenomena, including a quantum phase transition and an associated quantum critical point. A quantum dot (acting as an artificial magnetic impurity) attached to conducting reservoirs displays a many-body screening effect known as the Kondo effect. Coupling the small quantum dot to an additional, finite-sized reservoir dramatically modifies transport through the dot, providing evidence for a quantum phase transition.

Title: Long-Spin Relaxation Time in InGaN Multi-Quantum Wells — Suppression of the Spin-Flip Process Caused by the Phase-Separated Dot Formation
Authors: T. Kawano, S. Nagahara, M. Arita, and Y. Arakawa
Institution: University of Tokyo
Presenter: Takeshi Kawano
We first observed spin relaxation at room temperature in InxGa1-xN multi-quantum wells using spin-dependent pump and probe measurements. The spin lifetime increases with In molar fraction x. The observed spin lifetime obeys 65374;x3.3, which shows different behavior from typical results for D’yakonov-Perel and Elliot-Yafet processes involving the alloy scattering. The increase of In molar fraction induces indium-composition fluctuations, which cause the electron localization. The remarkable increase of the spin lifetime is due to the formation of quantum dots through the phase separation in InxGa1-xN multi-quantum well layers.

Title: Multi-Peak Kondo Effect in a One- and Two-Electron Quantum Dot
Authors: A. Vidan, M. Stopa, R.M. Westervelt, M. Hanson, and A.C. Gossard
Institution: Harvard University
Presenter: Andy Vidan
Semiconductor quantum dots are tunable systems and can serve as probes of strongly correlated electron behavior. We have fabricated a few-electron quantum dot that can be tuned down to zero electrons while maintaining strong coupling to the leads. Using a nearby quantum point contact as a charge sensor, we can determine the absolute number of electrons in the quantum dot. 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, and investigate the magnetic field dependence of these additional Kondo resonances.

Title: Manipulation of the Kondo Effect in Vertical Single Quantum Dot with Multiple Gates
Authors: S. Amaha, T. Hatano, S. Sasaki, T. Kubo, Y. Tokura, and S. Tarucha
Institution: ICORP/Japan Science and Technology Agency
Presenter: Shinichi Amaha
We fabricated a vertical quantum dot (QD) with four separate gate electrodes placed on the side of the circular mesa to study the effect of symmetry or asymmetry of orbital wave function on the Kondo effect. The symmetry/asymmetry can be manipulated by applying different voltages to the four gates. We used this technique to tune various kinds of state degeneracies in the QD at zero and finite magnetic fields (B). We observed enhancement of the Kondo effect for the electron number N = 7 at B = 0 T, which can be associated with the SU(6) Kondo effect only expected for a QD with 2-D harmonic symmetry. For N = 8, we observed enhancement of the Kondo effect at B = 0 T, associated with singlet-triplet (S-T) degeneracy. This Kondo effect is stronger than the S-T Kondo effect previously observed at finite magnetic field, and is understood in terms of a concept of S-T degeneracy but associated with “two” singlet states and a triplet state. Note the S-T Kondo effect at finite B field is associated with “one” singlet state and a triplet state. We also observed reduction of the doublet-doublet Kondo effect at finite B field, probably due to asymmetry-induced anti-crossing of two Fock-Darwin states.

Title: The Kondo Effect in Laterally Coupled Vertical Double-Dot System
Authors: S.Y. Sok, T. Hatano, S. Amaha, and S. Tarucha
Institution: ICORP/Japan Science and Technology Agency
Presenter: Shin Yun Sok
We investigated the Kondo effect in laterally coupled vertical quantum dots. The system consists of two vertical dots in the parallel configuration, two side gates, and one center gate. The dots are defined in a two-dimensional electron gas of an AlGaAs/GaAs/AlGaAs double barrier structure. The two side gates are used to control the electrostatic potentials of the two dots and the center gate located between the dots is used to manipulate the inter-dot coupling. From measurement of a charging diagram or a so-called ‘honeycomb’ pattern, we can identify the strength of inter-dot coupling, and the electron numbers and charging energies for the corresponding dots. The Kondo effect will be enhanced due to degeneracy at vertices in the ‘honeycomb’ pattern presented by a Coqblin-Schrieffer model of SU(N) where N is a degeneracy factor. We observed the ‘honeycomb’ pattern and enhancement of the conductance at around vertices as a signature of the Kondo effect in the double quantum dots.

Title: Charge Detection of Weakly Coupled Vertical Quantum Dots
Authors: T. Kodera, Y. Iwai, Y. Kitamura, W.G. van der Wiel, T. Obata, T. Hatano, K. Ono, and S. Tarucha
Institution: University of Tokyo
Presenter: Tetsuo Kodera
We have fabricated laterally coupled quantum dot devices that can be used for reading out electron charge and spin information of single and coupled quantum dots. One of the coupled four dots is then used as a single electron transistor (SET) electrometer. In this device, the inter-dot coupling is tunable with coupling gate voltage and each dot can be addressed individually by the side gate. We first use coupled two dots to observe stability diagrams reflecting electrostatic or tunnel coupling with coupling gate voltage as a parameter. We then use one of the two dots as an SET electrometer to the other dot. Here the two dots are only weakly electrostatically coupled. We find that the SET electrometer is sensitive enough for detecting a change of averaged electron number in the dot between 0 and 1. We also propose a spin readout scheme using Pauli effect.

Title: Zeeman Effect and Hund’s First Rule in Strongly Coupled InAs Self-Assembled Dots
Authors: Y. Igarashi, T. Ota, K. Ono, Y. Nakata, H.Z. Song, T. Miyazawa, and S. Tarucha
Institution: University of Tokyo
Presenter: Yuichi Igarashi
InAs self-assembled quantum dots (SAQDs) have been attracting considerable interests because of the potential applications to quantum information technologies. The attractive features have been embodied by the observations of single photon emission and extremely long spin lifetime [1]. However, the electronic spin effects are not well resolved because of the large inhomogeneity included in the measurement of QD ensembles. We measure single electron transport through a single pair of vertically strongly coupled InAs SAQDs embedded in a SET [2,3] to study the Zeeman effect of few electron systems in the coupled QD. From measurement of the Coulomb peaks evolving with in-plane magnetic field (B), we derive the electron g-factor and find evidence of Hund’s first rule. The measured spacing between the first and second Coulomb peaks, DE21, and between the fourth and fifth peaks, DE54, both increase linearly with B, indicating the spin anti-parallel filling. In contrast, DE43 and E65 are constant with B, indicating the spin parallel filling. The spin parallel filling of the N = 3 and 4 states is due to Hund’s rule for the 2p shell. From the DE21 (DE54) vs. B, we estimate |g| = 1.0 ± 0.1(0.98 ± 0.1), which is consistent with those measured by optical spectroscopy [4].
[1] M. Kroutvar et al., Nature (2004).
[2] T. Ota et al., PRL (2004).
[3] T. Ota et al., PRL (2005).
[4] For example, Y. Toda et al., APL (1998).

Title: Two Electron Singlet-Triplet Spectroscopy
Authors: D. Zumbühl, I. Radu, C.R. Dillard, G. Granger, M.A. Kastner, M.P. Hanson, and
A.C. Gossard
Institution: Massachusetts Institute of Technology
Presenter: Dominik Zumbühl
We present measurements of few electron quantum dots formed by lateral depletion of a GaAs/AlGaAs 2-D 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.

Title: Mesoscopic Fermi-Edge Singularity
Authors: D. Abanin and L. Levitov
Institution: Massachusetts Institute of Technology
Presenter: Dmitry Abanin
We study resonant tunneling in an open mesoscopic dot. Coulomb interaction leads to a Fermi-Edge singularity of the tunneling current. 1) We demonstrate that scattering of the tunneling electron in the dot strongly modifies the power-law Fermi-Edge resonance. The control of scattering by varying the dot shape and coupling to the leads allows to explore a wide range of exponents. 2) At energies above Thouless energy, we study interplay of the local density of states fluctuations and the Fermi-Edge Singularity. We predict that fluctuations of the tunneling current as a function of energy have a power-law singularity with an exponent larger than that of the average current.

Title: Spontaneous Nuclear Polarization in Quantum Dots
Authors: M. Rudner and L. Levitov
Institution: Massachusetts Institute of Technology
Presenter: Mark Rudner
We investigate the spontaneous nuclear polarization that can be produced by the hyperfine interaction in double quantum dot systems with spin-blocked electron transport. In particular we consider the case of two dots where the following states are well separated in energy from all others: 1) one electron localized in the lowest spatial orbital of the second dot, 2) two electrons localized in the lowest spatial orbital of the second dot with singlet spin configuration, and 3) one electron localized in the lowest spatial orbital of each dot with either singlet or triplet spin configuration. We label these states by (0, 1), (0, 2)s, (1, 1)s and (1, 1)t, respectively. These conditions are motivated by the transport experiments of Tarucha et al. in which current is believed to flow via a 3-stage cycle: (0, 1) --> (1, 1){s/t} --> (0, 2)s --> (0,1) ... Spin-blockade occurs when the system occupies the spin-triplet state (1, 1)t at the second stage of the cycle. From this state, (0, 2)s cannot be reached without a spin-flip. Leakage current is due either to slow inelastic/co-tunneling processes which avoid the (0, 2)s state completely, or to spin-flip processes arising from hyperfine or spin-orbit interactions that mix the (1, 1)t and (1, 1)s states. In the case where the hyperfine interaction nearly resonantly couples the singlet and triplet states, we find that the spin-blockade is partially lifted and that the zero net-polarization nuclear spin state becomes unstable. At zero field, nuclear dipole-dipole interactions disorder the nuclear spins and prevent build-up of polarization. At small applied field the longitudinal part of these interactions is quenched and spontaneous polarization is possible. We study the nature and strength of this zero-polarization instability, and the dynamical behavior of the resulting magnetic moment.

Title: Shape-Dependent DNA Translocation through a Membrane Pore
Authors: M.G. Fyta, S. Melchionna, S. Succi, and E. Kaxiras
Institution: Harvard University
Presenter: Maria Fyta
We report multiscale simulations of DNA translocation through a membrane pore. The effects of the external electric field, the pore size, the DNA length and its shape on this phenomenon are investigated. Preliminary results show that the translocation time is strongly influenced by the shape of the DNA, and circular chains can penetrate more effectively into the cell.

Group B

Title: Scanning Probe Microscopy of Semiconducting Nanowires
Authors: A.C. Bleszynski, R.M. Westervelt, F.A. Zwanenburg, L.P. Kouwenhoven, A.L. Roest, and E.P.A.M. Bakkers
Institution: Harvard University; Kavli Institute of Nanoscience Delft, Delft University of Technology; Philips Research Laboratories, Eindhoven, The Netherlands
Presenter: Ania C. Bleszynski
We have used a liquid-He cooled scanning probe microscope (SPM) with a conducting tip to image electrical conduction through InAs nanowires. The charged SPM tip is scanned above the nanowire and the resulting change in nanowire conductance is recorded to form the image. These conductance images are used to study the behavior of electrons in the nanowire on a local scale. For example, the images reveal barriers to conduction at the contacts as well as sections of the wire that act as quantum dots. At 4K the wires exhibit Coulomb blockade oscillations in conductance versus backgate voltage that are indicative of multiple quantum dots in series. The images reveal the location of the quantum dots along the wire and the tip voltage can tune their charge state. The nanowires, grown catalytically from small gold particles, have diameters between 50 and 100 nm. Ti/Al source and drain contacts with a spacing of 1 to 2 µm were defined using e-beam lithography.

Title: Imaging Magnetic Focusing in a Two-Dimensional Electron Gas
Authors: K. Aidala, R.E. Parrott, T. Kramer, E.J. Heller, R.M. Westervelt, M.P. Hanson, and
A.C. Gossard
Institution: Harvard University
Presenter: Katherine Aidala
Using a liquid-He cooled scanning probe microscope (SPM), we have directly imaged cyclotron orbits of electrons in a two-dimensional electron gas (2DEG) traveling between two side-by-side quantum point contacts (QPCs). The images show magnetic focusing when the spacing between the QPCs is an integer multiple of twice the cyclotron radius. An image is created by deflecting electrons away from their original trajectories using a capacitively coupled SPM tip, and recording the change in conductance as the tip is raster scanned above the surface. The cyclotron orbits are clearly visualized, as well as fringes that demonstrate the coherent nature of the flow. Classical and quantum simulations show how electrons are deflected by the tip to produce the image. With an applied magnetic field, the simulated images of magnetic focusing agree very well with the measured images. The simulations also show the effect of small angle scattering due to the ionized donor atoms on the electron flow. Fully quantum simulations show that interference fringes can be produced. Imaging and understanding the motion of electrons in magnetic fields is useful for the development of devices for spintronics and quantum information processing.

Title: SPM Tip Scattering and Deflection Imaging: Beyond Backscattering
Authors: R.E. Parrott, K.E. Aidala, T. Kramer, E.J. Heller, and R.M. Westervelt
Institution: Harvard University
Presenter: Robert E. Parrott
We present recent work on the general scattering of electrons off an SPM tip in a 2DEG, and in particular SPM imaging of bouncing ball states in a magnetic field. Previous SPM imaging experiments used a single QPC in weak or no magnetic field, and a tip voltage that produced a strong effective tip potential V(r). This allowed for a simplified backscattering model of the scattering process, where the details of V(r) were not important. We extend this tip scattering model to examine the case of weak or even attractive tip potentials, and systems where the electron is deflected to a secondary target QPC. In particular we consider the SPM imaging version of the classic magnetic focusing experiment in a 2DEG. Preliminary experimental and theoretical results suggest that small-scale quantum interference fringes are obtained only for tip voltages that deplete, but that beyond this the actual strength of the tip potential is not important. In addition, results suggest that weak tip potentials, by acting as diffusing elements, can be used to map the classical branched flow to the target QPC, but that stronger tip potentials distort this branching. This suggests that by varying the SPM tip potential from weak to strong, both classical and phase information can be resolved independently in the general tip deflection case.

Title: Coulomb Blockade Imaging of Few-Electron Quantum Dots in a Magnetic Field
Authors: P. Fallahi, R.M. Westervelt, M. Stopa, M.P. Hanson, and A.C. Gossard
Institution: Harvard University
Presenter: Parisa Fallahi
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. Quantum 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. We 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.
[1] P. Fallahi et al., Nano Letters 5, 223 (2005).

Title: Spin Manipulation in Lateral Quantum Dots under Time-Dependent Confinement
Authors: J. Walls and E.J. Heller
Institution: Harvard University
Presenter: Jamie Walls
Single spin manipulations are a critical component for potential realizations of spintronic devices and quantum computers in lateral quantum dots. In this work, we demonstrate a new method for creating spin excitations in lateral quantum dots, which uses the interplay between the spin-orbit interaction and a time-dependent lateral confining potential. For an asymmetric dot in the presence of an in-plane magnetic field, the spin quantization axis can be tilted away from the applied magnetic field due to the Rashba spin-orbit coupling, with the degree of tilting depending parametrically upon the confinement potential. By making small modulations to the confinement potential at a frequency given roughly by the Zeeman splitting, efficient spin excitations can be performed. We have performed theoretical and numerical calculations which demonstrate that Rabi frequencies on the order of tens of megahertz can be achieved for experimentally accessible parameters.

Title: Imaging Electron Flow in Different High-Mobility 2DEG Heterostructures
Authors: M. Jura, A. Sciambi, D. Lo, D. Goldhaber-Gordon, K. West and L. Pfeiffer
Institution: Stanford University
Presenter: Mark Topinka
In this poster we present images of electron flow through QPCs in several different 2DEG heterostructures. The measurements were taken at 4.2 K with a home-built scanning gate system designed for insertion into He-4, He-3, and Dill-Fridge systems. We will concentrate on the observed differences in flow patterns as well as the variability in imageability between heterostructures from several sources with different mobility, density, and layer structures.

Title: Measuring Exchange Interactions by Tunneling Deep Into the Quantum Hall Liquid
Authors: O.E. Dial, R.C. Ashoori, L.N. Pfieffer, and K.W. West
Institution: MIT, Bell Labs, Lucent Technologies
Presenter: Oliver Dial
We present measurements of the tunneling density of states of a two-dimensional gas (2DEG) in GaAs at high magnetic fields and energies up to 10 meV above and below the Fermi energy. Using pulsed electric fields, we determine the current-voltage (IV) characteristics for tunneling perpendicularly between a gated 2DEG and a 3-D electron continuum separated by a thin tunneling barrier. In these IV curves we can clearly resolve the level spectrum of both filled and unfilled Landau levels. By varying the density and magnetic field, we can observe changes in the energy structure far from the Fermi surface as individual levels fill and empty, allowing us to separate contributions from single particle physics and electron-electron interactions in the spectra. This provides a unique measurement of the exchange enhanced spin splitting of empty and filled Landau levels.

Title: Low-Temperature Scanning Tunneling Microscopy of InGaAs Thin Films Epitaxially Grown on Lattice-Matched Substrates
Authors: S. Perraud, K. Kanisawa, Z.Z. Wang, and Y. Hirayama
Institution: NTT Basic Research Laborartories
Presenter: Simon Perraud
In solids, electrons can be elastically scattered by crystal defects or impurities. Quantum-mechanical interferences between incident and scattered electron waves result in the formation of standing waves around scattering centers. The corresponding local density of states (LDOS) show spatial oscillations, often referred to as Friedel oscillations. In this work, we have used low-temperature scanning tunneling microscopy (STM) to characterize Friedel oscillations at the surface of high-mobility InGaAs thin films epitaxially grown on lattice-matched (111)A substrates. The energy dependence of the electron wavelength was determined by Fourier transform of LDOS maps, giving access to the conduction-band dispersion relation. It was found that the Fermi level at the InGaAs(111)A surface is unpinned at high silicon doping levels.

Title: Near-Field Observation of Magneto-Photoluminescence of Two-Dimensional Electron Gas Systems
Authors: T. Tokizaki and H. Yokoyama
Institution: AIST, Nanotechnology Research Institute
Presenter: Takashi Tokizaki
We investigated local magneto-photoluminescence spectra of 2DEG in GaAs/AlGaAs mesa structures using a scanning near-field optical microscope operated in cryogenic temperatures and strong magnetic fields. The PL intensity change depending on the magnetic field and the blue shift at the mesa edges were observed.

Title: Ultrafast ElectricalNanoprobing by Scanning Maxwell-stress Microscopy
Authors: T. Inoue, K. Takami, T. Tokizaki, H. Yokoyama
Institution: Association for Iron and Steel Technology, SORST/Japan Science and Technology Agency
Presenter: Takahito Inoue
We have developed the heterodyne force-detected scanning Maxwell-stress microscopy (SMM), which makes use of the nonlinearity of the tip-sample interactions to measure ultrafast voltage signals of the sample with high spatial and time resolution. In this paper, we present a direct measurement of the microwave voltage at the tip apex by using the heterodyne force-detected SMM and a development of a customized cantilever with an integrated transmission line connected right up to the tip in order to improve the transmission efficiency of microwave signals through the cantilever in the GHz range.

Title: Solid State Quantum Computation: From Physical Gates to Architectures
Authors: J.M. Taylor, H.-A. Engel, W. Dür, A. Yacoby, C.M. Marcus, P. Zoller, and M.D. Lukin
Institution: Harvard University
Presenter: Jacob Taylor
Solid-state approaches to quantum computation offer intriguing prospects for large-scale integration and long-term stability. However, achieving fault tolerant quantum computation entails significant mitigation of environmental couplings, which is particularly challenging in the solid state. We will discuss the theoretical and experimental development of a scalable architecture for solid-state quantum computation based on actively protected two electron spin states in quantum dots. Specifically, we find a universal set of gates for two-spin states that can be implemented using only local electrical control, with explicit suppression of hyperfine interactions, the dominant source of error. The architecture allows for a modular, hierarchical design, and includes autonomous control and nonlocal coupling using controlled electron transport. Fault tolerance properties of the architecture will be considered.

Title: Hyperfine Interaction in Single and Double Quantum Dots
Authors: W.A. Coish and D. Loss
Institution: University of Basel
Presenter: William A. Coish
We have evaluated hyperfine-induced electron spin dynamics for two electrons confined to a double quantum dot. Our quantum solution accounts for decay of a singlet-triplet correlator even in the presence of a fully static nuclear spin system, with no ensemble averaging over initial conditions. In contrast to an earlier semiclassical calculation, which neglects the exchange interaction, we find that the singlet-triplet correlator shows a long-time saturation value that differs from 1/2, even in the presence of a strong magnetic field. Furthermore, we find that the form of the long-time decay undergoes a transition from a rapid Gaussian to a slow power law (~1/t3/2) when the exchange interaction becomes nonzero and the singlet-triplet correlator acquires a phase shift given by a universal (parameter independent) value of 3π/4 at long times. The oscillation frequency and time-dependent phase shift of the singlet-triplet correlator can be used to perform a precision measurement of the exchange interaction and Overhauser field fluctuations in an experimentally accessible system. We also address the effect of orbital dephasing on singlet-triplet decoherence, and find that there is an optimal operating point where orbital dephasing becomes negligible.

Title: Nuclear Spin State Narrowing via Gate-Controlled Rabi Oscillations in a Double Quantum Dot
Authors: D. Klauser, W.A. Coish, and D. Loss
Institution: University of Basel
Presenter: Daniel Klauser
We study spin dynamics for two electrons confined to a double quantum dot under the influence of an oscillating exchange interaction. This leads to driven Rabi oscillations in the two-electron system. The width of the Rabi resonance allows one to narrow the distribution of nuclear spin states and thereby to prolong the spin decoherence time. Further, we study decoherence of the two-electron states due to the hyperfine interaction and analize the “square-root-of-swap” gate taking into account hyperfine induced decoherence.

Title: Technical Development of On-chip Micro Coil Systems for Implementing Electron Spin Qubit
Authors: T. Obata, M. P. Ladriere, T. Kodera, W. G.van der Wiel, and S. Tarucha
Institution: ICORP/Japan Science and Technology Agency
Presenter: Toshiaki Obata
We study a technical approach for implementing electron-spin qubits with quantum dots (QD). Among many kinds of candidates for making solid-state qubits, electron spin with QDs is attractive because of the potentially long decoherent time. Single spin manipulation in QDs needs a high radio frequency (rf) magnetic field with low temperature environment, however, the technique has yet to be developed. The magnetic field frequency necessary for spin rotation is of the order of ~GHz, as is well known for ESR experiments. In these experiments rf magnetic field is usually introduced through a cavity resonator. However, the cavity technique is not suitable for our purpose because it strongly warms up the low temperature environment itself. To avoid the heating problem, it is crucial to feed an rf magnetic field in a concentrated manner to a QD. We are developing a combined setup of an on-chip micro-coil and a micro-probe, which has a good resonance condition even at dilution refrigerator temperatures. We use our on-chip micro-coil resonator to examine the performance of high frequency response and maximal magnetic field. In the workshop we discuss capability of our on-chip resonator including some trial measurements of electron spin resonance.

Title: Real-Time Measurement of Spin-Dependent Tunneling and Electronic Relaxation in a Lateral Quantum Dot
Authors: J. Sugawa and S. Tarucha
Institution: University of Tokyo
Presenter: Jun Sugawa
Single-electron tunneling between a quantum dot (QD) and the reservoir can be monitored in real time using a nearby quantum point contact (QPC) as an electrometer. We examine the difference between the tunnel rate for coming into and out of the ground state of the dot in the presence of a perpendicular magnetic field. When spin-resolved edge states are formed in the contact 2DEG leads, the tunnel rate through the ground state becomes different between spin-up and spin-down electrons, reflecting the distance of the spin-resolved edge states from the tunnel junction. We also examine the spin-dependent relaxation time in the QD. For some condition, the tunnel rate for the QD excited state is much larger (smaller) than that for the ground state. An electron can be non-adiabatically trapped by the excited state in the QD by application of a positive step voltage to the plunger gate, and remains in the excited state as long as the voltage duration is shorter than the relaxation time. Then when the voltage is turned off, we observe the tunnel rate through the excited state. Otherwise we observe the tunnel rate through the ground state. We use this pump and probe technique to examine the spin-dependent relaxation in the presence of a perpendicular magnetic field.

Title: Hybrid Quantum Dot Devices for Coherent Single Electron Spin Control in a Slanting Zeeman Field
Authors: T. Obata, T. Kodera, K. Yoshida, W. G. van der Wiel, and S. Tarucha
Institution: ICORP/Japan Science and Technology Agency
Presenter: Michel Pioro-Ladriere
Quantum dots (QDs) are man-made structures, which can confine conduction electrons in semiconductor to a nanometer size volume. For gated structures, the number of electrons trapped by a QD can be varied one-by-one down to zero. In this regime, the spin of individual electrons, which is a natural two-level system, can be used as a quantum bit for implementing scalable quantum computing. In this context various experiments have been performed on robustness of electron spin in single- and double-dot systems, spin correlation, electrical readout of a single electron spin, and coherent manipulation of spin states in artificial molecules. However, the rotation of a single electron spin or qubit operation, which is the most fundamental quantum operation, still needs to be demonstrated urgently. Applying conventional spin resonance method is difficult because of high-frequency radiations, which heat up the spin qubit to an unacceptable level. One viable approach to coherent single electron spin control consists of modulating a QD electric field in a position-dependent magnetic field. In this poster, we report on the design and fabrication of hybrid devices where the slanted magnetic field required for this novel technique is produced by micron size permanent magnets.

Title: Quantum Phase Coherence through Aharonov-Bohm Interferometer Containing Laterally Coupled Double Quantum Dots
Authors: T. Kubo, Y. Tokura, T Hatano, and S. Tarucha
Institution: ICORP/Japan Science and Technology Agency
Presenter: Toshiaki Kubo
Probing and manipulating quantum phase coherence are the heart of quantum information technology, and have long been studied for various mesoscopic systems. One of the most powerful techniques for detecting quantum phase coherence is to measure the phase difference using an Aharonov-Bohm (AB) interferometer. This technique has recently been applied to an AB interferometer containing a quantum dot (QD). The experiment shows modulation of tunneling current as a function of magnetic flux through the interferometer, indicating that the phase coherence of electrons is maintained during the tunneling through the QD. We theoretically investigate electron transport through an AB interferometer containing laterally coupled double QDs (DQDs). We particularly concentrate on the effect of the coupling parameter whose modulus characterizes the coupling strength between two QDs through the reservoirs. Although there exist two conduction modes in general, we have only a single conduction mode when the coupling is strong. We calculate linear conductance through the DQDs using Green’s function approach for non-interacting system. Based on the exact expression of the linear conductance, we investigate AB oscillations through the DQDs from the weak interdot tunnel coupling regime to the strong interdot tunnel coupling regime. The visibility of AB oscillations in the linear conductance becomes lower as the modulus of the coupling parameter decreases. The AB oscillation is independent of the magnetic flux when the coupling parameter is zero, and the visibility becomes zero.

Title: Gate Control of Pauli Spin Blockade in Vertical Quantum Dot Devices
Authors: Y. Kitamura, K. Ono, and S. Tarucha
Institution: University of Tokyo
Presenter: Yosuke Kitamura
Manipulating single electron spins with quantum dots is essential for implementing qubits in quantum computing. We previously demonstrated for vertical double quantum dots that Pauli effect leads to novel spin blockade in electron tunneling through the double dot system. This effect can be used to read out individual electron spin information, which is one of the ingredients for quantum computing. Ideal Pauli effect is observed in vertical double dot system with a relevant potential offset between the two dots, however, the tunability of Pauli effects is limited because the potential offset is not so tunable with gate voltages in vertical devices. To solve this problem, we made a novel vertical double-dot device that enables us to tune the spin blockade condition using separate two gate electrodes. The Schottky gate is separated into two parts by the mesa line. By applying different voltages to the two gates, we succeeded in changing the potential offset between vertically coupled two dots, and realized electrical control of Pauli spin blockade. This will enable us to carry out various experiments about electron spin.

Title: Probing Spin States in Quantum Dots by Spin-Resolved One-Dimensional Contacts
Authors: K. Hitachi, M. Yamamoto, and S. Tarucha
Institution: University of Tokyo
Presenter: Kenichi Hitachi
We fabricate a quantum dot (QD) with tunnel-coupled quantum wires (QWs) to study the electronic states of the QD with the QWs as spin filters. The propagating mode of the QWs is adjusted with the gate voltages to be either spin-resolved (n = 1) or spin degenerate (n = 2) under a finite magnetic field. For the n = 2 QW, we observe small and large Coulomb peaks, depending on whether the peak is related to the lowest Landau state or the first excited Landau state in the QD. The tunnel coupling to the wire is weaker for the first excited Landau state because the state is located more inside the QD. When we set the n = 1 QW mode, we observe four kinds of Coulomb peaks. Two of them arise from the difference of tunnel couplings described above, while the other two arise from the spin filtering effect. The QD state holding even number of electrons is usually either a spin singlet state or a triplet state, whereas that holding odd number of electrons is usually a spin-half state. So adding a spin down electron to form the singlet state is prohibited by the QW spin filtering effect. We also measure the relaxation time of singlet states and triplet state using a QPC charge readout technique. The time scale for the triplet state is nealy equal to half the value for the singlet state. These results are consistent with the data of Coulomb oscillations.

Title: Dynamical Enhancement of Nuclear-Spin Coherence in GaAs and Si
Authors: S. Sasaki, Y. Hirayama, S. Watanabe, S. Nagai, T. Sakashita, and J. Harada
Institution: Niigata University, SORST/Japan Science and Technology Agency
Presenter: Susumu Sasaki
We have found that, by using a pulse method, the T2 of nuclear spins in GaAs and Si can be enhanced 10-100 times as long as the intrinsic value. In the case of Si, the enhanced values of T2 depend on the carrier concentration, whereas the intrinsic T2 value does not. The origin of this enhancement is also discussed.

Title: Exchange in Multiple Quantum Dots: The Spin Funnel
Authors: M. Stopa and C.M. Marcus
Institution: Harvard University
Presenter: Mike Stopa
We employ density functional calculated eigenstates as a basis for exact diagonalization studies of semiconductor double quantum dots through the transition from the symmetric bias regime to the regime where both electrons occupy the same dot. We calculate the “spin funnel” characteristic of the singlet-triplet splitting as a function of bias detuning J(V) and explain its functional shape. For an applied magnetic field B we predict the existence of local minima where dJ(V,B)/dV = 0 and suppression of voltage noise can be expected.

Title: Creation of Effective Pure States Using All-Electrical GaAs Nanoscale Device toward NMR Quantum Computing
Authors: T. Ota, G. Yusa, N. Kumada, K. Muraki, S. Miyashita, and Y. Hirayama
Institution: NTT Basic Research Laboratories
Presenter: Takeshi Ota
The initialization of an NMR quantum computer is based on the preparation of effective pure states for the nuclei ensemble. In conventional NMR experiments, the effective pure states are prepared from thermal equilibrium states. However, in our all-electrical GaAs nanoscale device, we can use strongly polarized nuclear spin states as initial states, which are induced by dynamic nuclear spin polarization driven by electrical current, and prepare the effective pure states from these initial states, which make a large population available as the pure states. Here, we demonstrate creation of the effective pure states by applying appropriate RF pulse sequences to the initial state of 69Ga dynamic nuclear spin polarization, and find that nuclear spin population, which is far from thermal equilibrium, significantly depends on electron spin configurations.

Title: Electron Spin and Nuclear Spin Manipulation in Quantum Dots
Authors: K. Ono, Y. Kitamura, A. Takahashi, S. Yamaguchi, T. Inoshita, Y. Hirayama,
and S. Tarucha
Institution: Riken
Presenter: Keiji Ono
Electron spins and nuclear spins in semiconductor nanostructures are subject to intensive studies from the viewpoints of quantum information processing. Double quantum dots in the spin blockade (SB) regime, where the electron conduction is mostly blocked by Pauli effect unless the electron spin state is changed, are one of the platforms to explore the spin effects in quantum dots [1].We have proposed the spin-blocked double dots can act as a nuclear spin polarizer [2,3]. Transition from the spin-blocked triplet state to unblocked singlet state can be induced by the hyperfine flip-flop scatterings with the nuclei in the quantum dots. However at zero magnetic field, a finite tunnel coupling and exchange interaction between dots lifts the degeneracy of the singlet and the triplet states, and this energy separation greatly surpluses the most efficient elastic hyperfine scatterings. At a certain magnetic field B0 Zeeman splitting of the triplet states induces degeneracy of the SZ = +1 triplet and the singlet states. The elastic hyperfine scattering from SZ = +1 triplet to the singlet states is substantially favored over those from SZ = 0 or −1. Nuclear spin will be thus “pumped” from ↓ to ↑, and eventually leads to dynamic polarization of the nuclei. This novel nuclear polarization mechanism can be also tuned on/off electrically by means of the source-drain voltage VS, since the triplet-singlet energy separation depends sensitively on VS. When VS is tuned from a certain value at SB region to zero, at B0, the triplet-singlet energy separation increase and the triplet-singlet degeneracy is lifted. Thus the pumping of the nuclear spins is stopped and the nuclear spin system is nearly decoupled from the electronic systems. This polarized and electrically decoupled nuclear spin system can be manipulated following the continuous wave (CW) and pulsed NMR techniques. The manipulated nuclear spin state is measured electrically by means of a recovery time, a time needed to reestablish the polarized initial state after VS is tuned back to the value at SB region. In this paper we present results of these CW and pulsed NMR applied to InGaAs and GaAs quantum dots. In the GaAs dots a well-resolved quadrupole splitting is observed in the CW NMR spectrum of 71Ga nuclei. Asymmetric peak heights shows the nuclear spin is indeed polarized, i.e., IZ = 3/2 is mostly populated compared to IZ = 1/2, −1/2, or −3/2. We also demonstrate a logic gate operation of the four-level system using this initial state and transition selective rf pulses.
[1] K. Ono et al., Science 297, 1313 (2002).
[2] T. Inoshita et al., J. Phys. Soc. Jpn. 72, Suppl. A 183 (2003); T. Inoshita et al., ICPS2004.
[3] K. Ono et al., Phys. Rev. Lett. 92, 256803 (2004), cond-mat/0309062.

Title: Integration of Donor Electron Spin Qubits in Silicon
Authors: T. Schenkel, J. Bokor, J.A. Liddle, S. Lyon, A. Tyryshkin, R. deSousa, and K.B. Whaley
Institution: E.O. Lawrence Berkeley National Laboratory; AFRD
Presenter: Thomas Schenkel
Spins of electrons bound to donor atoms in silicon are promising candidates for the realization of quantum information processing devices. This promise is based on relatively long spin coherence times (>1 ms at 5 K), and the technology edge of silicon processing. We describe our efforts towards basic qubit demonstrations, which include the development of a single atom placement technique, studies of spin ensembles in predevice structures, and the design of single spin readout transistors.


  Last Modified February 14, 2006 by the NSEC Office.