Publications (84)242.18 Total impact
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Article: Charging and Spin Effects in Triple Dot Artificial Molecules
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ABSTRACT: We have fabricated artificial molecules consisting of three coupled quantum dots defined in the two-dimensional electron gas of a GaAs/AlGaAs heterostructure using lithographically patterned gates and trenches. The three dots are arranged in a ring structure, where each dot is coupled to the other two dots. We find that, when tuned to the Coulomb blockade regime, the triple quantum dot device acts as a charge rectifier: an electron enters the third dot where it is trapped, producing a jamming effect where no other electron may enter the first dot. Triple quantum dots coupled in a ring will allow for the study of new molecular phases using artificial molecules and may also serve as building blocks of two-dimensional arrays for quantum computation.Journal of Superconductivity 04/2012; 18(2):223-227. -
Article: Coaxial atomic force microscope probes for imaging with dielectrophoresis
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ABSTRACT: We demonstrate atomic force microscope (AFM) imaging using dielectrophoresis (DEP) with coaxial probes. DEP provides force contrast allowing coaxial probes to image with enhanced spatial resolution. We model a coaxial probe as an electric dipole to provide analytic formulas for DEP between a dipole, dielectric spheres, and a dielectric substrate. AFM images taken of dielectric spheres with and without an applied electric field show the disappearance of artifacts when imaging with DEP. Quantitative agreement between our model and experiment shows that we are imaging with DEP.Applied Physics Letters 06/2011; · 3.84 Impact Factor -
Article: Scaling of transverse nuclear magnetic relaxation due to magnetic nanoparticle aggregation.
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ABSTRACT: The aggregation of superparamagnetic iron oxide (SPIO) nanoparticles decreases the transverse nuclear magnetic resonance (NMR) relaxation time T2CP of adjacent water molecules measured by a Carr-Purcell-Meiboom-Gill (CPMG) pulse-echo sequence. This effect is commonly used to measure the concentrations of a variety of small molecules. We perform extensive Monte Carlo simulations of water diffusing around SPIO nanoparticle aggregates to determine the relationship between T2CP and details of the aggregate. We find that in the motional averaging regime T2CP scales as a power law with the number N of nanoparticles in an aggregate. The specific scaling is dependent on the fractal dimension d of the aggregates. We find T2CP∝N-0.44 for aggregates with d = 2.2, a value typical of diffusion limited aggregation. We also find that in two-nanoparticle systems, T2CP is strongly dependent on the orientation of the two nanoparticles relative to the external magnetic field, which implies that it may be possible to sense the orientation of a two-nanoparticle aggregate. To optimize the sensitivity of SPIO nanoparticle sensors, we propose that it is best to have aggregates with few nanoparticles, close together, measured with long pulse-echo times.Journal of Magnetism and Magnetic Materials 10/2010; 322(20):3122-3126. · 1.78 Impact Factor -
Article: Imaging coherent transport in graphene. Part I: mapping universal conductance fluctuations.
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ABSTRACT: Graphene provides a fascinating testbed for new physics and exciting opportunities for future applications based on quantum phenomena. To understand the coherent flow of electrons through a graphene device, we employ a nanoscale probe that can access the relevant length scales--the tip of a liquid-He-cooled scanning probe microscope (SPM) capacitively couples to the graphene device below, creating a movable scatterer for electron waves. At sufficiently low temperatures and small size scales, the diffusive transport of electrons through graphene becomes coherent, leading to universal conductance fluctuations (UCF). By scanning the tip over a device, we map these conductance fluctuations versus scatterer position. We find that the conductance is highly sensitive to the tip position, producing delta G approximately e(2)/h fluctuations when the tip is displaced by a distance comparable to half the Fermi wavelength. These measurements are in good agreement with detailed quantum simulations of the imaging experiment and demonstrate the value of a cooled SPM for probing coherent transport in graphene.Nanotechnology 07/2010; 21(27):274013. · 3.98 Impact Factor -
Article: High-Voltage Dielectrophoretic and Magnetophoretic Hybrid Integrated Circuit/Microfluidic Chip
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ABSTRACT: A hybrid integrated circuit (IC)/microfluidic chip is presented that independently and simultaneously traps and moves microscopic objects suspended in fluid using both electric and magnetic fields. This hybrid chip controls the location of dielectric objects, such as living cells and drops of fluid, on a 60 times 61 array of pixels that are 30 times 38 mum<sup>2</sup> in size, each of which can be individually addressed with a 50-V peak-to-peak dc-to-10-MHz radio-frequency voltage. These high-voltage pixels produce electric fields above the chip's surface with a magnitude |oarrE| ap 1 V/ mum, resulting in strong dielectrophoresis (DEP) forces |oarrF<sub>DEP</sub>| ap 1 nN. Underneath the array of DEP pixels, there is a magnetic matrix that consists of two perpendicular sets of 60 metal wires running across the chip. Each wire can be sourced with 120 mA to trap and move magnetically susceptible objects using magnetophoresis. The DEP pixel array and magnetic matrix can be used simultaneously to apply forces to microscopic objects, such as living cells or lipid vesicles, that are tagged with magnetic nanoparticles. The capabilities of the hybrid IC/microfluidic chip demonstrated in this paper provide important building blocks for a platform for biological and chemical applications.Journal of Microelectromechanical Systems 01/2010; · 2.10 Impact Factor -
Article: Mapping universal conductance fluctuations
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ABSTRACT: Graphene provides a fascinating testbed for new physics and exciting opportunities for future applications based on quantum phenomena. To understand the coherent flow of electrons through a graphene device, we employ a nanoscale probe that can access the relevant length scales - the tip of a liquid-He-cooled scanning probe microscope (SPM) capacitively couples to the graphene device below, creating a movable scatterer for electron waves. At sufficiently low temperatures and small size scales, the diffusive transport of electrons through graphene becomes coherent, leading to universal conductance fluctuations (UCF). By scanning the tip over a device, we map these conductance fluctuations \textit{vs.} scatterer position. We find that the conductance is highly sensitive to the tip position, producing $\delta G \sim e^2/h$ fluctuations when the tip is displaced by a distance comparable to half the Fermi wavelength. These measurements are in good agreement with detailed quantum simulations of the imaging experiment, and demonstrate the value of a cooled SPM for probing coherent transport in graphene. Comment: 16 pages, 6 figures07/2009; -
Article: Applied physics. Graphene nanoelectronics.
Science 05/2008; 320(5874):324-5. · 31.20 Impact Factor -
Article: Imaging a 1-electron InAs quantum dot in an InAs/InP nanowire
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ABSTRACT: Nanowire heterostructures define high-quality few-electron quantum dots for nanoelectronics, spintronics and quantum information processing. We use a cooled scanning probe microscope (SPM) to image and control an InAs quantum dot in an InAs/InP nanowire, using the tip as a movable gate. Images of dot conductance vs. tip position at T = 4.2 K show concentric rings as electrons are added, starting with the first electron. The SPM can locate a dot along a nanowire and individually tune its charge, abilities that will be very useful for the control of coupled nanowire dots.02/2008; -
Article: Integrated circuit/microfluidic chip to programmably trap and move cells and droplets with dielectrophoresis.
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ABSTRACT: We present an integrated circuit/microfluidic chip that traps and moves individual living biological cells and chemical droplets along programmable paths using dielectrophoresis (DEP). Our chip combines the biocompatibility of microfluidics with the programmability and complexity of integrated circuits (ICs). The chip is capable of simultaneously and independently controlling the location of thousands of dielectric objects, such as cells and chemical droplets. The chip consists of an array of 128 x 256 pixels, 11 x 11 microm(2) in size, controlled by built-in SRAM memory; each pixel can be energized by a radio frequency (RF) voltage of up to 5 V(pp). The IC was built in a commercial foundry and the microfluidic chamber was fabricated on its top surface at Harvard. Using this hybrid chip, we have moved yeast and mammalian cells through a microfluidic chamber at speeds up to 30 microm sec(-1). Thousands of cells can be individually trapped and simultaneously positioned in controlled patterns. The chip can trap and move pL droplets of water in oil, split one droplet into two, and mix two droplets into one. Our IC/microfluidic chip provides a versatile platform to trap and move large numbers of cells and fluid droplets individually for lab-on-a-chip applications.Lab on a Chip 02/2008; 8(1):81-7. · 5.67 Impact Factor -
Article: Scanned probe imaging of quantum dots inside InAs nanowires.
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ABSTRACT: We show how a scanning probe microscope (SPM) can be used to image electron flow through InAs nanowires, elucidating the physics of nanowire devices on a local scale. A charged SPM tip is used as a movable gate. Images of nanowire conductance versus tip position spatially map the conductance of InAs nanowires at liquid-He temperatures. Plots of conductance versus backgate voltage without the tip present show complex patterns of Coulomb-blockade peaks. Images of nanowire conductance identify their source as multiple quantum dots formed by disorder along the nanowire--each dot is surrounded by a series of concentric rings corresponding to Coulomb blockade peaks. An SPM image locates the dots and provides information about their size. In this way, SPM images can be used to understand the features that control transport through nanowires. The nanowires were grown from metal catalyst particles and have diameters approximately 80 nm and lengths 2-3 microm.Nano Letters 10/2007; 7(9):2559-62. · 13.20 Impact Factor -
Article: Imaging Magnetic Focusing of Coherent Electron Waves
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ABSTRACT: The magnetic focusing of electrons has proven its utility in fundamental studies of electron transport. Here we report the direct imaging of magnetic focusing of electron waves, specifically in a two-dimensional electron gas (2DEG). We see the semicircular trajectories of electrons as they bounce along a boundary in the 2DEG, as well as fringes showing the coherent nature of the electron waves. Imaging flow in open systems is made possible by a cooled scanning probe microscope. Remarkable agreement between experiment and theory demonstrates our ability to see these trajectories and to use this system as an interferometer. We image branched electron flow as well as the interference of electron waves. This technique can visualize the motion of electron waves between two points in an open system, providing a straightforward way to study systems that may be useful for quantum information processing and spintronics.05/2007; -
Article: The silicon that moves and feels small living things
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ABSTRACT: The silicon microelectronic chips that make today's computers possible are emerging as powerful tools for rapid and sensitive analysis of small biological objects, including cells, proteins, DNA, and viruses. Major new and exciting developments in the interface of solid-state circuits and biological entities are discussed in this article.IEEE Solid-State Circuits Newsletter 02/2007; -
Article: Smoothing a current-carrying atomic mirror
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ABSTRACT: We have measured the magnitude and roughness of the reflecting potential produced by a current-carrying magnetic mirror from which cold rubidium atoms have been bounced at normal incidence. By varying the current in the mirror, a study of the potential barrier from which the atoms in different magnetic substates bounce has been made. A combination of time-of-flight and imaging techniques allowed us to study the roughness of the reflecting potential. We have demonstrated that the observed roughness can be reduced by additional wires to compensate for the finite-size effects, and by careful control of the direction of the magnetic bias field.EPL (Europhysics Letters) 01/2007; 47(5):538. · 2.17 Impact Factor -
Conference Proceeding: CMOS Meets Bio
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ABSTRACT: There are burgeoning efforts to use CMOS ICs for biotechnology. This paper reviews one such effort, development of a CMOS/Microfluidic hybrid system for magnetic manipulation of biological cells originally reported by the authors in H. Lee et al. (2005, 2006). Programmable magnetic field patterns produced by a CMOS microcoil array IC efficiently manipulate individual cells (tagged by magnetic beads) inside a microfluidic system fabricated on top of the IC.Solid-State Circuits Conference, 2006. ASSCC 2006. IEEE Asian; 12/2006 -
Article: Ge/Si nanowire mesoscopic Josephson junctions.
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ABSTRACT: The controlled growth of nanowires (NWs) with dimensions comparable to the Fermi wavelengths of the charge carriers allows fundamental investigations of quantum confinement phenomena. Here, we present studies of proximity-induced superconductivity in undoped Ge/Si core/shell NW heterostructures contacted by superconducting leads. By using a top gate electrode to modulate the carrier density in the NW, the critical supercurrent can be tuned from zero to greater than 100 nA. Furthermore, discrete sub-bands form in the NW due to confinement in the radial direction, which results in stepwise increases in the critical current as a function of gate voltage. Transport measurements on these superconductor-NW-superconductor devices reveal high-order (n = 25) resonant multiple Andreev reflections, indicating that the NW channel is smooth and the charge transport is highly coherent. The ability to create and control coherent superconducting ordered states in semiconductor-superconductor hybrid nanostructures allows for new opportunities in the study of fundamental low-dimensional superconductivity.Nature Nanotechnology 12/2006; 1(3):208-13. · 27.27 Impact Factor -
Article: Dielectrophoresis tweezers for single cell manipulation.
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ABSTRACT: Positioning single cells is of utmost importance in areas of biomedical research as diverse as in vitro fertilization, cell-cell interaction, cell adhesion, embryology, microbiology, stem cell research, and single cell transfection. Here we describe dielectrophoretic tweezers, a sharp glass tip with electrodes on either side, capable of trapping single cells with electric fields. Mounted on a micromanipulator, dielectrophoresis tweezers can position a single cell in three dimensions, holding the cell against fluid flow of hundreds of microns per second with more than 10 pN of force. We model the electric field produced by the tweezers and the field produced by coaxial microelectrodes. We show that cells are trapped without harm while they divide in the trap. In addition, dielectrophoretic tweezers offer the possibility for trapping, electroporating, and microinjecting a single cell with one probe.Biomedical Microdevices 10/2006; 8(3):227-30. · 3.03 Impact Factor -
Article: IC/microfluidic hybrid system for magnetic manipulation of biological cells
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ABSTRACT: This paper introduces an integrated circuit (IC)/microfluidic hybrid system for magnetic manipulation of biological cells. The hybrid system consists of an IC and a microfluidic system fabricated on top. Biological cells attached to magnetic beads are suspended inside the microfluidic system that maintains biocompatibility. The IC contains a microcoil array circuit that produces spatially-patterned microscopic magnetic fields. Programmable, rapid reconfiguration of the field pattern made possible by the IC allows an efficient simultaneous manipulation of multiple individual bead-bound cells with precise position control. Two prototypes, SiGe/microfluidic and CMOS/microfluidic hybrid systems, validate the proposed approach.IEEE Journal of Solid-State Circuits 07/2006; · 3.23 Impact Factor -
Article: Multipeak Kondo effect in one- and two-electron quantum dots.
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ABSTRACT: 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. 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 Kondo resonances.Physical Review Letters 05/2006; 96(15):156802. · 7.37 Impact Factor -
Article: Imaging electrons in a magnetic field
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ABSTRACT: We present simulations of an imaging mechanism that reveals the trajectories of electrons in a two-dimensional electron gas (2DEG), as well as simulations of the electron flow in zero and small magnetic fields. The end goal of this work is to implement the proposed mechanism to image the flow of electrons inside a ballistic electron device from one specific point (A) to another (B) in a 2DEG, using a low temperature scanning probe microscope with a charged tip. The tip changes the electron density in the 2DEG beneath it and deflects the electrons traveling nearby, thereby changing the conductance from point A to point B. The simulations presented here show that by measuring the transmission of electrons from A to B versus tip position, one can image the electron flow. This forward scattering mechanism is well suited for imaging in a magnetic field, in contrast to previous probes that depended on backscattering. One could use this technique to image cyclotron orbits in an electron focusing geometry, in which electrons travel from point A to point B in semi-circular paths bouncing along a wall. Imaging the motion of electrons in magnetic fields is useful for the development of devices for spintronics and quantum information processing.03/2006; -
Conference Proceeding: IC/microfluidic hybrid system for biology: review
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ABSTRACT: This paper reviews our recent development of an integrated circuit (IC)/microfluidic hybrid system. The hybrid system consists of an IC chip and a microfluidic channel fabricated on top. Biological cells attached to magnetic beads are suspended inside the microfluidic system that maintains biocompatibility. A microcoil array in the IC produces programmable, spatially-patterned magnetic fields to simultaneously manipulate multiple individual bead-bound cells with precise position control. Two prototypes validate the proposed approach.Bipolar/BiCMOS Circuits and Technology Meeting, 2005. Proceedings of the; 11/2005
Top co-authors
Top Journals
Institutions
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1989–2012
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Harvard University
- • Department of Physics
- • School of Engineering and Applied Sciences
- • Area of Applied Physics
Cambridge, MA, USA
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2004
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University of California, Santa Barbara
Santa Barbara, CA, USA
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