Publications (352)1112.53 Total impact
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ABSTRACT: We investigate low temperature transport properties of splitgate devices lithographically patterned on a GaAs/AlGaAs heterostructure containing a 2D electron gas with mobility 2000 m^2/Vs in a perpendicular magnetic field. By using quantum point contacts (QPCs) with different lithographic widths and varying the voltage applied on the gates for each QPC, we can control the width of the conduction channel continuously from ˜3000 to ˜100nm. The width of the channel is estimated from the lowfield magnetic field dependence of the conductance through the QPC. We find that the spinsplitting of the Landau levels is suppressed in the QPCs compared to the bulk, and we measure the filling factor numax above which spin splitting can no longer be observed. Surprisingly, we find that numax is approximately half the number of quantum channels in the QPC for all widths less than 1200 nm. This work was partially supported by ARO (W911NF0510062), by the NSEC program of NSF (PHY0117795), by NSF (DMR0353209) and by Project Q of Microsoft.  [Show abstract] [Hide abstract]
ABSTRACT: We measure the relaxation rate $W \equiv T_{1}^{1}$ of a single electron spin in a quantum dot at magnetic fields from 7 T down to 1.75 T, much lower than previously measured. At 1.75 T we find that $T_{1}$ is 170 ms. We find good agreement between our measurements and theoretical predictions of the relaxation rate caused by the spinorbit interaction, demonstrating that spinorbit coupling can account for spin relaxation in quantum dots. Comment: 4 pages, 3 figures  [Show abstract] [Hide abstract]
ABSTRACT: We demonstrate low leakage surface gating of an indium arsenide heterostructure with the twodimensional electron gas close to the surface. Gating is made possible by growing an aluminum oxide layer on top of the device. We find that the depletion point can be changed by applying a positive gate voltage and we see hysteresis when the voltage is swept below depletion.  [Show abstract] [Hide abstract]
ABSTRACT: Singleelectron transistors (SETs) are attractive candidates for spin qubits. An AlGaAs/GaAs SET consists of a confined twodimensional droplet of electrons, called an artificial atom or quantum dot, coupled by tunnel barriers to two conducting leads. Controlling the voltages on the lithographic gates that define the quantum dot allows us to confine a single electron in the dot, as well as to adjust the tunnel barriers to the leads. By applying a magnetic field, we can split the spinup and spindown states of the electron by an energy gµ B B; the goal is to utilize coherent superpositions of these spin states to construct a qubit. We will discuss our attempts to observe electron spin resonance (ESR) in this system by applying magnetic fields at microwave frequencies. Observation of ESR would demonstrate that we can manipulate a single spin and allow us to measure the decoherence time T 2 *.  [Show abstract] [Hide abstract]
ABSTRACT: We measure the conductance of closepacked films of CdTe nanocrystals in fieldeffect structures in the dark and in the presence of light. We find that the majority carriers are holes, that they are injected from gold electrodes into the CdTe nanocrystal films, and that the hole density can be modulated with gate voltage. Secondary photocurrents have a photoconductive gain of ∼10 at 106 V∕cm showing that the hole mobility is higher than the electron mobility. A single phenomenological description of the field dependence of the hole mobility can explain the dependence of current on sourcedrain voltage for both dark and light currents.  [Show abstract] [Hide abstract]
ABSTRACT: Utilizing a currentbiased quantumpointcontact charge sensor, we observe electrons hopping on and off a AlGaAs/GaAs singleelectron transistor (SET) in real time. An electron tunnels between the extended states in the leads and the lowestenergy state localized in the lateral quantum dot created by nanometersize surface electrodes. We observe changes in the tunneling rates, caused by the spin splitting in a magnetic field B applied parallel to the 2DEG. We have also observed singleelectron photoionization of the SET by application of microwave radiation. This work is supported by the ARO (W911NF0510062), the NSF (DMR0353209) and in part by the NSEC Program of the NSF (PHY0117795).  [Show abstract] [Hide abstract]
ABSTRACT: Measurements of charge fluctuations in an AlGaAs/GaAs single electron transistor (SET) are presented. The SET consists of a lateral quantum dot created by confining a twodimensional electron gas using nanometersize surface electrodes. The charge on the quantum dot is detected by changes in conductance of a nearby quantum point contact. We discuss noise and bandwidth characteristics of our charge detection method, which uses commercially available voltage amplifiers. Our real time charge detection capabilities are used to investigate charge dynamics on the SET in a magnetic field parallel to the twodimensional electron gas. This work is supported by the U.S. Army Research Office under Contract No. W911NF0510062, by the National Science Foundation under Grant No. DMR0353209, and in part by the NSEC Program of the National Science Foundation under Award No. PHY0117795.  [Show abstract] [Hide abstract]
ABSTRACT: 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 singlettriplet energy splitting J over large ranges of gate voltages. We present the temperature, magnetic field and tunnelcoupling dependence of these transport features, which are in good agreement with recent theory. Further, we observe signatures of spinblockade that becomes visible for sourcedrain voltages exceeding the triplet energy. This work was partially supported by the ARO (W911NF0510062), by the NSEC program of the NSF (PHY0117795) and by NSF (DMR0353209). 
Article: Prospects for Quantum Dot Implementation of Adiabatic Quantum Computers for Intractable Problems
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ABSTRACT: Schemes are examined for implementing quantum computers to solve problems that cannot be solved in polynomial time on classical computers. Special attention is given to the newly proposed adiabatic quantum computation algorithms. We explore the ways in which the latter might be easier to implement than the more conventional approach.  [Show abstract] [Hide abstract]
ABSTRACT: We report the fabrication of multiisland singleelectron devices made by lithographic contacting of selfassembled alkanethiolcoated gold nanocrystals. The advantages of this method, which bridges the dimensional gap between lithographic and NC sizes, are (1) that all tunnel junctions are defined by selfassembly rather than lithography and (2) that the ratio of gate capacitance to total capacitance is high. The rich electronic behavior of a doubleisland device, measured at 4.2 K, is predicted in detail by combining finite element and Monte Carlo simulations with the standard theory of Coulomb blockade with very few adjustable parameters.  [Show abstract] [Hide abstract]
ABSTRACT: An artificial atom with four electrons is driven through a singlettriplet transition by varying the confining potential. In the triplet, a Kondo peak with a narrow dip at drainsource voltage V_ds=0 is observed. The low energy scale V_ds* characterizing the dip is consistent with predictions for the twostage Kondo effect. The phenomenon is studied as a function of temperature T and magnetic field B, parallel to the twodimensional electron gas. The low energy scales T* and B* are extracted from the behavior of the zerobias conductance and are compared to the low energy scale V_ds* obtained from the differential conductance. Good agreement is found between kT* and gmuB*, but eV_ds* is larger, perhaps because of nonequilibrium effects. Comment: 7 pages, 7 figures. Added labels on Fig. 3f and one reference  [Show abstract] [Hide abstract]
ABSTRACT: We present a photocurrent study of CdSe quantum dot films exhibiting unity internal quantum efficiency as a result of postdeposition treatments. While the photocurrent of untreated films is highly voltage dependent at all voltages, the treated films depend strongly on voltage at low voltage, linearly with voltage above a voltage threshold, and finally saturate at high voltage. The voltage dependence of the treated films can be reproduced with a model assuming blocking contacts and a field dependent exciton ionization efficiency that saturates to unity. The increase in exciton ionization efficiency is a result of increased surface passivation and decreased QD spacing.  [Show abstract] [Hide abstract]
ABSTRACT: We present a detailed study of the Kondo peak splitting as a function of Kondo temperature TK and magnetic field B parallel to the 2DEG in an AlGaAs/GaAs singleelectron transistor. We observe that, at fixed B, the Kondo splitting decreases logarithmically with Kondo temperature, in agreement with theory. However, we find that the magnitude of the prefactor of the logarithm is much larger than predicted. We also find that there exists a critical magnetic field Bc below which the Kondo peak does not split, in qualitative agreement with theory. However, our results indicate that Bc is smaller than predicted. These measurements show that the theory of nonequilibrium Kondo physics is still incomplete.  [Show abstract] [Hide abstract]
ABSTRACT: We address anomalous transport phenomena in arrays of semiconductor nanocrystals (quantum dots): Transient powerlaw decay of current as a response to a step in large bias voltage applied across the array, as well as memory effects observed after successive applications of the bias voltage. A novel phenomenological model of transport in such systems is proposed, capable of rationalizing both anomalous transport and memory. The model describes electron transport by a stationary Levy process of transmission events and therefore requires no time dependence of system properties. The long tail in the waiting time distribution gives rise to a nonstationary response in the presence of a voltage pulse. Noise measurements agree well with the predicted nonPoissonian fluctuations in current. We briefly discuss possible microscopic mechanisms that could cause the anomalous statistics in transmission.  [Show abstract] [Hide abstract]
ABSTRACT: We report the observation of a magneticfieldinduced transition between magnetically disordered and ordered phases in slightly underdoped La(2x)SrxCuO4 with x=0.144. Static incommensurate spindensitywave order is induced above a critical field of about 3 T, as measured by elastic neutron scattering. Our results allow us to constrain the location of a quantum critical point on the phase diagram.  [Show abstract] [Hide abstract]
ABSTRACT: A Kondo peak in the differential conductance of a singleelectron transistor is measured as a function of both magnetic field and the Kondo temperature. We observe that the Kondo splitting decreases logarithmically with Kondo temperature and that there exists a critical magnetic field Bc below which the Kondo peak does not split, in qualitative agreement with theory. However, we find that the magnitude of the prefactor of the logarithm is larger than predicted and is independent of B, in contradiction with theory. Our measurements also suggest that the value of Bc is smaller than predicted. Comment: 6 pages, 5 figures. Replaced with version submitted to PRB that includes corrected theoretical predictions, additonal analysis, and other suggested modifications  [Show abstract] [Hide abstract]
ABSTRACT: We measure the spin splitting in a magnetic field B of localized states in singleelectron transistors using a new method, inelastic spinflip cotunneling. Because it involves only internal excitations, this technique gives the most precise value of the Zeeman energy Delta=/g/mu(B)B. In the same devices we also measure the splitting with B of the Kondo peak in differential conductance. The Kondo splitting appears only above a threshold field as predicted by theory. However, the magnitude of the Kondo splitting at high fields exceeds 2/g/mu(B)B in disagreement with theory.  [Show abstract] [Hide abstract]
ABSTRACT: We present measurements of photoconductivity in CdSe quantum dot films treated with a variety of reagents. While the photocurrent of untreated samples is highly voltage dependent at all voltages, after treatment the photocurrent is much larger, depends strongly on voltage at low voltage, displays a linear region above a voltage threshold, and finally saturates at high voltage. All regions of the currentvoltage curves after treatment can be reproduced with a model that requires noninjecting contacts and a field dependent exciton ionization efficiency that saturates to unity. This model is shown to be consistent with the trends observed with different treatments. The changes in photocurrent with treatment are shown to be largely a consequence of increased quantum dot surface passivation and decreased quantum dot spacing, regardless of whether the molecules used for treatment are conjugated or able to crosslink the quantum dots.  [Show abstract] [Hide abstract]
ABSTRACT: We measure the differential conductance of a singleelectron transistor (SET) irradiated with microwaves. The spinentangled manyelectron Kondo state produces a zerobias peak in the dc differential conductance if the quantum dot in the SET contains an unpaired electron. When the photon energy hf is comparable to the energy width of the Kondo peak and to e (the charge on the electron) times the microwave voltage across the dot, satellites appear in the differential conductance shifted in voltage by +/hf/e from the zerobias resonance. We also observe an overall suppression of the Kondo features with increasing microwave voltage. 
Article: Magneticfield induced transition to static longrange magnetic order in underdoped La_2xSr_xCuO_4
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ABSTRACT: Hightemperature cuprate superconductors often have dynamic and sometimes static magnetic order, which coexist with superconductivity. Underdoped La_2xSr_xCuO4 has static magnetic order when x < 0.14. At higher doping, the static magnetic order is destroyed and a spingap opens when it is optimally or slightly overdoped (0.15
Publication Stats
20k  Citations  
1,112.53  Total Impact Points  
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Institutions

19742015

Massachusetts Institute of Technology
 • Department of Physics
 • Department of Materials Science and Engineering
 • Research Laboratory of Electronics
Cambridge, Massachusetts, United States


2002

Columbia University
 Department of Physics
New York, New York, United States


2001

Tel Aviv University
 Department of Physics and Astronomy
Tell Afif, Tel Aviv, Israel 
Rutgers, The State University of New Jersey
НьюБрансуик, New Jersey, United States


1988

Nippon Telegraph and Telephone
Edo, Tōkyō, Japan 
Boston University
 Center for Polymer Studies
Boston, Massachusetts, United States


1970

University of Illinois at Chicago
 Department of Physics
Chicago, Illinois, United States
