Session Y37 - Quantum Dots: Transport.
ORAL session, Friday morning, March 26
520D, Palais des Congres

[Y37.001] Tunnelling spectroscopy of a quantum dot out of equilibrium

We discuss the problem of tunnelling into a quantum dot out of equilibrium. The tunnel current is calculated through a probe weakly coupled to the dot, while a current is passed between two open contacts to drive the dot out of equilibrium. We treat Coulomb interactions within the charging approximation. Carefully separating the inelastic and elastic contributions, we find that the inelastic part of the tunnel current encodes information about the distribution function of the dot.

[Y37.002] Measuring Electron Tunneling Times as a Means of Single Shot/Single Electron Spin Readout

In this talk, we present our strategy for measuring the spin of a single electron trapped in an gate-defined quantum dot in a single shot measurement. The electron is allowed to tunnel out of the electron into a spin dependent final state, or conversely, an external electron with a definite spin is allowed to tunnel onto the dot. The measurement of spin is thereby turned into a measurement of time.

A necessary prerequisite is the ability to reliably and accurately measure the dwell time for single electrons on the quantum dot with microsecond accuracy. We report on our experimental progress in this area, where rather than using a relatively hard to fabricate SET for charge detection, we employ a quantum point contact FET that is cofabricated with the quantum dot.

[Y37.003] Inelastic Cotunneling Measurements of Zeeman Splitting in Single-Electron Transistors

We report measurements of the Zeeman splitting \Delta for orbitals in quantum dots of GaAs/AlGaAs single-electron transistors. \Delta is measured with the magnetic field B parallel to the GaAs/AlGaAs interface using two methods. First, the energy difference between spin up and spin down is observed as a splitting of the peak in differential conductance corresponding to the addition of an electron. This method is sensitive to random charge fluctuations and other sources of chemical potential variation. In the second method, when the dot contains an odd number of electrons, sharp thresholds for inelastic cotunneling are observed in the differential conductance at drain-source voltages V_ds=\pm\Delta/e. Inelastic cotunneling is the more precise method, since it measures the excitation energy, rather than the addition energy, and is therefore insensitive to the chemical potential. Furthermore, the threshold has a smaller intrinsic line width, providing a lower bound for the decoherence time of the excited spin state. We find that \Delta is highly linear in B, but g is nearly three times smaller than the bulk GaAs value of –0.44. This may result from the penetration of the electron wave function into the AlGaAs, for which g=+0.4.

[Y37.004] Signatures of exchange interaction in chaotic quantum dots

We derive the temperature dependence of the conductance peak-height and peak-spacing statistics for a chaotic quantum dot in the presence of an exchange interaction. Using a realistic value of the exchange interaction strength, we find significantly better agreement with the experimental data as compared with the statistics in the absence of exchange [1]. We also study the effects of temperature on the parametric correlator of the conductance peak height versus an orbital magnetic field. Our calculations are based on a novel formula for the conductance in the presence of a constant exchange interaction. In this formula the sum over all occupation numbers is carried out in a closed form using spin and particle-number projection methods. This work has been supported in part by the Department of Energy grant No. DE-FG-0291-ER-40608.

[1] Y. Alhassid and T. Rupp, Phys. Rev. Lett. 91, 056801 (2003).

[Y37.005] Spin of a quantum dot and Coulomb blockade peak motion in a parallel magnetic field

We study the finite temperature Coulomb blockade peak motion in a quantum dot in the presence of a parallel magnetic field. Parallel field dependence of Coulomb blockade peak position has been suggested as an experimental technique to determine the ground-state spin of a quantum dot [1]. We find that in the presence of an exchange interaction, the peak motion is significantly affected by finite temperature effects for k_BT \agt 0.1\Delta. For some of the peaks, this might lead to a misidentification of the ground-state spin at zero magnetic field. We propose an improved method to determine the ground-state spin from parallel field measurements. In this method both measured peak position and peak height are compared with analytic expressions that take into account temperature effects at finite exchange interaction. This work is supported in part by the U.S. Department of Energy grant No. DE-FG-0291-ER-40608.\par

[1] J.A. Folk \emphet al., Ground state spin and Coulomb blockade peak motion in chaotic quantum dots, Physica Scripta T90 26 (2001).

[Y37.006] Three Quantum Dots in a Ring

Few-electron quantum dots can serve as qubits for quantum information processing systems. We have fabricated three tunnel-coupled quantum dots in a ring in a GaAs/AlGaAs heterostructure containing a two-dimensional electron gas using lithographically patterned gates and trenches. Previously, we have characterized few-electron tunnel-coupled double quantum dots and observed the expected Coulomb blockade peak splitting [1]. Triple dots tunnel-coupled in a ring will allow us to study the electronic states of this new type of artificial molecule.

This work was supported at Harvard by DARPA DAAD19-01-1-0659 and at UCSB by iQuest.

1. I.H. Chan, P. Fallahi, A. Vidan, R.M. Westervelt, M. Hanson, A.C. Gossard, “Few-electron quantum dots for quantum computing”, cond-mat/0309205

[Y37.007] Engineering the electromagnetic environment in a semiconductor nanostructure for study of single electron tunneling.

Direct observation of single electron tunneling (SET) through a potential barrier requires embedding the barrier in a high-impedance electromagnetic environment. To this end, we have fabricated staggered arrays of quantum point contacts (QPCs) across a narrow conducting channel in a GaAs/AlGaAs heterostructure containing a two-dimensional electron gas. The conductance G versus gate voltage of such an array of 10 QPCs shows plateau-like structures at fractions of the conductance quantum, G_0=2e^2/h. The last plateau occurs at G\approx0.15G_0, beyond which G drops sharply to zero, suggesting this plateau corresponds to one open channel in each QPC. Placing a tunnel barrier between a pair of such arrays embeds it in a high-impedance environment at the frequencies relevant to SET. We will present further measurements on the nanostructure, made using an integrated radio frequency single electron transistor placed close to the tunnel barrier.

[Y37.008] Asymmetric Quantum Shot Noise in Quantum Dots

We analyze the frequency-dependent noise of a current through a quantum dot which is coupled to Fermi leads and which is in the Coulomb blockade regime. We show that the asymmetric shot noise as function of frequency shows steps and becomes super-Poissonian. This provides experimental access to the quantum fluctuations of the current. We present an exact calculation for a single dot level and a perturbative evaluation of the noise in Born approximation (sequential tunneling regime but without Markov approximation) for the general case of many levels with charging interaction.

[Y37.009] Latched Detection of Excited States in an Isolated Double Quantum Dot

We present measurements of the excited state spectrum of a GaAs double quantum dot, in the complete absence of transport, using pulsed gating in conjunction with local capacitive charge sensing. Charge sensing has been used previously to probe the ground states of mesoscopic systems too isolated for transport measurements. This method uses a nearby constriction, tuned so that its conductance is acutely sensitive to the local electrostatic potential, to measure changes in the number of nearby charges. In other work, pulsed gates have been used with transport measurements to investigate excited state dynamics in quantum dots. Here we present a combination of these methods. Two pulsed gates permit fast and independent control of the tunnel barrier and the energy difference between the dots, while the average charge configuration is measured via local charge sensing. Alignment of a filled state in one dot with an empty state in the other increases the probability that a charge will tunnel and alter the sensor conductance. Supported in part by DARPA SpinS, DARPA QuIST, and the NSF through Harvard NSEC.

[Y37.010] Quantum dot spectroscopy probed by scanning tunneling microscope

We fabricated InAs quantum dots of tens nanometers in diameter using AlSb/InAs/AlSb double-barrier resonant tunneling structure and probed their current-voltage characteristics with a 4K cryogenic scanning tunneling microscope. The dc current-voltage characteristics within low bias range less than 200mV show Coulomb gap and staircase due to Coulomb blockade. In contrast, at higher, fixed biases, the current displays two-level random telegraph signals (RTS), resulting from single electron charging effect. At yet higher biases, the pattern of current fluctuation switches to multi-level RTS. The power spectral density measurements at different biases further confirm the time domain observations, and suggest that the spectra evolve from Lorentzian distribution at low biases to 1/f -like at high biases. The bias-dependence of RTS indicates a trapping and detrapping mechanism, involving defects in the AlSb barriers.

[Y37.011] Transport characteristics of InAs self-assembled dot ensembles in an AlGaAs tunneling barrier of gated sub-micron vertical mesas that conduct near zero bias

Materials with an InAs self-assembled quantum dot ensemble located in a plane at the center of a single (AlGa)As tunnel barrier, and designed to be conducting at zero bias, were grown [1]. With these materials, we made small sub-micron sized multiple gated vertical mesas [2]. Measurements reveal Coulomb oscillation-like and Coulomb diamond-like features near zero bias from which we deduce, particularly from the shape and number of the diamond-like features, that the transport characteristics are dominated by electron tunneling through energy levels of just a "few" dots. These "active" dots are located at a different distance from the mesa side-wall, and so the dot energy levels up-shift at different rates when the gate adjustable depletion region front sweeps past each dot as it spreads towards the center of the mesa (gate voltage made more negative). We also discuss the magnetic field dependence of the tunneling current near zero bias (field applied parallel to the current). [1] R J A Hill et al J. Appl. Phys. 91, 3474 (2002). [2] D G Austing et al Appl. Phys. Lett. 75, 671 (1999).

[Y37.012] Metal-nanoparticle single-electron transistors fabricated using electromigration

We describe the fabrication of single-electron transistors from individual metal nanoparticles using a geometry that provides improved coupling between the particle and the gate electrode. This is accomplished by incorporating a metal particle into a nm-sized gap created between two electrodes using electromigration, all on top of an oxidized aluminum gate. We achieve sufficient gate coupling to access more than ten charge states of individual gold nanoparticles (5-15 nm in diameter). This indicates a particle-gate capacitance approximately ten times larger than achieved previously by our group. The fabrication process provides a yield of 15-20 percent of devices that exhibit single-electron-transistor behavior at 4.2 K with electrons flowing through a single particle only. In a few samples, we also observed current flow through a small network of particles. The devices are sufficiently stable to permit spectroscopic studies of the electron-in-a-box level spectrum within the nanoparticle as its charge state is varied.

[Y37.013] Tunneling spectroscopy of electron-in-a-box states in metal nanoparticles as a function of the number of electrons

If the island of a metallic single-electron transistor is sufficiently small, the discrete spectrum of "electron in a box" states in the island can be resolved by tunneling spectroscopy at low temperature. We report studies of such spectra from devices made by a new fabrication technique that allows the number of electrons in a metal nanoparticle to be tuned by more than 10 while measuring its discrete energy levels. Except for the effects of level filling, we find that the excited-state spectra in gold nanoparticles changes very little as electrons are added, in contrast to previous studies in GaAs quantum dots. We conclude that exchange interactions are sufficiently weak in gold that the (spin-degenerate) energy levels are filled as in a non-interacting model. In several gold samples, we also find larger g-factors for Zeeman splitting than were measured in a previous device geometry.

[Y37.014] Single-Electron Transistor Spectroscopy of InGaAs Self-Assembled Quantum Dots

An InGaAs self-assembled quantum dot exhibits large Coulomb interactions and quantum confinement within a GaAs matrix. As a consequence, InGaAs quantum dots exhibit the potential for novel devices, such as single-photon sources, if one can control the charge occupancy. We have fabricated single-electron transistors over InGaAs self-assembled quantum dots in order to measure the charge occupation of individual quantum dots. Electrons are tunneled into the quantum dot from an underlying n-doped GaAs layer. By monitoring the response of the single-electron transistor, we have been able to observe single-electron tunneling onto the quantum dots and construct an electron addition spectrum of a single quantum dot. With this technique we can also estimate the position of the quantum dot with respect to the transistor island. We are currently investigating how the electron addition spectra are influenced by the Stark effect.

[Y37.015] Magneto-Bloch Oscillations, Anomalous Hall Velocity and Nonlinear Magnetotransport in Polar Semiconductor Superlattices

We investigate transient coherent magneto-Bloch oscillations (MBO's) of electronic wave packet photoexcited in a biased polar semiconductor superlattice (SSL) in a crossed or tilted magnetic field, as well as temporal transition to nonlinear magnetotransport in the superlattice. Specific forms of the complex avoided crosssing and complex frequency gap between the full-miniband BO's and longitudinal optical phonons in a wide-miniband SSL and between the MBO's and bottom-miniband cyclotron oscillations in SSL in the tilted electric and magnetic fields are described. It is shown that the damping of the full-miniband BO's is increased while the damping of the longitudinal optical phonons or cyclotron oscillations in parallel component of magnetic field is decreased at the complex anticrossing. It is shown that the full-miniband MBO's in a biased SSL in a crossed magnetic field gives rise to anomalous Hall velocity which is inverse to the electric field, depends in general on the excitation conditions and determines the shift of the MBO's frequency with respect to the frequency of Bloch oscillations without magnetic field. The anomalous Hall velocity determines also the quadratic in magnetic field blue shift of the Wannier-Stark levels observed in photocurrent spectroscopy measurements. The connection between the value of anomalous DC Hall velocity and field position of the separatrix region which separates the full-miniband MBO's and bottom-miniband cyclotron oscillations in SSL is explained. The separatrix was recently clearly observed with femtosecond contactless optoelectronic measurements of the transient AC Hall currents in SSL in crossed electric and magnetic fields. Close to the separatrix, the frequency of MBO's of electronic wave packet decreases, the oscillations are suppressed and become strongly anharmonic. The electric-field-induced transition from the ordinary to anomalous Hall drift velocity determines the value of a peak stationary carrier velocity v^(p) and corresponding fields. The general criterion for the value of the electric field at which v^(p) is reached is found.

Part Y of program listing