Biomedical subjects

A C Gossard

Publications and source records attributed to A C Gossard.

At least 19 recordsLinked to original sources

PubMed2006

Coherence length of cold exciton gases in coupled quantum wells.

A Mach-Zehnder interferometer with spatial and spectral resolution was used to probe spontaneous coherence in cold exciton gases, which are implemented experimentally in the ring of indirect excitons in coupled quantum wells. A strong enhancement of the exciton coherence length is observed at temperatures below a few Kelvin. The increase of the coherence length is correlated with the macroscopic spatial ordering of excitons. The coherence length at the lowest temperature corresponds to a very narrow spread of the exciton momentum distribution, much smaller than that for a classical exciton gas.

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PubMed2006

Generating spin currents in semiconductors with the spin Hall effect.

We investigate electrically induced spin currents generated by the spin Hall effect in GaAs structures that distinguish edge effects from spin transport. Using Kerr rotation microscopy to image the spin polarization, we demonstrate that the observed spin accumulation is due to a transverse bulk electron spin current, which can drive spin polarization nearly 40 microns into a region in which there is minimal electric field. Using a model that incorporates the effects of spin drift, we determine the transverse spin drift velocity from the magnetic field dependence of the spin polarization.

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PubMed2006

Effect of exchange interaction on spin dephasing in a double quantum dot.

We measure singlet-triplet dephasing in a two-electron double quantum dot in the presence of an exchange interaction which can be electrically tuned from much smaller to much larger than the hyperfine energy. Saturation of dephasing and damped oscillations of the spin correlator as a function of time are observed when the two interaction strengths are comparable. Both features of the data are compared with predictions from a quasistatic model of the hyperfine field.

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PubMed2006

Suppression of spin relaxation in submicron InGaAs wires.

We investigate electron-spin dynamics in narrow two-dimensional n-InGaAs channels as a function of the channel width. The spin relaxation times increase with decreasing channel width, in accordance with recent theoretical predictions based on the dimensionally constrained D'yakonov-Perel' mechanism. Surprisingly, the suppression of the relaxation rate, which is anticipated for the one-dimensional limit, is observed for widths that are an order of magnitude larger than the electron mean free path. We find the spin precession length and the channel width to be the relevant length scales for interpreting these results.

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PubMed2006

Trapping of cold excitons in quantum well structures with laser light.

Optical trapping and manipulation of neutral particles has led to a variety of experiments from stretching DNA-molecules to trapping and cooling of neutral atoms. An exciting recent outgrowth of the technique is an experimental implementation of atom Bose-Einstein condensation. In this Letter, we propose and demonstrate laser-induced trapping for a new system--a gas of excitons in quantum well structures. We report on the trapping of a highly degenerate Bose gas of excitons in laser-induced traps.

Biophysics↗

PubMed2006

Resonant-optical-cavity photoconductive switch with 0.5% conversion efficiency and 1.0 W peak power.

We report a new photoconductive switch having an average output power of 44 microW, an instantaneous bandwidth of approximately 300 GHz, an output pulse width of approximately 2.2 ps, a peak output power of approximately 1.0 W, and an optical-to-electrical conversion efficiency of approximately 0.5% when pumped by a palm-sized mode-locked free-space laser at lambda=780.6 nm with an average power of 8.7 mW and an optical pulse width of approximately -230 fs. The switch is made from an ErAs:GaAs epitaxial layer inside a resonant optical cavity and coupled to a planar three-turn square spiral antenna.

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PubMed2006

Asymmetry of nonlinear transport and electron interactions in quantum dots.

The symmetry properties of transport beyond the linear regime in chaotic quantum dots are investigated experimentally. A component of differential conductance that is antisymmetric in both applied source-drain bias V and magnetic field B, absent in linear transport, is found to exhibit mesoscopic fluctuations around a zero average. Typical values of this component allow a measurement of the electron interaction strength.

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PubMed2006

Multipeak Kondo effect in one- and two-electron quantum dots.

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.

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PubMed2006

Excitation spectrum of two correlated electrons in a lateral quantum dot with negligible Zeeman splitting.

The excitation spectrum of a two-electron quantum dot is investigated by tunneling spectroscopy in conjunction with theoretical calculations. The dot made from a material with negligible Zeeman splitting has a moderate spatial anisotropy leading to a splitting of the two lowest triplet states at zero magnetic field. In addition to the well-known triplet excitation at zero magnetic field, two additional excited states are found at finite magnetic field. The lower one is identified as the second excited singlet state on the basis of an avoided crossing with the first excited singlet state at finite fields. The measured spectra are in remarkable agreement with exact-diagonalization calculations. The results prove the significance of electron correlations and suggest the formation of a state with Wigner-molecular properties at low magnetic fields.

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PubMed2006

Magnetic field symmetry and phase rigidity of the nonlinear conductance in a ring.

We have performed nonlinear transport measurements as a function of a perpendicular magnetic field in a semiconductor Aharonov-Bohm ring connected to two leads. While the voltage-symmetric part of the conductance is symmetric in the magnetic field, the voltage-antisymmetric part of the conductance is not symmetric. These symmetry relations are compatible with the scattering theory for nonlinear mesoscopic transport. The observed asymmetry can be tuned continuously by changing the gate voltages near the arms of the ring, showing that the phase of the nonlinear conductance in a two-terminal interferometer is not rigid, in contrast with the case for the linear conductance.

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PubMed2006

Enhancement of spin coherence using Q-factor engineering in semiconductor microdisc lasers.

Semiconductor microcavities offer unique means of controlling light-matter interactions in confined geometries, resulting in a wide range of applications in optical communications and inspiring proposals for quantum information processing and computational schemes. Studies of spin dynamics in microcavities, a new and promising research field, have revealed effects such as polarization beats, stimulated spin scattering and giant Faraday rotation. Here, we study the electron spin dynamics in optically pumped GaAs microdisc lasers with quantum wells and interface-fluctuation quantum dots in the active region. In particular, we examine how the electron spin dynamics are modified by the stimulated emission in the discs, and observe an enhancement of the spin-coherence time when the optical excitation is in resonance with a high-quality (Q approximately 5,000) lasing mode. This resonant enhancement, contrary to expectations from the observed trend in the carrier-recombination time, is then manipulated by altering the cavity design and dimensions. In analogy with devices based on excitonic coherence, this ability to engineer coherent interactions between electron spins and photons may provide new pathways towards spin-dependent quantum optoelectronics.

Biomedical Engineering↗

PubMed2006

Counting statistics of single electron transport in a quantum dot.

We have measured the full counting statistics of current fluctuations in a semiconductor quantum dot (QD) by real-time detection of single electron tunneling with a quantum point contact. This method gives direct access to the distribution function of current fluctuations. Suppression of the second moment (related to the shot noise) and the third moment (related to the asymmetry of the distribution) in a tunable semiconductor QD is demonstrated experimentally. With this method we demonstrate the ability to measure very low current and noise levels.

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PubMed2005

Probing the Kondo density of states in a three-terminal quantum ring.

We have measured the Kondo effect in a quantum ring connected to three terminals. In this configuration nonlinear transport measurements allow us to check which lead contributes to the Kondo density of states (DOS) and which does not. The ring geometry allows a fine-tuning of the coupling to each lead through the Aharonov-Bohm effect via application of a magnetic field. When the ring is connected to two strongly and one weakly coupled leads, conductance through the weakly coupled lead provides a direct measurement of the DOS in the Kondo regime. By applying a bias between the two strongly coupled leads, we demonstrate directly the splitting of the out-of-equilibrium Kondo DOS.

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PubMed2005

Coherent manipulation of coupled electron spins in semiconductor quantum dots.

We demonstrated coherent control of a quantum two-level system based on two-electron spin states in a double quantum dot, allowing state preparation, coherent manipulation, and projective readout. These techniques are based on rapid electrical control of the exchange interaction. Separating and later recombining a singlet spin state provided a measurement of the spin dephasing time, T2*, of approximately 10 nanoseconds, limited by hyperfine interactions with the gallium arsenide host nuclei. Rabi oscillations of two-electron spin states were demonstrated, and spin-echo pulse sequences were used to suppress hyperfine-induced dephasing. Using these quantum control techniques, a coherence time for two-electron spin states exceeding 1 microsecond was observed.

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PubMed2005

Antiferromagnetic s-d exchange coupling in GaMnAs.

Measurements of coherent electron spin dynamics in Ga1-xMnxAs/Al0.4Ga0.6As quantum wells with 0.0006%<x<0.03% show an antiferromagnetic (negative) exchange between s-like conduction band electrons and electrons localized in the d shell of the Mn2+ impurities. The magnitude of the s-d exchange parameter, N0alpha, varies as a function of well width indicative of a large and negative contribution due to kinetic exchange. In the limit of no quantum confinement, N0alpha extrapolates to -0.09+/-0.03 eV indicating that antiferromagnetic s-d exchange is a bulk property of GaMnAs. Measurements of the polarization-resolved photoluminescence show strong discrepancy from a simple model of the exchange enhanced Zeeman splitting, indicative of additional complexity in the exchange split valence band.

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PubMed2005

Triplet-singlet spin relaxation via nuclei in a double quantum dot.

The spin of a confined electron, when oriented originally in some direction, will lose memory of that orientation after some time. Physical mechanisms leading to this relaxation of spin memory typically involve either coupling of the electron spin to its orbital motion or to nuclear spins. Relaxation of confined electron spin has been previously measured only for Zeeman or exchange split spin states, where spin-orbit effects dominate relaxation; spin flips due to nuclei have been observed in optical spectroscopy studies. Using an isolated GaAs double quantum dot defined by electrostatic gates and direct time domain measurements, we investigate in detail spin relaxation for arbitrary splitting of spin states. Here we show that electron spin flips are dominated by nuclear interactions and are slowed by several orders of magnitude when a magnetic field of a few millitesla is applied. These results have significant implications for spin-based information processing.

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PubMed2005

Cotunneling-mediated transport through excited states in the Coulomb-blockade regime.

We present finite-bias transport measurements on a few-electron quantum dot. In the Coulomb-blockade regime, strong signatures of inelastic cotunneling occur which can directly be assigned to excited states observed in the nonblockaded regime. In addition, we observe structures related to sequential tunneling through the dot, occurring after it has been excited by an inelastic cotunneling process. We explain our findings using transport calculations within the real-time Green's function approach, including diagrams up to fourth order in the tunneling matrix elements.

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PubMed2005

Imaging electron interferometer.

An imaging interferometer was created in a two-dimensional electron gas by reflecting electron waves emitted from a quantum point contact with a circular mirror. Images of electron flow obtained with a scanning probe microscope at liquid He temperatures show interference fringes when the mirror is energized. A quantum phase shifter was created by moving the mirror via its gate voltage, and an interferometric spectrometer can be formed by sweeping the tip over many wavelengths. Experiments and theory demonstrate that the interference signal is robust against thermal averaging.

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