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K Ensslin

Publications and source records attributed to K Ensslin.

At least 19 recordsLinked to original sources
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

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

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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PubMed2004

Spatially resolved manipulation of single electrons in quantum dots using a scanned probe.

The scanning metallic tip of a scanning force microscope was coupled capacitively to electrons confined in a lithographically defined gate-tunable quantum dot at a temperature of 300 mK. Single electrons were made to hop on or off the dot by moving the tip or by changing the tip bias voltage owing to the Coulomb-blockade effect. Spatial images of conductance resonances map the interaction potential between the tip and individual electronic quantum dot states. Under certain conditions this interaction is found to contain a tip-voltage induced and a tip-voltage-independent contribution.

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PubMed2004

Kondo effect in a many-electron quantum ring.

The Kondo effect is investigated in a many-electron quantum ring as a function of the magnetic field. For fields applied perpendicular to the plane of the ring a modulation of the Kondo effect with the Aharonov-Bohm period is observed. This effect is discussed in terms of the energy spectrum of the ring and the parametrically changing tunnel coupling. In addition, we use gate voltages to modify the ground-state spin of the ring. The observed splitting of the Kondo-related zero-bias anomaly in this configuration is tuned with an in-plane magnetic field.

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PubMed2004

Magnetic-field-dependent transmission phase of a double-dot system in a quantum ring.

The Aharonov-Bohm effect is measured in a four-terminal open ring geometry. Two quantum dots are embedded in the structure, one in each of the two interfering paths. The number of electrons in the two dots can be controlled independently. The transmission phase is measured as electrons are added to or taken away from the individual quantum dots. Although the measured phase shifts are in qualitative agreement with theoretical predictions, the phase evolution exhibits unexpected dependence on the magnetic field. Phase lapses are found only in certain ranges of the magnetic field.

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PubMed2003

Singlet-triplet transition tuned by asymmetric gate voltages in a quantum ring.

Wave-function and interaction effects in the addition spectrum of a Coulomb-blockaded many-electron quantum ring are investigated as a function of asymmetrically applied gate voltages and magnetic field. Hartree and exchange contributions to the interaction are quantitatively evaluated at a crossing between states extended around the ring and states which are more localized in one arm of the ring. A gate tunable singlet-triplet transition of the two uppermost levels of this many-electron ring is identified at zero magnetic field.

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PubMed2001

Electrical control of spin coherence in semiconductor nanostructures.

The processing of quantum information based on the electron spin degree of freedom requires fast and coherent manipulation of local spins. One approach is to provide spatially selective tuning of the spin splitting--which depends on the g-factor--by using magnetic fields, but this requires their precise control at reduced length scales. Alternative proposals employ electrical gating and spin engineering in semiconductor heterostructures involving materials with different g-factors. Here we show that spin coherence can be controlled in a specially designed AlxGa1-xAs quantum well in which the Al concentration x is gradually varied across the structure. Application of an electric field leads to a displacement of the electron wavefunction within the quantum well, and because the electron g-factor varies strongly with x, the spin splitting is therefore also changed. Using time-resolved optical techniques, we demonstrate gate-voltage-mediated control of coherent spin precession over a 13-GHz frequency range in a fixed magnetic field of 6 T, including complete suppression of precession, reversal of the sign of g, and operation up to room temperature.

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PubMed2001

Energy spectra of quantum rings.

Quantum mechanical experiments in ring geometries have long fascinated physicists. Open rings connected to leads, for example, allow the observation of the Aharonov-Bohm effect, one of the best examples of quantum mechanical phase coherence. The phase coherence of electrons travelling through a quantum dot embedded in one arm of an open ring has also been demonstrated. The energy spectra of closed rings have only recently been studied by optical spectroscopy. The prediction that they allow persistent current has been explored in various experiments. Here we report magnetotransport experiments on closed rings in the Coulomb blockade regime. Our experiments show that a microscopic understanding of energy levels, so far limited to few-electron quantum dots, can be extended to a many-electron system. A semiclassical interpretation of our results indicates that electron motion in the rings is governed by regular rather than chaotic motion, an unexplored regime in many-electron quantum dots. This opens a way to experiments where even more complex structures can be investigated at a quantum mechanical level.

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PubMed2001

Signatures of spin pairing in chaotic quantum dots.

Coulomb blockade resonances are measured in a GaAs quantum dot in which both shape deformations and interactions are small. The parametric evolution of the Coulomb blockade peaks shows a pronounced pair correlation in both position and amplitude, which is interpreted as spin pairing. As a consequence, the nearest-neighbor distribution of peak spacings can be well approximated by a modified bimodal Wigner surmise, in which interactions are taken into account beyond the constant interaction model.

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PubMed2000

Intersubband electroluminescence from silicon-based quantum cascade structures.

The quantum cascade laser, which uses electronic transitions within a single band of a semiconductor, constitutes a possible way to integrate active optical components into silicon-based technology. This concept necessitates a transition with a narrow linewidth and an upper state with a sufficiently long lifetime. We report the observation of intersubband electroluminescence from a p-type silicon/silicon-germanium quantum cascade structure, centered at 130 millielectron volts with a width of 22 millielectron volts, with the expected polarization, and discernible up to 180 kelvin. The nonradiative lifetime is found to depend strongly on the design of the quantum well structure, and is shown to reach values comparable to that of an equivalent GaInAs/AlInAs laser structure.

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PubMed2000

Analysis of the metallic phase of two-dimensional holes in SiGe in terms of temperature dependent screening

We find that temperature dependent screening can quantitatively explain the metallic behavior of the resistivity on the metallic side of the so-called metal-insulator transition in p-SiGe. Interference and interaction effects exhibit the usual insulating behavior which is expected to overpower the metallic background at sufficiently low temperatures. We find empirically that the concept of a Fermi liquid describes our system with its large interaction parameter r(s) approximately 8.

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PubMed1999

The fermionic hanbury brown and twiss experiment

A Hanbury Brown and Twiss experiment for a beam of electrons has been realized in a two-dimensional electron gas in the quantum Hall regime. A metallic split gate serves as a tunable beam splitter to partition the incident beam into transmitted and reflected partial beams. In the nonequilibrium case the fluctuations in the partial beams are shown to be fully anticorrelated, demonstrating that fermions exclude each other. In equilibrium, the cross-correlation of current fluctuations at two different contacts is also found to be negative and nonzero, provided that a direct transmission exists between the contacts.

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