[Show abstract][Hide abstract] ABSTRACT: A three-terminal device based upon a two-dimensional electron system is investigated in the regime of nonequilibrium transport. Excited electrons scatter with the cold Fermi sea and transfer energy and momentum to other electrons. A geometry analogous to a water jet pump is used to create a jet pump for electrons. Because of its phenomenological similarity we name the observed behavior the “electronic Venturi effect.”
[Show abstract][Hide abstract] ABSTRACT: The energy relaxation channels of hot electrons far from thermal equilibrium in a degenerate two-dimensional electron system are investigated in transport experiments in a mesoscopic three-terminal device. We observe a transition from two dimensions at zero magnetic field to quasi-one-dimensional scattering of the hot electrons in a strong magnetic field. In the two-dimensional case, electron-electron scattering is the dominant relaxation mechanism, while the emission of optical phonons becomes more and more important as the magnetic field is increased. The observation of up to 11 optical phonons emitted per hot electron allows us to determine the onset energy of
longitudinal-optical phonons in GaAs at cryogenic temperatures with a high precision Eph=36.0±0.1 meV. Numerical calculations of electron-electron scattering and the emission of optical phonons underline our interpretation in terms of a transition to one-dimensional dynamics.
[Show abstract][Hide abstract] ABSTRACT: We explore the acoustic phonon-based interaction between two neighboring coplanar circuits containing semiconductor quantum point contacts in a perpendicular magnetic field B. In a drag-type experiment, a current flowing in one of the circuits (unbiased) is measured in response to an external current in the other. In moderate B the sign of the induced current is determined solely by the polarity of B. This indicates that the spatial regions where the phonon emission/reabsorption is efficient are controlled by magnetic field. The results are interpreted in terms of non-equilibrium transport via skipping orbits in two-dimensional electron system.
[Show abstract][Hide abstract] ABSTRACT: Time resolved electronic dynamics in coupled quantum dots is directly obtained by a pulsed-gate technique. While individual gate voltages are modulated with periodic pulse trains average charge occupations are measured. A nearby quantum point contact serves as detector. A key component of our setup is a radio frequency sample holder based on impedance matched micro strip lines virtually free of cross-talk. It allows to observe displacements of single electrons on time scales well below a nanosecond. Microscopically resolved tunneling rates as well as relaxation times are obtained by using an adapted rate equation model.
[Show abstract][Hide abstract] ABSTRACT: Quantum point contacts (QPCs) are commonly employed to detect capacitively the charge state of coupled quantum dots (QDs). An indirect backaction of a biased QPC onto a double QD laterally defined in a GaAs/AlGaAs heterostructure is observed. Energy is emitted by nonequilibrium charge carriers in the leads of the biased QPC. Part of this energy is absorbed by the double QD where it causes charge fluctuations that can be observed under certain conditions in its stability diagram. By investigating the spectrum of the absorbed energy, we find that both acoustic phonons and Coulomb interaction can be involved in the backaction, depending on the geometry and coupling constants.
[Show abstract][Hide abstract] ABSTRACT: We investigate an electronic Mach-Zehnder interferometer with high visibility in the quantum Hall regime. The superposition of the electrostatic potentials from a quantum point contact (QPC) and the residual disorder potential from doping impurities frequently results in the formation of inadvertent quantum dots (QDs) in one arm of the interferometer. This gives rise to resonances in the QPC transmission characteristics. While crossing the QD resonance in energy, the interferometer gains a phase shift of π in the interference pattern.
[Show abstract][Hide abstract] ABSTRACT: Scattering of otherwise ballistic electrons far from equilibrium is investigated in a cold two-dimensional electron system. The interaction between excited electrons and the degenerate Fermi liquid induces a positive charge in a nanoscale region which would be negatively charged for diffusive transport at local thermal equilibrium. In a three-terminal device we observe avalanche amplification of electrical current, resulting in a situation comparable to the Venturi effect in hydrodynamics. Numerical calculations using a random phase approximation are in agreement with our data and suggest Coulomb interaction as the dominant scattering mechanism. Comment: 4 pages, 4 figures
[Show abstract][Hide abstract] ABSTRACT: We investigate an electronic Mach-Zehnder interferometer with high visibility in the quantum Hall regime. The superposition of the electrostatic potentials from a quantum point contact (QPC) and the residual disorder potential from doping impurities frequently results in the formation of inadvertent quantum dots (QD) in one arm of the interferometer. This gives rise to resonances in the QPC transmission characteristics. While crossing the QD resonance in energy, the interferometer gains a phase shift of $\pi$ in the interference pattern. Comment: 5 pages, 4 figures
[Show abstract][Hide abstract] ABSTRACT: Interactions between mesoscopic devices induced by interface acoustic phonons
propagating in the plane of a two-dimensional electron system (2DES) are
investigated by phonon-spectroscopy. In our experiments ballistic electrons
injected from a biased quantum point contact emit phonons and a portion of them
are reabsorbed exciting electrons in a nearby degenerate 2DES. We perform
energy spectroscopy on these excited electrons employing a tunable
electrostatic barrier in an electrically separate and unbiased detector
circuit. The transferred energy is found to be bounded by a maximum value
corresponding to Fermi-level electrons excited and back-scattered by absorbing
interface phonons. Our results imply that phonon-mediated interaction plays an
important role for the decoherence of solid-state-based quantum circuits.
[Show abstract][Hide abstract] ABSTRACT: We report on nonequilibrium interaction phenomena between adjacent but electrostatically separated nanostructures in GaAs. A current flowing in one externally biased nanostructure causes an excitation of electrons in a circuit of a second nanostructure. As a result we observe a dc current generated in the unbiased second nanostructure. The results can be qualitatively explained in terms of acoustic phonon based energy transfer between the two mutually isolated circuits.
Physica E Low-dimensional Systems and Nanostructures 06/2008; 40(5-40):995-998. DOI:10.1016/j.physe.2007.08.107 · 2.00 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We briefly overview our recent results on nonequilibrium interactions between neighboring electrically isolated nanostructures. One of the nanostructures is represented by an externally biased quantum point contact (drive-QPC), which is used to supply energy quanta to the second nanostructure (detector). Absorption of these nonequilibrium quanta of energy generates a dc-current in the detector, or changes its differential conductance. We present results for a double quantum dot, a single quantum dot or a second QPC placed in the detector circuit. In all three cases a detection of quanta with energies up to ~1 meV is possible for bias voltages across the drive-QPC in the mV range. The results are qualitatively consistent with an energy transfer mechanism based on nonequilibrium acoustic phonons.
[Show abstract][Hide abstract] ABSTRACT: We study the visibility of Aharonov-Bohm interference in an electronic Mach-Zehnder interferometer (MZI) in the integer quantum Hall regime. The visibility is controlled by the filling factor $\nu$ and is observed only between $\nu \approx 2.0$ and 1.0, with an unexpected maximum near $\nu=1.5$. Three energy scales extracted from the temperature and voltage dependences of the visibility change in a very similar way with the filling factor, indicating that the different aspects of the interference depend sensitively on the local structure of the compressible and incompressible strips forming the quantum Hall edge channels.
[Show abstract][Hide abstract] ABSTRACT: We study an electronic Mach-Zehnder interferometer employing the edge channels of a two-dimensional electron gas (2DEG) in the quantum Hall regime and quantum point contacts (QPC) as beam splitters. The linear conductance of the QPC as a function of gate voltage shows resonances superimposed on quantized conductance steps. It was found that the visibility depends strongly on magnetic field and is highest near a filling factor of 1.5 in the interferometer arm. (c) 2007 Elsevier B.V. All rights reserved.
Physica E Low-dimensional Systems and Nanostructures 03/2008; 40(5-40):1706-1708. DOI:10.1016/j.physe.2007.10.060 · 2.00 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We study the interaction between two adjacent but electrically isolated quantum point contacts (QPCs). At high enough source-drain bias on one QPC, the drive QPC, we detect a finite electric current in the second, unbiased, detector QPC. The current generated at the detector QPC always flows in the opposite direction than the current of the drive QPC. The generated current is maximal, if the detector QPC is tuned to a transition region between its quantized conductance plateaus and the drive QPC is almost pinched-off. We interpret this counterflow phenomenon in terms of an asymmetric phonon-induced excitation of electrons in the leads of the detector QPC.
[Show abstract][Hide abstract] ABSTRACT: We observe an experimental signature of the role of phonons in spin relaxation between triplet and singlet states in a two-electron quantum dot. Using both the external magnetic field and the electrostatic confinement potential, we change the singlet-triplet energy splitting from 1.3 meV to zero and observe that the spin relaxation time depends nonmonotonously on the energy splitting. A simple theoretical model is derived to capture the underlying physical mechanism. The present experiment confirms that spin-flip energy is dissipated in the phonon bath.
[Show abstract][Hide abstract] ABSTRACT: We demonstrate experimentally a read-out method that distinguishes between two-electron spin states in a quantum dot. This scheme combines the advantages of the two existing mechanisms for spin-to-charge conversion with single-shot charge detection: a large difference in energy between the two states and a large difference in tunnel rate between the states and a reservoir. As a result, a spin measurement fidelity of 97% was achieved, which is much higher than previously reported fidelities.
[Show abstract][Hide abstract] ABSTRACT: We propose and implement a nondestructive measurement that distinguishes between two-electron spin states in a quantum dot. In contrast to earlier experiments with quantum dots, the spins are left behind in the state corresponding to the measurement outcome. By measuring the spin states twice within a time shorter than the relaxation time T1, correlations between the outcomes of consecutive measurements are observed. They disappear as the wait time between measurements becomes comparable to T1. The correlation between the postmeasurement state and the measurement outcome is measured to be ∼90% on average.
Physical Review B 11/2006; 74(19). DOI:10.1103/PhysRevB.74.195303 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We study a double quantum dot (DQD) coupled to a strongly biased quantum point contact (QPC), each embedded in independent electric circuits. For weak interdot tunneling we observe a finite current flowing through the Coulomb blockaded DQD in response to a strong bias on the QPC. The direction of the current through the DQD is determined by the relative detuning of the energy levels of the two quantum dots. The results are interpreted in terms of a quantum ratchet phenomenon in a DQD energized by a nearby QPC.
[Show abstract][Hide abstract] ABSTRACT: The activation gap Delta of the fractional quantum Hall state at constant filling nu=1/3 is measured in a wide range of perpendicular magnetic field B . Despite the full spin polarization of the incompressible ground state, we observe a sharp crossover between a low-field linear dependence of Delta on B and a Coulomblike square root behavior at large B . From the global gap reduction we get information about the mobility edges in the fractional quantum Hall regime.
Physical Review B 10/2006; 74(16). DOI:10.1103/PhysRevB.74.165325 · 3.74 Impact Factor