Hans-Peter Tranitz

Double-Dot Quantum Ratchet Driven by an Independently Biased Quantum Point Contact

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.

Activated transport in the separate layers that form the nuT=1 exciton condensate.

We observe the total filling factor nuT=1 quantum Hall state in a bilayer two-dimensional electron system with virtually no tunneling. We find thermally activated transport in the balanced system with a monotonic increase of the activation energy with decreasing d/lB below 1.65. In the imbalanced system we find activated transport in each of the layers separately, yet the activation energies show a striking asymmetry around the balance point, implying a different excitation spectrum for the separate layers forming the condensed state.

Experimental Signature of Phonon-Mediated Spin Relaxation in a Two-Electron Quantum Dot

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.

Nanometre spaced electrodes on a cleaved AlGaAs surface

We present a novel technique for fabricating nanometre spaced metal electrodes on a smooth crystal cleavage plane with precisely predetermined spacing. Our method does not require any high-resolution nanolithography tools, all lateral patterning being based on conventional optical lithography. Using molecular beam epitaxy we embedded a thin gallium arsenide (GaAs) layer in between two aluminium gallium arsenide (AlGaAs) layers with monolayer precision. By cleaving the substrate an atomically flat surface is obtained exposing the AlGaAs–GaAs sandwich structure. After selectively etching the GaAs layer, the remaining AlGaAs layers are used as a support for deposited thin film metal electrodes. We characterized these coplanar electrodes by atomic force microscopy and scanning electron microscopy; this revealed clean, symmetric and macroscopically flat surfaces with a maximum corrugation of less than 1.2 nm. In the case of a device with a 20 nm thick GaAs layer the measured electrode distance was 22.5 nm with a maximum deviation of less than 2.1 nm. To demonstrate the electrical functionality of our device we positioned single colloidal gold nanoparticles between the electrodes by an alternating voltage trapping method; this resulted in a drop of electrical resistance from ~11 G Ω to ~1.5 k Ω at 4.2 K. The device structure has large potential for the manipulation of nanosized objects like molecules or more complex aggregates on flat surfaces and the investigation of their electrical properties in a freely suspended configuration.

Decoherence and single electron charging in an electronic Mach-Zehnder interferometer

We investigate the temperature and voltage dependence of the quantum interference in an electronic Mach-Zehnder interferometer using edge channels in the integer quantum-Hall regime. The amplitude of the interference fringes is significantly smaller than expected from theory; nevertheless the functional dependence of the visibility on temperature and bias voltage agrees very well with theoretical predictions. Superimposed on the Aharonov-Bohm (AB) oscillations, a conductance oscillation with a six times smaller period is observed. The latter depends only on gate voltage and not on the AB phase, and may be related to single electron charging.

Density dependence of microwave induced magnetoresistance oscillations in a two-dimensional electron gas

We have measured the magnetoresistance of a two-dimensional electron gas (2DEG) under continuous microwave irradiation as a function of electron density and mobility tuned with a metallic top-gate. In the entire range of density and mobility we have investigated, we observe microwave induced oscillations of large amplitude that are B-periodic. These B-periodic oscillations are reminiscent of the ones reported by Kukushkin et al. [Phys. Rev. Lett. 92, 236803 (2004)] and which were attributed to the presence of edge-magneto-plasmons. We have found that the B-periodicity does not increase linearly with the density in our sample but shows a plateau in the range (2.4–3)×1011 cm−2. In this regime, the phase of the B-periodic oscillations is found to shift continuously by two periods.

Carbon-doped symmetric GaAs∕AlGaAs quantum wells with hole mobilities beyond 106cm2∕Vs

Utilizing a carbon filament doping source, we prepared two-dimensional hole gases in a symmetric quantum-well structure in the GaAs∕AlGaAs heterosystem. Low-temperature hole mobilities up to 1.2×106cm2∕Vs at a density of 2.3×1011cm−2 were achieved on GaAs (001) substrates. In contrast to electron systems, the hole mobility sensitively depends on variations of the quantum-well width and the spacer thickness. In particular, an increase of the quantum-well width from an optimal value of 15 nm to 18 nm is accompanied by a 35% reduction of the hole mobility. The quality of ultrahigh-mobility electron systems is not affected by the employed carbon-doping source.Utilizing a carbon filament doping source, we prepared two-dimensional hole gases in a symmetric quantum-well structure in the GaAs∕AlGaAs heterosystem. Low-temperature hole mobilities up to 1.2×106cm2∕Vs at a density of 2.3×1011cm−2 were achieved on GaAs (001) substrates. In contrast to electron systems, the hole mobility sensitively depends on variations of the quantum-well width and the spacer thickness. In particular, an increase of the quantum-well width from an optimal value of 15 nm to 18 nm is accompanied by a 35% reduction of the hole mobility. The quality of ultrahigh-mobility electron systems is not affected by the employed carbon-doping source.

Phonon-mediated versus coulombic backaction in quantum dot circuits.

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.

Nondestructive measurement of electron spins in a quantum dot

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.

Electric-field stabilization in a high-density surface superlattice

By application of the cleaved-edge overgrowth technique, we realize a two-channel superlattice (SL) device. The structure combines the parallel transport through a low-density SL under almost homogeneous electric field conditions with that through a surface SL (SSL) with large carrier density, which is, without parallel transport, subject to pronounced field instabilities. Direct control of the SSL density allows a separation of both transport contributions. With parallel transport through the low-density SL, the current carried by the SSL is characteristic for a SL with homogeneous field distribution. In particular, it exhibits negative differential conductivity over a wide range of applied electric fields. In contrast, for current only through the SSL clear electric-field instabilities, typical for SLs at high densities are observed. Thus, by means of the parallel transport channel, field instabilities are avoided and transport in high-density SLs with a homogeneous field distribution becomes accessible.