arXiv subjects

G. Abstreiter

Publications and source records attributed to G. Abstreiter.

50 records · Page 3Linked to original sources

arXiv2004

Direct Observation of Controlled Coupling in an Individual Quantum Dot Molecule

We report the direct observation of quantum coupling in individual quantum dot molecules and its manipulation using static electric fields. A pronounced anti-crossing of different excitonic transitions is observed as the electric field is tuned. Comparison of our experimental results with theory shows that the observed anti-crossing occurs between excitons with predominant spatially \emph{direct} and \emph{indirect} character. The electron component of the exciton wavefunction is shown to have molecular character at the anti-crossing and the quantum coupling strength is deduced optically. In addition, we determine the dependence of the coupling strength on the inter-dot separation and identify a field driven transition of the nature of the molecular ground state.

cond-mat.mes-hall↗

arXiv2004

Modulating the growth conditions: Si as an acceptor in (110) GaAs for high mobility p-type heterostructures

We implement metallic layers of Si-doped (110) GaAs as modulation doping in high mobility p-type heterostructures, changing to p-growth conditions for the doping layer alone. The strongly auto-compensated doping is first characterized in bulk samples, identifying the metal-insulator transition density and confirming classic hopping conduction in the insulating regime. To overcome the poor morphology inherent to Si p-type (110) growth, heterostructures are fabricated with only the modulation doping layer grown under p-type conditions. Such heterostructures show a hole mobility of 1.75x10^2 cm^2/Vs at density p=2.4x10^11 cm^-2. We identify the zero field spin-splitting characteristic of p-type heterostructures, but observe a remarkably isotropic mobility and a persistent photoconductivity unusual for p-heterojunctions grown using other doping techniques. This new modulated growth technique is particularly relevant for p-type cleaved-edge overgrowth and for III-V growth chambers where Si is the only dopant.

cond-mat.mes-hall↗

arXiv2004

Corner overgrowth: Bending a high mobility two-dimensional electron system by 90 degrees

Introducing an epitaxial growth technique called corner overgrowth, we fabricate a quantum confinement structure consisting of a high-mobility GaAs/AlGaAs heterojunction overgrown on top of an ex-situ cleaved substrate corner. The resulting corner-junction quantum-well heterostructure effectively bends a two-dimensional electron system (2DES) at an atomically sharp $90^{\rm o}$ angle. The high-mobility 2DES demonstrates fractional quantum Hall effect on both facets. Lossless edge-channel conduction over the corner confirms a continuum of 2D electrons across the junction, consistent with Schroedinger-Poisson calculations of the electron distribution. This growth technique differs distinctly from cleaved-edge overgrowth and enables a complementary class of new embedded quantum heterostructures.

cond-mat.mes-hall↗

arXiv2004

The structure of a single sharp quantum Hall edge probed by momentum-resolved tunneling

Momentum resolved magneto-tunnelling spectroscopy is performed at a single sharp quantum Hall edge. We directly probe the structure of individual integer quantum Hall (QH) edge modes, and find that an epitaxially overgrown cleaved edge realizes the sharp edge limit, where the Chklovskii picture relevant for soft etched or gated edges is no longer valid. The Fermi wavevector in the probe quantum well probes the real-space position of the QH edge modes, and reveals inter-channel distances smaller than both the magnetic length and the Bohr radius. We quantitatively describe the lineshape of principal conductance peaks and deduce an edge filling factor from their position consistent with the bulk value. We observe features in the dispersion which are attributed to fluctuations in the ground energy of the quantum Hall system.

cond-mat.mes-hall↗

arXiv2004

A new method to epitaxially grow long-range ordered self-assembled InAs quantum dots on (110) GaAs

We report on a new approach for positioning of self-assembled InAs quantum dots on (110) GaAs with nanometer precision. By combining self-assembly of quantum dots with molecular beam epitaxy on in-situ cleaved surfaces (cleaved-edge overgrowth) we have successfully fabricated arrays of long-range ordered InAs quantum dots. Both atomic force microscopy and micro-photoluminescence measurements demonstrate the ability to control size, position, and ordering of the quantum dots. Furthermore, single dot photoluminescence investigations confirm the high optical quality of the quantum dots fabricated.

cond-mat.mes-hall↗

arXiv2003

Quantum Hall Effect in a Two-Dimensional Electron System Bent by 90 Degrees

Using a new MBE growth technique, we fabricate a two-dimensional electron system which is bent around an atomically sharp 90 degree corner. In the quantum Hall regime under tilted magnetic fields, we can measure equilibration between both co- and counter-propagating edge channels of arbitrary filling factor ratio. We present here 4-point magnetotransport characterization of the corner junction with filling factor combinations which can all be explained using the standard Landauer-Buttiker edge channel picture. The success of this description confirms the realization of a new type of quantum Hall edge geometry.

cond-mat.mes-hall↗

arXiv2003

Remote-doping scattering and the local field corrections in the 2D electron system in a modulation-doped Si/SiGe quantum well

The small, about 30% magnetoresistance at the onset of full spin polarization in the 2D electron system in a modulation-doped Si/SiGe quantum well gives evidence that it is the remote doping that determines the transport scattering time. Measurements of the mobility in this strongly-interacting electron system with remote-doping scattering allow us to arrive at a conclusion that the Hubbard form underestimates the local field corrections by about a factor of 2.

cond-mat.str-el↗

arXiv2003

Probing the Electrostatics of Integer Quantum Hall Edges with Momentum-Resolved Tunnel Spectroscopy

We present measurements of momentum-resolved magneto-tunneling from a perpendicular two-dimensional (2D) contact into integer quantum Hall (QH) edges at a sharp edge potential created by cleaved edge overgrowth. Resonances in the tunnel conductance correspond to coincidences of electronic states of the QH edge and the 2D contact in energy-momentum space. With this dispersion relation reflecting the potential distribution at the edge we can directly measure the band bending at our cleaved edge under the influence of an external voltage bias. At finite bias we observe significant deviations from the flat-band condition in agreement with self-consistent calculations of the edge potential.

cond-mat↗

arXiv2003

Combined atomic force microscope and electron-beam lithography used for the fabrication of variable-coupling quantum dots

We have combined direct nanofabrication by local anodic oxidation with conventional electron-beam lithography to produce a parallel double quantum dot based on a GaAs/AlGaAs heterostructure. The combination of both nanolithography methods allows to fabricate robust in-plane gates and Cr/Au top gate electrodes on the same device for optimal controllability. This is illustrated by the tunability of the interdot coupling in our device. We describe our fabrication and alignment scheme in detail and demonstrate the tunability in low-temperature transport measurements.

cond-mat.mes-hall↗

arXiv2003

Kondo effect in a few-electron quantum ring

A small quantum ring with less than 10 electrons was studied by transport spectroscopy. For strong coupling to the leads a Kondo effect is observed and used to characterize the spin structure of the system in a wide range of magnetic fields. At small magnetic fields Aharonov-Bohm oscillations influenced by Coulomb interaction appear. They exhibit phase jumps by $π$ at the Coulomb-blockade resonances. Inside Coulomb-blockade valleys the Aharonov-Bohm oscillations can also be studied due to the finite conductance caused by the Kondo effect. Astonishingly, the maxima of the oscillations show linear shifts with magnetic field and gate voltage.

cond-mat.mes-hall↗

arXiv2003

Spin-photocurrent in p-SiGe quantum wells under terahertz laser irradiation

A detailed study of the circular photogalvanic effect (CPGE) in SiGe structures is presented. It is shown that the CPGE becomes possible due to the built-in asymmetry of quantum wells (QWs) in compositionally stepped samples and in asymmetrically doped structures. The photocurrent arises due to optical spin orientation of free carriers in QWs with spin splitting in k-space. It is shown that the effect can be applied to probe the macroscopic in-plane symmetry of low dimensional structures and allowing to conclude on Rashba or Dresselhaus terms in the Hamiltonian.

cond-mat↗

arXiv2002

Aharonov-Bohm oscillations of a tunable quantum ring

With an atomic force microscope a ring geometry with self-aligned in-plane gates was directly written into a GaAs/AlGaAs-heterostructure. Transport measurements in the open regime show only one transmitting mode and Aharonov-Bohm oscillations with more than 50% modulation are observed in the conductance. The tuning via in-plane gates allows to study the Aharonov-Bohm effect in the whole range from the open ring to the Coulomb-blockade regime.

cond-mat.mes-hall↗

arXiv2001

Optical excitations of a self assembled artificial ion

By use of magneto-photoluminescence spectroscopy we demonstrate bias controlled single-electron charging of a single quantum dot. Neutral, single, and double charged excitons are identified in the optical spectra. At high magnetic fields one Zeeman component of the single charged exciton is found to be quenched, which is attributed to the competing effects of tunneling and spin-flip processes. Our experimental data are in good agreement with theoretical model calculations for situations where the spatial extent of the hole wave functions is smaller as compared to the electron wave functions.

cond-mat.mes-hall↗

arXiv1999

Bandgap renormalization of modulation doped quantum wires

We measure the photoluminescence spectra for an array of modulation doped, T-shaped quantum wires as a function of the 1d density n_e which is modulated with a surface gate. We present self-consistent electronic structure calculations for this device which show a bandgap renormalization which, when corrected for excitonic energy and its screening, are largely insensitive to n_e and which are in quantitatively excellent agreement with the data. The calculation (cf. cond-mat/9908349) shows the importance of including orthogonality between the screening electrons and the electron(s) bound to the hole. The calculations show that electron and hole remain bound up to 3 x 10^6 cm^-1 and that therefore the stability of the exciton far exceeds the conservative Mott criterion.

cond-mat.mes-hall↗