arXiv subjects

W. Wegscheider

Publications and source records attributed to W. Wegscheider.

At least 55 records · Page 3Linked to original sources

arXiv2017

Non-linear and dot-dependent Zeeman splitting in GaAs/AlGaAs quantum dot arrays

We study the Zeeman splitting in lateral quantum dots that are defined in GaAs-AlGaAs het- erostructures by means of split gates. We demonstrate a non-linear dependence of the splitting on magnetic field and its substantial variations from dot to dot and from heterostructure to het- erostructure. These phenomena are important in the context of information processing since the tunability and dot-dependence of the Zeeman splitting allow for a selective manipulation of spins. We show that spin-orbit effects related to the GaAs band structure quantitatively explain the ob- served magnitude of the non-linear dependence of the Zeeman splitting. Furthermore, spin-orbit effects result in a dependence of the Zeeman splitting on predominantly the out-of-plane quantum dot confinement energy. We also show that the variations of the confinement energy due to charge disorder in the heterostructure may explain the dependence of Zeeman splitting on the dot position. This position may be varied by changing the gate voltages which leads to an electrically tunable Zeeman splitting.

cond-mat.mes-hall↗

arXiv2017

Scanning gate experiments: from strongly to weakly invasive probes

An open resonator fabricated in a two-dimensional electron gas is used to explore the transition from strongly invasive scanning gate microscopy to the perturbative regime of weak tip-induced potentials. With the help of numerical simulations that faithfully reproduce the main experimental findings, we quantify the extent of the perturbative regime in which the tip-induced conductance change is unambiguously determined by properties of the unperturbed system. The correspondence between the experimental and numerical results is established by analyzing the characteristic length scale and the amplitude modulation of the conductance change. In the perturbative regime, the former is shown to assume a disorder-dependent maximum value, while the latter linearly increases with the strength of a weak tip potential.

cond-mat.mes-hall↗

arXiv2017

Scattering mechanisms of highest-mobility InAs/Al$_{x}$Ga$_{1-x}$Sb quantum wells

We study molecular beam epitaxially grown, undoped Al$_{x}$Ga$_{1-x}$Sb/InAs/AlSb quantum wells with different buffer and barrier designs and varying quantum well width. The highest mobilities were achieved with Al$_{0.33}$Ga$_{0.67}$Sb buffers and lower barriers and a quantum well width of 24 nm. These quasi-single-interface InAs/AlSb quantum well devices reached a gate-tuned mobility of 2.4$~\times~10^{6}~$cm$^2$/Vs at a density of 1$\times 10^{12}$ cm$^{-2}$ and 1.3 K. In Hall bar devices boundary scattering is found to strongly influence the mobility determination in this mobility regime. Ionized background impurity scattering at low electron densities, device boundary scattering at intermediate electron densities, and intersubband scattering at high electron densities were identified as most likely dominant scattering processes. Ringlike structures in the Landau fan can be explained using a single-particle model of crossing Landau levels.

cond-mat.mtrl-sci↗

arXiv2017

Quantum simulation of a Fermi-Hubbard model using a semiconductor quantum dot array

Interacting fermions on a lattice can develop strong quantum correlations, which lie at the heart of the classical intractability of many exotic phases of matter. Seminal efforts are underway in the control of artificial quantum systems, that can be made to emulate the underlying Fermi-Hubbard models. Electrostatically confined conduction band electrons define interacting quantum coherent spin and charge degrees of freedom that allow all-electrical pure-state initialisation and readily adhere to an engineerable Fermi-Hubbard Hamiltonian. Until now, however, the substantial electrostatic disorder inherent to solid state has made attempts at emulating Fermi-Hubbard physics on solid-state platforms few and far between. Here, we show that for gate-defined quantum dots, this disorder can be suppressed in a controlled manner. Novel insights and a newly developed semi-automated and scalable toolbox allow us to homogeneously and independently dial in the electron filling and nearest-neighbour tunnel coupling. Bringing these ideas and tools to fruition, we realize the first detailed characterization of the collective Coulomb blockade transition, which is the finite-size analogue of the interaction-driven Mott metal-to-insulator transition. As automation and device fabrication of semiconductor quantum dots continue to improve, the ideas presented here show how quantum dots can be used to investigate the physics of ever more complex many-body states.

cond-mat.mes-hall↗

arXiv2017

Coherent shuttle of electron-spin states

We demonstrate a coherent spin shuttle through a GaAs/AlGaAs quadruple-quantum-dot array. Starting with two electrons in a spin-singlet state in the first dot, we shuttle one electron over to either the second, third or fourth dot. We observe that the separated spin-singlet evolves periodically into the $m=0$ spin-triplet and back before it dephases due to nuclear spin noise. We attribute the time evolution to differences in the local Zeeman splitting between the respective dots. With the help of numerical simulations, we analyse and discuss the visibility of the singlet-triplet oscillations and connect it to the requirements for coherent spin shuttling in terms of the inter-dot tunnel coupling strength and rise time of the pulses. The distribution of entangled spin pairs through tunnel coupled structures may be of great utility for connecting distant qubit registers on a chip.

cond-mat.mes-hall↗

arXiv2016

Nuclear magnetic resonance and nuclear spin relaxation in AlAs quantum well probed by ESR

The study of nuclear magnetic resonance and nuclear spin-lattice relaxation was conducted in an asymmetrically doped to $n\sim1.8\times10^{11}$ cm$^{-2}$ 16 nm AlAs quantum well grown in the $[001]$-direction. Dynamic polarization of nuclear spins due to the hyperfine interaction resulted in the so-called Overhauser shift of the two-dimensional conduction electron spin resonance. The maximum shifts achieved in the experiments are several orders of magnitude smaller than in GaAs-based heterostructures indicating that hyperfine interaction is weak. The nuclear spin-lattice relaxation time extracted from the decay of Overhauser shift over time turned out to depend on the filling factor of the two-dimensional electron system. This observation indicates that nuclear spin-lattice relaxation is mostly due to the interaction between electron and nuclear spins. Overhauser shift diminishes resonantly when the RF-radiation of certain frequencies was applied to the sample. This effect served as an indirect, yet powerful method for nuclear magnetic resonance detection: NMR quadrupole splitting of $^{75}$As nuclei was clearly resolved. Theoretical calculations performed describe well these experimental findings.

cond-mat.mes-hall↗

arXiv2016

Electron backscattering in a cavity: ballistic and coherent effects

Numerous experimental and theoretical studies have focused on low-dimensional systems locally perturbed by the biased tip of a scanning force microscope. In all cases either open or closed weakly gate-tunable nanostructures have been investigated, such as quantum point contacts, open or closed quantum dots, etc. We study the behaviour of the conductance of a quantum point contact with a gradually forming adjacent cavity in series under the influence of a scanning gate. Here, an initially open quantum point contact system gradually turns into a closed cavity system. We observe branches and interference fringes known from quantum point contacts coexisting with irregular conductance fluctuations. Unlike the branches, the fluctuations cover the entire area of the cavity. In contrast to previous studies, we observe and investigate branches under the influence of the confining stadium potential, which is gradually built up. We find that the branches exist only in the area surrounded by cavity top gates. As the stadium shrinks, regular fringes originate from tip-induced constrictions leading to quantized conduction. In addition, we observe arc-like areas reminiscent of classical electron trajectories in a chaotic cavity. We also argue that electrons emanating from the quantum point contact spread out like a fan leaving branch-like regions of enhanced backscattering.

cond-mat.mes-hall↗

arXiv2016

Overflow of a dipolar exciton trap at high magnetic fields

We study laterally trapped dipolar exciton ensembles in coupled GaAs quantum wells at high magnetic fields in the Faraday configuration. In photoluminescence experiments, we identify three magnetic field regimes. At low fields, the exciton density is increased by a reduced charge carrier escape from the trap, and additionally, the excitons' emission energy is corrected by a positive diamagnetic shift. At intermediate fields, magnetic field dependent correction terms apply which follow the characteristics of a neutral magnetoexciton. Due to a combined effect of an increasing binding energy and lifetime, the exciton density is roughly doubled from zero to about seven Tesla. At the latter high field value, the charge carriers occupy only the lowest Landau level. In this situation, the exciton trap can overflow independently from the electrostatic depth of the trapping potential, and the energy shift of the excitons caused by the so-called quantum confined Stark effect is effectively compensated. Instead, the exciton energetics seem to be driven by the magnetic field dependent renormalization of the many-body interaction terms. In this regime, the impact of parasitic in-plane fields at the edge of trapping potential is eliminated.

cond-mat.quant-gas↗

arXiv2016

Linear magnetoresistance in a quasi-free two dimensional electron gas in an ultra-high mobility GaAs quantum well

We report a magnetotransport study of an ultra-high mobility ($\barμ\approx 25\times 10^6$\,cm$^2$\,V$^{-1}$\,s$^{-1}$) $n$-type GaAs quantum well up to 33 T. A strong linear magnetoresistance (LMR) of the order of 10$^5$ % is observed in a wide temperature range between 0.3 K and 60 K. The simplicity of our material system with a single sub-band occupation and free electron dispersion rules out most complicated mechanisms that could give rise to the observed LMR. At low temperature, quantum oscillations are superimposed onto the LMR. Both, the featureless LMR at high $T$ and the quantum oscillations at low $T$ follow the empirical resistance rule which states that the longitudinal conductance is directly related to the derivative of the transversal (Hall) conductance multiplied by the magnetic field and a constant factor $α$ that remains unchanged over the entire temperature range. Only at low temperatures, small deviations from this resistance rule are observed beyond $ν=1$ that likely originate from a different transport mechanism for the composite fermions.

cond-mat.mes-hall↗

arXiv2016

Linear polarization study of microwave-radiation-induced magnetoresistance oscillations: Comparison of power dependence to theory

We present an experimental study of the microwave power and the linear polarization angle dependence of the microwave-induced magnetoresistance oscillations in the high-mobility GaAs/AlGaAs two-dimensional electron system. Experimental results show the sinusoidal dependence of the oscillatory magnetoresistance extrema as a function of the polarization angle. Yet, as the microwave power increases, the angular dependence includes additional harmonic content, and it begins to resemble the absolute value of the cosine function. We present a theory to explain such peculiar behavior.

cond-mat.mes-hall↗

arXiv2016

Comparative study of microwave radiation-induced magnetoresistive oscillations induced by circularly- and linearly- polarized photo-excitation

A comparative study of the radiation-induced magnetoresistance oscillations in the high mobility GaAs/AlGaAs heterostructure two dimensional electron system (2DES) under linearly- and circularlypolarized microwave excitation indicates a profound difference in the response observed upon rotating the microwave launcher for the two cases, although circularly polarized microwave radiation induced magnetoresistance oscillations observed at low magnetic fields are similar to the oscillations observed with linearly polarized radiation. For the linearly polarized radiation, the magnetoresistive response is a strong sinusoidal function of the launcher rotation (or linear polarization) angle, θ. For circularly polarized radiation, the oscillatory magnetoresistive response is hardly sensitive to θ.

cond-mat.mes-hall↗

arXiv2016

Nanosecond-timescale spin transfer using individual electrons in a quadruple-quantum-dot device

The ability to coherently transport electron-spin states between different sites of gate-defined semiconductor quantum dots is an essential ingredient for a quantum-dot-based quantum computer. Previous shuttles using electrostatic gating were too slow to move an electron within the spin dephasing time across an array. Here we report a nanosecond-timescale spin transfer of individual electrons across a quadruple-quantum-dot device. Utilizing enhanced relaxation rates at a so-called `hot spot', we can upper bound the shuttle time to at most 150 ns. While actual shuttle times are likely shorter, 150 ns is already fast enough to preserve spin coherence in e.g. silicon based quantum dots. This work therefore realizes an important prerequisite for coherent spin transfer in quantum dot arrays.

cond-mat.mes-hall↗

arXiv2016

Current-controlled Spin Precession of Quasi-Stationary Electrons in a Cubic Spin-Orbit Field

Space- and time-resolved measurements of spin drift and diffusion are performed on a GaAs-hosted two-dimensional electron gas. For spins where forward drift is compensated by backward diffusion, we find a precession frequency in absence of an external magnetic field. The frequency depends linearly on the drift velocity and is explained by the cubic Dresselhaus spin-orbit interaction, for which drift leads to a spin precession angle twice that of spins that diffuse the same distance.

cond-mat.mes-hall↗

arXiv2016

Coherent spin-exchange via a quantum mediator

Coherent interactions at a distance provide a powerful tool for quantum simulation and computation. The most common approach to realize an effective long-distance coupling 'on-chip' is to use a quantum mediator, as has been demonstrated for superconducting qubits and trapped ions. For quantum dot arrays, which combine a high degree of tunability with extremely long coherence times, the experimental demonstration of coherent spin-spin coupling via an intermediary system remains an important outstanding goal. Here, we use a linear triple-quantum-dot array to demonstrate a first working example of a coherent interaction between two distant spins via a quantum mediator. The two outer dots are occupied with a single electron spin each and the spins experience a superexchange interaction through the empty middle dot which acts as mediator. Using single-shot spin read-out we measure the coherent time evolution of the spin states on the outer dots and observe a characteristic dependence of the exchange frequency as a function of the detuning between the middle and outer dots. This approach may provide a new route for scaling up spin qubit circuits using quantum dots and aid in the simulation of materials and molecules with non-nearest neighbour couplings such as MnO, high-temperature superconductors and DNA. The same superexchange concept can also be applied in cold atom experiments.

cond-mat.mes-hall↗

arXiv2016

Computer-automated tuning of semiconductor double quantum dots into the single-electron regime

We report the computer-automated tuning of gate-defined semiconductor double quantum dots in GaAs heterostructures. We benchmark the algorithm by creating three double quantum dots inside a linear array of four quantum dots. The algorithm sets the correct gate voltages for all the gates to tune the double quantum dots into the single-electron regime. The algorithm only requires (1) prior knowledge of the gate design and (2) the pinch-off value of the single gate $T$ that is shared by all the quantum dots. This work significantly alleviates the user effort required to tune multiple quantum dot devices.

cond-mat.mes-hall↗

arXiv2016

Frequency-dependent polarization-angle-phase-shift in the microwave-induced magnetoresistance oscillations

Linear polarization angle, $θ$, dependent measurements of the microwave radiation-induced oscillatory magnetoresistance, $R_{xx}$, in high mobility GaAs/AlGaAs 2D electron devices have shown a $θ$ dependence in the oscillatory amplitude along with magnetic field, frequency, and extrema-dependent phase shifts, $θ_{0}$. Here, we suggest a microwave frequency dependence of $θ_{0} (f)$ using an analysis that averages over other smaller contributions, when those contributions are smaller than estimates of the experimental uncertainty.

cond-mat.mes-hall↗

arXiv2016

Scanning-gate-induced effects and spatial mapping of a cavity

Tailored electrostatic potentials are the foundation of scanning gate microscopy. We present several aspects of the tip-induced potential on the two-dimensional electron gas. First, we give methods on how to estimate the size of the tip-induced potential. Then, a ballistic cavity is formed and studied as a function of the bias-voltage of the metallic top gates and probed with the tip-induced potential. It is shown how the potential of the cavity changes by tuning the system to a regime where conductance quantization in the constrictions formed by the tip and the top gates occurs. This conductance quantization leads to a unprecedented rich fringe pattern over the entire structure. Finally, the effect of electrostatic screening of the metallic top gates is discussed.

cond-mat.mes-hall↗

arXiv2016

Scanning gate imaging in confined geometries

This article reports on tunable electron backscattering investigated with the biased tip of a scanning force microscope. Using a channel defined by a pair of Schottky gates, the branched electron flow of ballistic electrons injected from a quantum point contact is guided by potentials of a tunable height well below the Fermi energy. The transition from injection into an open two-dimensional electron gas to a strongly confined channel exhibits three experimentally distinct regimes: one in which branches spread unrestrictedly, one in which branches are confined but the background conductance is affected very little, and one where the branches have disappeared and the conductance is strongly modified. Classical trajectory-based simulations explain these regimes at the microscopic level. These experiments allow us to understand under which conditions branches observed in scanning gate experiments do or do not reflect the flow of electrons.

cond-mat.mes-hall↗