arXiv subjects

Werner Wegscheider

Publications and source records attributed to Werner Wegscheider.

At least 91 records · Page 5Linked to original sources

arXiv2017

Optical probing of the intrinsic spin Hall effect in a high mobility GaAs p-doped quantum well

We report on the detection of the intrinsic spin Hall effect in a modulation doped Al-GaAs/GaAs/AlGaAs heterostructure bounded by a self-aligned pn-junction, fabricated by the cleaved edge overgrowth method. Light emission due to the recombination of electrons and spin-polarized holes was generated and mapped with a spatial resolution of one micrometer. An edge accumulated spin polarization of up to 11% was measured, induced solely by application of an electric Field perpendicular to the pn-junction. Using a quantum dot structure as light source, a linear dependence of the effective spin polarization, and with that the dominance of the spin Hall effect, with the electric field is seen. Spatially resolved spectroscopy from an epitaxially fabricated LED is demonstrated to be a valuable tool to probe the edge states of electron and hole gases in reduced dimensions.

physics.app-ph↗

arXiv2017

Deterministic entanglement between a propagating photon and a singlet--triplet qubit in an optically active quantum dot molecule

Two-electron charged self-assembled quantum dot molecules exhibit a decoherence-avoiding singlet-triplet qubit subspace and an efficient spin-photon interface. We demonstrate quantum entanglement between emitted photons and the spin-qubit after the emission event. We measure the overlap with a fully entangled state to be $69.5\pm2.7\,\%$, exceeding the threshold of $50\,\%$ required to prove the non-separability of the density matrix of the system. The photonic qubit is encoded in two photon states with an energy difference larger than the timing resolution of existing detectors. We devise a novel heterodyne detection method, enabling projective measurements of such photonic color qubits along any direction on the Bloch sphere.

cond-mat.mes-hall↗

arXiv2017

Fermi edge singularities in transport through lateral GaAs quantum dots

We measure tunnelling currents through electrostatically defined quantum dots in a GaAs/AlGaAs heterostructure connected to two leads. For certain tunnelling barrier configurations and high sample bias we find a pronounced resonance associated with a Fermi edge singularity. This many-body scattering effect appears when the electrochemical potential of the quantum dot is aligned with the Fermi level of the lead less coupled to the dot. By changing the relative tunnelling barrier strength we are able to tune the interaction of the localised electron with the Fermi sea.

cond-mat.mes-hall↗

arXiv2017

Classical origin of conductance oscillations in an integrable cavity

Scanning gate microscopy measurements in a circular ballistic cavity with a tip placed near its center yield a non-monotonic dependence of the conductance on the tip voltage. Detailed numerical quantum calculations reproduce these conductance oscillations, and a classical scheme leads to its physical understanding. The large-amplitude conductance oscillations are shown to be of classical origin, and well described by the effect of a particular class of short trajectories.

cond-mat.mes-hall↗

arXiv2017

Strong Coupling Cavity QED with Gate-Defined Double Quantum Dots Enabled by a High Impedance Resonator

The strong coupling limit of cavity quantum electrodynamics (QED) implies the capability of a matter-like quantum system to coherently transform an individual excitation into a single photon within a resonant structure. This not only enables essential processes required for quantum information processing but also allows for fundamental studies of matter-light interaction. In this work we demonstrate strong coupling between the charge degree of freedom in a gate-detuned GaAs double quantum dot (DQD) and a frequency-tunable high impedance resonator realized using an array of superconducting quantum interference devices (SQUIDs). In the resonant regime, we resolve the vacuum Rabi mode splitting of size $2g/2π= 238$ MHz at a resonator linewidth $κ/2π= 12$ MHz and a DQD charge qubit dephasing rate of $γ_2/2π= 80$ MHz extracted independently from microwave spectroscopy in the dispersive regime. Our measurements indicate a viable path towards using circuit based cavity QED for quantum information processing in semiconductor nano-structures.

cond-mat.mes-hall↗

arXiv2016

Collective electronic excitation in a trapped ensemble of photogenerated dipolar excitons and free holes revealed by inelastic light scattering

Photogenerated excitonic ensembles confined in coupled GaAs quantum wells are probed by a complementary approach of emission spectroscopy and resonant inelastic light scattering. Lateral electrostatic trap geometries are used to create dense systems of spatially indirect excitons and excess holes with similar densities in the order of 10$^{11}$ cm$^{-2}$. Inelastic light scattering spectra reveal a very sharp low-lying collective mode that is identified at an energy of 0.44 meV and a FWHM of only ~50 $μ$eV. This mode is interpreted as a plasmon excitation of the excess hole system coupled to the photogenerated indirect excitons. The emission energy of the indirect excitons shifts under the application of a perpendicular applied electric field with the quantum-confined Stark effect unperturbed from the presence of free charge carriers. Our results illustrate the potential of studying low-lying collective excitations in photogenerated exciton systems to explore the many-body phase diagram, related phase transitions, and interaction physics.

cond-mat.quant-gas↗

arXiv2016

Experimental Signatures of the Inverted Phase in InAs/GaSb Coupled Quantum Wells

Transport measurements are performed on InAs/GaSb double quantum wells at zero and finite magnetic fields applied parallel and perpendicular to the quantum wells. We investigate a sample in the inverted regime where electrons and holes coexist, and compare it with another sample in the non-inverted semiconducting regime. Activated behavior in conjunction with a strong suppression of the resistance peak at the charge neutrality point in a parallel magnetic field attest to the topological hybridization gap between electron and hole bands in the inverted sample. We observe an unconventional Landau level spectrum with energy gaps modulated by the magnetic field applied perpendicular to the quantum wells. This is caused by strong spin-orbit interaction provided jointly by the InAs and the GaSb quantum wells.

cond-mat.mes-hall↗

arXiv2016

Combined atomic force microscopy and photoluminescence imaging to select single InAs/GaAs quantum dots for quantum photonic devices

We report on a combined photoluminescence imaging and atomic force microscopy study of single, isolated self-assembled InAs quantum dots (density <0.01 um^(-2) capped by a 95 nm GaAs layer, and emitting around 950 nm. By combining optical and scanning probe characterization techniques, we determine the position of single quantum dots with respect to comparatively large (100 nm to 1000 nm in-plane dimension) topographic features. We find that quantum dots often appear (>25% of the time) in the vicinity of these features, but generally do not exhibit significant differences in their non-resonantly pumped emission spectra in comparison to quantum dots appearing in defect-free regions. This behavior is observed across multiple wafers produced in different growth chambers. Our characterization approach is relevant to applications in which single quantum dots are embedded within nanofabricated photonic devices, where such large surface features can affect the interaction with confined optical fields and the quality of the single-photon emission. In particular, we anticipate using this approach to screen quantum dots not only based on their optical properties, but also their surrounding surface topographies.

cond-mat.mes-hall↗

arXiv2016

THz ultrastrong light-matter coupling

Cavity photon resonators with ultrastrong light-matter interactions are attracting interest both in semiconductor and superconducting systems displaying the capability to manipulate the cavity quantum electrodynamic ground state with controllable physical properties. Here we review a series of experiments aimed at probing the ultrastrong light-matter coupling regime, where the vacuum Rabi splitting $Ω$ is comparable to the bare transition frequency $ω$ . We present a new platform where the inter-Landau level transition of a two-dimensional electron gas (2DEG) is strongly coupled to the fundamental mode of deeply subwavelength split-ring resonators operating in the mm-wave range. Record-high values of the normalized light-matter coupling ratio $\fracΩω= 0.89$ are reached and the system appears highly scalable far into the microwave range.

physics.optics↗

arXiv2016

Heat dissipation and fluctuations in a driven quantum dot

While thermodynamics is a useful tool to describe the driving of large systems close to equilibrium, fluctuations dominate the distribution of heat and work in small systems and far from equilibrium. We study the heat generated by driving a small system and change the drive parameters to analyse the transition from a drive leaving the system close to equilibrium to driving it far from equilibrium. Our system is a quantum dot in a GaAs/AlGaAs heterostructure hosting a two-dimensional electron gas. The dot is tunnel-coupled to one part of the two-dimensional electron gas acting as a heat and particle reservoir. We use standard rate equations to model the driven dot-reservoir system and find excellent agreement with the experiment. Additionally, we quantify the fluctuations by experimentally test the theoretical concept of the arrow of time, predicting our ability to distinguish whether a process goes in the forward or backward drive direction.

cond-mat.mes-hall↗

arXiv2016

Improvement of the transport properties of a high-mobility electron system by intentional parallel conductance

We present a gating scheme to separate even strong parallel conductance from the magneto-transport signatures and properties of a two-dimensional electron system. By varying the electron density in the parallel conducting layer, we can study the impact of mobile charge carriers in the vicinity of the dopant layer on the properties of the two-dimensional electron system. It is found that the parallel conducting layer is indeed capable to screen the remote ionized impurity potential fluctuations responsible for the fragility of fractional quantum Hall states.

cond-mat.mes-hall↗

arXiv2016

On the impact of strain on the electronic properties of InAs/GaSb quantum well systems

Electron-hole hybridization in InAs/GaSb double quantum well structures leads to the formation of a mini band gap. We experimentally and theoretically studied the impact of strain on the transport properties of this material system. Thinned samples were mounted to piezo electric elements to exert strain along the [011] and [001] crystal directions. When the Fermi energy is tuned through the mini gap, a dramatic impact on the resistivity at the charge neutrality point is found which depends on the amount of applied external strain. In the electron and hole regimes, strain influences the Landau level structure. By analyzing the intrinsic strain from the epitaxial growth, the external strain from the piezo elements and combining our experimental results with numerical simulations of strained and unstrained quantum wells, we compellingly illustrate why the InAs/GaSb material system is regularly found to be semimetallic.

cond-mat.str-el↗

arXiv2016

Measuring the degeneracy of discrete energy levels using a GaAs/AlGaAs quantum dot

We demonstrate an experimental method for measuring quantum state degeneracies in bound state energy spectra. The technique is based on the general principle of detailed balance, and the ability to perform precise and efficient measurements of energy-dependent tunnelling-in and -out rates from a reservoir. The method is realized using a GaAs/AlGaAs quantum dot allowing for the detection of time-resolved single-electron tunnelling with a precision enhanced by a feedback-control. It is thoroughly tested by tuning orbital and spin-degeneracies with electric and magnetic fields. The technique also lends itself for studying the connection between the ground state degeneracy and the lifetime of the excited states.

cond-mat.mes-hall↗

arXiv2016

Investigating Energy Scales of Fractional Quantum Hall States using Scanning Gate Microscopy

We use the voltage biased tip of a scanning force microscope at a temperature of 35\,mK to locally induce the fractional quantum Hall state of $ν=1/3$ in a split-gate defined constriction. Different tip positions allow us to vary the potential landscape. From temperature dependence of the conductance plateau at $G=1/3 \times e^2/h$ we determine the activation energy of this local $ν=1/3$ state. We find that at a magnetic field of 6\,T the activation energy is between 153\,$μ$eV and 194\,$μ$eV independent of the shape of the confining potential, but about 50\% lower than for bulk samples.

cond-mat.mes-hall↗

arXiv2016

Non-local polarization feedback in a fractional quantum Hall ferromagnet

In a quantum Hall ferromagnet, the spin polarization of the two-dimensional electron system can be dynamically transferred to nuclear spins in its vicinity through the hyperfine interaction. The resulting nuclear field typically acts back locally, modifying the local electronic Zeeman energy. Here we report a non-local effect arising from the interplay between nuclear polarization and the spatial structure of electronic domains in a $ν=2/3$ fractional quantum Hall state. In our experiments, we use a quantum point contact to locally control and probe the domain structure of different spin configurations emerging at the spin phase transition. Feedback between nuclear and electronic degrees of freedom gives rise to memristive behavior, where electronic transport through the quantum point contact depends on the history of current flow. We propose a model for this effect which suggests a novel route to studying edge states in fractional quantum Hall systems and may account for so-far unexplained oscillatory electronic-transport features observed in previous studies.

cond-mat.mes-hall↗

arXiv2016

Structured Back Gates for High-Mobility Two-Dimensional Electron Systems Using Oxygen Ion Implantation

We present a new approach of back gate patterning that is compatible with the requirements of highest mobility molecular beam epitaxy. Contrary to common back gating techniques, our method is simple, reliable and can be scaled up for entire wafers. The back gate structures are defined by local oxygen implantation into a silicon doped GaAs epilayer, which suppresses the conductance without affecting the surface quality.

cond-mat.mes-hall↗

arXiv2015

Mapping an electron wave function by a local electron scattering probe

A technique is developed which allows for the detailed mapping of the electronic wave function in two-dimensional electron gases with low-temperature mobilities up to 15E6 cm^2/Vs. Thin ("delta") layers of aluminium are placed into the regions where the electrons reside. This causes electron scattering which depends very locally on the amplitude of the electron wave function at the position of the Al "delta"-layer. By changing the distance of this layer from the interface we map the shape of the wave function perpendicular to the interface. Despite having a profound effect on the electron mobiliy, the "delta"-layers do not cause a widening of the quantum Hall plateaus.

cond-mat.mes-hall↗

arXiv2015

Multistability and spin diffusion enhanced lifetimes in dynamic nuclear polarization in a double quantum dot

The control of nuclear spins in quantum dots is essential to explore their many-body dynamics and exploit their prospects for quantum information processing. We present a unique combination of dynamic nuclear spin polarization and electric-dipole-induced spin resonance in an electrostatically defined double quantum dot (DQD) exposed to the strongly inhomogeneous field of two on-chip nanomagnets. Our experiments provide direct and unrivaled access to the nuclear spin polarization distribution and allow us to establish and characterize multiple fixed points. Further, we demonstrate polarization of the DQD environment by nuclear spin diffusion which significantly stabilizes the nuclear spins inside the DQD.

cond-mat.mes-hall↗