New publication: Visualisierung von Messdaten eigener Sensormodule mit phyphox (in German)
In this German publication we demonstrate how the phyphox BLE library for Arduino can be used to easily visualize data from a custom build sensor on your smartphone. We use CO2 sensor kits as an example, which have been designed as an assembly kit for schools during the pandemic. But besides being a tool to monitor air quality, the easy visualization with phyphox allows for more advanced uses in STEM classes, with experiments in Biology, Chemistry and of course Physics.
New publication: Using a Smartphone pressure sensor as Pitot tube speedometer
As smartphones have become a part of our everyday life, their sensors have successfully been used to allow data acquisition with these readily available devices in a variety of different smartphone-based school experiments. Such experiments most commonly take advantage of the accelerometer and gyroscope. A less frequently used sensor in smartphone-based experiments is the pressure sensor or barometer. Pressure sensors in smartphones can improve indoor navigation, for example in multi-story shopping malls. In a popular smartphone experiment, the barometer is used to determine the current altitude in an elevator with the barometric height formula. Along with accelerometer data and by deriving the height data to calculate the velocity, z(t), v(t), and a(t) plots can be generated and shown to students in real time.
Short group excursion
Walked from the Kronenberg through the Aachenerwald to the Gut Entenpfuhl and played a round of mini-golf there. The weather was more than fine and being outside is always great.
New book chapter: "Überwachte Rollbewegung mit phyphox"
New publication: Probing Two-Electron Multiplets in Bilayer Graphene Quantum Dots
Phys. Rev. Lett. 127, 256802(2021) We report on finite bias spectroscopy measurements of the two-electron spectrum in a gate defined bilayer graphene (BLG) quantum dot for varying magnetic fields. The spin and valley degree of freedom in BLG give rise to multiplets of six orbital symmetric and ten orbital antisymmetric states. We find that orbital symmetric states are lower in energy and separated by ≈ 0.4–0.8 meV from orbital antisymmetric states. The symmetric multiplet exhibits an additional energy splitting of its six states of ≈ 0.15–0.5 meV due to lattice scale interactions. The experimental observations are supported by theoretical calculations, which allow to determine that intervalley scattering and “current-current” interaction constants are of the same magnitude in BLG.
New publication: Triggering phase-coherent spin packets by pulsed electrical spin injection across an Fe/GaAs Schottky barrier
Phys. Rev. B 104, 195202 (2021) The precise control of spins in semiconductor spintronic devices requires electrical means to generate spin packets with a well-defined initial phase. We demonstrate a pulsed electrical scheme that triggers the spin ensemble phase in a similar way as circularly polarized optical pulses generate phase coherent spin packets. Here, we use fast current pulses to initialize phase coherent spin packets, which are injected across an Fe/GaAs Schottky barrier into n-GaAs. By means of time-resolved Faraday rotation, we demonstrate phase coherence by the observation of multiple Larmor precession cycles for current pulse widths down to 500 ps at 17 K. We show that the current pulses are broadened by the charging and discharging time of the Schottky barrier. At high frequencies, the observable spin coherence is limited only by the finite bandwidth of the current pulses, which is of the order of 2 GHz. These results therefore demonstrate that all-electrical injection and phase control of electron spin packets at microwave frequencies is possible in metallic-ferromagnet–semiconductor heterostructures.
New publication: Contacts and upstream modes explain the electron-hole asymmetry in the graphene quantum Hall regime
Phys. Rev. B 104, L201406 (2021) Observations of electron-hole asymmetry in transport through graphene devices at high magnetic field challenge prevalent models of the graphene quantum Hall effect. Here we study this asymmetry both in conventional magnetotransport and in scanning gate microscopy maps measured in an encapsulated graphene constriction. We reveal that the presence of upstream modes and local doping in the vicinity of electrical contacts leads to a totally different picture of topological breakdown for electrons and holes, explaining the observed asymmetry.