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News 05.11.2020
Sebastian is a Falling Walls Winner in the category Digital Education 2020

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Sebastian is a Falling Walls Winner in the category Digital Education 2020

The nominating institution RWTH Aachen on the breakthrough:
With the free and open source app phyphox many sensors (even present in old and cheap smartphones) become accessible for students and turns their smartphones in mobile physics labs. Phyphox offers tools and instructions from school to university level and from individual labs with household items during Covid19 lockdown to global experiments to determine Earth’s axial tilt. The app, downloaded over 1.5 million and translated in 17 languages, makes science education worldwide more attractive. For more information see here.

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New publication: Electron–Hole Crossover in Gate-Controlled Bilayer Graphene Quantum Dots


Nano Letters 20, 7709 (2020)
Electron and hole Bloch states in bilayer graphene exhibit topological orbital magnetic moments with opposite signs, which allows for tunable valley-polarization in an out-of-plane magnetic field. This property makes electron and hole quantum dots (QDs) in bilayer graphene interesting for valley and spin-valley qubits. Here, we show measurements of the electron–hole crossover in a bilayer graphene QD, demonstrating opposite signs of the magnetic moments associated with the Berry curvature. Using three layers of top gates, we independently control the tunneling barriers while tuning the occupation from the few-hole regime to the few-electron regime, crossing the displacement-field-controlled band gap. The band gap is around 25 meV, while the charging energies of the electron and hole dots are between 3 and 5 meV. The extracted valley g-factor is around 17 and leads to opposite valley polarization for electrons and holes at moderate B-fields. Our measurements agree well with tight-binding calculations for our device.

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New publication: Fractional quantum Hall effect in CVD-grown graphene


2D Materials 7, 041007 (2020)
We show the emergence of fractional quantum Hall states in graphene grown by chemical vapor deposition (CVD) for magnetic fields from below 3 T to 35 T where the CVD-graphene was dry-transferred. Effective composite-fermion filling factors up to ν=4 are visible and higher order composite-fermion states (with four flux quanta attached) start to emerge at the highest fields. Our results show that the quantum mobility of CVD-grown graphene is comparable to that of exfoliated graphene and, more specifically, that the p/3 fractional quantum Hall states have energy gaps of up to 30 K, well comparable to those observed in other silicon-gated devices based on exfoliated graphene.

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New PhD position


Im Rahmen des von der Deutschen Forschungsgemeinschaft geförderten Exzellenzclusters „Materie und Licht für Quanteninformation ML4Q“ bietet unsere Arbeitsgruppe ab sofort eine Doktorandenstelle an. Hier erhalten Sie weitere Informationen.

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New publication: How Photoinduced Gate Screening and Leakage Currents Dynamically Change the Fermi Level in 2D Materials


Phys. Status Solidi RRL , 2000298 (2020)
An in‐depth analysis of physics in 2D materials like transition metal dichalcogenides requires the measurement of many material properties as a function of Fermi level position within the electronic band structure. This is normally done by changing the charge carrier density of the 2D material via the gate electric field effect. Herein, a comparison of gate‐dependent measurements, which are acquired under different measurement conditions, is shown to encounter significant problems due to the temporal evolution of the charging of trap states inside the dielectric layer or at its interfaces. The impact of, e.g., the gate sweep direction and the sweep rate on the overall gate dependence gets especially prominent in optical measurements due to photoexcitation of donor and acceptor states. Under such conditions, the same nominal gate voltage may lead to different gate‐induced charge carrier densities and, hence, Fermi level positions. It is demonstrated that a current flow from or even through the dielectric layer via leakage currents can significantly diminish the gate tunability in optical measurements of 2D materials.

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Radio contribution to LIEVITI - Episodio 4 - CORPO (in Italian)

The scientific voice comes from our institute: https://www.radiopapesse.org/it/archivio/sonora/lieviti-episodio-4-corpo.

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RWTH Start-Up Grant for Christian Volk


Christian Volk receives a RWTH Start-Up Grant for his proposal with the title: "Towards thermal conductance measurements as a probe for topological states in two-dimensional quantum systems". Congratulation!

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Smartphone Physics on the Rise

The free, online magazine “Physics” from the American Physical Society has published an article on “Smartphone Physics on the Rise” in their Arts & Culture section. It covers the possibilities and people involved in this field and includes a quote by Sebastian on the effects of the lock down on app downloads and feedback.
Here the link to the article: Smartphone Physics on the Rise.

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