New publication: Tuning the supercurrent distribution in parallel ballistic graphene Josephson junctions
Phys. Rev. Applied 20, 054049 (2023) We report on a ballistic and fully tunable Josephson-junction system consisting of two parallel ribbons of graphene in contact with superconducting molybdenum-rhenium. By electrostatic gating of the two individual graphene ribbons, we gain control over the real-space distribution of the superconducting current density, which can be continuously tuned between the two ribbons. We extract the respective gate-dependent spatial distributions of the real-space current density by employing Fourier and Hilbert transformations of the magnetic-field-induced modulation of the critical current. This approach is fast and does not rely on a symmetric current profile. It is therefore a universally applicable tool, potentially useful for carefully adjusting Josephson junctions.
New publication: Das Labor im Miniformat
Physik Journal 9, 75 (2023) Die an der RWTH Aachen entwickelte App phyphox hat seit ihrer Veröffentlichung im Herbst 2016 vielerorts Einzug in die Lehre in Schulen und Hochschulen gehalten. Sie ist vor allem als kostenfreies „alternatives“ Messgerät bekannt, da sie die in Smartphones und Tablets verbauten Sensoren für Physikexperimente nutzbar macht. Die Möglichkeiten der App gehen jedoch weit über den Einsatz als Messgerät hinaus.
New publication: Impact of competing energy scales on the shell-filling sequence in elliptic bilayer graphene quantum dots
Phys. Rev. B 108, 125128 (2023) We report on a detailed investigation of the shell-filling sequence in electrostatically defined elliptic bilayer graphene quantum dots (QDs) in the regime of low charge carrier occupation, N≤12, by means of magnetotransport spectroscopy and numerical calculations. We show the necessity of including both short-range electron-electron interaction and wave-function-dependent valley g-factors for understanding the overall fourfold shell-filling sequence. These factors lead to an additional energy splitting at half filling of each orbital state and different energy shifts in out-of-plane magnetic fields. Analysis of 31 different bilayer graphene (BLG) QDs reveals that both valley g-factor and electron-electron interaction-induced energy splitting increase with decreasing QD size, validating theory. However, we find that the electrostatic charging energy of such gate-defined QDs does not correlate consistently with their size, indicating complex electrostatics. These findings offer significant insights for future BLG QD devices and circuit designs.
New publication: Electronic and spin-orbit properties of h-BN encapsulated bilayer graphene
Phys. Rev. B 108, 125126 (2023) Van der Waals heterostructures consisting of Bernal bilayer graphene (BLG) and hexagonal boron nitride (hBN) are investigated. By performing first-principles calculations, we capture the essential BLG band structure features for several stacking and encapsulation scenarios. A low-energy model Hamiltonian, comprising orbital and spin-orbit coupling (SOC) terms, is employed to reproduce the hBN-modified BLG dispersion, spin splittings, and spin expectation values. Most important, the hBN layers open an orbital gap in the BLG spectrum, which can range from zero to tens of meV, depending on the precise stacking arrangement of the individual atoms. Therefore, large local band gap variations may arise in experimentally relevant moiré structures. Moreover, the SOC parameters are small (few to tens of µeV), just as in bare BLG, but are markedly proximity modified by the hBN layers. Especially when BLG is encapsulated by monolayers of hBN, such that inversion symmetry is restored, the orbital gap and spin splittings of the bands vanish. In addition, we show that a transverse electric field mainly modifies the potential difference between the graphene layers, which perfectly correlates with the orbital gap for fields up to about 1 V/nm. Moreover, the layer-resolved Rashba couplings are tunable by ∼5µeV per V/nm. Finally, by investigating twisted BLG/hBN structures, with twist angles between 6°–20°, we find that the global band gap increases linearly with the twist angle. The extrapolated 0° band gap is about 23 meV and results roughly from the average of the stacking-dependent local band gaps. Our investigations give insights into proximity spin physics of hBN/BLG heterostructures, which should be useful for interpreting experiments on extended as well as confined (quantum dot) systems.
New publication: Chemically detaching hBN crystals grown at atmospheric pressure and high temperature for high-performance graphene devices
Nanotechnology 34, 475703 (2023) In this work, we report on the growth of hexagonal boron nitride (hBN) crystals from an iron flux at atmospheric pressure and high temperature and demonstrate that (i) the entire sheet of hBN crystals can be detached from the metal in a single step using hydrochloric acid and that (ii) these hBN crystals allow to fabricate high carrier mobility graphene-hBN devices. By combining spatially-resolved confocal Raman spectroscopy and electrical transport measurements, we confirm the excellent quality of these crystals for high-performance hBN-graphene-based van der Waals heterostructures. The full width at half maximum of the graphene Raman 2D peak is as low as 16 cm−1, and the room temperature charge carrier mobilitiy is around 80000 cm2/(Vs) at a carrier density 1 × 1012 cm−12. This is fully comparable with devices of similar dimensions fabricated using crystalline hBN synthesized by the high pressure and high temperature method. Finally, we show that for exfoliated high-quality hBN flakes with a thickness between 20 and 40 nm the line width of the hBN Raman peak, in contrast to the graphene 2D line width, is not useful for benchmarking hBN in high mobility graphene devices.
Congratulations to Prof. Dr. Annika Kurzmann
Prof. Annika Kurzmann has been appointed ML4Q Professor for Experimental Solid-State Physics at the University of Cologne. In 2021 she joined our group with a RWTH Junior Principal Investigator (JPI) Fellowship, which she used as a steppingstone toward a tenure-track position. A real success story not only for Annika, but also for ML4Q, the RWTH and for the Aachen Graphene & 2D Materials Center. Congratulations!
New publication: Twist angle dependent interlayer transfer of valley polarization from excitons to free charge carriers in WSe2/MoSe2 heterobilayers
npj 2D Mater. Appl. 7, 58 (2023) Transition metal dichalcogenides (TMDs) have attracted much attention in the fields of valley- and spintronics due to their property of forming valley-polarized excitons when illuminated by circularly polarized light. In TMD-heterostructures it was shown that these electron-hole pairs can scatter into valley-polarized interlayer exciton states, which exhibit long lifetimes and a twist-angle dependence. However, the question how to create a valley polarization of free charge carriers in these heterostructures after a valley selective optical excitation is unexplored, despite its relevance for opto-electronic devices. Here, we identify an interlayer transfer mechanism in twisted WSe2/MoSe2 heterobilayers that transfers the valley polarization from excitons in WSe2 to free charge carriers in MoSe2 with valley lifetimes of up to 12 ns. This mechanism is most efficient at large twist angles, whereas the valley lifetimes of free charge carriers are surprisingly short for small twist angles, despite the occurrence of interlayer excitons.
New publication: Probing Enhanced Electron-Phonon Coupling in Graphene by Infrared Resonance Raman Spectroscopy
Phys. Rev. Lett. 130, 256901 (2023) We report on resonance Raman spectroscopy measurements with excitation photon energy down to 1.16 eV on graphene, to study how low-energy carriers interact with lattice vibrations. Thanks to the excitation energy close to the Dirac point at K, we unveil a giant increase of the intensity ratio between the double-resonant 2D and 2D′ peaks with respect to that measured in graphite. Comparing with fully ab initio theoretical calculations, we conclude that the observation is explained by an enhanced, momentum-dependent coupling between electrons and Brillouin zone-boundary optical phonons. This finding applies to two-dimensional Dirac systems and has important consequences for the modeling of transport in graphene devices operating at room temperature.
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