Nano Letters 24, 1867 (2024)
Few-layer graphene possesses low-energy carriers that behave as massive Fermions, exhibiting intriguing properties in both transport and light scattering experiments. Lowering the excitation energy of resonance Raman spectroscopy down to 1.17 eV, we target these massive quasiparticles in the split bands close to the K point. The low excitation energy weakens some of the Raman processes that are resonant in the visible, and induces a clearer frequency-separation of the substructures of the resonance 2D peak in bi- and trilayer samples. We follow the excitation-energy dependence of the intensity of each substructure, and comparing experimental measurements on bilayer graphene with ab initio theoretical calculations, we trace back such modifications on the joint effects of probing the electronic dispersion close to the band splitting and enhancement of electron–phonon matrix elements.

On 16.01.2024, Michael Schmitz successfully defended his PhD thesis. Congratulations to Dr. Michael Schmitz on this commendable accomplishment!

Phys. Rev. Research 6, L012003 (2024)
Motivated by the recent experimental realization of ABCB stacked tetralayer graphene [Wirth et al., ACS Nano 16, 16617 (2022)], we study correlated phenomena in moiré-less graphene tetralayers for realistic interaction profiles using an orbital resolved random phase approximation approach. We demonstrate that magnetic fluctuations originating from local interactions are crucial close to the van Hove singularities on the electron- and hole-doped side promoting layer selective ferrimagnetic states. Spin fluctuations around these magnetic states enhance unconventional spin-triplet, valley-singlet superconductivity with f-wave symmetry due to intervalley scattering. Charge fluctuations arising from long range Coulomb interactions promote doubly degenerate p-wave superconductivity close to the van Hove singularities. At the conduction band edge of ABCB graphene, we find that both spin and charge fluctuations drive f-wave superconductivity. Our analysis suggests a strong competition between superconducting states emerging from long- and short-ranged Coulomb interactions and thus stresses the importance of microscopically derived interaction profiles to make reliable predictions for the origin of superconductivity in graphene-based heterostructures.

Adv. Funct. Mat. 2312980 (2024)
Resistive switching devices based on metal oxides are candidates for nonvolatile memory storage. They often rely on the valence change mechanism, the field-induced movement of donor ions leading to nanoscale conductive paths in filamentary-type devices. Devices usually consist of a transition metal oxide like Ta₂O₅ sandwiched between two metal electrodes. Critical parameters of the devices, such as cycle-to-cycle variability, Roff/Ron ratio, and endurance depend on the morphology and composition of the filaments. However, investigating filaments on the nanoscale is cumbersome, and commonly applied techniques such as conductive atomic force or transmission electron microscopy require delaminating the metal top electrode, inhibiting in operando investigations over many switching cycles. Here, the authors use infrared scattering-type scanning near-field optical microscopy (s-SNOM) to investigate resistive switching in Ta₂O₅ films with a graphene top electrode in operando and reveal individual filaments on the device level. By selecting an appropriate illumination frequency, the authors can trace the evolution of filaments and the joule heating-induced retraction of the top electrode until device failure. s-SNOM promises a deeper understanding of resistive switching devices’ microscopic switching behavior and applies to a wide range of resistive switching oxides, such as HfO₂, SrTiO₃, and SiO₂.

As the festive season approaches, we at the 2D Materials and Quantum Devices Group would like to extend our warmest wishes to all for a Merry Christmas and a Happy New Year.

Meas. Sci. Technol. 24, 035501 (2023)
Optical micro-spectroscopy is an invaluable tool for studying and characterizing samples ranging from classical semiconductors to low-dimensional materials and heterostructures. To date, most implementations are based on point-scanning techniques, which are flexible and reliable, but slow. Here, we describe a setup for highly parallel acquisition of hyperspectral reflection and photoluminescence (PL) microscope images using a push-broom technique. Spatial as well as spectral distortions are characterized and their digital corrections are presented. We demonstrate close- to diffraction-limited spatial imaging performance and a spectral resolution limited by the spectrograph. The capabilities of the setup are demonstrated by recording a hyperspectral PL map of a MoSe₂–WSe₂ lateral heterostructure, grown by chemical vapor deposition (CVD), from which we extract the luminescence energies, intensities and peak widths across the interface.

Nat. Commun. 14, 7911 (2023)
The coherent dynamics of a quantum mechanical two-level system passing through an anti-crossing of two energy levels can give rise to Landau-Zener-Stückelberg-Majorana (LZSM) interference. LZSM interference spectroscopy has proven to be a fruitful tool to investigate charge noise and charge decoherence in semiconductor quantum dots (QDs). Recently, bilayer graphene has developed as a promising platform to host highly tunable QDs potentially useful for hosting spin and valley qubits. So far, in this system no coherent oscillations have been observed and little is known about charge noise in this material. Here, we report coherent charge oscillations and T₂* charge decoherence times in a bilayer graphene double QD. The charge decoherence times are measured independently using LZSM interference and photon assisted tunneling. Both techniques yield T₂* average values in the range of 400–500 ps. The observation of charge coherence allows to study the origin and spectral distribution of charge noise in future experiments.

At a ceremony, 18 students and doctoral candidates were honoured with the Friedrich Wilhelm Prize 2023 for their outstanding achievements. Among them were nine Master's students and nine doctoral students, including Luca Banszerus. Congratulations!

The Friedrich Wilhelm Prize 2023 was awarded to Lukas Grunwald M. Sc., Michel Virgil Heinz M. Sc., Dominik Steden M. Sc., Lukas Frenzel M. Sc., Nico Dirkes M. Sc., Mathias Krämer M. Sc., Abdullah Tokmak M. Sc., Devi Janani Ramesh M. Sc, Natalja Stojanović M. Sc, Dr rer. nat. Luca Felix Banszerus, Dr Felix Martin, Dr Abedulgader Baktheer, Dr Lukas Konstantin Willi Berger, Dr Jannik Burre, Dr rer. pol. Christina Diana Florence Dienhart, Dr rer. pol. Nicole Janine Hartwich, Dr rer. medic. Cláudia Régio Brambilla and Dr rer. Medic. Yaoxian Xu.

Support for research and teaching

The Friedrich Wilhelm Prizes are awarded by the foundation of the same name, which was established in 1865 by the legal predecessor of today's Aachener und Münchener Beteiligungsgesellschaft. Its main aim is to promote research and teaching and to support students and academics at RWTH Aachen University. The name of the foundation goes back to the Prussian Crown Prince and later Emperor Friedrich Wilhelm III. In 1858, he received a donation of 5,000 thalers from the Aachener und Münchener Feuerversicherungs-Gesellschaft for the establishment of a polytechnic institute in the Rhine province. This donation formed the basis for the later Friedrich Wilhelm Foundation, which in turn laid the foundation for the RWTH Aachen University.