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News 15.12.2021
New publication: Probing Two-Electron Multiplets in Bilayer Graphene Quantum Dots

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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.

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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.

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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.

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International standard for measuring the flatness of graphene has been published

Jointly with the Standardization Commission of the Graphene Flagship our institute (big thanks to Christoph Neumann and Jens Sonntag) have pushed through a new IEC standard for assessing the strain uniformity of single-layer graphene using Raman spectroscopy.

For more details see: IEC TS 62607-6-6 “Nanomanufacturing – Key control characteristics – Part 6-6: Graphene – Strain uniformity: Raman spectroscopy” https://webstore.iec.ch/publication/34162.

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New publication: Spin-valley coupling in single-electron bilayer graphene quantum dots


Nat. Commun. 12, 5250(2021)
Understanding how the electron spin is coupled to orbital degrees of freedom, such as a valley degree of freedom in solid-state systems, is central to applications in spin-based electronics and quantum computation. Recent developments in the preparation of electrostatically-confined quantum dots in gapped bilayer graphene (BLG) enable to study the low-energy single-electron spectra in BLG quantum dots, which is crucial for potential spin and spin-valley qubit operations. Here, we present the observation of the spin-valley coupling in bilayer graphene quantum dots in the single-electron regime. By making use of highly-tunable double quantum dot devices we achieve an energy resolution allowing us to resolve the lifting of the fourfold spin and valley degeneracy by a Kane-Mele type spin-orbit coupling of ≈ 60 μeV. Furthermore, we find an upper limit of a potentially disorder-induced mixing of the K and K′ states below 20 μeV.

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New publication: Dynamics of 2D material membranes


2D Materials 8, 042001 (2021)
The dynamics of suspended two-dimensional (2D) materials has received increasing attention during the last decade, yielding new techniques to study and interpret the physics that governs the motion of atomically thin layers. This has led to insights into the role of thermodynamic and nonlinear effects as well as the mechanisms that govern dissipation and stiffness in these resonators. In this review, we present the current state-of-the-art in the experimental study of the dynamics of 2D membranes. The focus will be both on the experimental measurement techniques and on the interpretation of the physical phenomena exhibited by atomically thin membranes in the linear and nonlinear regimes. We will show that resonant 2D membranes have emerged both as sensitive probes of condensed matter physics in ultrathin layers, and as sensitive elements to monitor small external forces or other changes in the environment. New directions for utilizing suspended 2D membranes for material characterization, thermal transport, and gas interactions will be discussed and we conclude by outlining the challenges and opportunities in this upcoming field.

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New publication: Upstream modes and antidots poison graphene quantum Hall effect


Nat. Commun.12, 4265 (2021)
The quantum Hall effect is the seminal example of topological protection, as charge carriers are transmitted through one-dimensional edge channels where backscattering is prohibited. Graphene has made its marks as an exceptional platform to reveal new facets of this remarkable property. However, in conventional Hall bar geometries, topological protection of graphene edge channels is found regrettably less robust than in high mobility semi-conductors. Here, we explore graphene quantum Hall regime at the local scale, using a scanning gate microscope. We reveal the detrimental influence of antidots along the graphene edges, mediating backscattering towards upstream edge channels, hence triggering topological breakdown. Combined with simulations, our experimental results provide further insights into graphene quantum Hall channels vulnerability. In turn, this may ease future developments towards precise manipulation of topologically protected edge channels hosted in various types of two-dimensional crystals.

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Christoph Stampfer becomes member of the Steering Committee of the profile area MatSE

Christoph Stampfer is appointed to the Steering Committee of the MatSE (Material science and engineering) profile area at RWTH Aachen University.

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