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News 27.07.2022
New publication: Transport spectroscopy of ultraclean tunable band gaps in bilayer graphene

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27.07.2022

New publication: Transport spectroscopy of ultraclean tunable band gaps in bilayer graphene

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Adv. Electron.Mater. 8, 2200510 (2022)
The importance of controlling both the charge carrier density and the band gap of a semiconductor cannot be overstated, as it opens the doors to a wide range of applications, including, for example, highly-tunable transistors, photodetectors, and lasers. Bernal-stacked bilayer graphene is a unique van-der-Waals material that allows tuning of the band gap by an out-of-plane electric field. Although the first evidence of the tunable gap is already found 10 years ago, it took until recent to fabricate sufficiently clean heterostructures where the electrically induced gap can be used to fully suppress transport or confine charge carriers. Here, a detailed study of the tunable band gap in gated bilayer graphene characterized by temperature-activated transport and finite-bias spectroscopy measurements is presented. The latter method allows comparing different gate materials and device technologies, which directly affects the disorder potential in bilayer graphene. It is shown that graphite-gated bilayer graphene exhibits extremely low disorder and as good as no subgap states resulting in ultraclean tunable band gaps up to 120 meV. The size of the band gaps are in good agreement with theory and allow complete current suppression making a wide range of semiconductor applications possible.

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18.07.2022

New publication: Charge-Induced Artifacts in Nonlocal Spin-Transport Measurements: How to Prevent Spurious Voltage Signals

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Phys. Rev. Appl. 18, 014028 (2022)
To conduct spin-sensitive transport measurements, a nonlocal device geometry is often used to avoid spurious voltages that are caused by the flow of charges. However, in the vast majority of reported nonlocal spin-valve, Hanle spin precession or spin Hall measurements, background signals have been observed that are not related to spins. We discuss seven different types of these charge-induced signals and explain how these artifacts can result in erroneous or misleading conclusions when falsely attributed to spin transport. The charge-driven signals can be divided into two groups: signals that are inherent to the device structure and/or the measurement setup and signals that depend on a common-mode voltage. We designed and built a voltage-controlled current source that significantly diminishes all spurious voltage signals of the latter group in both dc and ac measurements by creating a virtual ground within the nonlocal detection circuit. This is especially important for lock-in-based measurement techniques, where a common-mode voltage can create a phase-shifted, frequency-dependent signal with an amplitude several orders of magnitude larger than the actual spin signal. Measurements performed on graphene-based nonlocal spin-valve devices demonstrate how all spurious voltage signals that are caused by a common-mode voltage can be completely suppressed by such a current source.

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17.07.2022

New publication: Raman imaging of twist angle variations in twisted bilayer graphene at intermediate angles

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2D Materials 9, 045009 (2022)
Van der Waals layered materials with well-defined twist angles between the crystal lattices of individual layers have attracted increasing attention due to the emergence of unexpected material properties. As many properties critically depend on the exact twist angle and its spatial homogeneity, there is a need for a fast and non-invasive characterization technique of the local twist angle, to be applied preferably right after stacking. We demonstrate that confocal Raman spectroscopy can be utilized to spatially map the twist angle in stacked bilayer graphene for angles between 6.5° and 8° when using a green excitation laser. The twist angles can directly be extracted from the moiré superlattice-activated Raman scattering process of the transverse acoustic (TA) phonon mode. Furthermore, we show that the width of the TA Raman peak contains valuable information on spatial twist angle variations on length scales below the laser spot size of ∼500 nm.

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12.07.2022

Poster prize for Zachary Winter at the "Graphene 2022""

Zachary Winter won one of poster prizes at the "Graphene 2022" in Aachen with his poster on "Holistic approach to scalable 2D material processing from optimized CVD graphene catalysts to dry-transfer". Congratulations!

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11.07.2022

Graphene 2022 in Aachen

Our group was fairly present at the Graphene2022 conference in Aachen. Many interesting discussions! Not all of them made it to the group photo. Bernd Beschoten, Annika Kurzmann and Zach Winter were also there.

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30.06.2022

New publication: Spin relaxation in a single-electron graphene quantum dot

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Nat. Commun. 13, 3637 (2022)
The relaxation time of a single-electron spin is an important parameter for solid-state spin qubits, as it directly limits the lifetime of the encoded information. Thanks to the low spin-orbit interaction and low hyperfine coupling, graphene and bilayer graphene (BLG) have long been considered promising platforms for spin qubits. Only recently, it has become possible to control single-electrons in BLG quantum dots (QDs) and to understand their spin-valley texture, while the relaxation dynamics have remained mostly unexplored. Here, we report spin relaxation times (T1) of single-electron states in BLG QDs. Using pulsed-gate spectroscopy, we extract relaxation times exceeding 200 μs at a magnetic field of 1.9 T. The T1 values show a strong dependence on the spin splitting, promising even longer T1 at lower magnetic fields, where our measurements are limited by the signal-to-noise ratio. The relaxation times are more than two orders of magnitude larger than those previously reported for carbon-based QDs, suggesting that graphene is a potentially promising host material for scalable spin qubits.

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30.06.2022

New publication: Collaborative smartphone experiments for large audiences with phyphox

We present methods to implement collaborative experimentation with smartphone sensors for larger audiences ranging from typical university undergraduate courses of hundreds of students in a lecture hall to world-wide outreaches on the Internet. These methods are based on the app 'phyphox', which is being developed by the authors, and encompass simple data collection via web forms as well as a new network interface for 'phyphox', allowing to collect real-time experiment data from an audience on-site or easy data submission for remote participants. Examples are given with practical considerations derived from first implementations of this method in a lecture hall with 350 undergraduate students as well as a global experiment to determine the Earth's axial tilt with smartphones.

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23.06.2022

New publication: CVD Bilayer Graphene Spin Valves with 26 μm Spin Diffusion Length at Room Temperature

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Nano Lett. 22, 4949 (2022)
We present inverted spin-valve devices fabricated from CVD-grown bilayer graphene (BLG) that show more than a doubling in device performance at room temperature compared to state-of-the-art bilayer graphene spin valves. This is made possible by a polydimethylsiloxane droplet-assisted full-dry transfer technique that compensates for previous process drawbacks in device fabrication. Gate dependent Hanle measurements reveal spin lifetimes of up to 5.8 ns and a spin diffusion length of up to 26 μm at room temperature combined with a charge carrier mobility of about 24 000 cm2(Vs)−1 for the best device. Our results demonstrate that CVD-grown BLG shows equally good room temperature spin transport properties as both CVD-grown single-layer graphene and even exfoliated single-layer graphene.

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