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News 08.08.2017
Simulations on the Influence of Spatially Varying Spin Transport Parameters on the Measured Spin Lifetime in Graphene Non-Local Spin Valves

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Simulations on the Influence of Spatially Varying Spin Transport Parameters on the Measured Spin Lifetime in Graphene Non-Local Spin Valves


Physica Status Solidi B, 1700293(2017)
Spin transport properties of graphene non-local spin valve devices are typically determined from Hanle spin precession measurements by using a simplified solution of the one-dimensional Bloch-Torrey equation which assumes infinitely long transport channels and uniform spin transport parameter. We investigate the effects of a finite graphene size and explore the influence of spatially-varying transport parameters on the measured Hanle curves by finite element simulations. We assume enhanced spin dephasing in the contact-covered graphene areas with additional Fermi level pinning and explore the influence of non-magnetic reference electrodes which are not properly decoupled from graphene. In experiments, it is typically observed that the spin lifetime increases with increasing charge carrier density. None of our simulations can reproduce this trend indicating that this dependency originates from spin transport through graphene areas which are not covered by contacts. We find that the extracted spin lifetime might be overestimated in flakes which are shorter than the spin diffusion length. Moreover, contact-induced spin dephasing leads to an overall reduction of the extracted spin lifetime. Additionally, we show that non-magnetic reference electrodes may also influence the measured spin lifetime even though they are not part of the transport area under investigation.

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Kick-off for the Aachen Graphene & 2D-Materials Center


July 24th was the starting date for the Aachen Graphene & 2D-Materials Center, a joint competence center of RWTH Aachen University and AMO GmbH. The Aachen Graphene & 2D-Materials Center integrates the already ongoing activities of several research groups at RWTH Aachen University and AMO GmbH in the fields of physics, material science and electrical engineering with the primary goal to efficiently bridge the gap between fundamental science and applications.

The mission of the Aachen Graphene & 2D-Materials Center is to exploit the unique properties of graphene, two-dimensional (2D) materials and 2D-heterostructures from a fundamental and applied point of view. The activities of the Center will address the challenges of future technology including high-frequency electronics, flexible electronics, energy-efficient sensing, photonics as well as spintronics and valleytronics, for which graphene and related 2D-materials have proven to be a unique enabling platform. Therefore, the center brings together the complementary expertise of Aachen’s world leading research groups and puts the Aachen Graphene & 2D-Materials Center in a leading position in Germany and Europe.

The founding members are Prof. Christoph Stampfer (RWTH and spokesman of the center), Prof. Max Lemme (AMO and RWTH), Prof. Markus Morgenstern (RWTH), Prof. Renato Negra (RWTH) and Dr. Daniel Neumaier (AMO) who are also active in the EU Flagship Project Graphene. For further information please visit: www.graphene-center-aachen.de.

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SPICE YouTube Channel

To disseminate tutorials and talks of the "Spin Dynamics in the Dirac Systems Workshop" there is a YouTube channel containing all the workshop’s talks. Click here.

This interdisciplinary workshop offered a platform for the knowledge exchange between diverse novel condensed matter domains such as topological insulators and superconductors, Weyl physics, topological Josephson junctions, spintronics in graphene, spin valves, spin-logic devices, quantum magnetism, spin lattices, frustrated magnets, spin liquids, non-trivial spin states, etc.

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New publication: From Diffusive to Ballistic Transport in Etched Graphene Constrictions and Nanoribbons


Ann. Phys. 529, 1700082(2017)
Graphene nanoribbons and constrictions are envisaged as fundamental components of future carbon-based nanoelectronic and spintronic devices. At nanoscale, electronic effects in these devices depend heavily on the dimensions of the active channel and the nature of edges. Hence, controlling both these parameters is crucial to understand the physics in such systems. This review is about the recent progress in the fabrication of graphene nanoribbons and constrictions in terms of low temperature quantum transport. In particular, recent advancements using encapsulated graphene allowing for quantized conductance and future experiments towards exploring spin effects in these devices are presented. The influence of charge carrier inhomogeneity and the important length scales which play a crucial role for transport in high quality samples are also discussed.

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New publication: Dry transfer of CVD graphene using MoS2-based stamps


Phys. Status Solidi RRL 11, 1700136(2017)
Recently, a contamination-free dry transfer method for graphene grown by chemical vapor deposition (CVD) has been reported that allows to directly pick-up graphene from the copper growth substrate using a flake of hexagonal boron nitride (hBN), resulting in ultrahigh charge carrier mobility and low overall doping. Here, we report that not only hBN, but also flakes of molybdenum disulfide (MoS2) can be used to dry transfer graphene. This, on one hand, allows for the fabrication of complex van-der-Waals heterostructures using CVD graphene combined with different two-dimensional materials and, on the other hand, can be a route toward a scalable dry transfer of CVD graphene. The resulting heterostructures are studied using low temperature transport measurements revealing a strong charge carrier density dependence of the carrier mobilities (up to values of 12,000 cm2/(Vs)) and the residual charge carrier density fluctuations near the charge neutrality point when changing the carrier density in the MoS2 by applying a top gate voltage.

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“Lousberglauf” 2017

The participation of our Institute at the “Lousberglauf” 2017 was once again. Despite the suffocating heat, Michael (21:29), Isabell (29:46), Marc (23:25), Markus (23:41) and Alex (25.09) had a lot of fun and improved their performance compared to last year. Congratulations!

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JARA-Day 2017 -- Celebration of 10 years JARA

We actively contributed to the celebration of 10 years JARA. For more details see e.g. (both in german): Aachener Zeitung or "Pressemitteilungen" at Jara.org

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New publication: High mobility dry-transferred CVD bilayer graphene


Appl. Phys. Lett. 110, 263110 (2017)
We report on the fabrication and characterization of high-quality chemical vapor-deposited (CVD) bilayer graphene (BLG). In particular, we demonstrate that CVD-grown BLG can mechanically be detached from the copper foil by an hexagonal boron nitride (hBN) crystal after oxidation of the copper-to-BLG interface. Confocal Raman spectroscopy reveals an AB-stacking order of the BLG crystals and a high structural quality. From transport measurements on fully encapsulated hBN/BLG/hBN Hall bar devices we extract charge carrier mobilities up to 180,000 cm2/(Vs) at 2 K and up to 40,000 cm2/(Vs) at 300 K, outperforming state- of-the-art CVD bilayer graphene devices. Moreover, we show an on-off ratio of more than 10,000 and a band gap opening with values of up to 15 meV for a displacement field of 0.2 V/nm in such CVD grown BLG.

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