New publication: Dry-transferred CVD graphene for inverted spin valve devices
Appl. Phys. Lett. 111, 152402 (2017) Integrating high-mobility graphene grown by chemical vapor deposition (CVD) into spin transport devices is one of the key tasks in graphene spintronics. We use a van der Waals pick-up technique to transfer CVD graphene by hexagonal boron nitride (hBN) from the copper growth substrate onto predefined Co/MgO electrodes to build inverted spin valve devices. Two approaches are presented: (i) a process where the CVD-graphene/hBN stack is first patterned into a bar and then transferred by a second larger hBN crystal onto spin valve electrodes and (ii) a direct transfer of a CVD-graphene/hBN stack. We report record high spin lifetimes in CVD graphene of up to 1.75 ns at room temperature. Overall, the performances of our devices are comparable to devices fabricated from exfoliated graphene also revealing nanosecond spin lifetimes. We expect that our dry transfer methods pave the way towards more advanced device geometries not only for spintronic applications but also for CVD-graphene-based nanoelectronic devices in general where patterning of the CVD graphene is required prior to the assembly of final van der Waals heterostructures.
New publication: High quality factor graphene-based 2D heterostructure mechanical resonator
Nano Lett. 17, 5950 (2017) Ultralight mechanical resonators based on low-dimensional materials are well suited as exceptional transducers of minuscule forces or mass changes. However, the low dimensionality also provides a challenge to minimize resistive losses and heating. Here, we report on a novel approach that aims to combine different 2D materials to tackle this challenge. We fabricated a heterostructure mechanical resonator consisting of few layers of niobium diselenide (NbSe2) encapsulated by two graphene sheets. The hybrid membrane shows high quality factors up to 245'000 at low temperatures, comparable to the best few-layer graphene mechanical resonators. In contrast to few-layer graphene resonators, the device shows reduced electrical losses attributed to the lower resistivity of the NbSe2 layer. The peculiar low temperature dependence of the intrinsic quality factor points to dissipation over two-level systems which in turn relax over the electronic system. Our high sensitivity readout is enabled by coupling the membrane to a superconducting cavity which allows for the integration of the hybrid mechanical resonator as a sensitive and low loss transducer in future quantum circuits.
Recent press highlights about the newly established Aachen Graphene & 2D Materials Centre
Neue Publikation: Die akustische Stoppuhr in phyphox
MNU Journal 5, 322-327 (2017)
Die kostenlose App phyphox (Android und iOS) bietet viele neuartige Funktionen, um Physikexperimente mithilfe der gängigen Smartphone-Sensoren durchzuführen. Ein besonderes Werkzeug stellt hier die akustische Stoppuhr dar, welche für Schüler/innen eine intuitive Methode bietet, die Schallgeschwindigkeit zu bestimmen oder Zeitmessungen bei Fallexperimenten durchzuführen.
New publication: Establishment of a biophysical model to optimize endoscopic targeting of magnetic nanoparticles for cancer treatment
Int. J. Nanomedicine 12, 5933(2017) Superparamagnetic iron oxide nanoparticles (SPION) may be used for local tumor treatment by coupling them to a drug and accumulating them locally with magnetic field traps, that is, a combination of permanent magnets and coils. Thereafter, an alternating magnetic field generates heat which may be used to release the thermosensitively bound drug and for hyperthermia. Until today, only superficial tumors can be treated with this method. Our aim was to transfer this method into an endoscopic setting to also reach the majority of tumors located inside the body. To find the ideal endoscopic magnetic field trap, which accumulates the most SPION, we first developed a biophysical model considering anatomical as well as physical conditions. Entities of choice were esophageal and prostate cancer. The magnetic susceptibilities of different porcine and rat tissues were measured with a superconducting quantum interference device. All tissues showed diamagnetic behavior. The evaluation of clinical data (computed tomography scan, endosonography, surgical reports, pathological evaluation) of patients gave insight into the topographical relationship between the tumor and its surroundings. Both were used to establish the biophysical model of the tumors and their surroundings, closely mirroring the clinical situation, in which we could virtually design, place and evaluate different electromagnetic coil configurations to find optimized magnetic field traps for each tumor entity. By simulation, we could show that the efficiency of the magnetic field traps can be enhanced by 38-fold for prostate and 8-fold for esophageal cancer. Therefore, our approach of endoscopic targeting is an improvement of the magnetic drug-targeting setups for SPION tumor therapy as it holds the possibility of reaching tumors inside the body in a minimal-invasive way. Future animal experiments must prove these findings in vivo.
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.
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.
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
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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