Physica Status Solidi B (online)
We present spin transport studies on bi- and trilayer graphene non-local spin-valves which have been fabricated by a bottomup fabrication method. By this technique, spin injection electrodes are first deposited onto Si⁺⁺/SiO₂ substrates with subsequent mechanical transfer of a graphene/hBN heterostructure. We showed previously that this technique allows for nanosecond spin lifetimes at room temperature combined with carrier mobilities which exceed 20,000 cm₂Vs)−1. Despite strongly enhanced spin and charge transport properties, the MgO injection barriers in these devices exhibit conducting pinholes which still limit the measured spin lifetimes.We demonstrate that these pinholes can be partially diminished by an oxygen treatment of a trilayer graphene device which is seen by a strong increase of the contact resistance area products of the Co/MgO electrodes. At the same time, the spin lifetime increases from 1 to 2 ns.We believe that the pinholes partially result from the directional growth in molecular beam epitaxy. For a second set of devices, we therefore used atomic layer deposition of Al₂O₃ which offers the possibility to isotropically deposit more homogeneous barriers. While the contacts of the as-fabricated bilayer graphene devices are non-conductive, we can partially break the oxide barriers by voltage pulses. Thereafter, the devices also exhibit nanosecond spin lifetimes.

Sowmya Somanchi wins the 2015 „Falling Walls Lab Jülich“ competition. Under the slogan "Great Minds - 3 Minutes - 1 Day" 15 young scientsits did their best to convince the jury by a 3 minunts talk on their own research topic. Sowmya is now invited to the "Falling Walls Conference" on the 8th und 9th November in Berlin.

For more information please see: Pressemitteilung Forschungszentrum Jülich.

Alexander Braun (1.v.l.), Sowmya Somanchi (2.v.l.) und Moritz Nabel (rechts) fahren zum Finale und zur "Falling Walls Conference" nach Berlin. Prof. Wolfgang Marquardt (2.v.r.), Vorstandsvorsitzender des Forschungszentrums, war Mitglied der Jury des ersten "Falling Walls Lab Jülich". Copyright: Forschungszentrum Jülich.

Phys. Rev. B 92, 085417 (2015)
Freestanding Bi₂Se₃ single-crystal flakes of variable thicknesses are grown using a catalyst-free vapor-solid synthesis and are subsequently transferred onto a clean Si⁺⁺/SiO₂ substrate where the flakes are contacted in Hall bar geometry. Low-temperature magnetoresistance measurements are presented which show a linear magnetoresistance for high magnetic fields and weak antilocalization (WAL) at low fields. Despite an overall strong charge-carrier tunability for thinner devices, we find that electron transport is dominated by bulk contributions for all devices. Phase-coherence lengths l_ϕ as extracted from WAL measurements increase linearly with increasing electron density exceeding 1μm at 1.7 K. Although l_ϕ is in qualitative agreement with electron-electron interaction-induced dephasing, we find that spin-flip scattering processes limit l_ϕ at low temperatures.

Phys. Rev. B 92, 075417(2015)
We present a numerical study on the uniformity of the pseudomagnetic field in graphene as a function of the relative orientation between the graphene lattice and straining directions. We calculated the deformation of a regular micron-sized graphene hexagon by symmetrically displacing three of its edges. We found that the pseudomagnetic field is strongest if the strain is applied perpendicular to the armchair direction of graphene. For a hexagon with a side length of 1μm, the pseudomagnetic field has a maximum of 1.2 T for an applied strain of 3.5% and it is uniform (variance <1%) within a circle with a diameter of ∼520 nm. This diameter is on the order of the typical diameter of the laser spot in a state-of-the-art confocal Raman spectroscopy setup, which suggests that observing the pseudomagnetic field in measurements of shifted magnetophonon resonance is feasible.

Science Advances 1, e1500222 (2015)
Graphene research has prospered impressively in the past few years, and promising applications such as highfrequency transistors, magnetic field sensors, and flexible optoelectronics are just waiting for a scalable and costefficient fabrication technology to produce high-mobility graphene. Although significant progress has been made in chemical vapor deposition (CVD) and epitaxial growth of graphene, the carrier mobility obtained with these techniques is still significantly lower than what is achieved using exfoliated graphene. We show that the quality of CVD-grown graphene depends critically on the used transfer process, and we report on an advanced transfer technique that allows both reusing the copper substrate of the CVD growth and making devices with mobilities as high as 350,000 cm² V^–1 s^–1, thus rivaling exfoliated graphene.