Nature Comm. 6, 8429 (2015)
Confocal Raman spectroscopy has emerged as a major, versatile workhorse for the non-invasive characterization of graphene. Although it is successfully used to determine the number of layers, the quality of edges, and the effects of strain, doping and disorder, the nature of the experimentally observed broadening of the most prominent Raman 2D line has remained unclear. Here we show that the observed 2D line width contains valuable information on strain variations in graphene on length scales far below the laser spot size, that is, on the nanometre-scale. This finding is highly relevant as it has been shown recently that such nanometre-scaled strain variations limit the carrier mobility in high-quality graphene devices. Consequently, the 2D line width is a good and easily accessible quantity for classifying the crystalline quality, nanometre-scale flatness as well as local electronic properties of graphene, all important for future scientific and industrial applications.

Physica Status Solidi B 252, 2461 (2015)
We report on devices based on graphene charge detectors (CDs) capacitively coupled to graphene and carbon nanotube quantum dots (QDs). We focus on back action effects of the CD on the probed QD. A strong influence of the bias voltage applied to the CD on the current through the QD is observed. Depending on the charge state of the QD, the current through the QD can either strongly increase or completely reverse as a response to the applied voltage on the CD. To describe the observed behavior, we employ two simple models based on single electron transport in QDs with asymmetrically broadened energy distributions of the source and the drain leads. The models successfully explain the back action effects. The extracted distribution broadening shows a linear dependency on the bias voltage applied to the CD. We discuss possible mechanisms mediating the energy transfer between the CD and QD and give an explanation for the origin of the observed asymmetry.

Our institute trip of the 2nd Institute of Physics on the 4th of September 2015 was a great success.

Christoph Stampfer is invited as "Young Scientists" to the World Economic Forum’s (WEF) Annual Meeting of the New Champions, Global Summit on Innovation, Science and Technology.

For more information please see: www.weforum.org/events/annual-meeting-new-champions-2015.

For the ERC press release please see: erc.europa.eu/media-and-events/events/annual-meeting-new-champions-2015.

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.