Nano Lett. 19, 4083(2019)
We present time-resolved Kerr rotation measurements, showing spin lifetimes of over 100 ns at room temperature in monolayer MoSe₂. These long lifetimes are accompanied by an intriguing temperature-dependence of the Kerr amplitude, which increases with temperature up to 50 K and then abruptly switches sign. Using ab initio simulations, we explain the latter behavior in terms of the intrinsic electron–phonon coupling and the activation of transitions to secondary valleys. The phonon-assisted scattering of the photoexcited electron–hole pairs prepares a valley spin polarization within the first few ps after laser excitation. The sign of the total valley magnetization, and thus the Kerr amplitude, switches as a function of temperature, as conduction and valence band states exhibit different phonon-mediated intervalley scattering rates. However, the electron–phonon scattering on the ps time scale does not provide an explanation for the long spin lifetimes. Hence, we deduce that the initial spin polarization must be transferred into spin states, which are protected from the intrinsic electron–phonon coupling, and are most likely resident charge carriers, which are not part of the itinerant valence or conduction band states.

Nat. Commun. 10, 2428 (2019)
Dirac fermion optics exploits the refraction of chiral fermions across optics-inspired Klein-tunneling barriers defined by high-transparency p-n junctions. We consider the corner reflector (CR) geometry introduced in optics or radars. We fabricate Dirac fermion CRs using bottom-gate-defined barriers in hBN-encapsulated graphene. By suppressing transmission upon multiple internal reflections, CRs are sensitive to minute phonon scattering rates. Here we report on doping-independent CR transmission in quantitative agreement with a simple scattering model including thermal phonon scattering. As a signature of CRs, we observe Fabry-Pérot oscillations at low temperature, consistent with single-path reflections. Finally, we demonstrate high-frequency operation which promotes CRs as fast phonon detectors. Our work establishes the relevance of Dirac fermion optics in graphene and opens a route for its implementation in topological Dirac matter.

Christoph Stampfer became an Editorial Fellow at SciPost, joining the Editorial College (Physics).

After several months of preparation our Institute was ready to line up at another big sports competition - the Düsseldorf Marathon 2019. Together with over 20000 other runners and the best weather conditions Markus Müller (1:35:59) and Benedikt Frohn (1:42:39) went for the 21.1km half Marathon distance while Michael Schmitz successfully finished the full 42.2km Marathon in 2:59:33. Gratulations!

In the early sunday morning the at freezing temperatures and snow fall, our colleagues Michael Schmitz and Samuel Möller took part in the Powerman Alsdorf Duathlon together with over 700 other athletes. Competing in the relay race they had to alternately run, bike and run 10KM/60KM/10KM. With splits of 36:42, 1:34:02 and 37:27 and a total time of 2:50:20 they distanced by far all other relay competitors and took the first place. Gratulations!

The phyphox team has been awarded the "Archimedes award" 2019 by the "Verband zur Förderung des MINT-Unterrichts" (MNU, loosely translated: association to forward STEM education). The award entails 4,000€ sponsored by the Westermann group. It is awarded yearly, alternating in the fields of mathematics and physics, to decorate innovative concepts and teaching methods as well as particularly commited teachers.

The app allows students to use the sensors of their smartphones to conduct physics experiments in physics education. It has been developed at the 2nd Institute of Physics at the RWTH Aachen University under the direction of Prof. Christoph Stampfer and Dr. Sebastian Staacks statt and it is being continuously refined in cooperation with the 1st Institute of Physics under Prof. Heidrun Heinke and a team of PhD students.

The award does not only mention the app itself, but the MNU emphasizes the value of the accompanying material like the demonstration videos. With more than 600,000 installations worldwide phyphox has been establish in many schools since its first release in September 2016.

Hendrik Bluhm, Thomas Brückel, Markus Morgenstern, Gero Plessen, Christoph Stampfer (2019). Electrons in Solids: Mesoscopics, Photonics, Quantum Computing, Correlations, Topology (pp. I–IV). Berlin, Boston: De Gruyter. https://doi.org/10.1515/9783110438321

As a continuation of classical condensed matter physics texts, this graduate textbook on “Electrons in Solids” introduces advanced topics of correlated electron systems, mesoscopic transport, low-dimensional electron systems, unconventional superconductivity and topological insulators. These aspects are complemented by theoretical aspects of quantum information theory as well as solid state implementations. The book covers modern many body theory and effects arising from correlated electron systems and closes a gap between basic condensed matter textbooks and the forefront of research.