24 \textmu m length spin relaxation length in boron nitride encapsulated bilayer graphene

We have performed spin and charge transport measurements in dual gated high mobility bilayer graphene encapsulated in hexagonal boron nitride. Our results show spin relaxation lengths $\lambda_s$ up to 13~\textmu m at room temperature with relaxation times $\tau_s$ of 2.5~ns. At 4~K, the diffusion coefficient rises up to 0.52~m$^2$/s, a value 5 times higher than the best achieved for graphene spin valves up to date. As a consequence, $\lambda_s$ rises up to 24~\textmu m with $\tau_s$ as high as 2.9~ns. We characterized 3 different samples and observed that the spin relaxation times increase with the device length. We explain our results using a model that accounts for the spin relaxation induced by the non-encapsulated outer regions.Comment: 5 pages and 4 figure

Having Fun in Learning Formal Specifications

There are many benefits in providing formal specifications for our software. However, teaching students to do this is not always easy as courses on formal methods are often experienced as dry by students. This paper presents a game called FormalZ that teachers can use to introduce some variation in their class. Students can have some fun in playing the game and, while doing so, also learn the basics of writing formal specifications in the form of pre- and post-conditions. Unlike existing software engineering themed education games such as Pex and Code Defenders, FormalZ takes the deep gamification approach where playing gets a more central role in order to generate more engagement. This short paper presents our work in progress: the first implementation of FormalZ along with the result of a preliminary users' evaluation. This implementation is functionally complete and tested, but the polishing of its user interface is still future work

On the incorporation of trivalent rare earth ions in II-VI semiconductor nanocrystals

\u3cp\u3eNanocrystalline ZnS and CdS samples have been synthesized in the presence of Eu\u3csup\u3e3+\u3c/sup\u3e and Tb\u3csup\u3e3+\u3c/sup\u3e using various techniques (precipitation in water, methanol, or toluene and inverse micelle techniques) which have been reported to yield ZnS or CdS nanoparticles doped with luminescent rare earth ions. Nanocrystalline particles with a typical diameter of 4 nm were formed. In some cases, particles were heated (up to 800 °C) which resulted in an increase of the particle diameter (> 20 nm). To study the incorporation of rare earth ions in the particles, luminescence spectra have been measured. Upon excitation in the semiconductor host lattice, no emission or weak emission is observed from the rare earth ions. The excitation spectra of the characteristic rare earth emissions show excitation lines corresponding to intraconfigurational 4f\u3csup\u3en\u3c/sup\u3e-4f\u3csup\u3en\u3c/sup\u3e transitions of the rare earth ions but not the semiconductor host lattice excitation band. The absence of a host lattice excitation band indicates that with the presently used synthesis techniques the rare earth ions are not incorporated in the nanocrystalline semiconductor particles but are probably adsorbed at the surface.\u3c/p\u3

Temperature dependence of the luminescence of nanocrystalline CdS/Mn2+

\u3cp\u3eThe temperature dependence of the luminescence properties of nanocrystalline CdS/Mn\u3csup\u3e2+\u3c/sup\u3e particles is investigated. In addition to an orange Mn\u3csup\u3e2+\u3c/sup\u3e emission around 585nm a red defect related emission around 700 nm is observed. The temperature quenching of both emissions is similar (T\u3csub\u3eq\u3c/sub\u3e ≈ 100 K). For the defect emission the reduction in the lifetime follows the temperature dependence of the intensity. For the Mn\u3csup\u3e2+\u3c/sup\u3e emission however, the intensity decreases more rapidly than the lifetime with increasing temperature. To explain these observations a model is proposed in which the Mn\u3csup\u3e2+\u3c/sup\u3e ions are excited via an intermediate state involving shallowly trapped (≈ 40 meV) charge carriers.\u3c/p\u3

Eighty-Eight Percent Directional Guiding of Spin Currents with 90 μm Relaxation Length in Bilayer Graphene Using Carrier Drift

Electrical control of spin signals and long distance spin transport are major requirements in the field of spin electronics. Here, we report the efficient guiding of spin currents at room temperature in high mobility hexagonal boron nitride encapsulated bilayer graphene using carrier drift. Our experiments, together with modeling, show that the spin relaxation length, that is 7.7 μm at zero bias, can be tuned from 0.6 to 90 μm when applying a DC current of ∓90 μA, respectively. Our results also show that we are able to direct spin currents to either side of a spin injection contact. Eighty-eight percent of the injected spins flows to the left when <i>I</i>_dc = −90 μA and eighty-two percent flows to the right when the drift current is reversed. These results show the potential of carrier drift for spin-based logic operations and devices

Temporal dynamics of the frequency non-degenerate transient photoluminescence enhancement observed following excitation of inter-configurational f→d transitions in CaF2:Yb2+

We present a detailed investigation of the dynamics of impurity-trapped excitons in CaF2:Yb2+using transient photoluminescence enhancement induced via a two-frequency, sequential excitation process employing a UV optical parametric amplifier (OPA) synchronized to an infrared free electron laser (FEL). The temporal behaviour is well approximated by a multi-level rate equation model as relaxation between excited states of the exciton as well as a small contribution from local lattice heating by the FEL which becomes evident due to the 40 cm−1splitting of the exciton excited states giving rise to the transient photoluminescence enhancement itself