A recent experiment reports a non-local spin-signal that shows oscillatory behavior as a function of gate voltage when the contacts are magnetized along the direction of current flow, but not when they are magnetized perpendicular to the current, in agreement with the predictions from a simple theory. In this paper we first present a straightforward extension of this theory to include the angular spectrum of electrons and the extended injecting and detecting contacts. The results are in good agreement with those from a non-equilibrium Green function (NEGF)-based model with contact parameters adjusted to fit the experimental contact conductances. They also describe certain aspects of the experiment fairly well, but other aspects deserve further investigation.

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Identification of students’ difficulties is an important part of designing a quality learning. This research used survey method that aims to identify the difficulties experienced by students in understanding the concept of work and energy. The research subjects consisted of 142 undergraduate students in physics education (46 first-year students, 39 second-year students, and 57 third-year students). Data collection was carried out from 2018-2019. There are 10 multiple choices questions that are used to uncover the difficulties experienced by students. Based on the results obtained it can be concluded that many students still have difficulty in understanding the concepts of work and energy. The percentage of students who answered correctly are 20.42% in concept of work as a result of dot product multiplication; 11.26% in the work-energy theorem; 38.03% in the concept of spring potential energy, and 22.54% in the conservation law of mechanical energy.

Abstract. The quantum Hall effect in Graphene nano-ribbons (GNR) is investigated with the non-equilibriumGreen's function (NEGF) based quantum transport model in the ballistic regime. The nearest neighbor tightbinding model based on p z orbital constructs the device Hamiltonian. GNRs of different edge geometries (Zigzag and Armchair) are considered. The magnetic field is included in both the channels and contact through Peierls substitution. Efficient algorithms for calculating the surface Green function are used to save computation time while simulating realistically large dimensions comparable to those used in experiments. Hall resistance calculations exactly reproduce the quantum Hall plateaus observed in the experiments. Use of large dimensions in the simulation is crucial in order to capture the quantum Hall effect in magnetic fields within experimentally relevant 10-20T.