We report pressure-induced superconductivity in a single crystal of CaFe 2 As 2 . At atmospheric pressure, this material is antiferromagnetic below 170 K but under an applied pressure of 0.69 GPa becomes superconducting, with a transition temperature T c exceeding 10 K. The rate of T c suppression with applied magnetic field is −0.7 K T −1 , giving an extrapolated zero-temperature upper critical field of 10-14 T.

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Engagement in Massive Open Online Courses (MOOCs) is based on students who self-organize their participation according to their own goals and interests. Visual materials such as videos and discussion forums are basic ways of engaging students in MOOCs. Student achievement in MOOCs is typically measured using assessments distributed throughout the course. Although there is research on the basic forms of student's engagement and assessment in MOOCs, little is known about their effect on students' achievement in the form of students completing a MOOC. Using binomial logistic regression models, this paper addresses this gap in the literature by presenting the degree to which student engagement with videos and forum posts can predict students' probability of achievement in a MOOC. It also explores the extent to which participation behaviors and their intention to receive the course certification can be used to predict achievement in MOOCs. Using qualitative content analysis, this paper discusses the quality of the forum posts exchanged by participants in this MOOC. The findings from quantitative analysis support MOOC's pedagogical assumptions, showing that students' engagement in forums and with videos increases the probability of course achievement. It also shows that intention to certify plays a moderator effect on the number of videos watched, enhancing achievement in MOOCs. The findings from qualitative analysis reveal that most students' posts in forums display more information acquisition than critical thinking. Implications for practice suggest MOOC designers and MOOC instructors foster engagement in forums by implementing discussion prompts that foster interactions about deep meaning of concepts or application of concepts covered in the MOOC. In regard to videos, implications for practice suggest the creation of interactive videos that promote students' engagement and control such as inserting guiding questions and segmenting the video content. Future research comprising multiple MOOC cohorts is suggested to validate the empirical model presented in this study.

In this paper, we propose new algorithms to determine the antenna location for downlink distributed antenna systems (DASs) in single-cell and two-cell environments. We consider the composite fading channel which includes small and large scale fadings. First, for the single-cell DAS, we formulate the optimization problem of distributed antenna (DA) port locations by maximizing the lower bound of the expected signal to noise ratio (SNR). In comparison to the conventional algorithm based on the squared distance criterion which requires an iterative method, our problem generates a closed form solution. Next, for the two-cell DAS, we propose a gradient ascent algorithm which determines the optimum DA locations by maximizing the lower bound of the expected signal to leakage ratio (SLR). In our work, we consider selection transmission, maximal ratio transmission and zero-forcing beamforming (ZFBF) under sum power constraint and study equal gain transmission and scaled ZFBF under per-antenna power constraint. Simulation results show that our proposed algorithms based on both the SNR and the SLR criteria offer a capacity gain over the conventional centralized antenna systems.Index Terms-Distributed antenna systems, antenna placement.

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We investigate the relation of the critical current density (Jc) and the remarkably increased superconducting transition temperature (Tc) for the FeSe single crystals under pressures up to 2.43 GPa, where the Tc is increased by ~8 K/GPa. The critical current density corresponding to the free flux flow is monotonically enhanced by pressure which is due to the increase in Tc, whereas the depinning critical current density at which the vortex starts to move is more influenced by the pressure-induced magnetic state compared to the increase of Tc. Unlike other high-Tc superconductors, FeSe is not magnetic, but superconducting at ambient pressure. Above a critical pressure where magnetic state is induced and coexists with superconductivity, the depinning Jc abruptly increases even though the increase of the zero-resistivity Tc is negligible, directly indicating that the flux pinning property compared to the Tc enhancement is a more crucial factor for an achievement of a large Jc. In addition, the sharp increase in Jc in the coexisting superconducting phase of FeSe demonstrates that vortices can be effectively trapped by the competing antiferromagnetic order, even though its antagonistic nature against superconductivity is well documented. These results provide new guidance toward technological applications of high-temperature superconductors.