Materials Design using Correlated Oxides: Optical Properties of Vanadium Dioxide

Materials with strong electronic Coulomb interactions play an increasing role in modern materials applications. "Thermochromic" systems, which exhibit thermally induced changes in their optical response, provide a particularly interesting case. The optical switching associated with the metal-insulator transition of vanadium dioxide (VO2), for example, has been proposed for use in "intelligent" windows, which selectively filter radiative heat in hot weather conditions. In this work, we develop the theoretical tools for describing such a behavior. Using a novel scheme for the calculation of the optical conductivity of correlated materials, we obtain quantitative agreement with experiments for both phases of VO2. On the example of an optimized energy-saving window setup, we further demonstrate that theoretical materials design has now come into reach, even for the particularly challenging class of correlated electron systems.Comment: 4+x pages, 2 figure

Voltage-gating and assembly of split Kv10.1 channels

Voltage-gated ion channels allow ions to pass cell membrane upon changes of transmembrane electrical potential. Conformational changes in the voltage-sensing domain of the channel (VSD) are assumed to be transmitted to the pore domain (PD) through an alpha-helical linker between them (S4-S5 linker). We have previously shown that expression of VSD and PD as separate fragments results in functional Kv10.1 channels that retain voltage-dependence. Here we used such ‘split’ channels to investigate functional interactions between VSD and PD. We found that their electrophysiological properties greatly depend on where the S4-S5 linker is interrupted. Remarkably, wild-type-like channel behavior could be fully or largely restored by mutations of crucial linker amino acids, indicating that precise functional interactions between VSD and PD remain when they are not covalently bound. Voltage-Clamp Fluorometry measurements revealed that VSD motion is alerted in specific split channels, but these changes were subtler. Finally, the increased separation between VSD activation and channel opening in the split channel carrying a large deletion in the S4-S5 linker, as well as the failure of the PD expressed alone to give currents, suggest that the role of the VSD in the is to open the channel pore and prevent it from closing

The quantum Heisenberg antiferromagnet on the Sierpinski Gasket: An exact diagonalization study

We present an exact diagonalization study of the quantum Heisenberg antiferromagnet on the fractal Sierpinski gasket for spin quantum numbers s=1/2,s=1 and s=3/2. Since the fractal dimension of the Sierpinski gasket is between one and two we compare the results with corresponding data of one- and two-dimensional systems. By analyzing the ground-state energy, the low-lying spectrum, the spin-spin correlation and the low-temperature thermodynamics we find arguments, that the Heisenberg antiferromagnet on the Sierpinski gasket is probably disordered not only in the extreme quantum case s=1/2 but also for s=1 and s=3/2. Moreover, in contrast to the one-dimensional chain we do not find a distinct behavior between the half-integer and integer-spin Heisenberg models on the Sierpinski gasket. We conclude that magnetic disorder may appear due to the interplay of frustration and strong quantum fluctuations in this spin system with spatial dimension between one and two.Comment: 12 pages (LaTeX), 7 figures, 3 tables, to appear in Physica

Probabilistic Approach to Structural Change Prediction in Evolving Social Networks

We propose a predictive model of structural changes in elementary subgraphs of social network based on Mixture of Markov Chains. The model is trained and verified on a dataset from a large corporate social network analyzed in short, one day-long time windows, and reveals distinctive patterns of evolution of connections on the level of local network topology. We argue that the network investigated in such short timescales is highly dynamic and therefore immune to classic methods of link prediction and structural analysis, and show that in the case of complex networks, the dynamic subgraph mining may lead to better prediction accuracy. The experiments were carried out on the logs from the Wroclaw University of Technology mail server

Designing episode content for Design Squad, a new educational engineering children's television program : the human powered water pump as a design challenge

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2006.Includes bibliographical references (p. 53).In recent years, problems have emerged in the realm of engineering and engineering education in the United States. Technology literacy is low, there are insufficient numbers of engineering students, and there are misconceptions surrounding the engineering profession. To remedy these problems, WGBH Boston and MIT have created a reality-style engineering-based television program for 9- to 13-year-old children, entitled Design Squad. One episode of the show will challenge the 8 child contestants to build a human-powered waterslide pump, to be used at a community swimming pool. 'Two potential design solutions are proposed for the design challenge: a ball-and-chain pump and a positive-displacement plunger pump. The design process of each solution and an evaluation of each solution's feasibility are presented. Criteria for a successful episode of the show are discussed in relation to the challenge. Prototype experimentation and analysis suggest that the human-powered waterslide challenge will invoke an engaging episode of Design Squad.by Joel A. Sadder [and] Mika A. Tomczak.S.B

Investigation of quasi-periodic varaiations in hard X-rays of solar flares

The aim of the present paper is to use quasi-periodic oscillations in hard X-rays (HXRs) of solar flares as a diagnostic tool for investigation of impulsive electron acceleration. We have selected a number of flares which showed quasi-periodic oscillations in hard X-rays and their loop-top sources could be easily recognized in HXR images. We have considered MHD standing waves to explain the observed HXR oscillations. We interpret these HXR oscillations as being due to oscillations of magnetic traps within cusp-like magnetic structures. This is confirmed by a good correlation between periods of the oscillations and the sizes of the loop-top sources. We argue that a model of oscillating magnetic traps is adequate to explain the observations. During the compressions of a trap particles are accelerated, but during its expansions plasma, coming from chromospheric evaporation, fills the trap, which explains the large number of electrons being accelerated during a sequence of strong impulses. The advantage of our model of oscillating magnetic traps is that it can explain both the impulses of electron acceleration and quasi-periodicity of their distribution in time.Comment: 21 pages, 11 figures, 3 tables, submitted to Solar Physic