Superconductivity in carbon nanotubes coupled to transition metal atoms

The electronic structures of zig-zag and arm-chair single-walled carbon nanotubes interacting with a transitional-metal atomic nanowire of Ni have been determined. The Ni nanowire creates a large electron density of states (DOS)at the Fermi energy. The dependence of the enhanced DOS on the spin state and positioning of the transition-metal wire(inside or outside the nanotube) is studied. Preliminary estimates of the electron-phonon interaction suggest that such systems may have a superconducting transition temperature of $\sim$ 10-50 K. The signs of superconductivity seen in ``ropes'' of nanotubes may also be related to the effect of intrinsic transition-metal impurities.Comment: 4 pages and two figure

Addressing Some Physical Misconceptions in Electrostatics of Freshman Engineering Students

While the electrostatics is important topic consisting one among the four pillars of electromagnetic theory, as being represented by Gauss’s law, students persist encountering difficulties in its understanding. Even though the trend looks universal, we triggered an investigation to search for the main reasons behind the students’ misconceptions of electric field and flux aiming to analyze their origins and to search for intervention pathways to make corrections and, thus, to enhance the students learning of these physical concepts. The subject of our statistical sample is a group of 211 freshman engineering students taking the course of “Physics and Engineering Applications II”. Tests were designed for the diagnosis of the origins of students’ struggles in an attempt to find plausible interventional solutions to reduce the effects. Indeed, the statistics of results shows evidence of deviation of students in figuring out the correct answers and characterizes the depths of the misconceptions and shortcomings. Under the light of our discussions, alternative interventions are suggested to reduce the size of difficulties in learning and to pave the way for better understanding of the physical concepts and achieving better results

Functionalized Hf3C2 and Zr3C2 MXenes for suppression of shuttle effect to enhance the performance of sodium–sulfur batteries

Sodium-sulfur batteries show great potential for storing large amounts of energy due to their ability to undergo a double electron-redox process, as well as the plentiful abundance of sodium and sulfur resources. However, the shuttle effect caused by intermediate sodium polysulfides (Na2Sn) limits their performance and lifespan. To address this issue, here we propose using Hf3C2T2 and Zr3C2T2 (T = F, O), two functionalized MXenes, as cathode additives to suppress the shuttle effect. By using density-functional theory calculations, we investigate nature of the interactions between Na2Sn and MXene, such as the strength of adsorption energy, the electronic density of states, the charge exchange, and the dissociation energy of the Na2S molecule. Our findings show that both Hf3C2T2 and Zr3C2T2 systems inhibit the shuttle effect by binding to Na2Sn with a binding energy stronger than the commonly used electrolyte solvents. These MXenes retain their metallicity during this process and the decomposition barrier for Na2Sn on the oxygen-functionalized MXenes gets reduced which enhances the electrochemical process. Among the MXene systems studied, Zr3C2T2 shows the best performance in suppressing the shuttle effect and catalyzing the electrochemistry process and, thus, increasing the battery's reversible capacity and lifespan

Possibility of two types of localized states in a two-dimensional disordered lattice

We report results of our numerical calculations, based on the equation of motion method, of dc electrical conductivity, and of density of states for up to 40×40 two-dimensional square lattices modeling a tight-binding Hamiltonian for a binary (AB) compound, disordered by randomly distributed B vacancies up to 10%. Our results indicate strongly localized states away from band centers separated from the relatively weakly localized states towards midband. This is in qualitative agreement with the idea of a "mobility edge" separating exponentially localized states from the power-law localized states as suggested by the two-parameter scaling theory of Kaveh in two dimensions. An alternative explanation, consistent with one-parameter scaling theory, is that the observed numerical effects may arise as a consequence of the variation of the localization length over the band

Misconceptions about Atomic Models Amongst the Chemistry Students

Bohr’s model is a semi-classical model which involves both classical and quantum principles. Although more sophisticated Schrödinger model has been presented to students, the residual picture in their minds persists to consider Bohr’s model to be the closest to the physical reality. We included few questions about Bohr’s model in tests to assess the students’ understandings of realistic atomic models in general-chemistry courses offered for freshmen in two universities in the Middle-East (namely, Yarmouk University at Irbid, Jordan, and the United Arab Emirates University at Al-Ain, UAE, from both a statistical sample of 687 students was collected). The results reveal the existence of huge misconceptions amongst a large portion of the students’ sample (i.e., ≥ 85%). Alternative solutions are discussed and suggested to draw a strategy to better dissimilate the knowledge in order to overcome the existing learning difficulties

Length-scale-dependent ensemble-averaged conductance of a 1D disordered conductor: Conductance minimum

An exact numerical calculation of ensemble-averaged length-scale-dependent conductance for the one-dimensional Anderson model is shown to support an earlier conjecture for a conductance minimum. The numerical results can be understood in terms of the Thouless expression for the conductance and the Wigner level-spacing statistics

Length-scale-dependent ensemble-averaged conductance of a 1D disordered conductor: conductance minimum

An exact numerical calculation of ensemble-averaged length-scale-dependent conductance for the one-dimensional Anderson model is shown to support an earlier conjecture for a conductance minimum. The numerical results can be understood in terms of the Thouless expression for the conductance and the Wigner level-spacing statistics

Origins of visible-light emissions in porous silicon

The electronic and optical properties of porous silicon (p-Si) have been theoretically investigated using the minimal sp 3-basis set tight-binding method, with inclusion of second nearest neighbors and spin-orbit interactions. A hypothetical model of p-Si was assumed and calculations of band structures, with focus on bandgap energy E g, oscillator strength (OS) and recombination rate (RR), were carried varying the porosity and the mean distance d between pores. Similar calculations were also performed for other confined silicon nanostructures as hydrogen-passivated silicon nanocrystals (Si:H NCs) and silicon nanowires (Si-NWs). For these two latter systems, the results of E g versus the size d are found to be in excellent agreement with the available measured photoluminescence (PL) data of experimental p-Si samples. On one hand, the results show that sizes in the range d = 1-3 nm are responsible for emission in the visible-light energy window. On the other hand, our results also suggest that the emission properties of p-Si is strongly affected by the 2D and 3D quantum confinement (QC) characters of the involved band edge states. Furthermore, our theoretical values for the RR are found to be larger and closer to the experimental ones in the case of Si:H NCs with respect to the case of Si-NWs. This fact suggests that the intense measured PL features are mainly due to 3D confined nanostructures. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim