Thermal decomposition of a honeycomb-network sheet - A Molecular Dynamics simulation study

The thermal degradation of a graphene-like two-dimensional triangular membrane with bonds undergoing temperature-induced scission is studied by means of Molecular Dynamics simulation using Langevin thermostat. We demonstrate that the probability distribution of breaking bonds is highly peaked at the rim of the membrane sheet at lower temperature whereas at higher temperature bonds break at random anywhere in the hexagonal flake. The mean breakage time $\tau$ is found to decrease with the total number of network nodes $N$ by a power law $\tau \propto N^{-0.5}$ and reveals an Arrhenian dependence on temperature $T$. Scission times are themselves exponentially distributed. The fragmentation kinetics of the average number of clusters can be described by first-order chemical reactions between network nodes $n_i$ of different coordination. The distribution of fragments sizes evolves with time elapsed from a $\delta$-function through a bimodal one into a single-peaked again at late times. Our simulation results are complemented by a set of $1^{st}$-order kinetic differential equations for $n_i$ which can be solved exactly and compared to data derived from the computer experiment, providing deeper insight into the thermolysis mechanism.Comment: 21pages, 9 figures, LaTeX, revised versio

Optical spectra, crystal-field parameters, and magnetic susceptibility of the new multiferroic NdFe3(BO3)4

We report high-resolution optical absorption spectra for NdFe3(BO3)4 trigonal single crystal which is known to exhibit a giant magnetoelectric effect below the temperature of magnetic ordering TN = 33 K. The analysis of the temperature-dependent polarized spectra reveals the energies and, in some cases, symmetries and exchange splittings of Nd3+ 84 Kramers doublets. We perform crystal-field calculations starting from the exchange-charge model, obtain a set of six real crystal-field parameters, and calculate wave functions and magnetic g-factors. In particular, the values g(perpendicular) = 2.385, g(parallel) = 1.376 were found for the Nd3+ ground-state doublet. We obtain Bloc=7.88 T and |JFN|= 0.48 K for the values of the local effective magnetic field at liquid helium temperatures at the Nd3+ site and the Nd - Fe exchange integral, respectively, using the experimentally measured Nd3+ ground-state splitting of 8.8 cm-1. To check reliability of our set of crystal field parameters we model the magnetic susceptibility data from literature. A dimer containing two nearest-neighbor iron ions in the spiral chain is considered to partly account for quasi-one-dimensional properties of iron borates, and then the mean-field approximation is used. The results of calculations with the exchange parameters for Fe3+ ions Jnn = -6.25 K (intra-chain interactions) and Jnnn = -1.92 K (inter-chain interactions) obtained from fitting agree well with the experimental data.Comment: 13 pages, 8 figures, 2 table

Cracking the Quantum Advantage threshold for Gaussian Boson Sampling

Scientists in quantum technology aspire to quantum advantage: a computational result unattainable with classical computers. Gaussian boson sampling experiment has been already claimed to achieve this goal. In this setup squeezed light states interfere in a mid-sized linear optical network. The exact simulation of the counting statistics of $n$ detectors is far beyond the possibilities of modern supercomputers once $n$ exceeds $50$. We challenge quantum advantage for a mid-sized Gaussian boson sampling setup and propose the approximate algorithm to obtain the probability of any specific measurement outcome. For an idealized 70-mode device, our approximation achieves accuracy competitive with the experimental one.Comment: The supplementary material is uploaded as a separate fil

Tutor's role in online courses

The increased use of information and communication technologies in education demands a critical review of the approaches used for supporting teaching and learning activities in online environment. The paper discusses the special role of the tutor who guides and supports the learning process in the online course supplementing traditional face-to-face activities in the blended learning model. Students' evaluations of tutor's role were analyzed and underlay this study. The findings show that professional and successful tutoring helps students in improving their learning efficacy and having greater personal responsibility for their outcomes. Рост использования информационно-коммуникационных технологий в образовании требует критического анализа и пересмотра подходов, используемых для поддержки образовательной деятельности в онлайн среде. Данная работа рассматривает особую роль тьютора, который поддерживает и сопровождает учебный процесс в онлайн курсе, дополняющем традиционное контактное обучение в смешанной модели обучения. В основе данного исследования лежит анализ студенческого восприятия роли тьютора. Результаты исследования показывают, что профессиональная и успешная деятельность тьютора помогает повысить эффективность обучения и способствует пониманию личной ответственности студентов за результаты обучения

Inversionless light amplification and optical switching controlled by state-dependent alignment of molecules

We propose a method to achieve amplification without population inversion by anisotropic molecules whose orientation by an external electric field is state-dependent. It is based on decoupling of the lower-state molecules from the resonant light while the excited ones remain emitting. The suitable class of molecules is discussed, the equation for the gain factor is derived, and the magnitude of the inversionless amplification is estimated for the typical experimental conditions. Such switching of the sample from absorbing to amplifying via transparent state is shown to be possible both with the aid of dc and ac control electric fields.Comment: AMS-LaTeX v1.2, 4 pages with 4 figure

The effect of alloying on the ordering processes in near-alpha titanium alloys

The substructure of near-alpha Ti-Al-Sn-Zr-Mo-Si alloys containing up to 12.5. at% aluminum was studied by transmission electron microscopy (TEM). It was shown that long-range order sections are formed at aging temperatures up to 500. °C in alloys, high in aluminum, and the ordered phase is formed by the nucleation and growth mechanism at 700. °C aging temperatures. Causes of changing the phase transformation mechanism have been discussed, and the relationship between the structure and properties of alloys, depending on modes of heat treatment has been analyzed. Also the influence of aluminides and silicides precipitation on the mechanical alloy properties after aging was examined. It was shown that the aluminide formation led to a slight hardening and a significant viscosity decrease. The silicide particles formation reduced the heat resistance properties, due to the depletion of the solid solution by silicon. © 2012 Elsevier B.V