94,603 research outputs found

    Novel electronic and magnetic properties of BN sheet decorated with hydrogen and fluorine

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    First principles calculations based on density functional theory reveal some unusual properties of BN sheet functionalized with hydrogen and fluorine. These properties differ from those of similarly functionalized graphene even though both share the same honeycomb structure. (1) Unlike graphene which undergoes a metal to insulator transition when fully hydrogenated, the band gap of the BN sheet significantly narrows when fully saturated with hydrogen. Furthermore, the band gap of the BN sheet can be tuned from 4.7 eV to 0.6 eV and the system can be a direct or an indirect semiconductor or even a half-metal depending upon surface coverage. (2) Unlike graphene, BN sheet has hetero-atomic composition, when co-decorated with H and F, it can lead to anisotropic structures with rich electronic and magnetic properties. (3) Unlike graphene, BN sheets can be made ferromagnetic, antiferromagnetic, or magnetically degenerate depending upon how the surface is functionalized. (4) The stability of magnetic coupling of functionalized BN sheet can be further modulated by applying external strain. Our study highlights the potential of functionalized BN sheets for novel applications.Comment: 18 pages, 6 figures, and 1 tabl

    Spin-one ferromagnets with single-ion anisotropy in a perpendicular external field

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    In this paper, the conventional Holstein-Primakoff method is generalized with the help of the characteristic angle transformation [Lei Zhou and Ruibao Tao, J. Phys. A {\bf 27} 5599 (1994)] for the spin-one magnetic systems with single-ion anisotropies. We find that the weakness of the conventional method for such systems can be overcome by the new approach. Two models will be discussed to illuminate the main idea, which are the ``easy-plane" and the ``easy-axis" spin-one ferromagnet, respectively. Comparisons show that the current approach can give reasonable ground state properties for the magnetic system with ``easy-plane" anisotropy though the conventional method never can, and can give a better representation than the conventional one for the magnetic system with ``easy-axis" anisotropy though the latter is usually believed to be a good approximation in such case. Study of the easy-plane model shows that there is a phase transition induced by the external field, and the low-temperature specific heat may have a peak as the field reaches the critical value.Comment: Using LaTex. To be published in the September 1 issue of Physical Review B (1996). Email address: [email protected]

    Melt-growth dynamics in CdTe crystals

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    We use a new, quantum-mechanics-based bond-order potential (BOP) to reveal melt-growth dynamics and fine-scale defect formation mechanisms in CdTe crystals. Previous molecular dynamics simulations of semiconductors have shown qualitatively incorrect behavior due to the lack of an interatomic potential capable of predicting both crystalline growth and property trends of many transitional structures encountered during the melt →\rightarrow crystal transformation. Here we demonstrate successful molecular dynamics simulations of melt-growth in CdTe using a BOP that significantly improves over other potentials on property trends of different phases. Our simulations result in a detailed understanding of defect formation during the melt-growth process. Equally important, we show that the new BOP enables defect formation mechanisms to be studied at a scale level comparable to empirical molecular dynamics simulation methods with a fidelity level approaching quantum-mechanical method

    Calculation of renormalized viscosity and resistivity in magnetohydrodynamic turbulence

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    A self-consistent renormalization (RG) scheme has been applied to nonhelical magnetohydrodynamic turbulence with normalized cross helicity σc=0\sigma_c =0 and σc→1\sigma_c \to 1. Kolmogorov's 5/3 powerlaw is assumed in order to compute the renormalized parameters. It has been shown that the RG fixed point is stable for d≥dc≈2.2d \ge d_c \approx 2.2. The renormalized viscosity ν∗\nu^* and resistivity η∗\eta^* have been calculated, and they are found to be positive for all parameter regimes. For σc=0\sigma_c=0 and large Alfv\'{e}n ratio (ratio of kinetic and magnetic energies) rAr_A, ν∗=0.36\nu^*=0.36 and η∗=0.85\eta^*=0.85. As rAr_A is decreased, ν∗\nu^* increases and η∗\eta^* decreases, untill rA≈0.25r_A \approx 0.25 where both ν∗\nu^* and η∗\eta^* are approximately zero. For large dd, both ν∗\nu^* and η∗\eta^* vary as d−1/2d^{-1/2}. The renormalized parameters for the case σc→1\sigma_c \to 1 are also reported.Comment: 19 pages REVTEX, 3 ps files (Phys. Plasmas, v8, 3945, 2001

    Population of human ventricular cell models calibrated with in vivo measurements unravels ionic mechanisms of cardiac alternans

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    Cardiac alternansis an important risk factor in cardiac physiology, and is related to the initiation of many pathophysiological conditions. However, the mechanisms underlying the generation of alternans remain unclear. In this study, we used a population of computational human ventricle models based onthe O’Hara model [1] to explore the effect of 11 key factors experimentally reported to be related to alternans. In vivo experimental datasets coming from patients undergoing cardiac surgery were used in the calibration of our in silico population of models. The calibrated models in the population were divided into two groups (Normal and Alternans) depending on alternans occurrence. Our results showed that there were significant differences in the following 5 ionic currents between the two groups: fast sodium current, sodium calcium exchanger current, sodium potassium pump current, sarcoplasmic reticulum (SR) calcium release flux and SR calcium reuptake flux. Further analysis indicated that fast sodium current and SR calcium uptake were the two most significant currents that contributed to voltage and calcium alternans generation, respectively
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