Crossing Statistics of Anisotropic Stochastic Surface [CL]

We use crossing statistics and its generalization to determine the anisotropic direction imposed on a stochastic fields in $(2+1)$Dimension. This approach enables us to examine not only the rotational invariance of morphology but also we can determine the Gaussianity of underlying stochastic field in various dimensions. Theoretical prediction of up-crossing statistics (crossing with positive slope at a given threshold $\alpha$ of height fluctuation), $\nu^+_{\diamond}(\alpha)$, and generalized roughness function, $N^{\diamond}_{tot}(q)$, for correlation length ($\xi_{\diamond}$) and/with scaling exponent ($\gamma_{\diamond}$) anisotropies are calculated. The strategy to examine the anisotropy nature and to determine its direction is as follows: we consider a set of normal axes, and sign them $||$ (parallel) and $ \bot$ (normal) with respect to unknown anisotropic direction. Then we determine $\nu_{\diamond}^+ (\alpha)$ and $N^{\diamond}_{tot}(q)$ in both directions. The directional dependency of difference between computed results in mentioned directions are clarify. Finally we systematically recognize the anisotropy direction at $3\sigma$ confidence interval using P-value approach. In order to distinguish between nature of anisotropies, after applying a typical method in determining the scaling exponents in both mentioned directions with respect to the recognized anisotropy direction using up-crossing statistics, the kind and the ratio of correlation length anisotropy are specified. Our algorithm can be mounted with a simple software on various instruments for surface analysis, such as AFM, STM and etc.

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M. Nezhadhaghighi, S. Movahed, T. Yasseri, et. al.
Fri, 7 Aug 15

Comments: 14 pages and 11 figures

High-pressure, temperature elasticity of Fe- and Al-bearing MgSiO3: implications for the Earth's lower mantle [CL]

Fe and Al are two of the most important rock-forming elements other than Mg, Si, and O. Their presence in the lower mantle’s most abundant minerals, MgSiO_3 bridgmanite, MgSiO_3 post-perovskite and MgO periclase, alters their elastic properties. However, knowledge on the thermoelasticity of Fe- and Al-bearing MgSiO_3 bridgmanite, and post-perovskite is scarce. In this study, we perform ab initio molecular dynamics to calculate the elastic and seismic properties of pure, Fe^{3+}- and Fe^{2+}-, and Al^{3+}-bearing MgSiO_3 perovskite and post-perovskite, over a wide range of pressures, temperatures, and Fe/Al compositions. Our results show that a mineral assemblage resembling pyrolite fits a 1D seismological model well, down to, at least, a few hundred kilometers above the core-mantle boundary, i.e. the top of the D” region. In D”, a similar composition is still an excellent fit to the average velocities and fairly approximate to the density. We also implement polycrystal plasticity with a geodynamic model to predict resulting seismic anisotropy, and find post-perovskite with predominant (001) slip across all compositions agrees best with seismic observations in the D”.

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S. Zhang, S. Cottaar, T. Liu, et. al.
Mon, 20 Jul 15

Comments: 26 pages, 6 figures; submitted to journal 8 June 2015

Two phase coexistence for the hydrogen-helium mixture [CL]

We use our newly constructed quantum Gibbs ensemble Monte Carlo algorithm to perform computer experiments for the two phase coexistence of a hydrogen-helium mixture. Our results are in quantitative agreement with the experimental results of C. M. Sneed, W. B. Streett, R. E. Sonntag, and G. J. Van Wylen. The difference between our results and the experimental ones is in all cases less than 15% relative to the experiment, reducing to less than 5% in the low helium concentration phase. At the gravitational inversion between the vapor and the liquid phase, at low temperatures and high pressures, the quantum effects become relevant. At extremely low temperature and pressure the first component to show superfluidity is the helium in the vapor phase.

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R. Fantoni
Wed, 8 Jul 15

Comments: 7 pages, 4 figures, 4 tables

Modeling astronomically observed interstellar infrared spectra by ionized carbon pentagon-hexagon molecules (c9h7) n+ [CL]

Modeling a promising carrier of the astronomically observed polycyclic aromatic hydrocarbon (PAH), infrared (IR) spectra of ionized molecules (C9H7) n+ were calculated based on density functional theory (DFT). In a previous study, it was found that void induced coronene C23H12++ could reproduce observed spectra from 3 to 15 micron, which has carbon two pentagons connected with five hexagons. In this paper, we tried to test the simplest model, that is, one pentagon connected with one hexagon, which is indene like molecule (C9H7) n+ (n=0 to 4). DFT based harmonic frequency analysis resulted that observed spectrum could be almost reproduced by a suitable sum of ionized C9H7n+ molecules. Typical example is C9H7++. Calculated peaks were 3.2, 7.4, 7.6, 8.4, and 12.7 micron, whereas observed one 3.3, 7.6, 7.8, 8.6 and 12.7 micron. By a combination of different degree of ionized molecules, we can expect to reproduce total spectrum. For a comparison, hexagon-hexagon molecule naphthalene (C10H8) n+ was studied. Unfortunately, ionized naphthalene shows little coincidence with observed one. Carbon pentagon- hexagon molecules may play an important role as interstellar molecular dust.

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N. Ota
Fri, 19 Jun 15

Comments: 7 pages, 4 figures, 2 tables

Magneto-structural transformations via a solid-state nudged elastic band method: Application to iron under pressure [CL]

We extend the solid-state nudged elastic band method to handle a non-conserved order parameter – in particular, magnetization, that couples to volume and leads to many observed effects in magnetic systems. We apply this formalism to the well-studied magneto-volume collapse during the pressure-induced transformation in iron – from ferromagnetic body-centered cubic (bcc) austenite to hexagonal close-packed (hcp) martensite. We find a bcc-hcp equilibrium coexistence pressure of 8.4 GPa, with the transition-state enthalpy of 156 meV/Fe at this pressure. A discontinuity in magnetization and coherent stress occurs at the transition state, which has a form of a cusp on the potential-energy surface (yet all the atomic and cell degrees of freedom are continuous); the calculated pressure jump of 25 GPa is related to the observed 25 GPa spread in measured coexistence pressures arising from martensitic and coherency stresses in samples. Our results agree with experiments, but necessarily differ from those arising from drag and restricted parametrization methods having improperly constrained or uncontrolled degrees of freedom.

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N. Zarkevich and D. Johnson
Fri, 5 Jun 15

Comments: 7 pages, 7 figures

Van't Hoff law for temperature dependent Langmuir constants in clathrate hydrate nanocavities [EPA]

This work gives a van’t Hoff law expression of Langmuir constants of different species for determining their occupancy in the nanocavities of clathrate hydrates. The van’t Hoff law’s parameters are derived from a fit with Langmuir constants calculated using a pairwise site-site interaction potential to model the anisotropic potential environment in the cavities, as a function of temperature. The parameters can be used for calculating clathrates compositions. Results are given for nineteen gas species trapped in the small and large cavities of structure types I and II [1]. The accuracy of this approach is based on a comparison with available experimental data for ethane and cyclo- propane clathrate hydrates. The numerical method applied in this work, was recently validated from a comparison with the spherical cell method based on analytical considerations [1]

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A. Lakhlifi and P. Dahoo
Tue, 17 Mar 15

Comments: 2 figures

Magnetic Fields of Uranus and Neptune: Metallic Fluid Hydrogen [CL]

The magnetic fields of the Ice Giant Planets Uranus and Neptune (U/N) are unique in the solar system. Based on a substantial database measured on Earth for representative planetary fluids at representative dynamic pressures up to 200 GPa (2 Mbar) and a few 1000 K, the complex magnetic fields of U/N are (i) probably made primarily by degenerate metallic fluid H (MFH) at or near the crossover from the H-He envelopes to Ice cores at ~100 GPa (Mbar) pressures and normalized radii of ~90% of the radii of U/N; (ii) because those magnetic fields are made relatively close to the surfaces of U/N, non-dipolar fields can be expected; (iii) the Ice cores are most probably a heterogeneous fluid mixture of H, N, O, C, Fe/Ni and silicate-oxides and their mutual reaction products at high pressures and temperatures; (iv) the shapes of the magnetic fields are probably caused by weak coupling between rotational motions of U/N and convective motions of conducting fluids in dynamos that make those magnetic fields. Ironically, there is probably little nebular Ice in the Ice Giant Planets.

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W. Nellis
Wed, 4 Mar 15

Comments: 10 pages