Strain-induced phase transformation under compression in a diamond anvil cell: Simulations of a sample and gasket

Combined high pressure phase transformations (PTs) and plastic flow in a sample within a gasket compressed in diamond anvil cell (DAC) are studied for the first time using finite element method. The key point is that phase transformations are modelled as strain-induced, which involves a completely different kinetic description than for traditional pressure-induced PTs. The model takes into account, contact sliding with Coulomb and plastic friction at the boundaries between the sample, gasket, and anvil. A comprehensive computational study of the effects of the kinetic parameter, ratio of the yield strengths of high and low-pressure phases and the gasket, sample radius, and initial thickness on the PTs and plastic flow is performed. A new sliding mechanism at the contact line between the sample, gasket, and anvil called extrusion-based pseudoslip is revealed, which plays an important part in producing high pressure. Strain-controlled kinetics explains why experimentally determined phase transformation pressure and kinetics (concentration of high pressure phase vs. pressure) differ for different geometries and properties of the gasket and the sample: they provide different plastic strain, which was not measured. Utilization of the gasket changes radial plastic flow toward the center of a sample, which leads to high quasi-homogeneous pressure for some geometries. For transformation to a stronger high pressure phase, plastic strain and concentration of a high-pressure phase are also quasi-homogeneous. This allowed us to suggest a method of determining strain-controlled kinetics from experimentation, which is not possible for weaker and equal-strength high-pressure phases and cases without a gasket. Some experimental phenomena are reproduced and interpreted. Developed methods and obtained results represent essential progress toward the understanding of PTs under compression in the DAC. This will allow one optimal design of experiments and conditions for synthesis of new high pressure phases

Coupled phase transformations and plastic flows under torsion at high pressure in rotational diamond anvil cell: Effect of contact sliding

A three-dimensional large-sliding contact model coupled with strain-induced phase transformations (PTs) and plastic flow in a disk-like sample under torsion at high pressure in rotational diamond anvil cell (RDAC) is formulated and studied. Coulomb and plastic friction are combined and take into account variable parameters due to PT. Results are obtained for weaker, equal-strength, and stronger high pressure phases, and for three values of the kinetic coefficient in a strain-controlled kinetic equation and friction coefficient. All drawbacks typical of problem with cohesion are overcome, including eliminating mesh-dependent shear band and artificial plastic zones. Contact sliding intensifies radial plastic flow, which leads to larger reduction in sample thickness. Larger plastic strain and increased pressure in the central region lead to intensification of PT. However, the effect of the reduction in the friction coefficient on PT kinetics is nonmonotonous. Sliding increases away from the center and with growing rotation and is weakly dependent on the kinetic coefficient. Also, cyclic back and forth torsion is studied and compared to unidirectional torsion. Multiple experimental phenomena, e.g., pressure self-multiplication effect, steps (plateaus) at pressure distribution, flow to the center of a sample, and oscillatory pressure distribution for weaker high-pressure phase, are reproduced and interpreted. Reverse PT in high pressure phase that flowed to the low pressure region is revealed. Possible misinterpretation of experimental PT pressure is found. Obtained results represent essential progress toward understanding of strain-induced PTs under compression and shear in RDAC and may be used for designing experiments for synthesis of new high pressure phases and reduction in PT pressure for known phases, as well as for determination of PT kinetics from experiments

Beta-decay branching ratios of 62Ga

Beta-decay branching ratios of 62Ga have been measured at the IGISOL facility of the Accelerator Laboratory of the University of Jyvaskyla. 62Ga is one of the heavier Tz = 0, 0+ -> 0+ beta-emitting nuclides used to determine the vector coupling constant of the weak interaction and the Vud quark-mixing matrix element. For part of the experimental studies presented here, the JYFLTRAP facility has been employed to prepare isotopically pure beams of 62Ga. The branching ratio obtained, BR= 99.893(24)%, for the super-allowed branch is in agreement with previous measurements and allows to determine the ft value and the universal Ft value for the super-allowed beta decay of 62Ga

Differential Response of Female Deer Mice to Short Photoperiod

Author Institution: Department of Biological Sciences, Kent State UniversityIndividual male deer mice (Perotnyscus maniculatus} respond to inhibitory (short) photoperiod with gonadal responses that range from azoospermia to normal spermatogenesis. We undertook the present study to determine if female deer mice exhibit similar variation in reproductive response to inhibitory daylength. Following 8 wk exposure to short days, reproductive tract weights of 25% of all individual females did not differ from those displayed by mice housed on stimulatory (long) photoperiod; reproductive tracts of all remaining short day mice weighed significantly less. Short photoperiod also significantly reduced body weight, albeit only in those mice with regressed reproductive tracts. These results demonstrate that female deer mice respond differentially to the inhibitory effects of short photoperiod. Taken together with previous results, the present findings indicate that populations of deer mice are composed of subsets of males and females that differ in reproductive response to short daylength

Oscillatory Patterns In Angular Differential Ion-Atom Charge Exchange Cross Sections: The Role Of Electron Saddle Swaps

In this work, we have performed an experimental/theoretical study of state selective charge exchange cross sections in 1-10 keV/amu Ne8+ +Na(3s) collisions. Theoretical calculations provided by the classical trajectory Monte Carlo method (CTMC) are contrasted to data obtained at KVI by means of the magneto-optical trap recoil-ion momentum spectroscopy technique (MOTRIMS). We find that for electron capture to n 10, a two-step mechanism which involves an initial electronic excitation followed by electron capture at a later stage of the collision applies. Oscillatory structures in the n-state selective capture cross sections and recoil ion transverse momentum distributions are present in the experimental data as well as in the theoretical results, and are ascribed to the number of swaps the electron undergoes across the potential energy saddle during the collision process.Fil: Otranto, Sebastian. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico - CONICET - Bahía Blanca. Instituto de Física del Sur; Argentina. Universidad Nacional del Sur. Departamento de Física; ArgentinaFil: Blank, I.. Kernfysisch Versneller Instituut. Atomic Physics; Países BajosFil: Olson, R. E.. Missouri University of Science and Technology; Estados UnidosFil: Hoekstra, R.. Kernfysisch Versneller Instituut. Atomic Physics; Países Bajo

The Discovery of a Giant H-alpha Filament in NGC 7213

The nearby Seyfert galaxy NGC 7213 has been imaged in H-alpha and HI with the CTIO 1.5 m telescope and with the Australia Telescope Compact Array (ATCA), respectively. Optically NGC 7213 looks undisturbed and relatively featureless but the continuum-subtracted H-alpha image shows a 19 kpc long filament located approximately 18.6 kpc from the nucleus. The H-alpha filament could be neutral gas photo-ionized by the active nucleus, as has been suggested for the Seyfert galaxy NGC 5252, or shock-ionized by a jet interacting with the surrounding HI, as has been suggested for the radio galaxy PKS 2240-41. The HI map reveals NGC 7213 to be a highly disturbed system suggesting a past merging event.Comment: 14 pages including 4 figures and 1 table. Figures 1-4 are in jpeg format; Better quality images can be retrieved in postscript format at ftp://charon.nmsu.edu/pub/shameed/ ; Accepted for publication in ApJ Letter

Plastic flows and phase transformations in materials under compression in diamond anvil cell: Effect of contact sliding

Modeling of coupled plastic flows and strain-induced phase transformations (PTs) under high pressure in a diamond anvil cell is performed with the focus on the effect of the contact sliding between sample and anvils. Finite element software ABAQUS is utilized and a combination of Coulomb friction and plastic friction is considered. Results are obtained for PTs to weaker, equal-strength, and stronger high pressure phases, using different scaling parameters in a strain-controlled kinetic equation, and with various friction coefficients. Compared to the model with cohesion, artificial shear banding near the constant surface is eliminated. Sliding and the reduction in friction coefficient intensify radial plastic flow in the entire sample (excluding a narrow region near the contact surface) and a reduction in thickness. A reduction in the frictioncoefficient to 0.1 intensifies sliding and increases pressure in the central region. Increases in both plastic strain and pressure lead to intensification of strain-induced PT. The effect of self-locking of sliding is revealed. Multiple experimental phenomena are reproduced and interpreted. Thus, plastic flow and PT can be controlled by controlling friction