Simple Model of Propagating Flame Pulsations

A simple model which exhibits dynamical flame properties in 1D is presented. It is investigated analytically and numerically. The results are applicable to problems of flame propagation in supernovae Ia.Comment: 10 pages, 8 figures, revised version accepted by MNRA

Flame fronts in Supernovae Ia and their pulsational stability

The structure of the deflagration burning front in type Ia supernovae is considered. The parameters of the flame are obtained: its normal velocity and thickness. The results are in good agreement with the previous works of different authors. The problem of pulsation instability of the flame, subject to plane perturbations, is studied. First, with the artificial system with switched-off hydrodynamics the possibility of secondary reactions to stabilize the front is shown. Second, with account of hydrodynamics, realistic EOS and thermal conduction we can obtain pulsations when Zeldovich number was artificially increased. The critical Zeldovich numbers are presented. These results show the stability of the flame in type Ia supernovae against pulsations because its effective Zeldovich number is small.Comment: 12 pages, 11 figure

Novel hydrodynamic cumulation mechanism caused by quantum shell effects

The computational and theoretical analysis carried out in this article demonstrates the existence of a nontrivial mechanism for the compression of a submicron-sized gas bubble formed by a gas of classical ions and a gas of degenerate electrons. This mechanism fundamentally differs from conventional compression mechanisms. It is shown that taking into account the quantum effect of a large spatial scale in the distribution of electrons qualitatively changes the character of cumulative processes. Because of a large-scale electric field caused by quantum shell effects, the compression process is characterized by the formation of multiple shock waves. The values of gas temperature and pressure achieved during compression occur higher by two orders of magnitude as compared with the classical adiabatic regime. The analysis is carried out within the framework of the following model: the dynamics of the electron subsystem is described by equations of a quantum electron fluid, while the hydrodynamic approximation is adopted for the ionic subsystem. The large scale effect is taken into account by means of effective external field acting on electrons. The theoretical analysis carried out within this approach clarifies the nature of the cumulative process in the system under consideration; some quantitative characteristics obtained with numerical simulation are presented. The possibility of experimental observation of this cumulative mechanism is analyzed. It is suggested that the manifestation of the effect can be observed during laser compression of a system of submicron targets by measuring the neutron yield.Comment: 49 pages, pdf onl

Optimizing Sales Using Mobile Sales Ticketing Application

A pressure-induced collapse of magnetic ordering in $β−Li_{2}IrO_3$ at $P_m ∼ 1.5–2$ GPa has previously been interpreted as evidence for possible emergence of spin liquid states in this hyperhoneycomb iridate, raising prospects for experimental realizations of the Kitaev model. Based on structural data obtained at room temperature, this magnetic transition is believed to originate in small lattice perturbations that preserve crystal symmetry, and related changes in bond-directional anisotropic exchange interactions. Here we report on the evolution of the crystal structure of $β−Li_{2}IrO_3$ under pressure at low temperatures $(T≤50 K)$ and show that the suppression of magnetism coincides with a change in lattice symmetry involving Ir-Ir dimerization. The critical pressure for dimerization shifts from 4.4(2) GPa at room temperature to $∼1.5–2$ GPa below 50 K. While a direct $Fddd→C2/c$ transition is observed at room temperature, the low temperature transitions involve new as well as coexisting dimerized phases. Further investigation of the Ir ($L3/L_2$) isotropic branching ratio in x-ray absorption spectra indicates that the previously reported departure of the electronic ground state from a $J_{eff} = 1/2$ state is closely related to the onset of dimerized phases. In essence, our results suggest that the predominant mechanism driving the collapse of magnetism in $β−Li_{2}IrO_3$ is the pressure-induced formation of $Ir_2$ dimers in the hyperhoneycomb network. The results further confirm the instability of the $J_{eff} = 1/2$ moments and related noncollinear spiral magnetic ordering against formation of dimers in the low-temperature phase of compressed $β−Li_{2}IrO_3$

High-Pressure Synthesis of Dirac Materials: Layered van der Waals Bonded BeN4 Polymorph

High-pressure chemistry is known to inspire the creation of unexpected new classes of compounds with exceptional properties. Here, we employ the laser-heated diamond anvil cell technique for synthesis of a Dirac material BeN4. A triclinic phase of beryllium tetranitride tr-BeN4 was synthesized from elements at ∼85 GPa. Upon decompression to ambient conditions, it transforms into a compound with atomic-thick BeN4 layers interconnected via weak van der Waals bonds and consisting of polyacetylene-like nitrogen chains with conjugated π systems and Be atoms in square-planar coordination. Theoretical calculations for a single BeN4 layer show that its electronic lattice is described by a slightly distorted honeycomb structure reminiscent of the graphene lattice and the presence of Dirac points in the electronic band structure at the Fermi level. The BeN4 layer, i.e., beryllonitrene, represents a qualitatively new class of 2D materials that can be built of a metal atom and polymeric nitrogen chains and host anisotropic Dirac fermions. © 2021 American Physical Society.Parts of this research were carried out at the Extreme Conditions Beamline (P02.2) at DESY, a member of Helmholtz Association (HGF). Portions of this work were performed on beamline ID15 at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. Portions of this work were performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13) and at HPCAT (sector 16) of the Advanced Photon Source (APS), Argonne National Laboratory. Research was sponsored by the Army Research Office and was accomplished under the Cooperative Agreement No. W911NF-19-2-0172. N. D. and L. D. thank the Deutsche Forschungsgemeinschaft (DFG Projects No. DU 954-11/1, No. DU 393-9/2, and No. DU 393-13/1) and the Federal Ministry of Education and Research, Germany (BMBF, Grant No. No. 05K19WC1) for financial support. D. L. thanks the Alexander von Humboldt Foundation for financial support. Theoretical analysis of chemical bonding was supported by the Russian Science Foundation (Project No. 18-12-00492). Calculations of the phonon dispersion relations were supported by the Ministry of Science and Higher Education of the Russian Federation in the framework of Increase Competitiveness Program of NUST MISIS (No. K2-2020-026) implemented by governmental decree No. 211. Support from the Knut and Alice Wallenberg Foundation (Wallenberg Scholar Grant No. KAW-2018.0194), the Swedish Government Strategic Research Areas in Materials Science on Functional Materials at Linköping University (Faculty Grant SFO-Mat-LiU No. 2009 00971) and SeRC, the Swedish Research Council (VR) Grant No. 2019-05600 and Vinnova VINN Excellence Center Functional Nanoscale Materials (FunMat-2) Grant No. 2016–05156 is gratefully acknowledged. The computations were enabled by resources provided by the Swedish National Infrastructure for Computing (SNIC) partially funded by the Swedish Research Council through Grant Agreement No. 2016-07213. The work of M. I. K. was supported by the JTC-FLAGERA Project GRANSPORT. GeoSoilEnviroCARS is supported by the National Science Foundation–Earth Sciences (EAR–1634415) and Department of Energy-Geosciences (DE-FG02-94ER14466). HPCAT operations are supported by DOE-NNSA’s Office of Experimental Sciences. Advanced Photon Source is U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357

Pressure tuning of charge ordering in iron oxide

A Verwey-type charge-ordering transition in magnetite at 120 K leads to the formation of linear units of three iron ions with one shared electron, called trimerons. The recently-discovered iron pentoxide (Fe$_4$O$_5$) comprising mixed-valent iron cations at octahedral chains, demonstrates another unusual charge-ordering transition at 150 K involving competing formation of iron trimerons and dimerons. Here, we experimentally show that applied pressure can tune the charge-ordering pattern in Fe$_4$O$_5$ and strongly affect the ordering temperature. We report two charge-ordered phases, the first of which may comprise both dimeron and trimeron units, whereas, the second exhibits an overall dimerization involving both the octahedral and trigonal-prismatic chains of iron in the crystal structure. We link the dramatic change in the charge-ordering pattern in the second phase to redistribution of electrons between the octahedral and prismatic iron chains, and propose that the average oxidation state of the iron cations can pre-determine a charge-ordering pattern