Magnetic ground state and multiferroicity in BiMnO3_3

We argue that the centrosymmetric $C2/c$ symmetry in BiMnO$_3$ is spontaneously broken by antiferromagnetic (AFM) interactions existing in the system. The true symmetry is expected to be $Cc$, which is compatible with the noncollinear magnetic ground state, where the ferromagnetic order along one crystallographic axis coexists with the the hidden AFM order and related to it ferroelectric polarization along two other axes. The $C2/c$ symmetry can be restored by the magnetic field $B \sim 35$ Tesla, which switches off the ferroelectric polarization. Our analysis is based on the solution of the low-energy model constructed for the 3d-bands of BiMnO$_3$, where all the parameters have been derived from the first-principles calculations. Test calculations for isostructural BiCrO$_3$ reveal an excellent agreement with experimental data.Comment: 5 pages, 5 figure

Synthesis and photocatalytic properties of materials based on bismuth silicates

The influence of the preparation technique of bismuth silicate-based catalysts on their formation, phase composition, absorption characteristics, and photocatalytic properties is investigated. Samples the with initial ratio of Bi: Si = 2: 1 are prepared via the hydrothermal method with varied temperature conditions in the hydrothermal aging and calcination stages. The synthesized catalysts demonstrate photocatalytic activity in the decomposition of the methanol equilibrium vapor and visible light-induced decolorization of a methylene blue (MB) aqueous solution

Levy flights and Levy -Schroedinger semigroups

We analyze two different confining mechanisms for L\'{e}vy flights in the presence of external potentials. One of them is due to a conservative force in the corresponding Langevin equation. Another is implemented by Levy-Schroedinger semigroups which induce so-called topological Levy processes (Levy flights with locally modified jump rates in the master equation). Given a stationary probability function (pdf) associated with the Langevin-based fractional Fokker-Planck equation, we demonstrate that generically there exists a topological L\'{e}vy process with the very same invariant pdf and in the reverse.Comment: To appear in Cent. Eur. J. Phys. (2010

Structural Stability of CuAl2O4 under Pressure

Structural properties of CuAl2O4, which was recently argued to show unusual suppression of the Jahn–Teller distortions by the spin–orbit coupling, are investigated under pressures up to 6 GPa. Analysis of x-ray powder diffraction experiments shows that CuAl2O4 gets unstable and decomposes onto CuO and Al2O3 at pressures ∼6 GPa and temperature ∼1000 K. This finding is complemented by the density-functional theory + U + spin–orbit coupling calculations, which demonstrate that this instability is partially driven by a (relatively) large compressibility of strongly Jahn–Teller distorted CuO. © 2020 IOP Publishing Ltd Printed in the UK.SVS is grateful to I Leonov for discussion of structural stability of normal and inverse spinels. Theoretical calculations were supported by the Russian Science Foundation via program RSF 20-62-46047. AAB was partly supported by JSPS KAKENHI Grant Number JP20H05276, a research grant (40-37) from Nippon Sheet Glass Foundation for Materials Science and Engineering, and Innovative Science and Technology Initiative for Security (Grant Number JPJ004596) from Acquisition, Technology, and Logistics Agency (ATLA), Japan. We also thank 02.A03.21.0006 project of Russian Ministry of Education

Strong spin-phonon coupling in infrared and Raman spectra of SrMnO3

Infrared reflectivity spectra of cubic SrMnO3 ceramics reveal 18% stiffening of the lowest-frequency phonon below the antiferromagnetic phase transition occurring at T-N = 233 K. Such a large temperature change of the polar phonon frequency is extraordinary and we attribute it to an exceptionally strong spin-phonon coupling in this material. This is consistent with our prediction from first-principles calculations. Moreover, polar phonons become Raman active below T-N, although their activation is forbidden by symmetry in the Pm (3) over barm space group. This gives evidence that the cubic Pm (3) over barm symmetry is locally broken below T-N due to a strong magnetoelectric coupling. Multiphonon and multimagnon scattering is also observed in Raman spectra. Microwave and THz permittivity is strongly influenced by hopping electronic conductivity, which is caused by small nonstoichiometry of the sample. Thermoelectric measurements show room-temperature concentration of free carriers n(e) = 3.6 x 10(20) cm(-3) and the sample composition Sr2+Mn0.984+Mn0.023+O2.992-. The conductivity exhibits very unusual temperature behavior: THz conductivity increases on cooling, while the static conductivity markedly decreases on cooling. We attribute this to different conductivity of the ceramic grains and grain boundariesclose

A Metric of Influential Spreading during Contagion Dynamics through the Air Transportation Network

The spread of infectious diseases at the global scale is mediated by long-range human travel. Our ability to predict the impact of an outbreak on human health requires understanding the spatiotemporal signature of early-time spreading from a specific location. Here, we show that network topology, geography, traffic structure and individual mobility patterns are all essential for accurate predictions of disease spreading. Specifically, we study contagion dynamics through the air transportation network by means of a stochastic agent-tracking model that accounts for the spatial distribution of airports, detailed air traffic and the correlated nature of mobility patterns and waiting-time distributions of individual agents. From the simulation results and the empirical air-travel data, we formulate a metric of influential spreading––the geographic spreading centrality––which accounts for spatial organization and the hierarchical structure of the network traffic, and provides an accurate measure of the early-time spreading power of individual nodes