Condensation of the atomic relaxation vibrations in lead-magnesium-niobate at T=T∗T=T^*

We present neutron diffraction, dielectric permittivity and photoconductivity measurements, evidencing that lead-magnesium niobate experiences a diffuse phase transformation between the spherical glass and quadrupole glass phases, in the temperature interval between 400 K and 500 K, with the quadrupole phase possessing extremely high magnitudes of dielectric permittivity. Our analysis shows that the integral diffuse scattering intensity may serve as an order parameter for this transformation. Our experimental dielectric permittivity data support this choice. These data are important for the aplications desiring giant dielectric responses, in a wide temperature intervals and not related to electron's excitations.Comment: 6 figure

Cluster superconductivity in the magnetoelectric Pb(Fe1/2Sb1/2)O3 ceramics

We report the observation of cluster (local) superconductivity in the magnetoelectric Pb(Fe1/2Sb1/2)O3 ceramics prepared at a hydrostatic pressure of 6 GPa and temperatures 1200-1800 K to stabilize the perovskite phase. The superconductivity is manifested by an abrupt drop of the magnetic susceptibility at the critical temperature TC 7 K. Both the magnitude of this drop and TC decrease with magnetic field increase. Similarly, the low-field paramagnetic absorption measured by EPR spectrometer drops significantly below TC as well. The observed effects and their critical magnetic field dependence are interpreted as manifestation of the superconductivity and Meissner effect in metallic Pb nanoclusters existing in the ceramics. Their volume fraction and average size were estimated as 0.1-0.2% and 140-150 nm, respectively. The superconductivity related effects disappear after oxidizing annealing of the ceramics.Comment: 9 pages, 5 figure

Broken Local Symmetry in Paraelectric BaTiO3 Proved by Second Harmonic Generation

Precursor dynamics of a cubic to tetragonal ferroelectric phase transition in BaTiO3 is studied by the accurate measurement of the second harmonic generation (SHG) integral intensities. A finite signal holds for the SHG integrated intensity above the ferroelectric Curie temperature Tc=403  K. Above the Burn’s temperature Td≈580  K, the power law with the exponent γ=1 shows normal SHG nature originating from the hyper-Raman scattering by dynamical polar excitations, while, below Td, a SHG signal from polar nanoregions becomes dominant with the larger exponent γ=2. Such a crossover of the power law exponent near Td is discussed on the basis of the effective Hamiltonian method and Monte Carlo simulation

High potential for weathering and climate effects of non-vascular vegetation in the Late Ordovician

It has been hypothesized that predecessors of today’s bryophytes significantly increased global chemical weathering in the Late Ordovician, thus reducing atmospheric CO2 concentration and contributing to climate cooling and an interval of glaciations. Studies that try to quantify the enhancement of weathering by non-vascular vegetation, however, are usually limited to small areas and low numbers of species, which hampers extrapolating to the global scale and to past climatic conditions. Here we present a spatially explicit modelling approach to simulate global weathering by non-vascular vegetation in the Late Ordovician. We estimate a potential global weathering flux of 2.8 (km3 rock) yr−1, defined here as volume of primary minerals affected by chemical transformation. This is around three times larger than today’s global chemical weathering flux. Moreover, we find that simulated weathering is highly sensitive to atmospheric CO2 concentration. This implies a strong negative feedback between weathering by non-vascular vegetation and Ordovician climate

New data on acritarchs from the Upper Ordovician of the Tungus basin, Siberian platform

© 2014, Estonian Academy Publishers. All rights reserved. Distinctive late Ordovician acritarch assemblages have been discovered for the first time from about a 100 m sedimentary succession exposed along the Bol′shaya Nirunda River in Siberia. The studied stratigraphic interval includes the uppermost Baksian, Dolborian, Nirundian and Burian regional stages, which correspond to the Katian-?lowermost Hirnantian global stages. Acritarch assemblages from the Dolbor Regional Stage are exceptionally diverse and include aside from the longranging taxa several unique (endemic) morphotypes and a number of distinctive stratigraphically valuable species, well known outside Siberia. The occurrence of the acritarchs widespread outside Siberia is potentially important for interregional biostratigraphic correlations and might also play a significant role in biogeographic reconstructions. Having in mind that the Siberian palaeocontinent was located in a low-latitude tropical area during the entire Ordovician, the presence of taxa typical of cold-water settings along the Peri-Gondwana margin can be regarded as an additional evidence for penetration of cool-water currents into the epicontinental Tungus basin in the Upper Ordovician