59 research outputs found
Magnetic structures and reorientation transitions in noncentrosymmetric uniaxial antiferromagnets
A phenomenological theory of magnetic states in noncentrosymmetric tetragonal
antiferromagnets is developed, which has to include homogeneous and
inhomogeneous terms (Lifshitz-invariants) derived from Dzyaloshinskii-Moriya
couplings. Magnetic properties of this class of antiferromagnets with low
crystal symmetry are discussed in relation to its first known members, the
recently detected compounds Ba2CuGe2O7 and K2V3O8. Crystallographic symmetry
and magnetic ordering in these systems allow the simultaneous occurrence of
chiral inhomogeneous magnetic structures and weak ferromagnetism. New types of
incommensurate magnetic structures are possible, namely, chiral helices with
rotation of staggered magnetization and oscillations of the total
magnetization. Field-induced reorientation transitions into modulated states
have been studied and corresponding phase diagrams are constructed. Structures
of magnetic defects (domain-walls and vortices) are discussed. In particular,
vortices, i.e. localized non-singular line defects, are stabilized by the
inhomogeneous Dzyaloshinskii-Moriya interactions in uniaxial noncentrosymmetric
antiferromagnets.Comment: 18 pages RevTeX4, 13 figure
The distribution of plant fossils and their palaeoecology in Duckmantian (Bashkirian, Lower Pennsylvanian) strata at Brymbo, North Wales, UK
Changes to the Fossil Record of Insects through Fifteen Years of Discovery
The first and last occurrences of hexapod families in the fossil record are compiled from publications up to end-2009. The major features of these data are compared with those of previous datasets (1993 and 1994). About a third of families (>400) are new to the fossil record since 1994, over half of the earlier, existing families have experienced changes in their known stratigraphic range and only about ten percent have unchanged ranges. Despite these significant additions to knowledge, the broad pattern of described richness through time remains similar, with described richness increasing steadily through geological history and a shift in dominant taxa, from Palaeoptera and Polyneoptera to Paraneoptera and Holometabola, after the Palaeozoic. However, after detrending, described richness is not well correlated with the earlier datasets, indicating significant changes in shorter-term patterns. There is reduced Palaeozoic richness, peaking at a different time, and a less pronounced Permian decline. A pronounced Triassic peak and decline is shown, and the plateau from the mid Early Cretaceous to the end of the period remains, albeit at substantially higher richness compared to earlier datasets. Origination and extinction rates are broadly similar to before, with a broad decline in both through time but episodic peaks, including end-Permian turnover. Origination more consistently exceeds extinction compared to previous datasets and exceptions are mainly in the Palaeozoic. These changes suggest that some inferences about causal mechanisms in insect macroevolution are likely to differ as well
The nesting of titanopteran insects within tcholmanvissiids reassured and the earliest caeliferan identified: a reply to Huang et al
New Permian stoneflies of the family Palaeonemouridae (Insecta: Perlida = Plecoptera) from the Isady locality
Collaboration between the JINR and Research Institutions of the Republic of Belarus in the Area of Superconducting Accelerating Cavities: A Review
Researchers and specialists of the Institute of Nuclear Problems of the Belarusian State University, Belarusian State University of Informatics and Radio Electronics, Physical-Technical Institute, and the Research and Production Center for Material Science of the National Academy of Sciences of Belarus in collaboration with JINR physicists have developed, produced, and tested at the liquid helium temperature of 4.2 K, three pilot elliptical superconducting niobium cavities at a frequency of 1.3 GHz for accelerators of electrons and positrons. The cavities are constructed in the framework of the International Linear Collider (ILC) project. For the first time in Belarus, the cavities operated in superconducting mode, and the maximum value of the loaded Q-factor of the resonator equal to 2.8101 × 10 was obtained with the absolute matching of the cavity and the RF path at the standing wave ratio of 1.0000
Diagnostics of a spatial spin-modulated structure using nuclear magnetic resonance and Mössbauer spectroscopy
A new lycopod, Novgorodendron conophorum gen. et sp. nov., from the Lower Carboniferous of the Moscow Syneclise
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