3,152 research outputs found
Orbital density wave induced by electron-lattice coupling in orthorhombic iron pnictides
In this paper we explore the magnetic and orbital properties closely related
to a tetragonal-orthorhombic structural phase transition in iron pnictides
based on both two- and five-orbital Hubbard models. The electron-lattice
coupling, which interplays with electronic interaction, is self-consistently
treated. Our results reveal that the orbital polarization stabilizes the spin
density wave (SDW) order in both tetragonal and orthorhombic phases. However,
the ferro-orbital density wave (F-ODW) only occurs in the orthorhombic phase
rather than in the tetragonal one. Magnetic moments of Fe are small in the
intermediate Coulomb interaction region for the striped antiferromangnetic
phase in the realistic five orbital model. The anisotropic Fermi surface in the
SDW/ODW orthorhombic phase is well in agreement with the recent angle-resolved
photoemission spectroscopy experiments. These results suggest a scenario that
the magnetic phase transition is driven by the ODW order mainly arising from
the electron-lattice coupling.Comment: 21 pages, 10 figure
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Repeated evolution of durophagy during ichthyosaur radiation after mass extinction indicated by hidden dentition.
Marine tetrapods quickly diversified and were established as marine top predators after the end-Permian Mass extinction (EPME). Ichthyosaurs were the forerunner of this rapid radiation but the main drivers of the diversification are poorly understood. Cartorhynchus lenticarpus is a basal ichthyosauriform with the least degree of aquatic adaptation, holding a key to identifying such a driver. The unique specimen appeared edentulous based on what was exposed but a CT scanning revealed that the species indeed had rounded teeth that are nearly perpendicular to the jaw rami, and thus completely concealed in lateral view. There are three dental rows per jaw ramus, and the root lacks infoldings of the dentine typical of ichthyopterygians. The well-developed and worn molariform dentition with three tooth rows supports the previous inference that the specimen is not of a juvenile. The premaxilla and the corresponding part of the dentary are edentulous. Molariform dentition evolved three to five times independently within Ichthyosauriformes in the Early and Middle Triassic. Convergent exploitation of hard-shelled invertebrates by different subclades of ichthyosauriforms likely fueled the rapid taxonomic diversification of the group after EPME
NH (1,1) hyperfine intensity anomalies in the Orion A molecular cloud
In LTE, the two inner satellite lines (ISLs) and the two outer satellite
lines (OSLs) of the NH (1,1) transition are each predicted to have equal
intensities. However, hyperfine intensity anomalies (HIAs) are observed to be
omnipresent in star formation regions, which is still not fully understood. In
addressing this issue, we find that the computation method of the HIA by the
ratio of the peak intensities may have defects, especially when being used to
process the spectra with low velocity dispersions. Therefore we define the
integrated HIAs of the ISLs (HIA) and OSLs (HIA) by the
ratio of their redshifted to blueshifted integrated intensities and develop a
procedure to calculate them. Based on this procedure, we present a systematic
study of the integrated HIAs in the northern part of the Orion A MC. We find
that integrated HIA and HIA are commonly present in the
Orion A MC and no clear distinction is found at different locations of the MC.
The medians of the integrated HIA and HIA are
0.9210.003 and 1.4220.009, respectively, which is consistent with the
HIA core model and inconsistent with the CE model. Selecting those 170
positions where both integrated HIAs deviate by more than 3- from
unity, most (166) are characterized by HIA1,
which suggests that the HIA core model plays a more significant role than the
CE model. The remaining four positions are consistent with the CE model. We
compare the integrated HIAs with the para-NH column density
((para-NH)), kinetic temperature (), total velocity
dispersion (), non-thermal velocity dispersion (), and the total opacity of the NH (1,1) line (). Their
correlations can not be fully explained by neither the HIA core nor the CE
model.Comment: 12 pages, 10 figures, accepted for publication in A&
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