726 research outputs found
Observation of superconductivity in the noncentrosymmetric nodal chain semimetal Ba5In4Bi5
The combination with superconductivity and topological nontrivial band
structure provides a promising route towards novel quantum states such as
topological superconductivity. Here, we report the first observation of
superconductivity (4.1 K) in Ba5In4Bi5 single crystal, a noncentrosymmetric
topological semimetal featuring nodal chain loops at the high-symmetry points R
and X. The magnetization, resistivity, and specific heat capacity measurements
reveal that Ba5In4Bi5 is a moderately coupled type-II Bardeen-Cooper-Schrieffer
superconductor. Bulk superconductivity is suggested from the magnetic
susceptibility and specific heat measurements. The results show that Ba5In4Bi5
provides a new platform for exploring the relationship of superconductivity and
topological nontrivial band topology
Photospheric flux cancellation and associated flux rope formation and eruption
We study an evolving bipolar active region that exhibits flux cancellation at
the internal polarity inversion line, the formation of a soft X-ray sigmoid
along the inversion line and a coronal mass ejection. The evolution of the
photospheric magnetic field is described and used to estimate how much flux is
reconnected into the flux rope. About one third of the active region flux
cancels at the internal polarity inversion line in the 2.5~days leading up to
the eruption. In this period, the coronal structure evolves from a weakly to a
highly sheared arcade and then to a sigmoid that crosses the inversion line in
the inverse direction. These properties suggest that a flux rope has formed
prior to the eruption. The amount of cancellation implies that up to 60% of the
active region flux could be in the body of the flux rope. We point out that
only part of the cancellation contributes to the flux in the rope if the arcade
is only weakly sheared, as in the first part of the evolution. This reduces the
estimated flux in the rope to or less of the active region flux. We
suggest that the remaining discrepancy between our estimate and the limiting
value of of the active region flux, obtained previously by the flux
rope insertion method, results from the incomplete coherence of the flux rope,
due to nonuniform cancellation along the polarity inversion line. A hot linear
feature is observed in the active region which rises as part of the eruption
and then likely traces out field lines close to the axis of the flux rope. The
flux cancellation and changing magnetic connections at one end of this feature
suggest that the flux rope reaches coherence by reconnection shortly before and
early in the impulsive phase of the associated flare. The sigmoid is destroyed
in the eruption but reforms within a few hours after a moderate amount of
further cancellation has occurred.Comment: Astron. Astrophys., in pres
Phototrophic Fe(II)-oxidation in the chemocline of a ferruginous meromictic lake
© 2014 Walter, Picazo, Miracle, Vicente, Camacho, Aragno and Zopfi. Precambrian Banded Iron Formation (BIF) deposition was conventionally attributed to the precipitation of iron-oxides resulting from the abiotic reaction of ferrous iron (Fe(II)) with photosynthetically produced oxygen. Earliest traces of oxygen date from 2.7 Ga, thus raising questions as to what may have caused BIF precipitation before oxygenic photosynthesis evolved. The discovery of anoxygenic phototrophic bacteria thriving through the oxidation of Fe(II) has provided support for a biological origin for some BIFs, but despite reports suggesting that anoxygenic phototrophs may oxidize Fe(II) in the environment, a model ecosystem of an ancient ocean where they are demonstrably active was lacking. Here we show that anoxygenic phototrophic bacteria contribute to Fe(II) oxidation in the water column of the ferruginous sulfate-poor, meromictic lake La Cruz (Spain). We observed in-situ photoferrotrophic activity through stimulation of phototrophic carbon uptake in the presence of Fe(II), and determined light-dependent Fe(II)-oxidation by the natural chemocline microbiota. Moreover, a photoferrotrophic bacterium most closely related to Chlorobium ferrooxidans was enriched from the ferruginous water column. Our study for the first time demonstrates a direct link between anoxygenic photoferrotrophy and the anoxic precipitation of Fe(III)-oxides in a ferruginous water column, providing a plausible mechanism for the bacterial origin of BIFs before the advent of free oxygen. However, photoferrotrophs represent only a minor fraction of the anoxygenic phototrophic community with the majority apparently thriving by sulfur cycling, despite the very low sulfur content in the ferruginous chemocline of Lake La Cruz
Pressure-induced magnetic transition and volume collapse in FeAs superconductors: An orbital-selective Mott scenario
Motivated by pressure experiments on FeAs-122 superconductors, we propose a
scenario based on local-moment physics to explain the simultaneous
disappearance of magnetism, reduction of the unit cell volume, and decrease in
resistivity. In this scenario, the low-pressure magnetic phase derives from Fe
moments, which become screened in the paramagnetic high-pressure phase. The
quantum phase transition can be described as an orbital-selective Mott
transition, which is rendered first order by coupling to the lattice, in
analogy to a Kondo volume collapse. Spin-fluctuation driven superconductivity
competes with antiferromagnetism and may be stabilized at low temperatures in
the high-pressure phase. The ideas are illustrated by a suitable mean-field
analysis of an Anderson lattice model.Comment: 9 pages, 3 figs; (v2) robustness of OS Mott transition vs. fragility
of superconductivity discussed, final version to be publishe
On the multi-orbital band structure and itinerant magnetism of iron-based superconductors
This paper explains the multi-orbital band structures and itinerant magnetism
of the iron-pnictide and chalcogenides. We first describe the generic band
structure of an isolated FeAs layer. Use of its Abelian glide-mirror group
allows us to reduce the primitive cell to one FeAs unit. From
density-functional theory, we generate the set of eight Fe and As
localized Wannier functions for LaOFeAs and their tight-binding (TB)
Hamiltonian, . We discuss the topology of the bands, i.e. allowed and
avoided crossings, the origin of the d6 pseudogap, as well as the role of the
As orbitals and the elongation of the FeAs tetrahedron. We then
couple the layers, mainly via interlayer hopping between As orbitals,
and give the formalism for simple and body-centered tetragonal stackings. This
allows us to explain the material-specific 3D band structures. Due to the high
symmetry, several level inversions take place as functions of or
pressure, resulting in linear band dispersions (Dirac cones). The underlying
symmetry elements are, however, easily broken, so that the Dirac points are not
protected, nor pinned to the Fermi level. From the paramagnetic TB Hamiltonian,
we form the band structures for spin spirals with wavevector by coupling
and . The band structure for stripe order is studied as a
function of the exchange potential, , using Stoner theory. Gapping of
the Fermi surface (FS) for small requires matching of FS dimensions
(nesting) and -orbital characters. The origin of the propeller-shaped FS is
explained. Finally, we express the magnetic energy as the sum over
band-structure energies, which enables us to understand to what extent the
magnetic energies might be described by a Heisenberg Hamiltonian, and the
interplay between the magnetic moment and the elongation of the FeAs4
tetrahedron
Anisotropic Impurity-States, Quasiparticle Scattering and Nematic Transport in Underdoped Ca(Fe1-xCox)2As2
Iron-based high temperature superconductivity develops when the `parent'
antiferromagnetic/orthorhombic phase is suppressed, typically by introduction
of dopant atoms. But their impact on atomic-scale electronic structure, while
in theory quite complex, is unknown experimentally. What is known is that a
strong transport anisotropy with its resistivity maximum along the crystal
b-axis, develops with increasing concentration of dopant atoms; this
`nematicity' vanishes when the `parent' phase disappears near the maximum
superconducting Tc. The interplay between the electronic structure surrounding
each dopant atom, quasiparticle scattering therefrom, and the transport
nematicity has therefore become a pivotal focus of research into these
materials. Here, by directly visualizing the atomic-scale electronic structure,
we show that substituting Co for Fe atoms in underdoped Ca(Fe1-xCox)2As2
generates a dense population of identical anisotropic impurity states. Each is
~8 Fe-Fe unit cells in length, and all are distributed randomly but aligned
with the antiferromagnetic a-axis. By imaging their surrounding interference
patterns, we further demonstrate that these impurity states scatter
quasiparticles in a highly anisotropic manner, with the maximum scattering rate
concentrated along the b-axis. These data provide direct support for the recent
proposals that it is primarily anisotropic scattering by dopant-induced
impurity states that generates the transport nematicity; they also yield simple
explanations for the enhancement of the nematicity proportional to the dopant
density and for the occurrence of the highest resistivity along the b-axis
Risks of nonchromosomal birth defects, small-for-gestational age birthweight, and prematurity with in vitro fertilization: effect of number of embryos transferred and plurality at conception versus at birth
PURPOSE: Excess embryos transferred (ET) (> plurality at birth) and fetal heartbeats (FHB) at 6 weeks' gestation are associated with reductions in birthweight and gestation, but prior studies have been limited by small sample sizes and limited IVF data. This analysis evaluated associations between excess ET, excess FHB, and adverse perinatal outcomes, including the risk of nonchromosomal birth defects. METHODS: Live births conceived via IVF from Massachusetts, New York, North Carolina, and Texas included 138,435 children born 2004-2013 (Texas), 2004-2016 (Massachusetts and North Carolina), and 2004-2017 (New York) were classified by ET and FHB. Major birth defects were reported by statewide registries within the first year of life. Logistic regression was used to estimate adjusted odds ratios (AORs) and 95% CIs of the risks of a major nonchromosomal birth defect, small-for-gestational age birthweight (SGA), low birthweight (LBW), and preterm birth (≤36 weeks), by excess ET, and excess ET + excess FHB, by plurality at birth (singletons and twins). RESULTS: In singletons with [2 ET, FHB =1] and [≥3 ET, FHB=1], risks [AOR (95% CI)] were increased, respectively, for major nonchromosomal birth defects [1.13 (1.00-1.27) and 1.18 (1.00-1.38)], SGA [1.10 (1.03-1.17) and 1.15 (1.05-1.26)], LBW [1.09 (1.02-1.13) and 1.17 (1.07-1.27)], and preterm birth [1.06 (1.00-1.12) and 1.14 (1.06-1.23)]. With excess ET + excess FHB, risks of all adverse outcomes except major nonchromosomal birth defects increased further for both singletons and twins. CONCLUSION: Excess embryos transferred are associated with increased risks for nonchromosomal birth defects, reduced birthweight, and prematurity in IVF-conceived births
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