Several lines of geological and geochemical evidence indicate that the level of atmospheric oxygen was extremely low before 2.45 billion years (Gyr) ago, and that it had reached considerable levels ...by 2.22 Gyr ago. Here we present evidence that the rise of atmospheric oxygen had occurred by 2.32 Gyr ago. We found that syngenetic pyrite is present in organic-rich shales of the 2.32-Gyr-old Rooihoogte and Timeball Hill formations, South Africa. The range of the isotopic composition of sulphur in this pyrite is large and shows no evidence of mass-independent fractionation, indicating that atmospheric oxygen was present at significant levels (that is, greater than 10(-5) times that of the present atmospheric level) during the deposition of these units. The presence of rounded pebbles of sideritic iron formation at the base of the Rooihoogte Formation and an extensive and thick ironstone layer consisting of haematitic pisolites and oölites in the upper Timeball Hill Formation indicate that atmospheric oxygen rose significantly, perhaps for the first time, during the deposition of the Rooihoogte and Timeball Hill formations. These units were deposited between what are probably the second and third of the three Palaeoproterozoic glacial events.
The strange form factors of the nucleon are studied with the nonlocal chiral effective Lagrangian. One loop contributions from both octet and decuplet intermediate states are included. The ...relativistic regulator is obtained by the nonlocal Lagrangian where the gauge link is introduced to guarantee the local gauge symmetry. With the kaon loop, the calculated charge form factor is positive, while the magnetic form factor is negative. The strange magnetic moment is −0.041−0.014+0.012 with Λ=0.9±0.1 determined from the nucleon electromagnetic form factors. Our results are comparable with the recent lattice simulation.
We propose a mechanism for the Sivers distribution function in a proton with the chiral Lagrangian. By introducing the gauge link of the vector meson, the transverse momentum dependent distribution ...of a pion in the nucleon is redefined, which is locally SU(2)V invariant as the Lagrangian. The eikonal propagator is generated from the gauge link, and this scenario is proven to be equivalent to the final state interaction. By combining the calculated splitting function and the valence q¯ distribution in π from the recent fit, the sea quark Sivers function in a proton is obtained. We find reasonable numerical results for the first momentum xΔNfq¯(1)(x) without any fine tuning of the free parameters.
Retrotransposons play an important role in genome evolution but pose acute challenges to host genome integrity, particularly in early stage germ cells where epigenetic control is relaxed to permit ...genome-wide reprogramming. In most species, the inability to silence retrotransposons in the germline is usually associated with sterility. LINE1 is the most abundant retrotransposon type in the mammalian genome. Mammalian germ cells employ multiple mechanisms to suppress retrotransposon activity, including small non-coding piRNAs, DNA methylation, and repressive histone modifications. Novel factors contributing to the epigenetic silencing of retrotransposons in the germline continue to be identified. Recent studies have provided insight into how epigenetic changes associated with retrotransposon activation impact on fertility.
By combining angle-resolved photoemission spectroscopy and quantum oscillation measurements, we performed a comprehensive investigation on the electronic structure of LaSb, which exhibits ...near-quadratic extremely large magnetoresistance (XMR) without any sign of saturation at magnetic fields as high as 40 T. We clearly resolve one spherical and one intersecting-ellipsoidal hole Fermi surfaces (FSs) at the Brillouin zone (BZ) center Γ and one ellipsoidal electron FS at the BZ boundary X. The hole and electron carriers calculated from the enclosed FS volumes are perfectly compensated, and the carrier compensation is unaffected by temperature. We further reveal that LaSb is topologically trivial but shares many similarities with the Weyl semimetal TaAs family in the bulk electronic structure. Based on these results, we have examined the mechanisms that have been proposed so far to explain the near-quadratic XMR in semimetals.
Abstract Radio observations of pulsars offer a potential method to probe the intricate microstructure in the turbulent interstellar medium. Here we report on a high-resolution dynamic spectral ...analysis of the “swooshing pulsar” B0919+06 observed with the Five-hundred-meter Aperture Spherical radio Telescope over multiple epochs and with the ultrawideband receiver on the Parkes radio telescope. For all observations, the dynamic scintillation spectra, two-dimensional autocovariance functions, and secondary spectra are presented. At 1250 MHz, the decorrelation bandwidth, diffraction timescale, and the drift rate are determined to be Δ ν d = 25.89 ± 7.55 MHz, Δ τ d = 14.42 ± 3.98 minutes, and dt / d ν = 0.07 ± 0.14 minutes MHz −1 , respectively. The frequency dependencies of the scintillation parameters exhibit single power-law spectral behaviors, indicating that the electron density fluctuations in the interstellar medium approximately follow the Kolmogorov spectrum. The secondary spectra exhibit two distinct parabolic arcs with well-determined curvatures of 0.002 and 0.02 s 3 for the outer and inner arcs, respectively. The locations of the scattering screens are approximately determined to be 157.3 and 726.0 pc, respectively, from the pulsar for isotropic scattering. The inner scintillation arc is present contemporaneously over a wide frequency range, indicating that the scintillation arc is a broadband phenomenon. The arc curvature scales with observing frequency as a power law with an index of −2.05 ± 0.05, which implies that the scattering screen spans a physical distance from 689.7 to 883.3 pc from the pulsar.
A theoretical model for the thermoelectric coupling analysis of thermal materials with an inclined elliptic hole is constructed. The extended problem of an elliptic hole under biaxial loading is also ...discussed. Theoretical and numerical results for the thermoelectric coupling resulting thermal stress are obtained. Results show that, for the biaxial loaded elliptic hole, the solution cannot be the linear superposition of the solutions of two uniaxial loaded cases due to the nonlinear coupling. For inclined elliptic hole, the maximum thermoelectric concentration occurs when the major axis is perpendicular to the loading direction. However, the maximum stress concentration occurs when the major axis is parallel to the loading direction. The solution of a circle hole problem is given as a special case in the framework of elliptic hole problem. The crack problem is also discussed when the ellipse degenerates into a crack. It is found that all field intensity factors exhibit the traditional inverse square-root singularity at the crack tip. The mode I stress intensity factor only depends on the applied electric current. The mode II stress intensity factor only depends on the applied energy flux when the crack line is perpendicular to the loading direction. However, in the biaxial loading case, the mode II stress intensity factor relies on the applied electric current density and energy flux. This is the first paper to conduct a strict closed-form solution for an inclined elliptic hole in thermoelectric materials.
This paper analyses a two-dimensional problem in thermoelectric materials with an inclined elliptic inclusion. We have obtained the closed-form solutions of electric current density and temperature ...considering the inclusion's electrical and thermal permeability. Based on the derived thermoelectric field, the thermal stresses are given in the explicit form and the effect of inclusion on effective thermoelectric properties is investigated. The electrically impermeable and thermally impermeable inclusions will respectively cause maximum electric current concentration and heat concentration. The thermally impermeable and rigid inclusion will cause maximum stress concentration around the inclusion. Furthermore, we find that the effective electric conductivity (effective thermal conductivity) of the matrix-inclusion system is increased when the inclusion have higher electric conductivity (thermal conductivity) than the matrix. It is possible to enhance the effective figure of merit by inserting inclusions with specific electric conductivity and heat conductivity. It predicts a new way for the design of high-performance thermoelectric devices. The results in this paper can be directly used for reliability consideration in design and optimization of thermoelectric devices in engineering.
The amplification of primordial magnetic fields via a small-scale turbulent dynamo during structure formation might be able to explain the observed magnetic fields in galaxy clusters. The ...magnetization of more tenuous large-scale structures such as cosmic filaments is more uncertain, as it is challenging for numerical simulations to achieve the required dynamical range. In this work, we present magnetohydrodynamical cosmological simulations on large uniform grids to study the amplification of primordial seed fields in the intracluster medium (ICM) and in the warm–hot-intergalactic medium (WHIM). In the ICM, we confirm that turbulence caused by structure formation can produce a significant dynamo amplification, even if the amplification is smaller than what is reported in other papers. In the WHIM inside filaments, we do not observe significant dynamo amplification, even though we achieve Reynolds numbers of R
e ∼ 200–300. The maximal amplification for large filaments is of the order of ∼100 for the magnetic energy, corresponding to a typical field of a few ∼nG starting from a primordial weak field of 10−10 G (comoving). In order to start a small-scale dynamo, we found that a minimum of ∼102 resolution elements across the virial radius of galaxy clusters was necessary. In filaments we could not find a minimum resolution to set off a dynamo. This stems from the inefficiency of supersonic motions in the WHIM in triggering solenoidal modes and small-scale twisting of magnetic field structures. Magnetic fields this small will make it hard to detect filaments in radio observations.
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