164 research outputs found
Large Magnetoresistance in Compensated Semimetals TaAs and NbAs
We report large magnetoresistance (MR) at low temperatures in
single-crystalline nonmagnetic compounds TaAs and NbAs. Both compounds
exhibit parabolic-field-dependent MR larger than in a magnetic
field of 9 Tesla at 2 K. The MR starts to deviate from parabolic dependence
above 10 T and intends to be saturated in 45 T for TaAs at 4.2 K. The Hall
resistance measurements and band structural calculations reveal their
compensated semimetal characteristics. The large MR at low temperatures is
ascribed to a resonance effect of the balanced electrons and holes with large
mobilities. We also discuss the relation of the MR and samples' quality for
TaAs and other semimetals. We found that the magnitudes of MR are strongly
dependent on the samples' quality for different compounds.Comment: 26 pages, 11 figures, 2 table
Functional divergence of the NIP III subgroup proteins involved altered selective constraints and positive selection
Abstract Background Nod26-like intrinsic proteins (NIPs) that belong to the aquaporin superfamily are unique to plants. According to homology modeling and phylogenetic analysis, the NIP subfamily can be further divided into three subgroups with distinct biological functions (NIP I, NIP II, and NIP III). In some grasses, the NIP III subgroup proteins (NIP2s) were demonstrated to be permeable to solutes with larger diameter, such as silicic acid and arsenous acids. However, to date there is no data-mining or direct experimental evidences for the permeability of such larger solutes for dicot NIP2s, although they exhibit similar three-dimensional structures as those in grasses. It is therefore intriguing to investigate the molecular mechanisms that drive the evolution of plant NIP2s. Results The NIP III subgroup is more ancient with a divergence time that predates the monocot-dicot split. The proliferation of NIP2 genes in modern grass species is primarily attributed to whole genome and segmental chromosomal duplication events. The structure of NIP2 genes is relatively conserved, possessing five exons and four introns. All NIP2s possess an ar/R filter consisting of G, S, G, and R, except for the cucumber CsNIP2;2, where a small G in the H2 is substituted with the bulkier C residue. Our maximum likelihood analysis revealed that NIP2s, especially the loop A (LA) region, have undergone strong selective pressure for adaptive evolution. The analysis at the amino acid level provided strong statistical evidences for the functional divergence between monocot and dicot NIP III subgroup proteins. In addition, several SDPs (Specificity Determining Positions) responsible for functional specificity were predicted. Conclusions The present study provides the first evidences of functional divergence between dicot and monocot NIP2s, and suggests that positive selection, as well as a radical shift of evolutionary rate at some critical amino acid sites is the primary driver. These findings will expand our understanding to evolutionary mechanisms driving the functional diversification of monocot and dicot NIP III subgroup proteins.</p
Tip induced unconventional superconductivity on Weyl semimetal TaAs
Weyl fermion is a massless Dirac fermion with definite chirality, which has
been long pursued since 1929. Though it has not been observed as a fundamental
particle in nature, Weyl fermion can be realized as low-energy excitation
around Weyl point in Weyl semimetal, which possesses Weyl fermion cones in the
bulk and nontrivial Fermi arc states on the surface. As a firstly discovered
Weyl semimetal, TaAs crystal possesses 12 pairs of Weyl points in the momentum
space, which are topologically protected against small perturbations. Here, we
report for the first time the tip induced superconductivity on TaAs crystal by
point contact spectroscopy. A conductance plateau and sharp double dips are
observed in the point contact spectra, indicating p-wave like unconventional
superconductivity. Furthermore, the zero bias conductance peak in low
temperature regime is detected, suggesting potentially the existence of
Majorana zero modes. The experimentally observed tunneling spectra can be
interpreted with a novel mirror-symmetry protected topological superconductor
induced in TaAs, which can exhibit zero bias and double finite bias peaks, and
double conductance dips in the measurements. Our work can open a broad avenue
in search for new topological superconducting phases from topological Weyl
materials and trigger intensive investigations for pursuing Majorana fermions
Teacher Feedback Practices, Student Feedback Motivation, and Feedback Behavior: How Are They Associated With Learning Outcomes?
In spite of much recent theorizing about teacher provision of feedback, relatively fewer studies look at the dynamic relationships between teacher feedback practices, student feedback experience, and their learning outcomes in higher education settings. To fill this gap, this study looked at 308 university students' perceived teacher feedback practices and their feedback experiences in an English Studies course context at a key and non-key university, and explored how teacher feedback, student feedback motivation and feedback behavior were associated with students' course satisfaction and course exam performance. Results showed that students from the key university reported a higher level of teacher feedback use as well as student feedback motivation and behavior. Structural equation modeling (SEM) suggested that in the case of the non-key university, student feedback behavior significantly predicted course satisfaction and course exam results; teacher feedback also indirectly influenced course satisfaction and course exam results. In the case of the key university, while teacher feedback and student feedback behavior each had significant influence on course satisfaction, student feedback behavior showed no direct significant effect on course exam results, and teacher feedback also showed no significant indirect influence on course exam results
Divergence in function and expression of the NOD26-like intrinsic proteins in plants
<p>Abstract</p> <p>Background</p> <p>NOD26-like intrinsic proteins (NIPs) that belong to the aquaporin superfamily are plant-specific and exhibit a similar three-dimensional structure. Experimental evidences however revealed that functional divergence should have extensively occurred among NIP genes. It is therefore intriguing to further investigate the evolutionary mechanisms being responsible for the functional diversification of the NIP genes. To better understand this process, a comprehensive analysis including the phylogenetic, positive selection, functional divergence, and transcriptional analysis was carried out.</p> <p>Results</p> <p>The origination of NIPs could be dated back to the primitive land plants, and their diversification would be no younger than the emergence time of the moss <it>P. patens</it>. The rapid proliferation of NIPs in plants may be primarily attributed to the segmental chromosome duplication produced by polyploidy and tandem duplications. The maximum likelihood analysis revealed that <it>NIPs </it>should have experienced strong selective pressure for adaptive evolution after gene duplication and/or speciation, prompting the formation of distinct <it>NIP </it>groups. Functional divergence analysis at the amino acid level has provided strong statistical evidence for shifted evolutionary rate and/or radical change of the physiochemical properties of amino acids after gene duplication, and DIVERGE2 has identified the critical amino acid sites that are thought to be responsible for the divergence for further investigation. The expression of plant NIPs displays a distinct tissue-, cell-type-, and developmental specific pattern, and their responses to various stress treatments are quite different also. The differences in organization of <it>cis</it>-acting regulatory elements in the promoter regions may partially explain their distinction in expression.</p> <p>Conclusion</p> <p>A number of analyses both at the DNA and amino acid sequence levels have provided strong evidences that plant NIPs have suffered a high divergence in function and expression during evolution, which is primarily attributed to the strong positive selection or a rapid change of evolutionary rate and/or physiochemical properties of some critical amino acid sites.</p
A New Feature Points Reconstruction Method in Spacecraft Vision Navigation
The important applications of monocular vision navigation in aerospace are spacecraft ground calibration tests and spacecraft relative navigation. Regardless of the attitude calibration for ground turntable or the relative navigation between two spacecraft, it usually requires four noncollinear feature points to achieve attitude estimation. In this paper, a vision navigation system based on the least feature points is designed to deal with fault or unidentifiable feature points. An iterative algorithm based on the feature point reconstruction is proposed for the system. Simulation results show that the attitude calculation of the designed vision navigation system could converge quickly, which improves the robustness of the vision navigation of spacecraft
Recommended from our members
Open millifluidics based on powder-encased channels
Millifluidics, the manipulation of liquid flow in millimeter-sized channels, has been a revolutionary concept in chemical processing and engineering. The solid channels that contain the liquids, though, are not flexible in their design and modification, and prevent contact with the external environment. All-liquid constructs, on the other hand, while flexible and open, are imbedded in a liquid environment. Here, we provide a route to circumvent these limitations by encasing the liquids in a hydrophobic powder in air that jams on the surface, containing and isolating flowing fluids, offering flexibility and adaptability in design, as manifest in the ability to reconfigure, graft, and segment the constructs. Along with the open nature of these powder-contained channels that allow arbitrary connections/disconnections and substance addition/extraction, numerous applications can be opened in the biological, chemical, and material arenas
Ultraquantum magnetoresistance in Kramers Weyl semimetal candidate -Ag2Se
The topological semimetal -Ag2Se features a Kramers Weyl node at the
origin in momentum space and a quadruplet of spinless Weyl nodes, which are
annihilated by spin-orbit coupling. We show that single crystalline
-Ag2Se manifests giant Shubnikov-de Haas oscillations in the
longitudinal magnetoresistance which stem from a small electron pocket that can
be driven beyond the quantum limit by a field less than 9 T. This small
electron pocket is a remainder of the spin-orbit annihilatedWeyl nodes and thus
encloses a Berry-phase structure. Moreover, we observed a negative longitudinal
magnetoresistance when the magnetic field is beyond the quantum limit. Our
experimental findings are complemented by thorough theoretical band structure
analyses of this Kramers Weyl semimetal candidate, including first-principle
calculations and an effective k*p model.Comment: A new version based on arXiv:1502.0232
- …