496 research outputs found
Quasiparticle interference of C2-symmetric surface states in LaOFeAs parent compound
We present scanning tunneling microscopy studies of the LaOFeAs parent
compound of iron pnictide superconductors. Topographic imaging reveals two
types of atomically flat surfaces, corresponding to the exposed LaO layer and
FeAs layer respectively. On one type of surface, we observe strong standing
wave patterns induced by quasiparticle interference of two-dimensional surface
states. The distribution of scattering wavevectors exhibits pronounced two-fold
symmetry, consistent with the nematic electronic structure found in the
Ca(Fe1-xCox)2As2 parent state.Comment: 13 pages, 4 figure
Structure and composition of the superconducting phase in alkali iron selenide KFeSe
We use neutron diffraction to study the temperature evolution of the average
structure and local lattice distortions in insulating and superconducting
potassium iron selenide KFeSe. In the high temperature
paramagnetic state, both materials have a single phase with crystal structure
similar to that of the BaFeAs family of iron pnictides. While the
insulating KFeSe forms a iron
vacancy ordered block antiferromagnetic (AF) structure at low-temperature, the
superconducting compounds spontaneously phase separate into an insulating part
with iron vacancy order and a superconducting phase
with chemical composition of KFeSe and BaFeAs structure.
Therefore, superconductivity in alkaline iron selenides arises from alkali
deficient KFeSe in the matrix of the insulating block AF phase.Comment: 10 pages, 5 figure
The antenna of horse stomach bot flies:morphology and phylogenetic implications (Oestridae, Gasterophilinae: <i>Gasterophilus</i> Leach)
Antennae are among the most elaborate sensory organs in adult flies, and they provide rich information for phylogenic studies. The antennae of five out of eight species of Gasterophilus Leach (G. haemorrhoidalis (Linnaeus), G. intestinalis (De Geer), G. nasalis (Linnaeus), G. nigricornis (Loew) and G. pecorum (Fabricius)), were examined using scanning electron microscopy. The general morphology, including distribution, type, size, and ultrastructure of antennal sensilla were presented, and the definition of auriculate sensilla and sensory pits were updated and clarified. Eighteen antennal characters were selected to construct the first species-level phylogeny of this genus. The monophyly of Gasterophilus was supported by the presence of coeloconic sensilla III on the antennal arista. The species-level cladogram showed G. pecorum branching off at the base, and the remaining species forming the topology (G. intestinalis+ (G. haemorrhoidalis+ (G. nasalis+ G. nigricornis))). Our research shows the importance of the antennal ultrastructure as a reliable source for phylogenetic analysis
Strong similarities between the local electronic structure of insulating iron pnictide and lightly doped cuprate
One of the major puzzles regarding unconventional superconductivity is how
some of the most interesting superconductors are related to an insulating phase
that lies in close proximity. Here we report scanning tunneling microscopy
studies of the local electronic structure of Cu doped NaFeAs across the
superconductor to insulator transition. We find that in the highly insulating
regime the electronic spectrum develops an energy gap with diminishing density
of state at the Fermi level. The overall lineshape and strong spatial
variations of the spectra are strikingly similar to that of lightly doped
cuprates close to the parent Mott insulator. We propose that the suppression of
itinerant electron state and strong impurity potential induced by Cu dopants
lead to this insulating iron pnictide.Comment: 6 figures, to appear in Phys. Rev.
Spin-density-wave transition in double-layer nickelate La3Ni2O7
Recently, a signature of high-temperature superconductivity above the liquid
nitrogen temperature (77 K) was reported for La3Ni2O7 under pressure. This
finding immediately stimulates intense interest in the possible high-Tc
superconducting mechanism in double-layer nickelates. Interestingly, the
pressure-dependent phase diagram inferred from transport measurements indicates
that superconductivity under high pressure emerges from the suppression of a
density-wave-like transition at ambient pressure, which is similar to
high-temperature superconductors. Therefore, clarifying the exact nature of the
density-wave-like transition is important for determining the mechanism of
superconductivity in double-layer nickelates. Here, nuclear magnetic resonance
(NMR) spectroscopy of 139La nuclei was performed to study the density-wave-like
transition in a single crystal of La3Ni2O7. The temperature-dependent 139La NMR
spectrum and nuclear spin-lattice relaxation rate (1/T1) provide unambiguous
evidence for a spin-density-wave (SDW) transition with a transition temperature
TSDW of ~ 150 K. Furthermore, the anisotropic splitting of the NMR spectrum
suggests a possible double spin stripe with magnetic moments along the c axis.
In addition, the present NMR measurements also revealed spatial inhomogeneity
of magnetism due to inner apical oxygen vacancies. All these results will be
helpful for building a connection between superconductivity and magnetic
interactions in double-layer nickelates.Comment: 14 pages, 4 figure
Genomic analysis of a sexually-selected character: EST sequencing and microarray analysis of eye-antennal imaginal discs in the stalk-eyed fly Teleopsis dalmanni (Diopsidae)
<p>Abstract</p> <p>Background</p> <p>Many species of stalk-eyed flies (Diopsidae) possess highly-exaggerated, sexually dimorphic eye-stalks that play an important role in the mating system of these flies. Eye-stalks are increasingly being used as a model system for studying sexual selection, but little is known about the genetic mechanisms producing variation in these ornamental traits. Therefore, we constructed an EST database of genes expressed in the developing eye-antennal imaginal disc of the highly dimorphic species <it>Teleopsis dalmanni</it>. We used this set of genes to construct microarray slides and compare patterns of gene expression between lines of flies with divergent eyespan.</p> <p>Results</p> <p>We generated 33,229 high-quality ESTs from three non-normalized libraries made from the developing eye-stalk tissue at different developmental stages. EST assembly and annotation produced a total of 7,066 clusters comprising 3,424 unique genes with significant sequence similarity to a protein in either <it>Drosophila melanogaster </it>or <it>Anopheles gambiae</it>. Comparisons of the transcript profiles at different stages reveal a developmental shift in relative expression from genes involved in anatomical structure formation, transcription, and cell proliferation at the larval stage to genes involved in neurological processes and cuticle production during the pupal stages. Based on alignments of the EST fragments to homologous sequences in <it>Drosophila </it>and <it>Anopheles</it>, we identified 20 putative gene duplication events in <it>T. dalmanni </it>and numerous genes undergoing significantly faster rates of evolution in <it>T. dalmanni </it>relative to the other Dipteran species. Microarray experiments identified over 350 genes with significant differential expression between flies from lines selected for high and low relative eyespan but did not reveal any primary biological process or pathway that is driving the expression differences.</p> <p>Conclusion</p> <p>The catalogue of genes identified in the EST database provides a valuable framework for a comprehensive examination of the genetic basis of eye-stalk variation. Several candidate genes, such as <it>crooked legs</it>, <it>cdc2</it>, <it>CG31917 </it>and <it>CG11577</it>, emerge from the analysis of gene duplication, protein evolution and microarray gene expression. Additional comparisons of expression profiles between, for example, males and females, and species that differ in eye-stalk sexual dimorphism, are now enabled by these resources.</p
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