2,432 research outputs found
Superconductivity and magnetism in platinum-substituted SrFe2As2 single crystals
Single crystals of SrFe2-xPtxAs2 (0 < x < 0.36) were grown using the self
flux solution method and characterized using x-ray crystallography, electrical
transport, magnetic susceptibility, and specific heat measurements. The
magnetic/structural transition is suppressed with increasing Pt concentration,
with superconductivity seen over the range 0.08 < x < 0.36 with a maximum
transition temperature Tc of 16 K at x = 0.16. The shape of the phase diagram
and the changes to the lattice parameters are similar to the effects of other
group VIII elements Ni and Pd, however the higher transition temperature and
extended range of superconductivity suggest some complexity beyond the simple
electron counting picture that has been discussed thus far.Comment: 6 pages, 6 figure
NHEJ protects mycobacteria in stationary phase against the harmful effects of desiccation
The physiological role of the non-homologous end-joining (NHEJ) pathway in the repair of DNA double-strand breaks (DSBs) was examined in Mycobacterium smegmatis using DNA repair mutants (DeltarecA, Deltaku, DeltaligD, Deltaku/ligD, DeltarecA/ku/ligD). Wild-type and mutant strains were exposed to a range of doses of ionizing radiation at specific points in their life-cycle. NHEJ-mutant strains (Deltaku, DeltaligD, Deltaku/ligD) were significantly more sensitive to ionizing radiation (IR) during stationary phase than wild-type M. smegmatis. However, there was little difference in IR sensitivity between NHEJ-mutant and wild-type strains in logarithmic phase. Similarly, NHEJ-mutant strains were more sensitive to prolonged desiccation than wild-type M. smegmatis. A DeltarecA mutant strain was more sensitive to desiccation and IR during both stationary and especially in logarithmic phase, compared to wild-type strain, but it was significantly less sensitive to IR than the DeltarecA/ku/ligD triple mutant during stationary phase. These data suggest that NHEJ and homologous recombination are the preferred DSB repair pathways employed by M. smegmatis during stationary and logarithmic phases, respectively
Electronic Structure and Lattice dynamics of NaFeAs
The similarity of the electronic structures of NaFeAs and other Fe pnictides
has been demonstrated on the basis of first-principle calculations. The global
double-degeneracy of electronic bands along X-M and R-A direction indicates the
instability of Fe pnictides and is explained on the basis of a tight-binding
model. The de Haas-van Alphen parameters for the Fermi surface (FS) of NaFeAs
have been calculated. A spin density wave (SDW)
instead of a charge density wave (CDW) ground state is predicted based on the
calculated generalized susceptibility and a criterion
derived from a restricted Hatree-Fock model. The strongest electron-phonon
(e-p) coupling has been found to involve only As, Na z-direction vibration with
linear-response calculations. A possible enhancement mechanism for e-p coupling
due to correlation is suggested
Determination of characteristic muon precession and relaxation signals in FeAs and FeAs2, possible impurity phases in pnictide superconductors
We report muon-spin relaxation measurements of highly homogeneous samples of
FeAs and FeAs2, both previously found as impurity phases in some samples of
recently synthesized pnictide superconductors. We observe well defined muon
precession in the FeAs sample with two precession frequencies of 38.2(3) and
22.7(9) MHz at 7.5 K, with the majority of the amplitude corresponding to the
lower frequency component. In FeAs2 we confirm previous measurements showing
that no long-ranged magnetic order occurs above 2 K and measure the muon spin
relaxation rate, which increases on cooling. Our results exclude the
possibility that previous muon-spin relaxation measurements of pnictide
superconductors have been measuring the effect of these possible impurities.Comment: 4 pages, 3 figures, corrected Figure
Enhancement of Superfluid Stiffness, Suppression of Superconducting T_c and Field-induced Magnetism in the Pnictide Superconductor LiFeAs
Transverse-field muon-spin rotation measurements performed on two samples of
LiFeAs demonstrate that the superfluid stiffness of the superconducting
condensate in relation to its superconducting transition temperature is
enhanced compared to other pnictide superconductors. Evidence is seen for a
field-induced magnetic state in a sample with a significantly suppressed
superconducting transition temperature. The results in this system highlight
the role of direct Fe-Fe interactions in frustrating pairing mediated by
antiferromagnetic fluctuations and suggest that, in common with other pnictide
superconductors, the system is close to a magnetic instability.Comment: 4 pages, 5 figure
Pressure shift of the superconducting T_c of LiFeAs
The effect of hydrostatic pressure on the superconductivity in LiFeAs is
investigated up to 1.8 GPa. The superconducting transition temperature, T_c,
decreases linearly with pressure at a rate of 1.5 K/GPa. The negative pressure
coefficient of T_c and the high ambient pressure T_c indicate that LiFeAs is
the high-pressure analogue of the isoelectronic SrFe_2As_2 and BaFe_2As_2.Comment: 3 pages, 2 figure
Charge carrier localization induced by excess Fe in the Fe1+y(Te,Se) superconductor system
We have investigated the effect of Fe nonstoichiometry on properties of the
Fe1+y(Te, Se) superconductor system by means of resistivity, Hall coefficient,
magnetic susceptibility, and specific heat measurements. We find that the
excess Fe at interstitial sites of the (Te, Se) layers not only suppresses
superconductivity, but also results in a weakly localized electronic state. We
argue that these effects originate from the magnetic coupling between the
excess Fe and the adjacent Fe square planar sheets, which favors a short-range
magnetic order.Comment: 15 pages, 6 figures accepted for publication in PR
A new family of iron pnictides: BaFeAs2 and BaFeSb2
We investigate the structural, electronic, and magnetic properties of the
hypothetical compound BaFePn2 (Pn = As and Sb), which is isostructural to the
parent compound of the high temperature superconductor LaFeAsO1-xFx. Using
density functional theory, we show that the Fermi surface, electronic structure
and the spin density wave instability of BaFePn2 are very similar to the Fe
based superconductors. Additionally, there are very dispersive metallic bands
of a spacer Pn layer, which are almost decoupled from FePn layer. Our results
show that experimental study of BaFePn2 can test the role of charge and
polarization fluctuation, importance of two dimensionality in mechanism of
superconductivity.Comment: 4.2 pages, 4 figure
Biological surrogacy in tropical seabed assemblages fails
Surrogate taxa are used widely to represent attributes of other taxa for which data are sparse or absent. Because surveying and monitoring marine biodiversity is resource intensive, our understanding and management of marine systems will need to rely on the "availability of effective surrogates. The ability of any marine taxon to adequately, represent another, however, is largely unknown because there are rarely sufficient data for multiple taxa in the same region(s). Here, we defined a taxonomic group to be a surrogate for another taxonomic group if they possessed similar assemblage patterns. We investigated effects on surrogate performance of (1) grouping species by taxon at various levels of resolution, (2) selective removal of rare species from analysis, and (3) the number of clusters used to define assemblages, using samples for 11 phyla distributed across 1189 sites sampled from the seabed of Australia's Great Barrier Reef. This spatially and taxonomically comprehensive data set provided an opportunity for extensive testing of surrogate performance in a tropical marine system using these three approaches. for the first time, as resource and data constraints were previously limiting. We measured surrogate performance as to how similarly sampling sites were divided into assemblages between taxa. For each taxonomic group independently, we grouped sites into assemblages using. He linger distances and medoid clustering. We then used a similarity index to quantify the concordance of assemblages between all pairs Of taxonomic groups. Surrogates performed better when taxa were grouped at a phylum level, compared to taxa grouped at a finer taxonomic resolution, and were unaffected by the exclusion of spatially rare species. Mean surrogate performance increased as the number of clusters decreased. Moreover, no taxonomic group was a particularly good surrogate for any other, suggesting that the use of any one (or few) group(s) for mapping seabed biodiversity patterns is imprudent; sampling several taxonomic groups appears to be essential for understanding tropical/subtropical seabed communities. Consequently, where resource constraints do not allow complete surveying of biodiversity, it may be preferable to exclude rare species to allow investment in a broader range of taxonomic groups
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