Synthesis and search for superconductivity in LiBC

Following the recent theoretical prediction of superconductivity in hole doped LiBC by Rosner et al [1], we have attempted to synthesise Li deficient LixBC (x = 1, 0.8, 0.6 and 0.4) and look for superconductivity in this system. Our synthesis procedure, following the recipe for MgB2, involves reaction of elemental components in a Ta crucible at 900o C under 50 bar of argon pressure. X-ray diffraction measurements indicate the formation of P63/mmc structure up to x=0.6. However, no diamagnetic signal or zero resistance, corresponding to the superconducting transition, was observed in the temperature range of 300 to 4 K. This is possibly related to the presence of disorder in the B-C stacking; evidence for which is suggested from a study of the vibrational modes of LixBC through infrared spectroscopy.Comment: 7 pages, 6 figures 1 tabl

Critical properties of superconducting Ba1-xKxFe2As2

Magnetisation and magnetoresistance measurements have been carried out on superconducting Ba1-xKxFe2As2 samples with x=0.40 and 0.50. From low field magnetization data carried out at different temperatures below TC, HC1 has been extracted. The plot of HC1 versus temperature shows an anomalous increase at low temperatures. From high field magnetization hysterisis measurements carried out in fields up to 16 T at 4.2 K and 20 K, the critical current density has been evaluated using the Bean critical state model. The JC determined from the high field data is >104A/cm2 at 4.2 K and 5 T. The superconducting transitions were also measured resistively in increasing applied magnetic fields up to 12 Tesla. From the variation of the TC onset with applied field, dHC2/dT at TC was obtained to be -7.708 T/K and -5.57 T/K in the samples with x=0.40 and 0.50.Comment: 14 pages; 7 figure

Drastic ground state changes induced by Ni substitution in NaxCoO2

We report on the effect of Ni substitution at the Co site on the physical properties of NaxCoO2 system by investigating the series NaxCo1-yNiyO2 (x=0.75, 0<y<0.15). An upturn in the resistivity is observed in all Ni substituted samples as the temperature is lowered, suggestive of the occurrence of a Metal-Insulator Transition (MIT). The temperature at which this transition occurs increases with Ni content. The temperature dependence of the resistivity in the metallic region in the Ni substituted samples shows a T2 dependence, which is qualitatively different from that observed in the pristine sample. The evolution of the Fano asymmetry parameter, extracted by analyzing the lineshape of the IR active in-plane Co-O mode, both as a function of Ni concentration and temperature corroborates the occurrence of the MIT. It is argued that the progressive substitution of the Co4+ ions with Ni increases the probability of double occupancy and therefore the on-site Coulomb interaction energy leading to a shift in the thermodynamically driven MIT to higher temperatures.Comment: 17 pages 9 figures; submitted to Journal Of Phys. Cond. Ma

Dislocation Driven Chromium Precipitation in Fe-9Cr Binary Alloy: A Positron Lifetime Study

The influence of initial heat treatment on anomalous Cr precipitation within high temperature solubility region in Fe-9Cr alloy has been investigated using positron lifetime studies. Air-quenched samples with pre-existing dislocations exhibited a distinct annealing stage in positron lifetime between 800 and 1100 K corresponding to Cr-precipitation. During this stage, Transmission Electron Microscopy showed fine precipitates of average size 4 nm, dispersed throughout the sample and from EDS analysis they are found to be Cr-enriched. The existence of dislocations is found to be responsible for Cr precipitation.Comment: Revised version Submitted to Phys. Rev.

Large expert-curated database for benchmarking document similarity detection in biomedical literature search

Document recommendation systems for locating relevant literature have mostly relied on methods developed a decade ago. This is largely due to the lack of a large offline gold-standard benchmark of relevant documents that cover a variety of research fields such that newly developed literature search techniques can be compared, improved and translated into practice. To overcome this bottleneck, we have established the RElevant LIterature SearcH consortium consisting of more than 1500 scientists from 84 countries, who have collectively annotated the relevance of over 180 000 PubMed-listed articles with regard to their respective seed (input) article/s. The majority of annotations were contributed by highly experienced, original authors of the seed articles. The collected data cover 76% of all unique PubMed Medical Subject Headings descriptors. No systematic biases were observed across different experience levels, research fields or time spent on annotations. More importantly, annotations of the same document pairs contributed by different scientists were highly concordant. We further show that the three representative baseline methods used to generate recommended articles for evaluation (Okapi Best Matching 25, Term Frequency-Inverse Document Frequency and PubMed Related Articles) had similar overall performances. Additionally, we found that these methods each tend to produce distinct collections of recommended articles, suggesting that a hybrid method may be required to completely capture all relevant articles. The established database server located at https://relishdb.ict.griffith.edu.au is freely available for the downloading of annotation data and the blind testing of new methods. We expect that this benchmark will be useful for stimulating the development of new powerful techniques for title and title/abstract-based search engines for relevant articles in biomedical research.Peer reviewe

Towards Simulation of Dislocation-Based Creep Mechanisms: Application to Oxide Dispersion Strengthening Steels

International audienc

Emerging trends in molecular systematics and molecular phylogeny of mayflies (Insecta: Ephemeroptera)

Current trends are reviewed in the molecular systematics and phylogeny of the Ephemeroptera (mayflies), an ancient monophyletic lineage of pterygote insects. Theories of mayfly origins are analyzed, followed by a discussion of higher classification schemes in light of recent developments in molecular systematics. Ephemeroptera evolution is a classic example of ancient rapid radiation, presenting challenges for phylogenetic analysis. The utility of combined studies of morphological and molecular data is substantiated with examples and the role of molecular systematics in unraveling the taxonomy of cryptic species complexes is highlighted. The importance of DNA barcoding in mayfly taxonomy is discussed in the light of recent progress, and future contributions of genetics to the study of taxonomy, ecology and evolution in mayflies are discussed