Muon-spin rotation measurements of the penetration depth of the Mo_3Sb_7 superconductor

Measurements of the magnetic field penetration depth \lambda in superconductor Mo_3Sb_7 (T_c~2.1 K) were carried out by means of muon-spin-rotation. The absolute values of \lambda, the Ginzburg-Landau parameter \kappa, the first H_{c1} and the second H_{c2} critical fields at T=0 are \lambda(0)=720(100)nm, \kappa(0)=55(9), \mu_0H_{c1}(0)=1.8(3)mT, and \mu_0H_{c2}(0)=1.9(2)T. The zero temperature value of the superconducting energy gap \Delta(0) was found to be 0.35(1)meV corresponding to the ratio 2\Delta(0)/k_BT_c=3.83(10). At low temperatures \lambda^{-2}(T) saturates and becomes constant below T~0.3T_c, in agreement with what is expected for s-wave BCS superconductors. Our results suggest that Mo_3Sb_7 is a BCS superconductor with the isotropic energy gapComment: 5 pages, 4 figure

Scaling Relationships across Chemically Related Adsorbates for Fast Screening of Alloy Catalysts for Propane Dehydrogenation

A catalyst is a material which speeds up the rate of a specific reaction. A reaction that is of significant importance to the chemical industry is the selective transformations of light alkanes (ethane, propane), which are largely available in shale gas, to olefins. These olefins can then be converted into higher-value chemicals, materials, and fuels. However, there are several undesired reactions that take place alongside the main dehydrogenation reaction, so a catalyst of high selectivity is desired. Apart from the industrially used PtSn catalyst, work by our experimental collaborators has shown that various other Pt and Pd alloys (In, Zn) are also highly selective and active for propane dehydrogenation (PDH). We aim to use Density Functional theory (DFT) to determine the trends in catalytic reactivity and selectivity across a wider space of alloy catalysts. This can be achieved through the development of linear scaling relationships (LSR), which relate the thermodynamic adsorption properties of chemically related species on various alloys. Initial results on the Pd alloys (Pd3Sn, PdIn, PdZn) show that the alloys have lower binding energies as compared to a pure metal. These results can have implications on the higher selectivity observed on the alloys experimentally

Single crystal of superconducting SmFeAsO1-xFy grown at high pressure

Single crystals of SmFeAsO1-xFy of a size up to 120 micrometers have been grown from NaCl/KCl flux at a pressure of 30 kbar and temperature of 1350-1450 C using the cubic anvil high-pressure technique. The superconducting transition temperature of the obtained single crystals varies between 45 and 53 K.Obtained crystals are characterized by a full diamagnetic response in low magnetic fields and by a high critical current density in high magnetic fields. Structural refinement has been performed on single crystal. Differential thermal analysis investigations at 1 bar Ar pressure show decomposition of SmFeAsO1-xFy at 1302 C.Comment: 12 pages, 3 tables, 6 figure

Microscopic Study of the Superconducting State of the Iron Pnictide RbFe_2As_2

A study of the temperature and field dependence of the penetration depth \lambda of the superconductor RbFe_2As_2 (T_c=2.52 K) was carried out by means of muon-spin rotation measurements. In addition to the zero temperature value of the penetration depth \lambda(0)=267(5) nm, a determination of the upper critical field B_c2(0)=2.6(2) T was obtained. The temperature dependence of the superconducting carrier concentration is discussed within the framework of a multi-gap scenario. Compared to the other "122" systems which exhibit much higher Fermi level, a strong reduction of the large gap BCS ratio 2\Delta/k_B T_c is observed. This is interpreted as a consequence of the absence of interband processes. Indications of possible pair-breaking effect are also discussed.Comment: 5 pages, 4 figure

Bulk electronic structure of superconducting LaRu2P2 single crystals measured by soft x-ray angle-resolved photoemission spectroscopy

We present a soft X-ray angle-resolved photoemission spectroscopy (SX-ARPES) study of the stoichiometric pnictide superconductor LaRu2P2. The observed electronic structure is in good agreement with density functional theory (DFT) calculations. However, it is significantly different from its counterpart in high-temperature superconducting Fe-pnictides. In particular the bandwidth renormalization present in the Fe-pnictides (~2 - 3) is negligible in LaRu2P2 even though the mass enhancement is similar in both systems. Our results suggest that the superconductivity in LaRu2P2 has a different origin with respect to the iron pnictides. Finally we demonstrate that the increased probing depth of SX-ARPES, compared to the widely used ultraviolet ARPES, is essential in determining the bulk electronic structure in the experiment.Comment: 4 pages, 4 figures, 1 supplemental material. Accepted for publication in Physical Review Letter

The Reliability of the Factor Structure of the Pediatric Daytime Sleepiness Scale in both a Spanish-Colombian and French-Canadian Version

Daytime sleepiness is characterized by an increased likelihood of falling asleep and adversely impacts youth’s academic performance, behavior, and mood. The National Sleep Foundation Sur­vey (2006) found almost 50% of youth sleep 1 to 2 hours less than the recommended 9 hours per night and 60% report daytime sleepiness. The Pediatric Daytime Sleepiness Scale (PDSS; Drake et al., 2003) is a self-report questionnaire used to evaluate the likelihood of youth falling asleep in various everyday situations. The original PDSS was developed with an English-speaking American sample (Mage=11.8; SD=.6 years), and the measure was thought to assess a uni-dimensional construct: day­time sleepiness. The PDSS has previously been translated into a Spanish version for an Argentinean sample (Mage=13.3; SD=1.5 years)

Evidence of spin-density-wave order in RFeAsO from measurements of thermoelectric power

Data on the magneto-thermopower and specific heat of three compounds belonging to '1111' oxypnictides family are reported. One specimen (SmAsFeO0.8F0.2) is a superconductor with Tc = 53 K, while two others (SmAsFeO and NdAsFeO) are nonsuperconducting parent compounds. Our results confirm that spin density wave (SDW) order is present in SmAsFeO and NdAsFeO. In these two samples a strict connection between the thermoelectric power and electronic specific heat is found in the vicinity of SDW transition, what indicates that the chemical potential of charge carriers strongly depends on temperature in this region. Low temperature data suggest presence of significant contribution magnon-drag to the thermoelectric power.Comment: 14 pages, 5 figures; adjusted to referees' suggestions; to appear in Phys. Rev.