Microwave Properties of Ba(0.6)K(0.4)BiO(3) Crystals

We report on field-induced variations of the microwave surface resistance at 9.6 GHz of Ba(0.6)K(0.4)BiO(3) crystals. Energy losses have been investigated as a function of the static magnetic field in the range of temperatures 4.2 K - Tc. By analyzing the experimental results in the framework of the Coffey and Clem model we determine the temperature dependence of the first-penetration field, upper critical field and depinning frequency. The results show that the pinning energy of this bismuthate superconductor is weaker than those of cuprates.Comment: 6 pages, 8 embedded figure

Helium irradiation effects in polycrystalline Si, silica, and single crystal Si

Transmission electron microscopy (TEM) has been used to investigate the effects of room temperature 6 keV helium ion irradiation of a thin (≈55 nm thick) tri-layer consisting of polycrystalline Si, silica, and single-crystal Si. The ion irradiation was carried out in situ within the TEM under conditions where approximately 24% of the incident ions came to rest in the specimen. This paper reports on the comparative development of irradiation-induced defects (primarily helium bubbles) in the polycrystalline Si and single-crystal Si under ion irradiation and provides direct measurement of a radiation-induced increase in the width of the polycrystalline layer and shrinkage of the silica layer. Analysis using TEM and electron energy-loss spectroscopy has led to the hypothesis that these result from helium-bubble-induced swelling of the silicon and radiation-induced viscoelastic flow processes in the silica under the influence of stresses applied by the swollen Si layers. The silicon and silica layers are sputtered as a result of the helium ion irradiation; however, this is estimated to be a relatively minor effect with swelling and stress-related viscoelastic flow being the dominant mechanisms of dimensional change

Muon spin rotation study of the magnetic penetration depth in the intercalated graphite superconductor CaC6

We report temperature- and magnetic field-dependent bulk muon spin rotation measurements in a c-axis oriented superconductor CaC6 in the mixed state. Using both a simple second moment analysis and the more precise analytical Ginzburg-Landau model, we obtained a field independent in-plane magnetic penetration depth {\lambda}ab (0) = 72(3) nm. The temperature dependencies of the normalized muon spin relaxation rate and of the normalized superfluid density result to be identical, and both are well represented by the clean limit BCS model with 2\Delta/kB Tc = 3.6(1), suggesting that CaC6 is a fully gapped BCS superconductor in the clean limit regime.Comment: Accepted for publication in PR

The Effect of Condensed Tannins in Lotus spp. on the Lesser Migratory Grasshopper

Hatchling grasshoppers (Melanoplus sanguinipes Fab.) fed one accession of field grown big trefoil (Lotus uliginosis Schkuhr) containing a tannin content of 56 mg. g-1 FW, had a significantly lower mean weight than hatchlings fed field grown Lotus species and accessions with lower tannin content. Adult female grasshoppers fed L. uliginosis foliage with 32.3 mg.g-1 FW tannin also ate more feed and had a 2-fold higher gut trypsin and chymotrypsin activity than adults fed low tannin L. corniculatus foliage (0.13 mg.g-1 FW tannin). In more controlled experiments, growth at 20o C or 30o C was used to increase tannin in rooted cuttings of L. uliginosis to concentrations ranging from 7 to 195 mg.g-1 FW. Hatchling weight decreased as a function of dietary tannin concentration when fed these clones. Both hatchling weight and survival were affected when hatchlings were fed pelleted wheat seedlings supplemented with purified L. uliginosis tannin preparations

Negative thermal expansion of MgB2_{2} in the superconducting state and anomalous behavior of the bulk Gr\"uneisen function

The thermal expansion coefficient $\alpha$ of MgB$_2$ is revealed to change from positive to negative on cooling through the superconducting transition temperature $T_c$. The Gr\"uneisen function also becomes negative at $T_c$ followed by a dramatic increase to large positive values at low temperature. The results suggest anomalous coupling between superconducting electrons and low-energy phonons.Comment: 5 figures. submitted to Phys. Rev. Let

Field Dependence of Electronic Specific Heat in Two-Band Superconductors

The vortex structure is studied in light of MgB$_2$ theoretically based on a two-band superconducting model by means of Bogoliubov-de Gennes framework. The field dependence of the electronic specific heat coefficient $\gamma (H)$ is focused. The exponent $\alpha$ in $\gamma (H)\propto H^{\alpha}$ is shown to become smaller by adjusting the gap ratio of the two gaps on the major and minor bands. The observed extremely small value $\alpha\sim 0.23$ could be explained reasonably well in this two-band model with the gap ratio $\sim 0.3$.Comment: 5 pages, 4 figures, to be published in J. Phys. Soc. Jp

Retention of Two-Band Superconductivity in Highly Carbon-Doped MgB2

Tunneling data on MgB_{1.8}C_{0.2} show a reduction in the energy gap of the pi-bands by a factor of two from undoped MgB2 that is consistent with the Tc reduction, but inconsistent with the expectations of the dirty limit. Dirty-limit theory for undoped MgB2 predicts a single gap about three times larger than measured and a reduced Tc comparable to that measured. Our heavily-doped samples exhibit a uniform dispersion of C suggestive of significantly enhanced scattering, and we conclude that the retention of two-band superconductivity in these samples is caused by a selective suppression of interband scattering.Comment: 4 pages, 4 figures; added one figure, added one reference, minor changes to the text, manuscript accepted for publication as a Phys. Rev. B Rapid Communicatio