Direct observation by resonant tunneling of the B^+ level in a delta-doped silicon barrier

We observe a resonance in the conductance of silicon tunneling devices with a delta-doped barrier. The position of the resonance indicates that it arises from tunneling through the B^+ state of the boron atoms of the delta-layer. Since the emitter Fermi level in our devices is a field-independent reference energy, we are able to directly observe the diamagnetic shift of the B^+ level. This is contrary to the situation in magneto-optical spectroscopy, where the shift is absorbed in the measured ionization energy.Comment: submitted to PR

Frequency of alpha- and beta-haemolysin in Staphylococcus aureus of bovine and human origin - A comparison between pheno- and genotype and variation in phenotypic expression

The phenotypic expression of haemolysins and the presence of genes encoding alpha and beta-haemolysin were determined in 105 Sraphylococcus aureus isolates from bovine mastitis, 100 isolates from the nostrils of healthy humans, and 60 isolates from septicaemia in humans. Furthermore, the possible change in expression of haemolysins after subcultivation in human and bovine blood and milk was studied in selected isolates. alpha-haemolysin was expressed phenotypically in 39 (37%) of the bovine isolates, in 59 (59%) of the human carrier isolates, and in 40 (67%) of the isolates from septicaemia. beta-haemolysin was expressed in 76 (72%) bovine, 11 (11%) carrier, and 8 (13%) septicaemia isolates. Significantly more bovine than human isolates expressed beta-haemolysin and significantly fewer expressed alpha-haemolysin. Genotypically, the gene encoding alpha-haemolysin was detected in all isolates. A significant difference in the prevalence of the gene encoding P-haemolysin between the bovine (96%), human carrier (56%) and isolates from septicaemia (57%) was found. Of the bovine isolates, 75% of those carrying the gene encoding beta-haemolysin expressed beta-haemolysin phenotypically, whereas only 20% of the carrier isolates and 24% of the septicaemia isolates did so. No change in expression of haemolysins could be observed after subcultivation of bovine isolates in human blood and milk. After 5 to 10 subcultures in bovine blood and 1 to 4 in bovine milk, 9 of 10 human isolates originally phenotypically beta-haemolysin negative initiated the expression of beta-haemolysin. This study showed that a larger proportion of S. aureus of bovine origin carry the beta-haemolysin gene compared to isolates from humans. Furthermore, a larger number of the isolates of bovine origin carrying the beta-haemolysin gene express this gene phenotypically compared to isolates of human origin

Plates of melt-textured YBa₂Cu₃O_7-x superconductors with long crystals

Melt-textured plates of YBa2Cu3O7-x measuring 400×(15-21)×(2.5-3.5) mm3 have been manufactured. They are composed of crystal bundles whose crystal size can be varied from some mm to more than 100×10×2.5 mm3 and recently up to 300×(5- 10)×3 mm3 depending on the process parameters. The crystals contain the usual Y2BaCuO5 inclusions, (001) microcracking and twinning. An unusual feature is the presence of continuous transitions between slightly misorientated subgrains or domains. The current-carrying capabilities of the samples have been tested by direct and alternating current measurements up to 1650 ADC with current densities of 3600 A/cm2 at 4 μV/cm (77 K). Magnetic characterization indicates low field critical current densities of the order of 105 A/cm2 at 77 K in small monocrystalline samples characterized by weakly developed (001) microcracking. Apparently appreciable flux flow occurs along defects in the long polycrystalline samples and the effective cross-section of these may be reduced due to microcracking. The plates are intended for relatively short current connectors, and could be used for current limiters where the very low voltage can be accepted. Relations between the microtextual features and the orientation dependence of the magnetic hysteresis have further been studied in the field range up to 1 T.</p