High transport currents in mechanically reinforced MgB2 wires

We prepared and characterized monofilamentary MgB2 wires with a mechanically reinforced composite sheath of Ta(Nb)/Cu/steel, which leads to dense filaments and correspondingly high transport currents up to Jc = 10^5 A/cm^2 at 4.2 K, self field. The reproducibility of the measured transport currents was excellent and not depending on the wire diameter. Using different precursors, commercial reacted powder or an unreacted Mg/B powder mixture, a strong influence on the pinning behaviour and the irreversibility field was observed. The critical transport current density showed a nearly linear temperature dependency for all wires being still 52 kA/cm^2 at 20 K and 23 kA/cm^2 at 30 K. Detailed data for Jc(B,T) and Tc(B) were measured.Comment: 21 pages, 13 figures, revised version, to be published in Supercond. Sci. Techno

Pressure Effect and Specific Heat of RBa2Cu3Ox at Distinct Charge Carrier Concentrations: Possible Influence of Stripes

In YBa2Cu3Ox, distinct features are found in the pressure dependence of the transition temperature, dTc/dp, and in DeltaCp*Tc, where DeltaCp is the jump in the specific heat at Tc: dTc/dp becomes zero when DeltaCp*Tc is maximal, whereas dTc/dp has a peak at lower oxygen contents where DeltaCp*Tc vanishes. Substituting Nd for Y and doping with Ca leads to a shift of these specific oxygen contents, since oxygen order and hole doping by Ca influences the hole content nh in the CuO2 planes. Calculating nh from the parabolic Tc(nh) behavior, the features coalesce for all samples at nh=0.11 and nh=0.175, irrespective of substitution and doping. Hence, this behavior seems to reflect an intrinsic property of the CuO2 planes. Analyzing our results we obtain different mechanisms in three doping regions: Tc changes in the optimally doped and overdoped region are mainly caused by charge transfer. In the slightly underdoped region an increasing contribution to dTc/dp is obtained when well ordered CuO chain fragments serve as pinning centers for stripes. This behavior is supported by our results on Zn doped NdBa2Cu3Ox and is responsible for the well known dTc/dp peak observed in YBa2Cu3Ox at x=6.7. Going to a hole content below nh=0.11 our results point to a crossover from an underdoped superconductor to a doped antiferromagnet, changing completely the physics of these materials.Comment: 6 pages, 5 figures Proccedings of the 'Stripes 2000' Conference, Rome (2000

How) do work placements work: scrutinising the quantitative evidence for a theory-driven future research agenda

While supervised work placements are increasingly popular in higher education, evidence regarding their effects on career outcomes remain somewhat sparse and atheoretical. The aim of this systematic literature review is to evaluate the effectiveness of placements for career outcomes and to identify any underpinning core psychological processes and to offer a theoretically grounded framework for future research. Drawing on transition theory (Schlossberg, 1981) and career construction theory (Savickas, 1997), we argue that supervised work experiences are central transition experiences that enable social learning processes and trigger changes in a person’s identity development as a professional, thereby increasing career resources and employability which in turn affect future career outcomes positively. We screened 2,394 systematically selected abstracts across several databases and disciplines. Only quantitative studies that either offered a control-group or a longitudinal design were included, resulting in an in-depth review of 40 studies, applying a rigorous evaluation protocol. Placement participation elicits an overall positive (but small) effect on career outcomes: Graduates who completed a work placement found employment more quickly. Work placements also changed students’ perceptions of self-efficacy, their knowledge, skills, and attitudes. We suggest that these changes could be seen as indicative of the proposed social learning processes and identity changes that positively affect career resources. Our review points to several gaps in the literature, and building on existing career theories, we develop a theoretical model and offer new avenues for future research to integrate the heterogenic field of placement research and inform career research in other areas