Structural and Superconducting Property Variations with Nominal Mg Non‐Stoichiometry in Mg_xB₂ and Its Enhancement of Upper Critical Field

Abstract

By applying a combination of characterisation tools, changes in structural and superconducting properties with nominal Mg non‐stoichiometry in MgxB2 are found. The non‐stoichiometry produces enhanced in‐field critical current densities (Jc's) and upper critical field / irreversibility field (Hc2/Hirr(T)) values. Upper critical fields of ∼ 21 T (4.2 K) were obtained in nominal Mg‐deficient samples compared to ∼ 17 T (4.2 K) for near‐stoichiometric samples.

Evaluation of Mg size dependence on superconductivity of MgB2

“…Fig.4 shows the normalized resistivity plots three representative samples BM500, BM100 and BM30 corresponding to the highest, intermediate and lowest particle size of Mg precursor. The data is in concurrence with the reported results [12] for the MgB 2 bulk samples that are prepared by varying Mg:B ratio. The sample with larger Mg precursor size has more temperature dependence of resistivity as compared to the sample with smaller Mg precursor size.…”

“…(f)) BM500 along with the presence of large pore like opening again stresses upon the effects of larger Mg particle size of precursor. Since there was no evidence of any significant peak of Mg in the XRD for BM500, the presence of excessive metallic Mg can be negated, which can be explained by the possibility of quick evaporation of liquid Mg [12] during the reaction leaving behind pore like openings. Nevertheless the possibility of XRD-blind Mg traces cannot be completely ruled out.…”

Influence of Mg deficiency on crystal structure and superconducting properties inMgB2single crystals

“…Indeed, recent point contact Andreev reflection measurements of polycrystalline samples [25] reveal that deliberately introducing Mg deficiency dirties the π band without introducing significant interband scattering. Earlier investigations performed on the same samples [26] also found that T c remains constant across the sample series. These observations agree with our results.…”

Collective effect of hydrogen in argon and Mg as ambiance for the heat treatment on MgB2

“…Meanwhile, in case of samples with Mg ambiance, the resistivity shows much improved power law dependence in temperature with a reduced transition width of 1 K. This can indicate the increased dependence on electron-phonon scattering due to the decrease in defect and improved intra-grain as well as inter-grain connectivity [18]. Further, it can be observed that the actual superconducting current carrying fraction is much higher for ArMg sample as compared to ArHMg sample.…”

“…From inset it can be easily understood that the overall resistivity decreases from Ar to ArH sample, while it is highly reduced for the samples annealed with Mg ambiance. The values for effective current carrying area, A f calculated using Rowell analysis [17][18], onset (T c, on ), offset (T c, off ) and width (∆T c ) of the superconducting transition is given in table 1. The decrease in the resistivity of ArH samples supports our argument of H 2 with some reducing effect, which more or less cleans the grain as well as the grain boundary from the oxides rendering the improved overall connectivity and hence the reduced resistivity.…”