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Register a new userFormation of MgB2 at low temperatures by reaction of Mg with B6Si
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Formation of MgB2 by reactions of Mg with B6Si and Mg with B were compared, the former also producing Mg2Si as a major product. Compared to the binary system, the ternary reactions for identical time and temperature were more complete at 750 C and below, as indicated by higher diamagnetic shielding and larger x-ray diffraction peak intensities relative to those of Mg. MgB2 could be produced at temperatures as low as 450 C by the ternary reaction. Analyses by electron microscopy, x-ray diffraction, and of the upper critical field show that Si does not enter the MgB2 phase.
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A Hybrid Physical-Chemical Vapour Deposition (HPCVD) system consisting of separately controlled Mg-source heater and substrate heater is used to grow MgB2 thin films and thick films at various temperatures. We are able to grow superconducting MgB2 thin films at temperatures as low as 350 C with a Tc0 of 35.5 K. MgB2 films up to 4 um in thickness grown at 550 C have Jc over 10E6 A/cm2 at 5 K and zero applied field. The low deposition temperature of MgB2 films is desirable for all-MgB2 tunnel junctions and MgB2 thick films are important for applications in coated conductors.
A commercially available calorimeter has been used to investigate the specific heat of a high-quality kn single crystal. The addenda heat capacity of the calorimeter is determined in the temperature range $0.02 , mathrm{K} leq T leq 0.54 , mathrm{K}$. The data of the kn crystal imply the presence of a large $T^2$ contribution to the specific heat which gives evidence of $d$-wave order parameter symmetry in the superconducting state. To improve the measurements, a novel design for a calorimeter with a paramagnetic temperature sensor is presented. It promises a temperature resolution of $Delta T approx 0.1 , mathrm{mu K}$ and an addenda heat capacity less than $200 , mathrm{pJ/K}$ at $ T < 100 , mathrm{mK}$.
Here we describe the results of an atomic resolution study of the structure and composition of both the interior of the grains, and the grain boundaries in polycrystalline MgB2. We find that there is no oxygen within the bulk of the grains but significant oxygen enrichment at the grain boundaries. The majority of grain boundaries contain BOx phases smaller than the coherence length, while others contain larger areas of MgO sandwiched between BOx layers. Such results naturally explain the differences in connectivity between the grains observed by other techniques.
Core level X-ray Photoelectron Spectroscopy (XPS) studies have been carried out on polycrystalline MgB_2 pellets over the whole binding energy range with a view to having an idea of the charge state of Magnesium (Mg). We observe 3 distinct peaks in Mg 2p spectra at 49.3 eV (trace), 51.3 eV (major) and 54.0 eV (trace), corresponding to metallic Mg, MgB_2 and MgCO_3 or, divalent Mg species respectively. Similar trend has been noticed in Mg 2s spectra. The binding energy of Mg in MgB_2 is lower than that corresponding to Mg(2+), indicative of the fact that the charge state of Mg in MgB2 is less than (2+). Lowering of the formal charge of Mg promotes the sigma to pi electron transfer in Boron (B) giving rise to holes on the top of the sigma-band which are involved in coupling with B E_2g phonons for superconductivity. Through this charge transfer, Mg plays a positive role in hole superconductivity. B 1s spectra consist of 3 peaks corresponding to MgB_2, boron and B_2O_3. There is also evidence of MgO due to surface oxidation as seen from O 1s spectra.
Using a membrane-driven diamond anvil cell and both ac magnetic susceptibility and electrical resistivity measurements, we have characterized the superconducting phase diagram of elemental barium to pressures as high as 65 GPa. We have determined the superconducting properties of the recently discovered Ba-VI crystal structure, which can only be accessed via the application of pressure at low temperature. We find that Ba-VI exhibits a maximum Tc near 8 K, which is substantially higher than the maximum Tc found when pressure is applied at room temperature.
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