We extensively explore the experimentally proposed metallic structure of hcp ๐ 63 for the hydrogen rich compound, SiH4. It is found that the lattice dynamic of this structure is severely unstable. By freezing the soften mode, an orthorhombic Pbcn structure is discovered to be dynamically stable up to 226 GPa. Within the conventional BCS theory, the calculated critical temperature ๐๐ within the proposed Pbcn structure is 16.5 K at 188 GPa, in good agreement with the experimental result (17.5 K). Thus, we propose that the current predicted orthorhombic phase is a better candidate for the metallic phase of SiH4.
Large grain niobium has the potential of simplifying the production sequence and consequently reducing the cost of the superconducting RF cavities for ILC. To investigate the feasibility of fabrication and the possibility to achieve high gradient by large grain cavities, two 1.3 GHz cavities were made of China large grain niobium and a series of vertical tests were carried out following several different surfaces treatment procedures. Two cavities have both reached the high gradient of more than 43 MV/m repeatedly and the maximum accelerating field of 47.9 MV/m has been achieved by China large grain niobium. This paper introduces the features of the fabrication and surface treatments on the large grain cavities and presents the preliminary results of the research.
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