Chemical Control of Electronic Structure and Superconductivity in Layered Borides and Borocarbides: Understanding the Absence of Superconductivity in LixBC
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The synthetic search for materials related to the 39 K superconductor MgB2 has been difficult. The most promising theoretical suggestion, hole doping of LiBC, does not lead to a new superconductor. We show here that a combination of density functional theory (DFT) calculations, materials synthesis, and structural characterization reveals the origin of the puzzling absence of superconductivity in Li1/2BC as a subtle change in the electronic structure driven by structural response to the introduction of holes. This indicates that the unique aspects of the electronic structure of MgB2 will be demanding to replicate in other systems.Keywords:
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Over the past six years, superconductivity at high temperatures has been reported in a variety of hydrogen-rich compounds under high pressure. That high temperature superconductivity should exist in these materials is expected according to the conventional theory of superconductivity, as shown by detailed calculations. However, here we argue that experimental observations rule out conventional superconductivity in these materials. Our results indicate that either these materials are unconventional superconductors of a novel kind, which we term ``nonstandard superconductors,'' or alternatively, that they are not superconductors. If the first is true, we point out that the critical current in these materials should be several orders of magnitude larger than in standard superconductors, potentially opening up the way to important technological applications. If the second is the case, which we believe is more likely, we suggest that the signals interpreted as superconductivity are either experimental artifacts or they signal other interesting physics but not superconductivity.
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This chapter contains sections titled: Geometrical Characterization Layer Thickness and Vertical Structure Dimensions Lateral Dimensions Structures that Assist Measurement Characterization of Composition of Layers and Surfaces Atomic Composition Characterization of the Chemical Surface Functional Characterization of Nanostructures
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Materials characterization is a crucial issue in the development and application of new materials. Materials characterization aims to mine and acquire characteristic information and their evolution in the materials. It mainly includes three important topics which are microstructural characterization, properties characterization, and environmental degradation. In this paper, characterization techniques about these topics were discussed for C/SiC composites and a characterization system was preliminarily established. All these characterization research and their results further the better understanding of the relationship between microstructure and properties and of the failure mechanisms in the C/SiC composites.
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Here we report a pressure-induced reemergence of superconductivity in a recently discovered superconductor ${\mathrm{K}}_{2}{\mathrm{Mo}}_{3}{\mathrm{As}}_{3}$, which is an experimental case observed in quasi-one-dimensional superconductors. We find that, after full suppression of the ambient-pressure superconducting (SC-I) state at 8.7 GPa, an intermediary nonsuperconducting state sets in and prevails to the pressure up to 18.2 GPa, however, above this pressure a different superconducting (SC-II) state appears unexpectedly. High pressure x-ray diffraction measurements demonstrate that the pressure-induced dramatic change of the lattice parameter $c$ contributes mainly to the emergence of the SC-II state. Combined with the theoretical calculations on band structure, our results suggest that the reemergence of superconductivity is associated with the change of the complicated interplay among different orbital electrons, driven by the pressure-induced anisotropic change of the lattice.
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The synthetic search for materials related to the 39 K superconductor MgB2 has been difficult. The most promising theoretical suggestion, hole doping of LiBC, does not lead to a new superconductor. We show here that a combination of density functional theory (DFT) calculations, materials synthesis, and structural characterization reveals the origin of the puzzling absence of superconductivity in Li1/2BC as a subtle change in the electronic structure driven by structural response to the introduction of holes. This indicates that the unique aspects of the electronic structure of MgB2 will be demanding to replicate in other systems.
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