Superconductivity of various borides and the role of carbon in their high performance (original) (raw)
2009, Superconductor Science and Technology
The superconductivity of MgB 2 , Mg 1−x Al x B 2 and NbB 2+x is compared. The stretched c-lattice parameter (c = 3.52Å) of MgB 2 in comparison to NbB 2.8 (c = 3.32Å) and AlB 2 (c = 3.25Å) decides empirically the population of their π and σ bands and, as a result, their T c values at 39 and 11 K, respectively, for the first two and no superconductivity for the latter. Besides stretching of the c-lattice parameter not only the density of the carriers but also their signs change in these isostructural di-borides. The thermoelectric power of these compounds clearly demonstrates their changing π and σ band contributions and the ensuing appearance/disappearance of superconductivity. An increased c parameter increases the boron plane constructed hole type σ band population and decreases the contribution from the Mg or Al plane electron type π band. This turns the hole type (mainly σ band conduction) MgB 2 superconductor (39 K) into the electron type (mainly π band conduction) non-superconducting AlB 2 . The importance of hole type σ band conduction dominating the superconductivity of the various borides is further established by the high performance of intrinsically pinned MgB 2−x C x . Our results on MgB 2 added with nano-diamond, nano-SiC and various organics such as glucose, PVA and adipic acid, when compared with MgB 2−x C x , clearly demonstrate that the main role is played by C substitution at the B site in the host MgB 2 and the ensuing σ plane disorder and vortex pinning. The best strategy could be to add (<10 nm) nanoparticles to MgB 1.8 C 0.2 to ensure both extrinsic pinning by the former and intrinsic pinning by the latter.
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