Awana, V. P. S. and Vajpayee, Arpita and Mudgel, Monika and Kishan, H. (2008) Superconductivity of various borides and the role of carbon in their high performance. Superconductor Science and Technology , 22 (3). 034015-1-034015-7. ISSN 0953-2048

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Abstract

The superconductivity of MgB2, Mg1−xAlxB2 and NbB2+x is compared. The stretched c-lattice parameter (c = 3.52 Å) of MgB2 in comparison to NbB2.8 (c = 3.32 Å) and AlB2 (c = 3.25 Å) decides empirically the population of their π and σ bands and, as a result, their Tc 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) MgB2 superconductor (39 K) into the electron type (mainly π band conduction) non-superconducting AlB2. 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 MgB2−xCx. Our results on MgB2 added with nano-diamond, nano-SiC and various organics such as glucose, PVA and adipic acid, when compared with MgB2−xCx, clearly demonstrate that the main role is played by C substitution at the B site in the host MgB2 and the ensuing σ plane disorder and vortex pinning. The best strategy could be to add (<10 nm) nanoparticles to MgB1.8C0.2 to ensure both extrinsic pinning by the former and intrinsic pinning by the latter.

Item Type:	Article
Additional Information:	Copyright for this article belongs to M/s Institute of Physics.
Subjects:	Physics
Divisions:	UNSPECIFIED
Depositing User:	Ms Neetu Chandra
Date Deposited:	12 Oct 2012 11:55
Last Modified:	12 Oct 2012 11:55
URI:	http://npl.csircentral.net/id/eprint/688

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