Superconductivity of various borides and the role of carbon in their high performance

The superconductivity of MgB2, Mg1-xAlxB2 and NbB2+x is compared. The stretched c-lattice parameter (c = 3.52 angstrom) of MgB2 in comparison to NbB2.8 (c = 3.32 angstrom) and AlB2 (c = 3.25 angstrom) decides empirically the population of their pi and sigma 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 pi and sigma band contributions and the ensuing appearance/disappearance of superconductivity. An increased c parameter increases the boron plane constructed hole type sigma band population and decreases the contribution from the Mg or Al plane electron type pi band. This turns the hole type (mainly sigma band conduction) MgB2 superconductor (39 K) into the electron type (mainly pi band conduction) non-superconducting AlB2. The importance of hole type sigma 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 sigma 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.