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

Abstract

The superconductivity of MgB2, Mg1−xAlxB2 andNbB2+x is compared. Thestretched c-latticeparameter (c = 3.52 Å) of MgB2 incomparison to NbB2.8 (c = 3.32 Å)and AlB2 (c = 3.25 Å) decides empirically the population of theirπ andσ bands and, asa result, their Tc values at 39 and 11 K, respectively, for the first two and no superconductivity for the latter. Besides stretchingof the c-lattice parameter not only the density of the carriers but also their signs change in these isostructuraldi-borides. The thermoelectric power of these compounds clearly demonstrates their changingπ and σ band contributions and the ensuing appearance/disappearance of superconductivity. An increasedc parameter increases the boron plane constructed hole typeσ band population and decreases the contribution from the Mg or Al plane electron typeπ band. This turns thehole type (mainly σ band conduction) MgB2 superconductor (39 K) into the electron type (mainlyπ band conduction)non-superconducting AlB2. Theimportance of hole type σ band conduction dominating the superconductivity of the various boridesis further established by the high performance of intrinsically pinnedMgB2−xCx. Our resultson MgB2 addedwith nano-diamond, nano-SiC and various organics such as glucose, PVA and adipic acid, when comparedwith MgB2−xCx, clearly demonstrate that the main role is played by C substitution at the B site in the hostMgB2 and theensuing 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.