Superconductivity of Various Borides: The Role of Stretched c- Parameter

TL;DR

This paper compares the superconductivity of various borides, emphasizing how stretching the c-lattice parameter influences their electronic structure and transition temperatures, with experimental evidence showing enhanced superconductivity in non-stoichiometric samples.

Contribution

It demonstrates that increasing the c-parameter through non-stoichiometry can induce superconductivity in NbB2+x, highlighting the importance of lattice parameter tuning for superconductivity in borides.

Findings

MgB2 has a higher Tc (39K) linked to its stretched c-parameter.

Non-stoichiometric NbB2+x shows superconductivity with Boron excess.

Stoichiometric NbB2 and TaB2+x do not exhibit superconductivity.

Abstract

The superconductivity of MgB2, AlB2, NbB2+x and TaB2+x is inter-compared. The stretched c-lattice parameter (c = 3.52 A) of MgB2 in comparison to NbB2.4 (c = 3.32 A) and AlB2 (c = 3.25 A) decides empirically the population of their p and s bands and as a result their transition temperature, Tc values respectively at 39K and 9.5K for the first two and no superconductivity for the later. Besides the electron doping from substitution of Mg+2 by Al+3, the stretched c-parameter also affects the Boron plane constructed hole type Sigma-band population and the contribution from Mg or Al plane electron type Pi band. This turns the electron type (mainly Pi-band conduction) non-superconducting AlB2 to hole type (mainly s-band conduction) MgB2 superconductor (39 K) as indicated by the thermoelectric power study. Keeping this strategy in mind that stretching of c-parameter enhances superconductivity, the NbB2+x and TaB2+x samples are studied for existence of superconductivity. The non-stoichiometry induces an increase in c parameter with Boron excess in both borides. Magnetization (M-T) and Resistivity measurements (R-T) in case of niobium boride samples show the absence of superconductivity in stoichiometric NbB2 sample (c = 3.26 A) while a clear diamagnetic signal and a R = 0 transition for Boron excess NbB2+x samples. On the other hand, superconductivity is not achieved in TaB2+x case. The probable reason behind is the comparatively lesser or insufficient stretching of c-parameter.