Correlation between the transition temperature and the superfluid density in BCS superconductor NbB2+x

Abstract

The results of the muon-spin rotation experiments on BCS superconductors NbB2+x (x=0.2 and 0.34) are reported. Both samples, studied in the present work, exhibit rather broad transitions to the superconducting state, suggesting a distribution of the volume fractions with different transition temperatures (Tc's). By taking these distributions into account, the dependence of the inverse squared zero-temperature magnetic penetration depth (lambda₀^-2) on Tc was reconstructed for temperatures in the range 1.5 K<~Tc<~8.0 K. lambda₀^-2 was found to obey the power law dependence lambda₀^-2[proportional]T_c^3.1(1) which appears to be common for some families of BCS superconductors as, e.g., Al doped MgB2 and high-temperature cuprate superconductors as underdoped YBa2Cu3O7−delta.