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

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 (T-c's). By taking these distributions into account, the dependence of the inverse squared zero-temperature magnetic penetration depth (lambda(-2)(0)) on T-c was reconstructed for temperatures in the range 1.5 K less than or similar to T-c less than or similar to 8.0 K. lambda(-2)(0) was found to obey the power law dependence lambda(-2)(0)proportional to 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.