The results of the muon-spin rotation experiments on BCS superconductors ${\mathrm{NbB}}_{2+x}$ ($x=0.2$ 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 $\left(T_c\text{'s}\right)$. By taking these distributions into account, the dependence of the inverse squared zero-temperature magnetic penetration depth $\left({\lambda }_0^{-2}\right)$ on $T_c$ was reconstructed for temperatures in the range $1.5\mathrm{K}\lesssim T_c\lesssim 8.0\mathrm{K}$. ${\lambda }_0^{-2}$ was found to obey the power law dependence ${\lambda }_0^{-2}\propto T_c^{3.1\left(1\right)}$ which appears to be common for some families of BCS superconductors as, e.g., Al doped ${\mathrm{MgB}}_2$ and high-temperature cuprate superconductors as underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$.

• Received 24 April 2007

DOI:https://doi.org/10.1103/PhysRevB.77.064506