Correlation between Superconducting Carrier Density and Transition Temperature in NbB$_{2+x}$

We report on the magnetic penetration depth, lambda, in a type II superconductor NbB_{2+x} determined by muon spin rotation method. We show in the sample with x=0.1 that lambda at 2.0 K is independent of an applied magnetic field. This suggests that the superconducting order parameter in NbB_{2+x} is isotropic, as expected for conventional BCS superconductors. Meanwhile, the superconducting carrier density (proportional to lambda^{-2}) exhibits an interesting tendency of increase with increasing T_c (where T_c varies with x). Possible origin of such behavior is discussed in comparison with the case of exotic superconductors.