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. 
H. Takagiwa, S. Kuroiwa, M. Yamazawa, J. Akimitsu, K. Ohishi, A. Koda,   W. Higemoto, and R. Kadono