Comment on “Muon-spin-rotation study of the superconducting properties of Mo 3 Sb 7 ”

In a recent article, Tran et al. [Phys. Rev. B 78, 172505 (2008)] reported results from muon-spin-rotation $(\ensuremath{\mu}\text{SR})$ measurements of an ${\text{Mo}}_{3}{\text{Sb}}_{7}$ superconductor. Based on the analysis of the temperature and the magnetic field dependences of the Gaussian relaxation rate, ${\ensuremath{\sigma}}_{sc}$, it was suggested that ${\text{Mo}}_{3}{\text{Sb}}_{7}$ is a superconductor with two isotropic $s$-wavelike energy gaps. This relates to results found previously in the specific-heat measurements by several of the same authors in Acta Mater. 56, 5694 (2008). The purpose of this Comment is to point out that from the analysis made by Tran et al., the presence of two superconducting energy gaps in ${\text{Mo}}_{3}{\text{Sb}}_{7}$ cannot be justified. The analysis of $\ensuremath{\mu}\text{SR}$ data does not account for the reduction in ${\ensuremath{\sigma}}_{sc}$ with increasing temperature. The specific-heat data can be satisfactorily described within the framework of the one-gap model by assuming the presence of a small residual specific-heat component. The experimental data of Tran et al., as well as our earlier published $\ensuremath{\mu}\text{SR}$ results [Phys. Rev. B 78, 014502 (2008)], all seem to be consistent with the presence of single isotropic superconducting energy gap in ${\text{Mo}}_{3}{\text{Sb}}_{7}$.