We present the microscopic properties of superconducting state in ${\text{Mo}}_3{\text{Sb}}_7$ $\left(T_c=2.2\text{K}\right)$ using muon-spin rotation measurements. The zero-field-cooled and field-cooled (FC) data with an applied transverse field of 40 mT reveal an irreversibility in the muon relaxation rates and precessional frequencies below 2 K. We have also found an anomaly around 0.5 K, which may be related to a process of the vortex melting or some change in vortex-lattice symmetry. The temperature dependence of FC muon relaxation rate can be analyzed using a phenomenological double-gap $s$-wave model. The observation of a nonlinear field dependence of the muon relaxation rate is consistent with the occurrence of two superconducting gaps. Moreover, the magnetic penetration depth $\lambda$, coherence length $\xi$, superconducting carrier density $n_s$, and effective-mass enhancement $m^{\ast }$ have been found to be $\lambda \approx 665\text{nm}$, $\xi \approx 12.5\text{nm}$, $n_s\approx 1.2\times {10}^{27}\text{carriers}/{\text{m}}^3$, and $m^{\ast }\approx 18.7m_e$, respectively.

• Received 3 October 2008

DOI:https://doi.org/10.1103/PhysRevB.78.172505