We have measured the magnetic penetration depth of the recently discovered binary superconductor ${\mathrm{MgB}}_2$ using muon spin rotation and low-field ac susceptibility. From the damping of the muon precession signal we find the penetration depth at zero temperature is $\sim 85\hspace{0.5em}\mathrm{nm}.$ The low-temperature penetration depth shows a quadratic temperature dependence, indicating either the presence of nodes in the gap or that the gap minimum must be smaller than the lowest temperature measured (4.2 K) if the pairing has anisotropic s-wave symmetry.

• Received 2 March 2001

DOI:https://doi.org/10.1103/PhysRevB.64.094514