Penetration depth measurements in MgB2: Evidence for unconventional superconductivity

We have measured the magnetic penetration depth of the recently discovered binary superconductor MgB2 using Mon spin rotation and low-field ac susceptibility. From the damping of the muon precession signal we find the penetration depth at zero temperature is similar to 85 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.