Magnetic penetration depth of MgB2

The low temperature behaviour of the magnetic penetration depth λ(T) provides a useful probe of the low lying excitations in superconductors and hence of the anisotropy of the superconducting energy gap. In this paper, we will present measurements of the penetration depth in the Meissner state of both single crystal and polycrystalline samples of MgB2, using a very low field RF technique. λ(T) shows a clear exponential temperature dependence T/Tc≲0.2, strongly indicating that this material is fully gapped. Detailed fits to the superfluid density over the full temperature range of our measurements (1>T/Tc>0.04) gives strong support for the existence of two distinct superconducting gaps, Δs=29 K and Δb=75 K, in agreement with recent theoretical predictions and other probes.