The temperature dependence of the London penetration depth, λ(T), was measured in both single crystal and polycrystalline samples by using a high-resolution radio frequency technique. A clear exponential temperature dependence of λ(T) was observed at low temperatures, indicating s-wave pairing. A BCS fit to the lowest-temperature data gave an in-plane energy gap, Δ, of 30+/-2K (2Δ/TC = 1.5+/-0.1), which was significantly smaller than the standard BCS weak coupling value of 3.5. It was found that the data were best described by a 2-gap model.

Exponential Temperature Dependence of the Penetration Depth in Single Crystal MgB2. F.Manzano, A.Carrington, N.E.Hussey, S.Lee, A.Yamamoto, S.Tajima: Physical Review Letters, 2002, 88[4], 047002 (3pp)