Reflection of a two-gap nature in penetration-depth measurements of MgB2 film -: art. no. 064511

The magnetic penetration depth lambda(T) in the basal plane of a magnesium diboride (MgB2) film was measured using a two-coil mutual-inductance technique at 50 kHz. This film has T(c)similar or equal to38 K, DeltaT(c)less than or equal to1 K, and lambda(0)similar to1500+/-100 Angstrom. At low temperatures, lambda(-2)(T) shows a clear exponential temperature dependence, indicating s-wave superconducting order-parameter symmetry. However, the data are not quantitatively well described by the theory assuming a single gap. From the data fitted by the full BCS calculation assuming a double gap, the values of the two distinct gaps were obtained: Delta(S)(0)=2.61+/-0.41 meV and Delta(L)(0)=6.50+/-0.33 meV. The contributions of the small and large gaps to the total superfluid density at T=0 were estimated to be 21% and 79%, respectively. Finally, we investigate how the penetration depth lambda(T) is affected by the gap-size distribution on the Fermi surface.