Reflection of two-gap nature in penetration depth measurements of MgB₂ film

Original · EN

The magnetic penetration depth, λ⁻²(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 Tc≃ 38 K, ΔTc ≤ 1 K, and λ(0) 1500 Å. At low temperatures, λ⁻²(T) shows a clear exponential temperature dependence, indicating s-wave superconducting order parameter symmetry. However, the data are not quantitatively well described by theory assuming a single gap. From the data fit by the full BCS calculation assuming a double gap, the values of the two distinct gaps were obtained: Δₛ(0)=2.61±0.41 meV and Δₗ(0)=6.50±0.33 meV. The contributions of the small and the large gaps to the total superfluid density at T=0 were estimated to be 21% and 79%, respectively. Finally, we consider the effect of gap anisotropy on the penetration depth measurements, and find that the anisotropy does not play a significant role in determining the temperature dependence of the penetration depth.