2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/17848The temperature dependence of the London penetration depth, $λ(T)$, was measured in polycrystalline MgB$_2$ samples by a high-resolution, radio frequency technique. A clear exponential temperature dependence of $λ(T)$ was observed at low temperature, indicating s-wave pairing. A fit to the data gives an effective energy gap $Δ$ of $2.8\pm0.4$ meV ($2Δ/k_BT_c=1.7\pm0.2$), which is significantly smaller than the standard BCS weak coupling value of 3.5. This may be explained either by an anisotropic gap or the influence of low frequency phonon modes.4 pages - EPS figures includedSuperconductivitytext