Exponential Temperature Dependence of Penetration Depth in MgB_2

Abstract

The 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.