Exponential Temperature Dependence of Penetration Depth in MgB_2

Abstract

The temperature dependence of the London penetration depth, $\lambda (T)$, was measured in polycrystalline MgB${}_2$ samples by a high-resolution, radio frequency technique. A clear exponential temperature dependence of $\lambda (T)$ was observed at low temperature, indicating s-wave pairing. A fit to the data gives an effective energy gap $\Delta$ of $2.8\pm 0.4$ meV ($2\Delta /k_B{T}_c=1.7\pm 0.2$), 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 included