Spatially heterogeneous character of superconductivity in MgB2 as revealed by local probe and bulk measurements

Controversial tunnel and point-contact experimental data on superconducting gap values in MgB2 are analyzed and interpreted in terms of the spatial inhomogeneity and the proximity effect appropriate to this layered compound. Existing specific heat measurements are explained in the same way. Break-junction tunnel studies of polycrystalline MgB2 samples are carried out and a clear-cut multiplicity of gaps is observed, although the shapes of the I-V characteristics vary substantially.