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The electronic gap structure of Nb 3 Sn was measured by the break-junction (BJ) tunneling technique. The superconducting gap values are estimated to be in the range 2∆ = 4-5.5 meV at T = 4.2 K as follows from the observed distinct conductance peaks. In addition to the superconducting gap structure, we observed reproducible hump-like structures at the biases of about ± 20 and ± 50 mV. Such a coexistence of gap and hump structures resembles the situation found in the high-T c copper-oxide superconductors. Above the superconducting critical temperature T c ~ 18 K, the humps appear as the only gap-like structures. Their possible origin is discussed in connection to the structural phase transition occurring in Nb 3 Sn.

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Grain boundaries and their electric potential were studied in connection with the electric conduction in polycrystalline copper using a scanning tunneling microscope (STM). It was found that the grain boundaries consist mainly of cuprous oxide ( Cu 2 O ) and electric potential barriers are formed at most grain boundaries.