One- and two-electron tunneling in superconductors with local pairs

It is shown that the tunnel current from a superconductor with local pairs to a normal metal or a BCS superconductor has two components. One of them is exactly the same as in the BCS theory, but the one-particle energy gap has a different temperature dependence and can be strongly anisotropic. The other component is due to the transformation of local pairs into band states at the metal boundary. Its voltage dependence can be used to find the “pair” gap R along with the one-particle gap. The pair number density v can be estimated from the asymmetry of the current-voltage characteristic of the quasiparticle current in the normal state.

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