Multiple electron scattering by an impurity and at the <i>N–S</i> boundary. Negative differential conductivity of the <i>N–S–N</i> system

It is shown that on account of the specific nature of the Andreev reflection, the scattering of conduction electrons in a normal metal by an impurity and at the N–S boundary has a complex quantum-interference nature due to multiple (and coherent) scattering of electron and hole waves by the impurity and at the N–S boundary. Conditions are indicated under which the transition of a superconductor to the normal state in a N–S–N system is accompanied by a decrease in the total resistance of the sample and the current–voltage characteristic is S-shaped.

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