MEASUREMENT OF THE INCREMENTAL CHARGE OF A SUPERCONDUCTING ISLAND

We have measured the total charge of a superconducting electrode or 'i sland' which is free to exchange electrons with a metallic ground thro ugh a tunnel junction and which is capacitively coupled to a voltage s ource at potential U. We find that the island charge varies stepwise w ith U, the incremental charge being equal to 2e inside a low-temperatu re, low-magnetic-field domain, and to e elsewhere. In this latter case , steps corresponding to different parities of the number of electrons inside the island have unequal lengths. The odd-even free energy diff erence deduced from the asymmetry of the steps is well explained by th e theory of dirty superconductors.