We consider the hybrid setup formed by a metallic dot, capacitively coupled
to a superconducting island S connected to a bulk superconductor by a Josephson
junction. Charge fluctuations in S act as a dynamical gate and overscreen the
electronic repulsion in the metallic dot, producing an attractive interaction
between two additional electrons. As the offset charge of the metallic dot is
increased, the dot charging curve shows positive steps (+2e) followed by
negative ones (−e) signaling the occurrence of a negative differential
capacitance. A proposal for experimental detection is given, and potential
applications in nanoelectronics are mentioned.Comment: Revised version, 4 pages, 4 figure