Narrow Results

Vortices and single charges have particle-like properties when they are excited in a medium with extremely low damping at low temperatures. We have experimentally studied the behavior of arrays of superconducting islands, weakly coupled by high quality, small capacitance Josephson tunnel junctions. In the islands, the phase of the order parameter and the electrical charge act as conjugate variables. The response is determined by the ratio between the Josephson coupling energy E_J and the charging energy E_C. For E_C/E_J<<1 vortices are the dominant excitations; for E_C/E_J>>1 single charges are dominant. We have observed the phase transition as a function of E_C/E_J from superconducting to insulating behavior in zero applied field. It is also possible to observe the phase transition induced by a perpendicular magnetic field, characterized by Bose condensation of vortices. We have studied vortices as ballistic particles. Recently we have obtained first indications of quantum interference of vortices.