Tunneling between Pb and c-axis oriented Bi2Sr2CaCu2O8 single crystals

We investigated c-axis tunnel junctions between Bi2Sr2CaCu2O8+x (BSCCO) sin gle crystals and Pb. At low temperatures, the current-voltage characteristi cs exhibit a well developed Pb gap as well as a nonzero Josephson current. In magnetic fields the critical current follows closely a Fraunhofer diffra ction pattern showing that the critical current density is homogeneous. In microwave fields, Shapiro steps appear at voltages that are multiples of hf /2e confirming that the Josephson currents are of first order. These result s strongly indicate that the superconducting order parameter of BSCCO has a nonzero s-component. (C) 1999 Elsevier Science B.V. All rights reserved.