We have observed Josephson coupling between CuO double layers in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ single crystals by direct measurements of ac and dc Josephson effects with current flow along the c axis. The results show that a small ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_8$ single crystal behaves like a series array of Josephson junctions which can exhibit mutual phase locking.

• Received 11 February 1992

DOI:https://doi.org/10.1103/PhysRevLett.68.2394