Dynamic behavior of Josephson-coupled layered structures

We have investigated Josephson effects in stacks of both artificial and natural Josephson junctions. The measurements have been performed on Nb/Al-${\mathrm{AlO}}_{\mathit{x}}$/Nb multilayers and on small single crystals of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_8$. Both systems exhibit multiple branched I-V characteristics in zero magnetic field. In finite magnetic fields coupling via currents flowing along the superconducting layers is essential, since the layers are thinner than the London penetration depth. All observations are in good agreement with numerical simulations of stacks of coupled Josephson junctions. These simulations predict that a large number of junctions can be phase locked in large magnetic fields via Fiske resonances excited in all junctions.