Single fluxon dynamics in Nb-based Josephson junction stacks

J. Zitzmann*, A.V. Ustinov, M. Levichev and S. Sakai


Experiments on single-fluxon states in vertically stacked 5- 
and 10-junction Josephson rings are presented.  We find
that the current-voltage characteristic (CVC) of a fluxon moving in a
stack is distinctly different from the conventional case of a fluxon
in a long Josephson junction.

Motion of the fluxon in the stack of junctions creates plasma waves
via the Cherenkov mechanism. In the low damping regime at low
temperatures, interaction of the fluxon with these waves leads to
resonances in the CVC. We confirm observations made for two junction
rings [1] also for multi-junction rings. Good qualitative agreement
of the experimental results with simulation using the inductive
coupling model is found.

In the case of higher temperature and damping, resonant interaction
does not occur and we observe an S-shaped CVC, i.e. a non-monotonic
behavior of its derivative as suggested by our simulations. Depending
on the layer in which the fluxon is placed, we also observe a negative
differential resistance in the CVC. Simulations indicate that this is
due to the creation of a fluxon-antifluxon pair in a neighbouring
layer.

[1] E. Goldobin et al., Phys. Rev. B 57, 130 (1998)