Solitons in Long Josephson Junctions

Abstract

The Josephson junction transmission line (JTL) is approximately described by the sine-Gordon equation, and hence it is a convenient experimental solid state system for the study of solitons. In the JTL the physical manifestation of a soliton is a (propagatingj) fluxon, i.e. a magnetic flux quantum, Φ_o = h/2e = 2,064 X 10⁻¹⁵V•s. This and the corresponding relation to simple observable quantities (zero field steps) was noted first in a pioneering paper by Fulton and Dynes, 1973. The field has since attracted a large number of researchers and considerable progress has been made both theoretically and experimentally. A number of applications have been suggested; they include a soliton microwave generator, a vortex transistor, and the use of a fluxon as the basic bit in data processing circuits. The spectacular advances in fabrication technology connected with programmes on a Josephson junction computer implies that such ideas are not a priori unrealistic. In general there have been four methods of investigating perturbed sine-Gordon solitons on the JTL: (i) analytical or perturbation methods, (ii) numerical simulation, (iii) measurements on mechanical models, and (iv) experiments on real Josephson junctions. Much of the progress has taken place through interaction and stimulation between researchers working in those four directions. As a result we are moving in a direction where large Josephson junctions with solitons are understood almost as well as small junctions with spatial homogeneity.