Directional motion of fluxons in a parallel Josephson-Junction chain.
  
  F. Falo(a), P.J Martinez (a), J.J. Mazo (a,b), S. Cilla(a), E. Trias (b) and
  T. P. Orlando (b)
  
  (a) Grupo de Teoria y Simulacion de Sistemas Complejos. Universidad de
  Zaragoza -CSIC. Spain
  
  (b)Department of Electrical Engineering and Computer Science. MIT.
  USA.
  
  
  Directional motion (DM) of overdamped particles in asymmetric potential
  is currently an active field of research. The ideas of DM have been
  applied in proposals for devices in a large variety of systems from phase
  separation engines to array of quantum dots. In this communication we
  present a theoretical proposal and an experimental realization of DM of
  fluxons in a parallel chain of Josephson junctions. We propose (1) a
  simple configuration of the array in which the pinning potential for the
  trapped fluxon lacks of inversion symmetry (ratchet potential). This
  system can be modelised by a set of nonlinear pendula with alternating
  harmonic couplings and lengths. This corresponds to alternated critical
  currents and area plaquettes. We show, by numerical simulations , that
  fluxons behave as single particles in which the predictions for
  overdamped thermal ratchet can be verified. To check the validity of this
  design (2) we have measured the deppining of trapped fluxon in a
  fabricated circular array of JJ with alternated cell inductances and
  junctions areas. We find experimentally that the deppining current
  depends on the direction of the applied current in our ratchet ring. We
  also find other properties of de deppining current versus an applied
  magnetic field, such as a long period and a lack of reflection symmetry,
  which we can explain analytically.
  
  
  (1) Ratchet potential for fluxons in Josephson-junction arrays. 
  F. Falo, P.J. Martínez, J.J. Mazo and S. Cilla 
  Europhysics Letters 45(6), 700-706 (1999) 
                                          
  (2) Depinning of kinks in a Josephson-junction ratchet array. 
  E. Trías, J.J. Mazo, F. Falo and T.P. Orlando 
  Phys. Rev. E 61, 2257 (2000).