Fernando Falo

Ratchet potential for fluxons in Josephson-junction arrays

We propose a simple configuration of a one-dimensional parallel array of Josephson junctions in which the pinning potential for trapped fluxons lacks inversion symmetry (ratchet potential). This system can be modelised by a set of non-linear pendula with alternating lenghts and asymmetric harmonic couplings. We show, by molecular dynamics simulation, that fluxons behave as single particles in which the predictions for overdamped thermal ratchets can be easily verified.

Depinning of kinks in a Josephson-junction ratchet array

We have measured the depinning of trapped kinks in a ratchet potential using a fabricated circular array of Josephson junctions. Our ratchet system consists of a parallel array of junctions with alternating cell inductances and junctions areas. We have compared this ratchet array with other circular arrays. We find experimentally and numerically that the depinning current depends on the direction of the applied current in our ratchet ring. We also find other properties of the depinning current versus applied field, such as a long period and a lack of reflection symmetry, which we can explain analytically.

Mirror symmetry breaking through an internal degree of freedom leading to directional motion.

We analyze here the minimal conditions for directional motion (net flow in phase space) of a molecular motor placed on a mirror-symmetric environment and driven by a center-symmetric and time-periodic force field. The complete characterization of the deterministic limit of the dissipative dynamics of several realizations of this minimal model reveals a complex structure in the phase diagram in parameter space, with intertwined regions of pinning (closed orbits) and directional motion. This demonstrates that the mirror symmetry breaking, which is needed for directional motion to occur, can operate through an internal degree of freedom coupled to the translational one.

Josephson junction ladders: Ground state and relaxation phenomena

This paper considers a Josephson junction array with the geometry of a ladder and anisotropy in the Josephson couplings. The ground-state problem for the ladder corresponds to the one for the one-dimensional chiral {ital XY} model in a twofold anisotropy field, which allows for a rigorous characterization of the ground-state phase diagram and the relevant elementary excitations for the system. The approach to equilibrium, which we study using Langevin dynamics, shows slow relaxation, typical of systems whose energy landscape in the configuration space consists of a wealth of metastable states, dynamically disconnected.

Mode-locking of mobile discrete breathers.

We study numerically synchronization phenomena of mobile discrete breathers in dissipative nonlinear lattices periodically forced. When varying the driving intensity, the breather velocity generically locks at rational multiples of the driving frequency. In most cases, the locking plateau coincides with the linear stability domain of the resonant mobile breather and desynchronization occurs by the regular appearance of type-I intermittencies. However, some plateaus also show chaotic mobile breathers with locked velocity in the locking region. The addition of a small subharmonic driving tames the locked chaotic solution and enhances the stability of resonant mobile breathers.

Josephson-junction ladder: a benchmark for nonlinear concepts

The theoretical analysis of the ground state properties and dissipative dynamics of an anisotropic ladder of Josephson junctions has revealed interesting features associated to the nonlinear character of the Josephson effect, combined with the inherent discreteness of the system and the peculiarities of the ladder geometry. We analyse some aspects of its underdamped dynamics when spatially homogeneous time-periodic currents are injected into the islands, and predict the existence of attracting time-periodic spatially localised modes, for some ranges of junction characteristic parameters. These elementary dynamical excitations are of two different types, associated to oscillatory and rotating motion of a few superconducting island phases, respectively, revealing a dynamical mechanism of creation of vortex‐antivortex pairs. These results are physical applications of recent advances in the theory of nonlinear dynamics of discrete macroscopic systems. Their experimental confirmation would probe the physical relevance of localisation in superconducting devices. Copyright