Observation of progressive motion of ac-driven solitons

We report the first experimental observation of phase-locked motion of a topological soliton at a nonzero average velocity in a periodically modulated lossy medium, under the action of an ac force with no dc component [the effect was predicted by G. Filatrella, B.A. Malomed, and R.D. Parmentier, Phys. Lett. A 198, 43 (1995)]. The velocity is related by a resonant condition to the driving frequency. The observation is made in terms of the current-voltage, I(V), characteristics for a fluxon trapped in an annular Josephson junction placed into dc magnetic field. Large zero-crossing constant-voltage steps, exactly corresponding to the resonantly locked soliton motion at different orders of the resonance, are found on the experimental I(V) curves. A measured dependence of the size of the steps vs. the external magnetic field is in good agreement with predictions of an analytical model based on the balance equation for the fluxon's energy. The effect has a potential application as a low-frequency voltage standard. The work was supported by a grant from the German-Israeli Foundation.Comment: Physical Review B, in press (Rapid Communication

AC induced damping of a fluxon in long Josephson junction

We present a theoretical and experimental study of Josephson vortex (fluxon) moving in the presence of spatially homogeneous dc and ac bias currents. By mapping this problem to the problem of calculating the current-voltage characteristic of a small Josephson junction, we derive the dependence of the average fluxon velocity on the dc bias current. In particular we find that the low frequency ac bias current results in an additional nonlinear damping of fluxon motion. Such ac induced damping crucially depends on the intrinsic damping parameter and increases drastically as this parameter is reduced. We find a good agreement of the analysis with both the direct numerical simulations and the experimentally measured current-voltage characteristics of a long annular Josephson junction with one trapped fluxon.Comment: Physical Review B, in pres

Bunching of fluxons by the Cherenkov radiation in Josephson multilayers

A single magnetic fluxon moving at a high velocity in a Josephson multilayer (e.g., high-temperature superconductor such as BSCCO) can emit electromagnetic waves (Cherenkov radiation), which leads to formation of novel stable dynamic states consisting of several bunched fluxons. We find such bunched states in numerical simulation in the simplest cases of two and three coupled junctions. At a given driving current, several different bunched states are stable and move at velocities that are higher than corresponding single-fluxon velocity. These and some of the more complex higher-order bunched states and transitions between them are investigated in detail.Comment: 6 pages + 6 Figures, to be published in Phys. Rev. B on July 1, 200

Nonlinear Seebeck Effect in a Model Granular Superconductor

The change of the Josephson supercurrent density of a weakly-connected granular superconductor in response to externally applied arbitrary thermal gradient dT/dx (nonlinear Seebeck effect) is considered within a model of 3D Josephson junction arrays. For dT/dx>(dT/dx)_c, where (dT/dx)_c is estimated to be of the order of 10^4 K/m for YBCO ceramics with an average grain's size of 10 microns, the weak-links-dominated thermopower S (Seebeck coefficient) is predicted to become strongly dT/dx-dependent.Comment: REVTEX, no figure

A tunable rf SQUID manipulated as flux and phase qubit

We report on two different manipulation procedures of a tunable rf SQUID. First, we operate this system as a flux qubit, where the coherent evolution between the two flux states is induced by a rapid change of the energy potential, turning it from a double well into a single well. The measured coherent Larmor-like oscillation of the retrapping probability in one of the wells has a frequency ranging from 6 to 20 GHz, with a theoretically expected upper limit of 40 GHz. Furthermore, here we also report a manipulation of the same device as a phase qubit. In the phase regime, the manipulation of the energy states is realized by applying a resonant microwave drive. In spite of the conceptual difference between these two manipulation procedures, the measured decay times of Larmor oscillation and microwave-driven Rabi oscillation are rather similar. Due to the higher frequency of the Larmor oscillations, the microwave-free qubit manipulation allows for much faster coherent operations.Comment: Proceedings of Nobel Symposium "Qubits for future quantum computers", Goeteborg, Sweden, May 25-28, 2009; to appear in Physica Script

Experimental Critical Current Patterns in Josephson Junction Ladders

We present an experimental and theoretical study of the magnetic field dependence of the critical current of Josephson junction ladders. At variance with the well-known case of a one-dimensional (1D) parallel array of Josephson junctions the magnetic field patterns display a single minimum even for very low values of the self-inductance parameter $\beta_{\rm L}$. Experiments performed changing both the geometrical value of the inductance and the critical current of the junctions show a good agreement with numerical simulations. We argue that the observed magnetic field patterns are due to a peculiar mapping between the isotropic Josephson ladder and the 1D parallel array with the self-inductance parameter $\beta_{\rm L}^{\rm eff}=\beta_{\rm L}+2$.Comment: 4 pages, 4 picture

Observation of Stochastic Resonance in Percolative Josephson Media

Measurements of the electrical response of granular Sn-Ge thin films below the superconducting transition temperature are reported. The addition of an external noise to the magnetic field applied to the sample is found to increase the sample voltage response to a small externally applied ac signal. The gain coefficient for this signal and the signal-to-noise ratio display clear maxima at particular noise levels. We interpret these observations as a stochastic resonance in the percolative Josephson media which occurs close to the percolation threshold.Comment: 4 pages, 5 figure

Origin of Multikinks in Dispersive Nonlinear Systems

We develop {\em the first analytical theory of multikinks} for strongly {\em dispersive nonlinear systems}, considering the examples of the weakly discrete sine-Gordon model and the generalized Frenkel-Kontorova model with a piecewise parabolic potential. We reveal that there are no $2\pi$-kinks for this model, but there exist {\em discrete sets} of $2\pi N$-kinks for all N>1. We also show their bifurcation structure in driven damped systems.Comment: 4 pages 5 figures. To appear in Phys Rev

Josephson Vortex Bloch Oscillations: Single Pair Tunneling Effect

We consider the Josephson vortex motion in a long one--dimensional Josephson junction in a thin film. We show that this Josephson vortex is similar to a mesoscopic capacitor. We demonstrate that a single Cooper pair tunneling results in nonlinear Bloch--type oscillations of a Josephson vortex in a current-biased Josephson junction. We find the frequency and the amplitude of this motion.Comment: 7 pages, 2 figures included as postscript files, LaTe