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

Microscopic examination of hot spots giving rise to nonlinearity in superconducting resonators

We investigate the microscopic origins of nonlinear rf response in superconducting electromagnetic resonators. Strong nonlinearity appearing in the transmission spectra at high input powers manifests itself through the emergence of jumplike features near the resonant frequency that evolve toward lower quality factor with higher insertion loss as the rf input power is increased. We directly relate these characteristics to the dynamics of localized normal regions (hot spots) caused by microscopic features in the superconducting material making up the resonator. A clear observation of hot-spot formation inside a Nb thin film self-resonant structure is presented by employing the microwave laser scanning microscope, and a direct link between microscopic and macroscopic manifestations of nonlinearity is established.Comment: 5 pages, 4 figure

Electrical breakdown of a dielectric for the formation of a superconducting nanocontact

Electrical breakdown of the dielectric nanolayer between film electrodes of niobium and an alloy of 50% indium and 50% tin forms a bridge of this alloy between the electrodes. The bridge resistance depends on the breakdown current. The length of the bridge is equal to the thickness of the dielectric (30 nm), and its diameter is 25 nm. The calculated coherence length of the alloy at 0 K is close to the length of the bridge. The calculated critical current of a bridge with a resistance of 1 {\Omega} at a temperature of 0 K is 2 mA. It is concluded that such a bridge should have the properties of a Josephson contact at a temperature lower than the critical temperature of the alloy (6.5 K).Comment: 11 pages, 7 figure

Broken symmetry of row switching in 2D Josephson junction arrays

We present an experimental and theoretical study of row switching in two-dimensional Josephson junction arrays. We have observed novel dynamic states with peculiar percolative patterns of the voltage drop inside the arrays. These states were directly visualized using laser scanning microscopy and manifested by fine branching in the current-voltage characteristics of the arrays. Numerical simulations show that such percolative patterns have an intrinsic origin and occur independently of positional disorder. We argue that the appearance of these dynamic states is due to the presence of various metastable superconducting states in arrays.Comment: 4 Pages, 6 Figure

Observation of breathers in Josephson ladders

We report on the observation of spatially-localized excitations in a ladder of small Josephson junctions. The excitations are whirling states which persist under a spatially-homogeneous force due to the bias current. These states of the ladder are visualized using a low temperature scanning laser microscopy. We also compute breather solutions with high accuracy in corresponding model equations. The stability analysis of these solutions is used to interpret the measured patterns in the I-V characteristics