81 research outputs found
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
- …