S. G. Lachenmann

Frequency dependence of the emission from 2D array Josephson oscillators

Coherent emission from 2-D arrays of Josephson junctions, coupled to a detector junction through a DC blocking stripline capacitor, was detected over a frequency range from 50 to 210 GHz. A power of 0.26 mu W, which is larger than the 0.1 mu W expected from the resistively shunted-junction model, was detected in a range from 140 to 150 GHz. Frequencies where no emission was detected correspond to standing waves in the capacitor when multiples of the half-wavelength match the capacitor length. Low-temperature scanning electron microscopy confirmed the presence of standing waves at these frequencies, but also revealed standing waves at other frequencies, indicating an impedance mismatch and a possible extension of the standing waves into the array.<<ETX>>

Direct observation of vortex dynamics in two-dimensional Josephson-junction arrays

Spatially resolved images of the dynamic states of current-biased overdamped two-dimensional arrays of Nb/AlO/sub x//Nb Josephson junctions were obtained using low-temperature scanning electron microscopy. We present two-dimensional imaging results describing various vortex dynamic regimes in zero applied magnetic field. The nucleation of current-induced vortices at the array boundaries and their subsequent motion into the array interior are observed for bias currents slightly above the array critical current. With increasing bias current, vortex-vortex interaction becomes important. Discussions on the coherent microwave radiation emission are presented.<<ETX>>

OBSERVATION OF A SPATIALLY-COHERENT RESONANCE MODE IN STACKED JOSEPHSON JUNCTIONS

Abstract Underdamped flux-flow dynamics in double-barrier Josephson tunnel junctions is investigated by a spatially-resolving technique. Under the influence of a bias current, Josephson vortices can move simultaneously in both Josephson junctions resulting in the excitation of cavity waves in the junctions. The spatial structure of the cavity resonances is imaged by scanning the sample with a focused electron beam. We find clear indication that the vortices in the two stacked junctions move coherently.

Flux-flow steps in overlap Josephson junctions - imaging of different states by low temperature scanning electron microscopy (LTSEM)

Abstract Modelling a long Josephson junction with the perturbed sine-Gordon equation one possible solution represents a continous, unidirectional motion of magnetic flux quanta (fluxons). In the dc I-V curve this fluxon motion manifests itself as a current step. Using LTSEM we investigated flux-flow behavior of different types of Josephson junctions. The electron beam acts as a local perturbation influencing the “fluxon chain” in the Josephson junction. For different bias points on the same flux-flow step we observed different patterns inside the junction.

Soliton dynamics in Josephson tunnel junctions

The non-linear dynamics of Josephson junctions is well described by the perturbed sine-Gordon equation together with the appropriate boundary conditions. A possible solution of the pure sine-Gordon equation are solitons. In the junction the (quasi) solitons manifest themselves as Josephson vortices, each of them carrying one magnetic flux quantum. We used the technique of Low Temperature Scanning Electron Microscopy to study the soliton dynamics in superconducting tunnel junctions of various geometries. The electron beam power can be adjusted to work in different regimes. For high power, the beam acts as an active probe and we were able to introduce individual solitons in annular junctions one by one and studied multisoliton behavior without the influence of boundaries. For low power, the beam acts as a passive probe influencing the soliton dynamics in a well controlled way. Here, we investigated details of the dynamic soliton behavior.