J. Mygind

Optimization of the phase-locked flux-flow oscillator for the submm integrated receiver

The Superconducting Integrated Receiver (SIR) comprises in one chip a planar antenna integrated with an SIS mixer, a superconducting Flux Flow Oscillator (FFO) acting as Local Oscillator (LO) and a second SIS harmonic mixer (HM) for FFO phase locking. Free-running FFO linewidth well below 10 MHz is required to ensure phase-locked operation of an SIR. Comprehensive experimental study of the Nb-AlOx-Nb FFO linewidth and other main parameters has been carried out in order to achieve this goal. Essential dependence of the FFO linewidth on its width and idle region dimension has been found. It makes possible an optimization of the FFO design and selection of the best FFO parameters for practical operation of the SIR.

Half-harmonic parametric oscillations in Josephson junctions

Half-harmonic generation in Josephson tunnel junctions and microbridges is investigated theoretically, numerically, and experimentally. It is found that such generation may take place both at zero voltage and on rf-induced steps in tunnel junctions, but not in microbridges. A simplified theory, which leads to the definition of a plasma resonance on rf-induced steps, is presented. The experiments are made on junctions with different current densities and with pump frequencies at 18 and 70 GHz.

Josephson oscillations and noise temperatures in YBa2Cu3O7-x grain-boundary junctions

The ac Josephson effect was studied in YBa2Cu3O7−x grain‐boundary junctions (GBJ) in the temperature range from 4 to 90 K. The temperature dependence of the linewidth of millimeter‐wave Josephson oscillations was measured and it is shown that the derived effective noise temperatures may be as low as the physical temperature in the temperature range investigated. In the millimeter‐wave range, linewidths as low as 380 MHz were found at liquid‐nitrogen temperatures.

Zurek-Kibble mechanism for the spontaneous vortex formation in Nb-Al/Al(ox)/Nb Josephson tunnel junctions: new theory and experiment.

New scaling behavior has been both predicted and observed in the spontaneous production of fluxons in quenched Nb-Al/Al(ox)/Nb annular Josephson tunnel junctions (JTJs) as a function of the quench time, tau(Q). The probability f(1) to trap a single defect during the normal-metal-superconductor phase transition clearly follows an allometric dependence on tau(Q) with a scaling exponent sigma = 0.5, as predicted from the Zurek-Kibble mechanism for realistic JTJs formed by strongly coupled superconductors. This definitive experiment replaces one reported by us earlier, in which an idealized model was used that predicted sigma = 0.25, commensurate with the then much poorer data. Our experiment remains the only condensed matter experiment to date to have measured a scaling exponent with any reliability.

Spontaneous Fluxoid Formation in Superconducting Loops

We report on the experimental verification of the Zurek-Kibble scenario in an isolated superconducting ring over a wide parameter range. The probability of creating a single flux quantum spontaneously during the fast normal-superconducting phase transition of a wide Nb loop clearly follows a scaling relation on the quenching time

Zurek-Kibble Domain Structures: The Dynamics of Spontaneous Vortex Formation in Annular Josephson Tunnel Junctions

{\ensuremath{\tau}}_{Q}

Radiation linewidth of flux-flow oscillators

, as one would expect if the transition took place as fast as causality permits. However, the observed Zurek-Kibble scaling exponent

Phase‐locked flux‐flow Josephson oscillator

\ensuremath{\sigma}=0.62\ifmmode\pm\else\textpm\fi{}0.15

Towards a Phase-Locked Superconducting Integrated Receiver: Prospects and Limitations

is two times larger than anticipated for large loops. Assuming Gaussian winding number densities we show that this doubling is well founded for small annuli.

Phase locked 270-440 GHz local oscillator based on flux flow in long Josephson tunnel junctions

Phase transitions create a domain structure with defects, which has been argued by Zurek and Kibble (ZK) to depend in a characteristic way on the quench rate. We present an experiment to measure the ZK scaling exponent sigma. Using long symmetric Josephson tunnel junctions, for which the predicted index is sigma=0.25, we find sigma=0.27+/-0.05. Further, we agree with the ZK prediction for the overall normalization.