M. Cirillo

Microwave-Induced Thermal Escape in Josephson Junctions

We investigate, by experiments and numerical simulations, thermal activation processes of Josephson tunnel junctions in the presence of microwave radiation. When the applied signal resonates with the Josephson plasma frequency oscillations, the switching current may become multivalued in a temperature range far exceeding the classical to quantum crossover temperature. Plots of the switching currents traced as a function of the applied signal frequency show very good agreement with the functional forms expected from Josephson plasma frequency dependencies on the bias current. Throughout, numerical simulations of the corresponding thermally driven classical Josephson junction model show very good agreement with the experimental data.

On magnetic flux dynamics in 1D arrays of underdamped Josephson junctions

Abstract We have measured the current-voltage characteristics of parallel biased arrays of small Nb/NbO x /Pb Josephson junctions connected by superconducting loops. Depending on the temperature of the sample, the application of a magnetic field perpendicular to the planes of the loops gives rise to linear branches and flux-flow steps in the current-voltage characteristics of the arrays. For increasing temperatures the linear branches evolve in resonant flux-flow like singularities. Good agreement between the experimental data and the direct numerical simulations based on the discrete Josephson transmission line model is found.

Millimeter wave generation by fluxon oscillations in a Josephson junction

A small‐area Josephson tunnel junction is pumped by the radiation emitted by a long Josephson junction dc biased on a zero‐field singularity. Josephson constant voltage steps are observed in the current‐voltage characteristic of the small junction at voltages of ±150 μV corresponding to the fundamental frequency of emission of the oscillator. The radiation also gives rise to quasiparticle steps around the gap‐sum voltage of the small junction. From the measured current amplitude of the steps we estimate that the maximum power coupled to the small junction is about 90 nW.

Catastrophe observation in a Josephson-junction system

We report on experiments performed to probe quantum coherence in a system consisting of an rf-SQUID in which the Josephson junction is replaced by a small loop containing two junctions in parallel. At temperatures of the order of 10 mK the system may develop three potential energy wells, which modify the usual two well energy profile and thereby verify the qubit manipulation strategy. The appearance of the third potential well can be interpreted as evidence of a butterfly Catastrophe, namely, a catastrophe expected for a system described by four control parameters and one state variable.

Coherence of Josephson soliton oscillators in the millimeter-wave range

Abstract We report on the observation of phase-locking and radiation enhancement from two dc series-biased Josephson fluxon oscillators ac-coupled by a thin film-integrated capacitor. The radiation is detected by a small area Josephson junction displaying, under the effect of the radiation, critical current suppression and Shapiro steps at a voltage of 220μV. The phenomena that we observe are the higher frequency counterpart of the superradiant effects measured by means of X-band room temperature receivers detecting the radiation emitted by series arrays of long Josephson junctions.

Experiments on tunable Josephson millimeter-wave oscillators

The coupling of millimeter-wave radiation from a long Josephson junction to a small-area junction is discussed. The coupling is obtained by means of thin-film technology and the design parameters allowing radiation coupling are discussed. Since the frequency of the oscillator radiation is smaller than the plasma frequency of the detector junction, the latter shows evidence of chaotic dynamics in the current-voltage characteristic. However, very stable zero-crossing bias steps appear for high values of the applied radiation power. The tunability of the oscillator power allows measuring the dependence of the detector critical current on the external radiation to be measured; from this measured dependence and from the observed amplitude of the photon-assisted tunneling steps it is estimated that maximum coupled power is the range of tens of nanowatts.

Radiation detection from Fiske steps in Josephson junctions above 200 GHz

We have measured the Josephson radiation emitted at millimeter‐wave frequencies by a long Josephson junction dc‐current biased on Fiske steps of the current‐voltage characteristic. Our measurements demonstrate that differences between dynamic excitation generating Fiske steps and fluxon oscillations giving rise to zero‐field steps in long junctions may exist. The radiation, whose maximum measured frequency is 240 GHz, is generated by an in‐line junction and detected on a chip by a small planar tunnel junction. From measurements of Shapiro steps amplitude and photon‐assisted tunneling steps we find that the coupled radiation has a power tunability of 350 nW and the maximum coupled power is 20 nW in a 30‐Ω normal‐state resistance tunnel junction.We have measured the Josephson radiation emitted at millimeter‐wave frequencies by a long Josephson junction dc‐current biased on Fiske steps of the current‐voltage characteristic. Our measurements demonstrate that differences between dynamic excitation generating Fiske steps and fluxon oscillations giving rise to zero‐field steps in long junctions may exist. The radiation, whose maximum measured frequency is 240 GHz, is generated by an in‐line junction and detected on a chip by a small planar tunnel junction. From measurements of Shapiro steps amplitude and photon‐assisted tunneling steps we find that the coupled radiation has a power tunability of 350 nW and the maximum coupled power is 20 nW in a 30‐Ω normal‐state resistance tunnel junction.

Coupling of long Josephson junction oscillators at millimeter-wave frequencies

The authors report on results of phase-locking experiments in a system composed of two long Josephson junctions oscillating at millimeter-wave frequencies. The effects of the coherent behavior of the two junctions are visible on the current-voltage characteristic of the long junctions and in the power enhancement effects observed on the current-voltage characteristic of a detector junction. It is shown that is possible to couple two long junctions in such a way that the enhanced radiation can be transmitted to other devices. Results from another mode of operation of the long Josephson junction as an oscillator are also presented. This mode is based on magnetic-field-induced singularities in long junctions (Fiske steps) generated by the interaction of the cavity modes of the junction seen as an open-ended transmission line and the Josephson oscillations. This mode has given very promising results above 200 GHz, where a power of 20 nW was coupled in a 30- Omega normal state resistance tunnel junction.<<ETX>>

Analysis of linear branches in the I- characteristics of Josephson junctions

Resistive branches have often been observed in the current-voltage characteristics of long Josephson junctions. In the present paper we perform a systematic numerical analysis of the generation of these branches when an external magnetic field is applied to the junctions. We show that the linear branches exist only over a limited voltage interval which is determined quantitatively. We also clarify to what extent the linear branches can be attributed to flux-flow effects.

Magnetic field dependence of thermal excitations in Josephson junctions

We have measured the rate of escape out of the zero-voltage state in Josephson tunnel junctions as a function of the applied magnetic field. A marked difference is found in the behaviour of long and small junctions. In all cases, the statistical distribution of the switching currents can be described using a Kramers model for the escape process, where the barrier to be overcome is the Josephson barrier and the activation energy is due to an effective temperature T/sub e/. For small junctions T/sub e/ coincides, as expected, with the thermodynamic temperature, regardless of the applied magnetic field. For long junctions instead it is found that the escape temperature depends markedly on the magnetic field and on the junction geometry (inline or overlap), suggesting a close relationship with the magnetic field distribution inside the junction.