The current-voltage characteristics of microwave-irradiated superconducting weak links have been calculated on the analog computer. Experimental results of several authors can be interpreted correctly by taking into consideration the resistive feedback.
The dc characteristics of superconducting weak links, irradiated by one and two microwave frequencies and the dependence of the absorbed microwave power on the dc current, are evaluated. Within the constant voltage steps, the ac current generated in the weak link is synchronous with the incident microwaves or their mixing products. In the case of two incident microwave frequencies within the constant voltage steps, the dependence of the ratio of power-absorption variation on the impressed dc current at the two frequencies is governed by modified Manley-Rowe equations.
The current–voltage characteristics of a micrometre bridge of intrinsic Josephson junctions under microwave irradiation are studied. The collective switching of the group of four junctions splits up as the AC signal amplitude is gradually increased. The switching current of the remaining group of junctions is increased with increasing radiation power. We consider that microwave irradiation injects an additional quasiparticle current into the Josephson junction array. We use ideas of breakdown of quasineutrality and quasiparticle charge imbalance in the superconducting layers and explain the experimental results by the competition between the 'current effect' and the effect of suppression of the switching current by irradiation.
We report an experimental and theoretical study of the phase diffusion in small Josephson junctions under microwave irradiation. A peculiar enhancement of the phase diffusion by microwaves is observed. The enhancement manifests itself by a pronounced current peak in the current-voltage characteristics. The voltage position V(top) of the peak increases with the power P of microwave radiation as V(top) proportional to sqrt[P], while its current amplitude weakly decreases with P. As the microwave frequency increases, the peak feature evolves into Shapiro steps with a finite slope. Our theoretical analysis, taking into account the enhancement of incoherent superconducting current by multiphoton absorption, is in good agreement with experimental data.
In the presence of a battery source and a radiation field, a superconducting weaklink can be represented by a simplified model consisting of a resistance shunted Josephson junction driven by a constant current source. Using such a model, we give a general criterion for the occuring of constant voltage current steps in the current-voltage characteristics of the weaklink. Following this criterion, we have also discussed the absence of sub-harmonic current steps. For the case while the radiation field is very weak, the current step heights are calculated by pertubation method. The current step heights are also obtained by numerical compution for the case of strong field.
The electrical equations of a resonant circuit fed with a radio-frequency current and inductively coupled to a weakly superconducting loop have been derived. The current-voltage characteristic of the rf superconducting quantum interference device can then be calculated and studied as a function of the junction resistance or of the resonance frequency, enabling its behavior to be better understood. As shown by Kurkijärvi, the large positive slope of the steps and the rounding of the characteristic result from thermal noise. However, in order to explain quantitatively the experimental results concerning, namely, point contact weak links which are generally oxidized or which have several contact points, it is useful to remark that the connection between the two superconductors constituting the Josephson junction may be made from n resistive paths. The mean thermal energy of the fluctuating flux is then close to nkbT instead of kbT if the inductances linking the contact points are large enough, that is, on the order of 10−11 H.