Alexander Grib

The influence of standing waves on synchronization and self-heating of Josephson junctions in resonant systems

We investigated numerically synchronization of Josephson junctions inside the transmission line. We have found that due to the resonance behavior of the system there appear the self-induced resonance steps, strong synchronization of junctions on these steps and the inhomogeneous distribution of the Joule heat extraction (the self heating) along the line which can lead to the formation of “hot spots” in the line. The developed model can be applied to explain recent experiments in which these effects were obtained.

Synchronization of serial intrinsic Josephson junction arrays on vicinal substrates

A resonator together with a resistive shunting cover over the microbridge was applied to synchronize intrinsic Josephson junctions. We made both numerical and experimental investigations of the electrical properties of such microbridges: the multibranch behaviour above critical currents and switching between branches. The stability analyses revealed that in-phase states are sensitive to noise. Optimal parameters for synchronization of the system of two junctions up to 26–27% spread of critical currents are calculated.

Quasiparticle current and phase locking of intrinsic Josephson junctions

On the base of our experiments on thin film Josephson junctions in mesa geometry we discuss the quasiparticle branches of the intrinsic arrays within a tunnelling model using d-wave superconductor density of states. We find temperature dependent current contributions and a zero bias anomaly. The coherent behaviour is studied for intrinsic arrays with an additional side-wall shunt. The existence of thresholds of phase locking at small as well as at large inductances is demonstrated. We discuss the problems with experimental realisation of the shunts as well as with an alternative concept to enhance phase locking in such arrays towards application as oscillators in the frequency range up to some THz.

Phase locking in a stack of two Josephson junctions with a sidewall shunt

Abstract The coherent behaviour (phase locking) was investigated by the method of slowly varying amplitudes for different inductances of the shunt as well as for different capacitances of junctions. The existence of thresholds of phase locking at small inductances as well as at large inductances was explained in the developed model.

Experimental and theoretical investigation on high-Tc superconducting intrinsic Josephson junctions

Within the last years many groups have realized and investigated different types of intrinsic Josephson junction (IJJ) arrays out of high-temperature superconducting single crystals or thin films. We tried to improve the synchronization between the junctions by external shunts. Mesa structures as well as microbridges on vicinal cut substrates showed multi-branch behaviour in their IV characteristics and random switching between branches. Theoretical modelling was done investigating phase dynamics and stability numerically as well as analytically. Branch structure in current voltage characteristics of IJJ is studied in the framework of different models, particularly, in capacitevely coupled Josephson junctions (CCJJ) model and CCJJ model with diffusion current. Results of modelling of return current in IV characteristics for stacks with different number of IJJ are presented. We discussed the possible mechanisms of synchronization and the ranges of stability. Conclusions with respect to application of such arrays such as radiation sources were given.

The Coherent Dynamic State of Intrinsic Josephson Junctions

We developed a numerical model that described the dynamics of many long layered intrinsic Josephson junctions with normal edges. The IV characteristics of stacks containing two and three layers were calculated. We found that collective dynamics of junctions leads to the appearance of zero-field steps in the IV curves. We showed that due to the mutual inductive interaction of junctions, each of these zero-field steps was split into two steps. For the stack of two junctions, it was found that the dependence relations of voltages of split steps on the parameter of inductive coupling obeyed the simple relations following the theory of inductively coupled resonant contours. We also analyzed coherent radiation produced by the stack of long junctions.

Coherent emission of intrinsic Josephson junctions

The model of synchronization of intrinsic Josephson junctions in a mesa structure of a high-temperature superconductor is developed. According to our model, the mesa structure does not consist of continuous layers but is divided into small stacks of layers. These stacks form the continuous environment which is considered as a transmission line. We showed that the conventional s-wave dynamic model for the Josephson junction and the d-wave dynamic model give nearly the same particularities in IV-characteristics. We showed also that the model describes numerically the dependence of the frequency of the coherent emission on the width of samples obtained in experiments.

Resonant Modes in a Stack of Intrinsic Josephson Junctions

In order to study the origin of coherent emission of intrinsic junctions in high-temperature superconductors, we investigated numerically ac power of emission at zero-field steps in IV-characteristics of a stack of two inductively coupled long Josephson junctions as well as emission from the solitary long junction. We found that zero-field steps appeared due to noise, random distribution of critical currents or due to normal edges of junctions in the stack. Due to the formation of collective excitations in the stack of coupled junctions, the zero-field step was split into two steps, which correspond to two normal modes of excitations. We investigated the dependence of heights of steps on the length of the stack and found that there existed two different threshold values of the length, so above the threshold length the zero-field step existed and below the threshold length it vanished. We calculated the dependence of ac power on the length of the stack and found the optimal value of the length for larger emission.

The Resonant Interaction of Intrinsic Josephson Junctions With Standing Waves

IV-characteristics of shunted microbridges made of Tl2Ba2CaCu2O8 (TBCCO) were measured. The TBCCO films were grown epitaxially on 20° misaligned LaAlO3 substrates and then covered by the gold shunting layer. Each of the microbridges consisted of about 360 shunted intrinsic Josephson junctions. Self-induced resonant particularities with a main period of about 17 mV were found on IV-curves. We showed that these particularities did not depend on the characteristics of the individual junctions and that they were connected with the geometrical dimensions of the system. The origin of particularities was the interaction of Josephson generation with the standing wave appeared in the system. We modeled this interaction considering radiation of junctions into the transmission line and found that the model described the main resonant structure of IV-characteristics. Strong synchronization of junctions appeared in the vicinity of the first particularity.

Coherent radiation in serial arrays of Josephson junctions

We measured IV-characteristics of a planar array of about 362 shunted intrinsic Josephson junctions in microbridges made from the Tl2Ba2CaCu2O8 layers which were grown epitaxially on 20° misaligned LaAlO3 substrates. The IV-characteristics contained resonant particularities (jumps of voltages) with a main period of 17 mV. We showed that the origin of particularities was the interaction of Josephson generation with the internal resonant mode of the system. We modeled this interaction considering the radiation of junctions into the transmission line and found that strong synchronization of junctions appeared in the vicinity of the first particularity. The estimate of the ac power emitted into free space was 0.05-0.1 microwatt, whereas the power of 0.3-0.5 mW was emitted inside the resonant system.