Nanoscale physics

The influence of disorder on emission of a stack of two Josephson junctions
Shaternik V.1, Suvorov O.1, Grib A.2, Vovk R.2, Savich S.2
1G.V. Kurdyumov Institute for Metal Physics, 36, Vernadsky bl., 03142 Kyiv
E-mail: shaternikvolod@gmail.com 
2Kharkiv V. N. Karazin National University, Svobody sq., 4, 61022, Kharkiv

Investigations of self-resonant steps in IV-characteristics of long Josephson junctions which appear without applied external magnetic field (the so-called zero-field steps) become of large importance because coherent emission of sub-THz and THz waves in stacks of many intrinsic Josephson junctions in high-temperature superconductors was found at these steps. Zero-field steps appear in the IV-curves due to mixing of Josephson currents with standing waves which arise when the distribution of critical currents along the junction is inhomogeneous. In the present work we investigated the important problem of the determination of maximum tolerant spread of critical currents at which coherent emission exists. IV-characteristics and ac power of emission were calculated for the model of the stack of two long Josephson junctions with inductive coupling. Distributions of critical currents along junctions had Gaussian spreads. Zero-field steps at averaged voltages which correspond to frequencies of in-phase modes of oscillations of voltage over junctions as well as maxima of ac power of emission at these steps were found. The dependence of ac power at the maximum on the value of spread was investigated in the hysteretic region at the backward movement of the bias current. We found that coherent emission existed at large Gaussian spreads of critical currents. We believe that such strong synchronization of junctions in the stack appears due to the influence of strong ac currents in neighbor segments of junctions. These currents come into existence due to inequality of critical currents of these segments. Strong ac currents synchronize oscillations of voltage across segments of long junctions in the whole stack, increase the tolerant spread of critical currents and lead to strong phase locking. Our data support conclusions that the increase of the disorder of critical currents leads at certain conditions to the increase of the height of the zero-field step.
This publication is based on the research provided by the grant support of the State Fund for Fundamental Research (project Ф76/36725).