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تسجيل مستخدم جديدSpectral characteristics of noisy Josephson flux flow oscillator
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The current-voltage and spectral characteristics of a flux flow oscillator (FFO), based on a long Josephson junction, are studied. The investigations are performed in the range of small bias currents and magnetic fields, where the FFO radiates a quasi-chaotic signal with extremely large radiation linewidth, and the displaced linear slope (DLS) is observed at the current-voltage characteristic. By direct numerical simulation of the sine-Gordon equation it is demonstrated that for large lengths of the Josephson junction or in the case of finite matching of the FFO with external waveguide system, the DLS with extremely large linewidth is transformed into Fiske steps with very narrow linewidth. While there is the common belief that the chaotic regime of the FFO is due to excitation of the internal oscillation modes in the soft fluxon chain, it is demonstrated that this regime is inspired by multiple reflections of the traveling waves from junction ends.
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We elaborate a theoretical description of large Josephson junctions which is based on the Werthamers microscopic tunneling theory. The model naturally incorporates coupling of electromagnetic radiation to the tunnel currents and, therefore, is partic
ularly suitable for description of the self-coupling effect in Josephson junction. In our numerical calculations we treat the arising integro-differential equation, which describes temporal evolution of the superconducting phase difference coupled to the electromagnetic field, by the Odintsov-Semenov-Zorin algorithm. This allows us to avoid evaluation of the time integrals at each time step while taking into account all the memory effects. To validate the obtained microscopic model of large Josephson junction we focus our attention on the Josephson flux flow oscillator. The proposed microscopic model of flux flow oscillator does not involve the phenomenological damping parameter, rather, the damping is taken into account naturally in the tunnel current amplitudes calculated at a given temperature. The theoretically calculated current-voltage characteristics is compared to our experimental results obtained for a set of fabricated flux flow oscillators of different lengths. Our theoretical calculation agrees well with the obtained experimental results, and, to our knowledge, is the first where theoretical description of Josephson flux flow oscillator is brought beyond the perturbed sine-Gordon equation.
Since the very first experimental realization of Josephson flux-flow oscillator (FFO), its theoretical description has been limited by the phenomenological per- turbed sine-Gordon equation (PSGE). While PSGE can qualitatively describe the topological
excitations in Josephson junctions that are sine-Gordon solitons or flux- ons, it is unable to capture essential physical phenomena of a realistic system such as the coupling between tunnel currents and electromagnetic radiation. Furthermore, PSGE neglects any dependence on energy gaps of superconductors and makes no distinction between symmetric and asymmetric junctions: those made of two iden- tical or two different superconducting materials. It was not until recently when it became possible to calculate properties of FFO by taking into account information about energy gaps of superconductors [D. R. Gulevich et al., Phys. Rev. B 96, 024215 (2017)]. Such approach is based on the microscopic tunneling theory and has been shown to describe essential features of symmetric Nb-AlOx-Nb junctions. Here we extend this approach to asymmetric Nb-AlN-NbN junctions and compare the calculated current-voltage characteristics to our experimental results.
For the first time the linewidth of Flux-Flow Oscillator has been calculated by direct computer simulation of the sine-Gordon equation with noise. Nearly perfect agreement of the numerical results with the formula derived in [Phys. Rev. B, {bf 65}, 0
54504 (2002)] has been achieved. It has been demonstrated that for homogeneous bias current distribution the linewidth actually does not depend on the junction length for practically interesting parameters range. Depending on the length of the unbiased tail, the power may be maximized and the linewidth may be minimized in a broad range of bias currents. The linewidth can be decreased further by 1.5 times by proper load matching.
The computer simulations of fluctuational dynamics of the long overlap Josephson junction in the frame of the sine-Gordon model with a white noise source have been performed. It has been demonstrated that for the case of constant critical current den
sity the mean life time (MLT) of superconductive state increases with increasing the junctions length and for homogeneous bias current distribution MLT tends to a constant, while for inhomogeneous current distribution MLT quickly decreases after approaching of a few Josephson lengths. The mean voltage versus junction length behaves inversely in comparison with MLT.
High-resolution scanning superconducting quantum interference device (SQUID) microscopy was used to study the flux quantization phenomenon in multiply-connected anisotropic high-Tc NdBa2Cu3O7-d single crystalline thin films. The spatial distribution
of internal flux in a hole was found to be non-uniform and changed drastically for applied small fields. With increased fields above 10uT, a local magnetic dipole flux developed inside the hole, in contrast to an isotropic Nb superconductor. The total net flux trapped in a hole was kept to be constant for larger holes, but the abrupt transition of flux quantization state was observed for smaller holes. The possible explanation is given based on the anisotropic dx2-y2-wave order parameter of high-Tc superconductors.
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