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Josephsonovy jevy a jejich aplikace

Z. (Zdeněk) Janů

Published Jan 1, 2011• 0 citations
OpenAlex

Abstract

Chapter deals with Josephson effects, Feynman's derivation of DC and AC Josephson effects, Josephson effects in superfluid helium, calibration invariance, influence of DC magnetic field, temperature dependence of critical current, phase difference electrodynamics in the junction, RCSJ model, voltage-current characteristics, microwave irradiated junction, macroscopic quantum interference, technology, DC voltage standard, sub mm radiation receivers, microwave generators, SQUIDS and its applications.

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Chapter deals with Josephson effects, Feynman's derivation of DC and AC Josephson effects, Josephson effects in superfluid helium, calibration invariance, influence of DC magnetic field, temperature dependence of critical current, phase difference electrodynamics in the junction, RCSJ model, voltage-current characteristics, microwave irradiated junction, macroscopic quantum interference, technology, DC voltage standard, sub mm radiation receivers, microwave generators, SQUIDS and its applications.
Effects of magnetic fields and microwave radiation on I-V characteristics of YBa2Cu3Oy high-T, Josephson junctions, SQUIDS and Josephson arrays were reported in this experiment, with special emphasis focused on the effects of microwave radiation. In addition to the ordinary constant-voltage steps resulting from the ac Josephson effect, noninteger constant voltage steps are occasionally observed under microwave radiation. We attribute these noninteger constant-voltage steps to the flux coupled to junctions or SQUIDS. Resistive SQUIDS disclose V-Φmodulation without a biased current. The peak to peak value AV of a V-Φ curve is enhanced by microwave radiation.
Properties of weak links between two superconductors, or Josephson junctions, make them interesting for fundamental physics research. Since their discovery over four decades ago, they have provided a unique way to study the behavior of the superconducting quantum phase. More recently, ultra small, or mesoscopic, Josephson junctions with substantial single Cooper pair charging energy have gained interest due to their behavior as macroscopic quantum objects. In addition to the theoretical interest, Josephson junctions can be used as active elements in circuit applications. Particularly, in this Thesis we study two different devices. We develop the required theoretical treatments, derive device properties, and compare the results with experimental data. The first application is a Josephson voltage standard based on externally damped Superconductor - Insulator - Superconductor junctions. It consists of an array of large Josephson junctions connected in series and irradiated with a 70 GHz microwave signal. Phase locking the Josephson dynamics into the signal leads to the quantization of the voltage. This is utilized in metrology. We introduce a new circuit solution based on frequency dependent damping of the junctions. Optimization and some designs for practical arrays are presented. The purpose is to find such a design that the array is fast, has low power consumption and is as stable as possible. Arrays able to generate DC voltages of order 1 volt with metrological accuracy are demonstrated experimentally and their applicability in AC voltage calibrations is analyzed. The second application is the Bloch Oscillating Transistor (BOT). The BOT is based on controlling the Cooper pair current in an ultra small Josephson junction by means of quasiparticles tunneling through a normal junction. As part of the thesis work, the principle of operation is first demonstrated computationally. The model is then refined to yield quantitative predictions of the characteristics. Finally, an analytic theory for the device is developed and the properties as an amplifier are derived.
Applications to electronics are mainly based on Josephson junctions special properties. As it was pointed before, a single Josephson junction is a perfect voltage to frequency converter (AC Josephson effect). One example is the application into RF amplifiers where a radiofrequency signal generates a magnetic flux in a circuit coupled to, for example, DC-SQUID. Josephson effect allows converting the flux into the SQUID to an amplified voltage signal, proportional to the number of 0. That is the base of the microwave applications. The SQUID inductance together with the capacitance of the junction, allows also using the device as resonator.
The influence of microwave phase noise on rf-induced constant voltage steps generated by the ac Josephson effect is studied. Different circuit models for the junction excitation are considered: dc and rf voltage generators, dc and rf current generators, dc current and rf voltage generators. The analysis has been carried out assuming a power and band limited phase noise modulation in the first two cases, while in the last one a white frequency modulation spectrum is required for a treatment based on the Fokker-Planck equation. Some experimental results are reported, in order to display the basic phenomenological behavior.
This paper describes the I-V characteristics (as a function of applied magnetic field) and magnetic field response of a dc SQUID which incorporates two weak-link Josephson junctions in a superconducting ring. These characteristics are derived by digital computer simulation, using the Stewart-McCumber model. The effects of asymmetry (dissimilarity between the junctions) on SQUID critical current I c, normal resistance R and capacitance C are also simulated. With microwave excitation applied, the Josephson oscillations of the dc SQUID are found to be “frequency-locked” to the applied microwave signal even when critical currents I c of the two junctions differed, Subharmonic voltage steps are obtained for a symmetric dc SQUID in a magnetic field even when the current-phase relationship is purely sinusoidal.