Supersensitive high-Tc superconducting multijunction Josephson devices for environment investigation and biomagnetic applications

Supported by: The NATO "Science for Peace" Program, Project number SfP 973559 Duration: March 2001 - March 2003 Co-directors: Gennady A. Ovsyannikov, Doctor of Sciences, IREE RAS, Moscow, Russia,     Partner Country Project Director, Niels F. Pedersen, Prof., Technical University of Denmark, Lyngby, Denmark,     NATO Country Project Director, Nikolaos Uzunoglu, Prof., Athens, Greece, Victor K. Kornev, Dr., Moscow, Russia, Arsen Hakhoumian, Dr., Ashtarak, Armenia Background The project goal is design of novel extremely sensitive devices for environmental monitoring and medicine. Modern technology of high-temperature superconductors preparation and processing will be applied to design and production of such devices. Two types of devices prepared by common technology from same materials and based on similar physical phenomena will be investigated. Atmosphere monitoring is one of important parts of ecological monitoring of the environment. The upper layers of the atmosphere can hardly be tested using standard sampling technique, while contamination of these layers leads to ozone depletion, one of the most dangerous problems of the XX century. These problems are especially severe in parts of former Soviet Union, e.g. Russia or Armenia, due to insufficient efforts aimed on the ecological monitoring and extremely high pollution level. Distant measurements of these contaminations can be performed in the sub-TheraHerz and TheraHerz diapazone of electromagnetic waves, where spectral lines of various light molecules, like NO, OH, etc. can be observed. Receiver devices based on superconducting Josephson junction dynamics provide easy and extremely sensitive way for measurements of contents of these molecules. Improvement of device properties can be achieved by utilization of more then one Josephson junction on a chip and precise tuning of the junction-containing circuit. Application of a Josephson junction as a LO for the receiver circuit allows tuning of the heterodyne frequency and improves bandwidth of the resulting device. Devices made of high-temperature superconductors can be operated at intermediate temperatures without expensive liquid helium cooling or extremely sophisticated cryocoolers. Another advantage of high-temperature superconductors is the wide gap in the electron spectra allowing observations using Josephson junctions at frequencies more than 5 THz. Another field of superconductor devices application is measurement of low frequency magnetic fields in biology and geophysics using superconducting quantum interference devices (SQUIDs). SQUID consists of one or more Josephson junctions closed in a superconducting loop. Fast development of SQUIDs resulted in fabrication of number of devices for supersensitive physical experiments and production of sophisticated multichannel measurement systems used in medicine for magnetocardiography and neural investigations. The modern devices are based on low-temperature (niobium) SQUIDs and demand liquid helium cooling. Application of high-temperature superconductors for SQUID systems allows not only decrease of cooling price, but also substantial simplification of the measurement unit, resulting in decrease of size and increase of mobility of such system. Magnetocradiogram provides deeper insight in hart misfunctions compared to the standard electrocardiogram and mobile magnetocardiography system will make possible early diagnosis of infarction. Both suggested devices are based on multijunction high-temperature superconductor chips, applied for electromagnetic wave of TheraHerz frequency or sensitive low frequency magnetic field detection. No reproducible multijunction technology of high-temperature superconductors is available now, mainly due to complicated nature of HTSC and severe conditions of its formation (about 800 °C in oxygen atmosphere). In the proposed project we will develop such a technology and will design, fabricate and test multijunction structures for environmental monitoring and for supersensitive magnetic measurements. Tested structures can be passed to the end-users for incorporation into practical measurement systems. Project objectives The goals of theoretical investigation of the phase-locking Josephson structures includes the following aspects: Study of the phase-locked state regions versus oscillation frequency and JJs critical current margins. The research is directed to pinpointing the optimal values of the coupling circuit parameters and the McCumber parameter, characterising the Josephson-junction capacitance, to realise the strongest JJ interaction at frequencies f ~ fc. Search for the stability regions of parameters with maximum tolerable spread in JJs' critical currents. The impact of thermal noise on the locking states is also planned to be analysed by PSCAN program. Study of the phase-locked oscillation linewidth and its dependence on (a) number N of JJs in the array and (b) critical current margins. An auto-regressive method will be used to increase the efficiency of the spectral characteristic calculations. This study is very important to evaluate the potential applications of phase-locked arrays as coherent oscillators for tuneable submm wave receivers. Taking into account the distributed character of the JJA a numerical simulation will be carried out for arrays integrated into transmission lines. This lines can be modelled by chain circuits. It will allow the study of the interactions just as between Josephson elements, as well as between JJs and the electromagnetic wave. This should give the proper way to develop appropriate matching and phase-locking of multyjunction Josephson structures. We plan to develop: the technologies for high-Tc bicrystal Josephson junctions and multi-junction structures on sapphire substrates, the technique for optimum coupling of the Josephson-junction systems with an external submillimeter wave (500-1000 GHz) irradiation, the layout design for SQUID systems based on high-Tc bicrystal junctions. We plan also to carry out device elaboration and parameters testing: the study of microwave response of the junctions and multi-junction systems in mm and sub-mm wave ranges, the investigation of mutual phase-locking in one-dimensional and two-dimensional (1D and 2D) arrays of Josephson junctions in order to reveal phase-locking range, oscillation linewidth, power of emitted radiation, impedance matching, the investigation of self selected detectors and self-pumped mixers based on both single junction and multi-junction structures. In cooperation with the organizations interested in practical applications of such supersensitive devices for environmental investigations and biomagnetic multichannel systems, the novel supersensitive high-Tc Josephson-junction devices will be fabricated (laboratory-use version). The constructions of those high-Tc devices and fabrication technique of Josephson junction active elements will be optimized for operation in realistic conditions.