|Supported by:||Russian Foundation for Basic Research, Project number 98-02-17366|
|Duration:||January 1999 - January 2001|
|Supervisor:||Evgueni A. Stepantsov, Ph.D., Shoubnikov Institute of Crystallography RAS, Moscow, Russian Federation|
|Supervisor in IREE RAS:||Gennady A. Ovsyannikov, Doctor of Sciences in Physics and Mathematics|
More than 10 years have passed since the discovery of the high-temperature superconductivity in the metal oxide ceramics, but the nature of the superconducting state in these remains unclear. It was found out that while ordinary, low-temperature, metal, superconductors always show the s-type of the wave function of the superconducting electrons, isotropic on the impulse directions, the metal oxide superconductors wave function has more complex structure. A number of experiments proposes the d-type of the wave function, having two directions with positive and two (in orthogonal direction) - with negative sign of the order parameter. This corresponds to the 180-degree shift of the wave function phase. The effect of the wave function phase is most efficiently revealed in the experiments with Josephson structures, that are used for determination of the dependences on the pulse direction. Some experiments still cannot be explained using simple d-pairing model. This demands new experiments to determine the order parameter symmetry. The information on the order parameter anisotropy itself doesn't provide us with the superconducting pairing mechanism, but can be an important arguement when choosing the certain theoretical model.
The phase sensitivity of the grain boundary Josephson junctions was supposed to be used for measurements of the phase anisotropy of the order parameter in HTSC. The project goals are:
to study in detail the dependences of the electrophysical and magnetic parameters of the HTSC grain boundary junctions at different orientations of the current flow direction relatively to the boundary;
to study in detail the dependences of the microwave parameters of the HTSC grain boundary junctions at different orientations of the current flow direction relatively to the boundary;
to study more complex superconducting structures, where the effects, sensitive to the phase differences of the superconducting wave functions of the junction, can be revealed.
In the framework of the project the HTSC group carries out the following tasks:
The current-phase relation was studied experimentally for superconducting current component, flowing through the symmetric bicrystal high-Tc Josephson junction, using Fourie-transform technique for experimentally obtained dependences of critical current and Shapiro step amplitudes on the mm-wave diapason electromagnetic irradiation. It was shown that the current-phase relation for the discussed case is close to the sin-type probably due to the twins in the superconducting electrodes, forming the junction. This, as well as the inhomogeinity of the grain-boundary interface results in deviations from the theory, developed for the d-type superconducting junctions with bound Andereev levels. It was revealed experimentally that the asymmetry in the current feed under an angle with respect to the bicrystal interface yields a change in the current-phase function, enhancing with increase in rate of asymmetry.
The density of the critical current in a symmetric bicrystal high-Tc Josephson junction was experimentally found to be proportional to the square root of the barrier transparency due to existence of bound states with vanishing energies at interface boundary in the d-type superconducting junctions. These states are caused by the difference in phase for incident and reflected electrons in the tunnelling processes in the DID junctions. This gives an additional channel for the current flow in comparison to an ordinary tunnelling.
In order to obtain the current-phase relation in the (a-b) plane of the YBCO Josephson junctions 18 types of Y-ZrO2 bicrystal substrates with different crystal-axis orientations have been fabricated. A multijunction structure was designed with novel topology, fabricated and tested at microwaves. It consists of: a) 3/5 Josephson junction submm wave oscillating unit, b) two detectors, and c) properly coupled antenna.
To improve the experimental investigations reproducibility additional studies were carried out to enhance the HTSC thin film quality:
A.D. Mashtakov, K.I. Konstantinyan, G.A. Ovsyannikov, E.A. Stepantsov, YBCO Josephson junctions on a bicrystal sapphire substrate for devices in the millimeter and submillimeter wavelenght ranges, Technical Physics Letters, v. 25, No 4, p. 249-252, 1999.
A.D. Mashtakov G.A. Ovsyannikov, I.V. Borisenko, I.M. Kotelyanskii, K.Y. Constantinian, Z.G. Ivanov, D. Erts, High-Tc Superconducting Step-Edge Josephson on Sapphire Fabricated by Non-Etching Technique, IEEE Tr. On Appl. Supercond., v. 9, N 2, p. 3001-3004, 1999
G.A. Ovsyannikov, I.V. Borisenko, K.I. Kostantinyan, A.D. Mashtakov, E.A, Stepantsov, Phase dependence of the superconducting current in YBCO Josephson junctions on a bicrystal substrate, Technical Physics Letters, v. 25, No 11, p. 913-916, 1999.
G.A. Ovsyannikov, I.V. Borisenko, A.D. Mashtakov, K.Y. Constantinian, Current transport mechanism in YBCO bicrystal junctions on sapphire, IOP Conf.Ser., No 167, pt. II, pp. 253-256, 2000
G.A. Ovsyannikov, I.V. Borisenko, K.Y. Constantinian, Electron Transport in High-Tc Superconducting Grain Boundary Junctions, Preprint cond-mat/9911009 http://xxx.lanl.gov, 1999.
K.Y. Constantinian, A.D. Mashtakov, G.A. Ovsyannikov, V.K. Kornev, A.V. Arzumanov, N.A. Shcherbakov, M. Darula, J. Mygind, N.F. Pedersen, MM wave Josephson radiation in High-Tc bicrystal junction arrays, IOP Conf. Ser., No 167, pp. 717-720, 2000
K. Lee, I. Iguchi, K.Y. Constantinian, On-Chip Detection of Radiation from HTS Josephson Junction Arrays, IEEE Tr. of Appl. Supercond., v. 9, N 2, p. 4333-4336, 1999.
K.Y. Constantinian, A.D. Mashtakov, G.A. Ovsyannikov, K. Lee, I. Iguchi, High Frequency Detector response from an Array of HTSC Bicrystal Josephson Junctions, IEEE Tr. On Appl. Supercond., v. 9 N 2, p. 2947-2950, 1999.