"Determination of the space anisotropy of order parameter phase of the high temperature superconductivity"

Summary

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.

Project objectives

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:

1. 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;

2. 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;

3. 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.

Group activity

In the framework of the project the HTSC group carries out the following tasks:

• development of technologies for high T_c bicrystal junction on NdGaO₃ and Al₂O₃ substrates;
• preparation of HTSC circuits with Josephson junction with different orientations of of the current flow direction relatively to the boundary;
• measurement of the electrophysical and magnetic properties of the grain boundary Josephson junctions;
• measurement of the microwave properties of the grain boundary Josephson junctions;
• theoretical investigations and simulations of the electrophysical properties of the HTSC-HTSC interfaces.

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Scientific relults

January 2000

1. The current-phase relation was studied experimentally for superconducting current component, flowing through the symmetric bicrystal high-T_c 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.

2. The density of the critical current in a symmetric bicrystal high-T_c 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.

3. 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.

4. To improve the experimental investigations reproducibility additional studies were carried out to enhance the HTSC thin film quality:

   • the effect of laser ablation parameters on the surface roughness and crystalline quality of the YBCO thin films was determined;
   • the YBCO thin films were grown on the NdGaO₃ substrates with orientation differing from standard both with and without implementation of the CeO₂ buffer layer;
   • thin films of the NBCO material (HTSC of the Re123 family) were obtained. Application of this material provides films with higher crystal quality compared with the usual YBCO superconductor.

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Group publications

1. 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.

2. A.D. Mashtakov G.A. Ovsyannikov, I.V. Borisenko, I.M. Kotelyanskii, K.Y. Constantinian, Z.G. Ivanov, D. Erts, High-T_c Superconducting Step-Edge Josephson on Sapphire Fabricated by Non-Etching Technique, IEEE Tr. On Appl. Supercond., v. 9, N 2, p. 3001-3004, 1999

3. 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.

4. 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

5. G.A. Ovsyannikov, I.V. Borisenko, K.Y. Constantinian, Electron Transport in High-T_c Superconducting Grain Boundary Junctions, Preprint cond-mat/9911009 http://xxx.lanl.gov, 1999.

6. 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-T_c bicrystal junction arrays, IOP Conf. Ser., No 167, pp. 717-720, 2000

7. 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.

8. 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.