"Phase-coherent transport in contacts of conventional and unconventional superconductors, normal metals and magnetics "

Summary

The main question of high-T_c superconductivity is the question of the order parameter symmetry in these materials. In the case of d-wave, due to the sign change of the order parameter associated with this symmetry, strong deviations from the standard sinusoidal dependence have been predicted for the current-phase relations of various configurations of Josephson junctions employing such unconventional superconductors. These predictions are highly unusual. Therefore, an experimental clarification of the CPR for high-T_c junctions for which deviations from a standard harmonic behaviour are expected is desirable. Such measurements will further enhance our understanding of the order parameter symmetry in the high-T_c cuprates and its influence on the properties of grain boundaries and high-T_c Josephson junctions. In addition, they will provide valuable information for the design and use of Josephson junction-based circuits, of which many characteristics depend directly on the current-phase relations. However, to our knowledge, such experiments have not been carried out. All data available refer to Josephson junctions for which nominally sinusoidal current-phase relations are expected.

The idea of "p-contact" is connected with unconventional order parameter symmetry for high-T_c superconductors. It should be noted, that the existence of p-contact was proposed firstly for conventional superconductor-ferromagnet-superconductor Josephson junctions, where exchange ferromagnetic field was responsible for the additional phase difference. However p-contacts were not realised and investigated in detail both for the superconductor-ferromagnet-superconductor junctions and for the high-T_c contacts.

Therefore the aim of the investigation is to clarify the nature of transport mechanisms in high-T_c as well as in low-T_c specially prepared superconducting weak links.

Project objectives

• To investigate the current-phase relation of high-T_c superconducting grain boundary junctions, formed in high-T_c films, grown on bicrystal substrates with the same angle of misorientation (30°), but tilted in every 5° relatively to each other around (001) axis.

• To determinate the conductivity type in heterojunctions consisting of normal metal, unconventional and conventional superconductors with transport in [001] and tilted [001] crystallographic directions, including the current-phase relation measurements at low temperatures.

• To investigate experimentally the transport mechanisms current-phase relation and bound states in high-T_c bicrystal junctions with the in-plane tilt grain-boundaries.

• To measure the perpendicular transport characteristics of conventional superconductor/normal metal-ferromagnetic/conventional superconductor (Ta/Cu-Ni/Ta) junctions with the exchange energy of the normal metal/ferromagnetic-interlayer comparable with Josephson coupling energy to detect "pi"-contact behaviour.

• To investigate experimentally and theoretically peculiarities of the Andreev reflection of spin-polarised quasiparticles on the superconductor-ferromagnetic interfaces in conventional superconductor/ferromagnetic/conventional superconductor (Nb/Fe/Nb and Ta/Ni/Ta) sandwiches.

Group activity

February 2000

• High-T_c (YBCO) thin film deposition by DC-sputtering at high pressure and grain boundary junction preparation using ordinary photolithography, RF-etching and wet etching techniques. Optimisation of bicrystal junction fabrication on NdGaO₃ and Al₂O₃ (sapphire) substrates with a proper buffer layer. A newly developed magnetic field stimulated plasma etching technique was applied for patterning the bottom YBCO electrode of the junction. The advantages of this technique compared to the ordinary RF-sputtering is the improvement of discharge uniformity and stability.

• Nb/Au/YBCO heterojunctions for current-phase and dc transport measurements fabrication on the epitaxial c-axis oriented and tilted YBCO thin films by using lithography and etching technique. DC measurements of test junction showed the heterojunctions have the characteristics suitable for measurements of current-phase relation.

• Study of DC electrical properties of both the bi-crystal junctions and thin film heterostructures, autonomous and under influence of applied d.c. magnetic and/or microwave fields, provided with high accuracy power and frequency control both in the mm wave range at frequencies f_e=37-110 GHz, and in the sub-mm wave range at frequencies f_e=500-1000 GHz

Scientific relults

February 2000

1. Current transport in Au/YBa₂Cu₃O_x heterojunctions was investigated for (001) YBa₂Cu₃O_x and (109)/(101) YBa₂Cu₃O_x (c-axis tilted) films. The conductivity of Au/YBa₂Cu₃O_x heterostructure is of the tunnel and metallic type for the cases of (001) and (109)/(101)-oriented films correspondingly. R_NS values obtained for in-situ Au layers for (001)-oriented Au/YBa₂Cu₃O_x contacts were 10^-5Wcm² and fell down to 10^-6Wcm² with annealing at 600°C in oxygen atmosphere (R_N is the resistance in the normal state, S is the junction area). Contacts to basic (a-b) planes of YBa₂Cu₃O_x with R_NS~10^-7Wcm² were realized for hetero-junctions with (109)/(101) YBCO films. The probable reason of these facts is an oxygen diffusion to the YBa₂Cu₃O_x surface layer along tilted (a,b)-planes, which provides an oxygen content saturation of the YBa₂Cu₃O_x surface layer, which is involved in the current transport process across Au/YBa₂Cu₃O_x interface. Nb/Au/YBa₂Cu₃O_x Josephson junctions were fabricated on the basis of (001)-oriented YBa₂Cu₃O_x films. R(T) dependences and I-V curves of these junctions were measured at temperature 4.2 K. The I-V curves shows the behaviour similar to resistive shunted model of Josephson junction with the critical current density 1-10 A/cm², R_NS~10^-5-10^-6Wcm²

2. The dependences of the amplitudes of the harmonic and sub-harmonic Shapiro steps were used to study current-phase relation (CPR) of YBCO Josephson junction on a bicrystal sapphire substrate. It is shown that for a symmetric current bias across the mirror-symmetric bicrystal junction with interface transparency of the order of D = 10^-4, the CPR is close to sinusoidal one that differs from the theoretical predictions based on d-type of superconducting wave function in electrode. The divergence from symmetry in the current biasing across JJ causes the CPR to deviate from sinusoidal, which increases with increasing degree of asymmetry. This change in the CPR is described accurately by a model, which takes into account the formation of bounded Andreev states in Josephson junctions of superconductors with a d-type of superconducting wave function and twinning of YBCO films.

3. Several attempts have been made for optimization of high-T_c epitaxial film growth. Optimization of laser deposition parameter for the YBCO thin films was performed in order to obtain smooth particle-free film surfaces. Effect of Ar addition in oxygen atmosphere during deposition and influence of overheating of the target were studied. The Nd/Ba substitutions were investigated for the NdBa₂Cu₃O_y thin films .

4. Re-entrant superconducting behaviour of the critical supercurrent temperature dependencies has been observed for the Nb-Cu/Ni-Nb SFS (superconductor-ferromagnetic-superconductor) JJs. The detected critical supercurrent temperature dependencies are associated with a crossover of the SFS junctions from "0"- to "pi"-state that is related to a special feature of superconducting pair flow through a ferromagnetic (spatial oscillations of induced superconducting order parameter in presence of the exchange field).

5. We have argued advantages of SFS "pi"-junction use for the quantum bit implementation based on the superconducting loop with quantum JJs. While previously proposed designs are based on magnetically frustrated superconducting loops, we discuss the advantages offered by the "pi"-junctions in obtaining naturally degenerate two-level systems.

6. For investigation of the complete anisotropy of the phase component of the high-T_c order parameter in (a-b) plane, 18 types of (001) Y-ZrO₂ bicrystal substrates with 32-degrees artificial grain boundaries were designed and fabricated. In every type of these substrates the plane of the grain boundary was tilted in every 5 degrees around <001> axis relatively <100> axes. These 18 grain boundaries cover the complete space of the crystal lattice of high-T_c film, taking into account its crystallographic symmetry and presence of twins.

7. The method of YBCO bicrystal thin film patterning over artificial grain boundary was developed for getting nano-meter resolution up to 300 nm and bicrystal Josephson junctions on the base of nanobridges with width up to 300 nm were fabricated. The junctions demonstrate I_cR_N up to 5 mV and R_N up to 50 W at 4.2 K and no excess current.

8. It was shown experimentally that the suppression of the critical current of the Josephson junction due to the magnetic field leads to a modulation of the amplitude of the SQUID output signal. The drop of the SQUID signal for high-T_c bicrystal at a finite magnetic field is originated by the penetration of the magnetic field into the unwanted junction.

9. As expected, single Nb/AlO_x/Nb tunnel junctions with relatively large I_c as well as highly asymmetrical double junction structures exhibited the sinusoidal CPR. In contrast to that, the CPR found for the nominally symmetrical structures indeed distinctly deviated from sinf.

10. We have shown that the CPR of high-Tc ramp-edge Josephson junctions (YBa₂Cu₃O_7-x/ PrBa₂Cu₃O₇/YBa₂Cu₃O_7-x) with sufficiently small critical current I_c is sinusoidal. In the case of larger critical current, deviations of the apparent CPR from sinusoidal dependence were found. These deviations can be described by thermal fluctuations of the magnetic flux in the loop. So we conclude that for all investigated junctions the "true" CPR is sinusoidal.

11. For the CPR of symmetrical 45° grain-boundary junctions a model of strongly inhomogeneous Josephson junctions with randomly alternating current has been developed. Good agreement between theory and experiment has been found.

12. For the CPR of assymmetrical 45° grain-boundary junctions the remarkable amplitude of the p-periodical component has been experimentally found.

Group publications

1. P.B. Mozhaev, G.A. Ovsyannikov, J.L. Skov, Influence of laser deposition parameters on the orientation of cerium oxide buffer layer on sapphire and properties of superconducting thin film YBCO, Technical Physics, v. 44, No 2, p. 242-245, 1999.

2. P.V. Komissinski, G.A. Ovsyannikov, N.A. Tulina, V.V. Ryazanov, c-axis tunneling in Nb/Au/YBCO structures preprint Cond-mat/9903065.

3. F.V. Komissinski, G.A. Ovsyannikov, N.A. Tulina, and V.V. Ryazanov, Current transport along the [001] axis of YBCO in low-temperature superconductor - normal metal - high-temperature superconductor heterostructures, JETP Vol. 89, p. 1160, 1999.

4. 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.

5. 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

6. 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.

7. G.A. Ovsyannikov, A.D. Mashtakov, I.V. Borisenko, K.Y. Constantinian, YBCO Bicrystal Junction on Sapphire: d-Wave Impact and Posssible Application, Journal of Low temperature Physics, v. 117, No 3/4, p. 605-610, 1999.

8. A.Yu. Kidiyarova-Shevchenko, D.E. Kirichenko, Z. Ivanov, F. Komissinsky, E.A. Stepantsov, M.M. Khapaev, T. Claeson, Single flux quantum comparators for HTS ADS Converters, Physica C, v. 326-327, pp. 83-92, 1999.