"Current transport and high frequency interactions in high-T_c superconducting multijunction Josephson structures"

Summary

The main objectives of the project are both a theoretical and an experimental investigation of current transport and high-frequency non-linear dynamics in high-T_c superconducting weak links and multyjunction Josephson structures. Investigations of conductivity and magnetic properties of high-T_c superconducting weak links in a wide range of temperatures, applied magnetic fields, as well as under microwave irradiation in order to determine the mechanism of quasiparticle and superconducting currents transport through the grain-boundary interface will be carried out. One-dimensional parallel arrays of Josephson junctions will be studied. Investigations of the high-frequency dynamics of one-dimensional arrays of JJs connected in series with passive coupling circuits providing strong nonlocal interaction between Josephson elements, in particular, a search for phase-locked oscillation states, oscillation linewidth will be conducted. A study of two-dimensional arrays as distributed structures with active coupling circuits providing strong JJ interaction due to fluxon flow will be performed. This includes an insight into the dynamics of the breakdown phenomena in superconducting grain boundary multijunction structures, and an observation of a new type of vortices predicted for Josephson media, as well as the self-field effects on flux flow and the dissipative processes, related to excitation of plasma oscillations ("spin waves").

Experimental investigations of phase-locked Josephson-junction arrays include sample fabrication and dc and rf testing. It is planned to fabricate multyjunction structures using both low-T_c and high-T_c superconductors. Nb-tunnel Josephson-junction technology will be used to fabricate arrays of JJs connected in series or in parallel. Basic cells of Josephson-junction arrays will be also fabricated using high-T_c Josephson-junction technology. Dc and rf measurements include the studies of

1. I-V curves in a wide range of applied magnetic fields as well as under external microwave irradiation up to THz region,
2. frequency mixing and detector response on external microwave signal, and
3. power and linewidth of the microwave radiation emitted by arrays in the phase-locked state regime.

Project objectives

• Investigation of conductivity and magnetic properties of high-T_c superconducting weak links in wide range of temperatures, applied magnetic fields, as well as under microwave radiation in order to determine the intrinsic nature of quasiparticle and superconducting currents transport through the grain-boundary (GB) interface.

• Theoretical and experimental study of high frequency interactions in one-dimensional parallel arrays of GB Josephson junctions (JJs). It includes the investigation of both the applied magnetic field influence and electromagnetic wave impact on the phases of the Josephson oscillations to provide the narrow linewidth radiation, emitted by the array.

• Investigation the high-frequency dynamics of one-dimensional arrays of JJs connected in series with passive coupling circuits, providing strong non-local interaction between JJs. In particular, it involves a search for phase-locked oscillation states, which are insensitive to a spread in critical currents.

• Study of one-dimensional and two-dimensional arrays as distributed structures with active coupling circuits providing strong interaction of JJs due to fluxon flow. It includes an insight into the dynamics of the breakdown phenomena in superconducting GB multijunction structures and the self-field induced effects on the flux-flow regimes and on the dissipative processes, related to excitation of plasma oscillations.

Group activity

February 2000

1. Preparation of experimental JJs and arrays:
   • Bicrystal YBCO junctions on sapphire substrates, covered with cerium oxide buffer layer;
   • Arrays of bicrystal JJs on sapphire substrates.
2. Measurements:

   • 4-probe dc transport characteristics (I-V curves and dV/dI(V,I) functions, temperature dependencies of junction resistance R(T), critical current I_c(T), product I_cR_N(T) in the range of T = 4.2-300 K and magnetic field up to ±100 G) of HTS JJs;

   • low noise measurements of HTS JJ on sapphire substrate;

   • amplitudes of Shapiro steps vs applied microwave at frequencies 40-120 GHz;

   • self-radiation linewidths, background noise dependencies of JJs and arrays.

Scientific relults

January 2000

1. Josephson oscillation linewidth reduction with number of JJs was studied for promising JJ arrays with lumped and distributed coupling circuits by means of numerical simulation technique. It was shown that the linewidth reduction is proportional to the number N of JJs or unit cells in the case of 1D or 2D arrays with lumped coupling circuits. However, the reduction is always limited by a finite value of the coupling radius. In other words, a "saturation" effect for the linewidth reduction takes place, and the linewidth does not decrease with number N, if the system size exceeds the coupling radius. So, the case of parallel array of JJs coupled by small superconducting inductances (normalized value l ~ 0.5…1) the number N corresponding to the coupling radius runs up to 60…100. In the case of 2D array based on the 4-junction interferometer cell the coupling radius corresponds approximately to K ~ 3 unit cells for each dimension at l ~ 1, and the factor of oscillation linewidth reduction is 16 times K. If the coupling radius is exceeded, the decrease in the line width stops, and the spectrum of coherent oscillations transforms from a single spectral line to a set of lines. Despite of that the average voltages across JJs of the array are the same, the instant oscillation frequency is switched from one value to another with a beat frequency w_b. Thus, the factor of the linewidth reduction is unlikely to exceed 100 for multijunction arrays with the lumped coupling circuits. At the same time, the distributed coupling circuits are able to provide an other mechanism of linewidth reduction by impact of the standing electromagnetic waves excited in the coupling circuits.

2. Parallel array of JJs inserted into superconducting microstrip line was studied by means of numerical simulation. Each section of the line between the neighbouring JJs was modelled by a LC-chain of 10…20 element. It was shown that a significant interaction between JJs and electromagnetic wave takes place at r/R_N > 1, where r is the wave impedance of the microstrip line, and R_N is normal state resistance. The resonance peculiarities on IV-curve of the system correspond to the resonance of either series type at small values of McCumber parameter b or, straight conversely, of parallel type at large values of b, when the high-frequency impedance of JJ is a capacitive and much smaller than r. Despite of that the JJs are included into the dc superconducting loops, the phase-locked oscillation in-phase state can be provided by the strong interaction of JJs with the standing waves.

3. HTS bicrystal JJ arrays with superconducting thin-film loops as coupling circuits were fabricated and tested. A special feature of the array was that the junctions were connected in series for high frequency signals, and dc biased in parallel. The mm wave self-radiation of the arrays was measured by an external receiver. The system was also studied theoretically by means of numerical simulation. It was shown that according to the array topology the loops should be described by the shorted-circuit pieces of superconducting slit line. Unlike the previously studied parallel array inserted into microstrip line, the system is characterized by the interactions of Josephson junctions with the standing waves of the current. The crucial role of high frequency losses has been also studied. Two basic requirements for JJ arrays with distributed coupling circuits should be fulfilled to provide maximum oscillation linewidth reduction. First, the total microwave losses of the loaded system should be less then threshold value. Second, the relationship between the JJ impedance and characteristic impedance of the distributed circuits should correspond to the low wave reflection from the junctions.

4. Grain boundary bicrystal JJs on sapphire with misorientation angle of 24° were fabricated and tested at 100 GHz. The high resistance 10-20 W and I_cR_N = 1-2 mV for junction width of 4-5 mm and tolerance of R_NS about 30% allow design of a microwave Josephson circuit with at least 10 junctions on chip for mm and submm wavelengths set-up.

5. Mm-wave 40-140 GHz and sub-mm wave 270-550 GHz frequency down conversion and selective detection in JJs, prepared by 100 nm thick and 4 mm wide YBCO thin film deposition on bicrystal sapphire substrates. The linewidths of Josephson oscillation, obtained from selective detector response function, measured at 505 GHz and ambient temperature 17-20 K, were 3.5 - 4 times wider, than predictions of RSJ model. The heterodyne mixing, using the 3-rd harmonic pumping at 149 GHz has demonstrated strong down converted signal 450 GHz. Reducing the frequency of heterodyne oscillator down to 65 GHz, the mixing has been observed also for harmonics of signal source at frequencies of order of 500 GHz.

6. We developed a technology for performing high frequency experiments on HTS JJs using a STO-buffered MgO substrate. Using a self-mixing mode it was demonstrated that Josephson oscillations up to at least 2.5 THz are present. Using a hot-cold method the mixer noise temperature of a few thousand Kelvin at 100 GHz was measured.

7. Novel dynamic states are manifested in a broken symmetry of row switching were directly observed in 2D JJ arrays by means of visualization using laser scanning microscopy. The appearance of fine branching in the current-voltage characteristics of the array and the broken symmetry of row switching lines may be ascribed to the presence of various metastable superconducting states in arrays.

8. Vortex dynamics have been studied in high-quality 2D Nb-Pb JJ arrays by means of transport and magnetic measurements. 100x100 cell square array with the period 20 mm and the junction critical current of about 150 mA has manifested large self-field effects (b_L>>1). Magnetization measurements with sensitivity 10^-10 Am² show pronounced hysteresis loop. Multiple jumps observed at the loop can evidence for the self-organizing critical state in the array.

9. 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 IC(T) oscillations detected 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).

10. A crossover of the SFS junctions from "0"- to "pi"-state was observed. 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.

11. Transverse transport in the multilayered SN structures (Nb/Cu, Nb/Au/YBCO) and in HTSC quasi-2D superconductors (BSCCO) were studied. Both for the artificial Nb/Cu multilayers and for the extremely anisotropic HTSC's we have observed evidences for the existence of the multiple quasi-equilibrium Josephson fluxon modes: multi-valued and fluctuated critical currents branched I-V characteristics etc.

12. We study numerically and analytically phenomena in stacked JJs. Critical current suppression observed for LTSC-N-HTSC (Nb-Au-YBCO) structures we can explain both by small Au/YBCO interface transparency and in terms of contact between d-wave superconductors with normal metal.

Group publications

1. A.D. Mashtakov, K.Y. 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. K. Lee, I. Iguchi, K.Y. Constantinian, Tunable detection of radiation from HTS Josephson Junction Arrays, Physica C v. 320, pp. 65-70, 1999.

3. 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.

4. 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.

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.