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Electron transport and microwave dynamics of hybrid Nb/Au/CaSrCuO/YBaCuO planar Josephson junctions

This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2010 J. Phys.: Conf. Ser. 234 042004 (http://iopscience.iop.org/1742-6596/234/4/042004) View the table of contents for this issue, or go to the journal homepage for more

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9th European Conference on Applied Superconductivity (EUCAS 09) Journal of Physics: Conference Series 234 (2010) 042004

IOP Publishing doi:10.1088/1742-6596/234/4/042004

Electron transport and microwave dynamics of hybrid Nb/Au/CaSrCuO/YBaCuO planar Josephson junctions

K Y Constantinian1, G A Ovsyannikov1,2, Y V Kislinskii1, A V Shadrin1, I V Borisenko1, P V Komissinskiy1,3, A V Zaitsev1, J Mygind4 and D Winkler
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Kotel'nikov Institute of Radio Engineering and Electronics RAS, Moscow, Russia Chalmers University of Technology, Gothenburg, Sweden Department of Materials Science Darmstadt University of Technology, Germany Technical University of Denmark, Institute of Physics, Kgs. Lyngby, Denmark

E-mail: karen@hitech.cplire.ru
Abstract. We report on measurements of dc electron transport and microwave dynamics of thin film hybrid Josephson junctions with an oxide antiferromagnetic interlayer. The base superconducting electrode YBaCuO and the antiferromagnetic (AF) interlayer CaSrCuO (with thickness d = 20 - 70 nm) were grown by laser ablation on NdGaO3 substrates. I-V curves were well fitted to RSJ model and had no excess current, ICRN products were of order 0.2 mV at T=4.2 K. We did not observe any noticeable reduction of ICRN with increasing d. Such "tunnellike" behaviour also resulted in appearance of singularities on I-V curve when magnetic field was applied. Oscillating with microwave power integer and half-integer Shapiro steps were registered along with sub-harmonic detector response. Moreover, for some of junctions a "devil" staircase structure was observed on I-V curves and giant noise-like signals were measured in 1-2 GHz band at the certain levels of microwave power. Observed features, noise performance and the impact of the second harmonic in current-phase relation on junction dynamics are discussed taking into account data for structures without AF interlayer.

1. Introduction Unusual properties of superconducting hybrid junctions with an interlayer comprising magnetic materials are of great interest for fundamental physical and electronic applications. For instance, in ferromagnetic/superconducting/ferromagnetic (F/S/F) structures the superconductivity can be controlled by spin manipulation in F electrodes [1]. Another effect is the oscillatory behaviour of the superconducting order parameter induced in the ferromagnetic layer that may lead to -phase shift in the ground state of S/F/S Josephson junction. Interesting effects occur also at superconducting and antiferromagnetic (AF) S/AF interfaces. Recently junctions Nb/Au/Ca1-xSrxCuO2/YBa2Cu3O7- with S/AF interface have been fabricated [2], where YBa2Cu3O7- (YBCO) is oxide d-wave superconductor, Au - normal metal, and Ca1-xSrxCuO2 (CSCO)­ AF layer. Josephson junctions driven far from equilibrium at microwave frequencies may demonstrate noticeable deviations of high frequency dynamics predicted by RSJ model [3]. That's why the I-V

c 2010 IOP Publishing Ltd

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9th European Conference on Applied Superconductivity (EUCAS 09) Journal of Physics: Conference Series 234 (2010) 042004

IOP Publishing doi:10.1088/1742-6596/234/4/042004

characteristics affected by microwaves and magnetic field; noise performance and detector response functions have been measured and discussed. 2. Samples and measurements The CSCO/YBCO epitaxial double-layer was grown in situ by pulsed laser deposition on NdGaO3 (NGO) substrates. In order to avoid micro-shorts the following precautions have been made: (i) CSCO films (d=20 В 50 nm) were thicker than the RMS surface roughness of the YBCO layer; (ii) Nb/Au bilayer serves as superconducting counter-electrode. Junctions had shape of a square with linear sizes L = 10, 20, ...50 µm. If Nb deposited directly on the top of YBCO film it results in formation of Nb/YBCO interface with very high resistance (~1 вcm2) due to Nb surface oxidation. Temperature dependences of specific normal state resistance RNS, where S=L2 are shown on figure 1 for junctions with different thickness d; resistance d/L2 of a bare Ca0.5Sr0.5CuO2 is also plotted, demonstrating crucial change of interlayer properties, being sandwiched between superconductors. Thickness dependence of specific resistance RNS for junctions with x=0.5 is shown in figure 2. It's seen that RNS values for junctions without AF layer are apparently larger than RNS for samples with d=1.2 nm. For measurements at microwaves experimental variables were microwave frequency f=36 - 120 GHz and power P, controlled by attenuators in the range of =0В70 dB. The background noise of the junctions was measured within 1-2 GHz using low noise cooled HEMT preamplifier and simultaneously controlled by spectrum analyser. All measurements were done at T=4.2 K in magnetically shielded cryostat in microwave-screened environment. 3. Results and discussion Most of experimental samples had symmetric RSJ-type I-V curve without excess current and the product of critical current by normal resistance ICRN ~ 200 µV at T=4.2 K. All junctions had dimensions L 10
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Figure 1. Temperature dependences of junction resistance: (1) d=20 nm, S=10x10 µm2, (2) d=40 nm, S=50x50 µm2 and (3) resistance d/S of a bare CSCO film with x=0.5, d=40 nm, S=50x50 µm2 deposited on NGO substrate.

Figure 2. Thickness dependencies of specific resistance RNS for CSCO with x=0.5 Bold line corresponds to AF=7±1 nm. Open circles are data for junctions without AF interlayer. Dash line is calculated resistance for bare AF layer.

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9th European Conference on Applied Superconductivity (EUCAS 09) Journal of Physics: Conference Series 234 (2010) 042004

IOP Publishing doi:10.1088/1742-6596/234/4/042004

10 10 Jc, A/cm
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Figure 3. Thickness dependencies of critical current density for junctions with (and without at d=0) AF interlayer. Dashes are theoretical dependences for SAF-S junctions with different intensity of exchange field h (numbers) of F layers in AF.

Figure 4. Magnetic-field dependence IC(H) for structures with L = 50 µm. With AF interlayer - black points, and thin line is calculated curve by theory [4]. A central part for a structure without AF layer - open circles.

Figure 4 demonstrate magnetic-field dependence IC(H) for junctions with and without AF interlayer, the both had the same dimensions, L = 50 nm. The calculation of IC(H) by theory [4] for SAF-S junctions is shown as well, demonstrating a huge increase in sensitivity to applied magnetic field for structures with AF interlayer [5]. Oscillating dependences of integer Shapiro step amplitudes with microwave power demonstrate good fit to RSJ model. At the same time, half-integer Shapiro steps were registered. Superposition of magnetic and microwave fields did not lead to distortion in symmetry of I-V curves and equal heights of Shapiro steps at positive and negative current bias were observed. In high frequency limit >>1, =2eICRN/hf, a deviation from sinusoidal IS()=IC1 sin() ­ IC2 sin(2).superconducting current-phase relation (CPR) could be responsible for half-integer Shapiro steps. Half-integer Shapiro steps in single junction could be observed [6-8] depending of various combinations of parameters , C, L/J and applied microwave power P. In order to estimate the weigh q=IC2/IC of the 2nd harmonic in CPR when L
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9th European Conference on Applied Superconductivity (EUCAS 09) Journal of Physics: Conference Series 234 (2010) 042004

IOP Publishing doi:10.1088/1742-6596/234/4/042004

was large enough and a frequency mixing effect also takes place due to existence of self-Josephson radiation. In order to compare intensities of the frequency mixing products with the chaotic oscillations we estimated the output signal saturation in frequency mixing regime. Figure 6d shows the dependences of output signal levels vs. applied power of microwave signals at fe =45 GHz and 70 GHz when current bias was fixed, keeping the voltage near the half-integer Shapiro step V1/2=(2e/h)/fe, where chaotic behaviour was observed. Frequency mixing saturation level is also shown. Experimental conditions for chaotic oscillations in "short" Josephson junctions were analysed [9] and were experimentally observed [10] in the case of superconducting tunnel junctions with C >25. However, our junctions had relatively small C parameters and probe signals were in range 0.5 В 0.9.

-35

(d)
Output Signal (dB)
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70 GHz

45 GHz

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Figure 6. Families of I-V curves under different power levels of microwave irradiation f=45 GHz (a, b) and f=70 GHz (c). Critical current IC, integer I1, I2 and half-integer I1/2 Shapiro steps are indicated. (b): n=2 Shapiro step distortion is pointed by circle. (c): Chaotic noise rise corresponds to the jerks on I1/2 steps. All curves are shifted by voltage to the right with increase of applied power. AF interlayer was d=20 nm Ca0.5Sr0.5CuO2, L=20 µm, J = 88 µm, IC,=55 µA, RN=5.5 , and C=2. (d): Output signal measured in 1-2 GHz frequency band under microwave power P at 45 GHz (black symbols) and 70 GHz (open symbols). Dash line shows saturation level for frequency mixing.

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9th European Conference on Applied Superconductivity (EUCAS 09) Journal of Physics: Conference Series 234 (2010) 042004

IOP Publishing doi:10.1088/1742-6596/234/4/042004

Recently chaotic dynamics was predicted [11] for S/F/S Josephson structures with magnetic interlayer consisted of 3 separated F-layers with rotated magnetization. Although we did not obtained experimental evidence for the triplet Josephson effect in our structures, the latter finding points on very complicated high frequency dynamics in Josephson junctions with magnetic interlayer. These results show that along with the search for promising materials of magnetic interlayer aiming at applications at microwaves the dynamics of such junctions with magnetic interlayer should be studied in details in order to avoid chaotic behaviour and unstable operation. 4. Conclusions Hybrid superconducting Josephson junctions with an antiferromagnetic d=10 - 50 nm thick interlayer were fabricated on NdGaO3 substrates. Exponential decrease of critical current density with AF layer thickness was observed. Superposition of magnetic and microwave fields did not lead to distortion in symmetry of I-V curves: equal Shapiro step amplitudes were registered at positive and negative voltage biasing. The sensitivity to applied magnetic field for these junctions was found much higher than for conventional Josephson junctions. Then, half integer Shapiro steps observed along with the sub-harmonic frequency selective detector response. That points on deviation of current-hase relation from sinusoidal one. At the same time at the certain experimental conditions a "devil" staircase appears on I-V curve and the giant noise-like signal was registered for the junction fabricated using coriented YBCO electrode. Acknowledgment This work was supported in parts by Department of Physical Sciences of the Russian Academy of Sciences, EU Proj. NMP3-CT-2006-033191, ISTC Proj. 3743, RFBR 08-02-00487. References [1] Gu J Y, You C-Y, Jiang J S, Pearson J, Bazaliy Ya B and Bader S D 2002 Phys. Rev. Lett. 89 267001 [2] Komissinskiy P, Ovsyannikov G A, Borisenko I V, Kislinskii Y V, Constantinian K Y, Zaitsev A V and Winkler D 2007 Phys. Rev. Lett. 99 017004 [3] Barone A, Paterno G 1982 Physics and Applications of the Josephson Effect N.-Y. AwaleyInterscience Pub. John Wiley&Sons, 620 p. [4] Gorkov L P and Kresin V Z 2002 Physica C 367 193 [5] Kislinskii Y V, Konstantinian K Y, G.A. Ovsyannikov, P.V. Komissinskiy, I.V. Borisenko, A.V. Shadrin, 2008 JETP, 106, 800-805. [6] Terpstra D, Ijsselsteijn R P J and Rogalla H 1995 Appl. Phys. Lett. 66 2286 [7] Seidel P, Siegel M and Heinz E 1991 Physica C, 180 284 [8] Kislinskii Y V, Komissinski P V, Constantinian K Y, Ovsyannikov G A, Karminskaya T Y, Soloviev I I and Kornev V K 2005 JETP 101 494 [9] Kautz R and Monaco R 1985 J. Appl. Phys. 57 875 [10] Gubankov V N, Constantinian K Y, Koshelets V P, Ovsyannikov G A and Vystavkin AN 1983 IEEE Tr. on MAG-19 968 [11] Braude V and Blanter Ya M 2008 Phys. Rev. Lett. 100 207001

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