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Millimeter wave study of hybrid superconducting heterostructures with magnetic active interlayer
Y.V. Kislinskii1, K.Y. Constantinian1 P.V. Komissinskij1,2, G.A. Ovysuannikov1. Institute of Radio Engineering and Electronics, RAS, Mokhovaya 11-7, 125009 Moscow, Russia. Fax 7495 2038414, e-mail yulii@ns.crys.ras.ru 2 Chalmers University of Technology, S 41296 Geteborg, Sweden.

1

Introduction. Josephson junctions with ferromagnetic barriers were fabricated and studied [1] for their ground state change from 0 to : IS=ICsin() to IS = ICsin(+), where IS is superconducting current, IC critical current, is the phase difference of pair potential between superconducting electrodes. Circuits, consisting from 0-junctions and -junctions could be used for qubits ­elements of quantum computer [2]. We report on hybrid Nb/Au/(Ca1-xSrx)CuO2/YBa2Cu3O7 Josephson heterostructures made with antiferromagnetic (AF) barrier between superconducting electrodes, exhibiting current ­ phase relation (CPR), modeled as IS = (1q)ICsin()+ q ICsin(2). The epitaxial grown AF barrier layer Ca1-xSrxCuO2 was up to 50 nm thick. To investigate the origin of the zero voltage current for our hybrid heterostructures we applied microwave radiation and studied the ac Josephson effect. Measuring Shapiro step amplitudes and detector response functions we estimated the CPR for the junctions by methods described in [3, 4]. Studies of critical current versus magnetic field were performed for testing the uniformity of critical current density in the junctions. Recently L. Gorkov and V. Kresin predicted a reduced period of IC(H) magnetic field pattern for superconductor­antiferromagnetic­ superconductor junction, comparing with a SIS tunnel junction having the same size [5]. Obtained IC(H) experimental results for our heterojunctions were in qualitatively agreement with this prediction. Experimental methods YBa2Cu3O7 (YBCO) films were deposited by eximer laser on NdGaO3 substrates. At the same run the Ca0.85Sr0.15CuO2 (CSCO) was deposited epitaxially. A 20 nm thick Au film was deposited over the CSCO, then 200 nm Nb top electrode. The technology includes a deposition of SiO insulator and ion beam etching. The heterostructures were squares, from 10 to 50 µm in size L, as described in [6]. Structure of the junctions and dc measurement method are shown in Fig. 1.

Fig. 1. Crossection of the hybrid heterostructure. Four point measurements of I-V curve are indicated. Four point measurement of voltage ­ current characteristics, when microwave was applied, were performed. Detector responses were measured by modulated microwave signal. A response from junction at the modulation frequency was measured by PAR 124 Lock-in amplifier with input voltage resolution 2 nV/(Hz)1/2. Magnetic field could be applied in the both directions: in parallel and perpendicular to the substrate plane. Dependencies of critical current versus magnetic field The junctions show the IC(H) dependencies, which had a pronounced central peak. The central peak was much higher, than others, but its width H was not double of other peaks widths, as known for Fraunhofer pattern. We have compared the IC(H) dependencies for Nb/Au/(Ca1-xSrx)CuO2/ YBa2Cu3O7 and Nb/Au/YBa2Cu3O7 junctions, as shown in Fig. 2.


Fig. 2. The IC(H) dependences for Nb/Au/(Ca1-xSrx)CuO2/YBa2Cu3O7 ­ solid line, and for Nb/Au/YBa2Cu3O7 junction ­ triangles. Both junctions are 50 µm in size. The structure with antiferromagnetic has CSCO thickness 50 nm, H=1 µT. Fields were applied parallel to the substrate. Field periods H of the heterostructures with 50 nm CSCO layers are 30 times smaller, compare to Nb/Au/YBa2Cu3O7 junctions. However, the periods were proportional to L-1 for the both structures. Electrical properties of the junctions at millimeter waves If the current ­ phase relation of the junction has the both first and second harmonics, two kinds of Shapiro steps: integer one at voltages V1=hf/2e and half integer at V1/2 = hf/4e appear. The dependency of integer steps versus power P is described by: i1 ( P) = 2 max ( J 1 ( x) · sin() + qJ 2 (2 x) sin( 2)) (1) Here i1 = I1(P)/IC is normalized height of the first step, J1 and J2 are the first and second order Bessel function, x=IRF/(IC ) ­ normalized microwave amplitude. Normalized frequency =2eICRN/hf, RN is normal state resistance of the junction, f ­ external frequency. is the phase difference between external signal and the self Josephson generation. The minima of i1(P) become non-zero, caused by the second harmonic. Critical current: i0 ( P ) = max ( J 0 ( x) · sin() + qJ 0 (2 x) sin(2)) (2) also have nonzero minima due to the second harmonic. The half integer step versus power:

i1 / 2 ( P) = 2 max (sin() · qJ 1 (2 x))

(3)

The formulas were derived from [4, 6] in high frequency and zero capacitance limits. Experimental data demonstrate good agreement with theory (1) ­ (3) as shown on figure 3.

Fig. 3. Shapiro steps at f= 56 GHz, IC ­ circles, I1 ­ triangles, and of half integer steps ­ rhombuses. Solid lines ­ fit by (1) and (2). Dashed line is J1 approximation. The junction has 50 nm CSCO and L=10 µm. For data shown on Fig. 3, we obtain second harmonic term q 20% for CPR, using (1) ­ (3). Note, the approach [4, 6] is more accurate for higher frequencies and low microwave currents. Detector responses of the


junctions in 90 Â 120 GHz range were measured. If the outputs from junction are 1 for integer response at V1, and 1/2 for half integer response at V1/2, the ratio of 1/2/1 4q2 in the high frequency limit.

Fig. 4. Detector response (circles) on 120 GHz signal, =1, voltage ­ current characteristic ­ solid line. The junction had L = 10 µm, CSCO was 20 nm thick. Biasing voltages for integer and half integer responses are shown by arrows. Applied microwave power to the junction was of 6 µW and the junction was in small signal regime, which shows linear dependency of response on microwave power. The ratio of 1/2/1 for the data in Fig. 4 is of 0.6 and we estimate q 0.4. Because of high ICRN product of 250 µV, more accurate estimation of the second harmonic portion requires to apply higher microwave frequencies, than 120 GHz to meet requirement >>1. In conclusion we fabricated Nb/Au/(Ca1-xSrx)CuO2/YBa2Cu3O7 superconducting heterostructures with ICRN product 100 Â 300 µV at T=4.2 K. Critical current density was 10 A/cm2 through 50 nm thick Ca1xSrxCuO2 layer, and increased for thinner layers. Critical currents of the heterostructures had much smaller periods in IC(H) dependencies compare to Josephson junctions without antiferromagnetic layer. Half ­ integer Shapiro steps and detector responses were observed for the most of heterostructures. Current phase relations of the heterostructures deviated from sinusoidal with 10 Â 40% portion of the second harmonic. Acknowledgement Support of the Russian Academy of Sciences, Russian Foundation for Basic Research, Russian Ministry of Sciences and Education, the AQDJJ and THIOX programs of ESF, Swedish KVA, SI and SSF "OXIDE" programs and NANOXIDETTC project of EU are acknowledged. References 1. 2. 2. 3. 4. 5. V. V. Ryazanov, V.A Obosnov, et al., Phys. Rev. Lett. Vol. 86, 2427 (2001). L.B. Ioffe, V.B. Geshkenbein, M. V. Feigel'man, et al., Nature London Vol. 398, 679 (1999). G.A. Ovsyannikov, I.V. Borisenko, P.V. Komissinki et al., JETP Letters Vol. 84, 262 (2006). V.K. Kornev, T.Y. Karminskaya, Y.V. Kislinskii et al., Physica C Vol. 435, 27, (2006). L.P. Gorkov, V.Z. Kresin, Physica C Vol. 367, 103 (2002). Y.V. Kislinskii, P.V. Komissinki, K.Y. Costantinian, et al., JETP Vol. 101, 494 ( 2005).