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(1 1 0)YBa2Cu3O7−δ Josephson junctions: evidence against atomic scaled “corner junctions”
Physica C 340 (2000) 269±275
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Nb/Au/(1 1 0)YBa2Cu3O7ÿd Josephson junctions: evidence against atomic scaled ``corner junctions'' Toshiyuki Usagawa *,1, Jianguo Wen, Tadashi Utagawa, Satoshi Koyama, Youichi Enomoto Superconductivity Research Laboratory, ISTEC, 10-13 Shinonome 1-chome, Koto-ku, Tokyo 135-0062, Japan Received 8 May 2000; accepted 12 June 2000
Abstract We report on I±V characteristics for in situ formed Nb/Au/(1 1 0)YBa2 Cu3 O7ÿd (YBCO) Josephson junction, where the homoepitaxial (1 1 0)YBCO ®lm shows ultra-smooth surface morphology. The ®eld dependence of critical supercurrent Ic shows anisotropic large junction behavior with normal Fraunhofer patterns expected from BCS model of dx2 ÿy 2 wave superconductors. This strongly suggests that the Nb/Au/(1 1 0)YBCO junctions cannot be regarded as atomic scaled corner junctions, in contrast with (0 0 1)/(1 1 0)YBCO grain boundary junctions to show ``p-junction'' with a pronounced dip near zero ®elds in ®eld modulation of Ic . Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: I±V characteristics; Josephson junctions; Surface morphology; Grain boundary junctions
1. Introduction A large number of works on the physical nature of YBa2 Cu3 O7ÿd (YBCO) high-Tc oxide superconductors have revealed that the principal symmetry of order parameter for Cooper pairs consists of two-dimensional d-wave (dx2 ÿy 2 ). One of the direct evidences for d-wave (dx2 ÿy 2 ) has been believed to be the ``corner junction'' experiments [1,2] based on Pb/Au/a±b corners of YBCO single crystal. It shows p-junction type Fraunhofer diraction pat-
* Corresponding author. Tel.: +81-42-323-1111; fax: +81-42327-7722. E-mail address: [email protected] (T. Usagawa). 1 Present address: Advanced Research Laboratory, Hitachi, Ltd., Higashi-Koigakubo 1-280, Kokubunnji, Tokyo 185-8601, Japan.
tern (hereafter referred as p-pattern) with a pronounced dip near zero applied ®eld in the ®eld modulation of critical supercurrent Ic . Recently, Ishimaru et al. [3] have observed ``p-pattern'' on YBCO grain boundary junctions with atomically ¯at interfaces between (0 0 1) of the a-axis oriented grain and (1 1 0) of the c-axis oriented grain. Such ®eld dependence on Ic is also observed by NdBa2 Cu3 O7ÿd grain boundary junctions [4] with zero Ic under zero magnetic ®elds. The conventional BCS model [5] with d-wave symmetry (dx2 ÿy 2 ) naturally leads to the p-pattern for the above corner junction con®guration [1,2]. However, it leads to a normal Fraunhofer diraction pattern for SD junction con®guration between s-wave superconductors and (1 1 0) oriented dx2 ÿy 2 wave superconductors [5]. If we assume a simpli®ed picture to regard the (0 0 1)YBCO plane acting as eective s-wave superconductors from
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symmetry consideration, the experiments [3,4] and calculation [5] contradict each other. A direct method to respond the puzzle is to investigate the ®eld modulation of Ic for the Josephson junction between s-wave superconductors and the (1 1 0) oriented YBCO single crystals. In this case, we need a high-quality junction structure with very ¯at surfaces of (1 1 0) oriented YBCO regarded as a single surface orientation. In fact, the (1 1 0)/(1 0 0)YBCO bicrystal junctions [6] show anomalous Fraunhofer diraction patterns. The Cooper pairs will face various interface orientations across the junction because the interface meanders and its roughness caused by facets will be much larger than the size of Cooper pairs, the coherence lengths (nab 1:5 nm, nc 0:3 nm). The signi®cance of the transport experiments through smooth (1 1 0)YBCO plane should also be emphasized because of expected zero or reduced supercurrent [5,7] due to the gapless direction of dx2 ÿy 2 wave superconductors. For the last two years, we have reported ultrasmooth surface morphology of (1 1 0) oriented YBCO ®lms with high Tc s [8] of 91.5±92.5 K grown on (1 1 0)YBCO single crystal substrates [9] by transmission electron microscopy (TEM) [8,10], re¯ection high energy electron diraction (RHEED) [8], atomic force microscopy (AFM) [8] and scanning electron microscopy (SEM) [10] observations. In the last paper [10], we have shown that the stable and ¯at (1 1 0)YBCO interface can be formed by the in situ formed Nb/Au/ (1 1 0)YBCO junction structure, where the YBCO ®lm is grown on (1 1 0)YBCO single crystal substrates. The YBCO ®lm within the junction area can be regarded as a single crystal without grain boundaries. In this paper, we have studied the transport properties of the in situ formed Nb/Au/(1 1 0)YBCO SNS junction [10].
device fabrication process were reported elsewhere [8,10]. The schematic view of the fabricated fourprobe device is described in Fig. 1. The device area is de®ned by focused ion beam isolation and Ar ion milling of Au/Nb/Au/(1 1 0)YBCO ®lms with the aid of hand-written photoresist mask. The junction area S is 3:3 10ÿ4 cm2 . The TEM image for the ex situ sputtered Au(420 nm)/in situ deposited Au(270 nm)/Nb(200 nm)/Au(28 nm)/(1 1 0)YBCO(150 nm) homoepitaxial ®lm is shown in Fig. 2(a). The best interface ¯atness between homoepitaxial YBCO ®lms and in situ deposited Au(28 nm) of N-layer has been kept
2. Experimental
Fig. 1. (a) Schematic planar view and (b) cross-sectional view of the fabricated four-probe Nb/Au/(1 1 0)YBCO SNS junction. The magnetic ®eld direction, H, is shown for the ®eld modulation experiments on Ic (Fig. 5). In the inset of the extension of Au/YBCO interface, the concept of critical size kc is given to represent the ``corner'' to show ``p-junction type Fraunhofer pattern.''
The homoepitaxial process is introduced to remove microcracks [8] caused by mechanical polishing process of single crystal substrates. The detailed conditions for YBCO ®lm growth and
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Fig. 2. (a) TEM image for the fabricated Nb/Au/(1 1 0)YBCO SNS junction. The surface reconstructed area is denoted by arrows. (b) TEM image for (0 0 1)/(1 1 0)YBCO grain boundary junctions in Ref. [11]. Possible interface atomic positions are described for discussions.
over the large area beyond the presented low magni®cation TEM image. The high magni®cation
TEM image in Fig. 2(a) shows the interface between Au and (1 1 0)YBCO is atomically ¯at. The
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Fig. 4. I±V characteristics for the fabricated four-probe Nb/Au/ (1 1 0)YBCO SNS junction at T 5:1 K with microwave radiation of f 2:00 GHz.
Fig. 3. qT curve for the fabricated four-probe Nb/Au/ (1 1 0)YBCO SNS junction. Two superconducting transition temperatures Tc1 91:7 K and Tc2 8:44 K are associated with YBCO and Nb ®lms, respectively.
detailed TEM analysis [11] of IshimaruÕs (0 0 1)/ (1 1 0)YBCO grain boundary junctions is cited in Fig. 2(b) for later discussions. The vertical four-probe transport measurements for the Nb/Au/(1 1 0)YBCO junction (Fig. 1) are carried out from the current levels of 0.1 to 10 mA. The observed resistance R was nearly the same in these current levels. One of the measured qT curve with I 1 mA is plotted in Fig. 3. As an evidence of Josephson eects for the fabricated Nb/Au/(1 1 0)YBCO SNS structure, we show typical I±V characteristics with microwave irradiation of f 2:00 GHz in Fig. 4, which gives a clear Shapiro step at hm=2e 4:2 lV. The junction resistance Rn can be estimated to be 0.65 mX from the I±V characteristics and the Ic Rn product is evaluated to be a low value of 7.8 lV. The junction resistance Rn at 5.1 K agrees quite well with the intermediate residual resistance, R, 0.65±0.80 mX, between two superconducting transition temperatures of Tc1 91:7 K and Tc2 8:44 K associated with YBCO and Nb from qT measurements (Fig. 3). The R shows small temperature dependence. The bulk resistance of Nb ®lm (29 lX cm) and Au
®lm (0.5 lX cm) at T 10 K gives negligible contribution to the R. It means that the R and Rn are originated from Au/(1 1 0)YBCO interface speci®c contact resistance qYN c R or Rn =S of 2:2±2:6 10ÿ7 X cm2 instead of the Au ®lm bulk resistance of N-layer. The ®eld dependence of critical supercurrent Ic for this sample shows an anisotropic ``large'' junction behavior with normal Fraunhofer patterns (Fig. 5). The magnetic ®eld con®gurations of the sample are schematically shown in Fig. 1(a). The magnetic ®eld was supplied by a solenoid coil, and was changed sequentially from a plus maximum ®eld to a minus maximum ®eld. The independent measurements for two dierent magnetic ®eld con®gurations show nearly the same critical current density Jc H 0 of 30 A/cm2 at T 5:1 K. The corresponding Josephson penetration depth, kJ , is estimated as 56 and 26 lm for parallel (Hjj [0 0 1]) and perpendicular (Hjj [1 1 0]) magnetic ®eld to the c-axis of homoepitaxial YBCO ®lm if we assume the corresponding ®eld penetration length as kLab 0:15 lm and kLc 1:2 lm [12]. The anisotropic large junction behavior is qualitatively understood by these parameters as listed in Table 1. As the junction length L is much larger than kJ , we introduce a speci®c quantity 2Hc0 [13] 2
2 The 2Hc0 values are estimated by the circular shaped SNS ``large'' junction model.
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3. Discussion
Fig. 5. Fraunhofer patterns for the fabricated four-probe Nb/ Au/(1 1 0)YBCO SNS junction at T 5:1 K. (Hjj [0 0 1]) parallel magnetic ®eld H to c-axis and (Hjj [ 1 1 0]) perpendicular magnetic ®eld H to c-axis as is shown in Fig. 1(a).
de®ned by Table 1 to represent an extrapolated minimum magnetic ®eld to intercept the external magnetic ®eld in Fig. 5. In the case of perpendic1 1 0]), the observed 2Hc0 , ular magnetic ®eld (Hjj [ 0.5 G, agrees well with the rough estimated value, 0.39 G, by ``large junction'' model [13]. The ®eld suppression of Ic (T) is beyond 95% of Ic H 0 as shown in Fig. 5. The large ®eld suppression rate means very small excess current for the Nb/Au/ (1 1 0)YBCO homoepitaxial ®lms junction, which represents high quality of SNS junction.
Wollman and IshimaruÕs p-pattern [1,3] is understandable if the (1 1 0) plane and the a±b planesÕ corner of YBCO can be regarded to play the same role in the ®eld dependent transport experiments. In this case, however, it sheds a serious question whether the atomically ¯at (1 1 0) plane of YBCO single crystal can be regarded or not as the assembly of atomic scaled corner junctions, i.e., the limit of in®nitely divided corners. If a single corner junction [1,2] is divided into equal two corners junction, the two corners junction will also show the p-junction type Fraunhofer pattern. If we continue to repeat equal division of a single corner junction, does the Fraunhofer pattern change to normal Fraunhofer diraction pattern or keep ppattern? Our experimental results of the normal Fraunhofer patterns instead of p-pattern strongly suggest that the ultra-smooth surface of (1 1 0)YBCO homoepitaxial ®lms cannot be regarded as atomic scaled corner junctions corresponding to in®nitely divided WollmanÕs corner junction. Our results suggest that there exists a minimum size kc to represent the corner, schematically, shown in the inset of Au/(1 1 0)YBCO interface in Fig. 1, to show the p-pattern. We think the possible origin of such a discrepancy arises from the microscopic dierences of interface structures and relevant size of Cooper pairs. Although a few layers of YBCO near the interface was structurally damaged by TEM sample preparation process, the reconstructed two layers from the bulk atomic positions, observed in the TEM image of Nb/Au/(1 1 0)YBCO, will be caused by surface reconstruction of YBCO. The atomic positions of surface two layers are shifted from the atomic lines from [1 0 0] or [0 1 0] direction
Table 1 London penetration depth of Nb and YBCO is denoted by k1 0:10 lm and k2 , where k2 is kLab 0:15 lm or kLc 1:2 lm depending on the ®eld direction to YBCO single crystala Hjjc H?c
k (lm)
L (lm)
d (lm)
kJ (lm)
L=kJ
2Hc0 (G)
0.15 1.2
170 190
0.278 1.328
56 26
3 7.4
0.85 (4) 0.39 (0.5)
a The thickness of Au ®lm as N-layer is denoted by t (28 nm). The observed 2Hc0 is shown in the parenthesis. d k1 k2 t and Hc0 U0 =pdkJ .
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(Fig. 2(a)). The possible origin of high speci®c of 2:2±2:6 10ÿ7 X cm2 contact resistance qYN c arises from the surface reconstructed layers of YBCO, which is about 10 times larger than the qc of (0 0 1)/(1 1 0)YBCO grain boundary junction [3]. On the other hand, the IshimaruÕs (0 0 1)/(1 1 0)YBCO grain boundaryÕs interfaces show periodic microstructures with minimum size kc of the corner, comparable with the coherence length of nab or nc , to look like atomic scaled corner junctions. The atomic structure of (0 0 1)/(1 1 0)YBCO grain boundary is schematically shown in Fig. 2(b) which is basically dierent from the reconstructed (1 1 0)YBCO surface (Fig. 2(a)). The atomic positions of c-axis oriented ®lm around interfaces are on the atomic lines from [1 0 0] or [0 1 0] direction (Fig. 2(b)). According to the discussion of the growth mechanism [11] of needle-like a-axis oriented YBCO ®lms on (1 0 0)MgO substrates, the facet of a-axis oriented ®lm terminates with BaO plane. As the crystal growth rate is higher than the adjacent c-axis oriented ®lm, the (0 0 1)/(1 1 0)YBCO grain boundary always keeps BaO plane [11]. Possible atomic structures of the interface could be described in Fig. 2(b). In this case, we have to explain why such microscopic periodic structures produce p-pattern. In fact, the TanakaÕs calculation [5] assumes a perfect ¯atness neglecting the real atomic structures of crystal surface. As the coherence length of nab or nc , i.e., Cooper pairÕs size of YBCO is comparable to the typical size of spatial potential variation caused by the real atomic structures of crystals, the above periodic microscopic structures at the interface will produce p-pattern for the ®eld modulation on Ic s. On the other hand, as the Cooper pairÕs size of Nb (coherence length nNb 90 nm) is much larger than the interface microscopic size of ¯at (1 1 0)YBCO surfaces, the Cooper pairs from the Nb side cannot recognize such microscopic potential variation at the interface. The minimum size kc of the corner for the for Nb/Au/(1 1 0)YBCO SNS junctions should be comparable to nNb . In the above discussions, we assume no ¯ux pinning at the SNS interfaces. As was discussed by Klemm and Wollman [14] and recognized by Harlingen (see Fig. 12 in Ref. [2]), the ¯ux pinning at the corner changes the Fraunhofer diraction
pattern from normal pattern to p-pattern and vice versa depending on the s-wave or dx2 ÿy 2 wave. Furthermore, the several self-generated delocalized ¯ux pinning are observed at the junction boundaries along 500 lm of the (1 1 0)/(1 0 0) bicrystal junctions [6] by scanning SQUID microscope. The anomalous Fraunhofer patterns have been understood by such ¯ux pinning [6]. However, the low magni®cation TEM image [11] shows that the IshimaruÕs (0 0 1)/(1 1 0)YBCO interface is very smooth over the junction area strongly different from (1 1 0)/(1 0 0) bicrystal junctions [6]. As the junction width of 5 lm is small enough, the ¯ux trapping possibility seems to be small for the IshimaruÕs (0 0 1)/(1 1 0)YBCO grain boundary junction [3]. 4. Conclusion In conclusion, the microscopic structural difference of (1 1 0)YBCO interfaces and relevant size of Cooper pairs seems to suggest the existence of minimum size kc to represent the corner to show pjunction type Fraunhofer pattern. Acknowledgements The authors would like to express their sincere thanks to Y. Ishimaru, Y. Mizuno, and Dr. S. Tanaka for their fruitful discussions. This work was partly supported by New Energy and Industrial Technology Development Organization. References [1] D.A. Wollman, D.J. Van Harlingen, J. Giapintzakis, D.M. Ginsberg, Phys. Rev. Lett. 74 (1995) 797. [2] D.J. Van Harlingen, Rev. Mod. Phys. 67 (1995) 515. [3] Y. Ishimaru, J.G. Wen, N. Koshizuka, Y. Enomoto, Phys. Rev. B55 (1997) 11851. [4] Y. Mizuno, Y. Ishimaru, J.G. Wen, Y. Enomoto, Jpn. J. Appl. Phys. 36 (1997) L764. [5] Y. Tanaka, Phys. Rev. Lett. 72 (1994) 3871. [6] J. Mannhart, H. Hilgenkamp, B. Mayer, Ch. Gerber, J.R. Kirtley, K.A. Moler, M. Sigrist, Phys. Rev. Lett. 77 (1996) 2782. [7] M. Sigrist, T.M. Rice, J. Phys. Soc. Jpn 61 (1992) 4283.
T. Usagawa et al. / Physica C 340 (2000) 269±275 [8] T. Usagawa, Y. Ishimaru, J.G. Wen, S. Koyama, Y. Enomoto, Jpn J. Appl. Phys. 36 (1997) L100. [9] Y. Namikawa, E. Egami, S. Koyama, Y. Shiohara, J. Mater. Res. 11 (1996) 804. [10] T. Usagawa, J.G. Wen, Y. Ishimaru, S. Koyama, T. Utagawa, Y. Enomoto, Appl. Phys. Lett. 72 (1998) 3202.
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[11] J.G. Wen, Y. Ishimaru, K. Hayashi, Y. Enomoto, N. Koshizuka, Mater. Sci. Engng. B41 (1996) 82. [12] J. Schutzmann, S. Tajima, S. Miyamoto, S. Tanaka, Phys. Rev. Lett. 73 (1994) 174. [13] A. Barone, G. Paterno, Physics and Applications of the Josephson Eect, Wiley, New York, 1982 (Chapter 5). [14] R.A. Klemm, Phys. Rev. Lett. 73 (1994) 1871.
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Nb/Au/(1 1 0)YBa2Cu3O7ÿd Josephson junctions: evidence against atomic scaled ``corner junctions'' Toshiyuki Usagawa *,1, Jianguo Wen, Tadashi Utagawa, Satoshi Koyama, Youichi Enomoto Superconductivity Research Laboratory, ISTEC, 10-13 Shinonome 1-chome, Koto-ku, Tokyo 135-0062, Japan Received 8 May 2000; accepted 12 June 2000
Abstract We report on I±V characteristics for in situ formed Nb/Au/(1 1 0)YBa2 Cu3 O7ÿd (YBCO) Josephson junction, where the homoepitaxial (1 1 0)YBCO ®lm shows ultra-smooth surface morphology. The ®eld dependence of critical supercurrent Ic shows anisotropic large junction behavior with normal Fraunhofer patterns expected from BCS model of dx2 ÿy 2 wave superconductors. This strongly suggests that the Nb/Au/(1 1 0)YBCO junctions cannot be regarded as atomic scaled corner junctions, in contrast with (0 0 1)/(1 1 0)YBCO grain boundary junctions to show ``p-junction'' with a pronounced dip near zero ®elds in ®eld modulation of Ic . Ó 2000 Elsevier Science B.V. All rights reserved. Keywords: I±V characteristics; Josephson junctions; Surface morphology; Grain boundary junctions
1. Introduction A large number of works on the physical nature of YBa2 Cu3 O7ÿd (YBCO) high-Tc oxide superconductors have revealed that the principal symmetry of order parameter for Cooper pairs consists of two-dimensional d-wave (dx2 ÿy 2 ). One of the direct evidences for d-wave (dx2 ÿy 2 ) has been believed to be the ``corner junction'' experiments [1,2] based on Pb/Au/a±b corners of YBCO single crystal. It shows p-junction type Fraunhofer diraction pat-
* Corresponding author. Tel.: +81-42-323-1111; fax: +81-42327-7722. E-mail address: [email protected] (T. Usagawa). 1 Present address: Advanced Research Laboratory, Hitachi, Ltd., Higashi-Koigakubo 1-280, Kokubunnji, Tokyo 185-8601, Japan.
tern (hereafter referred as p-pattern) with a pronounced dip near zero applied ®eld in the ®eld modulation of critical supercurrent Ic . Recently, Ishimaru et al. [3] have observed ``p-pattern'' on YBCO grain boundary junctions with atomically ¯at interfaces between (0 0 1) of the a-axis oriented grain and (1 1 0) of the c-axis oriented grain. Such ®eld dependence on Ic is also observed by NdBa2 Cu3 O7ÿd grain boundary junctions [4] with zero Ic under zero magnetic ®elds. The conventional BCS model [5] with d-wave symmetry (dx2 ÿy 2 ) naturally leads to the p-pattern for the above corner junction con®guration [1,2]. However, it leads to a normal Fraunhofer diraction pattern for SD junction con®guration between s-wave superconductors and (1 1 0) oriented dx2 ÿy 2 wave superconductors [5]. If we assume a simpli®ed picture to regard the (0 0 1)YBCO plane acting as eective s-wave superconductors from
0921-4534/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 0 ) 0 1 5 0 3 - 3
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symmetry consideration, the experiments [3,4] and calculation [5] contradict each other. A direct method to respond the puzzle is to investigate the ®eld modulation of Ic for the Josephson junction between s-wave superconductors and the (1 1 0) oriented YBCO single crystals. In this case, we need a high-quality junction structure with very ¯at surfaces of (1 1 0) oriented YBCO regarded as a single surface orientation. In fact, the (1 1 0)/(1 0 0)YBCO bicrystal junctions [6] show anomalous Fraunhofer diraction patterns. The Cooper pairs will face various interface orientations across the junction because the interface meanders and its roughness caused by facets will be much larger than the size of Cooper pairs, the coherence lengths (nab 1:5 nm, nc 0:3 nm). The signi®cance of the transport experiments through smooth (1 1 0)YBCO plane should also be emphasized because of expected zero or reduced supercurrent [5,7] due to the gapless direction of dx2 ÿy 2 wave superconductors. For the last two years, we have reported ultrasmooth surface morphology of (1 1 0) oriented YBCO ®lms with high Tc s [8] of 91.5±92.5 K grown on (1 1 0)YBCO single crystal substrates [9] by transmission electron microscopy (TEM) [8,10], re¯ection high energy electron diraction (RHEED) [8], atomic force microscopy (AFM) [8] and scanning electron microscopy (SEM) [10] observations. In the last paper [10], we have shown that the stable and ¯at (1 1 0)YBCO interface can be formed by the in situ formed Nb/Au/ (1 1 0)YBCO junction structure, where the YBCO ®lm is grown on (1 1 0)YBCO single crystal substrates. The YBCO ®lm within the junction area can be regarded as a single crystal without grain boundaries. In this paper, we have studied the transport properties of the in situ formed Nb/Au/(1 1 0)YBCO SNS junction [10].
device fabrication process were reported elsewhere [8,10]. The schematic view of the fabricated fourprobe device is described in Fig. 1. The device area is de®ned by focused ion beam isolation and Ar ion milling of Au/Nb/Au/(1 1 0)YBCO ®lms with the aid of hand-written photoresist mask. The junction area S is 3:3 10ÿ4 cm2 . The TEM image for the ex situ sputtered Au(420 nm)/in situ deposited Au(270 nm)/Nb(200 nm)/Au(28 nm)/(1 1 0)YBCO(150 nm) homoepitaxial ®lm is shown in Fig. 2(a). The best interface ¯atness between homoepitaxial YBCO ®lms and in situ deposited Au(28 nm) of N-layer has been kept
2. Experimental
Fig. 1. (a) Schematic planar view and (b) cross-sectional view of the fabricated four-probe Nb/Au/(1 1 0)YBCO SNS junction. The magnetic ®eld direction, H, is shown for the ®eld modulation experiments on Ic (Fig. 5). In the inset of the extension of Au/YBCO interface, the concept of critical size kc is given to represent the ``corner'' to show ``p-junction type Fraunhofer pattern.''
The homoepitaxial process is introduced to remove microcracks [8] caused by mechanical polishing process of single crystal substrates. The detailed conditions for YBCO ®lm growth and
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271
Fig. 2. (a) TEM image for the fabricated Nb/Au/(1 1 0)YBCO SNS junction. The surface reconstructed area is denoted by arrows. (b) TEM image for (0 0 1)/(1 1 0)YBCO grain boundary junctions in Ref. [11]. Possible interface atomic positions are described for discussions.
over the large area beyond the presented low magni®cation TEM image. The high magni®cation
TEM image in Fig. 2(a) shows the interface between Au and (1 1 0)YBCO is atomically ¯at. The
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Fig. 4. I±V characteristics for the fabricated four-probe Nb/Au/ (1 1 0)YBCO SNS junction at T 5:1 K with microwave radiation of f 2:00 GHz.
Fig. 3. qT curve for the fabricated four-probe Nb/Au/ (1 1 0)YBCO SNS junction. Two superconducting transition temperatures Tc1 91:7 K and Tc2 8:44 K are associated with YBCO and Nb ®lms, respectively.
detailed TEM analysis [11] of IshimaruÕs (0 0 1)/ (1 1 0)YBCO grain boundary junctions is cited in Fig. 2(b) for later discussions. The vertical four-probe transport measurements for the Nb/Au/(1 1 0)YBCO junction (Fig. 1) are carried out from the current levels of 0.1 to 10 mA. The observed resistance R was nearly the same in these current levels. One of the measured qT curve with I 1 mA is plotted in Fig. 3. As an evidence of Josephson eects for the fabricated Nb/Au/(1 1 0)YBCO SNS structure, we show typical I±V characteristics with microwave irradiation of f 2:00 GHz in Fig. 4, which gives a clear Shapiro step at hm=2e 4:2 lV. The junction resistance Rn can be estimated to be 0.65 mX from the I±V characteristics and the Ic Rn product is evaluated to be a low value of 7.8 lV. The junction resistance Rn at 5.1 K agrees quite well with the intermediate residual resistance, R, 0.65±0.80 mX, between two superconducting transition temperatures of Tc1 91:7 K and Tc2 8:44 K associated with YBCO and Nb from qT measurements (Fig. 3). The R shows small temperature dependence. The bulk resistance of Nb ®lm (29 lX cm) and Au
®lm (0.5 lX cm) at T 10 K gives negligible contribution to the R. It means that the R and Rn are originated from Au/(1 1 0)YBCO interface speci®c contact resistance qYN c R or Rn =S of 2:2±2:6 10ÿ7 X cm2 instead of the Au ®lm bulk resistance of N-layer. The ®eld dependence of critical supercurrent Ic for this sample shows an anisotropic ``large'' junction behavior with normal Fraunhofer patterns (Fig. 5). The magnetic ®eld con®gurations of the sample are schematically shown in Fig. 1(a). The magnetic ®eld was supplied by a solenoid coil, and was changed sequentially from a plus maximum ®eld to a minus maximum ®eld. The independent measurements for two dierent magnetic ®eld con®gurations show nearly the same critical current density Jc H 0 of 30 A/cm2 at T 5:1 K. The corresponding Josephson penetration depth, kJ , is estimated as 56 and 26 lm for parallel (Hjj [0 0 1]) and perpendicular (Hjj [1 1 0]) magnetic ®eld to the c-axis of homoepitaxial YBCO ®lm if we assume the corresponding ®eld penetration length as kLab 0:15 lm and kLc 1:2 lm [12]. The anisotropic large junction behavior is qualitatively understood by these parameters as listed in Table 1. As the junction length L is much larger than kJ , we introduce a speci®c quantity 2Hc0 [13] 2
2 The 2Hc0 values are estimated by the circular shaped SNS ``large'' junction model.
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3. Discussion
Fig. 5. Fraunhofer patterns for the fabricated four-probe Nb/ Au/(1 1 0)YBCO SNS junction at T 5:1 K. (Hjj [0 0 1]) parallel magnetic ®eld H to c-axis and (Hjj [ 1 1 0]) perpendicular magnetic ®eld H to c-axis as is shown in Fig. 1(a).
de®ned by Table 1 to represent an extrapolated minimum magnetic ®eld to intercept the external magnetic ®eld in Fig. 5. In the case of perpendic1 1 0]), the observed 2Hc0 , ular magnetic ®eld (Hjj [ 0.5 G, agrees well with the rough estimated value, 0.39 G, by ``large junction'' model [13]. The ®eld suppression of Ic (T) is beyond 95% of Ic H 0 as shown in Fig. 5. The large ®eld suppression rate means very small excess current for the Nb/Au/ (1 1 0)YBCO homoepitaxial ®lms junction, which represents high quality of SNS junction.
Wollman and IshimaruÕs p-pattern [1,3] is understandable if the (1 1 0) plane and the a±b planesÕ corner of YBCO can be regarded to play the same role in the ®eld dependent transport experiments. In this case, however, it sheds a serious question whether the atomically ¯at (1 1 0) plane of YBCO single crystal can be regarded or not as the assembly of atomic scaled corner junctions, i.e., the limit of in®nitely divided corners. If a single corner junction [1,2] is divided into equal two corners junction, the two corners junction will also show the p-junction type Fraunhofer pattern. If we continue to repeat equal division of a single corner junction, does the Fraunhofer pattern change to normal Fraunhofer diraction pattern or keep ppattern? Our experimental results of the normal Fraunhofer patterns instead of p-pattern strongly suggest that the ultra-smooth surface of (1 1 0)YBCO homoepitaxial ®lms cannot be regarded as atomic scaled corner junctions corresponding to in®nitely divided WollmanÕs corner junction. Our results suggest that there exists a minimum size kc to represent the corner, schematically, shown in the inset of Au/(1 1 0)YBCO interface in Fig. 1, to show the p-pattern. We think the possible origin of such a discrepancy arises from the microscopic dierences of interface structures and relevant size of Cooper pairs. Although a few layers of YBCO near the interface was structurally damaged by TEM sample preparation process, the reconstructed two layers from the bulk atomic positions, observed in the TEM image of Nb/Au/(1 1 0)YBCO, will be caused by surface reconstruction of YBCO. The atomic positions of surface two layers are shifted from the atomic lines from [1 0 0] or [0 1 0] direction
Table 1 London penetration depth of Nb and YBCO is denoted by k1 0:10 lm and k2 , where k2 is kLab 0:15 lm or kLc 1:2 lm depending on the ®eld direction to YBCO single crystala Hjjc H?c
k (lm)
L (lm)
d (lm)
kJ (lm)
L=kJ
2Hc0 (G)
0.15 1.2
170 190
0.278 1.328
56 26
3 7.4
0.85 (4) 0.39 (0.5)
a The thickness of Au ®lm as N-layer is denoted by t (28 nm). The observed 2Hc0 is shown in the parenthesis. d k1 k2 t and Hc0 U0 =pdkJ .
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(Fig. 2(a)). The possible origin of high speci®c of 2:2±2:6 10ÿ7 X cm2 contact resistance qYN c arises from the surface reconstructed layers of YBCO, which is about 10 times larger than the qc of (0 0 1)/(1 1 0)YBCO grain boundary junction [3]. On the other hand, the IshimaruÕs (0 0 1)/(1 1 0)YBCO grain boundaryÕs interfaces show periodic microstructures with minimum size kc of the corner, comparable with the coherence length of nab or nc , to look like atomic scaled corner junctions. The atomic structure of (0 0 1)/(1 1 0)YBCO grain boundary is schematically shown in Fig. 2(b) which is basically dierent from the reconstructed (1 1 0)YBCO surface (Fig. 2(a)). The atomic positions of c-axis oriented ®lm around interfaces are on the atomic lines from [1 0 0] or [0 1 0] direction (Fig. 2(b)). According to the discussion of the growth mechanism [11] of needle-like a-axis oriented YBCO ®lms on (1 0 0)MgO substrates, the facet of a-axis oriented ®lm terminates with BaO plane. As the crystal growth rate is higher than the adjacent c-axis oriented ®lm, the (0 0 1)/(1 1 0)YBCO grain boundary always keeps BaO plane [11]. Possible atomic structures of the interface could be described in Fig. 2(b). In this case, we have to explain why such microscopic periodic structures produce p-pattern. In fact, the TanakaÕs calculation [5] assumes a perfect ¯atness neglecting the real atomic structures of crystal surface. As the coherence length of nab or nc , i.e., Cooper pairÕs size of YBCO is comparable to the typical size of spatial potential variation caused by the real atomic structures of crystals, the above periodic microscopic structures at the interface will produce p-pattern for the ®eld modulation on Ic s. On the other hand, as the Cooper pairÕs size of Nb (coherence length nNb 90 nm) is much larger than the interface microscopic size of ¯at (1 1 0)YBCO surfaces, the Cooper pairs from the Nb side cannot recognize such microscopic potential variation at the interface. The minimum size kc of the corner for the for Nb/Au/(1 1 0)YBCO SNS junctions should be comparable to nNb . In the above discussions, we assume no ¯ux pinning at the SNS interfaces. As was discussed by Klemm and Wollman [14] and recognized by Harlingen (see Fig. 12 in Ref. [2]), the ¯ux pinning at the corner changes the Fraunhofer diraction
pattern from normal pattern to p-pattern and vice versa depending on the s-wave or dx2 ÿy 2 wave. Furthermore, the several self-generated delocalized ¯ux pinning are observed at the junction boundaries along 500 lm of the (1 1 0)/(1 0 0) bicrystal junctions [6] by scanning SQUID microscope. The anomalous Fraunhofer patterns have been understood by such ¯ux pinning [6]. However, the low magni®cation TEM image [11] shows that the IshimaruÕs (0 0 1)/(1 1 0)YBCO interface is very smooth over the junction area strongly different from (1 1 0)/(1 0 0) bicrystal junctions [6]. As the junction width of 5 lm is small enough, the ¯ux trapping possibility seems to be small for the IshimaruÕs (0 0 1)/(1 1 0)YBCO grain boundary junction [3]. 4. Conclusion In conclusion, the microscopic structural difference of (1 1 0)YBCO interfaces and relevant size of Cooper pairs seems to suggest the existence of minimum size kc to represent the corner to show pjunction type Fraunhofer pattern. Acknowledgements The authors would like to express their sincere thanks to Y. Ishimaru, Y. Mizuno, and Dr. S. Tanaka for their fruitful discussions. This work was partly supported by New Energy and Industrial Technology Development Organization. References [1] D.A. Wollman, D.J. Van Harlingen, J. Giapintzakis, D.M. Ginsberg, Phys. Rev. Lett. 74 (1995) 797. [2] D.J. Van Harlingen, Rev. Mod. Phys. 67 (1995) 515. [3] Y. Ishimaru, J.G. Wen, N. Koshizuka, Y. Enomoto, Phys. Rev. B55 (1997) 11851. [4] Y. Mizuno, Y. Ishimaru, J.G. Wen, Y. Enomoto, Jpn. J. Appl. Phys. 36 (1997) L764. [5] Y. Tanaka, Phys. Rev. Lett. 72 (1994) 3871. [6] J. Mannhart, H. Hilgenkamp, B. Mayer, Ch. Gerber, J.R. Kirtley, K.A. Moler, M. Sigrist, Phys. Rev. Lett. 77 (1996) 2782. [7] M. Sigrist, T.M. Rice, J. Phys. Soc. Jpn 61 (1992) 4283.
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