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Flux pinning effects of twin boundaries studied with unidirectionally twinned YBCO films
Physica C 372–376 (2002) 1885–1889 www.elsevier.com/locate/physc
Flux pinning effects of twin boundaries studied with unidirectionally twinned YBCO films H. Yamasaki *, Y. Nakagawa, A. Sawa, H. Obara, K. Develos National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
Abstract We have investigated the flux pinning effects of the twin boundary (TB) of YBCO, using TB-aligned YBCO films on YAlO3 and NdGaO3 (0 0 1) substrates. We observed a large anisotropy in critical current densities Jc and normal-state resistivities, when four-probe transport measurements were performed with currents in different directions. To our surprise, along TBs Jc was lower and the normal-state resistivity was higher. We observed microcracks perpendicular to the TBs, and found that the observed large anisotropy in transport properties is due to microcracks. It is strongly suggested that the previously reported anisotropic transport properties in unidirectionally twinned YBCO films [Phys. Rev. Lett. 77 (1996) 3913] are caused by microcracks and/or related planar defects, not by TBs. We have observed homogeneous Jc in YBCO films without apparent microcracks whose TBs are aligned but not perfectly. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Twin boundary; Flux pinning; Anisotropy in critical current density; Microcrack
1. Introduction So far there have been many studies on the flux pinning mechanisms of YBa2 Cu3 O7d (YBCO) thin films [1–9]. Hylton and Beasley proposed, based on a simple model, that the pinning is due to a large density of point defects [1]. However, Feenstra et al. investigated the effect of oxygen deficiency and concluded that chain-site oxygen vacancies are not strong pinning centers in high-Jc YBCO films [2]. Besides point defects, there are several extended defects in YBCO films such as screw dislocations, edge dislocations, artificially
*
Corresponding author. Fax: +81-298-61-5264. E-mail address: [email protected] (H. Yamasaki).
introduced columnar defects, twin boundaries, etc. It was reported that there is poor correlation between Jc and screw dislocation density [3,4]. In contrast, Dıas et al. gave clear evidence for vortex pinning by edge dislocations in low-angle grain boundaries [5]. Strong flux pinning by artificially introduced columnar defects was reported in YBCO films on miscut LaAlO3 substrates [6]. Twin boundaries (TBs) are present in any orthorhombic YBCO film. In an early paper by Lairson et al. it was reported that there is no correlation between the TB microstructure and the flux pinning strength in YBCO thin films [7]. Recently, however, Safar et al. observed anisotropic Jc with different current directions in c-axis-oriented YBCO thick films in high magnetic fields (B P 5 T, parallel to the c-axis) [8,9]. The Jc was lower with
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 0 0 1 - 8
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H. Yamasaki et al. / Physica C 372–376 (2002) 1885–1889
currents along and perpendicular to the TBs than that with currents 45° to TBs, and they explained this phenomenon by the channel flow flux motion along TBs [8,9]. Thus, it is not yet clear if TBs work as strong flux pinning centers in YBCO films or not. In this study we have investigated the flux pinning effects of twin boundaries in YBCO, by using TB-aligned YBCO films on YAlO3 (YAO) and NdGaO3 (NGO) substrates. We observed a large anisotropy in Jc with different current directions, parallel, perpendicular and 45° to TBs. However, we found that this large anisotropy is not due to TBs, but due to microcracks and/or related planar defects formed perpendicular to TBs.
2. Preparation of twin-boundary-aligned YBCO films We prepared YBCO films on YAO and NGO (0 0 1) substrates, using a pulsed laser deposition method. Orthorhombic YAO (a ¼ 0:5179 nm, b ¼ 0:5329 nm) and NGO (a ¼ 0:5428 nm, b ¼ 0:5493 nm) have a- and b- lattice constants that are close to the length of the diagonal of YBCO in the 1=2 1=2 a–b plane: ða2 þ b2 Þ ¼ ð0:38212 þ 0:38852 Þ ¼ 0:5449 nm. It was reported that the twinning directions of YBCO align along the b-direction of YAO [10]. We confirmed this result and found that TBs of YBCO also align along the b-direction of NGO, which is contrary to the observation by Villard et al. [11]. In this paper we present the results for four typical specimens, Samples A, B, C and D, which are c-axis-oriented YBCO films on YAO (A–C) and on NGO (D). Sample A (thickness t 250 nm) was prepared with a usual method: a YBCO film was deposited in 0.3 Torr O2 and cooled to room temperature in 100 Torr O2 to form the orthorhombic phase. Samples B ðt 250 nm), C ðt 80 nm) and D ðt 80 nm) were prepared with a tetra-ortho conversion method. YBCO films deposited in 0.3 Torr O2 are rapidly cooled to 600 °C, and then evacuated and cooled to room temperature. We annealed the obtained tetragonal films in 1 atm O2 at 450 °C to convert them to the orthorhombic phase. The deposition temperatures (temperature of the substrate heater) were 670–690 °C.
Fig. 1. (a) A /-scan on the 113 reflections of YBCO deposited on YAO (Sample B). Note a single peak around / 60° and double peaks around / 150°. (b) A large central peak observed between the double peaks in Sample A (114 reflection). (c) A small central peak observed in Sample C (113 reflection). (d) A /-scan on the 112 reflection of YBCO deposited on NGO (Sample D), showing imperfect double-peak separation.
The /-scan on the 113 reflections of YBCO in Sample B (Fig. 1a) shows that the TBs of YBCO align along one direction. In the configuration around / 150° the plane composed of the incident and diffracted X-ray beams is parallel to the a-axis of the YAO substrate. The double peaks of the YBCO 113 reflection mean that TBs align perpendicular to this direction [10]. The TB forms the diagonal of the a–b plane of adjacent YBCO domains, and the [1 1 3] planes of adjacent YBCO that diffract X-ray are not exactly parallel but form a small angle, D/ ¼ 4f45 arctanða=bÞg 2° [12]. On the contrary, around / 60° the [1 1 3] planes of adjacent YBCO domains that diffract Xray are exactly parallel to each other, and a single X-ray diffraction peak is observed. For Sample B the double-peak separation was perfect, and this means the perfect orientation of TBs. However, for other samples there was a central peak besides the double peaks (Samples A and C, Fig. 1b and c, respectively), or the double peak separation was not perfect (Sample D, Fig. 1d). This means that the TB alignment of these samples is poorer than that of Sample B. Fig. 2a and b shows plan-view transmission electron micrographs for Samples B and C, re-
H. Yamasaki et al. / Physica C 372–376 (2002) 1885–1889
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spectively. The horizontal direction of the micrographs is parallel to the b-axis of YAO substrate. We can clearly see that TBs of YBCO perfectly align along the b-axis of YAO for Sample B. The microcracks running perpendicular to TBs may be formed by different thermal expansion coefficients in YBCO and YAO. Such microcracks have been often observed with a scanning electron microscope (SEM) in YBCO/YAO and YBCO/NGO films along the a-axis of YAO and NGO. The micrograph for Sample C (Fig. 2b) shows that TBs align along the b-axis of YAO considerably, but not at all perfect, which is consistent with the X-ray diffraction result (Fig. 1c). 3. Transport properties
Fig. 2. Plan-view transmission electron micrographs for (a) Sample B, and (b) Sample C.
We made four-probe bridges (20 lm 2 mm) with different current directions in the YBCO films, and transport properties were measured. Fig. 3 shows the temperature dependence of resistivities for different directions in Sample B. Along the microcracks (perpendicular to TBs) we observe the R–T curve that is typical for highquality YBCO thin films. However, we observed
Fig. 3. Temperature dependence of resistivities measured for six bridges of different directions in Sample B, unidirectionally-twinned specimen with microcracks running perpendicular to TBs (Fig. 2a). Two bridges are along microcracks, two are perpendicular to them, and the last two are 45° to them. Inset shows the R–T data in Sample A, poorly-TB-aligned specimen with microcracks.
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three orders of magnitude higher resistivities and semiconductor-like R–T curves with currents perpendicular or 45° to microcracks. This means that microcracks are extending almost the full area of the cross-section of the bridge. Actually we observed many microcracks longer than the bridge width (20 lm) by SEM after the transport measurements. We speculate that the cross-section of the microcracks has the form of a wedge and that there is a strained, semiconducting YBCO layer between the microcrack and the substrate. The inset of Fig. 3 shows the R–T curves in Sample A prepared with a usual method. In this specimen we observed short microcracks (<10 lm) along the a-axis of YAO with SEM. Perpendicular to the microcrack direction the room temperature resistivity qRT was about twice as high as that along the microcracks. For Sample C in which we have not observed any microcracks with SEM, we observed homogeneous resistivities ( 5%) for five bridges with different directions, qRT ¼ 1:8–2:0 lX m. We have measured the critical current density Jc by using a pulse current (pulse width ¼ 25 ms for I > 300 mA and 100 ms for I 6 300 mA) to avoid the heating effect at the current terminals. Fig. 4a shows the Jc –B curves measured in Sample A. A large anisotropy in Jc with different current directions was observed, and the anisotropy was larger at higher temperatures and magnetic-field dependent: 10 at 20 K and 20 at 60 K and 4 T. The Jc was higher along microcracks (perpendicular to the direction of TB alignment), and this large anisotropy is definitely not due to TBs. Since the anisotropy in Jc was much larger than that of normal-state resistivities ( 2, inset of Fig. 3), and was temperature and field dependent, this is not caused by a simple reduction of the area of current paths due to microcracks. It is not reasonable to think that the current can flow across the microcracks. We think that there are planar defects parallel to the microcracks whose superconducting order parameter is less than that of the matrix, most probably strained YBCO planes adjacent to the microcracks. Then the flux lines will flow easily along the planar defects with transport currents across them. If there are planar defects adjacent to microcracks, this channeling effect is signifi-
Fig. 4. Critical current densities Jc measured for four-probe bridges of different directions in (a) Sample A, and (b) Samples C and D. The critical current was determined by a criterion of 10 lV/cm.
cant due to the enhanced current flow detouring around the microcracks. Another possibility is the enhanced electric field near the planar obstacles in transport current flow, reported recently by Gurevich and Friesen [13]. The local electric-field enhancement is much larger in superconductors than in normal metals, and this might explain our experimental results. Anisotropic transport prop-
H. Yamasaki et al. / Physica C 372–376 (2002) 1885–1889
erties similar to those of Sample A were reported in YBCO/NGO films, and the channeling effect by TBs was suggested by Villard et al. [11]. However, here we have definitely demonstrated that the observed Jc anisotropy cannot be due to TBs, and it is probably due to the planar defects parallel to the microcracks. As for the microcrack-free Sample C with TBs partly aligned (Figs. 1c and 2b), we observed highly homogeneous Jc for three bridges with different directions at 77.3 K (Fig. 4b). Although the J?TB bridge was broken with an overcurrent after the measurement at 77.3 K, we observed similar Jc for the J kTB and 45° bridges also at 60 and 20 K. For Sample D with better TB alignment we observed similar Jc s for the J kTB and J?TB bridges, but Jc for the 45° bridge was somewhat higher (Fig. 4b). Since we observed an increase of resistivities almost probably due to the formation of microcracks after thermal cycles for the J kTB (J?microcracks) and 45° bridges in this sample, we cannot tell that this tendency in Jc is reproducible or not. Up to present we have not obtained the experimental results supporting that TBs work as strong flux pinning centers in high-Jc YBCO films. Acknowledgements This work has been carried out as a part of the Super-ACE project (R&D of fundamental tech-
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nologies for superconducting AC power equipment) of the Ministry of Economy, Trade and Industry (METI). References [1] T.L. Hylton, M.R. Beasley, Phys. Rev. B 41 (1990) 11669. [2] R. Feenstra, D.K. Christen, C.E. Klabunde, J.D. Budai, Phys. Rev. B 45 (1992) 7555. [3] J. Mannhart, D. Anselmetti, J.G. Bednorz, A. Catana, C. Gerber, K.A. Muller, D.G. Schlom, Z. Phys. B, Condensed Matter 86 (1992) 177. [4] B. Dam, J.M. Huijbregtse, F.C. Klaassen, R.C.F. van der Geest, G. Doornbos, J.H. Rector, A.M. Testa, S. Freisem, J.C. Martinez, B. St€auble-P€ umpin, R. Griessen, Nature 399 (1999) 439. [5] A. Dıas az, L. Mechin, P. Berghuis, J.E. Evetts, Phys. Rev. Lett. 80 (1998) 3855. [6] D.H. Lowndes, D.K. Christen, C.E. Klabunde, Z.L. Wang, D.M. Kroeger, J.D. Budai, S. Zhu, D.P. Norton, Phys. Rev. Lett. 74 (1995) 2355. [7] B.M. Lairson, S.K. Streiffer, J.C. Bravman, Phys. Rev. B 42 (1990) 10067. [8] H. Safar, S. Foltyn, H. Kung, M.P. Maley, J.O. Willis, P. Arendt, X.D. Wu, Appl. Phys. Lett. 68 (1996) 1853. [9] H. Safar, S.R. Foltyn, Q.X. Jia, M.P. Maley, Phil. Mag. B 74 (1996) 647. [10] C. Dubourdieu, J.P. Senateur, O. Thomas, F. Weiss, B.P. Thrane, M. Brunel, Appl. Phys. Lett. 69 (1996) 1942. [11] C. Villard, G. Koren, D. Cohen, E. Polturak, B. Thrane, D. Chateignier, Phys. Rev. Lett. 77 (1996) 3913. [12] Y. Zhu, M. Suenaga, Y. Xu, R.L. Sabatini, A.R. Moodenbaugh, Appl. Phys. Lett. 54 (1989) 374. [13] A. Gurevich, M. Friesen, Phys. Rev. B 62 (2000) 4004.
Flux pinning effects of twin boundaries studied with unidirectionally twinned YBCO films H. Yamasaki *, Y. Nakagawa, A. Sawa, H. Obara, K. Develos National Institute of Advanced Industrial Science and Technology, AIST Tsukuba Central 2, 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
Abstract We have investigated the flux pinning effects of the twin boundary (TB) of YBCO, using TB-aligned YBCO films on YAlO3 and NdGaO3 (0 0 1) substrates. We observed a large anisotropy in critical current densities Jc and normal-state resistivities, when four-probe transport measurements were performed with currents in different directions. To our surprise, along TBs Jc was lower and the normal-state resistivity was higher. We observed microcracks perpendicular to the TBs, and found that the observed large anisotropy in transport properties is due to microcracks. It is strongly suggested that the previously reported anisotropic transport properties in unidirectionally twinned YBCO films [Phys. Rev. Lett. 77 (1996) 3913] are caused by microcracks and/or related planar defects, not by TBs. We have observed homogeneous Jc in YBCO films without apparent microcracks whose TBs are aligned but not perfectly. Ó 2002 Elsevier Science B.V. All rights reserved. Keywords: Twin boundary; Flux pinning; Anisotropy in critical current density; Microcrack
1. Introduction So far there have been many studies on the flux pinning mechanisms of YBa2 Cu3 O7d (YBCO) thin films [1–9]. Hylton and Beasley proposed, based on a simple model, that the pinning is due to a large density of point defects [1]. However, Feenstra et al. investigated the effect of oxygen deficiency and concluded that chain-site oxygen vacancies are not strong pinning centers in high-Jc YBCO films [2]. Besides point defects, there are several extended defects in YBCO films such as screw dislocations, edge dislocations, artificially
*
Corresponding author. Fax: +81-298-61-5264. E-mail address: [email protected] (H. Yamasaki).
introduced columnar defects, twin boundaries, etc. It was reported that there is poor correlation between Jc and screw dislocation density [3,4]. In contrast, Dıas et al. gave clear evidence for vortex pinning by edge dislocations in low-angle grain boundaries [5]. Strong flux pinning by artificially introduced columnar defects was reported in YBCO films on miscut LaAlO3 substrates [6]. Twin boundaries (TBs) are present in any orthorhombic YBCO film. In an early paper by Lairson et al. it was reported that there is no correlation between the TB microstructure and the flux pinning strength in YBCO thin films [7]. Recently, however, Safar et al. observed anisotropic Jc with different current directions in c-axis-oriented YBCO thick films in high magnetic fields (B P 5 T, parallel to the c-axis) [8,9]. The Jc was lower with
0921-4534/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 2 ) 0 1 0 0 1 - 8
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H. Yamasaki et al. / Physica C 372–376 (2002) 1885–1889
currents along and perpendicular to the TBs than that with currents 45° to TBs, and they explained this phenomenon by the channel flow flux motion along TBs [8,9]. Thus, it is not yet clear if TBs work as strong flux pinning centers in YBCO films or not. In this study we have investigated the flux pinning effects of twin boundaries in YBCO, by using TB-aligned YBCO films on YAlO3 (YAO) and NdGaO3 (NGO) substrates. We observed a large anisotropy in Jc with different current directions, parallel, perpendicular and 45° to TBs. However, we found that this large anisotropy is not due to TBs, but due to microcracks and/or related planar defects formed perpendicular to TBs.
2. Preparation of twin-boundary-aligned YBCO films We prepared YBCO films on YAO and NGO (0 0 1) substrates, using a pulsed laser deposition method. Orthorhombic YAO (a ¼ 0:5179 nm, b ¼ 0:5329 nm) and NGO (a ¼ 0:5428 nm, b ¼ 0:5493 nm) have a- and b- lattice constants that are close to the length of the diagonal of YBCO in the 1=2 1=2 a–b plane: ða2 þ b2 Þ ¼ ð0:38212 þ 0:38852 Þ ¼ 0:5449 nm. It was reported that the twinning directions of YBCO align along the b-direction of YAO [10]. We confirmed this result and found that TBs of YBCO also align along the b-direction of NGO, which is contrary to the observation by Villard et al. [11]. In this paper we present the results for four typical specimens, Samples A, B, C and D, which are c-axis-oriented YBCO films on YAO (A–C) and on NGO (D). Sample A (thickness t 250 nm) was prepared with a usual method: a YBCO film was deposited in 0.3 Torr O2 and cooled to room temperature in 100 Torr O2 to form the orthorhombic phase. Samples B ðt 250 nm), C ðt 80 nm) and D ðt 80 nm) were prepared with a tetra-ortho conversion method. YBCO films deposited in 0.3 Torr O2 are rapidly cooled to 600 °C, and then evacuated and cooled to room temperature. We annealed the obtained tetragonal films in 1 atm O2 at 450 °C to convert them to the orthorhombic phase. The deposition temperatures (temperature of the substrate heater) were 670–690 °C.
Fig. 1. (a) A /-scan on the 113 reflections of YBCO deposited on YAO (Sample B). Note a single peak around / 60° and double peaks around / 150°. (b) A large central peak observed between the double peaks in Sample A (114 reflection). (c) A small central peak observed in Sample C (113 reflection). (d) A /-scan on the 112 reflection of YBCO deposited on NGO (Sample D), showing imperfect double-peak separation.
The /-scan on the 113 reflections of YBCO in Sample B (Fig. 1a) shows that the TBs of YBCO align along one direction. In the configuration around / 150° the plane composed of the incident and diffracted X-ray beams is parallel to the a-axis of the YAO substrate. The double peaks of the YBCO 113 reflection mean that TBs align perpendicular to this direction [10]. The TB forms the diagonal of the a–b plane of adjacent YBCO domains, and the [1 1 3] planes of adjacent YBCO that diffract X-ray are not exactly parallel but form a small angle, D/ ¼ 4f45 arctanða=bÞg 2° [12]. On the contrary, around / 60° the [1 1 3] planes of adjacent YBCO domains that diffract Xray are exactly parallel to each other, and a single X-ray diffraction peak is observed. For Sample B the double-peak separation was perfect, and this means the perfect orientation of TBs. However, for other samples there was a central peak besides the double peaks (Samples A and C, Fig. 1b and c, respectively), or the double peak separation was not perfect (Sample D, Fig. 1d). This means that the TB alignment of these samples is poorer than that of Sample B. Fig. 2a and b shows plan-view transmission electron micrographs for Samples B and C, re-
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spectively. The horizontal direction of the micrographs is parallel to the b-axis of YAO substrate. We can clearly see that TBs of YBCO perfectly align along the b-axis of YAO for Sample B. The microcracks running perpendicular to TBs may be formed by different thermal expansion coefficients in YBCO and YAO. Such microcracks have been often observed with a scanning electron microscope (SEM) in YBCO/YAO and YBCO/NGO films along the a-axis of YAO and NGO. The micrograph for Sample C (Fig. 2b) shows that TBs align along the b-axis of YAO considerably, but not at all perfect, which is consistent with the X-ray diffraction result (Fig. 1c). 3. Transport properties
Fig. 2. Plan-view transmission electron micrographs for (a) Sample B, and (b) Sample C.
We made four-probe bridges (20 lm 2 mm) with different current directions in the YBCO films, and transport properties were measured. Fig. 3 shows the temperature dependence of resistivities for different directions in Sample B. Along the microcracks (perpendicular to TBs) we observe the R–T curve that is typical for highquality YBCO thin films. However, we observed
Fig. 3. Temperature dependence of resistivities measured for six bridges of different directions in Sample B, unidirectionally-twinned specimen with microcracks running perpendicular to TBs (Fig. 2a). Two bridges are along microcracks, two are perpendicular to them, and the last two are 45° to them. Inset shows the R–T data in Sample A, poorly-TB-aligned specimen with microcracks.
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three orders of magnitude higher resistivities and semiconductor-like R–T curves with currents perpendicular or 45° to microcracks. This means that microcracks are extending almost the full area of the cross-section of the bridge. Actually we observed many microcracks longer than the bridge width (20 lm) by SEM after the transport measurements. We speculate that the cross-section of the microcracks has the form of a wedge and that there is a strained, semiconducting YBCO layer between the microcrack and the substrate. The inset of Fig. 3 shows the R–T curves in Sample A prepared with a usual method. In this specimen we observed short microcracks (<10 lm) along the a-axis of YAO with SEM. Perpendicular to the microcrack direction the room temperature resistivity qRT was about twice as high as that along the microcracks. For Sample C in which we have not observed any microcracks with SEM, we observed homogeneous resistivities ( 5%) for five bridges with different directions, qRT ¼ 1:8–2:0 lX m. We have measured the critical current density Jc by using a pulse current (pulse width ¼ 25 ms for I > 300 mA and 100 ms for I 6 300 mA) to avoid the heating effect at the current terminals. Fig. 4a shows the Jc –B curves measured in Sample A. A large anisotropy in Jc with different current directions was observed, and the anisotropy was larger at higher temperatures and magnetic-field dependent: 10 at 20 K and 20 at 60 K and 4 T. The Jc was higher along microcracks (perpendicular to the direction of TB alignment), and this large anisotropy is definitely not due to TBs. Since the anisotropy in Jc was much larger than that of normal-state resistivities ( 2, inset of Fig. 3), and was temperature and field dependent, this is not caused by a simple reduction of the area of current paths due to microcracks. It is not reasonable to think that the current can flow across the microcracks. We think that there are planar defects parallel to the microcracks whose superconducting order parameter is less than that of the matrix, most probably strained YBCO planes adjacent to the microcracks. Then the flux lines will flow easily along the planar defects with transport currents across them. If there are planar defects adjacent to microcracks, this channeling effect is signifi-
Fig. 4. Critical current densities Jc measured for four-probe bridges of different directions in (a) Sample A, and (b) Samples C and D. The critical current was determined by a criterion of 10 lV/cm.
cant due to the enhanced current flow detouring around the microcracks. Another possibility is the enhanced electric field near the planar obstacles in transport current flow, reported recently by Gurevich and Friesen [13]. The local electric-field enhancement is much larger in superconductors than in normal metals, and this might explain our experimental results. Anisotropic transport prop-
H. Yamasaki et al. / Physica C 372–376 (2002) 1885–1889
erties similar to those of Sample A were reported in YBCO/NGO films, and the channeling effect by TBs was suggested by Villard et al. [11]. However, here we have definitely demonstrated that the observed Jc anisotropy cannot be due to TBs, and it is probably due to the planar defects parallel to the microcracks. As for the microcrack-free Sample C with TBs partly aligned (Figs. 1c and 2b), we observed highly homogeneous Jc for three bridges with different directions at 77.3 K (Fig. 4b). Although the J?TB bridge was broken with an overcurrent after the measurement at 77.3 K, we observed similar Jc for the J kTB and 45° bridges also at 60 and 20 K. For Sample D with better TB alignment we observed similar Jc s for the J kTB and J?TB bridges, but Jc for the 45° bridge was somewhat higher (Fig. 4b). Since we observed an increase of resistivities almost probably due to the formation of microcracks after thermal cycles for the J kTB (J?microcracks) and 45° bridges in this sample, we cannot tell that this tendency in Jc is reproducible or not. Up to present we have not obtained the experimental results supporting that TBs work as strong flux pinning centers in high-Jc YBCO films. Acknowledgements This work has been carried out as a part of the Super-ACE project (R&D of fundamental tech-
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nologies for superconducting AC power equipment) of the Ministry of Economy, Trade and Industry (METI). References [1] T.L. Hylton, M.R. Beasley, Phys. Rev. B 41 (1990) 11669. [2] R. Feenstra, D.K. Christen, C.E. Klabunde, J.D. Budai, Phys. Rev. B 45 (1992) 7555. [3] J. Mannhart, D. Anselmetti, J.G. Bednorz, A. Catana, C. Gerber, K.A. Muller, D.G. Schlom, Z. Phys. B, Condensed Matter 86 (1992) 177. [4] B. Dam, J.M. Huijbregtse, F.C. Klaassen, R.C.F. van der Geest, G. Doornbos, J.H. Rector, A.M. Testa, S. Freisem, J.C. Martinez, B. St€auble-P€ umpin, R. Griessen, Nature 399 (1999) 439. [5] A. Dıas az, L. Mechin, P. Berghuis, J.E. Evetts, Phys. Rev. Lett. 80 (1998) 3855. [6] D.H. Lowndes, D.K. Christen, C.E. Klabunde, Z.L. Wang, D.M. Kroeger, J.D. Budai, S. Zhu, D.P. Norton, Phys. Rev. Lett. 74 (1995) 2355. [7] B.M. Lairson, S.K. Streiffer, J.C. Bravman, Phys. Rev. B 42 (1990) 10067. [8] H. Safar, S. Foltyn, H. Kung, M.P. Maley, J.O. Willis, P. Arendt, X.D. Wu, Appl. Phys. Lett. 68 (1996) 1853. [9] H. Safar, S.R. Foltyn, Q.X. Jia, M.P. Maley, Phil. Mag. B 74 (1996) 647. [10] C. Dubourdieu, J.P. Senateur, O. Thomas, F. Weiss, B.P. Thrane, M. Brunel, Appl. Phys. Lett. 69 (1996) 1942. [11] C. Villard, G. Koren, D. Cohen, E. Polturak, B. Thrane, D. Chateignier, Phys. Rev. Lett. 77 (1996) 3913. [12] Y. Zhu, M. Suenaga, Y. Xu, R.L. Sabatini, A.R. Moodenbaugh, Appl. Phys. Lett. 54 (1989) 374. [13] A. Gurevich, M. Friesen, Phys. Rev. B 62 (2000) 4004.