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Long Gd-123 coated conductor by PLD method
Physica C 468 (2008) 1510–1513
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Long Gd-123 coated conductor by PLD method H. Fuji *, M. Igarashi, Y. Hanada, T. Miura, S. Hanyu, K. Kakimoto, Y. Iijima, T. Saitoh Fujikura Ltd., 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan
a r t i c l e
i n f o
Article history: Available online 27 June 2008 PACS: 74.72.Bk 75.47. m 85.25.Kx Keywords: IBAD PLD Y-123 Gd-123
a b s t r a c t We have developed long Gd-123 coated conductors by ion-beam-assisted deposition (IBAD) and pulsedlaser-deposition (PLD) method. Recently, large-scale reel-to-reel apparatus with the 110 cm 15 cm assisting ion source was introduced to IBAD system. It was enable to produce 500 m-class IBAD–Gd2Zr2O7 (GZO) tapes with D/ of below 15° and high throughputs of 3 m/h. Furthermore, apparatus with multilane and laser scanning was introduced to PLD system. As a result, end to end Ic of 318 A were obtained for a 201.5 m long tape, and Ic L values were 64,077 Am. Furthermore, 500 m-class deposition was carried out by improving PLD conditions. As a result, Ic L values of 112,166 Am was obtained and it’s a world record on August 2007. In the short samples, Ic of over 500 A was obtained with Gd-123 thickness of 2.0 lm and over 100 A was obtained in magnetic field of 3 T, perpendicular to c-axis. Ó 2008 Published by Elsevier B.V.
1. Introduction The developments of YBCO coated conductors have been performed all over the world. Recently, over 100 m long YBCO coated conductors with the Ic of over 100 A were prepared in several research organizations. For the length over 100 m, a large area apparatus of IBAD was successfully developed with long lifetime operation even in oxygen atmosphere [1,2]. The PLD method has been used as a deposition technique of Y-123 films. It is characterized with stable stoichiometry, easy oxygen pressure control, very high growth rate, etc. The combination of IBAD and PLD is one of the most promising method to fabricate long Y-123 coated conductors. In this paper, we first describe large-scale apparatus for IBAD and PLD systems for coated conductors so as to get higher process throughput for long length conductors over 500 m. Next, we show characterization of high-performance 200 m and 500 m class Gd-123 conductors by combination of IBAD and PLD method. Furthermore, in order to supply the coated conductors to electrical applications, technique of stabilizing and insulation to conductor are developed. Finally, we describe the high Ic vales of thicker films and Ic characteristics in magnetic field. 2. Experiments Gd2Zr2O7 (GZO) films for a 200 m long tape were deposited on roll-milled Hastelloy C276 with 10 mm in width and the 100 lm * Corresponding author. Tel.: +81 3 5606 1064; fax: +81 3 5606 1512. E-mail address: [email protected] (H. Fuji). 0921-4534/$ - see front matter Ó 2008 Published by Elsevier B.V. doi:10.1016/j.physc.2008.05.284
in thickness by reel-to-reel dual-ion-beam sputtering system with two sets of radio frequency (RF) discharged 66 cm 6 cm squareshaped ion sources. Continuous growth of GZO films was performed with the production speed of 0.5–1.0 m/h under the conventional condition [2]. Large-scale reel-to-reel IBAD system to prepare an over 500 m long was constructed as shown in Fig. 1. It has a specially designed 110 cm 15 cm assisting ion source. It has sophisticated ion-extracting grids by which quite low divergent and low energy Ar+ ion beam can be generated. Longitudinal ion current distribution was measured by an array of Faraday cups. A large sintered GZO target was sputtered by sputtering Ar+ ion beam. Film deposition area is 100 cm 25 cm, which is 4–5 times larger than the former system. CeO2 secondary buffer layers and YBCO films were deposited by a reel-to-reel PLD system with Kr-F (k = 248 nm) excimer laser. The large scale PLD system was equipped with the 180 W laser, which has three times higher power in comparison with the former PLD system was shown Fig. 2 [3,4]. This PLD system has the multi-lane deposition in order to improve the production speed and the yield of targets. Superconducting layers were deposited by PLD system (Fig. 2) on CeO2/GZO/Hastelloy taps. It enhances the manufacturing speed of the wires by practically increasing the deposition area in which the tape is passed through multiple plumes created by scanning the laser beam. Also, recently, since the critical current conductivity in a magnetic field is excellent, we are developing the GdBa2Cu3OX (Gd-123) wires by substituting Gd for the rare-earth (RE) portion of YBa2Cu3OX (Y-123). It is possible to form a film of the wires under nearly the same conditions as those of Y-123 that we conventionally have used. Oxygen partial pressure and deposi-
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H. Fuji et al. / Physica C 468 (2008) 1510–1513
deposited on YBCO films by RF magnetron sputtering and thereafter annealed for several hours at 500 °C in 760 Torr oxygen. Crystalline alignment was measured by X-ray diffraction pole figure. Transport properties were measured at 77 K by reel to reel continuance Ic measurement system. And Ic characterizations in magnetic field were measured in short samples. The Ic values were determined by the criterion of 10 6 V/cm. In electric power devices, it is necessary to make the superconducting wire into a conductor by adding stability and insulation to it. To add stability, after forming a 10 lm thick silver (Ag) layer by sputtering method and annealing it, we laminate a Cu metal tape with 100 lm by a Sn solder using the continuous stabilization layer combined instrument. The lamination speed can attain 100 m/h. Fig. 3 shows laminating apparatus. Finally, to isolate the wire, we wrap a polyimide tape around it. 3. Results and discussion Fig. 1. Large scale reel-to-reel IBAD system to prepare an over 500 m long.
Fig. 4 shows 540 m long GZO/Hastellloy tape by large scale IBAD system with the production speed of about 3 m/h. The firm thickness was 1.0 lm and the in-plane mosaic spread (Du) were below 15° by X-ray (2 2 2) pole figure for the both ends of GZO film. Secondary buffer layer of CeO2 was grown by the large scale PLD system. Below 4° of CeO2 was obtained from Du of 10° GZO/IBAD buffer layer with production speed of 10 m/h. Using below Du of 15° GZO/IBAD buffer layer, good crystallization of CeO2 buffer layer under Du value of 5.5° was obtained by once to three times depositions with production 10 m/h. The real production speed was 3.3–10 m/h.
Fig. 2. Large scale PLD system equipped with the 180 W laser.
Fig. 4. 540 m long IBAD-GZO/Hastellloy tape.
350 300
Ic(77 K) / A
250 200 150 100 Fig. 3. Laminating apparatus with high throughputs 100 m/h.
0
tion temperature were optimized for long tape depositions. Furthermore, in order to obtain uniform Ic values, target control was optimized for stability of temperature. 20 lm thick Ag films were
90 Pa 70 Pa
50 810
820
830
840
850
860
870
T(substrate) / C° Fig. 5. Relationship between Ic value and temperature of deposition.
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H. Fuji et al. / Physica C 468 (2008) 1510–1513
200A
former
70A
optimized
Fig. 6. Ic distribution compared optimized target operating with former target operating by Tapestar.
Fig. 5 shows relationship between Ic value and temperature of deposition. In the case of higher oxygen pressure at Gd-123 deposition, we can obtain higher Ic values. Fig. 6 shows Ic distribution compared optimized target operating with former target operating by Tapestar that is continuous Ic measurement system using magnetization operating. In the case of optimized target operating, stability of temperature was observed. As a result, we obtain uniform Ic values. Table 1 shows studied condition in advance of deposition for 200 m and 500 m tapes. Ic distribution of 200 m Gd-123 coated conductor as shown in Fig. 7. We obtained results of 300 A or more, which is our final target value, for the Ic value over the entire length of tapes. Stabilizing and insulation as shown in Fig. 8 and double pancake winding for
no inductance were carried out to the tapes. As a result, we obtain end to end Ic of 318 A and Ic L values of 64,077 Am. Fig. 9 shows Ic distribution of 500 m Gd-123 coated conductor. We obtained results of 300 A or more for the Ic value over the entire length of 500 m tapes. However, a few part of Ic degradation by defects of substrate or buffer layer were observed. Ic L values were 122,166 Am, it is world record (2007.8), by minimum Ic values maximum length of non Ic degradation.
Table 1 Condition for long PLD deposition 500
600 m 120 Hz 6c 3.3 m/ 840 °C 70 Pa
? ? ? 6.6 m/ ? 90 Pa
400 350 300 250 200 150 100 50 00
Fig. 8. Views of (A) stabilizing by Cu laminating, and (B) insulation by polyimide
Ic (A)
200
Laser condition Strength Frequency Scan Tape speed Temperature Oxygen
Ic (A)
Tape length
50
100
150
Position on Tape (m) Fig. 7. Ic distribution of 200 m Gd-123 coated conductor.
200
450 400 350 300 250 200 150 100 50 00
368m 304.8A 50
100 150 200 250 300 350 400 450 500
Position on Tape (m) Fig. 9. Ic distribution of 500 m Gd-123 coated conductor.
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H. Fuji et al. / Physica C 468 (2008) 1510–1513
Bright-field image
Dark-field image
GdBCO
GdBCO g=200
g=200
BaCeO3
BaCeO3
BaCeO3
BaCeO3
CeO2
CeO2
Fig. 10. TEM observation on cross-section of Gd-123 long tapes.
1000
1000
Gd123
Field : 3T
Width:10mm 300
I c (A)
Ic (A)
100
10
65K 100
77K
1
30
B⊥ c
B//c 0.1
0
2
4
6
10 -30
8
B (T)
0
30
60
90
120
Theta (deg.)
Fig. 11. Ic–B properties of higher Ic sample of thicker film. Fig. 12. Ic–h properties of higher Ic sample of thicker film compared 77 K with 65 K.
Fig. 10 shows TEM observation on cross-section of Gd-123 long tapes. BaCeO3 were observed at interface between G-123 and CeO2 layer. In order to obtain uniform Ic of entire long tapes, severe controlling of temperature less than ±5 °C was required during long deposition times. Fig. 11 shows Ic–B properties of higher Ic values of thicker film. We obtain the 500 A of Ic in self field and over 100 A in magnetic field of 3 T (perpendicular to c-axis). Fig. 12 shows Ic–h properties of higher Ic values of thicker film compared 77 K with 65 K. Minimum Ic values of below 30 A are obtained around 25° of h at 77 K. In order to increase Ic around low angle to c-axis, artificial pining technique were required at 77 K or applied at lower temperature. Finally, we discuss the cost performance and productivity of coated conductor by PLD process. In order to achieve a low cost, we show several proposals as follows: (a) Apparatus with high power excimer laser were introduced (from 180 W to 300 W). (b) Multi-lane were applied (from 2 lane to 5 lane). (c) Life time of laser tube were extended (from 1 109 to 4 109). (d) Yield of target (from 26% to 40%). Recently, laser technology makes progress in dramatic by applied to the industry of liquid crystal. If we resolve above subjects, we can expect to achieve a higher throughputs of 30–50 m/h and low cost about 5 yen/Am by a few years later.
4. Summary The IBAD and PLD system were scaled up to prepare an over 500 m long Gd-123 coated conductor with the Ic of over 300 A. Using these apparatuses, we deposited the 200 m and 500 m Gd-123 coated conductors. As a result, In the case of 200 m-class tape, end to end Ic of 318 A were obtained in end-to-end of a 201.5 m long tape and Ic L values were 64,077 Am. In the case of 500 m-class tape, Ic L values of 112,166 Am was obtained and it’s a world record on August 2007. In the short samples, Ic of over 500 A was obtained with Gd-123 thickness of 2.0 lm and over 100 A was obtained in magnetic field of 3 T, perpendicular to c-axis. Acknowledgement This work is/was supported by New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of Fundamental Technologies for Superconductivity Application. References [1] Y. Iijima, K. Kakimoto, Y. Sutoh, S. Ajimura, T. Saitoh, IEEE Trans. Appl. Supercond. 15 (2005) 2590. [2] K. Kakimoto, Y. Iijima, T. Saitoh, Physica C 392–396 (2003) 783. [3] H. Fuji, S. Hanyu, K. Kakimoto, Y. Iijima, T. Saitoh, IEEE Trans. Appl. Supercond. 17 (2007) 3383. [4] H. Fuji, Y. Hanada, T. Miura, S. Hanyu, K. Kakimoto, Y. Iijima, T. Saitoh, Abstr. CSJ Conf. 76 (2007) 23.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Long Gd-123 coated conductor by PLD method H. Fuji *, M. Igarashi, Y. Hanada, T. Miura, S. Hanyu, K. Kakimoto, Y. Iijima, T. Saitoh Fujikura Ltd., 1-5-1, Kiba, Koto-ku, Tokyo 135-8512, Japan
a r t i c l e
i n f o
Article history: Available online 27 June 2008 PACS: 74.72.Bk 75.47. m 85.25.Kx Keywords: IBAD PLD Y-123 Gd-123
a b s t r a c t We have developed long Gd-123 coated conductors by ion-beam-assisted deposition (IBAD) and pulsedlaser-deposition (PLD) method. Recently, large-scale reel-to-reel apparatus with the 110 cm 15 cm assisting ion source was introduced to IBAD system. It was enable to produce 500 m-class IBAD–Gd2Zr2O7 (GZO) tapes with D/ of below 15° and high throughputs of 3 m/h. Furthermore, apparatus with multilane and laser scanning was introduced to PLD system. As a result, end to end Ic of 318 A were obtained for a 201.5 m long tape, and Ic L values were 64,077 Am. Furthermore, 500 m-class deposition was carried out by improving PLD conditions. As a result, Ic L values of 112,166 Am was obtained and it’s a world record on August 2007. In the short samples, Ic of over 500 A was obtained with Gd-123 thickness of 2.0 lm and over 100 A was obtained in magnetic field of 3 T, perpendicular to c-axis. Ó 2008 Published by Elsevier B.V.
1. Introduction The developments of YBCO coated conductors have been performed all over the world. Recently, over 100 m long YBCO coated conductors with the Ic of over 100 A were prepared in several research organizations. For the length over 100 m, a large area apparatus of IBAD was successfully developed with long lifetime operation even in oxygen atmosphere [1,2]. The PLD method has been used as a deposition technique of Y-123 films. It is characterized with stable stoichiometry, easy oxygen pressure control, very high growth rate, etc. The combination of IBAD and PLD is one of the most promising method to fabricate long Y-123 coated conductors. In this paper, we first describe large-scale apparatus for IBAD and PLD systems for coated conductors so as to get higher process throughput for long length conductors over 500 m. Next, we show characterization of high-performance 200 m and 500 m class Gd-123 conductors by combination of IBAD and PLD method. Furthermore, in order to supply the coated conductors to electrical applications, technique of stabilizing and insulation to conductor are developed. Finally, we describe the high Ic vales of thicker films and Ic characteristics in magnetic field. 2. Experiments Gd2Zr2O7 (GZO) films for a 200 m long tape were deposited on roll-milled Hastelloy C276 with 10 mm in width and the 100 lm * Corresponding author. Tel.: +81 3 5606 1064; fax: +81 3 5606 1512. E-mail address: [email protected] (H. Fuji). 0921-4534/$ - see front matter Ó 2008 Published by Elsevier B.V. doi:10.1016/j.physc.2008.05.284
in thickness by reel-to-reel dual-ion-beam sputtering system with two sets of radio frequency (RF) discharged 66 cm 6 cm squareshaped ion sources. Continuous growth of GZO films was performed with the production speed of 0.5–1.0 m/h under the conventional condition [2]. Large-scale reel-to-reel IBAD system to prepare an over 500 m long was constructed as shown in Fig. 1. It has a specially designed 110 cm 15 cm assisting ion source. It has sophisticated ion-extracting grids by which quite low divergent and low energy Ar+ ion beam can be generated. Longitudinal ion current distribution was measured by an array of Faraday cups. A large sintered GZO target was sputtered by sputtering Ar+ ion beam. Film deposition area is 100 cm 25 cm, which is 4–5 times larger than the former system. CeO2 secondary buffer layers and YBCO films were deposited by a reel-to-reel PLD system with Kr-F (k = 248 nm) excimer laser. The large scale PLD system was equipped with the 180 W laser, which has three times higher power in comparison with the former PLD system was shown Fig. 2 [3,4]. This PLD system has the multi-lane deposition in order to improve the production speed and the yield of targets. Superconducting layers were deposited by PLD system (Fig. 2) on CeO2/GZO/Hastelloy taps. It enhances the manufacturing speed of the wires by practically increasing the deposition area in which the tape is passed through multiple plumes created by scanning the laser beam. Also, recently, since the critical current conductivity in a magnetic field is excellent, we are developing the GdBa2Cu3OX (Gd-123) wires by substituting Gd for the rare-earth (RE) portion of YBa2Cu3OX (Y-123). It is possible to form a film of the wires under nearly the same conditions as those of Y-123 that we conventionally have used. Oxygen partial pressure and deposi-
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H. Fuji et al. / Physica C 468 (2008) 1510–1513
deposited on YBCO films by RF magnetron sputtering and thereafter annealed for several hours at 500 °C in 760 Torr oxygen. Crystalline alignment was measured by X-ray diffraction pole figure. Transport properties were measured at 77 K by reel to reel continuance Ic measurement system. And Ic characterizations in magnetic field were measured in short samples. The Ic values were determined by the criterion of 10 6 V/cm. In electric power devices, it is necessary to make the superconducting wire into a conductor by adding stability and insulation to it. To add stability, after forming a 10 lm thick silver (Ag) layer by sputtering method and annealing it, we laminate a Cu metal tape with 100 lm by a Sn solder using the continuous stabilization layer combined instrument. The lamination speed can attain 100 m/h. Fig. 3 shows laminating apparatus. Finally, to isolate the wire, we wrap a polyimide tape around it. 3. Results and discussion Fig. 1. Large scale reel-to-reel IBAD system to prepare an over 500 m long.
Fig. 4 shows 540 m long GZO/Hastellloy tape by large scale IBAD system with the production speed of about 3 m/h. The firm thickness was 1.0 lm and the in-plane mosaic spread (Du) were below 15° by X-ray (2 2 2) pole figure for the both ends of GZO film. Secondary buffer layer of CeO2 was grown by the large scale PLD system. Below 4° of CeO2 was obtained from Du of 10° GZO/IBAD buffer layer with production speed of 10 m/h. Using below Du of 15° GZO/IBAD buffer layer, good crystallization of CeO2 buffer layer under Du value of 5.5° was obtained by once to three times depositions with production 10 m/h. The real production speed was 3.3–10 m/h.
Fig. 2. Large scale PLD system equipped with the 180 W laser.
Fig. 4. 540 m long IBAD-GZO/Hastellloy tape.
350 300
Ic(77 K) / A
250 200 150 100 Fig. 3. Laminating apparatus with high throughputs 100 m/h.
0
tion temperature were optimized for long tape depositions. Furthermore, in order to obtain uniform Ic values, target control was optimized for stability of temperature. 20 lm thick Ag films were
90 Pa 70 Pa
50 810
820
830
840
850
860
870
T(substrate) / C° Fig. 5. Relationship between Ic value and temperature of deposition.
1512
H. Fuji et al. / Physica C 468 (2008) 1510–1513
200A
former
70A
optimized
Fig. 6. Ic distribution compared optimized target operating with former target operating by Tapestar.
Fig. 5 shows relationship between Ic value and temperature of deposition. In the case of higher oxygen pressure at Gd-123 deposition, we can obtain higher Ic values. Fig. 6 shows Ic distribution compared optimized target operating with former target operating by Tapestar that is continuous Ic measurement system using magnetization operating. In the case of optimized target operating, stability of temperature was observed. As a result, we obtain uniform Ic values. Table 1 shows studied condition in advance of deposition for 200 m and 500 m tapes. Ic distribution of 200 m Gd-123 coated conductor as shown in Fig. 7. We obtained results of 300 A or more, which is our final target value, for the Ic value over the entire length of tapes. Stabilizing and insulation as shown in Fig. 8 and double pancake winding for
no inductance were carried out to the tapes. As a result, we obtain end to end Ic of 318 A and Ic L values of 64,077 Am. Fig. 9 shows Ic distribution of 500 m Gd-123 coated conductor. We obtained results of 300 A or more for the Ic value over the entire length of 500 m tapes. However, a few part of Ic degradation by defects of substrate or buffer layer were observed. Ic L values were 122,166 Am, it is world record (2007.8), by minimum Ic values maximum length of non Ic degradation.
Table 1 Condition for long PLD deposition 500
600 m 120 Hz 6c 3.3 m/ 840 °C 70 Pa
? ? ? 6.6 m/ ? 90 Pa
400 350 300 250 200 150 100 50 00
Fig. 8. Views of (A) stabilizing by Cu laminating, and (B) insulation by polyimide
Ic (A)
200
Laser condition Strength Frequency Scan Tape speed Temperature Oxygen
Ic (A)
Tape length
50
100
150
Position on Tape (m) Fig. 7. Ic distribution of 200 m Gd-123 coated conductor.
200
450 400 350 300 250 200 150 100 50 00
368m 304.8A 50
100 150 200 250 300 350 400 450 500
Position on Tape (m) Fig. 9. Ic distribution of 500 m Gd-123 coated conductor.
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H. Fuji et al. / Physica C 468 (2008) 1510–1513
Bright-field image
Dark-field image
GdBCO
GdBCO g=200
g=200
BaCeO3
BaCeO3
BaCeO3
BaCeO3
CeO2
CeO2
Fig. 10. TEM observation on cross-section of Gd-123 long tapes.
1000
1000
Gd123
Field : 3T
Width:10mm 300
I c (A)
Ic (A)
100
10
65K 100
77K
1
30
B⊥ c
B//c 0.1
0
2
4
6
10 -30
8
B (T)
0
30
60
90
120
Theta (deg.)
Fig. 11. Ic–B properties of higher Ic sample of thicker film. Fig. 12. Ic–h properties of higher Ic sample of thicker film compared 77 K with 65 K.
Fig. 10 shows TEM observation on cross-section of Gd-123 long tapes. BaCeO3 were observed at interface between G-123 and CeO2 layer. In order to obtain uniform Ic of entire long tapes, severe controlling of temperature less than ±5 °C was required during long deposition times. Fig. 11 shows Ic–B properties of higher Ic values of thicker film. We obtain the 500 A of Ic in self field and over 100 A in magnetic field of 3 T (perpendicular to c-axis). Fig. 12 shows Ic–h properties of higher Ic values of thicker film compared 77 K with 65 K. Minimum Ic values of below 30 A are obtained around 25° of h at 77 K. In order to increase Ic around low angle to c-axis, artificial pining technique were required at 77 K or applied at lower temperature. Finally, we discuss the cost performance and productivity of coated conductor by PLD process. In order to achieve a low cost, we show several proposals as follows: (a) Apparatus with high power excimer laser were introduced (from 180 W to 300 W). (b) Multi-lane were applied (from 2 lane to 5 lane). (c) Life time of laser tube were extended (from 1 109 to 4 109). (d) Yield of target (from 26% to 40%). Recently, laser technology makes progress in dramatic by applied to the industry of liquid crystal. If we resolve above subjects, we can expect to achieve a higher throughputs of 30–50 m/h and low cost about 5 yen/Am by a few years later.
4. Summary The IBAD and PLD system were scaled up to prepare an over 500 m long Gd-123 coated conductor with the Ic of over 300 A. Using these apparatuses, we deposited the 200 m and 500 m Gd-123 coated conductors. As a result, In the case of 200 m-class tape, end to end Ic of 318 A were obtained in end-to-end of a 201.5 m long tape and Ic L values were 64,077 Am. In the case of 500 m-class tape, Ic L values of 112,166 Am was obtained and it’s a world record on August 2007. In the short samples, Ic of over 500 A was obtained with Gd-123 thickness of 2.0 lm and over 100 A was obtained in magnetic field of 3 T, perpendicular to c-axis. Acknowledgement This work is/was supported by New Energy and Industrial Technology Development Organization (NEDO) as Collaborative Research and Development of Fundamental Technologies for Superconductivity Application. References [1] Y. Iijima, K. Kakimoto, Y. Sutoh, S. Ajimura, T. Saitoh, IEEE Trans. Appl. Supercond. 15 (2005) 2590. [2] K. Kakimoto, Y. Iijima, T. Saitoh, Physica C 392–396 (2003) 783. [3] H. Fuji, S. Hanyu, K. Kakimoto, Y. Iijima, T. Saitoh, IEEE Trans. Appl. Supercond. 17 (2007) 3383. [4] H. Fuji, Y. Hanada, T. Miura, S. Hanyu, K. Kakimoto, Y. Iijima, T. Saitoh, Abstr. CSJ Conf. 76 (2007) 23.