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Observation of superconductivity in Ba-Ca-Cu-O thin films prepared by RF thermal plasma deposition method
Physica C 231 (1994) 330-334
ELSEVIER
Observation of superconductivity in Ba-Ca-Cu-O thin films prepared by RF thermal plasma deposition method H . Y a k a b e *, J . G . W e n , A. K u m e , Y . S h i o h a r a , N . K o s h i z u k a , S. T a n a k a Superconductivity Research Laboratory, ISTEC, 10-13 Shinonome, 1-chome, Koto-ku, Tokyo 135, Japan
Received 27 June 1994; revised manuscript received 28 July 1994
Abstract
Superconducting thin t'rims in the Ba-Ca-Cu-O system were prepared on SrTiO3 substrates using the RF thermal plasma deposition method. High-resolution transmission electron microscopy (HRTEM) has revealed that the Cu-1212 phase exists in the thin films. The Cu-1212 phase is oriented with the lattice constant c = 11.7 A normal to the substrate surface. The superconducting properties of the thin films are very sensitive to the oxygen content. After annealing in flowing oxygen gas at 300"C, the film showed superconductivity at a T¢o~t of 90 K and zero resistivity below 35 K.
1. Introduction
The high-pressure technique has been demonstrated as a powerful method for realizing new highT~ superconductors. Takano et al. successfully synthesized superconducting "infinite-layer" compounds in the A - C u - O system under a high-pressure of 6 GPa [ 1 ]. Moreover, they found a new family of Srn+ 1CunO2~+ 1+a superconductors with Tc ~ 100 K [ 2 ]. In addition, during preparation of the "infinitelayer" compounds, Adachi et al. found another new S r - C a - C u - O family with 0 2 ( n - 1 )n structure which shows superconductivity at Tc~ 100 K [ 3 ]. Recently, Ihara et al. found a new high-T~ superconductor of the Agl_xCuxBa2Can_ ~CunO2n+3+6 (in short (Ag, C u ) - 1 2 ( n - 1 ) n ) family with T¢=117 K [4]. Jin et al. [ 5] and Alario-Franco et al. [6 ] successfully synthesized Cu- 12 ( n - 1 ) n superconductors where n = 3, 4 and 5 without Ag. The Cu- 12 (n - 1 ) n compound where n = 4 showed the highest T~ of 117 K. * Corresponding author.
So far, the vapor-phase growth technique has not been so effective in identifying new superconductors. Although many researchers have tried to make "infinite-layer" compound p-type superconductors using the RF sputtering [ 7 ] or laser ablation methods [ 8 ], the "infinite-layer" thin films showed no superconductivity. Only a few anomalies have been observed in the resistive property of Ca1 _xSrxCuO2thin films made by the laser molecular beam growth technique [ 9,10 ]. Recently, Kawai et al. found superconductivity in B a - C a - C u - O thin films prepared by the laser molecular beam layer-by-layer growth technique [ 11 ]. However, the B a - C a - C u - O system with Cu-12 ( n - 1 )n structure has not been composed yet by the vapor-phase growth technique. Recently, it was found that the thermal plasma deposition method has an advantage in the fabrication of high- Tc superconducting thin films [ 12 ]. Using this method, (Sr, Ca) CuO2 thin films with the "infinitelayer" structure were synthesized. Interestingly, the strong oxidizing ability made it possible to introduce excess oxygens into the "infinite-layer" compound
0921-4534/94/$07.00 @ 1994 ElsevierScienceB.V. All rights reserved SSDI0921-4534(94)00504-4
331
11. Yakabe et al. / Physica C231 (1994) 330-334
[ 13 ]. In the present paper, we report on the first preparation of superconducting B a - C a - C u - O thin films using the RF thermal plasma deposition method, and on the studies of structures and superconducting properties of these films.
2. Experimental
Thin films were prepared on SrTiOs single-crystal substrates with the (100) plane by the RF thermal plasma deposition technique. The experimental apparatus used here is shown elsewhere [ 12 ], and experimental conditions are listed in Table 1. The raw materials of BaCO3, CaO and CuO powders were mixed and calcined at 1000°C for I h. The nominal composition was varied around a typical atomic ratio of Ba: Ca: Cu = l : 1 : 2. After re-grinding, the powders were fed into the RF oxygen plasma environment with an Ar carrier gas. The substrates were spontaneously heated by the thermal plasma before feeding. The substrate temperature was monitored by a thermocouple and thin films were prepared at various substrate temperatures of 500°C ~ 6000C. The fed powders were vaporized and codeposited onto the substrates in 1 rain. The film thickness of the 1 min deposited samples was usually about 300 .~. After deposition, the samples were naturally quenched on releasing vacuum to ambient pressure. The samples were oxidized by annealing in flowing oxygen gas at 300°C for 24 h. The structures of the prepared films were characterized by X R D and H R T E M measurements. DC electrical resistivity was measured by the standard four-probe technique and the magnetoresistance was measured in pulsed magnetic fields up to 20 T. The
DC magnetic susceptibility was measured using a SQUID magnetometer in an applied field of 50 Oe.
3. Results and discussion
The X R D measurement shows that the superconducting B a - C a - C u - O films contain many phases. By means of changing the atomic ratio in the starting powder and the substrate temperature, it is possible to obtain nearly single-phase thin films the X R D peaks of which are seen also in the pattern of the multi-phase B a - C a - C u - O film. However, the resistivity measurement reveals that all the nearly singlephase thin films show insulating resistive properties and there is no correlation between the relative abundance of these phases in the superconducting B a - C a C u - O film and the superconductivity. This means that the superconducting phase is a minor phase mixed in the film. The temperature dependences of the resistivity for the as-grown and the annealed multi-phase B a - C a C u - O films are shown in Fig. 1. The as-grown sample shows a curious resistive property. Two humps are observed near 210 K and 90 K in the resistivity curve. On the other hand, after annealing, the sample shows no hump near 210 K but becomes metallic. Below 90 K, the resistivity of the annealed sample abruptly decreases and zero resistance is obtained at 35 K. Therefore, the resistive hump near 90 K may be due to the superconducting property of the as-grown sample. However, the broad transition width means that
f
j J
~ as-grown
15
Table 1 Summaryof film growth conditions "~
RF input power
46 kW
Gas
7 l/min Ar+50 l/rain 02 BaCO3, CaO, and CuO (calcined at 1000°Cfor lh) 200 Torr 500-600°C
Powder Pressure Substrate temperature Growth duration
1 min (film thickness=300 A)
10
/
~e
/
/ / ./ SOi
/
,'o
2£
2h
3O0
Temperature (K)
Fig. 1. Temperature dependence of the electrical resistivity for as-grownand annealed thin films.
H. Yakabe et al. / Physica C 231 (1994) 330-334
332
the superconducting volume is not so high and the superconducting regions are weakly coupled in the film. Or the resistive hump near 2 l0 K may originate from the coexistence of a superconducting phase which shows a metallic property above Tc and unknown semiconducting phases: after annealing, the metallic property of the superconducting phase is predominant and the semiconducting property may be screened. Fig. 2 shows the temperature dependence of the DC magnetic susceptibility obtained from the same annealed sample as shown in Fig. 1. Although the sample shows the resistive drop below 90 K, a remarkable diamagnetic signal is observed just below 25 K. Fig. 3 shows magnetoresistance data measured for another sample. The measurement was performed at 0.0
[
l.
i
J
-0.2 -0.4
• ZFC
-~- -0.6
several temperatures in pulsed magnetic fields perpendicular to the film surface. Although the sample does not show zero resistance at 4.2 K, positive magnetoresistance is recognized at every temperature at and below 80 K. However, the sample shows no magnetoresistance at 100 K. A positive magnetoresistance has been obtained for many high-To superconductors [ 14-16 ], and the positive magnetoresistance for our thin film may be due to superconductivity below 90 K in consistency with the resistivity drop below 90 K. Fig. 4 shows a HRTEM image of a typical sample which shows superconductivity. The incident electron beam was parallel to the [ 100] direction. The bright lines as indicated by arrows on the right side in the figure correspond to CuO2_~ layers. The dark dot lines sandwiching the CuO2_~ layer correspond to BaO layers. This image is similar to that of a Cu1 2 ( n - 1 )n compound measured by Wu et al. [17]. By the analogy of the Cu- 12 ( n - 1 ) n system, the "infinite-layer" type block with different numbers of layers exists between the BaO/CuO2_~/BaO blocks. The distance between the upper CuO2_~ layers indi-
~ -11,8 -1.0
Temperature(K) Fig. 2. Temperature dependence of the magnetization for the same sample used for the resistivity measurement. A magnetic field of 50 Oe was applied perpendicularly to the film surface.
0
"I0
"5
0
~
110
115
20
Magnetic Field (T) Fig. 3. Magnetoresistance in fields up to 20 T perpendicular to the film surface at various temperatures.
Fig. 4. Cross-sectional HRTEM image of a film with its [001] axis along the beam direction. The arrows indicate CUO2_6planes and the indicated numbers correspond to the total numbers of CuP2 planes between the arrows.
H. Yakabe et al. / Physica C 231 (1994) 330-334
cared by arrows is 11.7 A. This one-unit structure contains only one CuO2/Ca/CuO2 block and it corresponds to the n = 2 member of the Cu- 12 (n - 1 ) n structure. The Cu-1212 structure exists in almost all areas of the superconducting B a - C a - C u - O films. Another phase corresponding to the n = 3 member of the C u - 1 2 ( n - 1 )n can also be seen in the TEM image. Judging from the TEM image, we consider that, although the X R D patterns of the superconducting films showed no recognizable peaks corresponding to the Cu-1212 phase and the Cu-1223 phase, the minor Cu- 12 ( n - 1 ) n phases show superconductivity below 90 K. The structural model of Cu-1212 is illustrated in Fig. 5. This structure is similar to that of YBa2Cu307_, with deficient CuO2_, layers with the Y site being substituted by Ca. As Wu et al. mentioned about the C u - 1 2 ( n - 1 ) n system elsewhere [ 17 ], oxygen vacancies must exist also in the Cu- 1212 phase as well as other Cu-12 ( n - 1 )n phases in order to maintain charge neutrality of the crystals. We have not measured the transport properties such as the Seebeck coefficient and Hall coefficient to clarify the
• Cu
333
carrier sign. However, the annealing experiment suggests that O( 1 ) vacancies in the B a O / C u O 2 _ j B a O block tend to be filled, hole type carriers being thus generated. In conclusion, B a - C a - C u - O thin films were prepared by the RF thermal plasma deposition method. The resistivity measurements showed that the onset of resistivity drop occurs at 90 K and the zero resistance is achieved at 35 K. The DC magnetic susceptibility measurement confirmed that the sample shows superconductivity below 25 K, and the magnetoresistance measurement suggested the superconductivity to occur below 90 K. The results of the Xray diffraction measurement showed that the superconducting phase was a minor phase in the superconducting thin films and the TEM image revealed that the superconducting films have Cu-1212 structure with c = 11.7 A as well as the Cu-1223 structure. It is considered that these Cu- 12 (n - 1 ) n phases (where n = 2, 3) show superconductivity below 90 K in the B a - C a - C u - O thin films. The superconducting properties of the films were sensitive to the oxygen heat treatment.
Acknowledgements
O Ba @ Ca
@
The authors wish to thank H. Yamauchi, N. Sugii, S. Adachi and K. Nakanishi for their helpful discussions. They also wish to thank K. Nakao for his help in the magnetoresistance measurement and T. Egi for his help in the magnetization measurement by SQUID. This work was partially supported by New Energy and Industrial Technology Development Organization for the R&D of Industrial Science and Technology Frontier Program.
References
C
Fig. 5. Structural model of the Cu-1212 compound. The c and a parametersof the obtainedfdm are 11,7A and 3.9 A, respectively.
[ 1] M. Takano, Y. Takeda, H. Okada, M. Miyamoto and T. Kusaka, Physica C 159 (1989) 375. [2] Z. Hiroi, M. Takano, M. Azuma and Y. Takeda, Nature (London) 364 (1993) 315. [ 3 ] S. Adachi, H. Yarnauchi, S. Tanaka and N. Mori, PhysicaC 208 (1993) 226. [4]H. Ihara, K. Yokiwa, H. Ozawa, M. Hirabayashi, H. Matuhata, A. Negishi and Y.S. Song, Jpn. J. Appl. Phys. 33
(1994) L300.
334
H. Yakabe et al. / Physica C 231 (1994) 330-334
[ 5 ] C.Q. Jin, S. Adachi, X.J. Wu, H. Yamauchi and S. Tanaka, Physica C 223 (1994) 238. [6] M.A.A. Franco, C. Chaillout, J.J. Capponi, J.L. Tholence and B. Souletie, Physica C 222 (1994) 52. [7] IC Kubo and H. Yamauchi, Phys. Rev. B 49 (1994) 1289. [8] N. Sugii, M. Ichikawa, K. Hayashi, K. Kubo, K. Yamamoto and H. Yamauchi, Physica C 213 (1993) 345. [ 9 ] X. Li, M. Kanai, T. Kawai and S. Kawai, Jpn. J. Appl. Phys. 31 (1992) L217. [10]C. Niu and C.M. Lieber, J. Am. Chem. Soc. 115 (1993) 137. [ 11 ] X. Li, T. Kawai and S. Kawai, Jpn. J. Appl. Phys. 33 (1994) L18.
[ 12 ] S. Yuhya, K. Kikuchi and Y. Shiohara, J. Mater. Res. 7 (1992) 2673. [13]H. Yakabe, A. Kume, M. Kosuge, Y. Shiohara and N. Koshizuka, ISS Conf. in Hiroshima, 1993. [ 14 ] K. Nakao, N. Miura, S. Uchida, H. Takagi, S. Tanaka, K. Kishio, J. Shimoyama, K. Kitazawa and K. Fueki, Jpn. J. Appl. Phys. 26 (1987) 26. [ 15 ] W. Staguhn, K. Nakao, N. Miura, M. Orstenberg, S. Hata, Y. Ichikawa and K. Wasa, J. Phys. Soc. Jpn. 58 (1989) 2877. [ 16 ] H. Raffy, S. Labdi, O. Laborde and P. Monceau, Physica C 184 (1991) 159. [ 17 ] X.J. Wu, S. Adachi, C.Q. Jin and H. Yamauchi, Physica C 223 (1994) 242.
ELSEVIER
Observation of superconductivity in Ba-Ca-Cu-O thin films prepared by RF thermal plasma deposition method H . Y a k a b e *, J . G . W e n , A. K u m e , Y . S h i o h a r a , N . K o s h i z u k a , S. T a n a k a Superconductivity Research Laboratory, ISTEC, 10-13 Shinonome, 1-chome, Koto-ku, Tokyo 135, Japan
Received 27 June 1994; revised manuscript received 28 July 1994
Abstract
Superconducting thin t'rims in the Ba-Ca-Cu-O system were prepared on SrTiO3 substrates using the RF thermal plasma deposition method. High-resolution transmission electron microscopy (HRTEM) has revealed that the Cu-1212 phase exists in the thin films. The Cu-1212 phase is oriented with the lattice constant c = 11.7 A normal to the substrate surface. The superconducting properties of the thin films are very sensitive to the oxygen content. After annealing in flowing oxygen gas at 300"C, the film showed superconductivity at a T¢o~t of 90 K and zero resistivity below 35 K.
1. Introduction
The high-pressure technique has been demonstrated as a powerful method for realizing new highT~ superconductors. Takano et al. successfully synthesized superconducting "infinite-layer" compounds in the A - C u - O system under a high-pressure of 6 GPa [ 1 ]. Moreover, they found a new family of Srn+ 1CunO2~+ 1+a superconductors with Tc ~ 100 K [ 2 ]. In addition, during preparation of the "infinitelayer" compounds, Adachi et al. found another new S r - C a - C u - O family with 0 2 ( n - 1 )n structure which shows superconductivity at Tc~ 100 K [ 3 ]. Recently, Ihara et al. found a new high-T~ superconductor of the Agl_xCuxBa2Can_ ~CunO2n+3+6 (in short (Ag, C u ) - 1 2 ( n - 1 ) n ) family with T¢=117 K [4]. Jin et al. [ 5] and Alario-Franco et al. [6 ] successfully synthesized Cu- 12 ( n - 1 ) n superconductors where n = 3, 4 and 5 without Ag. The Cu- 12 (n - 1 ) n compound where n = 4 showed the highest T~ of 117 K. * Corresponding author.
So far, the vapor-phase growth technique has not been so effective in identifying new superconductors. Although many researchers have tried to make "infinite-layer" compound p-type superconductors using the RF sputtering [ 7 ] or laser ablation methods [ 8 ], the "infinite-layer" thin films showed no superconductivity. Only a few anomalies have been observed in the resistive property of Ca1 _xSrxCuO2thin films made by the laser molecular beam growth technique [ 9,10 ]. Recently, Kawai et al. found superconductivity in B a - C a - C u - O thin films prepared by the laser molecular beam layer-by-layer growth technique [ 11 ]. However, the B a - C a - C u - O system with Cu-12 ( n - 1 )n structure has not been composed yet by the vapor-phase growth technique. Recently, it was found that the thermal plasma deposition method has an advantage in the fabrication of high- Tc superconducting thin films [ 12 ]. Using this method, (Sr, Ca) CuO2 thin films with the "infinitelayer" structure were synthesized. Interestingly, the strong oxidizing ability made it possible to introduce excess oxygens into the "infinite-layer" compound
0921-4534/94/$07.00 @ 1994 ElsevierScienceB.V. All rights reserved SSDI0921-4534(94)00504-4
331
11. Yakabe et al. / Physica C231 (1994) 330-334
[ 13 ]. In the present paper, we report on the first preparation of superconducting B a - C a - C u - O thin films using the RF thermal plasma deposition method, and on the studies of structures and superconducting properties of these films.
2. Experimental
Thin films were prepared on SrTiOs single-crystal substrates with the (100) plane by the RF thermal plasma deposition technique. The experimental apparatus used here is shown elsewhere [ 12 ], and experimental conditions are listed in Table 1. The raw materials of BaCO3, CaO and CuO powders were mixed and calcined at 1000°C for I h. The nominal composition was varied around a typical atomic ratio of Ba: Ca: Cu = l : 1 : 2. After re-grinding, the powders were fed into the RF oxygen plasma environment with an Ar carrier gas. The substrates were spontaneously heated by the thermal plasma before feeding. The substrate temperature was monitored by a thermocouple and thin films were prepared at various substrate temperatures of 500°C ~ 6000C. The fed powders were vaporized and codeposited onto the substrates in 1 rain. The film thickness of the 1 min deposited samples was usually about 300 .~. After deposition, the samples were naturally quenched on releasing vacuum to ambient pressure. The samples were oxidized by annealing in flowing oxygen gas at 300°C for 24 h. The structures of the prepared films were characterized by X R D and H R T E M measurements. DC electrical resistivity was measured by the standard four-probe technique and the magnetoresistance was measured in pulsed magnetic fields up to 20 T. The
DC magnetic susceptibility was measured using a SQUID magnetometer in an applied field of 50 Oe.
3. Results and discussion
The X R D measurement shows that the superconducting B a - C a - C u - O films contain many phases. By means of changing the atomic ratio in the starting powder and the substrate temperature, it is possible to obtain nearly single-phase thin films the X R D peaks of which are seen also in the pattern of the multi-phase B a - C a - C u - O film. However, the resistivity measurement reveals that all the nearly singlephase thin films show insulating resistive properties and there is no correlation between the relative abundance of these phases in the superconducting B a - C a C u - O film and the superconductivity. This means that the superconducting phase is a minor phase mixed in the film. The temperature dependences of the resistivity for the as-grown and the annealed multi-phase B a - C a C u - O films are shown in Fig. 1. The as-grown sample shows a curious resistive property. Two humps are observed near 210 K and 90 K in the resistivity curve. On the other hand, after annealing, the sample shows no hump near 210 K but becomes metallic. Below 90 K, the resistivity of the annealed sample abruptly decreases and zero resistance is obtained at 35 K. Therefore, the resistive hump near 90 K may be due to the superconducting property of the as-grown sample. However, the broad transition width means that
f
j J
~ as-grown
15
Table 1 Summaryof film growth conditions "~
RF input power
46 kW
Gas
7 l/min Ar+50 l/rain 02 BaCO3, CaO, and CuO (calcined at 1000°Cfor lh) 200 Torr 500-600°C
Powder Pressure Substrate temperature Growth duration
1 min (film thickness=300 A)
10
/
~e
/
/ / ./ SOi
/
,'o
2£
2h
3O0
Temperature (K)
Fig. 1. Temperature dependence of the electrical resistivity for as-grownand annealed thin films.
H. Yakabe et al. / Physica C 231 (1994) 330-334
332
the superconducting volume is not so high and the superconducting regions are weakly coupled in the film. Or the resistive hump near 2 l0 K may originate from the coexistence of a superconducting phase which shows a metallic property above Tc and unknown semiconducting phases: after annealing, the metallic property of the superconducting phase is predominant and the semiconducting property may be screened. Fig. 2 shows the temperature dependence of the DC magnetic susceptibility obtained from the same annealed sample as shown in Fig. 1. Although the sample shows the resistive drop below 90 K, a remarkable diamagnetic signal is observed just below 25 K. Fig. 3 shows magnetoresistance data measured for another sample. The measurement was performed at 0.0
[
l.
i
J
-0.2 -0.4
• ZFC
-~- -0.6
several temperatures in pulsed magnetic fields perpendicular to the film surface. Although the sample does not show zero resistance at 4.2 K, positive magnetoresistance is recognized at every temperature at and below 80 K. However, the sample shows no magnetoresistance at 100 K. A positive magnetoresistance has been obtained for many high-To superconductors [ 14-16 ], and the positive magnetoresistance for our thin film may be due to superconductivity below 90 K in consistency with the resistivity drop below 90 K. Fig. 4 shows a HRTEM image of a typical sample which shows superconductivity. The incident electron beam was parallel to the [ 100] direction. The bright lines as indicated by arrows on the right side in the figure correspond to CuO2_~ layers. The dark dot lines sandwiching the CuO2_~ layer correspond to BaO layers. This image is similar to that of a Cu1 2 ( n - 1 )n compound measured by Wu et al. [17]. By the analogy of the Cu- 12 ( n - 1 ) n system, the "infinite-layer" type block with different numbers of layers exists between the BaO/CuO2_~/BaO blocks. The distance between the upper CuO2_~ layers indi-
~ -11,8 -1.0
Temperature(K) Fig. 2. Temperature dependence of the magnetization for the same sample used for the resistivity measurement. A magnetic field of 50 Oe was applied perpendicularly to the film surface.
0
"I0
"5
0
~
110
115
20
Magnetic Field (T) Fig. 3. Magnetoresistance in fields up to 20 T perpendicular to the film surface at various temperatures.
Fig. 4. Cross-sectional HRTEM image of a film with its [001] axis along the beam direction. The arrows indicate CUO2_6planes and the indicated numbers correspond to the total numbers of CuP2 planes between the arrows.
H. Yakabe et al. / Physica C 231 (1994) 330-334
cared by arrows is 11.7 A. This one-unit structure contains only one CuO2/Ca/CuO2 block and it corresponds to the n = 2 member of the Cu- 12 (n - 1 ) n structure. The Cu-1212 structure exists in almost all areas of the superconducting B a - C a - C u - O films. Another phase corresponding to the n = 3 member of the C u - 1 2 ( n - 1 )n can also be seen in the TEM image. Judging from the TEM image, we consider that, although the X R D patterns of the superconducting films showed no recognizable peaks corresponding to the Cu-1212 phase and the Cu-1223 phase, the minor Cu- 12 ( n - 1 ) n phases show superconductivity below 90 K. The structural model of Cu-1212 is illustrated in Fig. 5. This structure is similar to that of YBa2Cu307_, with deficient CuO2_, layers with the Y site being substituted by Ca. As Wu et al. mentioned about the C u - 1 2 ( n - 1 ) n system elsewhere [ 17 ], oxygen vacancies must exist also in the Cu- 1212 phase as well as other Cu-12 ( n - 1 )n phases in order to maintain charge neutrality of the crystals. We have not measured the transport properties such as the Seebeck coefficient and Hall coefficient to clarify the
• Cu
333
carrier sign. However, the annealing experiment suggests that O( 1 ) vacancies in the B a O / C u O 2 _ j B a O block tend to be filled, hole type carriers being thus generated. In conclusion, B a - C a - C u - O thin films were prepared by the RF thermal plasma deposition method. The resistivity measurements showed that the onset of resistivity drop occurs at 90 K and the zero resistance is achieved at 35 K. The DC magnetic susceptibility measurement confirmed that the sample shows superconductivity below 25 K, and the magnetoresistance measurement suggested the superconductivity to occur below 90 K. The results of the Xray diffraction measurement showed that the superconducting phase was a minor phase in the superconducting thin films and the TEM image revealed that the superconducting films have Cu-1212 structure with c = 11.7 A as well as the Cu-1223 structure. It is considered that these Cu- 12 (n - 1 ) n phases (where n = 2, 3) show superconductivity below 90 K in the B a - C a - C u - O thin films. The superconducting properties of the films were sensitive to the oxygen heat treatment.
Acknowledgements
O Ba @ Ca
@
The authors wish to thank H. Yamauchi, N. Sugii, S. Adachi and K. Nakanishi for their helpful discussions. They also wish to thank K. Nakao for his help in the magnetoresistance measurement and T. Egi for his help in the magnetization measurement by SQUID. This work was partially supported by New Energy and Industrial Technology Development Organization for the R&D of Industrial Science and Technology Frontier Program.
References
C
Fig. 5. Structural model of the Cu-1212 compound. The c and a parametersof the obtainedfdm are 11,7A and 3.9 A, respectively.
[ 1] M. Takano, Y. Takeda, H. Okada, M. Miyamoto and T. Kusaka, Physica C 159 (1989) 375. [2] Z. Hiroi, M. Takano, M. Azuma and Y. Takeda, Nature (London) 364 (1993) 315. [ 3 ] S. Adachi, H. Yarnauchi, S. Tanaka and N. Mori, PhysicaC 208 (1993) 226. [4]H. Ihara, K. Yokiwa, H. Ozawa, M. Hirabayashi, H. Matuhata, A. Negishi and Y.S. Song, Jpn. J. Appl. Phys. 33
(1994) L300.
334
H. Yakabe et al. / Physica C 231 (1994) 330-334
[ 5 ] C.Q. Jin, S. Adachi, X.J. Wu, H. Yamauchi and S. Tanaka, Physica C 223 (1994) 238. [6] M.A.A. Franco, C. Chaillout, J.J. Capponi, J.L. Tholence and B. Souletie, Physica C 222 (1994) 52. [7] IC Kubo and H. Yamauchi, Phys. Rev. B 49 (1994) 1289. [8] N. Sugii, M. Ichikawa, K. Hayashi, K. Kubo, K. Yamamoto and H. Yamauchi, Physica C 213 (1993) 345. [ 9 ] X. Li, M. Kanai, T. Kawai and S. Kawai, Jpn. J. Appl. Phys. 31 (1992) L217. [10]C. Niu and C.M. Lieber, J. Am. Chem. Soc. 115 (1993) 137. [ 11 ] X. Li, T. Kawai and S. Kawai, Jpn. J. Appl. Phys. 33 (1994) L18.
[ 12 ] S. Yuhya, K. Kikuchi and Y. Shiohara, J. Mater. Res. 7 (1992) 2673. [13]H. Yakabe, A. Kume, M. Kosuge, Y. Shiohara and N. Koshizuka, ISS Conf. in Hiroshima, 1993. [ 14 ] K. Nakao, N. Miura, S. Uchida, H. Takagi, S. Tanaka, K. Kishio, J. Shimoyama, K. Kitazawa and K. Fueki, Jpn. J. Appl. Phys. 26 (1987) 26. [ 15 ] W. Staguhn, K. Nakao, N. Miura, M. Orstenberg, S. Hata, Y. Ichikawa and K. Wasa, J. Phys. Soc. Jpn. 58 (1989) 2877. [ 16 ] H. Raffy, S. Labdi, O. Laborde and P. Monceau, Physica C 184 (1991) 159. [ 17 ] X.J. Wu, S. Adachi, C.Q. Jin and H. Yamauchi, Physica C 223 (1994) 242.