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Thin film growth of high-Tc YBa2Cu3O7-δ phase by an oxygen refilling process
Journal of Crystal Growth 99 (1990) 947-950 North-Holland
947
T H I N FILM G R O W T H OF H I G H - Tc YBa2Cu307_ 8 P H A S E BY AN OXYGEN R E F I L L I N G P R O C E S S
M. A K I N A G A Department of Physics, Fukuoka University of Education, Fukuoka 811-41, Japan
and D. A B U K A Y and L. R I N D E R E R Institute of Experimental Physics, University of Lausanne, CH-IO15 Lausanne-Dorigny, Switzerland
The high-Tc YBa2CU3OT_ 8 phase in thin films has been grown by an oxygen refilling process and low-temperature annealing. Such samples on an MgO substrate showed a small electrical resistivity with exactly linear temperature-dependence below room temperature and sharp transition with high To.
1. Introduction
Since the discovery of high-Tc superconductivity in perovskite-type oxides [1-5], a great deal of research has already been undertaken and characterization is now well advanced. Analyzing sintered samples, Ishikawa et al. [6] have reported that in addition to the orthorhombic 90 K superconducting phase (ortho-I), there exists another orthorhombic superconductor with Tc - 60 K (ortho-II) and non-superconducting tetragonal phase (tetra) in the YBa2Cu307_ ~ system. This phase sequence is supposed to be caused by the increasing number of oxygen vacancies, which must play a very important role in the mechanism of high-Tc superconductivity. In this work we have tried to prepare these three different phases, especially the ortho-I, in the form of thin films by an oxygen refilling process and low-temperature annealing. This is very interesting from the viewpoint of the crystallography and has great potential for applications in many technical fields.
MgO and sapphire by RF-magnetron sputtering from a stoichiometric target. After sputtering, post-annealing was carried out in a rather special way or in the usual manner described elsewhere [7]. Here we discuss the former, which was our key step towards obtaining a sharp transition at high
re. As shown in fig. 1, the typical process is as follows: after placing the as-deposited sample in a quartz tube in the furnace, the tube was evacuated while heating the sample to 820-840 o C at a ramp of 2 5 0 ° C / h . Subsequently, oxygen was slowly
tO00
Thin films with a thickness of 1.0-1.5/~m were deposited onto the substrates of polished SrTiO3, 0022-0248/90/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
in 02
800 ,,
l,
650 "C
600
6"',
(1
400
2O0
0
2. Experimental
vacuum i
f
to
21
3[0
40
T ( hour )
Fig. 1. Post-annealing process for the thin films deposited by RF-magnetron sputtering. The oxygen gas was slowly introduced at 820-840 o C.
948
M. Akinaga et al. / Thin film growth of high-T~ YBaeCu307 _ 8 phase by oxygen refilling
The electrical resistivity of the samples was measured by the standard DC or AC four-probe method, where the terminals were attached to the film surface by silver paste. The temperature was measured by a calibrated platinum resistance or silicon diode thermometer.
3. Results and discussion
Fig. 2. SEM photograph of Y - B a - C u - O film on (100) MgO substrate.
introduced into the tube at atmospheric pressure and the temperature was maintained constant for 5 h. Then, the samples were cooled down to 650 o C, which is in the vicinity of the o r t h o - t e t r a phase transition temperature. They were kept at this temperature for 10-15 h and slowly cooled to near room temperature. The heating of films in vacuum, the refilling of oxygen and the annealing at rather low temperature are characteristic in this processing technique. With the more usual post-annealing process, it was very difficult to grow YBa2Cu3OT_ 8 thin films with reasonable superconducting properties on a sapphire substrate with good adhesion. With our new processing technique, all the samples adhered well on the sapphire using low-temperature heating. Fig. 2 shows a SEM (scanning electron microscope) micrograph of a Y - B a - C u - O film on a (100) MgO substrate prepared using the process mentioned above. Two kinds of grains, small and large ones are apparent. Using E D X (energy dispersive X-ray spectra), small grains of the order of 0.1/~m in the photograph were found to consist of the 1 : 2 : 3 system. It is suggested that the origin of such a small grain size is due to the rather low annealing temperature from the correlation between the grain size and annealing temperature reported by Tsuchida et al. [8].
Fig. 3 shows a typical temperature dependence of the resistance ratio for YBa 2Cu307_6 thin-films on a MgO substrate (a) prepared by the process described in fig. 1, together with the results for the films on (100) SrTiO 3 substrates (b, c) prepared by usual heat treating without the oxygen-refilling process. The sample shows a sharp transition with Tc - 90 K, a low resistivity of 0(300) - 1.7 ml2 cm and an exactly linear temperature-dependence in the normal state below room temperature. This is very similar to the resistive behavior of ortho-I in sintered samples prepared by Ishikawa et al. [6]. Sample (a) in fig. 3 therefore is highly likely to consist of the ortho-I phase. U p till now, we have never succeeded in growing a thin film with the . . . .
i
. . . .
~,
0~
i
. . . .
i
. . . .
(d)
i
. . . .
i
YBa2Cu307_
i
,
,
,
i
i
,
i
~
co~ ! ~ ,
, . f
,o
I°' o.s
. . J
~ f
i" 0
, 50
I 100
. . . .
i
. . . .
150
i 200
. . . .
i 250
,
i
I
300
T(K)
Fig. 3. Typical temperature dependence of resistance ratio for YBa2Cu307_ 8 thin films. (a) R 0 (at 300 K) =17.1 $2; (b) R 0 (at 300 K ) = 232 g2; (c) R 0 (at 270 K ) = 40.8 ~2; (d) R 0 (at 300 K) > 1 k I2.
M. Akinaga et a L / Thin film growth ofhigh-T~. YBa2Cu307 ... ~ phase by oxygen refilling
YB°2Cu307-&
1.5
c
i
1,0~°
0.5
0 ~ 40
0 50
60
70
80
90
100
110
T(K)
Fig. 4. Typical temperature depenuence of the resistivity of the samples prepared by oxygen refilling process: (a) and (b) on MgO substrate; (c) on sapphire substrate.
ortho-I phase using the usual heat treatment; instead, behavior typical of the ortho-II phase with T o - 60 K is found, see fig. 3c. Moreover, many samples showed semiconductor-like behavior due to the tetra phase, as shown by the dotted line (d). Curve (b) probably corresponds to a mixed state of ortho-I and ortho-II, in which the I - V characteristics due to a superconducting weak link are clearly exhibited, as already reported [9]. In fig. 3, the extrapolated line of the normal resistance for the film (a) intersects the temperature axis at a positive value ( - 10 K). This tendency was also observed for highly oriented films or bulk samples in the YBa2Cu30 x system [10], the B i - S r - C a - C u - O system [11] and the T I - B a - C a - C u system [121 with high T~. Char et al. [13] described this point with respect to the Y2Ba4CuaO20_x system, but so far the physical significance has not been clarified. X-ray diffraction failed to reveal the so-called copper-rich 2 : 4 : 8 phase in our films. In fig. 4, three resistive transition curves are shown for samples prepared by our new method, on MgO (a, b) and on sapphire (c). Curves (a) in fig. 4 indicate a superconducting onset temperature of 90 K and resistance approximately to zero at 83 K. The sample on the MgO substrate was post-annealed using process (a) in fig. 1, annealing at 6 5 0 ° C for 15 h. The onset temperature for sample (b) on MgO post-annealed using process
949
(b) in fig. 1 was measured as 80 K, and the zero resistance state at 75 K, which is a rather sharp transition for a thin-film sample. For the film on sapphire (c) in fig. 4, the onset was measured as 80 K and zero resistance at 56 K, which is rather a broad transition. Compared with films on sapphire substrates prepared by a multiple-source electron-beam codeposition (by Naito et al. [14]), the superconductive properties are not so good. As shown in fig. 4, the value of the resistivity of the sample on sapphire is also larger (by about one order) than that of films on MgO substrates. This result might be because there is a reaction between the film and the sapphire, where aluminum is expected to diffuse, or because of the lack of preferential orientation of the grains due to the different growth kinetics associated with the crystal symmetry of the sapphire substrate. On the other hand, the weak negative temperature-coefficient of resistivity indicates that the concentration of oxygen vacancies is still high in sample (c) in fig. 4, which might require an even longer annealing time. Heating the films in vacuum up to 820-840 ° C led to higher oxygen deficiency, and hence larger number of vacancies. This results in a phase transformation as mentioned in section 1. This
YBa2Cu307_&
C
0
o(1) Cu(1)
Fig. 5. The crystal structure of orthorhombic form for YBazCU3OT_ 8 (after Izumi et al. [15]).
950
M. Akinaga et al. / Thin film growth ofhigh.TC YBa2Cu307_ ~ phase by o.rygen refilling
also helps to desorb the foreign gas atoms trapped d u r i n g the sputtering. Then, i n t r o d u c i n g oxygen back into the tube fills the oxygen vacancies i n a controlled m a n n e r b y keeping the t e m p e r a t u r e c o n s t a n t at 650 o C, which is the value j u s t below the t e t r a g o n a l - o r t h o r h o m b i c phase t r a n s i t i o n temperature. The crystal structure of the orthor h o m b i c form for YBa2Cu3OT_ ~ [15] is illustrated in fig. 5. The o c c u p a t i o n factor of the oxygen O(1) sites o n the a-axis determines the k i n d of phase, ortho-I, ortho-II a n d tetra phase. The supercond u c t i n g properties i m p r o v e consistently with increasing o c c u p a t i o n factor. The new p o s t - a n n e a l ing process with controlled refilling of the oxygen sites at low a n n e a l i n g temperature makes it possible to increase the o c c u p a t i o n factor, so that thin films of Y B a 2 C u 3 0 7 _ ~ can be grown with n a r r o w t r a n s i t i o n width a n d high T~.
Acknowledgements T h e authors wish to t h a n k Professor T. A o m i n e for valuable discussions a n d encouragement. They also express thanks to Mr. G. Burri for his excellent work o n the m i c r o p r o b e analysis of the sampies. O n e of the authors (M.A.) would like to acknowledge gratefully the m e m b e r s of the Institut de Physique Exp~rimentale de l'Universit~ de L a u s a n n e for their hospitality. The authors are also i n d e b t e d to the F o n d s N a t i o n a l Suisse de la Recherche Scientifique for financial support.
References [1] J.G. Bednorz and K.A. Miiller, Z. Physik B64 (1986) 189. [2] S. Uchida, H. Takagi, K. Kitazawa and S. Tanaka, Japan. J. Appl. Phys. 26 (1987) L1. [3] M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. Letters. 58 (1987) 908. [4] H. Maeda, Y. Tanaka, M. Fukutomi and T. Asano, Japan. J. Appl. Phys. 27 (1988) L209. [5] Z.Z. Sheng and A.M. Hermann, Nature 322 (1988) 138. [6] M. Ishikawa, T. Takabatake and Y. Nakazawa, Physica 148B (1987) 332. [7] M. Akinaga, D. Abukay and L. Rinderer, Helv. Phys. Acta 61 (1988) 861. [8] K. Tsuchida, M. Ishii, M. Takata and T. Yamashita, Japan. J. Appl. Phys. 27 (1988) L2081. [9] M. Akinaga, D. Abukay and L. Rinderer, in: Proc. Intern. Symp. on High Temperature Superconductivity, Jaipur, 1988 (Oxford & IBH, New Delhi, 1988) p. 51. [10] P.M. Mankiewich, R.E. Howard, W.J. Skocpol, A.H. Dayem, A. Ourmazd, M.G. Young and E. Good, Mater. Res. Soc. Symp. Proc. 99 (1988) 119. [11] for example, K. Kuroda, M. Mukaida and S. Miyazawa, Japan. J. Appl. Phys. 27 (1988) L2230. [12] For example, M. Kikuchi, S. Nakajima, Y. Syono, K. Hiraga, T. Oku, D. Shindo, N. Kobayashi, H. Iwasaki and Y. Muto, Physica C158 (1989) 79. [13] K. Char, M. Lee, R.W. Barton, A.F. Marshall, I. Bozovic, R.H. Hammond, M.R. Beasley, T.H. Geballe and A. Kapitulnik, Phys. Rev. B38 (1988) 834. [14] M. Naito, R.H. Hammond, B. Oh, M.R. Hahn, J.W.P. Hsu, P. Rosenthal, A.F. Marshall, M.R. Beasley, T.H. Geballe and A. Kapitulnik, J. Mater. Res. 2 (1987) 713. [15] F. Izumi, H. Asano, T. Ishigaki, E. Takayama-Muromachi, Y. Uchida, N. Watanabe and T. Nishikawa, Japan. J. Appl. Phys. 26 (1987) L649.
947
T H I N FILM G R O W T H OF H I G H - Tc YBa2Cu307_ 8 P H A S E BY AN OXYGEN R E F I L L I N G P R O C E S S
M. A K I N A G A Department of Physics, Fukuoka University of Education, Fukuoka 811-41, Japan
and D. A B U K A Y and L. R I N D E R E R Institute of Experimental Physics, University of Lausanne, CH-IO15 Lausanne-Dorigny, Switzerland
The high-Tc YBa2CU3OT_ 8 phase in thin films has been grown by an oxygen refilling process and low-temperature annealing. Such samples on an MgO substrate showed a small electrical resistivity with exactly linear temperature-dependence below room temperature and sharp transition with high To.
1. Introduction
Since the discovery of high-Tc superconductivity in perovskite-type oxides [1-5], a great deal of research has already been undertaken and characterization is now well advanced. Analyzing sintered samples, Ishikawa et al. [6] have reported that in addition to the orthorhombic 90 K superconducting phase (ortho-I), there exists another orthorhombic superconductor with Tc - 60 K (ortho-II) and non-superconducting tetragonal phase (tetra) in the YBa2Cu307_ ~ system. This phase sequence is supposed to be caused by the increasing number of oxygen vacancies, which must play a very important role in the mechanism of high-Tc superconductivity. In this work we have tried to prepare these three different phases, especially the ortho-I, in the form of thin films by an oxygen refilling process and low-temperature annealing. This is very interesting from the viewpoint of the crystallography and has great potential for applications in many technical fields.
MgO and sapphire by RF-magnetron sputtering from a stoichiometric target. After sputtering, post-annealing was carried out in a rather special way or in the usual manner described elsewhere [7]. Here we discuss the former, which was our key step towards obtaining a sharp transition at high
re. As shown in fig. 1, the typical process is as follows: after placing the as-deposited sample in a quartz tube in the furnace, the tube was evacuated while heating the sample to 820-840 o C at a ramp of 2 5 0 ° C / h . Subsequently, oxygen was slowly
tO00
Thin films with a thickness of 1.0-1.5/~m were deposited onto the substrates of polished SrTiO3, 0022-0248/90/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
in 02
800 ,,
l,
650 "C
600
6"',
(1
400
2O0
0
2. Experimental
vacuum i
f
to
21
3[0
40
T ( hour )
Fig. 1. Post-annealing process for the thin films deposited by RF-magnetron sputtering. The oxygen gas was slowly introduced at 820-840 o C.
948
M. Akinaga et al. / Thin film growth of high-T~ YBaeCu307 _ 8 phase by oxygen refilling
The electrical resistivity of the samples was measured by the standard DC or AC four-probe method, where the terminals were attached to the film surface by silver paste. The temperature was measured by a calibrated platinum resistance or silicon diode thermometer.
3. Results and discussion
Fig. 2. SEM photograph of Y - B a - C u - O film on (100) MgO substrate.
introduced into the tube at atmospheric pressure and the temperature was maintained constant for 5 h. Then, the samples were cooled down to 650 o C, which is in the vicinity of the o r t h o - t e t r a phase transition temperature. They were kept at this temperature for 10-15 h and slowly cooled to near room temperature. The heating of films in vacuum, the refilling of oxygen and the annealing at rather low temperature are characteristic in this processing technique. With the more usual post-annealing process, it was very difficult to grow YBa2Cu3OT_ 8 thin films with reasonable superconducting properties on a sapphire substrate with good adhesion. With our new processing technique, all the samples adhered well on the sapphire using low-temperature heating. Fig. 2 shows a SEM (scanning electron microscope) micrograph of a Y - B a - C u - O film on a (100) MgO substrate prepared using the process mentioned above. Two kinds of grains, small and large ones are apparent. Using E D X (energy dispersive X-ray spectra), small grains of the order of 0.1/~m in the photograph were found to consist of the 1 : 2 : 3 system. It is suggested that the origin of such a small grain size is due to the rather low annealing temperature from the correlation between the grain size and annealing temperature reported by Tsuchida et al. [8].
Fig. 3 shows a typical temperature dependence of the resistance ratio for YBa 2Cu307_6 thin-films on a MgO substrate (a) prepared by the process described in fig. 1, together with the results for the films on (100) SrTiO 3 substrates (b, c) prepared by usual heat treating without the oxygen-refilling process. The sample shows a sharp transition with Tc - 90 K, a low resistivity of 0(300) - 1.7 ml2 cm and an exactly linear temperature-dependence in the normal state below room temperature. This is very similar to the resistive behavior of ortho-I in sintered samples prepared by Ishikawa et al. [6]. Sample (a) in fig. 3 therefore is highly likely to consist of the ortho-I phase. U p till now, we have never succeeded in growing a thin film with the . . . .
i
. . . .
~,
0~
i
. . . .
i
. . . .
(d)
i
. . . .
i
YBa2Cu307_
i
,
,
,
i
i
,
i
~
co~ ! ~ ,
, . f
,o
I°' o.s
. . J
~ f
i" 0
, 50
I 100
. . . .
i
. . . .
150
i 200
. . . .
i 250
,
i
I
300
T(K)
Fig. 3. Typical temperature dependence of resistance ratio for YBa2Cu307_ 8 thin films. (a) R 0 (at 300 K) =17.1 $2; (b) R 0 (at 300 K ) = 232 g2; (c) R 0 (at 270 K ) = 40.8 ~2; (d) R 0 (at 300 K) > 1 k I2.
M. Akinaga et a L / Thin film growth ofhigh-T~. YBa2Cu307 ... ~ phase by oxygen refilling
YB°2Cu307-&
1.5
c
i
1,0~°
0.5
0 ~ 40
0 50
60
70
80
90
100
110
T(K)
Fig. 4. Typical temperature depenuence of the resistivity of the samples prepared by oxygen refilling process: (a) and (b) on MgO substrate; (c) on sapphire substrate.
ortho-I phase using the usual heat treatment; instead, behavior typical of the ortho-II phase with T o - 60 K is found, see fig. 3c. Moreover, many samples showed semiconductor-like behavior due to the tetra phase, as shown by the dotted line (d). Curve (b) probably corresponds to a mixed state of ortho-I and ortho-II, in which the I - V characteristics due to a superconducting weak link are clearly exhibited, as already reported [9]. In fig. 3, the extrapolated line of the normal resistance for the film (a) intersects the temperature axis at a positive value ( - 10 K). This tendency was also observed for highly oriented films or bulk samples in the YBa2Cu30 x system [10], the B i - S r - C a - C u - O system [11] and the T I - B a - C a - C u system [121 with high T~. Char et al. [13] described this point with respect to the Y2Ba4CuaO20_x system, but so far the physical significance has not been clarified. X-ray diffraction failed to reveal the so-called copper-rich 2 : 4 : 8 phase in our films. In fig. 4, three resistive transition curves are shown for samples prepared by our new method, on MgO (a, b) and on sapphire (c). Curves (a) in fig. 4 indicate a superconducting onset temperature of 90 K and resistance approximately to zero at 83 K. The sample on the MgO substrate was post-annealed using process (a) in fig. 1, annealing at 6 5 0 ° C for 15 h. The onset temperature for sample (b) on MgO post-annealed using process
949
(b) in fig. 1 was measured as 80 K, and the zero resistance state at 75 K, which is a rather sharp transition for a thin-film sample. For the film on sapphire (c) in fig. 4, the onset was measured as 80 K and zero resistance at 56 K, which is rather a broad transition. Compared with films on sapphire substrates prepared by a multiple-source electron-beam codeposition (by Naito et al. [14]), the superconductive properties are not so good. As shown in fig. 4, the value of the resistivity of the sample on sapphire is also larger (by about one order) than that of films on MgO substrates. This result might be because there is a reaction between the film and the sapphire, where aluminum is expected to diffuse, or because of the lack of preferential orientation of the grains due to the different growth kinetics associated with the crystal symmetry of the sapphire substrate. On the other hand, the weak negative temperature-coefficient of resistivity indicates that the concentration of oxygen vacancies is still high in sample (c) in fig. 4, which might require an even longer annealing time. Heating the films in vacuum up to 820-840 ° C led to higher oxygen deficiency, and hence larger number of vacancies. This results in a phase transformation as mentioned in section 1. This
YBa2Cu307_&
C
0
o(1) Cu(1)
Fig. 5. The crystal structure of orthorhombic form for YBazCU3OT_ 8 (after Izumi et al. [15]).
950
M. Akinaga et al. / Thin film growth ofhigh.TC YBa2Cu307_ ~ phase by o.rygen refilling
also helps to desorb the foreign gas atoms trapped d u r i n g the sputtering. Then, i n t r o d u c i n g oxygen back into the tube fills the oxygen vacancies i n a controlled m a n n e r b y keeping the t e m p e r a t u r e c o n s t a n t at 650 o C, which is the value j u s t below the t e t r a g o n a l - o r t h o r h o m b i c phase t r a n s i t i o n temperature. The crystal structure of the orthor h o m b i c form for YBa2Cu3OT_ ~ [15] is illustrated in fig. 5. The o c c u p a t i o n factor of the oxygen O(1) sites o n the a-axis determines the k i n d of phase, ortho-I, ortho-II a n d tetra phase. The supercond u c t i n g properties i m p r o v e consistently with increasing o c c u p a t i o n factor. The new p o s t - a n n e a l ing process with controlled refilling of the oxygen sites at low a n n e a l i n g temperature makes it possible to increase the o c c u p a t i o n factor, so that thin films of Y B a 2 C u 3 0 7 _ ~ can be grown with n a r r o w t r a n s i t i o n width a n d high T~.
Acknowledgements T h e authors wish to t h a n k Professor T. A o m i n e for valuable discussions a n d encouragement. They also express thanks to Mr. G. Burri for his excellent work o n the m i c r o p r o b e analysis of the sampies. O n e of the authors (M.A.) would like to acknowledge gratefully the m e m b e r s of the Institut de Physique Exp~rimentale de l'Universit~ de L a u s a n n e for their hospitality. The authors are also i n d e b t e d to the F o n d s N a t i o n a l Suisse de la Recherche Scientifique for financial support.
References [1] J.G. Bednorz and K.A. Miiller, Z. Physik B64 (1986) 189. [2] S. Uchida, H. Takagi, K. Kitazawa and S. Tanaka, Japan. J. Appl. Phys. 26 (1987) L1. [3] M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. Letters. 58 (1987) 908. [4] H. Maeda, Y. Tanaka, M. Fukutomi and T. Asano, Japan. J. Appl. Phys. 27 (1988) L209. [5] Z.Z. Sheng and A.M. Hermann, Nature 322 (1988) 138. [6] M. Ishikawa, T. Takabatake and Y. Nakazawa, Physica 148B (1987) 332. [7] M. Akinaga, D. Abukay and L. Rinderer, Helv. Phys. Acta 61 (1988) 861. [8] K. Tsuchida, M. Ishii, M. Takata and T. Yamashita, Japan. J. Appl. Phys. 27 (1988) L2081. [9] M. Akinaga, D. Abukay and L. Rinderer, in: Proc. Intern. Symp. on High Temperature Superconductivity, Jaipur, 1988 (Oxford & IBH, New Delhi, 1988) p. 51. [10] P.M. Mankiewich, R.E. Howard, W.J. Skocpol, A.H. Dayem, A. Ourmazd, M.G. Young and E. Good, Mater. Res. Soc. Symp. Proc. 99 (1988) 119. [11] for example, K. Kuroda, M. Mukaida and S. Miyazawa, Japan. J. Appl. Phys. 27 (1988) L2230. [12] For example, M. Kikuchi, S. Nakajima, Y. Syono, K. Hiraga, T. Oku, D. Shindo, N. Kobayashi, H. Iwasaki and Y. Muto, Physica C158 (1989) 79. [13] K. Char, M. Lee, R.W. Barton, A.F. Marshall, I. Bozovic, R.H. Hammond, M.R. Beasley, T.H. Geballe and A. Kapitulnik, Phys. Rev. B38 (1988) 834. [14] M. Naito, R.H. Hammond, B. Oh, M.R. Hahn, J.W.P. Hsu, P. Rosenthal, A.F. Marshall, M.R. Beasley, T.H. Geballe and A. Kapitulnik, J. Mater. Res. 2 (1987) 713. [15] F. Izumi, H. Asano, T. Ishigaki, E. Takayama-Muromachi, Y. Uchida, N. Watanabe and T. Nishikawa, Japan. J. Appl. Phys. 26 (1987) L649.