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Effect of nonmagnetic impurities of Al, Mo and Zn on the superconductivity of Ba2YCu3O7
~°% Solid State Communications, Vol.66,No.4, pp.413-416, 1988. ~%1~_~ Printed in Great Britain.
0038-1098/88 $3.00 + .00 Pergamon P r e s s p l c
EFFECT OF NONMAGNETIC IMPURITIES OF AI, Mo AND Zn ON THE SUPERCONDUCTIVITY OF Ba2YCu307 T. TAKABATAKE and M. ISHIKAWA Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan (Received 15 February 1988 by W. Sasaki)
We have investigated the structural and superconducting properties of Ba2Y(Cu I Mx)~O 7 for M = AI, Mo and Zn in the range of 0 ~ x ~ 0.i. The crystal s~ruc~u~-ey of both AI and Mo doped systems gradually loses the orthorhombicity approaching the solubility limit of x ~ 0 . 0 4 , while the Zn doped system remains orthorhombic without a significant change of lattice parameters. We also found that all the three nonmagnetic impurities depress both T and superconducting volume fraction in a similar way but more strongly tChan magnetic ones like Fe, Co and Ni do.
In ordinary superconductors, magnetic impurities such as Fe and Co significantly depress the superconducting transition temperature, T c , due to the paramagne tic pair-breaking effect. For the high T superconductor Ba_YCu307, however, Xiao et a~ Z reported that T is depressed more strongly by nonmagnetic Zn Catoms substituted for Cu than by magnetic atoms like Fe, Co and Ni [I]. Subsequently, Maeno et al confirmed that the partial substitution for Cu atoms by 3.3 % of Fe, Co and Ga induces a structural change from orthorhombic to tetragonal symmetry, whereas the Zn substitution does not [2]. Based on their finding of the smooth decrease of T • . c through the orthorhombic-tetragonal transltlon in the Fe substituted system, they claimed that the Cul-O chain is less important to the superconductivity [2,3] . It is, however, hardly understood in their model why Zn atoms which predominantly occupy Cul sites [4] more drastically degrade the superconductivity. These pioneer works posed an important question of what the key parameter responsible for the high-T of Ba^YCu~O. is, and triggered . c ~ I . . a serles of exper ~men~s of subst ~tut ~on [5-10]. But it is difficult to draw a definite conclusion from reported diverse concentration dependences of T which probably result from the difficulty c in preparing homogeneous samples. We initiated similar experiments by studying the effect of other nonmagnetic impurities with various ionic radius and valence state. In this C o ~ u n i c a t ion, we present the results of a systematic investigation of partially substituted systems with nonmagnetic AI, Mo and Zn for Cu atoms. Surprisingly, all these impurities depress T in a similar way in spite of the larg c difference in the ionic radius and valence state. From the consideration of their preferential site occupation, it is concluded that the superconductivity in Ba_YCu^O~ is Z J I . intimately associated with the Cul-O chaln between the Ba-O plains.
Samples of Ba^Y(Cu. M ).O~ were prepared for M = AI, Mo aid znla~dX~
414
AI, Mo AND Zn ON THE SUPERCONDUCTIVITY OF Ba2YCu307 3.92
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Vol. 66, No. 4
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0
AI 0
300
(K)
I
I
0.05 X
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I
I
,,~..~ -
X=0.05
l
0.1
Fig. i. Variation of the unit cell parameters b and c/3 of Ba^Y(Cu, M )~O~ for M = AI, ~, i-~ x ~ /-y and Zn as a function o K nomlnal concentration The filled data points represent those multi-phased samples.
a, Mo x. of
u C:I E
3
~
2
~""
0.01
n,-
"
....
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and Co doped systems, it has been reported that the orthorhombic-tetragonal transition takes place near x=0.02~0.03 [3,6], and the transition is induced by rearrangement of _ 3+ ~3+ • oxygen atoms near the doped Ee or ~o zons on the Cul site due to their octahedral site preference [5,9]. The loss of orthorhombicity in the AI doped system~ may be explained in a similar way because AI j+ ions have been found to occupy only Cul sites [i0]. The very similar variation in the lattice parameters of Al_and Mo doped systems strongly suggests that Mo J+ ions also occupy predominantly Cul sites. The substitution of Zn ions which are found to preferentially occupy Cul sites [4] does not change the lattice parameters as mentioned above. Consequently, we expect that the Zn 2+ ions substitute for Cul atoms without significant rearrangement of the neighbouring oxygen atoms. It is interesting to further note that the c-cell length increases upon doping AI or Mo ions with a smaller or comparable ionic radius compared to that of Cu ions. For the undoped system of Ba^YCu~O_ • . Z J /-y' an increase of c-cell length is known to result from the decrease of oxygen content [11]. In fact, we found the oxygen content in the AI and Mo doped samples of x=O.05 to be 6.55 and 6.81 for the formula unit respectively, being less than that of the undoped reference sample, 6.88. On the contrary, such decrease of oxygen content was not found for Zn doped samples whose c-parameter does not change much in the studied range of concentration, as can be seen in Fig. I. In Fig. 2, we show the temperature dependence of electrical resistivity of Ba~Y(Cu, M )~O~ separately for M = AI, Mo an~ Znl-X~n ~ ~I~ the three systems, the
0
2p
0
100 ~50 200 Temperature (K)
I ....
I ....
I ....
zo u v
.~
t ....
300
I ' ' ' ' 120
x:005
0 035L..---~
1.0
°o
i " ~
250
15
E
-H
. . . . . ,-,--
i ' . ,:. 50
....
- 6
"
0.5
E i
• • .'. ; ..." 50
Jl , , , , a .... 100 150 Temperature
, .... 200
, .... 250
0 300
(K)
Fig. 2. Temperature dependence of electrical resistivity of Ba~Y(Cu. M )~O~ for (a) M = AI, (b) M = Mo and (c~ M = l ~ . x J l-y
temperature of zero resistance shifts from 92 K to about 50 K regularly with x up to 0.035. Beyond this value of x, however, the resistive transition temperature of AI and Mo doped samples does not decrease any more. This is consistent with the solubility limit of x 0.04 estimated from the X-ray diffraction analysis. On the other hand, the T value of the single-phased Zn doped samples c continues decreasing and beyond x=0.075 they turn themselves to semiconductor-like behavior without exhibiting superconductivity down to
Vol. 66, No. 4
AI, Mo AND Zn ON THE SUPERCONDUCTIVITY
4.2 K. We briefly remark here that the T_2/3 resistivity of x=O.l sample follows a law between 20 and 250 K, as found in a similar system of Ba2Eu(Cu 0 9.Zn0 05)307 [12]. The power law dependence "o~ r e ~ i s t l v 1 ~ [13] may suggest that weak electron localization takes place in such heavily doped Zn samples. It must, here, be emphasized that the resistive superconducting transition does not necessarily probe the bulk nature of the superconductivity. This point becomes an important issue especially in the discussion of superconductivity in inhomogeneous or multi-phased samples. In this study, to better probe bulk nature we also made AC susceptibility measurements on finely powdered samples and display the results in Figs. 3 and 4. In Fig. 3 is plotted the superconducting transition temperature defined as the midpoint of the inductive transition and in Fig. 4 the normalized diamagnetic signal at 4.2 K to that of the undoped sample, which is a measure of
100
I
i
i
i
i
I
o A
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l
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o
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i
o
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0
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I
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100 - 0
o AI 0 Zn A Mo
o 80
T=4.2K o A O
60 --
•
o I
20
'I
l i l l l l l l l l
0.05 X
o
o 0.1
Fig. 4. Diamagnetic signal of AC susceptibility of Ba2Y(Cu. M )~O 7 for M = AI, Mo and Zn as a functlon o~ x. XT~e d~ta are normalized to that of the undoped sample at 4.2 K.
'~ M o
o
60
415
OF Ba2YCu307
0.0G
0.1
X Fig. 3. Superconducting transition temperature defined as the midpoint of inductive transition of Ba~Y(Cu. M )~O~ for M = AI, Mo and Zn as a z ~-x x J l-y function Ot X.
the volume fraction of superconducting part of the sample. It should be noted that the initial drop of both T and the volume fraction follows almost i~entical curves for the three systems. This signifies that the suppression of superconductivity is independent of either the ionic radius or the valence state of doped ions or the degree of orthorhombicity and furthermore that the superconductivity in the substituted samples precipitately loses the bulk nature as x
increases. In the present three systems, the doped ions preferentially occupy Cul sites, as aforementioned. Therefore, the rapid decrease of T and the volume fraction in these systems can ~e ascribed to the resultant disorder of Cul-O chains. We further stress that the observed depression of T is stronger than those reported for Ni and ~e impurities [3,9]. Note here that Ni atoms are known to occupy preferentially Cu2 site [4] and Fe atoms occupy both Cul and Cu2 sites [14]. It is worthwhile to mention that rare earth counterparts of the yttrium compound, Bap(RE)Cu307 are also a 90K superconductor, buE as soon as RE atoms go into the Ba sites, T immediately starts decreasing independent o~ particular RE element, as found in (Ba~ RE )(RE)Cu~O~ [15]. All these L-x x J z experlmental results accordingly lead to the conclusion that the superconductivity of Ba~YCu~O. is intimately associated with both th~ CJul!O chain and the Ba-O plain. However, the moderate depression of T by Fe, Co and Ni substitution compared to th~ nonmagnetic case studied in this work seems to he somewhat difficult to understand within conventional models. If it were not a result of extrinsic problems like inhomogeneity in samples, we may have to call for an unconventional electron pairing such as the one mediated by spin fluctuations. Acknowledgements - The authors are pleased to acknowledge F. Sakai for her chemical analysis of oxygen content and T. Shibuya for his assistance in sample preparations.
416
AI, Mo AND Zn ON THE SUPERCONDUCTIVITY OF Ba2YCu307
Vol. 66, No. 4
REFERENCES
I. G. Xiao, F.H. Streitz, A. Gavrin, Y.W. Du & C.L. Chien, Phys. Rev. B35, 8782 (1987). 2. Y. Maeno & T. Fujita, Proc. Workshop on Novel Mechanisms of Superconductivity, p.i073 (Eds. S.A. Wolf & V.Z. Kresin), Plenum, New York (1987). 3. Y. Maeno, M. Kato, Y. Aoki & T. Fujita, Jpn. J. Appl.Phys. 26, L1982 (1987). 4. T. Kajitani, K. Kusaba, M. Kikuchi, Y. Shono & M. Hirabayashi, to be published in Jpn. J. Appl. Phys. 27 (1988). 5. Y. Oda, H. Fujita, H. Toyoda, T. Kaneko, T. Kohara, I. Nakada & K. Asayama, Jpn. J. Appl. Phys. 26, L1660 (1987). 6. D. Shindo, K. Hiraga, M. Hirabayashi, A. Tokiwa, Y. Shono, O. Nakatsu, N. Kobayashi, Y. Muto & E. Aoyagi, Jpn. J. Appl. Phys. 26, L1667 (1987). 7. J.H. Tarascon, L.H. Green, P. Barboux, W.R. McKinnon, G.W. Hull, T.P. Orlando, K.A. Delin, S. Foner & E.J. McNiff, Jr., Phys. Rev. B36, 8393 (1987); M. Mehbod, P. Wyder, R. Deltour, Ph. Duvigneaud & G. Naessens, Phys. Rev. B36, 8819 (1987); M. Hiratani, Y. Ito, K. Miyauchi & T. Kudo, Jpn. J. Appl. Phys. 26, L1997 (1987).
8. P.B. Kirby, M.R. Harrison, W.G. Freeman, I. Samuel & H.J. Haines, Phys. Rev. B36, 8315 (1987). 9. E.T. Muromachi, Y. Uchida & K. Kato, Jpn. J. Appl. Phys. 2_66,L2087 (1987). 10. T. Siegrist, L.F. Schneemeyer, J.V. Waszczak, N.P. Singh, R.L. Opila, B. Batlogg, L.W. Rupp & D.W. Murphy, Phys. Rev. B36, 8365 (1987). II. Y. Nakazawa, M. Ishikawa, T. Takabatake, K. Koga & K. Terakura, Jpn. J. Appl. Phys. 26, L796 (1987). 12. H.A. Borges, G.L. Wells, S.W. Cheong, R.S. Kwok, J.D. Thompson, Z. Fisk & J.L. Smith, Physica 148B, 411 (1987). 13. Y. Imry, Percolation, Localization and Superconductivity, p.189 (Eds. A.M. Goldman & S.A. Wolf), Plenum, New York (1984). 14. S. Nasu, H. Kitagawa, Y. Oda, T. Kohara, T. Shinjo, K. Asayama & F.E. Fujita, Physica 148B, 484 (1987). 15. K. Zhang, B. Dabrowski, C.U. Segre, P.G. Hinks, I.K. Schuller, J.D. Jorgensen & M. Slaski, J. Phys. C 20, L935 (1987).
0038-1098/88 $3.00 + .00 Pergamon P r e s s p l c
EFFECT OF NONMAGNETIC IMPURITIES OF AI, Mo AND Zn ON THE SUPERCONDUCTIVITY OF Ba2YCu307 T. TAKABATAKE and M. ISHIKAWA Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo 106, Japan (Received 15 February 1988 by W. Sasaki)
We have investigated the structural and superconducting properties of Ba2Y(Cu I Mx)~O 7 for M = AI, Mo and Zn in the range of 0 ~ x ~ 0.i. The crystal s~ruc~u~-ey of both AI and Mo doped systems gradually loses the orthorhombicity approaching the solubility limit of x ~ 0 . 0 4 , while the Zn doped system remains orthorhombic without a significant change of lattice parameters. We also found that all the three nonmagnetic impurities depress both T and superconducting volume fraction in a similar way but more strongly tChan magnetic ones like Fe, Co and Ni do.
In ordinary superconductors, magnetic impurities such as Fe and Co significantly depress the superconducting transition temperature, T c , due to the paramagne tic pair-breaking effect. For the high T superconductor Ba_YCu307, however, Xiao et a~ Z reported that T is depressed more strongly by nonmagnetic Zn Catoms substituted for Cu than by magnetic atoms like Fe, Co and Ni [I]. Subsequently, Maeno et al confirmed that the partial substitution for Cu atoms by 3.3 % of Fe, Co and Ga induces a structural change from orthorhombic to tetragonal symmetry, whereas the Zn substitution does not [2]. Based on their finding of the smooth decrease of T • . c through the orthorhombic-tetragonal transltlon in the Fe substituted system, they claimed that the Cul-O chain is less important to the superconductivity [2,3] . It is, however, hardly understood in their model why Zn atoms which predominantly occupy Cul sites [4] more drastically degrade the superconductivity. These pioneer works posed an important question of what the key parameter responsible for the high-T of Ba^YCu~O. is, and triggered . c ~ I . . a serles of exper ~men~s of subst ~tut ~on [5-10]. But it is difficult to draw a definite conclusion from reported diverse concentration dependences of T which probably result from the difficulty c in preparing homogeneous samples. We initiated similar experiments by studying the effect of other nonmagnetic impurities with various ionic radius and valence state. In this C o ~ u n i c a t ion, we present the results of a systematic investigation of partially substituted systems with nonmagnetic AI, Mo and Zn for Cu atoms. Surprisingly, all these impurities depress T in a similar way in spite of the larg c difference in the ionic radius and valence state. From the consideration of their preferential site occupation, it is concluded that the superconductivity in Ba_YCu^O~ is Z J I . intimately associated with the Cul-O chaln between the Ba-O plains.
Samples of Ba^Y(Cu. M ).O~ were prepared for M = AI, Mo aid znla~dX~
414
AI, Mo AND Zn ON THE SUPERCONDUCTIVITY OF Ba2YCu307 3.92
]
I
I
I
I
I
I
I
I
I
A.Mo
_C/3
,.
5
I
. . . .
I
. . . .
I
Vol. 66, No. 4
. . . .
I
. . . .
m
,,
m
I
. . . .
I
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AI E
. . . .
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100
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200
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O&O
0
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. . . .
I
. . . .
I
. . . .
I
. . . .
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. . . .
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. . . .
8
0
AI 0
300
(K)
I
I
0.05 X
I
I
I
I
,,~..~ -
X=0.05
l
0.1
Fig. i. Variation of the unit cell parameters b and c/3 of Ba^Y(Cu, M )~O~ for M = AI, ~, i-~ x ~ /-y and Zn as a function o K nomlnal concentration The filled data points represent those multi-phased samples.
a, Mo x. of
u C:I E
3
~
2
~""
0.01
n,-
"
....
0
and Co doped systems, it has been reported that the orthorhombic-tetragonal transition takes place near x=0.02~0.03 [3,6], and the transition is induced by rearrangement of _ 3+ ~3+ • oxygen atoms near the doped Ee or ~o zons on the Cul site due to their octahedral site preference [5,9]. The loss of orthorhombicity in the AI doped system~ may be explained in a similar way because AI j+ ions have been found to occupy only Cul sites [i0]. The very similar variation in the lattice parameters of Al_and Mo doped systems strongly suggests that Mo J+ ions also occupy predominantly Cul sites. The substitution of Zn ions which are found to preferentially occupy Cul sites [4] does not change the lattice parameters as mentioned above. Consequently, we expect that the Zn 2+ ions substitute for Cul atoms without significant rearrangement of the neighbouring oxygen atoms. It is interesting to further note that the c-cell length increases upon doping AI or Mo ions with a smaller or comparable ionic radius compared to that of Cu ions. For the undoped system of Ba^YCu~O_ • . Z J /-y' an increase of c-cell length is known to result from the decrease of oxygen content [11]. In fact, we found the oxygen content in the AI and Mo doped samples of x=O.05 to be 6.55 and 6.81 for the formula unit respectively, being less than that of the undoped reference sample, 6.88. On the contrary, such decrease of oxygen content was not found for Zn doped samples whose c-parameter does not change much in the studied range of concentration, as can be seen in Fig. I. In Fig. 2, we show the temperature dependence of electrical resistivity of Ba~Y(Cu, M )~O~ separately for M = AI, Mo an~ Znl-X~n ~ ~I~ the three systems, the
0
2p
0
100 ~50 200 Temperature (K)
I ....
I ....
I ....
zo u v
.~
t ....
300
I ' ' ' ' 120
x:005
0 035L..---~
1.0
°o
i " ~
250
15
E
-H
. . . . . ,-,--
i ' . ,:. 50
....
- 6
"
0.5
E i
• • .'. ; ..." 50
Jl , , , , a .... 100 150 Temperature
, .... 200
, .... 250
0 300
(K)
Fig. 2. Temperature dependence of electrical resistivity of Ba~Y(Cu. M )~O~ for (a) M = AI, (b) M = Mo and (c~ M = l ~ . x J l-y
temperature of zero resistance shifts from 92 K to about 50 K regularly with x up to 0.035. Beyond this value of x, however, the resistive transition temperature of AI and Mo doped samples does not decrease any more. This is consistent with the solubility limit of x 0.04 estimated from the X-ray diffraction analysis. On the other hand, the T value of the single-phased Zn doped samples c continues decreasing and beyond x=0.075 they turn themselves to semiconductor-like behavior without exhibiting superconductivity down to
Vol. 66, No. 4
AI, Mo AND Zn ON THE SUPERCONDUCTIVITY
4.2 K. We briefly remark here that the T_2/3 resistivity of x=O.l sample follows a law between 20 and 250 K, as found in a similar system of Ba2Eu(Cu 0 9.Zn0 05)307 [12]. The power law dependence "o~ r e ~ i s t l v 1 ~ [13] may suggest that weak electron localization takes place in such heavily doped Zn samples. It must, here, be emphasized that the resistive superconducting transition does not necessarily probe the bulk nature of the superconductivity. This point becomes an important issue especially in the discussion of superconductivity in inhomogeneous or multi-phased samples. In this study, to better probe bulk nature we also made AC susceptibility measurements on finely powdered samples and display the results in Figs. 3 and 4. In Fig. 3 is plotted the superconducting transition temperature defined as the midpoint of the inductive transition and in Fig. 4 the normalized diamagnetic signal at 4.2 K to that of the undoped sample, which is a measure of
100
I
i
i
i
i
I
o A
[]
BO
l
i
o
AI
i
o
Zn
i
I
o
A v
o
40
o 20
o
0
l
I
I
I
I
I
i
I
I
I
I
100 - 0
o AI 0 Zn A Mo
o 80
T=4.2K o A O
60 --
•
o I
20
'I
l i l l l l l l l l
0.05 X
o
o 0.1
Fig. 4. Diamagnetic signal of AC susceptibility of Ba2Y(Cu. M )~O 7 for M = AI, Mo and Zn as a functlon o~ x. XT~e d~ta are normalized to that of the undoped sample at 4.2 K.
'~ M o
o
60
415
OF Ba2YCu307
0.0G
0.1
X Fig. 3. Superconducting transition temperature defined as the midpoint of inductive transition of Ba~Y(Cu. M )~O~ for M = AI, Mo and Zn as a z ~-x x J l-y function Ot X.
the volume fraction of superconducting part of the sample. It should be noted that the initial drop of both T and the volume fraction follows almost i~entical curves for the three systems. This signifies that the suppression of superconductivity is independent of either the ionic radius or the valence state of doped ions or the degree of orthorhombicity and furthermore that the superconductivity in the substituted samples precipitately loses the bulk nature as x
increases. In the present three systems, the doped ions preferentially occupy Cul sites, as aforementioned. Therefore, the rapid decrease of T and the volume fraction in these systems can ~e ascribed to the resultant disorder of Cul-O chains. We further stress that the observed depression of T is stronger than those reported for Ni and ~e impurities [3,9]. Note here that Ni atoms are known to occupy preferentially Cu2 site [4] and Fe atoms occupy both Cul and Cu2 sites [14]. It is worthwhile to mention that rare earth counterparts of the yttrium compound, Bap(RE)Cu307 are also a 90K superconductor, buE as soon as RE atoms go into the Ba sites, T immediately starts decreasing independent o~ particular RE element, as found in (Ba~ RE )(RE)Cu~O~ [15]. All these L-x x J z experlmental results accordingly lead to the conclusion that the superconductivity of Ba~YCu~O. is intimately associated with both th~ CJul!O chain and the Ba-O plain. However, the moderate depression of T by Fe, Co and Ni substitution compared to th~ nonmagnetic case studied in this work seems to he somewhat difficult to understand within conventional models. If it were not a result of extrinsic problems like inhomogeneity in samples, we may have to call for an unconventional electron pairing such as the one mediated by spin fluctuations. Acknowledgements - The authors are pleased to acknowledge F. Sakai for her chemical analysis of oxygen content and T. Shibuya for his assistance in sample preparations.
416
AI, Mo AND Zn ON THE SUPERCONDUCTIVITY OF Ba2YCu307
Vol. 66, No. 4
REFERENCES
I. G. Xiao, F.H. Streitz, A. Gavrin, Y.W. Du & C.L. Chien, Phys. Rev. B35, 8782 (1987). 2. Y. Maeno & T. Fujita, Proc. Workshop on Novel Mechanisms of Superconductivity, p.i073 (Eds. S.A. Wolf & V.Z. Kresin), Plenum, New York (1987). 3. Y. Maeno, M. Kato, Y. Aoki & T. Fujita, Jpn. J. Appl.Phys. 26, L1982 (1987). 4. T. Kajitani, K. Kusaba, M. Kikuchi, Y. Shono & M. Hirabayashi, to be published in Jpn. J. Appl. Phys. 27 (1988). 5. Y. Oda, H. Fujita, H. Toyoda, T. Kaneko, T. Kohara, I. Nakada & K. Asayama, Jpn. J. Appl. Phys. 26, L1660 (1987). 6. D. Shindo, K. Hiraga, M. Hirabayashi, A. Tokiwa, Y. Shono, O. Nakatsu, N. Kobayashi, Y. Muto & E. Aoyagi, Jpn. J. Appl. Phys. 26, L1667 (1987). 7. J.H. Tarascon, L.H. Green, P. Barboux, W.R. McKinnon, G.W. Hull, T.P. Orlando, K.A. Delin, S. Foner & E.J. McNiff, Jr., Phys. Rev. B36, 8393 (1987); M. Mehbod, P. Wyder, R. Deltour, Ph. Duvigneaud & G. Naessens, Phys. Rev. B36, 8819 (1987); M. Hiratani, Y. Ito, K. Miyauchi & T. Kudo, Jpn. J. Appl. Phys. 26, L1997 (1987).
8. P.B. Kirby, M.R. Harrison, W.G. Freeman, I. Samuel & H.J. Haines, Phys. Rev. B36, 8315 (1987). 9. E.T. Muromachi, Y. Uchida & K. Kato, Jpn. J. Appl. Phys. 2_66,L2087 (1987). 10. T. Siegrist, L.F. Schneemeyer, J.V. Waszczak, N.P. Singh, R.L. Opila, B. Batlogg, L.W. Rupp & D.W. Murphy, Phys. Rev. B36, 8365 (1987). II. Y. Nakazawa, M. Ishikawa, T. Takabatake, K. Koga & K. Terakura, Jpn. J. Appl. Phys. 26, L796 (1987). 12. H.A. Borges, G.L. Wells, S.W. Cheong, R.S. Kwok, J.D. Thompson, Z. Fisk & J.L. Smith, Physica 148B, 411 (1987). 13. Y. Imry, Percolation, Localization and Superconductivity, p.189 (Eds. A.M. Goldman & S.A. Wolf), Plenum, New York (1984). 14. S. Nasu, H. Kitagawa, Y. Oda, T. Kohara, T. Shinjo, K. Asayama & F.E. Fujita, Physica 148B, 484 (1987). 15. K. Zhang, B. Dabrowski, C.U. Segre, P.G. Hinks, I.K. Schuller, J.D. Jorgensen & M. Slaski, J. Phys. C 20, L935 (1987).