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Shielding studies of electrochemically oxidised La2CuO4
PHYSICA
Physica C 199 (1992) 139-142 North-Holland
Shielding studies of electrochemically oxidised La2CuO4 G. R a j a r a m 1, R. S u r y a n a r a y a n a n 2, N. L e N a g a r d , O. G o r o c h o v a n d L. O u h a m m o u Laboratoire de Physique des solides de Bellevue, CNRS, 92195 Meudon, France
W. P a u l u s a n d G. H e g e r Laboratoire Leon Brillouin, CEN Saclay, 91191 Gif sur Yvette, France
Received 18 May 1992 Revised manuscript received 15 June 1992
La2CuO4becomes superconducting at Tc= 44 K when subjected to an electrochemical oxydation in 1N NaOH at room temperature. Data on AC susceptibility of this compound as a function of temperature and DC fields from 0 to 85 Oe are presented and compared with those obtained on other known superconductors. Our studies indicate that the superconductivity in our sample is not confined to the surface as might be expected for a process dependent on small diffusion coefficients.
1. Introduction La2CuO4 prepared by the solid state reaction is an antiferromagnetic insulator and has been o f considerable interest because it becomes a superconductor when La is partially replaced by B a / S r [ 1 ] or when subjected to high pressure oxygen treatment [2] and also because o f its relatively simple structure. It has been recently demonstrated [ 3 ] and confirmed [ 4,5 ] that this c o m p o u n d becomes superconducting when subjected to an electrochemical oxidation at room temperature. It has been suggested that diffusion o f oxygen into interstitial sites [ 3 ], or o f traces o f H out of vacancies at La sites [4] induce mixed valence at Cu, thus making the material a superconductor. M a n y high temperature superconductors ( H T S C ) , prepared by the solid state sintering reaction exhibit diffusion related inhomogeneities in the ceramic, especially at the "weak links" between grains. These significantly affect some microstructure related properties such as bulk critical currents. AC magnetic susceptibility measurements, which have proved to be quite sensitive to microstructure, 1 Permanent address: School of Physics, University of Hyderabad, Hyderabad 500134, India. 2 To whom all correspondence should be addressed.
could be used to study the electromagnetic aspects of the granular superconductors. We report such measurements on electrochemically oxidised LaECuO4 pellets.
2. Experimental techniques La2CuO4 was prepared by mixing La203 and CuO in stoichiometric proportions, pelletising and then sintering in air at 1000°C for 12 h. One part of the pellet (6 × 6 × 0.5 m m ) was electrochemically oxidised in 1N N a O H solution for over 48 h as reported earlier [ 5 ]. The lattice parameters o f this sample were found to be a = 5 . 3 5 , b = 5 . 3 7 9 and c = 13.216 A as reported earlier [3]. AC susceptibility measurements were performed on this sample. For most measurements, the AC field strength used was about 0.I Oe rms (amplitude h o = 0 . 1 4 Oe) and the frequency used was 1.5 kHz. The DC fields applied ranged from 0 (earth's field) to 85 Oe. Signals proportional to the real (Z') and imaginary (Z") parts o f the susceptibility were measured as a function of temperature (T) while warming in the stated D C field after either cooling from above Tc with this field on ( F C R ) , or in zero field (ZFC).
0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All fights reserved.
140
G. Rajaram et al. /Shielding studies of electrochemically oxidised La2Cu04
3. Results and discussion
Figure 1 shows Z' (T) and Z" (T) data for the pellet at 0 DC field and AC field of 0 . 1 0 e . The transition onset is at 44 K and the width 4 K. To determine whether there were any large scale inhomogeneities, the same measurements were performed on a piece of the sample crushed to a powder manually using a mortar and pestle. These are also shown in fig. 1. There are three differences between the two data sets. (1) The transition for the powder is considerably broadened, with sharp drops in X' below 40 K (there is, however, a very small drop at 44 K). (2) Z' (T) for powder does show a tendency to level off at the lowest temperatures measured. The shielded fraction calculated from the drop in z ' ( T ) is about 68% that of the pellet. (3) X" (T) for the pellet peaks at 40 K with a width of 3 K, while that for the powder shows only a small increase as the temperature is lowered below 20 K with no peak. We conclude from (3) that the peak observed for the pellet is due to dissipation in the intergranular "weak links" in the presence of inter-granular shielding currents. The differences ( 1 ) and (2) can be explained in terms of a penetration depth 2 (T) increasing with T and diverging at To, and a single superconducting phase, if 2 ( 0 ) = 0 . 2 a where a is the radius of the shielding supercurrent loop. If 2(0) is 2000 A as is typical for HTSCs, then a is of the order of 1 ~tm.
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T (K) Fig. 1. Z' and Z" of the electrochemically oxidised pellet ( - - ) and after powdering it; ( - - - ) as a function of temperature (hAt=0.1 Oe and HDc=0).
The typical grain size in the powder is about 5 to 10 ~tm. However, a hierarchy of barriers to the supercurrents is known to exist even within particles in the HTSCs and this is a reasonable estimate. Of course, a much smaller 2(0) could imply non-superconducting phase to the level of 32% (or in fact greater, if the superconducting phase is selectively distributed on the powder grain surface). What the data does rule out, however, is the superconductivity being confined to the pellet surface. The pellet and the powder samples were stored in a dry dessicator. The data taken both on the pellet and on the powder a month after did not show any appreciable changes. We discuss now the data pertaining to the pellet. When the AC field strength is increased to 0 . 2 0 e , the peak in Z " ( T ) shifts to 39.3 K. If the flux change across the barrier is a thermally activated process, the peak in g" (T) is expected to occur when the flux jump rate equals the excitation field frequency. In such a case, the activation energy required of the barrier height (U) can be determined from the frequency dependence of the peak Tm. Measurements were performed at 150 Hz and 15 kHz in addition to 1.5 kHz for the AC field. Tm appears to increase monotonically with increasing frequency, but the total change is only 0.6 K which is comparable to the experimental errors originating from broadening of the peaks. This gives a lower bound for U of 1.2 eV. Shifts in Tm similar to this have been observed in pellets of YBa2Cu307 [ 6,7 ] and single crystals of Bi2-2-1-2 at low fields [8,9]. Larger shifts (and hence smaller U) have been reported in Bi(Pb)-2-2-2-3 [10]. The z ' ( T ) and z " ( T ) (ZFC) (fig. 2) show variations which are typical of the high-To granular superconductor, z ' ( T ) is independent of field, and there is no dissipation, for a small temperature region below To. Intergranular shielding currents, and hence further decrease of X', develop below a temperature Tirr which decreases with increasing DC field. This is also, within error, the temperature below which dissipation of (Z") starts (fig. 3). g " ( T ) shows a maximum at a temperature Tin, whose dependence on H is also shown in fig. 3. T~(H) shows a functional dependence of the form ( 1 - T m / To) =kH q [11 ], with q=0.5 and k is a constant. A variation with q= -] was found in the case of L a - B a C u - O for the temperature above which x(H, T) be-
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and field [ 13 ]. Shielding currents are limited by Jcj, which allows flux to penetrate the sample, leading to an inter-grain penetration depth that is much larger than the intra-grain penetration depth. "Critical state" models have been applied to estimate flux profiles by assuming the existence o f "Josephson vortices" consisting o f inter-granular shielding currents with the weak links as barriers [ 14,15 ]. The T > Tm regions represent regions o f the ( T, H ) diagram where there is substantial m o v e m e n t of flux in and out of the sample [ 14 ]. It has been suggested that a m a x i m u m in Z " ( T ) occurs when Jcj (B, T) is such that the flux changes due to the AC field reach the center o f the sample. The flux profile for the ZFC case is expected to be more complicated (convex outward) than a linear variation with distance given by Bean's model which assumes Jc independent o f B. The F C R case is more likely to resemble the latter. In this case, the m a x i m u m in Z " ( T ) occurs when J~j ( T, B) is o f the order o f h o / a , i.e. about 0.3 A / c m 2 (a-- area o f the sample). We expect better shielding in the ZFC case than the F C R one, since B is lower and J~j are larger in the interior o f the sample in the former. However, as shown in fig. 4 (and Tm(H) data for the F C R case in fig. 3), the results are in contradiction with this expectation at low temperatures. The ZFC and F C R data coincide at Ti=, but at lower temperatures, for T < Tm the ZFC data indicate poorer shielding than the F C R data. One explanation for this is that there are substantial demagne-
,;,
,
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30 T
,,:.~:~. 40 50
(K)
'
Fig. 3. T m for ZFC (1) and FCR (2), Ti= from ;(' (3) and ;(" (4) as a function of Hoc. Inset shows the same data as a function of ( 1- T/To). Lines are fits to data. See text for further explanation. comes field independent [ 12 ]. This temperature is probably closer to our T~,, which shows deviations from a power-law dependence. If force fitted, q is 1/ 1.6 and 1/1.96 respectively for Ti= from z ' ( T ) and Z"(T).
It has been suggested that the granular behaviour o f the high-T¢ superconductors can be characterised by a inter-grain/weak-link critical current JCj which is a sensitive (decreasing) function o f temperature
i
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20
m
30
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40 5'0 60 T(KJ Fig. 4. Z' and;(" (in arbitrary units) as a function of temperature at various applied DC fields with hAc=O.10e. Curves 1 (85.8, FCR); 2 (85.8, ZFC); 3 (57.2, FCR); 4 (57.2, ZFC); 5 (14.4, FCR); 6 (14.4, ZFC); 7 (57.20e applied at T<50 K and switched off upon warming ); 8 (0, ZFC).
142
G. Rajaram et al. / Shielding studies of electrochemically oxidised La2CuO+
tisation effects in these m e a s u r e m e n t s (the demagnetisation factor would be of the order of 8). The demagnetisation energy c o n t r i b u t i o n to the total free energy may likely result in a more complicated screening current structure as in ferromagnetic domains. Further complications arise if the resultant fields approach the intra-grain critical field Hcj. It is noteworthy in this context, that the Z F C x ' ( T ) at higher fields do not reach the full diamagnetic value even at the lowest temperatures. In summary, we find that superconductivity in an electrochemically oxidised La2CuO4 pellet is not confined to the surface as expected for a process with small diffusion coefficients. The real a n d imaginary parts of the AC magnetic susceptibility in D C fields upto 85 Oe show b e h a v i o u r qualitatively similar to other high-To superconductors. U n u s u a l differences in Z F C a n d F C R data are f o u n d which are probably due to substantial dernagnetisation effect.
Acknowledgements G R would like to thank U G C ( I n d i a ) , CSIR (India) for a travel grant a n d CNRS ( F r a n c e ) for local support.
References [ 1] J.G. Bednorz and K.A. Miiller,Z. Phys. B 64 (1986) 189.
[2]J. Beille, R. Cabanel, C. Chaillout, B. Chevalier, G. Demazeau, F. Deslandes,J. Etourneau, P. Lejay,C. Michel, J. Provost, B. Raveau, A. Sulpice, J. Tholence and R. Tournier, C.R. Acad. Sci. 304 (1987) 1087; J.E. Schirber, B, Morison, R.M. Merill, P.F. Hlava, E.L. Venturini, J.F. Kwak, P.J. Nigrey, R.J. Baughmanand D.S. Ginley, Physica C 152 ( 1988) 121. [3] A. Wattiaux, J.C. Park, J.C. Grenier and M. Pouchard, C.R.Aead. Sci., Srrie II, Paris 310(1990), 1047; N. Laguete, A. Wattiaux,J.C. Park, J.C. Grenier, L. Fournes and M. Pouchard, J. Phys. III (Paris) France 1 ( 1991 ) 1755+ [4] P. Rudolf, W. Paulus and R. SchSllhorn, Adv. Mater. 3 ( 1991 ) 438. [5]R. Suryanarayanan, O. Gorochov, M.S.R. Rao, L. Ouhammou, W. Paulus and G+ Heger, Physica C 185-189 (1991) 573. [ 6 ] C. Rillo, F. Lera, R. Navarro, J. Bartolome, J. Flokstra and D.H.A. Blank, Cryogenics29 (1989) 1128. [ 7 ] M. Nicoloand R,B. Goldfarb,Phys. Rev. B 39 (1989) 6615. t 8 ] J.H.P.M. Emmen, V.A.M. Brabers and H.W.J+M. de Jonge, Physica C 176 (1991) 137. [9] S.L. Shinde, J. Moriella, D. Goland, D.A. Chance and T.R. McGuire, Phys. Rev. B 41 (1990) 8838. [ 10] A. Hammer and G. Srinivasan, J. Appl. Phys. 69 ( 1991 ) 4899. [ 11 ] Y. Yeshurun and A.P. Malozemoff, Phys. Rev. Lett. 60 (1988) 2202; A.P. Malozemoff, T.K. Worthington, Y. Yeshurun, F. Holtzberg and P.H. Kes, Phys. Rev. B 38 (1988) 7203. [12] K.A. Miiller, M. Takashige and J.G. Bednorz, Phys. Rev. Lett. 58 (1987) 1143. [13] J. Manhart, P. Chaudhari, D. Dimos, C.C. Tsui and T.R. McGuire, Phys. Rev, Lett. 61 (1988 ) 2476. [ 14] J.R. Clem, Physica C 153-155 (1988) 50. [ 15] M. Tinkham and C.J. Lobb, Solid State Phys. 42 ( 1989) 91.
Physica C 199 (1992) 139-142 North-Holland
Shielding studies of electrochemically oxidised La2CuO4 G. R a j a r a m 1, R. S u r y a n a r a y a n a n 2, N. L e N a g a r d , O. G o r o c h o v a n d L. O u h a m m o u Laboratoire de Physique des solides de Bellevue, CNRS, 92195 Meudon, France
W. P a u l u s a n d G. H e g e r Laboratoire Leon Brillouin, CEN Saclay, 91191 Gif sur Yvette, France
Received 18 May 1992 Revised manuscript received 15 June 1992
La2CuO4becomes superconducting at Tc= 44 K when subjected to an electrochemical oxydation in 1N NaOH at room temperature. Data on AC susceptibility of this compound as a function of temperature and DC fields from 0 to 85 Oe are presented and compared with those obtained on other known superconductors. Our studies indicate that the superconductivity in our sample is not confined to the surface as might be expected for a process dependent on small diffusion coefficients.
1. Introduction La2CuO4 prepared by the solid state reaction is an antiferromagnetic insulator and has been o f considerable interest because it becomes a superconductor when La is partially replaced by B a / S r [ 1 ] or when subjected to high pressure oxygen treatment [2] and also because o f its relatively simple structure. It has been recently demonstrated [ 3 ] and confirmed [ 4,5 ] that this c o m p o u n d becomes superconducting when subjected to an electrochemical oxidation at room temperature. It has been suggested that diffusion o f oxygen into interstitial sites [ 3 ], or o f traces o f H out of vacancies at La sites [4] induce mixed valence at Cu, thus making the material a superconductor. M a n y high temperature superconductors ( H T S C ) , prepared by the solid state sintering reaction exhibit diffusion related inhomogeneities in the ceramic, especially at the "weak links" between grains. These significantly affect some microstructure related properties such as bulk critical currents. AC magnetic susceptibility measurements, which have proved to be quite sensitive to microstructure, 1 Permanent address: School of Physics, University of Hyderabad, Hyderabad 500134, India. 2 To whom all correspondence should be addressed.
could be used to study the electromagnetic aspects of the granular superconductors. We report such measurements on electrochemically oxidised LaECuO4 pellets.
2. Experimental techniques La2CuO4 was prepared by mixing La203 and CuO in stoichiometric proportions, pelletising and then sintering in air at 1000°C for 12 h. One part of the pellet (6 × 6 × 0.5 m m ) was electrochemically oxidised in 1N N a O H solution for over 48 h as reported earlier [ 5 ]. The lattice parameters o f this sample were found to be a = 5 . 3 5 , b = 5 . 3 7 9 and c = 13.216 A as reported earlier [3]. AC susceptibility measurements were performed on this sample. For most measurements, the AC field strength used was about 0.I Oe rms (amplitude h o = 0 . 1 4 Oe) and the frequency used was 1.5 kHz. The DC fields applied ranged from 0 (earth's field) to 85 Oe. Signals proportional to the real (Z') and imaginary (Z") parts o f the susceptibility were measured as a function of temperature (T) while warming in the stated D C field after either cooling from above Tc with this field on ( F C R ) , or in zero field (ZFC).
0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All fights reserved.
140
G. Rajaram et al. /Shielding studies of electrochemically oxidised La2Cu04
3. Results and discussion
Figure 1 shows Z' (T) and Z" (T) data for the pellet at 0 DC field and AC field of 0 . 1 0 e . The transition onset is at 44 K and the width 4 K. To determine whether there were any large scale inhomogeneities, the same measurements were performed on a piece of the sample crushed to a powder manually using a mortar and pestle. These are also shown in fig. 1. There are three differences between the two data sets. (1) The transition for the powder is considerably broadened, with sharp drops in X' below 40 K (there is, however, a very small drop at 44 K). (2) Z' (T) for powder does show a tendency to level off at the lowest temperatures measured. The shielded fraction calculated from the drop in z ' ( T ) is about 68% that of the pellet. (3) X" (T) for the pellet peaks at 40 K with a width of 3 K, while that for the powder shows only a small increase as the temperature is lowered below 20 K with no peak. We conclude from (3) that the peak observed for the pellet is due to dissipation in the intergranular "weak links" in the presence of inter-granular shielding currents. The differences ( 1 ) and (2) can be explained in terms of a penetration depth 2 (T) increasing with T and diverging at To, and a single superconducting phase, if 2 ( 0 ) = 0 . 2 a where a is the radius of the shielding supercurrent loop. If 2(0) is 2000 A as is typical for HTSCs, then a is of the order of 1 ~tm.
v
_
"N 10
""-.
J/
~<
I
i
!
I
20
30
40
50
60
T (K) Fig. 1. Z' and Z" of the electrochemically oxidised pellet ( - - ) and after powdering it; ( - - - ) as a function of temperature (hAt=0.1 Oe and HDc=0).
The typical grain size in the powder is about 5 to 10 ~tm. However, a hierarchy of barriers to the supercurrents is known to exist even within particles in the HTSCs and this is a reasonable estimate. Of course, a much smaller 2(0) could imply non-superconducting phase to the level of 32% (or in fact greater, if the superconducting phase is selectively distributed on the powder grain surface). What the data does rule out, however, is the superconductivity being confined to the pellet surface. The pellet and the powder samples were stored in a dry dessicator. The data taken both on the pellet and on the powder a month after did not show any appreciable changes. We discuss now the data pertaining to the pellet. When the AC field strength is increased to 0 . 2 0 e , the peak in Z " ( T ) shifts to 39.3 K. If the flux change across the barrier is a thermally activated process, the peak in g" (T) is expected to occur when the flux jump rate equals the excitation field frequency. In such a case, the activation energy required of the barrier height (U) can be determined from the frequency dependence of the peak Tm. Measurements were performed at 150 Hz and 15 kHz in addition to 1.5 kHz for the AC field. Tm appears to increase monotonically with increasing frequency, but the total change is only 0.6 K which is comparable to the experimental errors originating from broadening of the peaks. This gives a lower bound for U of 1.2 eV. Shifts in Tm similar to this have been observed in pellets of YBa2Cu307 [ 6,7 ] and single crystals of Bi2-2-1-2 at low fields [8,9]. Larger shifts (and hence smaller U) have been reported in Bi(Pb)-2-2-2-3 [10]. The z ' ( T ) and z " ( T ) (ZFC) (fig. 2) show variations which are typical of the high-To granular superconductor, z ' ( T ) is independent of field, and there is no dissipation, for a small temperature region below To. Intergranular shielding currents, and hence further decrease of X', develop below a temperature Tirr which decreases with increasing DC field. This is also, within error, the temperature below which dissipation of (Z") starts (fig. 3). g " ( T ) shows a maximum at a temperature Tin, whose dependence on H is also shown in fig. 3. T~(H) shows a functional dependence of the form ( 1 - T m / To) =kH q [11 ], with q=0.5 and k is a constant. A variation with q= -] was found in the case of L a - B a C u - O for the temperature above which x(H, T) be-
G. Rajaram et aLI Shielding studies of electrochemically oxidised LaeCuO,
I
I
I
.
.;~ . . . .
|
I
!
I
10
20
30
40
50
;...~
. ,- . .
"'~, ,, ,,: . , , ; .~
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•
60
T(K) Fig, 2. ZFC ;(' (top) and ;(" (bottom) of the pellet as a function of temperature at fixed hAc= 0.10e and at different Ht~cincreasing from right to left 0, 6.76, 14.3, 29.5, 42.5, 56.8 and 85.20e.
2
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' ='
60 ~
::'
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,,,
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,
,,
141
and field [ 13 ]. Shielding currents are limited by Jcj, which allows flux to penetrate the sample, leading to an inter-grain penetration depth that is much larger than the intra-grain penetration depth. "Critical state" models have been applied to estimate flux profiles by assuming the existence o f "Josephson vortices" consisting o f inter-granular shielding currents with the weak links as barriers [ 14,15 ]. The T > Tm regions represent regions o f the ( T, H ) diagram where there is substantial m o v e m e n t of flux in and out of the sample [ 14 ]. It has been suggested that a m a x i m u m in Z " ( T ) occurs when Jcj (B, T) is such that the flux changes due to the AC field reach the center o f the sample. The flux profile for the ZFC case is expected to be more complicated (convex outward) than a linear variation with distance given by Bean's model which assumes Jc independent o f B. The F C R case is more likely to resemble the latter. In this case, the m a x i m u m in Z " ( T ) occurs when J~j ( T, B) is o f the order o f h o / a , i.e. about 0.3 A / c m 2 (a-- area o f the sample). We expect better shielding in the ZFC case than the F C R one, since B is lower and J~j are larger in the interior o f the sample in the former. However, as shown in fig. 4 (and Tm(H) data for the F C R case in fig. 3), the results are in contradiction with this expectation at low temperatures. The ZFC and F C R data coincide at Ti=, but at lower temperatures, for T < Tm the ZFC data indicate poorer shielding than the F C R data. One explanation for this is that there are substantial demagne-
,;,
,
0.: ,.Vrt 10 20
30 T
,,:.~:~. 40 50
(K)
'
Fig. 3. T m for ZFC (1) and FCR (2), Ti= from ;(' (3) and ;(" (4) as a function of Hoc. Inset shows the same data as a function of ( 1- T/To). Lines are fits to data. See text for further explanation. comes field independent [ 12 ]. This temperature is probably closer to our T~,, which shows deviations from a power-law dependence. If force fitted, q is 1/ 1.6 and 1/1.96 respectively for Ti= from z ' ( T ) and Z"(T).
It has been suggested that the granular behaviour o f the high-T¢ superconductors can be characterised by a inter-grain/weak-link critical current JCj which is a sensitive (decreasing) function o f temperature
i
I0
20
m
30
8
I
40 5'0 60 T(KJ Fig. 4. Z' and;(" (in arbitrary units) as a function of temperature at various applied DC fields with hAc=O.10e. Curves 1 (85.8, FCR); 2 (85.8, ZFC); 3 (57.2, FCR); 4 (57.2, ZFC); 5 (14.4, FCR); 6 (14.4, ZFC); 7 (57.20e applied at T<50 K and switched off upon warming ); 8 (0, ZFC).
142
G. Rajaram et al. / Shielding studies of electrochemically oxidised La2CuO+
tisation effects in these m e a s u r e m e n t s (the demagnetisation factor would be of the order of 8). The demagnetisation energy c o n t r i b u t i o n to the total free energy may likely result in a more complicated screening current structure as in ferromagnetic domains. Further complications arise if the resultant fields approach the intra-grain critical field Hcj. It is noteworthy in this context, that the Z F C x ' ( T ) at higher fields do not reach the full diamagnetic value even at the lowest temperatures. In summary, we find that superconductivity in an electrochemically oxidised La2CuO4 pellet is not confined to the surface as expected for a process with small diffusion coefficients. The real a n d imaginary parts of the AC magnetic susceptibility in D C fields upto 85 Oe show b e h a v i o u r qualitatively similar to other high-To superconductors. U n u s u a l differences in Z F C a n d F C R data are f o u n d which are probably due to substantial dernagnetisation effect.
Acknowledgements G R would like to thank U G C ( I n d i a ) , CSIR (India) for a travel grant a n d CNRS ( F r a n c e ) for local support.
References [ 1] J.G. Bednorz and K.A. Miiller,Z. Phys. B 64 (1986) 189.
[2]J. Beille, R. Cabanel, C. Chaillout, B. Chevalier, G. Demazeau, F. Deslandes,J. Etourneau, P. Lejay,C. Michel, J. Provost, B. Raveau, A. Sulpice, J. Tholence and R. Tournier, C.R. Acad. Sci. 304 (1987) 1087; J.E. Schirber, B, Morison, R.M. Merill, P.F. Hlava, E.L. Venturini, J.F. Kwak, P.J. Nigrey, R.J. Baughmanand D.S. Ginley, Physica C 152 ( 1988) 121. [3] A. Wattiaux, J.C. Park, J.C. Grenier and M. Pouchard, C.R.Aead. Sci., Srrie II, Paris 310(1990), 1047; N. Laguete, A. Wattiaux,J.C. Park, J.C. Grenier, L. Fournes and M. Pouchard, J. Phys. III (Paris) France 1 ( 1991 ) 1755+ [4] P. Rudolf, W. Paulus and R. SchSllhorn, Adv. Mater. 3 ( 1991 ) 438. [5]R. Suryanarayanan, O. Gorochov, M.S.R. Rao, L. Ouhammou, W. Paulus and G+ Heger, Physica C 185-189 (1991) 573. [ 6 ] C. Rillo, F. Lera, R. Navarro, J. Bartolome, J. Flokstra and D.H.A. Blank, Cryogenics29 (1989) 1128. [ 7 ] M. Nicoloand R,B. Goldfarb,Phys. Rev. B 39 (1989) 6615. t 8 ] J.H.P.M. Emmen, V.A.M. Brabers and H.W.J+M. de Jonge, Physica C 176 (1991) 137. [9] S.L. Shinde, J. Moriella, D. Goland, D.A. Chance and T.R. McGuire, Phys. Rev. B 41 (1990) 8838. [ 10] A. Hammer and G. Srinivasan, J. Appl. Phys. 69 ( 1991 ) 4899. [ 11 ] Y. Yeshurun and A.P. Malozemoff, Phys. Rev. Lett. 60 (1988) 2202; A.P. Malozemoff, T.K. Worthington, Y. Yeshurun, F. Holtzberg and P.H. Kes, Phys. Rev. B 38 (1988) 7203. [12] K.A. Miiller, M. Takashige and J.G. Bednorz, Phys. Rev. Lett. 58 (1987) 1143. [13] J. Manhart, P. Chaudhari, D. Dimos, C.C. Tsui and T.R. McGuire, Phys. Rev, Lett. 61 (1988 ) 2476. [ 14] J.R. Clem, Physica C 153-155 (1988) 50. [ 15] M. Tinkham and C.J. Lobb, Solid State Phys. 42 ( 1989) 91.