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The effect of indium substitution for copper on the superconductivity of the electron-doped system Nd-Ce-Cu-O
Physica C 170 (1990) 211-214 North-Holland
The effect of indium substitution for copper on the superconductivity of the electron-doped system N d - C e - C u - O N . Y . A y o u b l, C.C. A l m a s a n , E.A. E a r l y , J.T. M a r k e r t 2, C.L. S e a m a n a n d M . B . M a p l e Department of Physics and Institute for Pure and Applied Physical Sciences, University of California, San Diego, La Jolla, CA 92093, USA Received 5 April 1990 Revised manuscript received 5 July 1990
Electrical resistivity and magnetic susceptibility measurements on the new electron-doped Nd2_xCexfut_zInzO4_ysuperconductors are reported. Replacing 3% of Cu2+ with In3+ in the non-superconducting compound NdLssCeo.t2CuO4_r yields superconductivity with an onset superconducting critical temperature T¢~ 23 K. The combined substitution of Ce4÷ and In3÷ with x+ z in the range 0.15 to 0.17 affects Tc in a similar way as the substitution of Ce4+ alone (z= 0) where electrons are supplied from outside the CuO2 planes. Furthermore, for x+ z= 0.15 (i.e., constant electron concentration ), we find that Tc remains relatively constant ( ~ 24 K), while ATe increases with increasing z from 4 K at z = 0 to 19.4 K at z = 0.04.
1. Introduction Substitutions o f various elements for Cu 2+ in the h o l e - d o p e d high t e m p e r a t u r e superconductors ( H T S C ) such as La2_xMxfuO4_y ( M = B a , Sr, Ca, Na, ...) [ 1 ] a n d RBa2Cu307_~ ( R = Y or a lanthanide e l e m e n t ) [2 ] have a t t r a c t e d m u c h attention in the literature [ 3 ]. All o f the Cu 2÷ substitutions that have been r e p o r t e d result in a decrease in Tc [4 ], p r e s u m a b l y because o f a decrease in the n u m b e r o f holes in the conducting CuO2 planes a n d superconducting electron p a i r b r e a k i n g due to exchange scattering o f mobile holes by localized magnetic moments. One way o f enhancing s u p e r c o n d u c t i v i t y in the a b o v e systems m a y be to increase the concentration o f holes in the CuO2 planes via m o n o v a l e n t element substitutions for the d i v a l e n t Cu. However, the discovery o f the new e l e c t r o n - d o p e d H T S C ' s , Ln2_xMxCuO4_y ( L n = P r , Nd, Sm, Eu; M = C e , T h ) [ 5 - 8 ] , and Nd2CuO4_x_yFx [9] yields a larger choice o f suitable substituents for Cu 2÷ which ini Permanent address: Department of Physics, Yarmouk University, Irbid, Jordan. 2 Present address: Department of Physics, University of Texas, Austin, Texas 78712.
crease the concentration o f electrons. T h e elements with c o m p a r a b l e radii to Cu 2÷ a n d that will increase the electron concentration include elements like G a 3+, In 3+, ml3+, Sn 4+ a n d T P +. In the system Nd2_xCexCuO4_y, the superconducting critical temperature Tc peaks a r o u n d x = 0.15 [5] and vanishes at about x = 0 . 1 3 [ 10]. In particular, no superconductivity is f o u n d at x = 0 . 1 2 . On the basis o f the valence count, the replacement o f 3% Cu 2+ with In 3+ in Nd2_xCexCuO4_y with x = 0.12 will yield the same concentration o f electrons as the i n d i u m free system with x = 0 . 1 5 . In this paper, we show that the system Nd2_xCexCUl_zInzO4_y has a similar superconducting behaviour as the Ce 4+-dOped system Nd2_~CexCuO4_y, a n d that substituting In 3+ for Cu 2+ can extend the range o f superconductivity down to x = 0.12. Finally, low field magnetic susceptibility m e a s u r e m e n t s for the samples with x = 0 . 1 2 a n d various z values indicate the presence o f bulk superconductivity in our i n d i u m - d o p e d system.
2. Experimental Nine polycrystalline samples of Nd2_xCexCu~_zlnzOa_y were prepared by solid-state
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N. Y. Ayoub et al. / Replacing Cu with In in the Nd-Ce-Cu-O system
reaction of mixtures of high purity (99.99%) CuO, Nd203, CeO2 and In203. These samples will be referred to as samples 1 to 9; i.e., ( 1 ) x=0.12, z=0.01, (2) x=0.12, z=0.02, (3) x=0.12, z=0.03, (4) x=0.12, z=0.04, (5) x=0.12, z=0.05, (6) x=0.13, z=0.02, (7) x=0.15, z = 0 , (8) x=0.15, z=0.01, (9) x=0.15, z=0.02. It is noteworthy that samples 1 through 5 have a constant x of 0.12, that samples 3 and 6 have the restriction x + z = 0 . 1 5 , and finally that samples 7,8, and 9 have a constant x of 0.15. Due to the volatile nature of In203 at about 850°C, the powders were mixed, ground and fired at 650°C for twelve days with intermediate grinding. These initial firings apparently facilitate incorporation of indium into the ultimate structure, either directly or perhaps through the formation of intermediate compounds. Subsequently, the specimens were fired twice at 1000°C for 12 h each time with intermediate grinding. Finally the samples were pressed into pellets at ~ 4 kbar pressure, which were fired at 1100 ° C for 15 h and then cooled to room temperature over a period of 4 h. In order to achieve superconductivity, the specimens were annealed in flowing helium at 950°C for 18 h and then cooled to room temperature over 2 h. These annealing conditions resulted in maximum transition temperatures in all samples. X-ray powder diffraction measurements showed that all nine samples are single-phase compounds with the Nd2fuO4 structure. Four probe electrical resistivity measurements were made using a Linear Research Model LR 400 AC resistance bridge on bar-shaped samples with dimensions 1 × 2 × 7 m m 3 which had been cut with a diamond wheel saw from the annealed pellets. Platinum electrical leads were attached with silver epoxy to gold film contact pads. Static magnetic susceptibility measurements were performed in a field of 10 Oe, using an SHE Model VTS-50 SQUID magnetometer. The demagnetizing effects were taken into account.
3. Results and discussion
Plots of the normalized electrical resistivity, p (T) / p(60 K), versus temperature for five samples are displayed in fig. 1. The superconducting transition temperatures Tc(o.~), TCto.5) and T~(o.9) at which p ( T )
drops to 10%, 50% and 90%, respectively, of its maximum value, and the transition widths ATe-Tc(o ~)-Tc(o.9) for seven samples are listed in table I. The first new result is that sample 3 with x = 0.12, z--0.03, superconducting, whereas the sample with x = 0.12, z = 0, is not. This result may be interpreted in terms of electrons donated by In 3÷ ions which reside in the CuO2 planes, where the concentration of electrons in sample 3 is similar to that in the superconducting compound with x = 0.15, z = 0. Similarly, from ref. [10] we note that the compound with x=0.13, z = 0 , is weakly superconducting with Tc(o.5)--14 K, whereas sample 6 with x=0.13, z=0.02, has a Tc(o.5)~20 K, which is closer to Tc(o.~) ~ 3 K of sample 7 with x = 0.15, z = 0. From the above results, we conclude that, in general, the electron concentration is an important factor in determing the superconducting critical temperature To. From samples 7 (x=0.15, z = 0 ) , 8 (x=0.15, z = 0 . 0 1 ) and 9 (x=0.15, z = 0 . 0 2 ) , where 0.15_< x+z_<0.17, we observe that Tc(o ~) is only slightly affected by the indium doping, which is similar to the effect of Ce 4+ substitutions on Tc(o.~) in the system Nd2_xCexCuO4_y for 0.15_
N. Y. Ayoub et al. / Replacing Cu with In in the Nd-Ce-Cu-O system
213
Nd2-xCexCu 1-zlnzO4-y 1.5 (a)
(b)
1.0
,
0.5 O 0 Q..
=x 0.13 , z = 0.02
floiioi2
(d)
(c) ca
~
• x = 0.12, z= 0.03
1.0
x=0.15,
0.5
0 0
20
40
0 20 TEMPERATURE (K)
40
60
Fig. 1. Normalized electrical resistivity, p( T)/p(60 K), vs. temperature Tfor five Nd2_xCexCUl_zlnzO4_ysuperconductors. Table I Compositions x, and z, superconducting transition temperatures Tcto.,)where the resistivity drops by 0.n of its maximum value for n = 1, 5 and 9, and the transition width ATe =- TctoI ) - Tcto.9),for Nd2_xCexCul-zlnzO4-r Sample
Composition (x, z)
T¢(ol ) (K)
T¢(o.5 ~(K)
Tc(o.9)(K)
ATe (K) ~ Te(o.1) - -
2 3 4 6 7 8 9
x-0.12, x-0.12, x=0.12, x=0.13, x=0.15, x=0.15, x-0.15,
21.0 23.0 22.0 23.8 24.8 24.3 24.3
15.4 17.0 10.5 19.5 22.7 20.5 19.5
10.7 12.2 2.6 17.8 20.8 19.2 17.0
10.3 10.8 19.4 6.0 4.0 5.1 7.3
z=0.02 z=0.03 z=0.04 z=0.02 z=0.00 z=0.01 z=0.02
width o f transition ATe is a p p a r e n t l y governed by other factors, such as the ionic radius o f the substituent ion, etc. Finally, displayed in fig. 2 are the zero-field-cooled ( Z F C ) d i a m a g n e t i c fraction, -4nZ, where Z is defined as M / H , a n d the resistive superconducting transition t e m p e r a t u r e Tc ( d e f i n e d here as the midpoint o f the resistive t r a n s i t i o n ) , plotted as a function o f x + z . The d a t a in fig. 2 reveal the narrow concentration range over which superconductivity occurs in the system with x = 0 . 1 2 a n d z = 0 . 0 1 , 0.02, 0.03,
T¢(0.9)
0.04 and 0.05. The values o f the field cooled ( F C ) diamagnetic fraction are about 50% o f the corresponding Z F C values. Although the values are relatively small ( ~ 3%), considering the fact that no subtraction o f the paramagnetic contribution o f N d 3+ was made, they clearly indicate the presence o f bulk superconductivity. Also, the magnetic b e h a v i o u r o f the system with constant x - - 0 . 1 2 a n d various ind i u m concentration is quite similar to the b e h a v i o u r o f the N d - C e - C u - O system with various Ce dopings [10].
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Acknowledgements
Ndl.88Ceo.l 2CUl.zlnzO4_y 40
We acknowledge useful discussions with Y. Dalichaouch. This work was supported by the U.S. Department of Energy under Grant No. DE-FG03-86ER45230 and the CIES Fulbright Fellowship Program (NYA).
H = 10Oe "1- To( P/Pmax = 0.5)
E
z(6K) ZFC
3o
"~ 20
"7, References
lO
0
0.10
0.12
0.14
0.16
0.18
0.20
X+Z
Fig. 2. Diamagnetic fraction, as determined by the zero-fieldcooled magnetic susceptibility in a field H = 10 Oe, and the resistive transition midpoint, vs. x + z , with x=0.12, for the system Nd j ssCe0, lzCu i - zlnzO4_y.
4. Conclusions
We have extended the range of superconductivity down to x = 0 . 1 2 for the N d - C e - C u - O system via In s+ substitution for Cu a+ and established that: (i) the electron concentration in these systems plays a major role in determining the onset of superconductivity; (ii) it is apparently unimportant whether the electrons involved in the onset of superconductivity are supplied from outside or within the CuO2 planes; and, finally, (iii) ATe increases with higher indium doping.
[ 1 ] J.G. Bednorz and K.A. Miiller, Z. Phys. B64 (1986) 189. [2] M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hu, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. Lett. 58 (1987) 908. [3] See, for example, J.T. Markert, Y. Dalichaouch and M.B. Maple, in: Physical Properties of High Temperature Superconductors I, ed. D.M. Ginsberg (World Scientific, Singapore, 1989) Ch. 6. [4] S.B. Oseroff, D.C. Vier, J.F. Smyth, C.T. Sailing, S. Schultz, Y. Dalichaouch, B.W. Lee, M.B. Maple, Z. Fisk, J.D. Thompson, J.L. Smith and E. Zirngiebl, Solid State Commun. 64 ( 1987)241. [5] Y. Tokura, H. Takagi and S. Uchida, Nature 337 (1986) 345. [6] J.T. Markert and M.B. Maple, Solid State Commun. 70 (1989) 145. [ 7 ] J.T. Markert, E.A. Early, T. Bjornholm, S. Ghamaty, B.W. Lee, J.J. Neumeier, R.D. Price, C.L. Seaman and M.B. Maple, Physica C 158 (1989) 178. [8] E.A. Early, N.Y. Ayoub, J. Beille, J.T. Markert and M.B. Maple, Physica C 160 (1989) 320. [9] A.C.W.P. James, S.M. Zahurak and D.W. Murphy, Nature 338 (1989) 240. [ 10] N.Y. Ayoub, J.T. Markert, E.A. Early, C.L. Seaman, L. Paulius and M.B. Maple, Physica C 165 (1990) 469. [ 11 ] I. Felner, U. Yaron, Y. Yeshurun, E.R. Yacoby and Y. Wolfus, Phys. Rev. B40 (1989) 11366.
The effect of indium substitution for copper on the superconductivity of the electron-doped system N d - C e - C u - O N . Y . A y o u b l, C.C. A l m a s a n , E.A. E a r l y , J.T. M a r k e r t 2, C.L. S e a m a n a n d M . B . M a p l e Department of Physics and Institute for Pure and Applied Physical Sciences, University of California, San Diego, La Jolla, CA 92093, USA Received 5 April 1990 Revised manuscript received 5 July 1990
Electrical resistivity and magnetic susceptibility measurements on the new electron-doped Nd2_xCexfut_zInzO4_ysuperconductors are reported. Replacing 3% of Cu2+ with In3+ in the non-superconducting compound NdLssCeo.t2CuO4_r yields superconductivity with an onset superconducting critical temperature T¢~ 23 K. The combined substitution of Ce4÷ and In3÷ with x+ z in the range 0.15 to 0.17 affects Tc in a similar way as the substitution of Ce4+ alone (z= 0) where electrons are supplied from outside the CuO2 planes. Furthermore, for x+ z= 0.15 (i.e., constant electron concentration ), we find that Tc remains relatively constant ( ~ 24 K), while ATe increases with increasing z from 4 K at z = 0 to 19.4 K at z = 0.04.
1. Introduction Substitutions o f various elements for Cu 2+ in the h o l e - d o p e d high t e m p e r a t u r e superconductors ( H T S C ) such as La2_xMxfuO4_y ( M = B a , Sr, Ca, Na, ...) [ 1 ] a n d RBa2Cu307_~ ( R = Y or a lanthanide e l e m e n t ) [2 ] have a t t r a c t e d m u c h attention in the literature [ 3 ]. All o f the Cu 2÷ substitutions that have been r e p o r t e d result in a decrease in Tc [4 ], p r e s u m a b l y because o f a decrease in the n u m b e r o f holes in the conducting CuO2 planes a n d superconducting electron p a i r b r e a k i n g due to exchange scattering o f mobile holes by localized magnetic moments. One way o f enhancing s u p e r c o n d u c t i v i t y in the a b o v e systems m a y be to increase the concentration o f holes in the CuO2 planes via m o n o v a l e n t element substitutions for the d i v a l e n t Cu. However, the discovery o f the new e l e c t r o n - d o p e d H T S C ' s , Ln2_xMxCuO4_y ( L n = P r , Nd, Sm, Eu; M = C e , T h ) [ 5 - 8 ] , and Nd2CuO4_x_yFx [9] yields a larger choice o f suitable substituents for Cu 2÷ which ini Permanent address: Department of Physics, Yarmouk University, Irbid, Jordan. 2 Present address: Department of Physics, University of Texas, Austin, Texas 78712.
crease the concentration o f electrons. T h e elements with c o m p a r a b l e radii to Cu 2÷ a n d that will increase the electron concentration include elements like G a 3+, In 3+, ml3+, Sn 4+ a n d T P +. In the system Nd2_xCexCuO4_y, the superconducting critical temperature Tc peaks a r o u n d x = 0.15 [5] and vanishes at about x = 0 . 1 3 [ 10]. In particular, no superconductivity is f o u n d at x = 0 . 1 2 . On the basis o f the valence count, the replacement o f 3% Cu 2+ with In 3+ in Nd2_xCexCuO4_y with x = 0.12 will yield the same concentration o f electrons as the i n d i u m free system with x = 0 . 1 5 . In this paper, we show that the system Nd2_xCexCUl_zInzO4_y has a similar superconducting behaviour as the Ce 4+-dOped system Nd2_~CexCuO4_y, a n d that substituting In 3+ for Cu 2+ can extend the range o f superconductivity down to x = 0.12. Finally, low field magnetic susceptibility m e a s u r e m e n t s for the samples with x = 0 . 1 2 a n d various z values indicate the presence o f bulk superconductivity in our i n d i u m - d o p e d system.
2. Experimental Nine polycrystalline samples of Nd2_xCexCu~_zlnzOa_y were prepared by solid-state
0921-4534/90/$03.50 © 1990 - Elsevier Science Publishers B.V. (North-Holland)
212
N. Y. Ayoub et al. / Replacing Cu with In in the Nd-Ce-Cu-O system
reaction of mixtures of high purity (99.99%) CuO, Nd203, CeO2 and In203. These samples will be referred to as samples 1 to 9; i.e., ( 1 ) x=0.12, z=0.01, (2) x=0.12, z=0.02, (3) x=0.12, z=0.03, (4) x=0.12, z=0.04, (5) x=0.12, z=0.05, (6) x=0.13, z=0.02, (7) x=0.15, z = 0 , (8) x=0.15, z=0.01, (9) x=0.15, z=0.02. It is noteworthy that samples 1 through 5 have a constant x of 0.12, that samples 3 and 6 have the restriction x + z = 0 . 1 5 , and finally that samples 7,8, and 9 have a constant x of 0.15. Due to the volatile nature of In203 at about 850°C, the powders were mixed, ground and fired at 650°C for twelve days with intermediate grinding. These initial firings apparently facilitate incorporation of indium into the ultimate structure, either directly or perhaps through the formation of intermediate compounds. Subsequently, the specimens were fired twice at 1000°C for 12 h each time with intermediate grinding. Finally the samples were pressed into pellets at ~ 4 kbar pressure, which were fired at 1100 ° C for 15 h and then cooled to room temperature over a period of 4 h. In order to achieve superconductivity, the specimens were annealed in flowing helium at 950°C for 18 h and then cooled to room temperature over 2 h. These annealing conditions resulted in maximum transition temperatures in all samples. X-ray powder diffraction measurements showed that all nine samples are single-phase compounds with the Nd2fuO4 structure. Four probe electrical resistivity measurements were made using a Linear Research Model LR 400 AC resistance bridge on bar-shaped samples with dimensions 1 × 2 × 7 m m 3 which had been cut with a diamond wheel saw from the annealed pellets. Platinum electrical leads were attached with silver epoxy to gold film contact pads. Static magnetic susceptibility measurements were performed in a field of 10 Oe, using an SHE Model VTS-50 SQUID magnetometer. The demagnetizing effects were taken into account.
3. Results and discussion
Plots of the normalized electrical resistivity, p (T) / p(60 K), versus temperature for five samples are displayed in fig. 1. The superconducting transition temperatures Tc(o.~), TCto.5) and T~(o.9) at which p ( T )
drops to 10%, 50% and 90%, respectively, of its maximum value, and the transition widths ATe-Tc(o ~)-Tc(o.9) for seven samples are listed in table I. The first new result is that sample 3 with x = 0.12, z--0.03, superconducting, whereas the sample with x = 0.12, z = 0, is not. This result may be interpreted in terms of electrons donated by In 3÷ ions which reside in the CuO2 planes, where the concentration of electrons in sample 3 is similar to that in the superconducting compound with x = 0.15, z = 0. Similarly, from ref. [10] we note that the compound with x=0.13, z = 0 , is weakly superconducting with Tc(o.5)--14 K, whereas sample 6 with x=0.13, z=0.02, has a Tc(o.5)~20 K, which is closer to Tc(o.~) ~ 3 K of sample 7 with x = 0.15, z = 0. From the above results, we conclude that, in general, the electron concentration is an important factor in determing the superconducting critical temperature To. From samples 7 (x=0.15, z = 0 ) , 8 (x=0.15, z = 0 . 0 1 ) and 9 (x=0.15, z = 0 . 0 2 ) , where 0.15_< x+z_<0.17, we observe that Tc(o ~) is only slightly affected by the indium doping, which is similar to the effect of Ce 4+ substitutions on Tc(o.~) in the system Nd2_xCexCuO4_y for 0.15_
N. Y. Ayoub et al. / Replacing Cu with In in the Nd-Ce-Cu-O system
213
Nd2-xCexCu 1-zlnzO4-y 1.5 (a)
(b)
1.0
,
0.5 O 0 Q..
=x 0.13 , z = 0.02
floiioi2
(d)
(c) ca
~
• x = 0.12, z= 0.03
1.0
x=0.15,
0.5
0 0
20
40
0 20 TEMPERATURE (K)
40
60
Fig. 1. Normalized electrical resistivity, p( T)/p(60 K), vs. temperature Tfor five Nd2_xCexCUl_zlnzO4_ysuperconductors. Table I Compositions x, and z, superconducting transition temperatures Tcto.,)where the resistivity drops by 0.n of its maximum value for n = 1, 5 and 9, and the transition width ATe =- TctoI ) - Tcto.9),for Nd2_xCexCul-zlnzO4-r Sample
Composition (x, z)
T¢(ol ) (K)
T¢(o.5 ~(K)
Tc(o.9)(K)
ATe (K) ~ Te(o.1) - -
2 3 4 6 7 8 9
x-0.12, x-0.12, x=0.12, x=0.13, x=0.15, x=0.15, x-0.15,
21.0 23.0 22.0 23.8 24.8 24.3 24.3
15.4 17.0 10.5 19.5 22.7 20.5 19.5
10.7 12.2 2.6 17.8 20.8 19.2 17.0
10.3 10.8 19.4 6.0 4.0 5.1 7.3
z=0.02 z=0.03 z=0.04 z=0.02 z=0.00 z=0.01 z=0.02
width o f transition ATe is a p p a r e n t l y governed by other factors, such as the ionic radius o f the substituent ion, etc. Finally, displayed in fig. 2 are the zero-field-cooled ( Z F C ) d i a m a g n e t i c fraction, -4nZ, where Z is defined as M / H , a n d the resistive superconducting transition t e m p e r a t u r e Tc ( d e f i n e d here as the midpoint o f the resistive t r a n s i t i o n ) , plotted as a function o f x + z . The d a t a in fig. 2 reveal the narrow concentration range over which superconductivity occurs in the system with x = 0 . 1 2 a n d z = 0 . 0 1 , 0.02, 0.03,
T¢(0.9)
0.04 and 0.05. The values o f the field cooled ( F C ) diamagnetic fraction are about 50% o f the corresponding Z F C values. Although the values are relatively small ( ~ 3%), considering the fact that no subtraction o f the paramagnetic contribution o f N d 3+ was made, they clearly indicate the presence o f bulk superconductivity. Also, the magnetic b e h a v i o u r o f the system with constant x - - 0 . 1 2 a n d various ind i u m concentration is quite similar to the b e h a v i o u r o f the N d - C e - C u - O system with various Ce dopings [10].
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N. Y. Ayoub et al. / Replacing Cu with In in the N d - C e - C u - O system
Acknowledgements
Ndl.88Ceo.l 2CUl.zlnzO4_y 40
We acknowledge useful discussions with Y. Dalichaouch. This work was supported by the U.S. Department of Energy under Grant No. DE-FG03-86ER45230 and the CIES Fulbright Fellowship Program (NYA).
H = 10Oe "1- To( P/Pmax = 0.5)
E
z(6K) ZFC
3o
"~ 20
"7, References
lO
0
0.10
0.12
0.14
0.16
0.18
0.20
X+Z
Fig. 2. Diamagnetic fraction, as determined by the zero-fieldcooled magnetic susceptibility in a field H = 10 Oe, and the resistive transition midpoint, vs. x + z , with x=0.12, for the system Nd j ssCe0, lzCu i - zlnzO4_y.
4. Conclusions
We have extended the range of superconductivity down to x = 0 . 1 2 for the N d - C e - C u - O system via In s+ substitution for Cu a+ and established that: (i) the electron concentration in these systems plays a major role in determining the onset of superconductivity; (ii) it is apparently unimportant whether the electrons involved in the onset of superconductivity are supplied from outside or within the CuO2 planes; and, finally, (iii) ATe increases with higher indium doping.
[ 1 ] J.G. Bednorz and K.A. Miiller, Z. Phys. B64 (1986) 189. [2] M.K. Wu, J.R. Ashburn, C.J. Torng, P.H. Hu, R.L. Meng, L. Gao, Z.J. Huang, Y.Q. Wang and C.W. Chu, Phys. Rev. Lett. 58 (1987) 908. [3] See, for example, J.T. Markert, Y. Dalichaouch and M.B. Maple, in: Physical Properties of High Temperature Superconductors I, ed. D.M. Ginsberg (World Scientific, Singapore, 1989) Ch. 6. [4] S.B. Oseroff, D.C. Vier, J.F. Smyth, C.T. Sailing, S. Schultz, Y. Dalichaouch, B.W. Lee, M.B. Maple, Z. Fisk, J.D. Thompson, J.L. Smith and E. Zirngiebl, Solid State Commun. 64 ( 1987)241. [5] Y. Tokura, H. Takagi and S. Uchida, Nature 337 (1986) 345. [6] J.T. Markert and M.B. Maple, Solid State Commun. 70 (1989) 145. [ 7 ] J.T. Markert, E.A. Early, T. Bjornholm, S. Ghamaty, B.W. Lee, J.J. Neumeier, R.D. Price, C.L. Seaman and M.B. Maple, Physica C 158 (1989) 178. [8] E.A. Early, N.Y. Ayoub, J. Beille, J.T. Markert and M.B. Maple, Physica C 160 (1989) 320. [9] A.C.W.P. James, S.M. Zahurak and D.W. Murphy, Nature 338 (1989) 240. [ 10] N.Y. Ayoub, J.T. Markert, E.A. Early, C.L. Seaman, L. Paulius and M.B. Maple, Physica C 165 (1990) 469. [ 11 ] I. Felner, U. Yaron, Y. Yeshurun, E.R. Yacoby and Y. Wolfus, Phys. Rev. B40 (1989) 11366.