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Comparative study of the pressure effect on critical parameters of GdBa2Cu4O8 and YBa2Cu4O8
ELSEVIER
Physica C 241 ( 1995) 383-388
Comparative study of the pressure effect on critical parameters of GdBaECU408 and YBa2Cu408 M. Baran *, V. Dyakonov 1, L. Gtadczuk, G. Levchenko ~, S. Piechota, H. Szymczak Institute of Physics, PolishAcademy of SciencesAl. Lotnik6w 32/46, 02-668 Warszawa,Poland Received 20 September 1994
Abstract
The influence of hydrostatic pressure up to 1.1 GPa on critical parameters (To, Hc~, j~) of YBa2Cu4Os (Y 124) and GdBa2Cu4Os (Gd124) ceramics has been investigated using a CuBe pressure bomb. A linear increase of Tc with the growth of pressure was found, with d Tc/dp = 5.5 K/GPa and 4.5 K/GPa for Y124 and Gd 124, respectively. The difference in Tc increase on pressure, dTJdp, between the two compounds is assumed to have the following possible mechanisms: first, the replacement of yttrium by gadolinium constrains the change of interionic distances within and between the CuO2 planes and thus the change of charge transfer integral having an effect on the density of electronic states at CuO2 planes; secondly, this replacement modifies the charge transfer into the CuO2 planes from rare-earth sites. It was found that the critical current density increased a little under pressure; nevertheless a different behaviour on temperature has been observed for the two compounds. Insignificant changes of Hot under pressure were found at temperatures up to 40 K. However, at higher temperatures a little growth of Hc~ was observed.
1. Introduction
From the very beginning of the HTSC era the importance of high pressure investigation has been realized. Chu et al. showed [ 1 ] that by applying a hydrostatic pressure to the LaBaCuO system a significant growth o f critical temperature could be achieved with the value of dTc/dp equal to 6.1 K / GPa. In fact, this large pressure effect led to the very successful idea [ 2 ] o f replacing La by the smaller Y, resulting in T~ above 90 K for YBaECU307_6. A large number of papers have appeared on the effect of pressure on high-temperature superconductors. However, it is difficult to say that the actual knowl* Corresponding author. ~Permanent address: Institute of Physics and Technology, Ukrainian Academy of Sciences 340114 Donetsk, Ukraine.
edge allows for a full understanding o f the microscopic mechanism responsible for changes o f the critical temperature under pressure. The reason for that is the very complex character of the pressure dependence o f the critical temperature. In the case of YBa2Cu3Ox with the value of x close to 7, the slope of the pressure effect on T¢, dTc/ dp, is equal to about 0.4 K / G P a . With the change of x down to 6.77 an abrupt increase ofdT¢/dp was observed [ 3 ] up to 4 K / G P a . A sharp maximum in dT¢/ dp dependence on x was observed for x ~ 6.7 [4,5]. From these experiments it was concluded that anomalies in the pressure dependence o f critical temperature for YBa2Cu306 +x (Y 123 ) should be connected with oxygen structure (oxygen content, distribution and ordering). To examine this view it is important to study the pressure dependence o f the superconducting properties for ReBa2Cu4Os compounds
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M. Baran et al. /Physica C 241 (1995) 383-388
(Re 124) in which the oxygen concentration is constant, oxygen vacancies are absent and oxygen migration does not occur. It should be noted that Rel24 compounds, in comparison to Y123, have one additional CuO chain layer in the unit cell and a lower value of T¢. The number of high pressure experiments carried out on YBa2Cu408 have made a significant contribution to our knowledge of this material. Unlike the fully oxygenated YBaECU307,the Y124 compound with Tc ~ 80 K has a rather large Tc pressure dependence, with dT¢/dp equal to 5-6 K / G P a [6,7]. Both the large and small values of dTc/dp for Y124 and Y123, respectively, have been attributed to a pressure-induced charge (hole) transfer mechanism [811 ]. As Y 124 shows superconductivity in the stoichiometric composition in which it has exactly one hole per chemical formula unit and has a very stable oxygen content, the various experiments can be performed under a well-defined hole concentration. It is established that there is a good correlation between transition temperature Tc and the density of holes in the CuO2 plane [ 12,13 ]. But, at present, the mechanisms of the change of the hole density in the CuO2 planes and of the charge redistribution in YBaCuO, especially under pressure, are not clear. The possible mechanisms playing a role in the change of hole density under pressure which should be discussed are the following: (1) A change of the charge density in the CuO2 plane due to variation in the Cu(2)-apical oxygen O ( 1 ) bond length [ 7 ]. (2) The charge density redistribution between Cu and O in the CuO2 plane and, respectively, the Tc relation to the change of the electronic state of the CuO2 layer due to modification of the Cu-O (2,3) squarepyramid shape [ 14 ]. (3) The charge density redistribution between the CuO2 plane and the ionic elements, Y and Ba [ 15 ]. Additional investigations are necessary to understand the role of these mechanisms in the formation of the superconducting state under pressure. Studies of the pressure effect on superconducting properties of ReBa:Cu4Os (Re - the rare-earth ion) can throw some light on these mechanisms. The Re ion as well as Y and Ba have to play a definite role in the formation of the electronic state of the CuO2 plane and to have some influence on the T¢ value. It is neces-
sary to note that the role of the ionic elements has been fully ignored in the interpretation of the pressure dependence of the superconducting transition temperature. In this paper we present some results of measurements of the pressure effect on critical parameters ( To, j~) of the GdBa2Cu4Oa (GD124) and YBa2Cu408 (Y 124 ) superconducting compounds.
2. Sample preparation and experimental technique The investigated ceramic samples were obtained using the conventional method of solid state reaction under atmospheric pressure of oxygen. Besides the main components of the investigated compounds, an admixture of calcium with the concentration of 0.1 atom per formula unit was also applied. It is known that in the case of Y124 the replacement of 10% of yttrium by calcium stabilizes the 124 structure [ 16,17 ] and shifts Tc toward higher values [ 18 ]. In our Rel24 samples Ca replaces both Y and Ba in practically the same amounts. The crystal structure and the phase purity of Re0.95Bal.95Cao.lCu408 compounds have been determined by powder X-ray diffraction at room temperature (using the Cu Ks line of diffractometer DRON-I.5). The samples were found to be of essentially a single-phase Re 124 material. The magnetic measurements also do not reveal any traces of 123-phase. The hydrostatic pressure was generated in the highpressure bomb (HPB) constructed as a piston-cylinder device made of fully hardened beryllium copper [ 19 ]. A 1 : 1 mixture of transformer oil and kerosene was used as a hydrostatic pressure-transmitting medium. The cylindrical samples of approximately 1 mm diameter and 5 mm length were placed in the HPB (external diameter - 4 mm, inner diameter - 1.43 mm). The necessary pressure was generated at room temperature. After cooling to low temperature, the pressure in the working canal of HPB decreased by about 0.15-0.2 GPa. At the low temperatures the pressure was verified by measuring the shift of Sn superconducting transition temperature. According to our estimation the error in the pressure value should not exceed + 0.05 GPa. It was verified that the con-
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M. Baran et al. / Physica C 241 (1995) 383-388
tribution of HPB to the total magnetization value was negligible. The change of the superconducting transition temperature under pressure has been determined from the temperature dependence of the magnetization at some fixed values of the pressure. The DC magnetization measurements of samples were carried out with a vibrating sample magnetometer (PAR, Model 4500) in a magnetic field of 1 mT, after cooling in a zero field (ZFC), and in the temperature range 4.2100 K. The critical current densities were determined from the magnitude of the hysteresis of the DC magnetization. The magnetic hysteresis loops were measured in magnetic fields up to 1.5 T at various temperatures.
3. Experimental results and discussion
0
20
40 T (K) 60
80
-0.2.
.z2.
100
~,o
0.0-
--o--p= 0 GPa --o--p=0.9 GPa
/i
-0,4-
[]
O
-0.6-0.8-
-1.0-
Fig. 2. Magnetization of Y 124 as a function of temperature in magnetic field of 1 mT, without pressure and under pressure of 0.93 GPa. 6
In Figs. 1 and 2 there are presented the temperature dependences of magnetization for Gd 124 and for Y 124 in magnetic field of I mT, both without pressure as well as under pressure. As can be seen, the M ( T ) curves are shifted to higher temperatures when measured under pressure, i.e. T¢ under pressure increases. The superconducting transition width decreases with pressure in the case of Gd 124, while for Y 124 it is practically constant. To determine the difference of Tc induced by pressure, ATe, it was necessary to choose what definition of Tc would be used. It
4
2- S
0~ 0.00
Gd124
0.25
0.50
0.75
1.00
P (GPa) Fig. 3. ATe as a function of pressure for Y 124 and Gd 124.
T (K) 40 60
20 ,
I
,
I
,
80
P
,
I
100 ~
I
0.0-0.2-0.4.
~
[]
-
[]
-0.6-0.8-1.0-
/
/
~ / / - - o - -
~mt~rg~W'Uo~o~O~°
p = 0 GPa
--o--p= i.i GPa
Fig. 1. Temperature dependence of magnetization of Gdo.95Bal.gsCao.]Cu4Os in magnetic field of I mT, without pressure and under pressure of 1.1 GPa.
was decided to use as the Tc value a temperature corresponding to the intersection of the tangent to the M ( T ) relation at its maximum slope with the level of M = 0 . It should be noted that in the case of Gd124, because of the influence of pressure on the transition width, values of ATe are lower than those determined from the midpoint of the transition. In Fig. 3 the pressure dependences of ATe, for both examined compounds, are presented. As can be seen, ATe increases linearly with growing pressure for both compounds. In the case of Y 124 the obtained value of dTJdp is equal to + 5.5 K / G P a , which is consistent with the former results [6,7]. The value of dTJdp for Gd124 has been estimated as equal to +4.5 K / GPa.
386
M. Baran et al. / Physica C 241 (1995) 383-388
The increase of Tc can be interpreted as being due to a change of hole transfer into the C u O 2 layers under pressure and pressure-induced charge redistribution among the ionic sites. It should be stressed that in Re 124 the double CuO chains are less compressible than the CuO2 layers. It was observed in Ref. [ 8 ] that Cu ( 2 )-O ( 1 ) bonds, bridging the CuO2 layers to the chains, shorten with pressure. These changes cause the charge response of the CuO chain electron reservoirs which affects the hole density in the CuO2 planes. Besides the charge transfer mechanism between CuO2 layers and CuO chains, a possible reason for Tc growth under pressure is such an intrinsic effect as the change in interatomic distances within CuO2 planes or between them. It is impossible to ignore the contribution of ionic elements (Y, Ba, Gd) in the electronic charge redistribution under pressure. This can be seen from the different values of dTddp for Gd124 and Y124. As is shown in Ref. [ 15], significant changes of the electronic charge occur at ionic sites (Y, Ba) under pressure. The electronic charge density at these positions always increases under pressure. This accumulation of electronic charge at the ionic sites has to be connected with a rise of the hole density at the CuO2 planes. The difference between dTJdp in Y124 and Gd 124 is obviously related to the difference of y3+ and Gd 3+ ionic radii. As the result of larger ionic radius, the Gd 3+ ions more weakly influence the electronic state of the CuO2 planes than y3+ ions. It would be noted that the replacement of the Y ion by a larger rare-earth Gd ion at the site located between two CuO2 planes increases both the lattice parameters and C u ( 2 ) - C u ( 2 ) distance. This also involves changes ofinterionic distances within the CuO2 layer itself. It is a consequence of the negative "internal structural pressure" which alters the lengths of interatomic bonds. It is known that the Cu-O bonds in the C u ( 2 ) - O layers are compressed. The negative "internal pressure", increasing the lattice parameters a and b, decreases the compression of these Cu-O bonds, i.e. both C u - O ( 2 ) and C u - O ( 3 ) bonds lengthen. The increase of Cu-O distances in the Cu ( 2 ) - O planes decreases the Cu-O molecular orbital overlap. This promotes a decrease of the hole density in the Cu (2) - O planes. The above shows that the changes of both electronic state of the CuO2 planes and T~ in Gd124 are of structural origin. The lower
value ofdTc/dP for Gd124 in comparison with Y124 confirms that the changes both in the Cu ( 2 ) - C u (2) distance and within the CuO2 plane play an important role in the pressure dependence of To. In the large rare-earth Gd 124 compound the additional holes are less effectively transferred into the CuO2 planes under pressure than in Y 124. A deflection of the M(H) relation from linearity allowed us to estimate values of lower critical He1 in the low magnetic field region (up to 100 mT). A practically linear decrease of H~ was found with increasing temperature in the range 4.2-60 K. In the range of temperature up to 50 K for Y124 and 40 K for Gd124 only insignificant changes of H~ were obtained. For higher temperatures (up to 60 K) a tendency of deviation from linear behaviour of H~ (to higher values) was observed. The hysteresis M-H loops of Gd124 and Y124 measured in magnetic fields up to 1.5 T have different shapes, which is determined by the strong paramagnetic contribution of gadolinium, especially at the lower temperatures. The area of the hysteresis loops increased under pressure for both compounds. In Figs. 4 and 5 the effect of pressure on the hysteresis loops at 4.2 K for Y 124 and Gd 124 is shown. From the hysteresis loops the change of the critical current density under pressure can be estimated on the basis of Bean's critical state model [20]. Assuming rather typical average grain size in the ceramics ( 10 p.m), values of intragranular critical current density j~ were estimated. In Figs. 6 and 7 the effect of pressure on the
2_2
,_ . . . . . . . . . . . . . . . .
.I k , x , _ , - / ' ~ P = 0 " 9 G P a -4~ ~ , , --~- p=0 GPa 0.0
0.5
H (T)
1.0
, 1.5
Fig. 4. Effect of pressure (0.9 GPa) on M - H hysteresis loop for Y124 at 4.2 K.
M. Baran et al. /Physica C 241 (1995) 383-388 15-
387
5•
4-
10
GdBa2Cu408 x.
""',.>~
="''"~'~m~m~
T=4.2 K"'''x"....
'~ 3<
E
m
"'x.....
~ ! X
..... X .......X.. "'"'X
5
O
J '_
..~
T=4.2 K --p=L1 GPa . . . . . p=0 GPa
.-S
0.0
015
ll,O
I-
---
o:5
115
H (T)
Fig. 5. Effect of pressure ( 1.1 GPa) on M-H hysteresis loop for Gd124 at 4.2 K. 8
YBa2Cu408
--m-- p=0.9 GPa T=4.2 K "'" x.. "~'l~..m.__x.__p=0 GPa
- - , - - p = 1.1 GPa ---x--- p=0 GPa
T=60 K
~ ---:':-- ~ ---'-:':"-~ ' ~ - - -
H (T)
1:0
I ~ ' ~ - - II
1.5
Fig. 7. The critical current density jc of Gd124 as a function of field, at 4.2 K and 60 K, without and under pressure of 1.1 GPa.
connected with intrinsic effects such as the change in interionic distances within or between CuO2 planes and the change of amount of the pressure-induced charge transfer into the CuO2 plane when the Gd ion is placed into the Y sites.
"-- x.... x
""
Acknowledgement
mNm~ m T=60 K "'m'~--m~,
2
"--'--,--,--,--,__,__,__,
o
I
I
0.25
0.50
~
I
I
0.75
1.00
1.25
H (T)
The work was supported by the Polish State Committee for Scientific Research (KBN) under the contract No. 2 P302 082 05.
Fig. 6. The critical current density Jc of Y124 as a function of field, at 4.2 K and 60 K, without and under pressure of 0.9 GPa.
References dependences ofjc versus magnetic field, at 4.2 K and 60 K, are shown for Y 124 and Gd 124, respectively.
4. Conclusion We have investigated the effect of pressure up to 1.1 GPa on the critical temperature and the critical current density in both Gd124 and Y124 compounds. T h e increase o f b o t h Tc a n d j¢ u n d e r c o m p r e s s i o n is o b s e r v e d . T h e pressure d e p e n d e n c e o f Tc ( d T ¢ / d p ) for G d 1 2 4 is s m a l l e r t h a n that for Y124. T h e possible m e c h a n i s m s o f increasing o f T¢ u n d e r pressure are discussed. T h e s e m e c h a n i s m s are
[ 1 ] C.W. Chu, P.H. Hor, R.L. Meng, L. Gao, Z.J. Huang and Y.Q. Wang, Phys. Rev. Lett. 58 (1987) 405. [2] 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. Lett. 58 (1987) 908. [3] R. Benischke, T. Weber, W.H. Fietz, J. Metzger, K. Grube, T. Wolf and H. W~ihl, Physica C 203 (1992) 293. [4] I.V. Medvedeva, Yu.S. Bersenev, B.A. Gizhensky, N.M. Chebotaev, S.V. Naumov and G.B. Denishev, Z. Phys. B 81 (1990) 311. [ 5 ] K. Murata, Y. Honda, H. Oyanagi, Y. Nishihara, H. Ihara, N. Terada, H. Sugise, M. Hirabayashi, M. Tokumoto and Y. Kimura, Int. Meeting on Adv. Mater., 1988, Tokyo, Japan. [ 6 ] B. Bucher, J. Karpinski, E. Kaldis and P. Wachter, Physica C 157 (1989) 478.
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[7]E.N. Van Eenige, R. Griessen, R.J. Wijngaarden, J. Karpinski, E. KaMis, S. Rusiecki and E. Jilek, Physica C 168 (1990) 482. [8] E. Kaldis, P. Fisher, A.W. Hewat, E.A. Hewat, J. Karpinski and S. Rusiecki, Physica C 159 (1989) 668. [9] R.J. Cava, A.W. Hewat, E.A. Hewat, B. Batlogg, M. Marezio, K.M. Rabe, J.J. Krajewski, W.F. Peck Jr. and L.W. Rupp Jr., Physica C 165 (1990) 419. [ 10] Y. Yamada, J.D. Jorgenscn, S. P¢i, P. Lightfoot, Y. Kodama, T. Matsumoto and F. Izumi, Physica C 173 ( 1991 ) 185. [11 ] J.J. Scholtz, E.N. van Eenige, R.J. Wijngaarden and R. Gries~n, Phys. Rev. B 45 (1992) 3077. [12]J.B. Torrance, Y. Tokura, A.I. Nazzal, A. Bezinge, T.C. Huang and S.S.P. Parkin, Phys. Rev. Lett. 61 (1988) 1127.
[ 13 ] Y. Tokura, J.B. Torrance, T.C. Huang and A.I. Nazzal, Phys. Rev. B 38 (1988) 7156. [ 14 ] X. Zhang and C.R.A. Catlow, Physica C 200 ( 1992 ) 323. [ 15 ] R.P. Gupta and M. Gupta, Physica C 204 ( 1993 ) 349. [16] R.J. Cava, J.J. Krajewski, W.F. Peck Jr., B. Batlogg, L.W. Rupp Jr., R.M. Fleming, A.C.W.P. James and P. Marsh, Nature 338 (1989) 328. [ 17 ] S. Adachi, H. Adachi, K. Setsune and K. Wasa, Physica C 175 (1991) 523. [ 18 ] T. Miyatake, S. Gotoh, N. Koshizuka and S. Tanaka, Nature 341 (1989) 41. [19] V. Dyakonov and G. Levchenko, Pribory i Technika Eksperimenta (Russia) 5 (1883) 236. [20] C.P. Bean, Phys. Rev. Lett. 8 (1962) 250.
Physica C 241 ( 1995) 383-388
Comparative study of the pressure effect on critical parameters of GdBaECU408 and YBa2Cu408 M. Baran *, V. Dyakonov 1, L. Gtadczuk, G. Levchenko ~, S. Piechota, H. Szymczak Institute of Physics, PolishAcademy of SciencesAl. Lotnik6w 32/46, 02-668 Warszawa,Poland Received 20 September 1994
Abstract
The influence of hydrostatic pressure up to 1.1 GPa on critical parameters (To, Hc~, j~) of YBa2Cu4Os (Y 124) and GdBa2Cu4Os (Gd124) ceramics has been investigated using a CuBe pressure bomb. A linear increase of Tc with the growth of pressure was found, with d Tc/dp = 5.5 K/GPa and 4.5 K/GPa for Y124 and Gd 124, respectively. The difference in Tc increase on pressure, dTJdp, between the two compounds is assumed to have the following possible mechanisms: first, the replacement of yttrium by gadolinium constrains the change of interionic distances within and between the CuO2 planes and thus the change of charge transfer integral having an effect on the density of electronic states at CuO2 planes; secondly, this replacement modifies the charge transfer into the CuO2 planes from rare-earth sites. It was found that the critical current density increased a little under pressure; nevertheless a different behaviour on temperature has been observed for the two compounds. Insignificant changes of Hot under pressure were found at temperatures up to 40 K. However, at higher temperatures a little growth of Hc~ was observed.
1. Introduction
From the very beginning of the HTSC era the importance of high pressure investigation has been realized. Chu et al. showed [ 1 ] that by applying a hydrostatic pressure to the LaBaCuO system a significant growth o f critical temperature could be achieved with the value of dTc/dp equal to 6.1 K / GPa. In fact, this large pressure effect led to the very successful idea [ 2 ] o f replacing La by the smaller Y, resulting in T~ above 90 K for YBaECU307_6. A large number of papers have appeared on the effect of pressure on high-temperature superconductors. However, it is difficult to say that the actual knowl* Corresponding author. ~Permanent address: Institute of Physics and Technology, Ukrainian Academy of Sciences 340114 Donetsk, Ukraine.
edge allows for a full understanding o f the microscopic mechanism responsible for changes o f the critical temperature under pressure. The reason for that is the very complex character of the pressure dependence o f the critical temperature. In the case of YBa2Cu3Ox with the value of x close to 7, the slope of the pressure effect on T¢, dTc/ dp, is equal to about 0.4 K / G P a . With the change of x down to 6.77 an abrupt increase ofdT¢/dp was observed [ 3 ] up to 4 K / G P a . A sharp maximum in dT¢/ dp dependence on x was observed for x ~ 6.7 [4,5]. From these experiments it was concluded that anomalies in the pressure dependence o f critical temperature for YBa2Cu306 +x (Y 123 ) should be connected with oxygen structure (oxygen content, distribution and ordering). To examine this view it is important to study the pressure dependence o f the superconducting properties for ReBa2Cu4Os compounds
0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10921-4534 (94)02359-X
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M. Baran et al. /Physica C 241 (1995) 383-388
(Re 124) in which the oxygen concentration is constant, oxygen vacancies are absent and oxygen migration does not occur. It should be noted that Rel24 compounds, in comparison to Y123, have one additional CuO chain layer in the unit cell and a lower value of T¢. The number of high pressure experiments carried out on YBa2Cu408 have made a significant contribution to our knowledge of this material. Unlike the fully oxygenated YBaECU307,the Y124 compound with Tc ~ 80 K has a rather large Tc pressure dependence, with dT¢/dp equal to 5-6 K / G P a [6,7]. Both the large and small values of dTc/dp for Y124 and Y123, respectively, have been attributed to a pressure-induced charge (hole) transfer mechanism [811 ]. As Y 124 shows superconductivity in the stoichiometric composition in which it has exactly one hole per chemical formula unit and has a very stable oxygen content, the various experiments can be performed under a well-defined hole concentration. It is established that there is a good correlation between transition temperature Tc and the density of holes in the CuO2 plane [ 12,13 ]. But, at present, the mechanisms of the change of the hole density in the CuO2 planes and of the charge redistribution in YBaCuO, especially under pressure, are not clear. The possible mechanisms playing a role in the change of hole density under pressure which should be discussed are the following: (1) A change of the charge density in the CuO2 plane due to variation in the Cu(2)-apical oxygen O ( 1 ) bond length [ 7 ]. (2) The charge density redistribution between Cu and O in the CuO2 plane and, respectively, the Tc relation to the change of the electronic state of the CuO2 layer due to modification of the Cu-O (2,3) squarepyramid shape [ 14 ]. (3) The charge density redistribution between the CuO2 plane and the ionic elements, Y and Ba [ 15 ]. Additional investigations are necessary to understand the role of these mechanisms in the formation of the superconducting state under pressure. Studies of the pressure effect on superconducting properties of ReBa:Cu4Os (Re - the rare-earth ion) can throw some light on these mechanisms. The Re ion as well as Y and Ba have to play a definite role in the formation of the electronic state of the CuO2 plane and to have some influence on the T¢ value. It is neces-
sary to note that the role of the ionic elements has been fully ignored in the interpretation of the pressure dependence of the superconducting transition temperature. In this paper we present some results of measurements of the pressure effect on critical parameters ( To, j~) of the GdBa2Cu4Oa (GD124) and YBa2Cu408 (Y 124 ) superconducting compounds.
2. Sample preparation and experimental technique The investigated ceramic samples were obtained using the conventional method of solid state reaction under atmospheric pressure of oxygen. Besides the main components of the investigated compounds, an admixture of calcium with the concentration of 0.1 atom per formula unit was also applied. It is known that in the case of Y124 the replacement of 10% of yttrium by calcium stabilizes the 124 structure [ 16,17 ] and shifts Tc toward higher values [ 18 ]. In our Rel24 samples Ca replaces both Y and Ba in practically the same amounts. The crystal structure and the phase purity of Re0.95Bal.95Cao.lCu408 compounds have been determined by powder X-ray diffraction at room temperature (using the Cu Ks line of diffractometer DRON-I.5). The samples were found to be of essentially a single-phase Re 124 material. The magnetic measurements also do not reveal any traces of 123-phase. The hydrostatic pressure was generated in the highpressure bomb (HPB) constructed as a piston-cylinder device made of fully hardened beryllium copper [ 19 ]. A 1 : 1 mixture of transformer oil and kerosene was used as a hydrostatic pressure-transmitting medium. The cylindrical samples of approximately 1 mm diameter and 5 mm length were placed in the HPB (external diameter - 4 mm, inner diameter - 1.43 mm). The necessary pressure was generated at room temperature. After cooling to low temperature, the pressure in the working canal of HPB decreased by about 0.15-0.2 GPa. At the low temperatures the pressure was verified by measuring the shift of Sn superconducting transition temperature. According to our estimation the error in the pressure value should not exceed + 0.05 GPa. It was verified that the con-
385
M. Baran et al. / Physica C 241 (1995) 383-388
tribution of HPB to the total magnetization value was negligible. The change of the superconducting transition temperature under pressure has been determined from the temperature dependence of the magnetization at some fixed values of the pressure. The DC magnetization measurements of samples were carried out with a vibrating sample magnetometer (PAR, Model 4500) in a magnetic field of 1 mT, after cooling in a zero field (ZFC), and in the temperature range 4.2100 K. The critical current densities were determined from the magnitude of the hysteresis of the DC magnetization. The magnetic hysteresis loops were measured in magnetic fields up to 1.5 T at various temperatures.
3. Experimental results and discussion
0
20
40 T (K) 60
80
-0.2.
.z2.
100
~,o
0.0-
--o--p= 0 GPa --o--p=0.9 GPa
/i
-0,4-
[]
O
-0.6-0.8-
-1.0-
Fig. 2. Magnetization of Y 124 as a function of temperature in magnetic field of 1 mT, without pressure and under pressure of 0.93 GPa. 6
In Figs. 1 and 2 there are presented the temperature dependences of magnetization for Gd 124 and for Y 124 in magnetic field of I mT, both without pressure as well as under pressure. As can be seen, the M ( T ) curves are shifted to higher temperatures when measured under pressure, i.e. T¢ under pressure increases. The superconducting transition width decreases with pressure in the case of Gd 124, while for Y 124 it is practically constant. To determine the difference of Tc induced by pressure, ATe, it was necessary to choose what definition of Tc would be used. It
4
2- S
0~ 0.00
Gd124
0.25
0.50
0.75
1.00
P (GPa) Fig. 3. ATe as a function of pressure for Y 124 and Gd 124.
T (K) 40 60
20 ,
I
,
I
,
80
P
,
I
100 ~
I
0.0-0.2-0.4.
~
[]
-
[]
-0.6-0.8-1.0-
/
/
~ / / - - o - -
~mt~rg~W'Uo~o~O~°
p = 0 GPa
--o--p= i.i GPa
Fig. 1. Temperature dependence of magnetization of Gdo.95Bal.gsCao.]Cu4Os in magnetic field of I mT, without pressure and under pressure of 1.1 GPa.
was decided to use as the Tc value a temperature corresponding to the intersection of the tangent to the M ( T ) relation at its maximum slope with the level of M = 0 . It should be noted that in the case of Gd124, because of the influence of pressure on the transition width, values of ATe are lower than those determined from the midpoint of the transition. In Fig. 3 the pressure dependences of ATe, for both examined compounds, are presented. As can be seen, ATe increases linearly with growing pressure for both compounds. In the case of Y 124 the obtained value of dTJdp is equal to + 5.5 K / G P a , which is consistent with the former results [6,7]. The value of dTJdp for Gd124 has been estimated as equal to +4.5 K / GPa.
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M. Baran et al. / Physica C 241 (1995) 383-388
The increase of Tc can be interpreted as being due to a change of hole transfer into the C u O 2 layers under pressure and pressure-induced charge redistribution among the ionic sites. It should be stressed that in Re 124 the double CuO chains are less compressible than the CuO2 layers. It was observed in Ref. [ 8 ] that Cu ( 2 )-O ( 1 ) bonds, bridging the CuO2 layers to the chains, shorten with pressure. These changes cause the charge response of the CuO chain electron reservoirs which affects the hole density in the CuO2 planes. Besides the charge transfer mechanism between CuO2 layers and CuO chains, a possible reason for Tc growth under pressure is such an intrinsic effect as the change in interatomic distances within CuO2 planes or between them. It is impossible to ignore the contribution of ionic elements (Y, Ba, Gd) in the electronic charge redistribution under pressure. This can be seen from the different values of dTddp for Gd124 and Y124. As is shown in Ref. [ 15], significant changes of the electronic charge occur at ionic sites (Y, Ba) under pressure. The electronic charge density at these positions always increases under pressure. This accumulation of electronic charge at the ionic sites has to be connected with a rise of the hole density at the CuO2 planes. The difference between dTJdp in Y124 and Gd 124 is obviously related to the difference of y3+ and Gd 3+ ionic radii. As the result of larger ionic radius, the Gd 3+ ions more weakly influence the electronic state of the CuO2 planes than y3+ ions. It would be noted that the replacement of the Y ion by a larger rare-earth Gd ion at the site located between two CuO2 planes increases both the lattice parameters and C u ( 2 ) - C u ( 2 ) distance. This also involves changes ofinterionic distances within the CuO2 layer itself. It is a consequence of the negative "internal structural pressure" which alters the lengths of interatomic bonds. It is known that the Cu-O bonds in the C u ( 2 ) - O layers are compressed. The negative "internal pressure", increasing the lattice parameters a and b, decreases the compression of these Cu-O bonds, i.e. both C u - O ( 2 ) and C u - O ( 3 ) bonds lengthen. The increase of Cu-O distances in the Cu ( 2 ) - O planes decreases the Cu-O molecular orbital overlap. This promotes a decrease of the hole density in the Cu (2) - O planes. The above shows that the changes of both electronic state of the CuO2 planes and T~ in Gd124 are of structural origin. The lower
value ofdTc/dP for Gd124 in comparison with Y124 confirms that the changes both in the Cu ( 2 ) - C u (2) distance and within the CuO2 plane play an important role in the pressure dependence of To. In the large rare-earth Gd 124 compound the additional holes are less effectively transferred into the CuO2 planes under pressure than in Y 124. A deflection of the M(H) relation from linearity allowed us to estimate values of lower critical He1 in the low magnetic field region (up to 100 mT). A practically linear decrease of H~ was found with increasing temperature in the range 4.2-60 K. In the range of temperature up to 50 K for Y124 and 40 K for Gd124 only insignificant changes of H~ were obtained. For higher temperatures (up to 60 K) a tendency of deviation from linear behaviour of H~ (to higher values) was observed. The hysteresis M-H loops of Gd124 and Y124 measured in magnetic fields up to 1.5 T have different shapes, which is determined by the strong paramagnetic contribution of gadolinium, especially at the lower temperatures. The area of the hysteresis loops increased under pressure for both compounds. In Figs. 4 and 5 the effect of pressure on the hysteresis loops at 4.2 K for Y 124 and Gd 124 is shown. From the hysteresis loops the change of the critical current density under pressure can be estimated on the basis of Bean's critical state model [20]. Assuming rather typical average grain size in the ceramics ( 10 p.m), values of intragranular critical current density j~ were estimated. In Figs. 6 and 7 the effect of pressure on the
2_2
,_ . . . . . . . . . . . . . . . .
.I k , x , _ , - / ' ~ P = 0 " 9 G P a -4~ ~ , , --~- p=0 GPa 0.0
0.5
H (T)
1.0
, 1.5
Fig. 4. Effect of pressure (0.9 GPa) on M - H hysteresis loop for Y124 at 4.2 K.
M. Baran et al. /Physica C 241 (1995) 383-388 15-
387
5•
4-
10
GdBa2Cu408 x.
""',.>~
="''"~'~m~m~
T=4.2 K"'''x"....
'~ 3<
E
m
"'x.....
~ ! X
..... X .......X.. "'"'X
5
O
J '_
..~
T=4.2 K --p=L1 GPa . . . . . p=0 GPa
.-S
0.0
015
ll,O
I-
---
o:5
115
H (T)
Fig. 5. Effect of pressure ( 1.1 GPa) on M-H hysteresis loop for Gd124 at 4.2 K. 8
YBa2Cu408
--m-- p=0.9 GPa T=4.2 K "'" x.. "~'l~..m.__x.__p=0 GPa
- - , - - p = 1.1 GPa ---x--- p=0 GPa
T=60 K
~ ---:':-- ~ ---'-:':"-~ ' ~ - - -
H (T)
1:0
I ~ ' ~ - - II
1.5
Fig. 7. The critical current density jc of Gd124 as a function of field, at 4.2 K and 60 K, without and under pressure of 1.1 GPa.
connected with intrinsic effects such as the change in interionic distances within or between CuO2 planes and the change of amount of the pressure-induced charge transfer into the CuO2 plane when the Gd ion is placed into the Y sites.
"-- x.... x
""
Acknowledgement
mNm~ m T=60 K "'m'~--m~,
2
"--'--,--,--,--,__,__,__,
o
I
I
0.25
0.50
~
I
I
0.75
1.00
1.25
H (T)
The work was supported by the Polish State Committee for Scientific Research (KBN) under the contract No. 2 P302 082 05.
Fig. 6. The critical current density Jc of Y124 as a function of field, at 4.2 K and 60 K, without and under pressure of 0.9 GPa.
References dependences ofjc versus magnetic field, at 4.2 K and 60 K, are shown for Y 124 and Gd 124, respectively.
4. Conclusion We have investigated the effect of pressure up to 1.1 GPa on the critical temperature and the critical current density in both Gd124 and Y124 compounds. T h e increase o f b o t h Tc a n d j¢ u n d e r c o m p r e s s i o n is o b s e r v e d . T h e pressure d e p e n d e n c e o f Tc ( d T ¢ / d p ) for G d 1 2 4 is s m a l l e r t h a n that for Y124. T h e possible m e c h a n i s m s o f increasing o f T¢ u n d e r pressure are discussed. T h e s e m e c h a n i s m s are
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