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Effect of pressure on critical parameters of electron-doped superconductors Nd2−xMxCuO4−y (M = Ce, Th) and Sm2−xCexCuO4−y
Volume 146, number 9
PHYSICS LETTERS A
18 June 1990
EFFECT OF PRESSURE ON CRITICAL PARAMETERS OF ELECTRON-DOPED SUPERCONDUCTORS Nd2~M~CuO4~ (M = Ce, Th) AND Sm2~Ce~CuO4~ S.L. BUD’KO, A.G. GAPOTCHENKO, A.E. LUPPOV instil utefor High Pressure Physics, USSR Academy ofSciences, Troitsk, Moscow Region 142092, USSR
E.A. EARLY, M.B. MAPLE and J.T. MARKERT Inst itutefor Pure and Applied Physical Sciences and Department of Physics, University of California, San Diego, La Jolla, CA 92093, USA Received 2 April 1990; accepted for publication 6 April 1990 Communicated by V.M. Agranovich
The effect of pressure on the temperature of the superconductive transition and the critical field of irreversibility for N-type polycrystalline high-T. compounds Nd153Ce015CuO4_~,Nd1,85Th01~CuO4_~ and Sm185Ceo.15CuO4_~was investigated. The critical parameters decrease under pressure with pressure derivatives which are smaller in magnitude than those for p-type high-i’. compounds. This fact may be accounted for by structure peculiarities of electron-doped HTSC.
1. Introduction An important development in high-Ta superconductivity (HTSC) has been the recent discovery of a new class of high-Ta superconductors: Ln2_~M~CuO4~compounds [1—3] whose critical temperatures are within the range 8—24 K. In contrast to all recently investigated superconductive cuprates for which holes were the charge carriers, electrons are the charge carriers for this new class of HTSC, i.e. this new compounds are n-type superconductors. Besides that, the crystal structures of HTSC which were known earlier contain two-dimensional arrays of Cu06 octahedra or Cu05 pyramids. In contrast to these crystal structures the electron-doped HTSC have a so-called T’-phase structure which is cornposed of sheets of Cu04 squares. The Ce or Th ions in these oxides are supposed to be tetravalent and they become donors of electrons in the Cu—O planes. Such substantial differences of the new HTSC class from the high-Ta superconductors known before have stimulated experimental investigations of various properties of the compounds from this class and theoretical discussions of superconductivity mechanisms in n-type HTSC. It seems to us that the in-
vestigations of the changes of the temperature T~of superconductive transition and the critical field of irreversibility “c2 under pressure can give some additional information on characteristic properties of the superconductive state in this HTCS class.
2. Experimental Polycrystalline samples of three different compounds Nd2_~Ce~CuO4_~ (NCCO), Nd2_~Th~Cu°4y (NTCO) and Sm2_~Ce~CuO4_~ (SCCO) with x=0.15 were investigated. X-ray investigations showed the (241) phase with no impurities at 100: 1 signal-to-noise level. Oxygen content analysis on similarly prepared specimens indicate a deficiency y=O.02. The compounds studied were prepared by solid state reaction from oxides as described elsewhere [2]. The measurements were performed under hydrostatic pressure up to 20—25 kbar by the inductive technique with modulation frequency 2 kHz. Three characteristic points which correspond to different possible definitions of the critical temperature (T~, ~ T~)(fig. 1) were chosen on the superconductive transition curve and the change of
0375-9601/90/S 03.50 © Elsevier Science Publishers B.V. (North-Holland)
543
Volume 146, number 9
PHYSICS LETTERS A
~/
T~°
arb.u~t
18 June 1990
change of the critical field of irreversibility H~2under pressure displays rather well (in sign and order of magnitude) just the same quantity as for the “true” second critical field. ative
~ Tb
~f
3. Results 3.1. Nd1.85Ce0.15CuO4_~ The change of the temperature of the superconductivearetransition and the(see critical of irreversibility quite negligible tablefield 1). Taking into
1’
t T~’T
account the accuracy of the measurements, the resuits can be considered as pointing to a slight decrease of T~under pressure (dT~/dP<0.03 K/kbar, dT~/dP<0) which is in qualitative agreement with the results obtained by the resistive technique: dT~/ dP=0.00+0.05 K/kbar in refs. [7] and [8] for T~ defined from the R=0lR~level, In ref. [81 the width of the superconductive transition in resistive measurements and the value of the normal state resistance decrease under pressure. The width of the transition and the value of the change of the signal ~ during the superconductive transition in our work increase slightly under pressure (the relative change is a fraction of that in ref. [81). These changes observed in both techniques are most prob-
K Fig. 1. Definition ofthe characteristic points in the superconduclive transition curve.
their position in magnetic field and under pressure was investigated. The superconductive transition was recorded during the internal heating of the sample from the temperature of the helium bath (4.2 K). Our experimental technique and the method of determination of physical quantities from the experimental data are described elsewhere [4]. Taking into account the analysis of the usage of various experimental techniques in measurements of critical parameters of HTSC in magnetic field in ref. [5] we can consider that the value of R~ 2obtained from our measurements corresponds to a greater extent to the so-called “field of irreversibility”. Nevertheless, according to our considerations [6], the rel-
ably connected with the changes of the properties of intergranular boundaries under pressure, which display themselves more violently in resistive measurements.
Table I Effect of pressure on critical parameters of electron-doped superconductors Compound
Definition ofT,
T~(P=0) (K)
dT~/dP (K/kbar)
—dH~2/dT~ (kOe/K)
H~2(kOe)~ (P—.0)
dH~2/dP~ (kOe/kbar)
dIn H~2/dP~ (kbar’)
NCCO
T,b T,°~ T,°~
21.6 18.4 15.7
—0.03 ±0.01 0.007±0.01 —0.08 ±0.01
11 4.8
62 21.5
—0.2 —0.07
—0.003 —0.003
NTCO
T~ ~
21.4 15.9
—0.1 ±0.05 —0.04 ±0.01
5.2 4.1
23 14
—0.5 —0.16
—0.02 —0.01
SCCO
T~ T~” T~
23.9 21.1 15.5
—0.07 ±0.02 —0.05 ±0.03 —0.08 ±0.02
12.1 4.9
61 28
—0.25 —0.05
—0.004 —0.002
~ T=7K.
544
Volume 146, number 9
PHYSICS LETTERS A
18 June 1990
kOe/K for a field directed in the basal plane obtamed for NCCO single crystals [9].
-
3.2. Nd,85Th015CuO4_~
-
a •
5
10
15
20
P, K
Fig. 2. 11,2 (T) dependences for NCCO (P-.. 0) determined from T~(a) and T,°”(b)
The P,~ ( T) dependences determined from T~and T~°~ have different curvature and temperature derivatives dP,2/dT (fig. 2). The curves ‘c2 (T) shift along the T-axis under pressure without distinct change of their shape. The representation of I?,2 (T) curves as 1 ~ t= T/TC, which is often used in the examination ofthe causes of the curvature of the H,2 (T) dependence gives for H> 5 kOe different values of q for the curves determined from T,b (q=0.26) and TC°~ (q=0.48). The critical field “c2 decreases a little under pressure. The pressure derivatives (PD) for J?,2 at T=7 K (when the “c2 (T) dependence tends towards linearity) are presented in table 1. Let us note that dlnP~dlnP~ dT dT
‘
dlnP~ dP
dlnP~2 dP
Our result for dP~2/dTis close to that obtained for P—~0 by the resistive technique for polycrystalline samples [8] (—dH,2/dT= 12 kOe/K at the R=0.5R~level and 18 kOe/KattheR=0.9R~level) and it is between the values of dH,2/dT= 4.3 kOe/ K for the magnetic field along the c-axis and 88.5 —
The width of the superconductive transition for NTCO investigated by the inductive technique is greater than that for NCCO. The temperature of superconductive transition decreases under pressure. The PD is somewhat larger in magnitude than for NCCO which is consistent with the results of ref. [8], where the resistance technique was used. The critical field Pc2 at fixed temperature also decreases under pressure. PD are given in table 1. The H~2(T) dependences for P—.. 0 are presented in fig. 3. The behaviour of the Pc2 (T) dependences is towards more gently sloping and the values of the power q are different to some extent (q=0.3 for the curves determined from T~ and q=0.35 for the dependences determined from T~) in comparison with those of NCCO. These results and also the greater width of the transition may be due to both the differences in the physical properties ofthe HTSC compounds under consideration and the different quality of the samples, which is connected with the fabrication route. We do not known any literature data on H~2( T) dependences for NTCO. 3.3. Sm1.85Ce0.15CuO4_~ We do not know any results of measurements of critical parameters of this compound under pres______________________________________
-
a -
~c’J
bV\~~
I I°
20
‘1, K
Fig. 3. 11,2(T) dependences for NTCO (P—0) determined from T~(a) and ~
(b).
545
Volume 146, number 9
PHYSICS LETTERS A
18 June 1990
ample, ref [11]) predict existence of a line which separates the region of reversibility of magnetic moment from the region of its irreversibility. This line follows the characteristic relationship 1 H213. Such a model was used, for example, to explain a set of experimental data obtained for some HTSC ceramic samples. It is necessary to point out that this model is suitable for a range of rather small magnetic fields (<1 kOe) and a narrow (a few degrees in width) ternperature region T,— T. The approach based upon the concept of of magnetic flux creep [12] seems to be the most promising for this work. The value of the power q in the expression 1 t = ~ is determined by the dependence of the pinning potential upon the temperature and magnetic field within this model. The mechanism of pinning is in an exploratory stage at present. The values q=2/3 and q=3/4 quoted in ref. [5] were obtamed from clear enough physical considerations and
-
—
a 0
è
-
—~
—
b ~
\
I 5
10
15
20T,K
they are in good agreement with the experimental re-
Fig. 4. 11, 2(T) dependences for SCCO (P-. 0) determined from T~’(a) and ~ (b).
sure. The results obtained in our work are in qualitative agreement with those of NCCO and NTCO. Both T~and P~2decrease under pressure (see table 1). The value of dT~/dP is close to that for Sm1,85Th05CuO4_~[10]. The width of the transition and the magnitude of the change of signal during the transition increase under pressure somewhat more slowly than for NCCO. The dependences H~2(T) for P-.~0 are presented in fig. 4. The values of the power q are equal to 0.32 and 0.39 for the curves determined from T,b and T~’~ respectively.
4. Discussion and conclusions At present there are several approaches within the scope of which attempts are made to explain the magnetic properties of HTSC compounds, in particular the behaviour of the H~2( T) dependence. One of such models is the model of superconductive glass which is based on the consideration of the HTSC ceramic sample as a system of superconductive granules with weak Josephson links. Theoretical analysis and numerical computation (see, for ex546
sults for different orientations of YBa2Cu3O7 ~ single crystals. Nevertheless, these values are unlikely to be the only ones possible (for example, they are in disagreement with experimental dependences J1c2 ( T) for Bi—Sr—Ca—Cu—O single crystals [131, we obtamed in ref. [14] q—. 0.2 for the kOe range of the magnetic field h> 10 for two orientations of Bi2Sr2CaCu2O~single crystals). The three compounds which were investigated in this work are similar in behaviour, the shape of P,2 (T) curves is in qualitative agreement with the model discussed in ref. [5]. The value of the power q in the equation 1 —t=aH depends upon the mechanism of pinning. If the potentials of pinning in NCCO grains for different field orientations have different dependences on magnetic field and ternperature and therefore different values of q are displayed, then the consequence of this is different q values for different manners of definition of T~in a magnetic field. Perhaps, in NTCO and SCCO we measure some “average” value of q and this fact may be due to peculiarities in the fabrication route or in the other characteristic features, which are displayed for example in a substantially greater width of the transition for the last two compounds in comparison with NCCO. The absence of a noticeable change of the shape of H,2 (T) curves points to absence of a
Volume 146, number 9
PHYSICS LETTERS A
noticeable effect of the hydrostatic pressure on the pinning potential in the range considered. At presence there is no theory that explains the changes in the critical parameters of HTSC while the interatomic distances in the crystal lattice change both for p-type and n-type HTSC. From the structural point of view the substantial difference between p- and n-type HTSC consists in the fact that the common element of the structures of the “hole” superconductors is the occurrence of layers of three-dimensional Cu06 octahedra or Cu05 pyramids. The corresponding layers in “electron” HTSC consist oftwo-dimensional Cu04 squares. The connections between an apical oxygen atom in the Cu05 pyramid or a CuO6 octahedron in a p-type HTSC and copper atoms are rather weak, but they are strong enough to have an effect on the Cu—O connection in the Cu02 plane [15,71. Thus the effect of pressure on T, for p-type HTSC is substantially greater than for n-type HTSC where the corresponding structure element is two-dimensional. As it seems to us the stated characteristic properties of HTSC structures give an opportunity to understand the common reason for a wide range ofvalues of pressure derivatives for different classes of HTSC. The whole complex of experimental data cited in the literature gives no opportunity to choose a single model to describe the mechanism of superconductivity in electron-doped HTSC compounds. However the decrease of their critical parameters (T,, ‘1,2) under pressure allows one to doubt the possibility to apply a bipolaronic model for the cornpounds under consideration.
18 June 1990
Acknowledgement We acknowledge support for this research from the USSR Interdepartmental Council for the HTSC Problem under Grant No. 32 and from the US Department of Energy under Grant No. DE-FGO386ER45230. We are greatly indebted to A.A. Abrikosov for his kind attention to this work and stimulating discussions and to E.S. Itskevich for his kind attention.
References [1] Y. Tokura,
H. Takagi and S. Uchida, Nature 337 (1989)
[2] J.T. Markert and M.B. Maple, Solid State Commun. 70 (1989) 145. [3]J.T.Markertetal.,PhysicaC 158 (1989) 178. [4] S.L. Bud’ko, AG. Gapotchenko and E.S. Itskevich, Solid State Commun. 69 (1989) 387. [SlAP. Malozemoff, T.K. Worthington, Y. Yeshurun and F. Holtzberg, Phys. Rev. B 38 (1988) 7203. [6] S.L. Bud’ko, AG. Gapotchenko, ES. Itskevich and A.E. Luppov, Phys. Lett. A 140 (1989) 197. [7] C. Murayama et al., Nature 339 (1989) 293. [8] C.L. Seaman et al., Physica C 159 (1989) 391. 19] Y. Hidaka and M. Suzuki, Nature 338 (1989) 320. [10] E.A. Early et al., Physica C 160 (1989) 320. [1111. Morgenstern, K.A. Muller and J.B. Bednorz, Physica B 152 (1988) 85. [12] Y. Yeshurun A.P. Malozemoff, Phys. Rev. Lett. 60 (1988) 2202. [13] Y. Yeshurun et al., Cryogenics 29 (1989) 258. [14] S.L. Bud’ko, AG. Gapotchenko, ES. Itskevich and A.E. Luppov, Superconduct. Phys. Chem. Tech. (USSR) 3 (1990) 36. [15] R.P. Gupta and M. Gupta, Physica C 160 (1989) 129.
547
PHYSICS LETTERS A
18 June 1990
EFFECT OF PRESSURE ON CRITICAL PARAMETERS OF ELECTRON-DOPED SUPERCONDUCTORS Nd2~M~CuO4~ (M = Ce, Th) AND Sm2~Ce~CuO4~ S.L. BUD’KO, A.G. GAPOTCHENKO, A.E. LUPPOV instil utefor High Pressure Physics, USSR Academy ofSciences, Troitsk, Moscow Region 142092, USSR
E.A. EARLY, M.B. MAPLE and J.T. MARKERT Inst itutefor Pure and Applied Physical Sciences and Department of Physics, University of California, San Diego, La Jolla, CA 92093, USA Received 2 April 1990; accepted for publication 6 April 1990 Communicated by V.M. Agranovich
The effect of pressure on the temperature of the superconductive transition and the critical field of irreversibility for N-type polycrystalline high-T. compounds Nd153Ce015CuO4_~,Nd1,85Th01~CuO4_~ and Sm185Ceo.15CuO4_~was investigated. The critical parameters decrease under pressure with pressure derivatives which are smaller in magnitude than those for p-type high-i’. compounds. This fact may be accounted for by structure peculiarities of electron-doped HTSC.
1. Introduction An important development in high-Ta superconductivity (HTSC) has been the recent discovery of a new class of high-Ta superconductors: Ln2_~M~CuO4~compounds [1—3] whose critical temperatures are within the range 8—24 K. In contrast to all recently investigated superconductive cuprates for which holes were the charge carriers, electrons are the charge carriers for this new class of HTSC, i.e. this new compounds are n-type superconductors. Besides that, the crystal structures of HTSC which were known earlier contain two-dimensional arrays of Cu06 octahedra or Cu05 pyramids. In contrast to these crystal structures the electron-doped HTSC have a so-called T’-phase structure which is cornposed of sheets of Cu04 squares. The Ce or Th ions in these oxides are supposed to be tetravalent and they become donors of electrons in the Cu—O planes. Such substantial differences of the new HTSC class from the high-Ta superconductors known before have stimulated experimental investigations of various properties of the compounds from this class and theoretical discussions of superconductivity mechanisms in n-type HTSC. It seems to us that the in-
vestigations of the changes of the temperature T~of superconductive transition and the critical field of irreversibility “c2 under pressure can give some additional information on characteristic properties of the superconductive state in this HTCS class.
2. Experimental Polycrystalline samples of three different compounds Nd2_~Ce~CuO4_~ (NCCO), Nd2_~Th~Cu°4y (NTCO) and Sm2_~Ce~CuO4_~ (SCCO) with x=0.15 were investigated. X-ray investigations showed the (241) phase with no impurities at 100: 1 signal-to-noise level. Oxygen content analysis on similarly prepared specimens indicate a deficiency y=O.02. The compounds studied were prepared by solid state reaction from oxides as described elsewhere [2]. The measurements were performed under hydrostatic pressure up to 20—25 kbar by the inductive technique with modulation frequency 2 kHz. Three characteristic points which correspond to different possible definitions of the critical temperature (T~, ~ T~)(fig. 1) were chosen on the superconductive transition curve and the change of
0375-9601/90/S 03.50 © Elsevier Science Publishers B.V. (North-Holland)
543
Volume 146, number 9
PHYSICS LETTERS A
~/
T~°
arb.u~t
18 June 1990
change of the critical field of irreversibility H~2under pressure displays rather well (in sign and order of magnitude) just the same quantity as for the “true” second critical field. ative
~ Tb
~f
3. Results 3.1. Nd1.85Ce0.15CuO4_~ The change of the temperature of the superconductivearetransition and the(see critical of irreversibility quite negligible tablefield 1). Taking into
1’
t T~’T
account the accuracy of the measurements, the resuits can be considered as pointing to a slight decrease of T~under pressure (dT~/dP<0.03 K/kbar, dT~/dP<0) which is in qualitative agreement with the results obtained by the resistive technique: dT~/ dP=0.00+0.05 K/kbar in refs. [7] and [8] for T~ defined from the R=0lR~level, In ref. [81 the width of the superconductive transition in resistive measurements and the value of the normal state resistance decrease under pressure. The width of the transition and the value of the change of the signal ~ during the superconductive transition in our work increase slightly under pressure (the relative change is a fraction of that in ref. [81). These changes observed in both techniques are most prob-
K Fig. 1. Definition ofthe characteristic points in the superconduclive transition curve.
their position in magnetic field and under pressure was investigated. The superconductive transition was recorded during the internal heating of the sample from the temperature of the helium bath (4.2 K). Our experimental technique and the method of determination of physical quantities from the experimental data are described elsewhere [4]. Taking into account the analysis of the usage of various experimental techniques in measurements of critical parameters of HTSC in magnetic field in ref. [5] we can consider that the value of R~ 2obtained from our measurements corresponds to a greater extent to the so-called “field of irreversibility”. Nevertheless, according to our considerations [6], the rel-
ably connected with the changes of the properties of intergranular boundaries under pressure, which display themselves more violently in resistive measurements.
Table I Effect of pressure on critical parameters of electron-doped superconductors Compound
Definition ofT,
T~(P=0) (K)
dT~/dP (K/kbar)
—dH~2/dT~ (kOe/K)
H~2(kOe)~ (P—.0)
dH~2/dP~ (kOe/kbar)
dIn H~2/dP~ (kbar’)
NCCO
T,b T,°~ T,°~
21.6 18.4 15.7
—0.03 ±0.01 0.007±0.01 —0.08 ±0.01
11 4.8
62 21.5
—0.2 —0.07
—0.003 —0.003
NTCO
T~ ~
21.4 15.9
—0.1 ±0.05 —0.04 ±0.01
5.2 4.1
23 14
—0.5 —0.16
—0.02 —0.01
SCCO
T~ T~” T~
23.9 21.1 15.5
—0.07 ±0.02 —0.05 ±0.03 —0.08 ±0.02
12.1 4.9
61 28
—0.25 —0.05
—0.004 —0.002
~ T=7K.
544
Volume 146, number 9
PHYSICS LETTERS A
18 June 1990
kOe/K for a field directed in the basal plane obtamed for NCCO single crystals [9].
-
3.2. Nd,85Th015CuO4_~
-
a •
5
10
15
20
P, K
Fig. 2. 11,2 (T) dependences for NCCO (P-.. 0) determined from T~(a) and T,°”(b)
The P,~ ( T) dependences determined from T~and T~°~ have different curvature and temperature derivatives dP,2/dT (fig. 2). The curves ‘c2 (T) shift along the T-axis under pressure without distinct change of their shape. The representation of I?,2 (T) curves as 1 ~ t= T/TC, which is often used in the examination ofthe causes of the curvature of the H,2 (T) dependence gives for H> 5 kOe different values of q for the curves determined from T,b (q=0.26) and TC°~ (q=0.48). The critical field “c2 decreases a little under pressure. The pressure derivatives (PD) for J?,2 at T=7 K (when the “c2 (T) dependence tends towards linearity) are presented in table 1. Let us note that dlnP~dlnP~ dT dT
‘
dlnP~ dP
dlnP~2 dP
Our result for dP~2/dTis close to that obtained for P—~0 by the resistive technique for polycrystalline samples [8] (—dH,2/dT= 12 kOe/K at the R=0.5R~level and 18 kOe/KattheR=0.9R~level) and it is between the values of dH,2/dT= 4.3 kOe/ K for the magnetic field along the c-axis and 88.5 —
The width of the superconductive transition for NTCO investigated by the inductive technique is greater than that for NCCO. The temperature of superconductive transition decreases under pressure. The PD is somewhat larger in magnitude than for NCCO which is consistent with the results of ref. [8], where the resistance technique was used. The critical field Pc2 at fixed temperature also decreases under pressure. PD are given in table 1. The H~2(T) dependences for P—.. 0 are presented in fig. 3. The behaviour of the Pc2 (T) dependences is towards more gently sloping and the values of the power q are different to some extent (q=0.3 for the curves determined from T~ and q=0.35 for the dependences determined from T~) in comparison with those of NCCO. These results and also the greater width of the transition may be due to both the differences in the physical properties ofthe HTSC compounds under consideration and the different quality of the samples, which is connected with the fabrication route. We do not known any literature data on H~2( T) dependences for NTCO. 3.3. Sm1.85Ce0.15CuO4_~ We do not know any results of measurements of critical parameters of this compound under pres______________________________________
-
a -
~c’J
bV\~~
I I°
20
‘1, K
Fig. 3. 11,2(T) dependences for NTCO (P—0) determined from T~(a) and ~
(b).
545
Volume 146, number 9
PHYSICS LETTERS A
18 June 1990
ample, ref [11]) predict existence of a line which separates the region of reversibility of magnetic moment from the region of its irreversibility. This line follows the characteristic relationship 1 H213. Such a model was used, for example, to explain a set of experimental data obtained for some HTSC ceramic samples. It is necessary to point out that this model is suitable for a range of rather small magnetic fields (<1 kOe) and a narrow (a few degrees in width) ternperature region T,— T. The approach based upon the concept of of magnetic flux creep [12] seems to be the most promising for this work. The value of the power q in the expression 1 t = ~ is determined by the dependence of the pinning potential upon the temperature and magnetic field within this model. The mechanism of pinning is in an exploratory stage at present. The values q=2/3 and q=3/4 quoted in ref. [5] were obtamed from clear enough physical considerations and
-
—
a 0
è
-
—~
—
b ~
\
I 5
10
15
20T,K
they are in good agreement with the experimental re-
Fig. 4. 11, 2(T) dependences for SCCO (P-. 0) determined from T~’(a) and ~ (b).
sure. The results obtained in our work are in qualitative agreement with those of NCCO and NTCO. Both T~and P~2decrease under pressure (see table 1). The value of dT~/dP is close to that for Sm1,85Th05CuO4_~[10]. The width of the transition and the magnitude of the change of signal during the transition increase under pressure somewhat more slowly than for NCCO. The dependences H~2(T) for P-.~0 are presented in fig. 4. The values of the power q are equal to 0.32 and 0.39 for the curves determined from T,b and T~’~ respectively.
4. Discussion and conclusions At present there are several approaches within the scope of which attempts are made to explain the magnetic properties of HTSC compounds, in particular the behaviour of the H~2( T) dependence. One of such models is the model of superconductive glass which is based on the consideration of the HTSC ceramic sample as a system of superconductive granules with weak Josephson links. Theoretical analysis and numerical computation (see, for ex546
sults for different orientations of YBa2Cu3O7 ~ single crystals. Nevertheless, these values are unlikely to be the only ones possible (for example, they are in disagreement with experimental dependences J1c2 ( T) for Bi—Sr—Ca—Cu—O single crystals [131, we obtamed in ref. [14] q—. 0.2 for the kOe range of the magnetic field h> 10 for two orientations of Bi2Sr2CaCu2O~single crystals). The three compounds which were investigated in this work are similar in behaviour, the shape of P,2 (T) curves is in qualitative agreement with the model discussed in ref. [5]. The value of the power q in the equation 1 —t=aH depends upon the mechanism of pinning. If the potentials of pinning in NCCO grains for different field orientations have different dependences on magnetic field and ternperature and therefore different values of q are displayed, then the consequence of this is different q values for different manners of definition of T~in a magnetic field. Perhaps, in NTCO and SCCO we measure some “average” value of q and this fact may be due to peculiarities in the fabrication route or in the other characteristic features, which are displayed for example in a substantially greater width of the transition for the last two compounds in comparison with NCCO. The absence of a noticeable change of the shape of H,2 (T) curves points to absence of a
Volume 146, number 9
PHYSICS LETTERS A
noticeable effect of the hydrostatic pressure on the pinning potential in the range considered. At presence there is no theory that explains the changes in the critical parameters of HTSC while the interatomic distances in the crystal lattice change both for p-type and n-type HTSC. From the structural point of view the substantial difference between p- and n-type HTSC consists in the fact that the common element of the structures of the “hole” superconductors is the occurrence of layers of three-dimensional Cu06 octahedra or Cu05 pyramids. The corresponding layers in “electron” HTSC consist oftwo-dimensional Cu04 squares. The connections between an apical oxygen atom in the Cu05 pyramid or a CuO6 octahedron in a p-type HTSC and copper atoms are rather weak, but they are strong enough to have an effect on the Cu—O connection in the Cu02 plane [15,71. Thus the effect of pressure on T, for p-type HTSC is substantially greater than for n-type HTSC where the corresponding structure element is two-dimensional. As it seems to us the stated characteristic properties of HTSC structures give an opportunity to understand the common reason for a wide range ofvalues of pressure derivatives for different classes of HTSC. The whole complex of experimental data cited in the literature gives no opportunity to choose a single model to describe the mechanism of superconductivity in electron-doped HTSC compounds. However the decrease of their critical parameters (T,, ‘1,2) under pressure allows one to doubt the possibility to apply a bipolaronic model for the cornpounds under consideration.
18 June 1990
Acknowledgement We acknowledge support for this research from the USSR Interdepartmental Council for the HTSC Problem under Grant No. 32 and from the US Department of Energy under Grant No. DE-FGO386ER45230. We are greatly indebted to A.A. Abrikosov for his kind attention to this work and stimulating discussions and to E.S. Itskevich for his kind attention.
References [1] Y. Tokura,
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