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Effect of pressure on critical parameters of electron-doped superconductors Ln2−xMxCuO4−y (Ln=Nd, Sm, Pr; M=Ce, Th)
Physica C 168 (1990) 530-538 North-Holland
EFFECT OF PRESSURE ON CRITICAL PARAMETERS OF ELECTRON-DOPED SUPERCONDUCTORS Ln2_~Vl~CuO4_, (Ln--Nd, Sm, Pr;, M--Ce, Th) S.L. BUD'KO, A.G. GAPOTCHENKO and A.E. LUPPOV Institutefor High PressurePhysics, USSR Academy of Sciences, Troitsk, MoscowRegion, 142092, USSR
E.A. EARLY, M.B. MAPLE and J.T. MARKERT Institute for Pure and Applied PhysicalSciences and Department of Physics, Universityof California, San Diego, La Jolla, CA 92093, USA Received 28 April 1990
The effects of pressure on the temperature of the superconductive transition and the critical field of irreversibility for n-type polycrystalline high-To compounds Ln2_~lxCuO4_y (Ln=Nd, Sin, Pr, M=Ce, Th) was investigated. The pressure derivatives of critical parameters are less in magnitude than these for p-type high-To compounds. This fact may be accounted for by structure peculiarities of electron-doped HTSC.
1. Introduction An important development in high-T¢ superconductivity (HTSC) has been a recent discovery of a new class of high-T¢ superconductors: Ln2_xMxCuO4_y ( L n = P r , Nd, Sm, Eu, ... ; M = C e , Th) compounds [ 1-3 ] whose critical temperature are within the range of 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. these new compounds are n-type superconductors. Besides that, the crystal structures of all recently known HTSC contain two-dimensional arrays of CuO6 octahedra or CuO5 pyramids. In contrast to these crystal structures the electron-doped HTSC have the so-called T'-phase structure which is composed of sheets of CuO4 squares. Ce or Th ions in these oxides are supposed to be tetravalent and they become donors of electrons in Cu-O planes. Such substantial differences of this new HTSC class from the high-T¢ 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 in0921-4534/90/$03.50 © Elsevier Science Publishers B.V. ( North-Holland )
vestigations of changes of the temperature of superconductive transition T¢ and the critical field of irreversibility/-7¢2 under pressure can give some additional information on characteristic properties of the superconductive state in this HTSC class.
2. Experimental The polycrystallincsamples of five different compounds, Nd2_xCCxCuO4_y (NCCO), Nd2_xTh~CuO4_y (NTCO), Sm2_~Cc~CuO4_y (SCCO), Pr2_xCexCuO4_y (PCCO) and Pr2_~ThxCuOd_y ( P T C O ) with x = 0.15 were studicd. X-ray investigations showed the (214) phasc with no impurities at I00: I signal/noise level. Oxygen content analysis on similarly prepared specimens indicate oxygen deficiency y--0.02. The compounds under consideration 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 inductive technique with modulation frequency 160 Hz-60 kHz. Three characteristic points which correspond to different possible definitions of criticaltemperature (T¢off , T °n, T b ) (fig. I ) were chosen on the super-
S.L. Bud'ko et al. / Effect o f pressure on electron-doped HTSC
arb.
T, K Fig. 1. The definition of the characteristic points at the superconductive transition curve.
conductive transition curve and the change of their position in the magnetic field and under pressure was investigated. The superconductive transition was recorded during the internal heating of the sample from the temperature of helium bath (4.2 K). Our experimental technique and the method of determination of physical quantities from the experimental data were described elsewhere [ 4 ]. Taking into account the analysis of the use of various experimental techniques in the measurements of critical parameters of HTSC in a magnetic field carried out in ref. [ 5 ] we can consider that the value of /7c2 obtained from our measurements corresponds to a great extent to the so called "field of itreversibility". Nevertheless, according to our considerations [ 6 ], the relative change of the critical field of irreversibility R~2 under pressure corresponds rather well (in sign and the order of magnitude) to the relative change of the "true" second critical field.
5 31
der pressure ( IdTc/dPI < 0.03 K/kbar, dTddP< O) which is in qualitative agreement with the results obtained by resistive technique: dTddP= 0.00 + 0.05 K / k b a r in ref. [ 7 ] and the same value in ref. [ 8 ] for T¢ defined from R = 0 . 1 R , level. In ref. [ 8 ] the width of superconductive transition in resistive measurements and the value of normal state resistance decrease under pressure. The width of the transition and the value of the change of the signal AX during superconductive transition in our work increase slightly under pressure (the relative change is a fraction of that in ref. [8] ). These changes observed in both techniques are most probably connected with the changes of properties of intergranular boundaries under pressure, which display themselves more strongly in resistive measurements. The/~¢2(T) dependences determined from Tcb and T °" have different curvatures and temperature derivatives d/-Ie2/dT (fig. 2). The curves/~2 (T) shift along T-axis under pressure without distinct change of their shape. The representation of/~c2 (T) curves as l - t = a H q, t= T/T~ which is often used in the examination of the reasons causing the curvature of /qc2 (T) dependency gives different values of q for the curves determined from T b (q=0.26) and T °" (q=0.48) for H > 5 kOe. The critical field /t~2 decreases slightly under pressure. The pressure derivatives (PD) for/t~2 at T = 7 K (when/t~2 (T) dependency tends towards linearity) are presented in table I. Note, that d(ln/~o~ ) / d T ~ d ( l n Heb2) / d T and d(ln ~o~ ) / d P ~ d ( l n / t b 2 )/dP. Our results for d/-I~2/dT is close to that obtained for P ~ 0 by resistive technique for polycrystalline samples [8] ( - d H c 2 / d T = 12 k O e / K at R = 0 . S R , level and 18 k O e / K at R = 0 . 9 R , level) and it is between the values of - d H e 2 / d T = 4.3 k O e / K for the direction of the magnetic field along c-axis and 88.5 k O e / K for the field directed in the basal plane, obtained for NCCO single crystals [9].
3. Results
3.2. Ndl.ssTho.15CuO~_y 3. I. Ndl.ssCeo.tsCuO4_y The change of the temperature of superconductive transition and the critical field of irreversibility are quite negligible (see table I). Taking into account the accuracy of the measurements, the results can be considered as pointing to a slight decrease of T¢ un-
The width of superconductive transition for NTCO, investigated by inductive technique is greater than that for NCCO. The temperature of superconductive transition decreases under pressure. The PD is somewhat greater in magnitude than for NCCO which is consistent with the results of ref. [ 8 ], where
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S.L. Bud'ko et al. / Effect o f pressure on electron-doped HTSC
Table I Effect of pressure on critical parameters of electron-doped superconductors. compound
def. of Tc
T¢(P=0) (K)
dTJdP (K/kbar)
_dlT]¢2/dT a) (kOe/K)
/7,2 ") (kOe) (P~0)
d/t~2/dT "> (kOe/kbar)
din ITc2/dT ") (kbar -1 )
NCCO
T~ 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
Tb 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
Tb T~°" T °ff
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
PCCO
T¢ T~ T~
24.4 22.4 12.9
0.01 _+0.05 0.02_+0.03 0.05 -+0.02
11.6 b) 5.3 b)
52 b) 28.4 b)
0.27 b) 0.1 b)
0.005 b) 0.004 b)
PTCO
T¢ Tc
19.6 15.5
-0.005_+ 0.01 0.003 _+0.006
25 9.2
-0.35 - 0.2
-0.01 - 0.02
") T = 7 K ,
8.8 4.3
") T = 9 K .
t h e r e s i s t a n c e t e c h n i q u e w a s used. T h e c r i t i c a l f i e l d /~c2 a t a f i x e d t e m p e r a t u r e also d e c r e a s e s u n d e r p r e s sure. P D a r e g i v e n i n t a b l e I. T h e / ~ c 2 ( T ) d e p e n d e n c e s f o r P ~ 0 a r e p r e s e n t e d i n fig. 3. T h e b e h a v i o u r o f / 7 c 2 ( T ) d e p e n d e n c e s is m o r e gently sloping and the indices of power q are differe n t t o s o m e e x t e n t ( q = 0.3 f o r t h e c u r v e s d e t e r m i n e d from T b and q=0.35 for the dependences determ i n e d f r o m T °n ) i n c o m p a r i s o n w i t h t h o s e o f NCCO. These results and also the greater width of
d
2
d
~.
IO
I5
20
T,
K
5
Fig. 2. The ~c2 (T) dependences for NCCO ( P ~ 0) determined from T~ (1) and T~°* (2).
IO
15
20
T, K
Fi~ 3. The no2 (T) dependences for NTCO ( P ~ 0 ) determined from T~ (1) and T~°* (2).
S.L. Bud'ka et al. / Effect of pressure on electron-doped HTSC
533
determined from T b and T °", respectively. The derivatives ( - d/7¢2/dT) lie between the values of 36 k O e / K fr H / c direction of magnetic field and 1 k O e / K for HIIc, which were obtained for SCCO single crystals by means of resistive technique [ 11 ].
the transition may be accounted for by both the distinctions in physical properties of HTSC compounds under consideration and the different qualities of the samples, which is due to fabrication. To our knowledge, there are no published data on H¢2 (T) dependences for NTCO.
3.4. Pr msCeo.15CuO,_y 3.3. S m l.ssCeo.l~CuO,_y
As is the case of SCCO, neither do we know any papers where the results of the measurements of critical parameters (T¢, H¢2) under pressure or the investigation of H¢2(T) dependency at P ~ 0 for this compound were published. Our results of/7c2(T) measurements for PCCO (fig. 5 ) are similar to those for NCCO and SCCO. The width of the superconductive transition decreases a little under pressure and the value of the change of the signal AX during superconductive transition slightly increases. As distinct from the three compounds mentioned above, the temperature of superconductive transition increases under pressure with the pressure derivative rather small in magnitude. The He2 (T) curves shift
We do not know any published results of the measurements of critical parameters of this compound under pressure. The results obtained in our work are in qualitative agreement with those of NCCO and NTCO. Both T¢ andA¢2 decrease under pressure (see table I). The value of d T d d P is close to that for Sml.85Tho.15CuO4_y [ 10]. The width of the transition and the magnitude of change of signal during the transition increase under pressure somewhat more slowly than for NCCO. The dependences ~¢2 (T) for P ~ 0 are presented in fig. 4. The indices of power q are equal to 0.32 and 0.39 for the curves
2\,
\
5
Io
z5
2o- ~, K"
Fig. 4. The I~c2(T) dependencesfor SCCO (P~ 0) determinedfrom T b ( 1) and T~ (2).
534
S.L, Bud'ko et al. / Effect of pressure on electron-doped HTSC
I0
I5
20
T, K
Fig. 5. The ~/~2(T) dependences for PCCO ( P ~ 0) determined from T~ ( 1 ) and T °" (2).
along T-axis under pressure without distinct change of their shape. The magnitude of/7~2 for constant temperature increases under pressure. The pressure derivatives for PCCO critical parameters are presented in table I.
The representation of/7c2 (T) curves as 1 - t = a H q, t = T/T¢, points to the existence of two regions of the magnetic field, H < H * , H > H * , ( H * ~ 15-20 kOe) (and, respectively, two regions of temperature z < z*, z>z*, with z = l - T/Tc, v*~0.4), for which indices
O
°
7 1
I
I
t
I
2
3
4
in H
Fig. 6. "Lines ofirreversibility" for PCCO (in logarithmic scale) determined from T~ ( 1 ) and T °" (2).
S.L. Bud'ko et al. / Effect of pressure on electron-doped HTSC
535
when the modulation frequency increases (fig. 7). This result is in qualitative agreement with the results of theoretical and experimental investigations of the "line of irreversibility" in HTSC with regard to the flux creep [ 5,12,13 ]. 3.5. Pr L85Tho.lsCuO4_y f~
5
IO
I5
20
T, K
Fig. 7. The/-7°2 (T) dependences for PTCO ( P ~ 0 ) determined from T b (1) and Y °° (2).
::~ 2
o
n
.....o.,.o~
~>~ --°''°~°'''-°-°
S
i
l
I
!
i
I
6
7
8
9
10
II
4. Discussion and conclusions
in f Fig. 8. The T ~ ( f ) dependence for P C C O
(I) and P T C O
The H~2 (T) dependences for PTCO (fig. 8 ) which were obtained in our experiments are similar in their appearance to those for NTCO. As in the case of the last two compounds the absence of suitable literature data for PTCO gives no opportunity to compare our results with those of other investigations. The pressure derivatives of critical parameters obtained for T~ and T °n, being compared, point to the conclusion that within the limits of the accuracy of measurements the temperature of superconductive transition and critical field/7c2 remain constant or slightly decrease under pressure (see table I). The value of the change of signal AX during superconductive transition decreases a little under pressure. The indices of power q for H > 5 kOe almost coincide in magnitude for both manners of Tc definition (q~0.26 for T b and q~0.32 for T°n). For PTCO we have also measured the critical temperature T ~ at constant magnetic field H = 10 kOe for various magnitudes of modulation frequency. Similar to the case of PCCO, T ~ increases when the modulation frequency increases (fig. 7).
(2)
(H= 10 kOe, P ~ 0 ) .
of power q are different (fig. 6). The indices of power for H < H * are q~0.64 (for Tcb) and q~0.68 (for T °" ) and, respectively, q~0.33 and q~0.39 for H~ /-/*. The measurements of the temperature of the superconductive transition T ~ at fixed magnetic field ( H = 10 kOe) at various values of modulation frequency were carried out for PCCO. T ~ increases
The superconductive properties for P ~ 0 of electron-doped superconductors under investigation (critical temperature, the width of the transition and ~ 2 (T) dependences) point to the similarity between NCCO, SCCO and PCCO on the one hand and NTCO and PTCO on the other. The magnitudes of T¢ a n d / ~ 2 for the second group are less than for the first group and the superconducting transition is more prolonged. The different values of Tc depending on chemical composition were also mentioned in ref. [ 3 ]. In our opinion, the reason for it lies in the difference between the radii of the rare earth ions. The difference between the radii of Nd 3+, Pr 3+ and Sin 3+ ions (0.99 A, 0.97 A and 1.00 A, respectively) is
536
S.L. Bud'ko et al. / Effect o f pressure on electron-doped H T S C
substantially less than the difference between the radii of Ce 4+ and Th 4+ ions (1.14 A and 1.01 A, respectively), this results in the fact that the differences between the two groups of electron-doped superconductors under considerations are appreciably greater than between the different compounds within the groups. The different values of width of inductive transition for different samples, as it seems, may be explained as an outcome of fabrication routes (note, that you can see the peculiarities in the resistive transition curves for NTCO and PTCO, which are present in ref. [ 3 ]; it is possible to consider these peculiarities to be a consequence of the inhomogeneity of the samples). In what follows we shall discuss the results of the measurements in a magnetic field. 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 behavior of the He2(T) dependence. One such model is the superconductive glass model, which is based on consideration of the HTSC ceramic sample as a system of superconductive granules with weak Josephson links. Theoretical analysis and numerical computation (see, for example, ref. [ 14 ] ) predict the existence of a line which separates a region of reversibility of magnetic moment from a region of its irreversibility. This line follows the characteristic relationship l - t ~/_/2/3. 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 ( < l kOe) and a narrow (a few degrees in width) temperature ( T c - T) region. The approach based upon the concept of magnetic flux creep [ 15 ] seems to be the most promising for this work. The value of the index of power q in the 1 - t = a H q expression is determined by the dependence of the pinning potential upon the temperature and magnetic field within the scope of this model. A study of the mechanism of pinning is under way at present. The values q = ] and q = ~4quoted in ref. [ 5 ] were obtained from rather clear physical considerations and they are in good agreement with the experimental results for different orientations of YBa2Cu307_6 single crystals. Nevertheless, these
values are unlikely to be the only possible ones (for example, they are in disagreement with experimental dependences HcE(T) for B i - S r - C a - C u - O single crystals [12], we obtained in ref. [14] q~0.2 for H > 10 kOe range of magnetic field for two orientations of Bi2SrECaCu2Ox single crystals). Five compounds which were investigated in this work are similar in behavior, the shape of/~¢2 (T) curves is in qualitative agreement with the model discussed in ref. [5]. The index of power q in 1 - t = a H q equation depends upon the mechanism of pinning. If the potentials of pinning in NCCO grains for different field orientations have different dependences upon a magnetic field and temperature and therefore different values of q appear in the equation, then the consequence of this is the difference of q values for different ways of definition of Tc in a magnetic field. Perhaps, in other electron-doped superconductors under consideration we measure some "average" value of q and this fact may be due to peculiarities in fabrication or in the other characteristic features, which are displayed for example in substantially greater width of the transition for these compounds in comparison with NCCO. The display of different indices of power q in 1 - t = a H q equation for PCCO for different ranges of a magnetic field (as it was, for sample, for BSCCO single crystals [ 16 ] ) may be caused by the presence of two mechanisms of pinning (which give rise to weak and strong pinning) [ 16,17 ]. It is worth mentioning that the index of power q for PCCO at H < H * agrees, within the range of accuracy of the measurements, with the value q = ] which is derived both from the superconductive glass model and from the approach based upon the conception of flux creep. The temperature range, where this value of q is observed, is rather wide ( 1 - t ~ 0 . 4 ) . Maybe this fact points to the agreement between the PCCO behavior and the theoretical model based upon flux creep concept. The value of the index of power q for H>/-/* is similar to that for NTCO, SCCO and PTCO for H > 5 kOe (q ~ ~ ). The experimental data for NTCO, SCCO and PTCO indicate that the index of power q in the initial range of a magnetic field ( H < 5 kOe) for compounds mentioned above is greater than that for a high magnetic field. The small amount of experimental points for the initial range of a magnetic field gives no opportunity to determine the magnitude of
S.L. Bud'ko et al. / Effect o f pressure on electron-doped H T S C
q with sufficient accuracy. As it seems to us, this indicates that the magnetic field H* for NTCO, SCCO and PTCO is lower than for PCCO (i.e./-/* ~ 2.5-5 kOe). In the measurements of the temperature of superconductive transition at a constant magnetic field as a function of modulation frequency T ~ ( f ) , which were carried out for PCCO and PTCO, T~n-f relation close to logarithmic was observed. For the modulation frequency within the range 1-60 kHz the experimental results for PCCO are in agreement with 1 - t ~ ~ (In f ) 2/3 relationship (fig. 9 ) obtained in ref. [ 8 ] for q = 2, which takes place in our experiments. The absence of noticeable change of the shape of H~2 (T) curves points to the absence of noticeable effect of hydrostatic pressure in the range under consideration upon the pinning potential. At present there is no theory for both p-type and n-type HTSC that explains the changes in critical parameters of HTSC while the interatomic distances in crystal lattice change. CuO2 planes are considered to be responsible for superconductivity in HTSC compounds. The common structural feature of all p-type HTSC is the presence of two-dimensional layers of three-dimensional CuO6-0ctahedra or CuOs-pyramids (for La2_xAxCuO4 ( A = C a , St, Ba) - array of CuO6-0ctahedra, for YBa2Cu307_6-1ike compounds - two layers of CuOs-pyramids and C u O - O chains, T1- and Bi-based HTSC B i ( T 1 ) - C a - S r ( B a ) - C u - O with general formula ( 2 2 ( n - 1 )n) - one layer of CuOroctahedral (n--1 ), two layers of CuOs-pyramids
b
0 0 0.~.
o~ "o.o °XO~o
o 1 6
i 7
I 8
~ 9
I IO
1 II
in £ Fig. 9. r3/2(ln f ) d e p e n d e n c e for P C C O ( H = 10 kOe, P ~ 0 ) , T= I - T/T¢.
537
(n = 2 ) or two layers of CuOs-pyramids with one or more (depending upon the composition) layers of "two-dimensional" CuO4-squares (n > 3). In contrast to p-type HTSC only the layer of two-dimensional CuO4 squares is present in the structure of electron-doped HTSC. The connections between apical oxygen atom at CuO5 pyramid or CuO6 octahedron at p-type HTSC and copper atoms are rather weak, but they are strong enough to influence C u - O connection in CuO2 plane [ 18,7 ]. Thus the effect of pressure on Tc 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 the wide range of values of pressure derivatives for different classes of HTSC. In order to compare pressure derivatives of To, let us consider the most simple compounds, which have one CuO2 layer in their structural unit: (La, Sr)ECuO 4 (T-phase), which contains an array of CuOr-octahedra, (Nd, Ce, Sr)ECuO 4 (T*-phase) recently discovered P-type HTSC [ 19 ], which contains an array of CuOs-pyramids and electron-doped HTSC (T'-phase), which contains CuO4 two-dimensional squares in a corresponding layer. In the first case there are two apical oxygen atoms away from the CuO2-plane for each CuO4-square, in the second case there is one such atom. The magnitude of pressure derivatives d T d d P for electron-doped HTSC is in turn less than that for (Nd, Ce, Sr)ECuO 4 [ 7 ], which is not greater than that for (La, Sr) 2CUO4. This fact serves as a qualitative confirmation of the assumption of substantial effect of apical oxygen atoms upon the pressure derivatives of critical temperature. 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-parameters (T~, H¢2) under pressure for NCCO, N T C O and SCCO allows to doubt the possibility to apply a bipolaronic model, within the scope of which the parameters under consideration increase while the interatomic distance decreases, to their behavior.
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S.L. Bud'ko et al. / Effect o f pressure on electron-doped H T S C
Acknowledgements W e a c k n o w l e d g e s u p p o r t for this r e s e a r c h f r o m USSR Interdepartmental Council on HTSC Probl e m u n d e r G r a n t N o . 32 a n d f r o m U S D e p a r t m e n t of Energy under Grant No. DE-FG03-86ER45230. W e are greatly i n d e b t e d to A.A. A b r i k o s o v for his k i n d a t t e n t i o n to this w o r k a n d s t i m u l a t e d i n t e r a c t i o n s a n d to E.S. I t s k e v i c h for his k i n d a t t e n t i o n .
References [ 1] Y. Tokura, H. Takagi and S. Uchida, Nature 337 (1989) 345. [2]J.T. Markert and M.B. Maple, Solid State Commun. 70 (1989) 145. [ 3 ] J.T. Markert, E.A. Early, T. Bjornholm et al., Physica C 158 (1989) 178. [4] S.L. Bud'ka, A.G. Gapotchenko, E.S. Itskevich, Solid State Commun. 69 (1989) 387. [ 5 ] A.P. Malozemoff, T.K. Worthington, Y. Yeshurun and F. Holtzberg, Phys. Rev. B38 (1988) 7203.
[6] S.L. Bud'ko, A.G. Gapotchenko, E.S. Itskevich and A.E. Luppov, Phys. Lett. A 140 (1989) 197. [ 7 ] C. Murayama, N. Mori, S. Yomo et al., Nature 339 (1989) 293. [8] C.L Seaman, N.Y. Ayouh, T. Bjornholm et al., Physica C 159 (1989) 391. [9 ] Y. Hidaka and M. Suzuki, Nature 338 ( 1989 ) 320. [ 10 ] E.A. Early, N.Y. Ayoub, J. BeiUeet al., Physica C 160 ( 1989 ) 320. [ 11 ] Y. Dalichaouch, B.W. Lee, C.L. Seamon et al., Phys. Rev. LetL 64 (1989) 599. [12 ] Y. Yeshurun, A.P. Malozemoff, T.K. Worthington et al., Cryogenics 29 (1989) 258. [13]J. van den Berg, C.J. van der Beek, P.H. Kes et al., Supercond. Sei. Techn. 1 (1989) 249. [ 14 ] I. Morgenstern, K.A. Muller, J.B. Bednorz, Physica C 152 (1988) 85. [15] Y. Yeshurun and A.P. Malozemoff, Phys. Rev. Lett. 60 (1988) 2202. [16] S.L Bud'ko, A.G. Gapotchenko, E.S. Itskevich, A.E. Luppov, Supercond.: Phys., Chem. Techn. (USSR) 3 (1990) 36. [ 17 ] A.V. Bezryadin, V.N. Kopylov, V.M. Krasnov et al., Pis'ma ZhETF (USSR) 51 (1990) 147. [ 18 ] R.P. Gupta and M. Gupta, Physica C 160 (1989) 129. [ 19 ] J. Akimitzu, S. Suzuki, M. Watanabe and H. Sawa, Jpn. J. Appl. Phys. 27 (1989) L1859.
EFFECT OF PRESSURE ON CRITICAL PARAMETERS OF ELECTRON-DOPED SUPERCONDUCTORS Ln2_~Vl~CuO4_, (Ln--Nd, Sm, Pr;, M--Ce, Th) S.L. BUD'KO, A.G. GAPOTCHENKO and A.E. LUPPOV Institutefor High PressurePhysics, USSR Academy of Sciences, Troitsk, MoscowRegion, 142092, USSR
E.A. EARLY, M.B. MAPLE and J.T. MARKERT Institute for Pure and Applied PhysicalSciences and Department of Physics, Universityof California, San Diego, La Jolla, CA 92093, USA Received 28 April 1990
The effects of pressure on the temperature of the superconductive transition and the critical field of irreversibility for n-type polycrystalline high-To compounds Ln2_~lxCuO4_y (Ln=Nd, Sin, Pr, M=Ce, Th) was investigated. The pressure derivatives of critical parameters are less in magnitude than these for p-type high-To compounds. This fact may be accounted for by structure peculiarities of electron-doped HTSC.
1. Introduction An important development in high-T¢ superconductivity (HTSC) has been a recent discovery of a new class of high-T¢ superconductors: Ln2_xMxCuO4_y ( L n = P r , Nd, Sm, Eu, ... ; M = C e , Th) compounds [ 1-3 ] whose critical temperature are within the range of 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. these new compounds are n-type superconductors. Besides that, the crystal structures of all recently known HTSC contain two-dimensional arrays of CuO6 octahedra or CuO5 pyramids. In contrast to these crystal structures the electron-doped HTSC have the so-called T'-phase structure which is composed of sheets of CuO4 squares. Ce or Th ions in these oxides are supposed to be tetravalent and they become donors of electrons in Cu-O planes. Such substantial differences of this new HTSC class from the high-T¢ 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 in0921-4534/90/$03.50 © Elsevier Science Publishers B.V. ( North-Holland )
vestigations of changes of the temperature of superconductive transition T¢ and the critical field of irreversibility/-7¢2 under pressure can give some additional information on characteristic properties of the superconductive state in this HTSC class.
2. Experimental The polycrystallincsamples of five different compounds, Nd2_xCCxCuO4_y (NCCO), Nd2_xTh~CuO4_y (NTCO), Sm2_~Cc~CuO4_y (SCCO), Pr2_xCexCuO4_y (PCCO) and Pr2_~ThxCuOd_y ( P T C O ) with x = 0.15 were studicd. X-ray investigations showed the (214) phasc with no impurities at I00: I signal/noise level. Oxygen content analysis on similarly prepared specimens indicate oxygen deficiency y--0.02. The compounds under consideration 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 inductive technique with modulation frequency 160 Hz-60 kHz. Three characteristic points which correspond to different possible definitions of criticaltemperature (T¢off , T °n, T b ) (fig. I ) were chosen on the super-
S.L. Bud'ko et al. / Effect o f pressure on electron-doped HTSC
arb.
T, K Fig. 1. The definition of the characteristic points at the superconductive transition curve.
conductive transition curve and the change of their position in the magnetic field and under pressure was investigated. The superconductive transition was recorded during the internal heating of the sample from the temperature of helium bath (4.2 K). Our experimental technique and the method of determination of physical quantities from the experimental data were described elsewhere [ 4 ]. Taking into account the analysis of the use of various experimental techniques in the measurements of critical parameters of HTSC in a magnetic field carried out in ref. [ 5 ] we can consider that the value of /7c2 obtained from our measurements corresponds to a great extent to the so called "field of itreversibility". Nevertheless, according to our considerations [ 6 ], the relative change of the critical field of irreversibility R~2 under pressure corresponds rather well (in sign and the order of magnitude) to the relative change of the "true" second critical field.
5 31
der pressure ( IdTc/dPI < 0.03 K/kbar, dTddP< O) which is in qualitative agreement with the results obtained by resistive technique: dTddP= 0.00 + 0.05 K / k b a r in ref. [ 7 ] and the same value in ref. [ 8 ] for T¢ defined from R = 0 . 1 R , level. In ref. [ 8 ] the width of superconductive transition in resistive measurements and the value of normal state resistance decrease under pressure. The width of the transition and the value of the change of the signal AX during superconductive transition in our work increase slightly under pressure (the relative change is a fraction of that in ref. [8] ). These changes observed in both techniques are most probably connected with the changes of properties of intergranular boundaries under pressure, which display themselves more strongly in resistive measurements. The/~¢2(T) dependences determined from Tcb and T °" have different curvatures and temperature derivatives d/-Ie2/dT (fig. 2). The curves/~2 (T) shift along T-axis under pressure without distinct change of their shape. The representation of/~c2 (T) curves as l - t = a H q, t= T/T~ which is often used in the examination of the reasons causing the curvature of /qc2 (T) dependency gives different values of q for the curves determined from T b (q=0.26) and T °" (q=0.48) for H > 5 kOe. The critical field /t~2 decreases slightly under pressure. The pressure derivatives (PD) for/t~2 at T = 7 K (when/t~2 (T) dependency tends towards linearity) are presented in table I. Note, that d(ln/~o~ ) / d T ~ d ( l n Heb2) / d T and d(ln ~o~ ) / d P ~ d ( l n / t b 2 )/dP. Our results for d/-I~2/dT is close to that obtained for P ~ 0 by resistive technique for polycrystalline samples [8] ( - d H c 2 / d T = 12 k O e / K at R = 0 . S R , level and 18 k O e / K at R = 0 . 9 R , level) and it is between the values of - d H e 2 / d T = 4.3 k O e / K for the direction of the magnetic field along c-axis and 88.5 k O e / K for the field directed in the basal plane, obtained for NCCO single crystals [9].
3. Results
3.2. Ndl.ssTho.15CuO~_y 3. I. Ndl.ssCeo.tsCuO4_y The change of the temperature of superconductive transition and the critical field of irreversibility are quite negligible (see table I). Taking into account the accuracy of the measurements, the results can be considered as pointing to a slight decrease of T¢ un-
The width of superconductive transition for NTCO, investigated by inductive technique is greater than that for NCCO. The temperature of superconductive transition decreases under pressure. The PD is somewhat greater in magnitude than for NCCO which is consistent with the results of ref. [ 8 ], where
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S.L. Bud'ko et al. / Effect o f pressure on electron-doped HTSC
Table I Effect of pressure on critical parameters of electron-doped superconductors. compound
def. of Tc
T¢(P=0) (K)
dTJdP (K/kbar)
_dlT]¢2/dT a) (kOe/K)
/7,2 ") (kOe) (P~0)
d/t~2/dT "> (kOe/kbar)
din ITc2/dT ") (kbar -1 )
NCCO
T~ 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
Tb 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
Tb T~°" T °ff
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
PCCO
T¢ T~ T~
24.4 22.4 12.9
0.01 _+0.05 0.02_+0.03 0.05 -+0.02
11.6 b) 5.3 b)
52 b) 28.4 b)
0.27 b) 0.1 b)
0.005 b) 0.004 b)
PTCO
T¢ Tc
19.6 15.5
-0.005_+ 0.01 0.003 _+0.006
25 9.2
-0.35 - 0.2
-0.01 - 0.02
") T = 7 K ,
8.8 4.3
") T = 9 K .
t h e r e s i s t a n c e t e c h n i q u e w a s used. T h e c r i t i c a l f i e l d /~c2 a t a f i x e d t e m p e r a t u r e also d e c r e a s e s u n d e r p r e s sure. P D a r e g i v e n i n t a b l e I. T h e / ~ c 2 ( T ) d e p e n d e n c e s f o r P ~ 0 a r e p r e s e n t e d i n fig. 3. T h e b e h a v i o u r o f / 7 c 2 ( T ) d e p e n d e n c e s is m o r e gently sloping and the indices of power q are differe n t t o s o m e e x t e n t ( q = 0.3 f o r t h e c u r v e s d e t e r m i n e d from T b and q=0.35 for the dependences determ i n e d f r o m T °n ) i n c o m p a r i s o n w i t h t h o s e o f NCCO. These results and also the greater width of
d
2
d
~.
IO
I5
20
T,
K
5
Fig. 2. The ~c2 (T) dependences for NCCO ( P ~ 0) determined from T~ (1) and T~°* (2).
IO
15
20
T, K
Fi~ 3. The no2 (T) dependences for NTCO ( P ~ 0 ) determined from T~ (1) and T~°* (2).
S.L. Bud'ka et al. / Effect of pressure on electron-doped HTSC
533
determined from T b and T °", respectively. The derivatives ( - d/7¢2/dT) lie between the values of 36 k O e / K fr H / c direction of magnetic field and 1 k O e / K for HIIc, which were obtained for SCCO single crystals by means of resistive technique [ 11 ].
the transition may be accounted for by both the distinctions in physical properties of HTSC compounds under consideration and the different qualities of the samples, which is due to fabrication. To our knowledge, there are no published data on H¢2 (T) dependences for NTCO.
3.4. Pr msCeo.15CuO,_y 3.3. S m l.ssCeo.l~CuO,_y
As is the case of SCCO, neither do we know any papers where the results of the measurements of critical parameters (T¢, H¢2) under pressure or the investigation of H¢2(T) dependency at P ~ 0 for this compound were published. Our results of/7c2(T) measurements for PCCO (fig. 5 ) are similar to those for NCCO and SCCO. The width of the superconductive transition decreases a little under pressure and the value of the change of the signal AX during superconductive transition slightly increases. As distinct from the three compounds mentioned above, the temperature of superconductive transition increases under pressure with the pressure derivative rather small in magnitude. The He2 (T) curves shift
We do not know any published results of the measurements of critical parameters of this compound under pressure. The results obtained in our work are in qualitative agreement with those of NCCO and NTCO. Both T¢ andA¢2 decrease under pressure (see table I). The value of d T d d P is close to that for Sml.85Tho.15CuO4_y [ 10]. The width of the transition and the magnitude of change of signal during the transition increase under pressure somewhat more slowly than for NCCO. The dependences ~¢2 (T) for P ~ 0 are presented in fig. 4. The indices of power q are equal to 0.32 and 0.39 for the curves
2\,
\
5
Io
z5
2o- ~, K"
Fig. 4. The I~c2(T) dependencesfor SCCO (P~ 0) determinedfrom T b ( 1) and T~ (2).
534
S.L, Bud'ko et al. / Effect of pressure on electron-doped HTSC
I0
I5
20
T, K
Fig. 5. The ~/~2(T) dependences for PCCO ( P ~ 0) determined from T~ ( 1 ) and T °" (2).
along T-axis under pressure without distinct change of their shape. The magnitude of/7~2 for constant temperature increases under pressure. The pressure derivatives for PCCO critical parameters are presented in table I.
The representation of/7c2 (T) curves as 1 - t = a H q, t = T/T¢, points to the existence of two regions of the magnetic field, H < H * , H > H * , ( H * ~ 15-20 kOe) (and, respectively, two regions of temperature z < z*, z>z*, with z = l - T/Tc, v*~0.4), for which indices
O
°
7 1
I
I
t
I
2
3
4
in H
Fig. 6. "Lines ofirreversibility" for PCCO (in logarithmic scale) determined from T~ ( 1 ) and T °" (2).
S.L. Bud'ko et al. / Effect of pressure on electron-doped HTSC
535
when the modulation frequency increases (fig. 7). This result is in qualitative agreement with the results of theoretical and experimental investigations of the "line of irreversibility" in HTSC with regard to the flux creep [ 5,12,13 ]. 3.5. Pr L85Tho.lsCuO4_y f~
5
IO
I5
20
T, K
Fig. 7. The/-7°2 (T) dependences for PTCO ( P ~ 0 ) determined from T b (1) and Y °° (2).
::~ 2
o
n
.....o.,.o~
~>~ --°''°~°'''-°-°
S
i
l
I
!
i
I
6
7
8
9
10
II
4. Discussion and conclusions
in f Fig. 8. The T ~ ( f ) dependence for P C C O
(I) and P T C O
The H~2 (T) dependences for PTCO (fig. 8 ) which were obtained in our experiments are similar in their appearance to those for NTCO. As in the case of the last two compounds the absence of suitable literature data for PTCO gives no opportunity to compare our results with those of other investigations. The pressure derivatives of critical parameters obtained for T~ and T °n, being compared, point to the conclusion that within the limits of the accuracy of measurements the temperature of superconductive transition and critical field/7c2 remain constant or slightly decrease under pressure (see table I). The value of the change of signal AX during superconductive transition decreases a little under pressure. The indices of power q for H > 5 kOe almost coincide in magnitude for both manners of Tc definition (q~0.26 for T b and q~0.32 for T°n). For PTCO we have also measured the critical temperature T ~ at constant magnetic field H = 10 kOe for various magnitudes of modulation frequency. Similar to the case of PCCO, T ~ increases when the modulation frequency increases (fig. 7).
(2)
(H= 10 kOe, P ~ 0 ) .
of power q are different (fig. 6). The indices of power for H < H * are q~0.64 (for Tcb) and q~0.68 (for T °" ) and, respectively, q~0.33 and q~0.39 for H~ /-/*. The measurements of the temperature of the superconductive transition T ~ at fixed magnetic field ( H = 10 kOe) at various values of modulation frequency were carried out for PCCO. T ~ increases
The superconductive properties for P ~ 0 of electron-doped superconductors under investigation (critical temperature, the width of the transition and ~ 2 (T) dependences) point to the similarity between NCCO, SCCO and PCCO on the one hand and NTCO and PTCO on the other. The magnitudes of T¢ a n d / ~ 2 for the second group are less than for the first group and the superconducting transition is more prolonged. The different values of Tc depending on chemical composition were also mentioned in ref. [ 3 ]. In our opinion, the reason for it lies in the difference between the radii of the rare earth ions. The difference between the radii of Nd 3+, Pr 3+ and Sin 3+ ions (0.99 A, 0.97 A and 1.00 A, respectively) is
536
S.L. Bud'ko et al. / Effect o f pressure on electron-doped H T S C
substantially less than the difference between the radii of Ce 4+ and Th 4+ ions (1.14 A and 1.01 A, respectively), this results in the fact that the differences between the two groups of electron-doped superconductors under considerations are appreciably greater than between the different compounds within the groups. The different values of width of inductive transition for different samples, as it seems, may be explained as an outcome of fabrication routes (note, that you can see the peculiarities in the resistive transition curves for NTCO and PTCO, which are present in ref. [ 3 ]; it is possible to consider these peculiarities to be a consequence of the inhomogeneity of the samples). In what follows we shall discuss the results of the measurements in a magnetic field. 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 behavior of the He2(T) dependence. One such model is the superconductive glass model, which is based on consideration of the HTSC ceramic sample as a system of superconductive granules with weak Josephson links. Theoretical analysis and numerical computation (see, for example, ref. [ 14 ] ) predict the existence of a line which separates a region of reversibility of magnetic moment from a region of its irreversibility. This line follows the characteristic relationship l - t ~/_/2/3. 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 ( < l kOe) and a narrow (a few degrees in width) temperature ( T c - T) region. The approach based upon the concept of magnetic flux creep [ 15 ] seems to be the most promising for this work. The value of the index of power q in the 1 - t = a H q expression is determined by the dependence of the pinning potential upon the temperature and magnetic field within the scope of this model. A study of the mechanism of pinning is under way at present. The values q = ] and q = ~4quoted in ref. [ 5 ] were obtained from rather clear physical considerations and they are in good agreement with the experimental results for different orientations of YBa2Cu307_6 single crystals. Nevertheless, these
values are unlikely to be the only possible ones (for example, they are in disagreement with experimental dependences HcE(T) for B i - S r - C a - C u - O single crystals [12], we obtained in ref. [14] q~0.2 for H > 10 kOe range of magnetic field for two orientations of Bi2SrECaCu2Ox single crystals). Five compounds which were investigated in this work are similar in behavior, the shape of/~¢2 (T) curves is in qualitative agreement with the model discussed in ref. [5]. The index of power q in 1 - t = a H q equation depends upon the mechanism of pinning. If the potentials of pinning in NCCO grains for different field orientations have different dependences upon a magnetic field and temperature and therefore different values of q appear in the equation, then the consequence of this is the difference of q values for different ways of definition of Tc in a magnetic field. Perhaps, in other electron-doped superconductors under consideration we measure some "average" value of q and this fact may be due to peculiarities in fabrication or in the other characteristic features, which are displayed for example in substantially greater width of the transition for these compounds in comparison with NCCO. The display of different indices of power q in 1 - t = a H q equation for PCCO for different ranges of a magnetic field (as it was, for sample, for BSCCO single crystals [ 16 ] ) may be caused by the presence of two mechanisms of pinning (which give rise to weak and strong pinning) [ 16,17 ]. It is worth mentioning that the index of power q for PCCO at H < H * agrees, within the range of accuracy of the measurements, with the value q = ] which is derived both from the superconductive glass model and from the approach based upon the conception of flux creep. The temperature range, where this value of q is observed, is rather wide ( 1 - t ~ 0 . 4 ) . Maybe this fact points to the agreement between the PCCO behavior and the theoretical model based upon flux creep concept. The value of the index of power q for H>/-/* is similar to that for NTCO, SCCO and PTCO for H > 5 kOe (q ~ ~ ). The experimental data for NTCO, SCCO and PTCO indicate that the index of power q in the initial range of a magnetic field ( H < 5 kOe) for compounds mentioned above is greater than that for a high magnetic field. The small amount of experimental points for the initial range of a magnetic field gives no opportunity to determine the magnitude of
S.L. Bud'ko et al. / Effect o f pressure on electron-doped H T S C
q with sufficient accuracy. As it seems to us, this indicates that the magnetic field H* for NTCO, SCCO and PTCO is lower than for PCCO (i.e./-/* ~ 2.5-5 kOe). In the measurements of the temperature of superconductive transition at a constant magnetic field as a function of modulation frequency T ~ ( f ) , which were carried out for PCCO and PTCO, T~n-f relation close to logarithmic was observed. For the modulation frequency within the range 1-60 kHz the experimental results for PCCO are in agreement with 1 - t ~ ~ (In f ) 2/3 relationship (fig. 9 ) obtained in ref. [ 8 ] for q = 2, which takes place in our experiments. The absence of noticeable change of the shape of H~2 (T) curves points to the absence of noticeable effect of hydrostatic pressure in the range under consideration upon the pinning potential. At present there is no theory for both p-type and n-type HTSC that explains the changes in critical parameters of HTSC while the interatomic distances in crystal lattice change. CuO2 planes are considered to be responsible for superconductivity in HTSC compounds. The common structural feature of all p-type HTSC is the presence of two-dimensional layers of three-dimensional CuO6-0ctahedra or CuOs-pyramids (for La2_xAxCuO4 ( A = C a , St, Ba) - array of CuO6-0ctahedra, for YBa2Cu307_6-1ike compounds - two layers of CuOs-pyramids and C u O - O chains, T1- and Bi-based HTSC B i ( T 1 ) - C a - S r ( B a ) - C u - O with general formula ( 2 2 ( n - 1 )n) - one layer of CuOroctahedral (n--1 ), two layers of CuOs-pyramids
b
0 0 0.~.
o~ "o.o °XO~o
o 1 6
i 7
I 8
~ 9
I IO
1 II
in £ Fig. 9. r3/2(ln f ) d e p e n d e n c e for P C C O ( H = 10 kOe, P ~ 0 ) , T= I - T/T¢.
537
(n = 2 ) or two layers of CuOs-pyramids with one or more (depending upon the composition) layers of "two-dimensional" CuO4-squares (n > 3). In contrast to p-type HTSC only the layer of two-dimensional CuO4 squares is present in the structure of electron-doped HTSC. The connections between apical oxygen atom at CuO5 pyramid or CuO6 octahedron at p-type HTSC and copper atoms are rather weak, but they are strong enough to influence C u - O connection in CuO2 plane [ 18,7 ]. Thus the effect of pressure on Tc 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 the wide range of values of pressure derivatives for different classes of HTSC. In order to compare pressure derivatives of To, let us consider the most simple compounds, which have one CuO2 layer in their structural unit: (La, Sr)ECuO 4 (T-phase), which contains an array of CuOr-octahedra, (Nd, Ce, Sr)ECuO 4 (T*-phase) recently discovered P-type HTSC [ 19 ], which contains an array of CuOs-pyramids and electron-doped HTSC (T'-phase), which contains CuO4 two-dimensional squares in a corresponding layer. In the first case there are two apical oxygen atoms away from the CuO2-plane for each CuO4-square, in the second case there is one such atom. The magnitude of pressure derivatives d T d d P for electron-doped HTSC is in turn less than that for (Nd, Ce, Sr)ECuO 4 [ 7 ], which is not greater than that for (La, Sr) 2CUO4. This fact serves as a qualitative confirmation of the assumption of substantial effect of apical oxygen atoms upon the pressure derivatives of critical temperature. 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-parameters (T~, H¢2) under pressure for NCCO, N T C O and SCCO allows to doubt the possibility to apply a bipolaronic model, within the scope of which the parameters under consideration increase while the interatomic distance decreases, to their behavior.
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S.L. Bud'ko et al. / Effect o f pressure on electron-doped H T S C
Acknowledgements W e a c k n o w l e d g e s u p p o r t for this r e s e a r c h f r o m USSR Interdepartmental Council on HTSC Probl e m u n d e r G r a n t N o . 32 a n d f r o m U S D e p a r t m e n t of Energy under Grant No. DE-FG03-86ER45230. W e are greatly i n d e b t e d to A.A. A b r i k o s o v for his k i n d a t t e n t i o n to this w o r k a n d s t i m u l a t e d i n t e r a c t i o n s a n d to E.S. I t s k e v i c h for his k i n d a t t e n t i o n .
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