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Effect of oxygen deficiency on the normal and superconducting properties of CaLaBaCu3Oy
PhysicaC 173 (1991) North-Holland
453-457
Effect of oxygen deficiency on the normal and superconducting properties of CaLaBaCu30, Takaaki
Yagi, Mikihiko
Domon,
Yoshitoshi
Okajima
and Kazuhiko
Yamaya
Department of Nuclear Engineering, Hokkaido University, Sapporo 060, Japan Received
27 November
1990
Measurements of superconducting transition temperature T, and lattice parameters of CaLaBaCugO,(CLBCO) indicate that CLBCO is a tetragonal bulk superconductor in the oxygen-content range from y= 6.7 to 7. The T, decreases monotonously with decreasing oxygen content and no superconducting transition is observed below y=6.7. It is concluded that disappearance of the superconductivity for y< 6.7 in CLBCO indicates the absence of self-doping of hole which induces the superconductivity. It was found further that samples for y< 6.67 exhibit insulating behaviors and the temperature dependences of the resistivity are well represented by an activated hopping term.
1. Introduction It is well known that YBa2CuJ0,(YBCO) exists over a range of oxygen content, 6< y<7, having its crystal structure changing from orthorhombic I phase for y- 7 to tetragonal one for y- 6 via orthorhombic II phase for y=6.4-6.6[ 1,2]. In the orthorhombic phases, the one-dimensional Cu-0 chain exists along the b-axis. The superconducting transition temperature T, in YBCO strongly depends on the oxygen content; T,=90 K for y= 7-6.8, T,=60 K for y=6.66.4 and T, < 30 K for y- 6.3. In the T,versus y curve, two plateaus are observed at T,= 60 K and 90 K [ 1,2 1. It is understood that the plateau is closely related with the long range ordering of the Cu-0 chain. Furthermore, as oxygen content decreases from 7 to 6, the temperature dependence of the resistivity changes systematically from metallic to insulating behavior. Total cation charge on the noncopper metals of YBCO is 7. When the oxygen content y is 6.5, the nominal hole concentration is zero if the valence value of Cu is 2: the sample should be insulating. However, YBCO with y=6.5 is not insulating, but superconducting with T,=60 K[ 1,2]. One of possible explanations for the enhancement of the superconductivity in YBCO is given by Kondo [ 31 in terms of self-doping in which hole is created in the 0921-4534/91/$03.50
0 1991 - Elsevier Science Publishers
CuOZ sheet even though the nominal hole concentration is zero. It is shown in the model that self-doping is induced by the alternate ordering of Cu-0 chain found in the orthorhombic II phase of YBCO [4-61. Thus, the T,of YBCO depends on not only the oxygen content but also self-doping through the ordering of the Cu-0 chain. CaLaBaCu30,(CLBCO) which is one of YBCOlike compounds is a bulk superconductor with a T, of 80 K [ 7-9 1. CLBCO is isomorphic to tetragonal YBCO, in which the Y-site is occupied by the Ca and La ions and the Ba-site is occupied by the Ba, La and Ca ions [ 8,9 1. It is reported by de Leeuw et al. [ 8 ] that transmission-electron-microscopy micrographs of CLBCO show no evidence for a/b twining, which is observed in YBa, (Cu, _xFex)307 [ lo]. Thus CLBCO with T,= 80 K is substantially tetragonal and has no oxygen deficiency (y- 7) [ 8,9]. Therefore, long range ordering of the Cu-0 chain does not exist in CLBCO, but the Cu-0 chain ordered in a short range may be isotropically distributed between the Ba(La/Ca)-0 planes which correspond to the Ba0 planes in YBCO. Since total cation charge on the noncopper metals of CLBCO as well as YBCO is 7, the nominal hole concentration is zero for ~~6.5. Therefore, it is very interesting to investigate whether self-doping of hole which induces the superconductivity exhibits in CLBCO for y=6.5. In this letter is
B.V. (North-Holland)
454
T. Yagi et al. / Oxygen deficiency effect in CaLaBaCu,O,
reported the experimental results of the oxygen-content dependence of crystal structure, T, and resistivity of CLBCO and discussed them in comparison with those of YBCO.
2. Experimental Samples were prepared by a solid-state reaction between CaC03, La203, BaCO, and CuO powders. Appropriate amounts of the powder were mixed and calcined at 900°C and 950°C for 20 h in air with intermediate grindings. The powder was pressed into into pellets of diameter 20 mm. The pellets were fired for 10 h at 950°C and then annealed for 20 h in air at temperatures between 400 and 1000°C at 50” C intervals. Oxygen contents in the pellets are varied by the different annealing temperature. Samples with various oxygen content were prepared by quenching the pellets from the annealing temperature to liquid nitrogen. Powder X-ray diffraction patterns were recorded on a diffractometer with CuKa radiation. Bars for resistivity measurements were cut from the centers of the quenched pellets, with approximate dimensions 1 x 1 x 5 mm3, and were measured by a fourprobe method with silver-paint contacts. Neighboring sections of each sample were cut for AC magnetic susceptibility and oxygen analysis. Oxygen content was determined by an iodometric titration method with an accuracy of kO.02.
3. Results and discussion The powder X-ray diffraction data showed that the samples are single phase with a tetragonal unit cell over the range of oxygen content from y=6.6 to 7: no change in the structure of CLBCO was observed. The lattice parameters a and c at room temperature of sample for y=6.88 are a=3.865 8, and c= 11.67 A. These values are almost the same as those reported previously [ 8,9]. The measured lattice parameters a and c/3 as a function of oxygen content are shown in fig. 1. As oxygen content decreases, the a increases gradually, while the c increases significantly and tends to saturate in the regime of y= 6.66.7. The present result is qualitatively consistent with
I
66
I
67 oxygen
I
G.8 content y
I, 6.9
7.0
Fig. 1. Lattice parameters a and c/3 of CaLaBaCu,O, as a function of oxygen content y. Vertical bars indicate the errors. Solid lines are provided to guide the eye.
that of the oxygen-content dependence of lattice parameters of YBCO determined by neutron powder diffraction by Jorgensen et al. [ 111. It is found that the oxygen deficiencies of YBCO is mainly due to the oxygen in the one-dimensional Cu-0 chain [ 111. Therefore, it is expected that the oxygen deficiencies of CLBCO are due to the oxygen in the Cu-0 chain ordered in the short range. Figure 2 shows the temperature dependence of the resistivity for several samples with various oxygen content. As oxygen content in samples decreases, metallic behavior varies to insulating one through nonmetallic behavior with a resistance minimum. The magnitude of the resistivity at room temperature p (RT) for sample of yc6.86 is about 3 mRcm which is a typical value for YBCO-like compounds. As decreasing oxygen content, p (RT) increases up to -40 m&m at y= 6.62. Comparing the present result with that of YBCO [ 121, it is found that the transition from the metallic state to insulating one in CLBCO occurs by a relatively small amount of oxygen deficiencies: electronic properties of CLBCO is quite sensitive for oxygen deficiencies. This may be related with the absence of long range ordering of the Cu-0 chain in CLBCO. Zero resistance due to the superconducting transition is observed in only samples for y> 6.7 in the temperature range above 4.2 K. On samples for ~~6.67 which are insulating, it is found that the temperature dependence of the resistivity can be well fitted to an activated hopping term with exponent l/3 or l/4 rather than a semicon-
T Yagi et al. /Oxygen deficiency effect in CaLaBaCuJO,
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Fig. 3. Temperature dependence of AC magnetic susceptibility of CaLaBaCu30,: sample-l (y=6.88), sample-2 (y=6.86), sample-3 (y=6.84), sample-4 (y=6.83), sample-5 (y=6.79), sample-6 (y=6,76),sample-7 (~~6.71) andsample-8(y=6.69). 1E3
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Fig. 2. Temperature dependence of resistivityp of CaLaBaCu30, with various oxygen content y: sample-l (y=6.88), sample2 (y=6.86), sample-3 (y=6.84), sample-4 (y=6.83), sample5 (y=6.79), sample-6 (y=6.76), sample-7 (y=6.71), sample8 (y=6.69), sample-9 (y=6.67), sample-10 (y=6.66) and sample-l 1 (y=6.62). activated tertn, which is also shown in YBCO for ~~6.0 [ 131. As oxygen content decreases, no change in the crystal structure of CLBCO is observed, while that of YBCO changes from orthorhombic phase to tetragonal one [ 1,2]. In spite of these facts, the overview of the temperature dependence of resistivity as a parameter of oxygen content qualitatively agrees well with each others. This common feature in the resistivity means that the electronic conduction is almost independent on the crystal structure and is mainly characterized by the carriers in the CuOZ sheet. Figure 3 shows the temperature dependence of AC magnetic susceptibility, xAc, for samples cooled in a magnetic field of circa 0.5 Oe. The diamagnetic signal is normalized to the value obtained for lead (Pb) as a standard sample. The amount of flux expulsion is estimated to be 80-90%, indicating that the materials are a good bulk superconductor. The transition widths are quite narrow, indicating good sample homogeneity, except at the oxygen content of y= 6.71. No diamagnetic signal is observed in sam-
-4
ducting
oxygen
content
y
Fig. 4. Oxygen-content dependence of the magnetic (0 ) and resistive (0 ) superconducting transition temperature in CaLaBaCulO, for 6.62~~~ 6.88. Vertical bars indicate the lo-90% resistive transition width. Solid triangle is T, of sample prepared in oxygen pressure at 20 atm. Solid lines are provided to guide the eye.
ples for y below 6.7. This result is consistent with that of the temperature dependence of resistivity shown in fig. 2 in which only samples for y above 6.7 exhibit zero resistance. It is concluded from both measurements of resistivity and AC magnetic susceptibility that CLBCO is a tetragonal bulk superconductor in the oxygen-content range from y=6.7 to 7. Figure 4 shows oxygen-content dependences of T,
456
T. Yagi et al. /Oxygen deficiency eflect in CaLaBaCu30,
obtained from the magnetic and resistive superconducting transition. The magnetic T, was defined as the temperature corresponding the intersection of the linear extraporations of the transition curve and the temperature dependence of xAc in the normal state. The resistive Tc’s are shown with vertical bars which indicate the lo-90% transition width with the midpoint marked with a solid circle. The oxygen-content dependences of both Tzs are substantially the same. We also measured T, for sample which was prepared in oxygen pressure of 20 atm at 400°C for 24 h. The result is denoted by the sign of triangle in fig. 4. It was found from these results that there is a narrow plateau near 80 K in the curve of T, versus y in CLBCO. This plateau may be related with a oxygen ordering in the Cu-0 chain ordered in a short range or in the disordered CuO plane between the Ba (La/ Ca)-0 planes proposed by Carim et al. [ 141, although the oxygen ordering in CLBCO is not yet confirmed. However, in the range of y< 6.9, any plateau is not observed and T, decreases monotonously with decreasing oxygen content. This suggests that there is no oxygen ordering in the region of ~~6.9. The superconductivity disappears near y= 6.7. This result is quite different from the case of YBCO in which the superconductivity exhibits even in sample for y=6.5. Since the nominal hole concentration of CLBCO for y=6.5 as well as that of YBCO is zero, the sample should be insulating. In fact, as shown in fig. 2 even sample for y=6.6 is insulating, but not metallic. As y increases from 6.5, the holes are doped. The superconducting transition appears near y=6.7, where the nominal hole concentration per Cu atom p is 0.133. The T, increases with increasing p. Thus we can conclude that the superconductivity in CLBCO is explained in terms of the nominal hole concentration, but not induced by selfdoping of hole. Since the hole concentration in the CuOZ sheet &, is one of the most important factors that govern T, in cuprate superconductors [ 3,7 1, the relation of T, and Psi, has been investigated for many cuprate superconductors [ 7,15-71. As the result, it is found that a maximum T, is universally observed near & = O.l0.2. Therefore, it is expected in CLBCO also with Tc,max= 80 K that the value of& will be nearly O.l0.2. Since the hole of CLBCO is distributed among the CuOZ sheets and the Cu-0 chain ordered in the short range or the disordered CuO plane, the nom-
inal hole concentration p will be given by an average over the hole concentration in them: p= (2p,,+p,,)/ 3. From this relation the value of the &h in CLBCO with T,, maxis estimated as 0.6-0.8, because T,,,, is observed near y=7 (~~0.33). Thus we find that as holes are doped in CLBCO, many holes are distributed in the Cu-0 chain and a small number of hole occupies the CuOZ sheet. This is qualitatively consistent with the results of electron spectroscopy [ 18 ] and the band calculation [ 19,201 obtained in YBCO.
5. Conclusions CaLaBaCu30, synthesized by the solid-solutionreaction method was found to be tetragonal in the range of oxygen content y from y=6.6 to 7.0. As y decreases, the temperature dependence of the resistivity changes systematically from metallic behavior to insulating one and the same time, the T, decreases monotonously. The superconducting transition disappears in the region which the nominal hole concentration is finite (p= 0.133 ). It is concluded that the superconductivity of CLBCO is not induced by self-doping of holes. From the universal relation of T, and Psi, in many cuprate superconductors, it is found in CLBCO that a small number of holes occupy the CuOl sheets and many holes are distributed in the Cu-0 chain.
Acknowledgements The authors thank Prof. T. Takama for useful discussion on the X-ray diffraction. This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Areas “Mechanism of Superconductivity” from the Ministry of Education, Science and Culture of Japan.
References [l] R.J. Cava, B. Batlogg, C.H. Chen, E.A. Rietman,
S.M. Zahurak and D. Werder, Phys. Rev. B36 ( 1987) 57 19. [ 21 T. Takabatake, M. Ishikawa, Y. Nakazawa and K. Koga, Physica C 152 ( 1988) 424; Y. Nakazawa and M. Ishikawa, Physica C 158 (1989) 381. [3] J. Kondo, J. Phys. Sot. Jpn. 59 ( 1990).
i? Yagi et al. /Oxygen deficiency effect in CaLaBaCu30, [4] D.J. Werder, C.H. Chen, R.J. Cava and B. Batlogg, Phys. Rev. B37 (1988) 2317. [ 5 ] C. Chaillout, M.A. Alario-France, J.J. Capponi, J. Chenevas and M. Marezio, Solid State Commun. 65 ( 1988) 283. [ 61 R.M. Fleming, L.F. Schneemeyer, PK. Gallagher, B. Batlog, L.W. Rupp and J.V. Waszczak, Phys. Rev. B37 (1988) 7920. [ 7 ] Y. Tokura, J.B. Torrance, T.C. Huang and AI. Nazal, Phys. Rev. B38 (1988) 7156. [8]D.M. de Leeuw, C.H.A. Mutsaers, H.A.M. van Hal, H. Verweij, A.H. Carim and H.C.A. Smoorenburg, Physica C 156 (1988) 126. [9] W.T. Fu, H.W. Zandbergen, C.J. van der Beek and L.J. de Jongh, Physica C 156 ( 1988) 133. [ IO] Y. Syono, M. Kikuchi, K. Oh-ishi, K. Hiraga, H. Arai, Y. Matsui, N. Kobayashi, T. Sasaoka and Y. Muto, Jpn. J. Appl. Phys. 26 ( 1987) L498. [ 111 J.D. Jorgensen, B.W. Veal, A.P. Paul&as, L.J. Nowicki, G.W. Crabtee, H. Claus and W.K. Kwok, Phys. Rev. B41 ( 1990) 1863.
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[ 121 S.I. Park, C.C. Tsuei and K.N. Tu, Phys. Rev. B37 (1987) 2305. [ 13 ] A. Matsushita, T. Oguchi, K. Kimura, T. Matsumoto, T. Hatano, K. Ogawa and S. Takayamagi, Jpn. J. Appl. Phys. 26 (1987) L1953. [ 141 A.H. Carin, A.F. de Jong and D.M. de Leeuw, Phys. Rev. B38 (1988) 7009. [ 151 J.B. Torrance, Y. Tokura, AI. Nazzal, A. Bezinge, T.C. Huangand S.S.P. Parkin, Phys. Rev. Lett. 61 (1988) 1127. [ 161 T. Manako, Y. Shimakawa, Y. Kubo, T. satoh and G. Igarashi, Physica C 158 (1989) 143. [ 171 M. Hase, I. Tsukada, K. Noda, S. Takebayashi and K. Uchinokura, Phys. Rev. B41 ( 1990) 64 18. [ 181 N. Nucker, Phys. Rev. B39 (1989) 6619. [ 191 J. Zaanen, A.T. Paxton, 0. Jepsen and O.K. Anderson, Phys. Rev. Lett. 60 (1988) 2685. [20] K-T. Park, K Terakura, T. Oguchi, A. Yanase and M. Ikeda, J. Phys. Sot. Jpn. 57 (1988) 3445.
453-457
Effect of oxygen deficiency on the normal and superconducting properties of CaLaBaCu30, Takaaki
Yagi, Mikihiko
Domon,
Yoshitoshi
Okajima
and Kazuhiko
Yamaya
Department of Nuclear Engineering, Hokkaido University, Sapporo 060, Japan Received
27 November
1990
Measurements of superconducting transition temperature T, and lattice parameters of CaLaBaCugO,(CLBCO) indicate that CLBCO is a tetragonal bulk superconductor in the oxygen-content range from y= 6.7 to 7. The T, decreases monotonously with decreasing oxygen content and no superconducting transition is observed below y=6.7. It is concluded that disappearance of the superconductivity for y< 6.7 in CLBCO indicates the absence of self-doping of hole which induces the superconductivity. It was found further that samples for y< 6.67 exhibit insulating behaviors and the temperature dependences of the resistivity are well represented by an activated hopping term.
1. Introduction It is well known that YBa2CuJ0,(YBCO) exists over a range of oxygen content, 6< y<7, having its crystal structure changing from orthorhombic I phase for y- 7 to tetragonal one for y- 6 via orthorhombic II phase for y=6.4-6.6[ 1,2]. In the orthorhombic phases, the one-dimensional Cu-0 chain exists along the b-axis. The superconducting transition temperature T, in YBCO strongly depends on the oxygen content; T,=90 K for y= 7-6.8, T,=60 K for y=6.66.4 and T, < 30 K for y- 6.3. In the T,versus y curve, two plateaus are observed at T,= 60 K and 90 K [ 1,2 1. It is understood that the plateau is closely related with the long range ordering of the Cu-0 chain. Furthermore, as oxygen content decreases from 7 to 6, the temperature dependence of the resistivity changes systematically from metallic to insulating behavior. Total cation charge on the noncopper metals of YBCO is 7. When the oxygen content y is 6.5, the nominal hole concentration is zero if the valence value of Cu is 2: the sample should be insulating. However, YBCO with y=6.5 is not insulating, but superconducting with T,=60 K[ 1,2]. One of possible explanations for the enhancement of the superconductivity in YBCO is given by Kondo [ 31 in terms of self-doping in which hole is created in the 0921-4534/91/$03.50
0 1991 - Elsevier Science Publishers
CuOZ sheet even though the nominal hole concentration is zero. It is shown in the model that self-doping is induced by the alternate ordering of Cu-0 chain found in the orthorhombic II phase of YBCO [4-61. Thus, the T,of YBCO depends on not only the oxygen content but also self-doping through the ordering of the Cu-0 chain. CaLaBaCu30,(CLBCO) which is one of YBCOlike compounds is a bulk superconductor with a T, of 80 K [ 7-9 1. CLBCO is isomorphic to tetragonal YBCO, in which the Y-site is occupied by the Ca and La ions and the Ba-site is occupied by the Ba, La and Ca ions [ 8,9 1. It is reported by de Leeuw et al. [ 8 ] that transmission-electron-microscopy micrographs of CLBCO show no evidence for a/b twining, which is observed in YBa, (Cu, _xFex)307 [ lo]. Thus CLBCO with T,= 80 K is substantially tetragonal and has no oxygen deficiency (y- 7) [ 8,9]. Therefore, long range ordering of the Cu-0 chain does not exist in CLBCO, but the Cu-0 chain ordered in a short range may be isotropically distributed between the Ba(La/Ca)-0 planes which correspond to the Ba0 planes in YBCO. Since total cation charge on the noncopper metals of CLBCO as well as YBCO is 7, the nominal hole concentration is zero for ~~6.5. Therefore, it is very interesting to investigate whether self-doping of hole which induces the superconductivity exhibits in CLBCO for y=6.5. In this letter is
B.V. (North-Holland)
454
T. Yagi et al. / Oxygen deficiency effect in CaLaBaCu,O,
reported the experimental results of the oxygen-content dependence of crystal structure, T, and resistivity of CLBCO and discussed them in comparison with those of YBCO.
2. Experimental Samples were prepared by a solid-state reaction between CaC03, La203, BaCO, and CuO powders. Appropriate amounts of the powder were mixed and calcined at 900°C and 950°C for 20 h in air with intermediate grindings. The powder was pressed into into pellets of diameter 20 mm. The pellets were fired for 10 h at 950°C and then annealed for 20 h in air at temperatures between 400 and 1000°C at 50” C intervals. Oxygen contents in the pellets are varied by the different annealing temperature. Samples with various oxygen content were prepared by quenching the pellets from the annealing temperature to liquid nitrogen. Powder X-ray diffraction patterns were recorded on a diffractometer with CuKa radiation. Bars for resistivity measurements were cut from the centers of the quenched pellets, with approximate dimensions 1 x 1 x 5 mm3, and were measured by a fourprobe method with silver-paint contacts. Neighboring sections of each sample were cut for AC magnetic susceptibility and oxygen analysis. Oxygen content was determined by an iodometric titration method with an accuracy of kO.02.
3. Results and discussion The powder X-ray diffraction data showed that the samples are single phase with a tetragonal unit cell over the range of oxygen content from y=6.6 to 7: no change in the structure of CLBCO was observed. The lattice parameters a and c at room temperature of sample for y=6.88 are a=3.865 8, and c= 11.67 A. These values are almost the same as those reported previously [ 8,9]. The measured lattice parameters a and c/3 as a function of oxygen content are shown in fig. 1. As oxygen content decreases, the a increases gradually, while the c increases significantly and tends to saturate in the regime of y= 6.66.7. The present result is qualitatively consistent with
I
66
I
67 oxygen
I
G.8 content y
I, 6.9
7.0
Fig. 1. Lattice parameters a and c/3 of CaLaBaCu,O, as a function of oxygen content y. Vertical bars indicate the errors. Solid lines are provided to guide the eye.
that of the oxygen-content dependence of lattice parameters of YBCO determined by neutron powder diffraction by Jorgensen et al. [ 111. It is found that the oxygen deficiencies of YBCO is mainly due to the oxygen in the one-dimensional Cu-0 chain [ 111. Therefore, it is expected that the oxygen deficiencies of CLBCO are due to the oxygen in the Cu-0 chain ordered in the short range. Figure 2 shows the temperature dependence of the resistivity for several samples with various oxygen content. As oxygen content in samples decreases, metallic behavior varies to insulating one through nonmetallic behavior with a resistance minimum. The magnitude of the resistivity at room temperature p (RT) for sample of yc6.86 is about 3 mRcm which is a typical value for YBCO-like compounds. As decreasing oxygen content, p (RT) increases up to -40 m&m at y= 6.62. Comparing the present result with that of YBCO [ 121, it is found that the transition from the metallic state to insulating one in CLBCO occurs by a relatively small amount of oxygen deficiencies: electronic properties of CLBCO is quite sensitive for oxygen deficiencies. This may be related with the absence of long range ordering of the Cu-0 chain in CLBCO. Zero resistance due to the superconducting transition is observed in only samples for y> 6.7 in the temperature range above 4.2 K. On samples for ~~6.67 which are insulating, it is found that the temperature dependence of the resistivity can be well fitted to an activated hopping term with exponent l/3 or l/4 rather than a semicon-
T Yagi et al. /Oxygen deficiency effect in CaLaBaCuJO,
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Fig. 3. Temperature dependence of AC magnetic susceptibility of CaLaBaCu30,: sample-l (y=6.88), sample-2 (y=6.86), sample-3 (y=6.84), sample-4 (y=6.83), sample-5 (y=6.79), sample-6 (y=6,76),sample-7 (~~6.71) andsample-8(y=6.69). 1E3
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Fig. 2. Temperature dependence of resistivityp of CaLaBaCu30, with various oxygen content y: sample-l (y=6.88), sample2 (y=6.86), sample-3 (y=6.84), sample-4 (y=6.83), sample5 (y=6.79), sample-6 (y=6.76), sample-7 (y=6.71), sample8 (y=6.69), sample-9 (y=6.67), sample-10 (y=6.66) and sample-l 1 (y=6.62). activated tertn, which is also shown in YBCO for ~~6.0 [ 131. As oxygen content decreases, no change in the crystal structure of CLBCO is observed, while that of YBCO changes from orthorhombic phase to tetragonal one [ 1,2]. In spite of these facts, the overview of the temperature dependence of resistivity as a parameter of oxygen content qualitatively agrees well with each others. This common feature in the resistivity means that the electronic conduction is almost independent on the crystal structure and is mainly characterized by the carriers in the CuOZ sheet. Figure 3 shows the temperature dependence of AC magnetic susceptibility, xAc, for samples cooled in a magnetic field of circa 0.5 Oe. The diamagnetic signal is normalized to the value obtained for lead (Pb) as a standard sample. The amount of flux expulsion is estimated to be 80-90%, indicating that the materials are a good bulk superconductor. The transition widths are quite narrow, indicating good sample homogeneity, except at the oxygen content of y= 6.71. No diamagnetic signal is observed in sam-
-4
ducting
oxygen
content
y
Fig. 4. Oxygen-content dependence of the magnetic (0 ) and resistive (0 ) superconducting transition temperature in CaLaBaCulO, for 6.62~~~ 6.88. Vertical bars indicate the lo-90% resistive transition width. Solid triangle is T, of sample prepared in oxygen pressure at 20 atm. Solid lines are provided to guide the eye.
ples for y below 6.7. This result is consistent with that of the temperature dependence of resistivity shown in fig. 2 in which only samples for y above 6.7 exhibit zero resistance. It is concluded from both measurements of resistivity and AC magnetic susceptibility that CLBCO is a tetragonal bulk superconductor in the oxygen-content range from y=6.7 to 7. Figure 4 shows oxygen-content dependences of T,
456
T. Yagi et al. /Oxygen deficiency eflect in CaLaBaCu30,
obtained from the magnetic and resistive superconducting transition. The magnetic T, was defined as the temperature corresponding the intersection of the linear extraporations of the transition curve and the temperature dependence of xAc in the normal state. The resistive Tc’s are shown with vertical bars which indicate the lo-90% transition width with the midpoint marked with a solid circle. The oxygen-content dependences of both Tzs are substantially the same. We also measured T, for sample which was prepared in oxygen pressure of 20 atm at 400°C for 24 h. The result is denoted by the sign of triangle in fig. 4. It was found from these results that there is a narrow plateau near 80 K in the curve of T, versus y in CLBCO. This plateau may be related with a oxygen ordering in the Cu-0 chain ordered in a short range or in the disordered CuO plane between the Ba (La/ Ca)-0 planes proposed by Carim et al. [ 141, although the oxygen ordering in CLBCO is not yet confirmed. However, in the range of y< 6.9, any plateau is not observed and T, decreases monotonously with decreasing oxygen content. This suggests that there is no oxygen ordering in the region of ~~6.9. The superconductivity disappears near y= 6.7. This result is quite different from the case of YBCO in which the superconductivity exhibits even in sample for y=6.5. Since the nominal hole concentration of CLBCO for y=6.5 as well as that of YBCO is zero, the sample should be insulating. In fact, as shown in fig. 2 even sample for y=6.6 is insulating, but not metallic. As y increases from 6.5, the holes are doped. The superconducting transition appears near y=6.7, where the nominal hole concentration per Cu atom p is 0.133. The T, increases with increasing p. Thus we can conclude that the superconductivity in CLBCO is explained in terms of the nominal hole concentration, but not induced by selfdoping of hole. Since the hole concentration in the CuOZ sheet &, is one of the most important factors that govern T, in cuprate superconductors [ 3,7 1, the relation of T, and Psi, has been investigated for many cuprate superconductors [ 7,15-71. As the result, it is found that a maximum T, is universally observed near & = O.l0.2. Therefore, it is expected in CLBCO also with Tc,max= 80 K that the value of& will be nearly O.l0.2. Since the hole of CLBCO is distributed among the CuOZ sheets and the Cu-0 chain ordered in the short range or the disordered CuO plane, the nom-
inal hole concentration p will be given by an average over the hole concentration in them: p= (2p,,+p,,)/ 3. From this relation the value of the &h in CLBCO with T,, maxis estimated as 0.6-0.8, because T,,,, is observed near y=7 (~~0.33). Thus we find that as holes are doped in CLBCO, many holes are distributed in the Cu-0 chain and a small number of hole occupies the CuOZ sheet. This is qualitatively consistent with the results of electron spectroscopy [ 18 ] and the band calculation [ 19,201 obtained in YBCO.
5. Conclusions CaLaBaCu30, synthesized by the solid-solutionreaction method was found to be tetragonal in the range of oxygen content y from y=6.6 to 7.0. As y decreases, the temperature dependence of the resistivity changes systematically from metallic behavior to insulating one and the same time, the T, decreases monotonously. The superconducting transition disappears in the region which the nominal hole concentration is finite (p= 0.133 ). It is concluded that the superconductivity of CLBCO is not induced by self-doping of holes. From the universal relation of T, and Psi, in many cuprate superconductors, it is found in CLBCO that a small number of holes occupy the CuOl sheets and many holes are distributed in the Cu-0 chain.
Acknowledgements The authors thank Prof. T. Takama for useful discussion on the X-ray diffraction. This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Areas “Mechanism of Superconductivity” from the Ministry of Education, Science and Culture of Japan.
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