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Rare-earth-ion-size control on magnetic irreversibility field in CuBa2RECu2O7−δ
Physica C 357±360 (2001) 230±233
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Rare-earth-ion-size control on magnetic irreversibility ®eld in CuBa2RECu2O7 d Yukiko Yasukawa a,b, Takayuki Nakane a,b, Maarit Karppinen a,c, Hisao Yamauchi a,b,* a
Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan b Department of Innovative and Engineered Materials, Interdisciplinary Graduate School, Tokyo Institute of Technology, Yokohama 226-8502, Japan c Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, FIN-02015 Espoo, Finland Received 16 October 2000; accepted 15 January 2001
Abstract For CuBa2 RECu2 O7 d (Cu-1212:P), the dependence of magnetic irreversibility ®eld (Hirr ) on the ionic radius of rare earth element (RE) has been studied. CuBa2 RECu2 O7 d samples with the RE ionic radii [r(RE)] varying from 0.982 A (for RE Yb0:6 Lu0:4 ) to 1.079 A (for RE Sm) were studied. The oxygen content of all the samples was set at a ®xed level, i.e. d 0:06 0:03. With decreasing r(RE), the Hirr characteristics, i.e. the Hirr vs 1 T =Tc plot, was clearly enhanced. The CuBa2 (Yb0:6 Lu0:4 )Cu2 O6:92 sample showed a record-high Hirr value of 9.0 T at 77 K. This result implies that through isovalent cation substitution the carrier-density distribution may be optimized to facilitate the superior Hirr property. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 74.25.Ha; 74.62.Dh; 74.72.-h Keywords: Magnetic irreversibility line; Rare earth substitution; Cu-1212:P; Isovalent substitution
1. Introduction Since the magnetic irreversibility-®eld line Hirr T determines the limit of high-®eld applications for high-Tc superconductors (HTSC), it is highly desirable to enhance the Hirr characteristics. It has been revealed that the Hirr characteristics of * Corresponding author. Address: Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohoma 226-8503, Japan. Tel.: +81-45-924-5315; fax: +81-45-924-5365. E-mail address: [email protected] (H. Yamauchi).
the CuBa2 RECu2 O7 d (RE rare earth element) phase, i.e. Cu-1212:P (P stands for a perovskitetype charge reservoir) or so-called ``123'', is the best among the known HTSC material systems [1]. Although many studies have been reported on the correlation between the radius of the rare earth ion [r(RE)] and Tc of CuBa2 RECu2 O7 d , the dependence of Hirr on r(RE) has not been established. Previously we reported the eects of oxygen doping and aliovalent cation substitutions on the hole density distribution and accordingly on the values of Tc and Hirr [2,3]. However, the consequences of isovalent or ``size-only-dependent'' cation substitutions are less understood. Here we present the
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 2 1 6 - 7
Y. Yasukawa et al. / Physica C 357±360 (2001) 230±233
results of a systematic study on the Hirr characteristics with respect to r(RE) in cation-stoichiometric CuBa2 RECu2 O7 d with a ®xed oxygen content.
2. Experimental CuBa2 RECu2 O7 d (RE Sm, Eu, Gd, Y, Y/Yb, Yb and Yb/Lu) samples were prepared through solid state reaction at 900±950°C for 12±120 h from stoichiometric mixtures of high-purity powders of RE2 O3 , BaCO3 and CuO. All the samples were fully oxygenated by post-annealing the assynthesized powders in a thermobalance in O2 gas ¯ow at 350±400°C for 10±96 h. The prepared samples were of single phase and the exact oxygen contents were established by the Cu(I)/(II) coulometric titration method [4] to be d 0:06 0:03 for all the samples. The structure parameters were re®ned from the X-ray powder diraction data (XRD, MAC Science: MXP18VAHF22 ; CuKa line). The temperature dependence of magnetic susceptibility was measured by a SQUID magnetometer (Quantum Design: MPMSR-5S). The ``eective'' superconductivity transition temperature Tc e values were estimated from the measured v vs T curves [5]. The Hirr characteristics were obtained through the procedure to be described later from the M vs H curves as measured by SQUID. In the magnetization measurements,
Table 1 Basic properties of the CuBa2 RECu2 O7
d
231
sample masses were kept quite constant. The average grain size was determined for each sample from a ®eld-emission scanning-electron micrograph (Hitachi: FE-SEM S-4500). Diraction data from microscopic areas were taken utilizing a transmission electron microscope (TEM; Hitachi: H-9000) operated at an accelerating voltage of 300 kV.
3. Results and discussion The lattice parameters, a, b and c, re®ned for the samples are given in Table 1. A clear tendency of ``lanthanide contraction'' is seen, that is, with decreasing r(RE) the lattice parameters decrease monotonically. Note that the tendency is not very clearly seen for solid solutions of RE Yb/Lu. Although the oxygen contents of the samples were constant within d 0:06 0:03, the Tc value decreases slightly with decreasing r(RE). This suggests that the hole-doping level of the CuO2 plane may shift towards the more overdoped side as r(RE) gets smaller. It should be emphasized that since the Hirr characteristics is strongly aected by the oxygen content, to reveal the eect of isovalent RE substitution, it is essential to standardize the oxygen content. We determined the Hirr line from the measured M vs H curves as the limit of H where the hysteresis in the magnetization (DM) disappears:
samples
RE
Ionic radius (A)
Tc e
Sm Eu Gd Y Y0:4 Yb0:6 Y0:2 Yb0:8 Yb Yb0:9 Lu0:1 Yb0:8 Lu0:2 Yb0:7 Lu0:3 Yb0:6 Lu0:4
1.079 1.066 1.053 1.019 0.999 0.992 0.985 0.984 0.983 0.983 0.982
92.9 91.4 94.2 91.4 91.1 91.4 89.5 89.5 89.9 89.7 89.7
(K)
Oxygen content d
Lattice parameters (A) a
b
c
N/A N/A 0.02 0.07 0.07 0.09 0.07 N/A 0.06 N/A 0.08
3.85(4) 3.84(2) 3.84(1) 3.82(6) 3.80(6) 3.81(4) 3.80(5) 3.80(4) 3.80(4) 3.80(5) 3.80(5)
3.90(3) 3.89(6) 3.90(1) 3.89(1) 3.87(4) 3.87(8) 3.87(4) 3.87(4) 3.87(4) 3.87(4) 3.87(4)
11.71(4) 11.69(1) 11.71(4) 11.66(8) 11.65(3) 11.66(2) 11.65(8) 11.66(2) 11.66(1) 11.66(3) 11.66(2)
232
Y. Yasukawa et al. / Physica C 357±360 (2001) 230±233
obtained in the present study is the highest Hirr value among those of the Cu-1212:P compounds not containing any extrinsic pinning centers. Possible reasons why the smaller RE Cu-1212:P samples have the better Hirr properties are as follows:
Fig. 1. Hirr (at T 77 K) of the CuBa2 RECu2 O6:940:03 samples with respect to r(RE).
Hirr H DM ! 0:
1
To satisfy the requirement given in Eq. (1), we assumed the following relation [6]: log H log Hirr b DM;
2
where b and Hirr are parameters determined by a least-square method applied to the experimental plot of log H vs DM. Note that, the DM data points which correspond to Jc values in the range of 500±1000 A/cm2 (according to Bean's model) were employed in this extrapolation procedure. In Fig. 1, thus determined values of Hirr at 77 K are plotted for all the samples with respect to r(RE). From Fig. 1, it is seen that the magnitude of Hirr strongly depends on r(RE) when the oxygen content is constant. With decreasing r(RE) the Hirr characteristics is clearly enhanced. For example, comparing CuBa2 (Yb0:6 Y0:4 )Cu2 O6:93 to CuBa2 (Yb0:6 Lu0:4 )Cu2 O6:92 , their Hirr characteristics are dierent by a factor of 2. Recently it was reported that a seeded-melt processed CuBa2 NdCu2 O7 d sample containing 15 mol% Nd4 Ba2 Cu2 O10 has a superb Hirr characteristics, i.e. 9.0 T in the case of H kc at 77 K and 10.5 T in the case of H?c at 88 K [7]. However, this is probably caused by ``extrinsic'' pinning centers, rather than an ``intrinsic'' nature of the electronic structure of the compound. Therefore, we consider that the Hirr value of 9.0 T at 77 K for CuBa2 (Yb0:6 Lu0:4 ) Cu2 O6:92
1. Some tiny impurity particles, e.g. RE2 BaCuO5 , may act as vortex pinning centers that might result in the better Hirr characteristics. 2. Some atomic disorder or concentration modulation along the ab plane may work as weak pinning centers to yield the better Hirr characteristics. 3. More homogeneous hole-density distribution along the c axis may induce the better Hirr characteristics. From SEM observations with EDX analyses of local compositions, it was revealed that there are hardly any impurities in the CuBa2 (Yb0:6 Lu0:4 )Cu2 O6:92 sample that shows the best Hirr characteristics. This implies that possibility (1) is not applicable to the present samples. Furthermore, a TEM electron diraction pattern from a CuBa2 (Yb0:6 Lu0:4 ) Cu2 O6:92 grain is shown in Fig. 2(a). Each spot in this diraction pattern for the ab plane is round (without streaks). The double spots indicate the existence of twin structures. Fig. 2(b) shows a TEM micrograph of a grain in the CuBa2 (Yb0:6 Lu0:4 )Cu2 O6:92 sample. It shows only twins but no impurity inclusions. Thus possibility (2) may not be applicable either, if twins as shown in Fig. 2(b) are benign. Since the oxygen content is precisely ®xed among the present CuB2 RECu2 O7 d samples, the total amount of holes must thus be ®xed. Using neutron diraction data obtained for ``optimized'' CuB2 RECu2 Oy samples [8], bond-valence-sum calculation reveals that with decreasing r(RE), the density of holes in the CuO2 plane increases while that in the CuO1 d chain decreases. For Cu-1212:P phases with large REs, the hole density in the CuO1 d chain is much larger than that in the CuO2 plane, i.e. the dierence in the hole density between the CuO2 plane and the CuO1 d chain is quite large. This indicates that the hole distribution along the c axis becomes more homogeneous with
Y. Yasukawa et al. / Physica C 357±360 (2001) 230±233
233
Fig. 2. (a) Electron diraction pattern from the ab plane and (b) TEM micrograph for a grain of the CuBa2 (Yb0:6 Lu0:4 )Cu2 O6:92 sample.
decreasing r(RE) in the Cu-1212:P system. Previously, it was established [1] that the more homogeneous the hole-density distribution along the c axis is, the better is the Hirr characteristics. Thus possibility (3) is believed to be most plausible.
4. Conclusion In this study, the Hirr characteristics of CuBa2 RECu2 O6:940:03 was systematically studied in terms of the RE ionic radius for RE Sm, Eu, Gd, Y, Y/ Yb, Yb and Yb/Lu. With decreasing r(RE), the Hirr characteristics was clearly enhanced in a monotonic way. For the CuBa2 (Yb0:6 Lu0:4 )Cu2 O6:92 sample with the smallest RE constituent, the Hirr value reached as high as 9.0 T at 77 K. This is the highest Hirr value ever reported for the Cu-1212:P phase at 77 K. The superb Hirr characteristics of the present CuBa2 (Yb0:6 Lu0:4 )Cu2 O6:92 sample is derived from size-only-dependent substitution. It was considered that the isovalent RE substitution induced more homogeneous hole-density distribution along the c axis and thus enhanced the Hirr characteristics. Further con®rmation with regard to the hole distribution by means of O K-edge XANES measurement is in progress.
Acknowledgements The present work has been supported by a grant-in-aid for Scienti®c Research (contract no. 11305002) from the Ministry of Education, Science and Culture of Japan, by an International Collaborative Research Project Grant-2000 of Materials and Structures Laboratory, Tokyo Institute of Technology, and by the Academy of Finland (decision no. 46039). References [1] M. Karppinen, H. Yamauchi, Mater. Sci. Eng. R 26 (1999) 51. [2] M. Karppinen, H. Yamauchi, K. Fujinami, T. Nakane, K. Peitola, H. Rundl of, R. Tellgren, Phys. Rev. B 60 (1999) 4378. [3] T. Nakane, K. Fujinami, M. Karppinen, H. Yamauchi, Supercond. Sci. Technol. 12 (1999) 242. [4] M. Karppinen, A. Fukuoka, L. Niinist o, H. Yamauchi, Supercond. Sci. Technol. 9 (1996) 121. [5] T. Matsushita, E.S. Otabe, T. Matsuno, M. Murakami, K. Kitazawa, Physica C 170 (1990) 375. [6] H. Yamauchi, M. Karppinen, K. Fujinami, T. Itoh, H. Suematsu, H. Sakata, K. Matsuura, K. Isawa, Supercond. Sci. Technol. 11 (1998) 1006. [7] N. Hari Babu, D.A. Cardwell, W. Lo, A.M. Campbell, Phys. Rev. B 61 (2000) 735. [8] H.B. Radousky, J. Mater. Res. 7 (1992) 1917.
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Rare-earth-ion-size control on magnetic irreversibility ®eld in CuBa2RECu2O7 d Yukiko Yasukawa a,b, Takayuki Nakane a,b, Maarit Karppinen a,c, Hisao Yamauchi a,b,* a
Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan b Department of Innovative and Engineered Materials, Interdisciplinary Graduate School, Tokyo Institute of Technology, Yokohama 226-8502, Japan c Laboratory of Inorganic and Analytical Chemistry, Helsinki University of Technology, FIN-02015 Espoo, Finland Received 16 October 2000; accepted 15 January 2001
Abstract For CuBa2 RECu2 O7 d (Cu-1212:P), the dependence of magnetic irreversibility ®eld (Hirr ) on the ionic radius of rare earth element (RE) has been studied. CuBa2 RECu2 O7 d samples with the RE ionic radii [r(RE)] varying from 0.982 A (for RE Yb0:6 Lu0:4 ) to 1.079 A (for RE Sm) were studied. The oxygen content of all the samples was set at a ®xed level, i.e. d 0:06 0:03. With decreasing r(RE), the Hirr characteristics, i.e. the Hirr vs 1 T =Tc plot, was clearly enhanced. The CuBa2 (Yb0:6 Lu0:4 )Cu2 O6:92 sample showed a record-high Hirr value of 9.0 T at 77 K. This result implies that through isovalent cation substitution the carrier-density distribution may be optimized to facilitate the superior Hirr property. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 74.25.Ha; 74.62.Dh; 74.72.-h Keywords: Magnetic irreversibility line; Rare earth substitution; Cu-1212:P; Isovalent substitution
1. Introduction Since the magnetic irreversibility-®eld line Hirr T determines the limit of high-®eld applications for high-Tc superconductors (HTSC), it is highly desirable to enhance the Hirr characteristics. It has been revealed that the Hirr characteristics of * Corresponding author. Address: Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohoma 226-8503, Japan. Tel.: +81-45-924-5315; fax: +81-45-924-5365. E-mail address: [email protected] (H. Yamauchi).
the CuBa2 RECu2 O7 d (RE rare earth element) phase, i.e. Cu-1212:P (P stands for a perovskitetype charge reservoir) or so-called ``123'', is the best among the known HTSC material systems [1]. Although many studies have been reported on the correlation between the radius of the rare earth ion [r(RE)] and Tc of CuBa2 RECu2 O7 d , the dependence of Hirr on r(RE) has not been established. Previously we reported the eects of oxygen doping and aliovalent cation substitutions on the hole density distribution and accordingly on the values of Tc and Hirr [2,3]. However, the consequences of isovalent or ``size-only-dependent'' cation substitutions are less understood. Here we present the
0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 2 1 6 - 7
Y. Yasukawa et al. / Physica C 357±360 (2001) 230±233
results of a systematic study on the Hirr characteristics with respect to r(RE) in cation-stoichiometric CuBa2 RECu2 O7 d with a ®xed oxygen content.
2. Experimental CuBa2 RECu2 O7 d (RE Sm, Eu, Gd, Y, Y/Yb, Yb and Yb/Lu) samples were prepared through solid state reaction at 900±950°C for 12±120 h from stoichiometric mixtures of high-purity powders of RE2 O3 , BaCO3 and CuO. All the samples were fully oxygenated by post-annealing the assynthesized powders in a thermobalance in O2 gas ¯ow at 350±400°C for 10±96 h. The prepared samples were of single phase and the exact oxygen contents were established by the Cu(I)/(II) coulometric titration method [4] to be d 0:06 0:03 for all the samples. The structure parameters were re®ned from the X-ray powder diraction data (XRD, MAC Science: MXP18VAHF22 ; CuKa line). The temperature dependence of magnetic susceptibility was measured by a SQUID magnetometer (Quantum Design: MPMSR-5S). The ``eective'' superconductivity transition temperature Tc e values were estimated from the measured v vs T curves [5]. The Hirr characteristics were obtained through the procedure to be described later from the M vs H curves as measured by SQUID. In the magnetization measurements,
Table 1 Basic properties of the CuBa2 RECu2 O7
d
231
sample masses were kept quite constant. The average grain size was determined for each sample from a ®eld-emission scanning-electron micrograph (Hitachi: FE-SEM S-4500). Diraction data from microscopic areas were taken utilizing a transmission electron microscope (TEM; Hitachi: H-9000) operated at an accelerating voltage of 300 kV.
3. Results and discussion The lattice parameters, a, b and c, re®ned for the samples are given in Table 1. A clear tendency of ``lanthanide contraction'' is seen, that is, with decreasing r(RE) the lattice parameters decrease monotonically. Note that the tendency is not very clearly seen for solid solutions of RE Yb/Lu. Although the oxygen contents of the samples were constant within d 0:06 0:03, the Tc value decreases slightly with decreasing r(RE). This suggests that the hole-doping level of the CuO2 plane may shift towards the more overdoped side as r(RE) gets smaller. It should be emphasized that since the Hirr characteristics is strongly aected by the oxygen content, to reveal the eect of isovalent RE substitution, it is essential to standardize the oxygen content. We determined the Hirr line from the measured M vs H curves as the limit of H where the hysteresis in the magnetization (DM) disappears:
samples
RE
Ionic radius (A)
Tc e
Sm Eu Gd Y Y0:4 Yb0:6 Y0:2 Yb0:8 Yb Yb0:9 Lu0:1 Yb0:8 Lu0:2 Yb0:7 Lu0:3 Yb0:6 Lu0:4
1.079 1.066 1.053 1.019 0.999 0.992 0.985 0.984 0.983 0.983 0.982
92.9 91.4 94.2 91.4 91.1 91.4 89.5 89.5 89.9 89.7 89.7
(K)
Oxygen content d
Lattice parameters (A) a
b
c
N/A N/A 0.02 0.07 0.07 0.09 0.07 N/A 0.06 N/A 0.08
3.85(4) 3.84(2) 3.84(1) 3.82(6) 3.80(6) 3.81(4) 3.80(5) 3.80(4) 3.80(4) 3.80(5) 3.80(5)
3.90(3) 3.89(6) 3.90(1) 3.89(1) 3.87(4) 3.87(8) 3.87(4) 3.87(4) 3.87(4) 3.87(4) 3.87(4)
11.71(4) 11.69(1) 11.71(4) 11.66(8) 11.65(3) 11.66(2) 11.65(8) 11.66(2) 11.66(1) 11.66(3) 11.66(2)
232
Y. Yasukawa et al. / Physica C 357±360 (2001) 230±233
obtained in the present study is the highest Hirr value among those of the Cu-1212:P compounds not containing any extrinsic pinning centers. Possible reasons why the smaller RE Cu-1212:P samples have the better Hirr properties are as follows:
Fig. 1. Hirr (at T 77 K) of the CuBa2 RECu2 O6:940:03 samples with respect to r(RE).
Hirr H DM ! 0:
1
To satisfy the requirement given in Eq. (1), we assumed the following relation [6]: log H log Hirr b DM;
2
where b and Hirr are parameters determined by a least-square method applied to the experimental plot of log H vs DM. Note that, the DM data points which correspond to Jc values in the range of 500±1000 A/cm2 (according to Bean's model) were employed in this extrapolation procedure. In Fig. 1, thus determined values of Hirr at 77 K are plotted for all the samples with respect to r(RE). From Fig. 1, it is seen that the magnitude of Hirr strongly depends on r(RE) when the oxygen content is constant. With decreasing r(RE) the Hirr characteristics is clearly enhanced. For example, comparing CuBa2 (Yb0:6 Y0:4 )Cu2 O6:93 to CuBa2 (Yb0:6 Lu0:4 )Cu2 O6:92 , their Hirr characteristics are dierent by a factor of 2. Recently it was reported that a seeded-melt processed CuBa2 NdCu2 O7 d sample containing 15 mol% Nd4 Ba2 Cu2 O10 has a superb Hirr characteristics, i.e. 9.0 T in the case of H kc at 77 K and 10.5 T in the case of H?c at 88 K [7]. However, this is probably caused by ``extrinsic'' pinning centers, rather than an ``intrinsic'' nature of the electronic structure of the compound. Therefore, we consider that the Hirr value of 9.0 T at 77 K for CuBa2 (Yb0:6 Lu0:4 ) Cu2 O6:92
1. Some tiny impurity particles, e.g. RE2 BaCuO5 , may act as vortex pinning centers that might result in the better Hirr characteristics. 2. Some atomic disorder or concentration modulation along the ab plane may work as weak pinning centers to yield the better Hirr characteristics. 3. More homogeneous hole-density distribution along the c axis may induce the better Hirr characteristics. From SEM observations with EDX analyses of local compositions, it was revealed that there are hardly any impurities in the CuBa2 (Yb0:6 Lu0:4 )Cu2 O6:92 sample that shows the best Hirr characteristics. This implies that possibility (1) is not applicable to the present samples. Furthermore, a TEM electron diraction pattern from a CuBa2 (Yb0:6 Lu0:4 ) Cu2 O6:92 grain is shown in Fig. 2(a). Each spot in this diraction pattern for the ab plane is round (without streaks). The double spots indicate the existence of twin structures. Fig. 2(b) shows a TEM micrograph of a grain in the CuBa2 (Yb0:6 Lu0:4 )Cu2 O6:92 sample. It shows only twins but no impurity inclusions. Thus possibility (2) may not be applicable either, if twins as shown in Fig. 2(b) are benign. Since the oxygen content is precisely ®xed among the present CuB2 RECu2 O7 d samples, the total amount of holes must thus be ®xed. Using neutron diraction data obtained for ``optimized'' CuB2 RECu2 Oy samples [8], bond-valence-sum calculation reveals that with decreasing r(RE), the density of holes in the CuO2 plane increases while that in the CuO1 d chain decreases. For Cu-1212:P phases with large REs, the hole density in the CuO1 d chain is much larger than that in the CuO2 plane, i.e. the dierence in the hole density between the CuO2 plane and the CuO1 d chain is quite large. This indicates that the hole distribution along the c axis becomes more homogeneous with
Y. Yasukawa et al. / Physica C 357±360 (2001) 230±233
233
Fig. 2. (a) Electron diraction pattern from the ab plane and (b) TEM micrograph for a grain of the CuBa2 (Yb0:6 Lu0:4 )Cu2 O6:92 sample.
decreasing r(RE) in the Cu-1212:P system. Previously, it was established [1] that the more homogeneous the hole-density distribution along the c axis is, the better is the Hirr characteristics. Thus possibility (3) is believed to be most plausible.
4. Conclusion In this study, the Hirr characteristics of CuBa2 RECu2 O6:940:03 was systematically studied in terms of the RE ionic radius for RE Sm, Eu, Gd, Y, Y/ Yb, Yb and Yb/Lu. With decreasing r(RE), the Hirr characteristics was clearly enhanced in a monotonic way. For the CuBa2 (Yb0:6 Lu0:4 )Cu2 O6:92 sample with the smallest RE constituent, the Hirr value reached as high as 9.0 T at 77 K. This is the highest Hirr value ever reported for the Cu-1212:P phase at 77 K. The superb Hirr characteristics of the present CuBa2 (Yb0:6 Lu0:4 )Cu2 O6:92 sample is derived from size-only-dependent substitution. It was considered that the isovalent RE substitution induced more homogeneous hole-density distribution along the c axis and thus enhanced the Hirr characteristics. Further con®rmation with regard to the hole distribution by means of O K-edge XANES measurement is in progress.
Acknowledgements The present work has been supported by a grant-in-aid for Scienti®c Research (contract no. 11305002) from the Ministry of Education, Science and Culture of Japan, by an International Collaborative Research Project Grant-2000 of Materials and Structures Laboratory, Tokyo Institute of Technology, and by the Academy of Finland (decision no. 46039). References [1] M. Karppinen, H. Yamauchi, Mater. Sci. Eng. R 26 (1999) 51. [2] M. Karppinen, H. Yamauchi, K. Fujinami, T. Nakane, K. Peitola, H. Rundl of, R. Tellgren, Phys. Rev. B 60 (1999) 4378. [3] T. Nakane, K. Fujinami, M. Karppinen, H. Yamauchi, Supercond. Sci. Technol. 12 (1999) 242. [4] M. Karppinen, A. Fukuoka, L. Niinist o, H. Yamauchi, Supercond. Sci. Technol. 9 (1996) 121. [5] T. Matsushita, E.S. Otabe, T. Matsuno, M. Murakami, K. Kitazawa, Physica C 170 (1990) 375. [6] H. Yamauchi, M. Karppinen, K. Fujinami, T. Itoh, H. Suematsu, H. Sakata, K. Matsuura, K. Isawa, Supercond. Sci. Technol. 11 (1998) 1006. [7] N. Hari Babu, D.A. Cardwell, W. Lo, A.M. Campbell, Phys. Rev. B 61 (2000) 735. [8] H.B. Radousky, J. Mater. Res. 7 (1992) 1917.