Effects of Ce on the crystal structure and superconductivity in TlSr2CaCu2O7−δ

PHYSICA ELSEVIER

PhysicaC253 (1995) 156-164

Effects of Ce on the crystal structure and superconductivity in T1SrECaCu207_ W.H. Lee *, D.C. Wang Department of Physics, National Chung Cheng University, Ming-Hsiung, Chia-Yi 621, Taiwan Received 25 July 1995

Abstract The partial substitution of Sr or Ca in TISr2CaCu207_ 8 (T1 1 : 2 : 1 : 2) with the rare-earth element Ce has the advantage to stabilize the tetragonal phase with space group P4/mmm and shows a prominent deviation from Vegard's law for the changes in the a, c and v parameters. The transitions ranging from non-superconducting metal to superconductor to insulator are displayed in both the Tl(Sr2_xCex)CaCu207 (0 ~
1. I n t r o d u c t i o n Superconductivity at 8 0 - 9 0 K has been resistively and magnetically observed [1] in the R - T 1 S r - C a - C u - O systems, where R represents the rare earths including Y, La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu and excepting Ce. In fact, not a few studies [2-40] o f carrier concentration controlled by using different aliovalent substitutions or changes in the oxygen stoichiometry in the parent undoped or overdoped cuprate oxides have been reported for the past years. However, comparatively few cases where Ce substitution can significantly increase the T~ o f an inherent over-hole-doped compound have been known in the literature [24,30,40]

* Corresponding author.

probably due to the unreactive nature o f CeO 2 [41]. It seems that the electron-doped cuprate superconductors o f the form R2_xCexCuO4_y (R = Pr, Nd, Sm or Eu) [42-45] is a more well-known discovery. F o r Ce doped R B C O and Bi2CaSr2Cu2Oy systems for which the hole, rather than the electron, is responsible for superconductivity, T¢ decreases with the dopant concentration [33,46,47]. Therefore, research on the suppression or enhancement o f superconducting properties in Ce containing materials is a subject o f sustained interest experimentally. The inherent over-hole-doped T1Sr2CaCu207_ 8 material g e n e r a l l y contains oxygen vacancies and is o f great interest because its crystal structure can be seen as a further variation o f the TIBa2YCu207_ ~ structure [48] which resembles the Y B a 2 C u 3 0 7 _ ~ structure with T1 occupying the corners o f the modified structure. Thus it is attractive to explore a comparative study o f Ce doping in the T1 1 : 2 : 1 : 2

0921-4534/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0921-4534(95)00505-6

W.H. Lee, D.C. Wang / Physica C 253 (1995) 156-164

system. To make the T1-O monolayer structure of T1 1 : 2 : 1 : 2 more stable, it may be helpful to make the charge compensation through feasible T I + 3 - p b 4+, Ca2+_R > 2+ (rare earth) or SrZ+-R > 2+ cation substitutions to reduce the effective valence of Cu. Nevertheless, it is not easy to prepare single-phase T1 based samples because the volatile property of thallium makes it difficult to keep the stoichiometry at the sintering temperature and multiphases with substantial amounts of intergrowths and defects are always formed. Taking advantage of the possibility to form single-phase samples with the same stoichiometry as those of the starting materials, we have focused our attention to the problem of forming Ce containing superconductors in T1 based compound system. The main aim of this study by Ce substitutions at the Sr or Ca site in the TISrzCaCu207_ ~ system is to gain further understanding of the change of physical properties as well as their synthetic conditions.

2. Experimental details All samples were prepared from > 99.9% pure TIEO 3, CeO 2 , SrCO 3, CaCO 3, and C u t by solid-state reactions and a two-step procedure. The well mixed powders in the stoichiometry of (Sra_xCex)CaCu20 w or Sr2(Cal_yCey)Cu20 z were first fired at 800 ~ 850°C in an A1203 crucible for 12 h with several intermediate grindings to obtain an uniform black Sr-Ce-Ca-Cu-O powder. Then approximate amounts of T1203 and S r - C e - C a - C u oxide powders with a certain stoichiometry were completely mixed, ground and pressed into cylindrical pellets. To alleviate possible decomposition of T1203 to TIEO and O 2 each pellet was wrapped in gold foil, sealed under ~ 1 atm pure oxygen in quartz tube and heated at 950 _+ 20°C for 2 h followed by an air quench to room temperature. Under our miscellaneous synthetic conditions, it was found that samples heated in an oxygen quartz tube had properties superior to those heated in air, Ar (a chemically reducing environment), vacuum or flowing oxygen. A microcomputer-controlled MXP3 diffractometer equipped with a copper target and graphite monochromator for Cu K o~ radiation was used to get the powder X-ray diffraction (XRD) patterns at a scan rate of 0.4°/min.

157

The refined lattice parameters of the unit cell were determined from powder X-ray diffraction patterns by the method of least squares [49] using the 16 most intense reflections for 2 0 < 50 ° and including an internal silicon standard (a = 0.543083 nm). Magnetic data were carried out in a Quantum Design SQUID magnetometer. The temperature dependence of magnetization was measured using a zero-field cooling (ZFC) process, i.e., the sample was initially cooled in zero field (actually ~ 5 × 10 -3 Oe) to 5 K and subsequently a small field ( ~ 10 Oe) was applied, and the ZFC curve was taken as a function of increasing temperature up to T > T~. The ambient pressure superconducting transition temperatures were determined from low-frequency ( ~ 25 Hz) AC magnetic-susceptibility measurements with an AC field amplitude of 1 Oe. Since some transitions are not sharp, it is difficult to define a single Tc. For comparison, the midpoint or the onset of the transition will be taken as the superconducting transition temperature T~. DC electrical-resistivity measurements were made between 5.0 and 300 K using a standard four-probe technique in a system fully automated for temperature stability and data acquisition [50]. Fine platinum wires ( ~ 2 mm diameter) were attached to the sample with conductive silver paint and served as the voltage and current leads. The disk-shaped sample is homogeneous in thickness. A Keithley Model 220 was taken as a constant current source and a Keithley Model 182 Nanovoltmeter was used to measure the output voltage. Data were taken with the current (1 mA) applied in both directions to eliminate possible thermal effects. Once the sample resistance R is measured, the resistivity p can be calculated via the van der Pauw law.

3. Results and discussion The observed powder X-ray diffraction patterns at room temperature for three representative samples T1SrECaCu207_~, Tl(Srl.sCeo.5)CaCu207_~ and T1Sr2(Cao.tCe0.4)CU2OT_ 8 are shown in Figs. l ( a c). The peaks of each observed pattern can be all indexed in a tetragonal structure with space group P 4 / m m m except for the undoped sample, where there is one unknown peak around 20 = 32 °, In fact, to the best of our knowledge, a single-phase TI

W.H. Lee, D.C. Wang/ Physica C 253 (1995) 156-164

158

1 : 2 : 1 : 2 sample without cation substitution was obtained only for a nonstoichiometric and Ca poor (Sr : Ca : Cu = 2 : 0.85 : 2) composition because the Ca site was found to be partially substituted with Tl [19,39,51,52]. This demonstrates that the partial substitution of Sr or Ca ion by Ce ion in the T1 1 : 2 : 1 : 2 system has the advantage to suppress the instability and to stabilize the tetragonal phase. The variation of refined lattice parameters of the pure

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Fig. 2. Variation of lattice parameters vs. dopant concentration. (a) a vs. x, (b) c vs. x and (c) v vs. x. ( O ) and (zx) represent the two series Tl(Sr2_xCex)CaCu2OT_8 and TISr2(Ca]_xCex)Cu207- 8, respectively.

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Fig. 1. Room-temperature powder X-ray diffraction patterns of (a) TISr2CaCu207-8, (b) Tl(SrLsCeo.5)CaCu2OT_8 and (c) TISr2(Cao,6Ceo.a)CU2OT- 8 using Cu Kot radiation.

samples with Figs. 2(a), (b) from Vegard's parameters of

dopant concentration is depicted in and (c). We note a marked deviation law for the changes in the a, c and v the solid solutions Tl(Sr2_xCex)-

159

W.H. Lee, D.C. Wang/ Physica C 253 (1995) 156-164

CaCu207- 8 and T1Sr2(Ca l_xcex)cu207_ a around x = 0.35, which seems to be an indication of inhomogeneous Ce distribution at higher concentrations due to approaching a solubility limit. This phenomenon was not observed in the other rare-earth doped T1 1 : 2 : 1 : 2 series [53]. The maximum Tc of 62 K observed in the Ce doped T1 1 : 2 : 1 : 2 series is less than the optimal superconducting phase transition temperatures (80 ~ 90 K) of the other rare-earth doped series in T1 1 : 2 : 1 : 2 series [53]. Therefore, at present stage, we do not exclude the possible strong electronic effects, which yield the deviation of Vegard's law and the lower Tc,max, between the Ce and C u - O layer in the two Ce doped systems. Considering the valence of the Ce atom and its corresponding size of ionic radius, we believe that the substitutional replacements for Sr or Ca atoms serve to change the number of conduction holes in CuO; plane, which results in the decrease of valence of Cu ion. The increase in the a parameter may be ascribed to the change in the planar C u - O bond distance due to hole filling in the system [54]. Moreover, the shortening in the c parameter is presumably either due to the increase in the oxygen content or due to the smaller size of Ce 3+ or Ce 4+ compared with the Sr 2+ ion. From the viewpoint of charge compensation for the sample Tl(Srl.aCe0.6)CaCu2Oy, the oxygen content y is probably to be greater than 7 or the valence of Ce ion is between 3 + and 4 + in order to keep the valence of Cu at least 2 -t-. In this calculation, the valence of T1, Sr, and Ca ions are supposed to be 3 + , 2 + , and 2 + , respectively. Therefore, excess oxygen introduced to the interstitial site between T10 monolayers is possible in this Ce doped T1 1 : 2 : 1 : 2 system. However, in the absence of the knowledge of oxygen concentration and distribution in the compound, the exact oxygen state is yet to be determined. Figs. 3(a) and (b) and 4(a) and (b) display the temperature dependence of the zero-field cooled magnetization for the nine samples (x = 0.05, 0.1, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, and 0.5) in the series Tl(Sr2_xCex)CaCu207_ a and seven samples ( x - 0.1, 0.2, 0.25, 0.3, 0.35, 0.4, and 0.45) in the series TISr2(Ca l_xcex)cueO7_a measured in a field of l0 Oe between 5 and 100 K. All measurements were performed on bulk samples with a mass of about 0.1 g. For both systems, it is noted that the shielding

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curves for constant field shift toward higher temperature with increasing Ce concentration up to x = 0.35, then backward to a lower temperature. In most cases the ZFC curves show a sharp transition and reach saturation at the lower temperature, which is an indication of the sample homogeneity. For the sample with a small Ce dopant concentration of x = 0.1, the transition is quite broad, reflecting sample inhomogeneity or some other imperfection, which is consistent with the apparent metastability of the parent compound T1Sr2CaCu20 7_ 8- These samples with x around 0.3 also show large shielding signals, by comparison to the ideal value of - 1/4~r for a long cylinder, with apparent volume fractions above 100%. This phenomenon can be explained in terms of the shielding current and the estimated geometrical demagnetization factor n ~ 0.5 because of the irregular shaped sample. The relatively small shield-

W.H. Lee, D.C. Wang / Physica C253 (1995) 156-164

160

ing fraction (several percent) of the x = 0.5 sample (Tc ~ 5 K) to the perfect diamagnetism suggests the coexistence of the insulating phase, which reveals that the sample of x = 0.5 is at the superconductorinsulator (SI) phase boundary. As reported earlier [26,31,55], an unusual property of the high-T~ materials in the overdoped region is the power-law temperature dependence of the resistivity ( p = P0 + A T " ) with n ~ 1 to 2. In the intrinsic over-hole-doped T1Sr2CaCu2OT_ 8 compound, we get an exponent n ~ 1.6 in the temperature range 30-300 K as shown in Figs. 5(a) and (b). Figs. 6(a) and (b) and 7(a) and (b) display the variation of the resistivity with temperature for the five samples in the series Tl(Sr2_xCex)CaCu2OT_ 8 and six samples in the series T1Sr2(Cal_xCe~)Cu207-8. It is seen that the zero-resistance temperature increases with increasing dopant concentration,

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Fig. 4. Zero-field cooling magnetization data for seven samples in the series T1Sr2(Cal_xCex)CU2OT_ 8. (a) x = 0.1, 0.2, 0.25, 0.3, 0.35; (b) x = 0.35, 0.4 and 0.45.

reaches a maximum of about 60 K at x = 0.3, and then decreases. These superconducting transition temperatures as determined by the resistive method are in good agreement with the magnetic measurements. Samples with x = 0.2 and 0.3 show a metallic behavior in the normal state with the nearly linear temperature dependence of resistivity. However, with increasing x, the normal-state resistance shows a change from metallic behavior to a semiconductorlike behavior. For both systems, the resistivity of the x = 0.4 sample increases to a maximum value around 60 K as the temperature is lowered from 100 K. Upon further cooling, the rather sharp drop in the resistivity is due to superconducting transition. For the Tl(Srl.5Ce0.s)CaCu207_ ~ sample, the resistivity peak shifts toward lower temperature and the position of the resistivity peak is around 10 K. Though

W.H. Lee, D.C. Wang/Physica C 253 (1995) 156-164

the unreactive nature of CeO 2 usually makes it difficult to obtain a uniform distribution o f Ce within each grain as demonstrated in Sm2_xCexCuO4_ 8 by Early et al. [56], no double resistive superconducting transition due to the granularity is present in all our superconducting polycrystalline samples. From the resistivity curves o f Fig. 6(b), it is seen that the metal-insulator ( M - I ) transition appears near the composition o f x = 0.6 in the series Tl(Sr2_=Cex)CaCu207_ ~. This sample is in a metallic behavior regime when the temperature T > 150 K and in a semiconductor-like behavior regime when the temperature T < 150 K, with no superconducting transition above 1.8 K as determined from the magnetic measurements. According to the d-dimensional variable-range-hopping (VRH) mechanism of Mott [57], the logarithm o f the resistivity 1n p in the semiconducting region behaves proportionally to T" with

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Fig. 6. Temperature dependence of resistivity for five samples in the series TI(Sr2_zCex)CaCu2OT_~. (a) x=0.2, 0.3, 0.4; (b) x = 0.5 and 0.6.

n = 1 / ( d + 1). In fact, the resistivity data of the x = 0.6 sample in the series Tl(Sr2_xCex)CaCu207_ 8 can be excellently fitted to the V R H dependence in the temperature range 3 0 - 1 0 0 K with n = ~ 1 as shown in Fig. 8, which suggests a three-dimensional V R H conduction mechanism. Recent experimental reports [31,58-60] have the trend that conduction is dominated by a two-dimensional V R H mechanism in the insulating phase and a crossover from 2D to 3D VRH behavior is generally admitted to take place as the metal-to-insulator transition is achieved. However, due to the solution limit o f Ce in Tl(Sr2_xCe~)CaCu207_ 8, the sample with higher Ce content ( x > 0.6) we have tried is not single phase and we are not permitted to determine the n value for the sample o f x > 0.6 with lower carder density, where the lower-dimension conduction is to be expected. In fact, similar resistivity results can also observed in

W.H. Lee, D.C. Wang / Physica C 253 (1995) 156-164

162 2.60

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Fig. 8, Logarithm of low-temperatureresistivity as a function of T -1/4 for the sample with x=0.6 in the series Ti(Sr2_xCex)CaCu2OT-8the series TlSr2(Cal_xCex)Cu207_8 and Fig. 9 shows the relationship between the low-temperature resistivity and T - I / 4 for the sample T1Sr2(Cao.3 C e o . 7 ) C u 2 0 7 - 8.

The T¢ dependence of the Ce content as determined by magnetic or electrical measurements is an undoubted manifestation of the role of the hole mechanism in both the Tl(Sr2_~Cex)CaCu2OT_ 8 and T1Sr2(Cal_xC%)Cu207_ 8 system. Recently, T. Kondo et al. [39] pointed out that the "charge carder density" calculated from the chemical composition is more reasonable than that determined from the Hall coefficient measurement in the "doped Mott 12.50

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Fig. 10. Tc normalizedat Tc,max as a function of hole content per Cue 2 layer in the two series Tl(Sr2_xCex)CaCu207_ ~ (C)) and TISr2(Cal_xCex)CU2OT_8 (I,). The solid line is a universal relationship curve proposedby Zhang and Sate.

insulator" picture. Therefore, to estimate the carrier density precisely, it is necessary to have the knowledge of the oxygen concentration and distribution in each sample. Although the 8 value of each sample has not been determined in the present work, some estimates can be taken from the earlier report [40], e.g., the number of hole carrier per C u e 2 layer of the oxygenated parent compound T1Sr2CaCu207_ 8 is 0.32. Assuming full occupancy of the oxygen sites in each Ce 4+ doped sample, Fig. 10 displays the correlation between Tc,onset o r Te,mid of Xac and the hole content per C u e 2 unit for the pure samples in the two systems Tl(Sr2_xCex)CaCu207 and T1Sr2(Cal_xCex)Cu2OT_8. With our experimental data, the interplay between the enhancement of superconductivity and the hole content behaves in conformity with the universal relationship as proposed by Zhang and Sato [61].

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Single-phase sample in the Ce doped T1Sr2CaCu207-8 system could be achieved under critical conditions including stoichiometry and heat-treatment method. The change of physical properties spanning the whole non-superconducting metal-superconductor-insulator transition regions has been observed through the varying hole concentration con-

W.H. Lee, D.C. Wang/ Physica C 253 (1995) 156-164 trolled by substitution o f C e > 3+ for Sr 2+ or C a 2+ in T 1 S r 2 C a C u 2 0 7 _ 8. A s the s y s t e m goes f r o m an o v e r - h o l e - d o p e d n o n - s u p e r c o n d u c t i n g m e t a l to a sup e r c o n d u c t o r , the resistivity u n d e r g o e s a c h a n g e f r o m a p o w e r - l a w t e m p e r a t u r e d e p e n d e n c e with e x p o n e n t n N 1.6 to a linear t e m p e r a t u r e d e p e n d e n c e . H i g h transition t e m p e r a t u r e s are f o u n d in a s o m e w h a t narrow w i n d o w o f C e c o n t e n t (0.2 < x ~< 0.4) in both series. A t l o w c o n c e n t r a t i o n o f c h a r g e carrier for the s a m p l e w i t h x around 0.4, holes go f r o m l o c a l i z e d to delocalized. A n a l y s i s o f the electrical resistivity in the insulating r e g i o n suggests that the c o n d u c t i o n is g o v e r n e d by a V R H m e c h a n i s m in the l o w - t e m p e r a ture r a n g e b e l o w ~ 100 K. In v i e w o f the crystal structure o f T1Sr2CaCu2OT_8, it is not surprising that similar c h a n g e s o f the p h y s i c a l properties o c c u r for either Sr 2+ or C a 2+ ion substituted by a C e 4+ ion b e c a u s e the C u - O layer sites b e t w e e n S r - O and C a - O layers and the state o f the C u - O layer is b e l i e v e d to be crucial to the p h y s i c a l p r o p e r t y in the cuprate oxides.

Acknowledgement T h i s w o r k was supported by the N a t i o n a l S c i e n c e C o u n c i l o f T a i w a n under C o n t r a c t No. N S C 8 4 - 2 1 1 2 M194-009.

References [1] Z.Z. Sheng, L. Sheng, X. Fei and A.M. Hermann, Phys. Rev. B 39 (1989) 2918. [2] R.J. Cava, B. Batlogg, C.H. Chen, E.A. Rietman, S.M. Zahurak and D. Werder, Nature (London) 329 (1987) 429. [3] M.W. Sharer, T. Penney and B.L. Olson, Phys. Rev. B 36 (1987) 4047. [4] J.B. Torrance, Y. Tokura, A.I. Nazzal, A. Bezinge, T.C. Huang and S.S.P. Parkin, Phys. Rev. LeU. 61 (1988) 1127. [5] A. Manthiram and J.B. Goodenough, Appl. Phys. Lett. 53 (1988) 420. [6] A.K. Ganguli, K.S. Nanjundaswamy and C.N.R. Rao, Physica C 156 (1988) 788. [7] M.A. Subramanian, C.C. Torardi, J. Gopalakrishnan, P.L. Gai, J.C. Calabrese, T.R. Askew, R.B. Flippen and A.M. Sleight, Science 242 (1988) 249. [8] P. Haldar, S. Sridhar, A. Roig-Janiki, W. Kennedy, D.H. Wu, C. Zahopoulos and B.C. Griessen, J. Supercond. 1 (1988) 211.

163

[9] J.M. Tarascon et al., Phys. Rev. B 39 (1989) 4316. [10] G. Xio, A. Bakshai, Z. Cieplak, Z. Teasanovic and C.L. Chien, Phys. Rev. B 39 (1989) 315. [11] Y. Shimakawa, Y. Kubo, T. Manako and H. Igarashi, Phys. Rev. B 40 (1989) 11400. [12] J.K. Liang, Y.L. Zhang, G.H. Rao, X.R. Chang, S.S. Xie and Z.X. Zhao, Solid State Commun. 70 (1989) 661. [13] S. Li and M. Greenblat, Physica C 157 (1989) 365. [14] R.S. Liu, J.M. Liang, Y.T. Huang, W.N. Wang, S.F. Wu, H.S. Koo, P.T. Wu and L.J. Chen, Physica C 162-164 (1989) 869. [15] Y.T. Huang, R.S. Liu, W.N. Wang and P.T. Wu, Physica C 162-164 (1989) 39. [16] A.K. Ganguli, V. Manivannan, A.K. Sood and C.N.R. Rao, Appl. Phys. Lett. 55 (1989) 2664. [17] Y. Shimakawa, Y. Kubo, T. Manako, H. Igarashi, F. Izumi and H. Asano, Phys. Rev. B 42 (1990) 10165. [18] H. Hattori, K. Nakamura and K. Ogawa, Jpn. J. Appl. Phys. 29 (1990) L36. [19] D.M. de Leeuw, W.A. Groen, J.C. Jol, H.B. Brom and H.W. Zandbergen, Physica C 166 (1990) 349. [20] S. Nakajima, M. Kikuchi, Y. Syono, K. Nagase, T. Oku, N. Kobayashi, D. Shindo and K. Hiraga, Physica C 170 (1990) 443. [21] Z.Z. Sheng, L.A. Burchfield and J.M. Meason, Mod. Phys. Lett. B 4 (1990) 967. [22] Z:Z. Sheng, Y. ]Kin, J.M. Meason and L.A. Burchfield, Physica C 172 (1990) 43. [23] Z.Z. Sheng, Y. Xin and J.M. Meason, Mod. Phys. Lett. B 4 (1990) 1281. [24] P.T. Wu, R.S. Liu and W.H. Lee, Mol. Cryst. Liq. Cryst. 184 (1990) 17. [25] J. Gopalakrishnan, R. Vijayaraghavan, R. Nagarajan and C. Shivakumara, J. Solid State Chem. 93 (1991) 272. [26] Y. Kubo, Y. Shimakawa, T. Manako and H. Igarashi, Phys. Rev. B 43 (1991) 7875. [27] Z.Z. Sheng, D.X. Gu, Y. Xin, D.O. Pederson, L.W. Finger, C.G. Hadisiacos and R.M. Hazen, Mod. Phys. Lett. B 5 (1991) 635. [28] Y. Kubo, T. Kondo, Y. Shimakawa, T. Manako and H. Igarashi, Phys. Rev. B 45 (1992) 5553. [29] Y. Miyazaki, H. Yamane, N. Kobayashi, T. Hirai, H. Nakata, K. Tomimoto and J. Akimitsu, Physica C 202 (1992) 162. [30] Q. Hong and J.H. Wang, Physica C 217 (1993) 439. [31] N.L. Wang, Y. Chong, C.Y. Wang, D.J. Huang, Z.Q. Mao, L.Z. Cao and Z.J. Chen, Phys. Rev. B 47 (1993) 3347. [32] T. Mertelj, D. Mihailovi6, F.C. Matacotta, R.S. Liu, J.R. Cooper, I. Gameson and P.P. Edwards, Phys. Rev. B 47 (1993) 12104. [33] V.P.S. Awana, S.K. Agarwal and A.V. Narlikar, Phys. Rev. B 48 (1993) 1211. [34] H.B. Liu, D.E. Morris and A.P.B. Sinha, Physica C 204 (1993) 262. [35] W.J. Zhu, J.J. Yne, Y.Z. Huang and Z.X. Zhao, Physica C 205 (1993) 118. [36] R.S. Liu, S.F. Wu, D.A. Jefferson, P.P. Edwards and P.D. Hunneyball, Physica C 205 (1993) 206.

164

W.H. Lee, D.C. Wang / Physica C 253 (1995) 156-164

[37] T.P. Beales, C. Dineen, S.R. Hall, M.R. Harrison and J.M. Parberry, Physica C 207 (1993) 1. [38] B. Dabrowski, V. Zhang-MacCoy, R. Hannon, B.A. Hunter, J.D. Jorgenson, J.L. Wagner and R.L. Hitterman, Physiea C 208 (1993) 183. [39] T. Kondo, Y. Kubo, Y. Shimakawa and T. Manako, Phys. Rev. B 50 (1994) 1244. [40] P.T. Wu, R.S. Liu, J.M. Liang, Y.T. Huang, S.F. Wn and L.J. Chen, Appl. Phys. Lett. 54 (1989) 2464. [41] M.E. L6pez-Morales, R.J. Savoy and P.M. Grant, Solid State Commun. 171 (1989) 1077. [42] Y. Tokura, H. Tagagi and S. Uchida, Nature (London) 337 (1989) 345. [43] J.T. Markert and M.B. Maple, Solid State Commun. 70 (1989) 145. [44] J.T. Markert, E.A. Em'ly, T. Bj~rnholm, S. Ghamaty, B.W. Lee, J.J. Neumeier, R.D. Price, C.L. Seaman and M.B. Maple, Physica C 158 (1989) 178. [45] E.A. Early, N.Y. Ayoub, J. Beille, J.T. Markert and M.B. Maple, Physica C 160 (1989) 320. [46] H. Uwe, Y. Motoi, H. Yamaguchi and T. Sakudo, Jpn. J. Appl. Phys. 26 (1987) L860. [47] I.K. Gopalakrishnan, J.V. Yakhmi and R.M. Iyer, Physica C 204 (1993) 413.

[48] T. Manoko, Y. Shimakawa, Y. Kubo, T. Satoh and H. Igarashi, Physica C 156 (1988) 315. [49] E.R. Hoverstreydt, J. Appl. Crystallogr. 16 (1983) 651. [50] Quantum Design, Inc., San Diego, CA 92121. [51] T. Doi, K. Usami and T. Kamo, Jpn. J. Appl. Phys. 29 (1990) L57. [52] F. Izumi, T. Kondo, Y. Shimakawa, T. Manako, Y. Kubo, H. Igarashi and H. Asano, Physica C 185-189 (1991) 615. [53] Unpublished work. [54] P. Mandal, A. Poddar, B. Ghosh and P. Choudhury, Phys. Rev. B 43 (1991) 13102. [55] Y. Kubo, M. Sofa, S. Yamagata, S. Kondoh, M. Onoda and M. Sato, Solid State Commun. 70 (1989) 303. [56] E.A. Early, C.C. Almasan, R.F Jardim and M.B. Maple, Phys. Roy. B 47 (1993) 433. [57] N.F. Mort, J. Non-Cryst. Solids 1 (1969) I; B.I. Shklovskii and A.L. Efros, Electronic Properties of Doped Semiconductors (Springer, Berlin, 1984). [58] P. Mandal, A. Poddar and B. Ghosh, Phys. Rev. B 43 (1991) 13102. [59] B. Ellman, J.M. Jaeger, D.P. Katz, T.F. Rosenbanm, A. Cooper and G.P. Espinosa, Phys. Rev. B 39 (1989) 9012. [60] A. Yamazaki et al., J. Phys. Soc. Jpn. 59 (1990) 1921. [61] H. Zhang and H. Sato, Phys. Rev. Lett. 70 (1993) 1697.