Compensation effect, impurity scattering and superconductivity in 123 compounds

PHYSICA

Physica B 194-196 (1994) 1957-1958 North-Holland

Compensation effect, impurity scattering and superconductivity in 123 compounds Y. Zhao ~b, H.K. Liu', X.B. Zhugeb, G. Yang', J.A. Xia a, Y.Y. He°, and S.X. Dou" School of Materials Science and Engineering, University of N. S. W. P.O Box 1, NSW 2033 Australia b Department of Physics, Zhejiang University, Hangzhou, Zhejiang 310027, P.R. China Cryogenic Laboratory, Academia Siniea, P.O. Box 2711, Beijing 100080, P R China

A series of samples of 123-compound with the composition of Cu(1)x_xMxBa2Ya_yCayCu(2)20~, Co(1)l_xM~Ba2YCu(2)2_y7_myOZ (M=Al and Co), and Cu(1)Ba2YCu(2)2_x_yZnxNiy have been synthesised and investigated. It is found that the dopants can be mainly classified into two types: For the first type, they merely influence the charge-transfer of the system; for the second type, they destroy directly the correlation between electrons and hence suppresses intrinsically the superconductivity in the euprates.

A very remarkable and important characteristic of copper oxide superconductors is that the electronic properties and superconductivity depend very much on the structural defects and their arrangement of them[I,2]. Concurrent substitutions have displayed a great importance in the investigation on this topic[3,4]. As a kind of defect, dopant can be put into the system according to some particular requests. For example, Ca preferentially substitutes for the Y; Zn and Ni for Co(2); and Co, Fe and AI for Cu(1). This provides us convenience to probe the effect of defect on electronic properties and superconductivity in various situations. Here a comprehensive study has been presented which reveal that the defects can be mainly classified into two types: the first type merely influences the charge-transfer of the system; the second type destroys the correlation between electrons and hence suppresses intrinsically the superconductivity in the cuprates. All samples in this study were prepared by a solid state reaction method as described before[3]. Each sample was carefully characterised by XRD, SEM and EDS to ensure that the specimen is homogeneous single phase. Resistivity, ae susceptibility, oxygen stoiehiometry, XPS and Hall effect measurements have been don,~ to characterise the electronic properties and superconductivity. As we have known, Co, Fe, and Al preferentially occupy the Cu(1) site in Y-123 compound. Once these elements are doped into the 0921-4526/94/$07.00 © 1994 S S D I 0921-4526(93) 1610-X

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T~ and hole number vs. dopant content.

system, the carrier concentration, and consequently the T c of the system will be largely reduced. This is due to the hole filling or charge-transfer effect in which the holes are filled by the extra electrons introduced by doping these elements or migrate from the CuO 2 planes to the Cu-O chains and are localised there. In this process, the CuO 2 is not disturbed except for the charge carriers being removed. If the holes can somehow be put into the CuO z plane, the conductivity of the CuO 2 plane and the superconductivity of the system may be restored. The typical example is shown in Fig.1 in which the solid lines represent the Tc and the dashed lines represent the hole numbers. For the series of samples doped with Co, the T e drops rapidly with increasing the dopant content, and the

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1958 hole number of the system is decreased as well. However, if doping Ca at Y site simultaneously, the To is redeemed and the hole number does not reduce again. This means that the influences of Co substitution for Cu(1) on electronic properties and superconductivity are considerably compensated by the Ca substitution for Y. Using the chargetransfer model it can be estimated that the Ca substitution for Y results in a so-called delocalisation effect which releases the hole localised at the Cu-O chains. Similar results have been obtained in Al and Ca or Fe and Ca concurrently doped system. In addition, the effect of oxygen vacancies in the CO-O chain can also be compensated by doping Ca at Y site, which has successfully restore the superconductivity in the YBa2CB3OT_ z system with the oxygen stoichiometry 7 - z near 6. Since the Y layer and the Cu(1)-O layer are merely the "charge-transfer" layers, the results presented here suggest that the effect of the defect in the "charge-transfer" layers is able to be compensated. This offers us an approach to "superconductinglize", by introducing a variety of defects in the system, such as some cuprates which contain CuO 2 plane but are nonsuperconducting so far. I00.00

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Table L The summary of the compensation effect in partially co-substituted YBazCu3OT_, compounds in which AI, Co, and Fe occupy at the C-~I) site, Za and Ni at Cu(2) site, and Ca at Y site. C: compensated; N:noncompensated.

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compensated by simultaneously doping Co or oxygen vacancies, but accelerates the suppression of T o. Similar result has got in Ca and Zn codoped system. If Ni and Zn concurrently substitute the Cu(2), the T c is also reduced, rather than compensated. XPS and Hall effect measurements reveal that the oxidation state of Cu as well as the hole concentration can be modulated by doping Ca, Co, AI, etc., but not by Zn and Ni. All these illustrate that the mechanism of Tc suppression in this case is beyond the hole filling or hole overdoping. Up to now no way has been found to compensate the effects of defect residing in the CuO 2 plane (as summarised in Table I). This hints that the search for high-T o superconductivity beyond the cuprates will be very difficult. From the measurements of electron transport properties it is found that the defects (such as Zn or Ni) residing in the CuO 2 plane introduce impurity scattering centres with a radius of scattering cross section large than the average Cu-O bond length, which directly destroys the correlation between the conduction electrons as wel as the superconducting path. This may be the main reason for the noncompensated depression of superconductivity.

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It is now dear that the CuO 2 sheet is the conductive plane where the conductive electrons reside in and superconductivity occurs. It is interesting to know whether the effect of the defects in this layer is able to be compensated or not. Fig.2, shows the typical results on the systems where the defects (Zn impurities) reside in CuO2 plane. In this case, the effects of Zn can not be

REFERENCES: 1. J.D. Jorgensen, Physics Today, 44, 34 (1991). 2. A.W. Sleight, Physics Today, 44, 24 (1991). 3. Y. Zhao et al, Physica C 179, 207; 185189. 753 (1991); J.Phys:C, 4, 2263 (1992). 4. A. Manthira and J.B. Googenough, Physica C 152-164, 67 (1989).