A Correlation Between the Optimum Levels of Hole Doping in the Sr- and Ba- Classes of Copper Oxide Superconductors
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A CORRELATION BETWEEN THE OPTIMUM LEVELS OF HOLE DOPING IN THE Sr- AND Ba- CLASSES OF COPPER OXIDE SUPERCONDUCTORS
John B. Parise, Mineral Physics Institute and Department of Earth and Space Sciences, State University of New York, StonyBrook, NY 11794 and Eugene M. McCarron m*, Central Research and Development Department, E. I. Du Pont de Nemours, P. 0. Box 80356, Wilmington, DE 19880-0356.
ABSTRACT Considerations of the in-plane Cu-O bond distance in a variety of cuprate-based superconducting systems indicate: (1) that an optimum level of hole doping exists in these systems, and (2) that this optimum value is roughly the same for Sr- and Ba-classes of superconductors when one normalizes for structural effects due to the difference in ionic size. Structural adjustment at the optimum level of doping leads to variations in Tc between structure-types.
INTRODUCTION The dependence of Tc upon the level of oxidation of the Cu02 sheets has been described in a number of recent papers [1-4]; in general, the Tc initially increases, reaches an optimum value, and then drops off as the number of holes in (or oxidation of) the CuO2 sheet increases. Introduced by doping with lower valent metals [5-71 or by oxygen insertion [8], the number of holes can be correlated with the Cu-O in-plane interatomic distance [1-41 - this distance decreasing monotonically as electrons are removed [9]. Recently [4] the relationship between in-plane Cu-O distance and Tc has been presented as a compendium in three parts: the classes and sub-classes of
the cuprate superconductors containing La, Sr, and Ba. Within each class and subclass a different optimum level of hole doping is proposed. This hypothesis is based both upon the observation that each class has a different optimum Cu-0 bond distance and upon calculations which suggest that the number of holes which can be accepted by the Cu-O sheets increases as the size of the ion separating the sheets increases [4]. As has been pointed out previously [4,101, the in-plane Cu-O bond distance depends not only upon the average oxidation state of the Cu0 2 sheet but also upon the nature of the intersheet cations; the larger cations necessarily increase the Cu-O distance compared to that for those materials having the same structure but containing smaller cations. An estimate of this structural effect can be made from careful studies of closely related materials in which the level of hole doping is the same. One such example is the comparison between the structurally related compounds (Yl-xCax)Ba 2Cu 3O6 [6,101 and (Yl-xCax)Sr2Cu3O6o2PbO [10,11-13]. The variation in Cu-O inplane bond distance for isoelectronic couples [specifically, YBa2Cu306 : YSr2Cu30692PbO and (YO.75Cao.25)Ba 2Cu 3O6 : (Y0.75Ca0.25)Sr2Cu306*2PbO [10-13] was such that the materials containing Sr rather than Ba consistently have a Cu-O bond length contracted by 0.02A at the same level of hole doping (in this case the holes are provided by substitution of Ca(II) for Y(III)). We had noted previously [14] that the Sr- and Ba-classes presen
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