Electronic Structures of Ga- and Zn- Substituted Yba2Cu3O7 Superconductors
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ELECTRONIC STRUCTURES OF Ga- AND Zn- SUBSTITUTED YBa2Cu3O7 SUPERCONDUCTORS YONG-NIAN XU
,
"*Department of
W.Y. CHING
AND K.W. WONG
Physics, University of Missouri-Kansas City, Kansas City, Missouri 64110 "*Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas 66045
ABSTRACT The band structures of YBa2Cu307 superconductors in which one of the Cu atoms was replaced by Ga or Zn have been studied by the first-principles orthogonalized LCAO method. Substitutions involving Cu on the chain site and Cu on the plane site were investigated. Results show significant modifications in the electronic structure due to the dopant-specific and site-specific substitutions. These results are discussed in light of some recent experimental observations.
I.
INTRODUCTION
There have been several experimental investigations on the Ga- and Znsubstituted YBa2Cu307 [1-2-3] compounds [1-3]. The idea is to probe the change in superconducting properties as Cu is replaced by Ga or Zn with a complete 3d shell structure, thereby gaining insight about the possible mechanism in high Tc superconducting oxides. Indeed, experimental data have shown a dramatic difference in the superconducting properties between Ga- and Zn- substituted samples; and these changes have been attributed to the changes in the electronic structures upon substitution of Cu by Ga or Zn at either the chain site (Cu I) or the plane site (Cu II). Therefore it is important to study the band structure of the substituted compound and to compare with that of the [1-2-31 in order to facilitate the interpretation of these data. Specifically, one would like to see which site, Cu I, Cu II or some other lattice sites are likely to be substituted by Ga or Zn; and what possible consequence it would have for Tc according to some proposed models. In this paper we present the band structures of the Ga- and Znsubstituted [1-2-3] compound (denoted by Ga-[l-2-3] and Zn-[l-2-3] respectively) calculated by the selfconsistent orthogonalized linear combination of atomic orbitals method (OLCAO). Substitutions involving both Cu I and Cu II sites were studied. The results are analyzed in the context of a theoretical model which is critically dependent on the normal state electronic structure as opposed to some other spin-mediated theories.
II.
METHOD OF CALCULATION
The OLCAO method was one of the first to be used to study the electronic structure of the [1-2-3] superconductor [4], the F-substituted [1-2-3] [5] and other high Tc superconductors [6]. Furthermore, the calculated electronic structure results have been used to study their optical properties [7]. The present calculation is similar to that for the [1-2-31 compound except one of the Cu atoms (either Cu I or Cu II) in the the orthorhombic unit cell is replaced by Ga or Zn. This means for the Cu I substitution, the Cu-0 chain becomes a Ga-0 or Zn-0 chain; while in the Cu II substitution, one of the Cu-0 planes remains intact while the other becomes a Ga-O or Zn-0 plane. This gives a very high substitution ratio of
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