The Dependence of T c on Structural and Electronic Parameters for Ceramic High-T c Superconductors
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THE DEPENDENCE OF Tc ON STRUCTURAL AND ELECTRONIC PARAMETERS FOR CERAMIC HIGH-Tc SUPERCONDUCTORS B.C. GIESSEN* AND R.S. MARKIEWICZ** *Northeastern University, Dept. of Chemistry and Barnett Institute iMaterials Science Division), 360 Huntington Avenue, Boston, MA 02115 Northeastern University, Dept. of Physics and Barnett Institute (Materials Science Division), 360 Huntington Avenue, Boston, MA 02115
ABSTRACT Based on the description of high-Tc cuprate superconductors as intercalation compounds and the van Hove singularity (vHs) concept for these phases, [with its consequences concerning their density of states (dos)] we give a model that explains Tc semi-quantitatively as a function of both hole concentration and stage number and, in a simplified variation, yields a functional dependence of Tc on the CuO 2 layer fraction of the unit cell that shows a maximum, as observed.
INTRODUCTION We present in this paper the structural foundations and the essential physics for understanding the dependence of Tc in high-Tc superconductors (HTSC's) on structural and electronic parameters. The structural arguments are based on a classification of HTSC's in terms of doping type, doping charge sign and the chemistry of the doping functional groups, especially for those HTSC's that can be described as intercalation compounds[l]. The physical arguments include a model for HTSC's that bases the occurrence of high-Tc ceramic superconductivity on the presence of a van Hove singularity (vHs) in these materials[2,3]. We conclude by presenting a model for the dependence of Tc on structural and electronic parameters based on this theory[4];
this
model
improves
upon
our
earlier,
"naive"
model
[5]
that
related Tc simply to the number of CuO 2 layers [6]. STRUCTURAL CONSIDERATIONS:
TYPES OF DOPING AND ROLE OF FUNCTIONAL GROUPS
Intercalation Layer Doping In analogy to other intercalation compounds such as graphite intercalation compounds[7], we visualize the HTSC cuprates as consisting of single layers or multilayers of the (semiconducting) "parent" cuprate layer compound (Ca,Sr)CuO2 [8] and intercalated layers of "doping units" such as TIBa02.5 which contain partially unfilled oxygen sites. These in ercalant sites can accept additional oxygen atoms which, upon ionizing to 0 -(and changing the doping group composition, e.g., to TlBa02.5+), oxidize the parent compound layers and, by introducing enough holes into the network of CuO2 squares, can produce conduction. [The number of consecutive cuprate layers (host units) between successive intercalant units is referred to as the "stage number".] We have demonstrated [5] that this concept qualitatively explains a maximum in the values of T7 seen in a plot of Tc versus the fraction f. of unit cell occupied by CuO2 S sheets[5,6]. This point is further discussed below.
Mat. Res. Soc. Symp. Proc. Vol. 169. ©1990 Materials Research Society
976
Table I:
Strctural Damilies of HIgh T, Dmeryondg=
O
-
I |
I
Intercalation IAy Doped .
I
&
I
I "Single layered" Phases T, T', V"
Ba(P
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