Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Patent
1994-12-30
1997-04-08
Wieder, Kenneth A.
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
324 716, 505843, G01N 2700, G01R 2714
Patent
active
056191413
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention comprises a method and means for determining the doped or chemical hole concentration in high-T.sub.c superconducting cuprates and related compounds.
The high-T.sub.c superconducting cuprates all have in common square-planar sheets of CuO.sub.2 with Cu occupying the B-site of a perovskite unit and O atoms in the perovskite anion sites linking the corner-shared Cu atoms. These CuO.sub.2 planes are the essential structural ingredient for superconductivity in these cuprate perovskites and superconductivity arises when holes or electrons are doped into the planes to a concentration exceeding a minimum (p.sub.min for holes, n.sub.min for electrons) and less than a maximum (p.sub.max and n.sub.max respectively). Without loss of generality, the bounds are established as p.sub.min .apprxeq.0.05 holes/Cu and p.sub.max .apprxeq.0.27 holes/Cu for the hole-type superconductors (see Presland et al, Physica C 176 (1991) 95-105), but are not yet clearly determined for the electron superconductors. In this specification the invention is described with reference to the hole-superconductors but qualitatively similar, although numerically different bounds are believed to apply for the electron superconductors and the invention and general principles disclosed herein also have application to the electron superconductors as will be apparent.
FIG. 1 shows this general superconducting phase behaviour for the hole-doped superconductors. Superconductivity occurs for about 0.05.ltoreq.p.ltoreq.0.27 and T.sub.c rises to a maximum T.sub.c (max) at p.about.0.16 following an approximately parabolic dependence on p, conveniently and without loss of generality, given by
For about p<0.05 the behaviour is semiconducting and insulating as T.fwdarw.0 while for about p>0.27 normal metallic behaviour occurs. The parabolic curve in FIG. 1 was fitted to the data reported by Torrance et al, Phys. Rev. B40 (1989) 8872 and Takagi et al, Phys. Rev. B40 (1989) 2254, but it is likely that all other superconducting cuprates follow a similar curve. The domain p<0.16 is referred to as underdoped and p>0.16 as overdoped. Because of this common phase behaviour which is general to the superconducting cuprates, the chemical hole concentration is an important parameter whose determination makes it possible to locate a compound on the superconducting phase diagram, and moreover indicates whether, and by how much, the doping need be altered to maximise T.sub.c at T.sub.c (max). Alternatively doping may be altered to maximise the critical current of the superconductor. To maximise critical current the material is generally overdoped to a degree. This doping can be controlled by alter-valent cation substitution or by changing the oxygen content in the cuprate. Such chemical manipulation is preferably carried out in such a way as to maintain the integrity of the CuO.sub.2 planes and minimise disorder on these planes which can diminish T.sub.c, by pair breaking, below the ideal phase curve T.sub.c =T.sub.c (p). Measurement and control of the value of p allows the superconducting state to be fine-tuned.
BACKGROUND ART
Standard measurement of p is by determination of the cation composition by chemical analysis and determination of the oxygen content by high temperature thermal gravimetry during reduction in hydrogen or by chemical titration. These methods are complex, slow and destructive to the sample and with many of the superconducting cuprates is ambiguous because of mixed valency in solution.
DISCLOSURE OF INVENTION
In broad terms the invention comprises a method for determining the hole or electron concentration, transition temperature, ratio T.sub.c /T.sub.c (max), or the state of doping of a material capable of exhibiting superconductivity when cooled below its critical temperature, comprising measuring the thermopower of a sample of the material above the critical temperature of the material and determining from the thermopower the hole or electron concentration, transition temperature, ratio T.sub.c /
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Cooper John R.
Obertelli Sandro D.
Tallon Jeffery L.
Bowser Barry C.
Wieder Kenneth A.
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