Crystal structures, charge carrier density and superconducting properties of high-T(c) cuprates

Huanbo Zhang, Purdue University

Abstract

The characteristics of crystal structure and superconducting properties of the p-type high-T$\sb{\rm c}$ cuprates have been studied. The existence of a universal relationship which can be expressed by T$\sb{\rm c}/{\rm T}\sb{\rm c,max}$ = exp$\{-$7.25 $\times$ 10$\sp5$ (p$\sb{\rm sh} -$ 0.1855)$\sp6\}$ is pointed out, where T$\sb{\rm c,max}$ is the maximum attainable T$\sb{\rm c}$ for a compound and p$\sb{\rm sh}$ is the hole content in the CuO$\sb2$ sheet. Since mobile holes for superconductivity are provided by nonstoichiometry, dopability of the high-T$\sb{\rm c}$ cuprates is investigated. Here, three major types of high-T$\sb{\rm c}$ cuprates, La$\sb{\rm 2-x}{\rm Sr}\sb{\rm x}{\rm CuO}\sb{4+\delta},$ YBa$\sb2{\rm Cu}\sb3{\rm O}\sb{7-\delta}$ and Bi$\sb2{\rm Sr}\sb2{\rm Ca}\sb{\rm n-1}{\rm Cu}\sb{\rm n}{\rm O}\sb{\rm 2n+4+\delta}$, which occupy different parts of the universal diagram, are discussed. The La$\sb{\rm 2-x}{\rm Sr}\sb{\rm x}{\rm Cuo}\sb{4+\delta}$ compound covers the whole range of the universal diagram. We find that the commonly obtained parabolic T$\sb{\rm c}---{\rm x}$ relation in the underdoped region could be attributed to a small variation of $\delta$, and the decrease of T$\sb{\rm c}$ in the "overdoped" region could be related to change in interplanar coupling upon doping. The YBa$\sb2{\rm Cu}\sb3{\rm O}\sb{7-\delta}$ compound exists only the under-doped region of the universal diagram. The two-plateau characteristics and the effects of oxygen ordering have been one of the most fascinating subjects in the field of high-T$\sb{\rm c}$ superconductivity. The key to understanding these complicated phenomena is the carrier distribution between the CuO$\sb2$ sheets and the CuO$\sb{\rm 1-\delta}$ plane. It is found that charge transfer consists of two peaks which occur at about the stoichiometric compositions for OI and OII ordered phases in the CuO$\sb{1-\delta}$ plane. The loss of superconductivity in the disordered state is concluded to be due to a small hole density on O(1) in the disordered state. The Bi$\sb2{\rm Sr}\sb2{\rm Ca}\sb{\rm n-1}{\rm Cu}\sb{\rm n}{\rm O}\sb{\rm 2n+4+\delta}$ compounds occupy mostly the over-doped region of the universal diagram. It is found that the dopability of this series of compounds is determined by the lattice mismatch between the perovskite block and the rock-salt block, and that excess oxygen or larger cation (such as Pb$\sp{2+})$ is necessary to stabilize the crystal structures of Bi-based cuprates. Critical values of excess oxygen and Pb are calculated based on this model, and good agreements are found with experimental results.

Degree

Ph.D.

Advisors

Sato, Purdue University.

Subject Area

Materials science|Condensation

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