The pair-breaking problem in high-temperature superconductors: A study of magnetic and nonisovalent impurities in yttrium barium copper oxide

Miguel A Castro, Purdue University

Abstract

By concurrently varying the magnetic moment concentration and hole density of $\rm YBa\sb2Cu\sb3O\sb{7-\delta}$ (YBCO) through simultaneously doping its Cu sites with magnetic and nonisovalent ions, we have observed a significant suppression of T$\sb{\rm c}$ when charge balance is attained, that is, when the hole density of the doped compound approaches that of pure YBCO. A constant amount of Cu (with an average valence of 2.33+) was replaced with varying ratios of (Ga$\sp{3+}$-Ni$\sp{2+}$) and (Fe$\sp{3+}$-Zn$\sp{2+}$) in YBa$\sb2$ (Cu$\sb{0.95}$(M$\sb{\rm x}$N$\sb{\rm 1-x})\sb{0.05}$) $\sb3$O$\sb{7-\delta},$ where M represents the trivalent dopant and N the divalent dopant. A strong depression of T$\sb{\rm c},$ relative to the T$\sb{\rm c}$ variation that would be expected if the dopants acted independently, was observed when x $\sim$ 0.5, the dopant concentration at which charge balance occurs as suggested by oxygen stoichiometry and Hall measurements. In contrast, a comparison study involving nonmagnetic (Ga$\sp{3+}$-Zn$\sp{2+}$) substitutions shows no such T$\sb{\rm c}$ depression, which strongly suggests that pair-breaking due to the presence of the magnetic Fe$\sp{3+}$ and Ni$\sp{2+}$ ions is responsible for the T$\sb{\rm c}$ suppression. Furthermore, a study involving isovalent (Ni$\sp{2+}$-Zn$\sp{2+})$ substitutions (where only the magnetic moment concentration is varied but the hole density remains approximately constant) reveals almost no deviations from the expected T$\sb{\rm c}$ variation which, when compared with the (Ga$\sp{3+}$-Ni$\sp{2+})$ and (Fe$\sp{3+}$-Zn$\sp{2+})$ results, suggests that the effect of magnetic pair-breaking is at a maximum when charge balance is achieved.

Degree

Ph.D.

Advisors

Durbin, Purdue University.

Subject Area

Condensation

COinS