Origin of the magnetic susceptibility maximum in CaCu3Ru4O12 and electronic states in the A-site substituted compounds

Abstract

${\mathrm{CaCu}}_{3}{\mathrm{Ru}}_{4}{\mathrm{O}}_{12}$ shows a broad maximum at around 200 K in temperature dependence of magnetic susceptibility, whose origin is under debate. The present study addresses this problem, using high-quality samples of ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{A}}_{x}{\mathrm{Cu}}_{3}{\mathrm{Ru}}_{4}{\mathrm{O}}_{12}$ (A = La, Na, and Sr) made by high-pressure synthesis technique. Unlike in a previous report, the maximum shifts to lower temperatures for the La substitution, becomes obscure by the Na substitution, and is less influenced by the Sr substitution. This behavior strongly suggests that the susceptibility maximum is caused by a sharp peak in the density of states just above the Fermi level, which induces strong spin fluctuations. Furthermore, the nature of electronic states of ${\mathrm{LaCu}}_{3}{\mathrm{Ru}}_{4}{\mathrm{O}}_{12}$ and ${\mathrm{NaCu}}_{3}{\mathrm{Ru}}_{4}{\mathrm{O}}_{12}$ are discussed; the former likely bears a Kondo character, and the latter has spin fluctuations of different origin below approximately 150 K.