Interplay between magnetism, charge localization, and structure inSr14−xCaxCu24O41

Abstract

Measurements of the thermal expansion of ${\mathrm{Sr}}_{14\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ $(0l~xl~12)$ single crystals were performed. At low temperature and small $x$ we observe anomalous magnetic contributions which are ascribed to the formation of the spin gap state in the chains. From our data we derive a positive pressure derivative of the spin gap and we show that the dimers are formed by next nearest Cu neighbors. Additional anomalous contributions to the thermal expansion are attributed to a low-dimensional charge order occurring at higher temperature. The relevance of the charge order for the spin gap formation is discussed.