Interplay between magnetism, charge localization, and structure in Sr14-xCaxCu24O41

Measurements of the thermal expansion of Sr14-xCaxCu24O41 (0 less than or equal to x less than or equal to 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.