Interplay of spin and charge dynamics in Sr14-xCaCu24O41 -: art. no. 104422

In single crystals of intrinsically hole-doped Sr14-xCaxCu24O41 a strong electron spin resonance (ESR) signal has been observed and studied as a function of temperature and Ca concentration (x=0-12). Since the. spin ladders show a large spin gap of about 400-500 K, the dominant contribution to the ESR signal below 300 K is attributed to the CuO2, spin-1/2 chains. For all values of x the data reveal a remarkable influence of the hole dynamics on the Cu-spin relaxation. This enables us to identify the onset of charge order in the chains for x=0 and 2 at 200 and 170 K, respectively. A further increase of x rapidly destroys the charge ordered state. A transition to an antiferromagnetically ordered state is observed at 2.5 K for x=12. However, the ESR signal shows critical broadening already for x greater than or equal to8 at low temperatures, which indicates the development of magnetic order for values of x smaller than 12. For comparison we show ESR data of La1Sr13Cu24O41 and La2Ca12Cu24O41, in which the hole concentration is reduced. The whole set of ESR data can be understood in terms of a transfer of only a small amount of holes from the chains to the ladders with increasing x in Sr14-xCaxCu24O41 and a simultaneous crossover from independent dimers to antiferromagnetically coupled chains, which order at low temperatures due to weak interchain interactions.