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

In single crystals of intrinsically hole-doped Sr14-xCaxCu24O41 a strong el ectron spin resonance (ESR) signal has been observed and studied as a funct ion of temperature and Ca concentration (x=0-12). Since the. spin ladders s how a large spin gap of about 400-500 K, the dominant contribution to the E SR signal below 300 K is attributed to the CuO2, spin-1/2 chains. For all v alues of x the data reveal a remarkable influence of the hole dynamics on t he Cu-spin relaxation. This enables us to identify the onset of charge orde r in the chains for x=0 and 2 at 200 and 170 K, respectively. A further inc rease of x rapidly destroys the charge ordered state. A transition to an an tiferromagnetically ordered state is observed at 2.5 K for x=12. However, t he 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 La1Sr1 3Cu24O41 and La2Ca12Cu24O41, in which the hole concentration is reduced. Th e 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 S r14-xCaxCu24O41 and a simultaneous crossover from independent dimers to ant iferromagnetically coupled chains, which order at low temperatures due to w eak interchain interactions.