Interplay of spin and charge dynamics in Sr_14-xCa_xCu₂₄O₄₁

In single crystals of intrinsically hole-doped Sr_14-xCa_xCu₂₄O₄₁ 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≥8 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 Sr_14-xCa_xCu₂₄O₄₁ and a simultaneous crossover from independent dimers to antiferromagnetically coupled chains, which order at low temperatures due to weak interchain interactions.