Charge order and dimer formation in the Cu (S=1/2) spin chains of Sr13LaCu24O41

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We present 63Cu NMR studies of the CuO2 chains in Sr13LaCu24O41 and Sr14Cu24O41. In both compounds the temperature dependence of the NMR frequency shift is described by an antiferromagnetic dimer model for the local susceptibility. For Sr14Cu24O41 the resulting spin excitation gap (133K) is consistent with macroscopic susceptibility and inelastic neutron scattering measurements and supports the suggested static charge order model for a hole-doping level of 0.6/Cu chain site. Surprisingly, Sr13LaCu24O41, a system with a reduced doping of ∼0.5 holes per Cu atom, for which macroscopic magnetic susceptibility measurements suggest a gapless S=1/2 antiferromagnetic chain, shows a similar behavior with a slightly reduced spin excitation gap of 108K.