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

Abstract We present 63 Cu NMR studies of the CuO 2 chains in Sr 13 LaCu 24 O 41 and Sr 14 Cu 24 O 41 . In both compounds the temperature dependence of the NMR frequency shift is described by an antiferromagnetic dimer model for the local susceptibility. For Sr 14 Cu 24 O 41 the resulting spin excitation gap (133 K) 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, Sr 13 LaCu 24 O 41 , 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 108 K.