NMR study on the competing spin chain Rb₂Cu₂Mo₃O₁₂

Quasi-one-dimensional S = 1/2 Heisenberg system Rb₂Cu₂Mo₃O₁₂ described by the J₁ - J₂ model with the nearest-neighbor ferromagnetic and next-nearest-neighbor antiferromagnetic exchange interactions have been investigated by ^87/85Rb-NMR under extended field range up to 18 T. From the thermal-activation-type temperature dependence of l/T₁, opening of the spin excitation gap was confirmed under the low field below 2 T. With increasing the field, the system becomes gapless, and the gap opens again in the high field saturation region, where the field dependence of the gap size showed the coherent excitation of the neighbouring two spins.