Energy dependence of commensurate neutron scattering peak in doped two-leg ladder antiferromagnet Sr_(14-x)Ca_(x)Cu₂₄O₄₁

The dynamical spin response of doped two-leg ladder antiferromagnets is investigated based on the fermion-spin approach. Our calculations clearly demonstrate a crossover from the incommensurate antiferromagnetism in the weak interchain coupling regime to commensurate spin fluctuation in the strong interchain coupling regime. In particular, the nuclear spin-lattice relaxation rate extracted from the commensurate spin fluctuation decreases exponentially with decreasing temperatures. The behaviors of the spin dynamics in the strong coupling regime are quantitatively close to the experimental results of Sr_{14-x}Ca_{x}Cu_{24}O_{41}.