We report a comparative study of 63Cu Nuclear Magnetic Resonance spin lattice relaxation rates, T_1^{-1}, on undoped SrCuO_2 and Ca doped Sr_{0.9}Ca_{0.1}CuO_2 spin chain compounds. A temperature independent T_1^{-1} is observed for SrCuO_2 as expected for an S=1/2 Heisenberg chain. Surprisingly, we observe an exponential decrease of T_1^{-1} for T < 90,K in the Ca-doped sample evidencing the opening of a spin gap. The data analysis within the J_1-J_2 Heisenberg model employing density-matrix renormalization group calculations suggests an impurity driven small alternation of the J_2-exchange coupling as a possible cause of the spin gap.
@article{arxiv.1103.1793,
title = {Disorder-induced Spin Gap in the Zigzag Spin-1/2 Chain Cuprate Sr_{0.9}Ca_{0.1}CuO_2},
author = {F. Hammerath and S. Nishimoto and H. -J. Grafe and A. U. B. Wolter and V. Kataev and P. Ribeiro and C. Hess and S. -L. Drechsler and B. Buechner},
journal= {arXiv preprint arXiv:1103.1793},
year = {2011}
}