Disorder-induced Spin Gap in the Zigzag Spin-1/2 Chain Cuprate   Sr_{0.9}Ca_{0.1}CuO_2
  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. 
F. Hammerath, S. Nishimoto, H.-J. Grafe, A.U.B. Wolter, V. Kataev, P.   Ribeiro, C. Hess, S.-L. Drechsler, and B. Buechner
cond-mat.str-el