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We report a comparative study of 63Cu Nuclear Magnetic Resonance spin latticerelaxation rates, T_1^{-1}, on undoped SrCuO_2 and Ca dopedSr_{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 inthe Ca-doped sample evidencing the opening of a spin gap. The data analysiswithin the J_1-J_2 Heisenberg model employing density-matrix renormalizationgroup calculations suggests an impurity driven small alternation of theJ_2-exchange coupling as a possible cause of the spin gap.

Disorder-induced Spin Gap in the Zigzag Spin-1/2 Chain Cuprate Sr_{0.9}Ca_{0.1}CuO_2