Spin gap in the single spin- 12 chain cuprate Sr1.9 Ca0.1 CuO3

We report 63Cu nuclear magnetic resonance and muon spin rotation measurements on the S = 1/2
antiferromagnetic Heisenberg spin chain compound Sr1.9Ca0.1CuO3. An exponentially decreasing spin-lattice
relaxation rate T −1
1 indicates the opening of a spin gap. This behavior is very similar to what has been observed
for the cognate zigzag spin chain compound Sr0.9Ca0.1CuO2, and it confirms that the occurrence of a spin gap
upon Ca doping is independent of the interchain exchange coupling J . Our results therefore suggest that the
appearance of a spin gap in an antiferromagnetic Heisenberg spin chain is induced by a local bond disorder of
the intrachain exchange coupling J . A low-temperature upturn of T −1
1 evidences growing magnetic correlations.
However, zero-field muon spin rotation measurements down to 1.5 K confirm the absence of magnetic order in
this compound, which is most likely suppressed by the opening of the spin gap.