Nuclear magnetic resonance spectra of Cu nuclei were measured in a typical spin ladder system, $\mathrm{Sr}({\mathrm{Cu}}_{1-x}{\mathrm{Zn}}_x{)}_2{\mathrm{O}}_3$ ( $x=0\mathrm{}\%$, 0.25%, and 0.5%) at temperatures ranging from room temperature to 20 K. The linewidth of the spectrum peaks drastically increased at temperatures below 100 K with x being only 0.25%. The width $\Delta W$ increased with the increase in Zn concentration and the decrease in temperatures $(\Delta W\sim x/T)$. These facts prove that a quite minute amount of impurity breaks the singlet ground state and the effect of impurity is observed in the whole system.

• Received 16 June 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.604