μSR study on the ground state of bond-disordered spin gap system (CH₃)₂CHNH₃Cu(Cl_1-xBr _x)₃ (x=0.95, 0.88)

Solid solution of the two spin gap systems (CH₃) ₂CHNH₃CuX₃ (X=Cl, Br) was reported from results of macroscopic measurements to be gapless when fraction of Cl content x is in between 0.44 and 0.87, and otherwise remains gapped. We have investigated by μSR the ground state of Cl-rich gapped region x=0.95 and 0.88 to find that they are microscopically phase separated into two components, magnetic tiny islands and singlet sea surrounding them. With decreasing temperature, the characteristic frequency of spin fluctuation in islands showed a significant slow down. However, the slow down ceased around 1 K, below which the spin fluctuation spectrum did not change down to 15 mK. These results indicate that unlike the Br-rich system x <0.44 where appearance of an exotic ground state has been argued, the Cl-rich system x >0.87 persists to be paramagnetic without showing a long-range order. This difference is considered to be originated in the robustness of Haldane gap against disorder.