5.3 Summary

We have investigated charge/vacancy-doped Haldane systems with magnetic susceptibility and $\mu$SR. The nominally pure sample does not show magnetic order down to 100 mK, supporting the singlet nature of the Haldane ground state. In the vacancy-doped systems, unpaired spins were created because of the doping, but they remained paramagnetic down to T=50 mK. In these two non-magnetic systems, muons are most likely detecting the dipolar fields from the unpaired spins.

In the charge-doped Haldane compounds, the susceptibility exhibited a spin-glass like history dependence below $T_{\rm g}$=1.5$\sim$3 K. However, $\mu$SR has revealed persistent spin dynamics down to 50 mK, which is well below the glass temperature $T_{\rm g}$. It was also found that the spin fluctuations of the charge-doped systems in the milli-Kelvin regime follow an unconventional, non-Markoffian processes.