Spin fluctuations in the spin-Peierls compound ${\mathrm{MEM}\left(\mathrm{TCNQ}\right)\mathrm{}}_2$ studied using muon spin relaxation

We report a muon spin relaxation $\left(\mu \mathrm{SR}\right)$ investigation of the organic spin-Peierls compound $\mathrm{MEM}(\mathrm{TCNQ}{)}_2$ at temperatures down to 39 mK. We have observed a slowing down of the electronic spins as the spin-Peierls gap widens at temperatures below the spin-Peierls transition and use this behavior to estimate the size of the gap. At the very lowest temperatures the electronic spin fluctuations freeze out and the muon spin depolarization is dominated by a persistent static mechanism which we ascribe to a defect-spin system. We relate the low-temperature depolarization rate to the concentration of these defects, and we propose a model for the creation of spin defects by the muon itself.