Altering TCNQ-TCNQ interactions and a study of the spin-Peierls state including muon-fluorine entanglement

Potassium TCNQ is a well known spin-Peierls system where a spin gap opens at ~390 K due to the strong electron-phonon coupling within the material, essentially forming 1D stack of antiferromagnetially coupled anion TCNQ dimers. We show that on substitution of TCNQF$_4$ for the TCNQ this changes the interaction of the anions and shifts $T_{\rm sp}$ to ~150 K. Additionally, substitution of TCNQ by TCNQBr$_2$ produces a sample with an even lower interaction strength. These observations demonstrate the ability to tune this system from a chemical point of view. Muon spin relaxation measurements show that in the vicinity of $T_{\rm sp}$ the system is dominated by magnetic fluctuations and these persist to lower temperatures until the moments exhibit a quasi-static state on the time scale of the $\mu$SR experiment (MHz range). Within the KTCNQF4 sample we also observe the emergence of a small contribution from an entangled F-$\mu^{+}$-F where the data could be modeled using two environments that can help suggest possible muon stopping sites.