Low-temperature states in quasi-one-dimensional charge transfer salt (TMTTF)$_{2}$SbF$_{6}$ studied by $^{13}$C NMR under hydrostatic high pressures

We report $^{13}$C NMR study of a quasi-one-dimensional charge transfer salt, (TMTTF)$_{2}$SbF$_{6}$, under the application of the hydrostatic pressure. The antiferromagnetic phase transition at ambient pressure was confirmed by the line splitting and the divergent increase of the spin-lattice relaxation rate 1/$T_{1}$. Under $\sim $5 kbar, 1/$T_{1}$ deceases abruptly at low temperatures without the enhancement, indicating that the ground state is a spin-gapped phase. However, the decrease of the Knight shift expected for spin-Peierls phase transition has not been observed. We observed additional lines, which split symmetrically, at low temperatures under the pressure of $\sim $17 kbar. The possible reentrant antiferromagnetic phases are discussed.