Features of a low-temperature charge density wave in the monoclinic phase of NbS$_3$ manifested in the NMR and in transport properties

The relaxation of the transverse nuclear magnetization in the monoclinic phase of NbS$_3$ has been studied by the $^{93}$Nb nuclear magnetic resonance method near the temperature $T_{\mathrm{P2}} = 150$ K, at which a low-temperature charge density wave is formed. It has been shown that the critical slowing down of one of the vibrational modes of the lattice, which is quite slow even above $T_{\mathrm{P2}}$, occurs slightly below $T_{\mathrm{P2}}$. The transition at $T_{\mathrm{P2}}$ occurs not only in low-resistance samples, as thought previously, but also in high-resistance ones, and involves Nb atoms in the bulk of a sample. The transport properties of high-resistance samples, namely, the smearing of the depinning threshold for the charge density wave below $T_{\mathrm{P2}}$, confirm that the phase transition in them occurs at $T_{\mathrm{P2}}$. It has been concluded that the distortion of the lattice at $T_{\mathrm{P2}}$ is not due to the Peierls mechanism and can be attributed to the Keldysh–Kopaev transition. Another possible mechanism is the fluctuation distortion of the lattice above $T_{\mathrm{P2}}$ that prevents the sliding of the charge density wave.