Change in the sign of the magnetoresistance and the two-dimensional conductivity of the layered quasi-one-dimensional semiconductor TiS$_3$

The dependences of the resistance of the layered quasi-one-dimensional semiconductor TiS$_3$ on the direction and magnitude of the magnetic field $\mathbf{B}$ have been measured. The anisotropy and angular dependences of the magnetoresistance indicate the two-dimensional character of the conductivity at $T < 100$ K. Below $T_0 \approx 50$ K, the magnetoresistance for the directions of the field in the plane of the layers (ab plane) increases sharply, whereas the transverse magnetoresistance ($\mathbf{B}\parallel c$) becomes negative. The results confirm the possibility of an electron phase transition to a collective state at $T_0$. The negative magnetoresistance (at $\mathbf{B}\parallel c$) below $T_0$ is explained by the magnetic-field-induced suppression of two-dimensional weak localization. The positive magnetoresistance (at $\mathbf{B}\parallel ab$) is explained by the effect of the magnetic field on the spectrum of electronic states.