Thermoelectric power ofTiSe2−xSxmixed crystals at low temperatures

We present thermoelectric power (TEP) measurements, as a function of temperature in the range 7-450 K, of $\mathrm{Ti}{\mathrm{Se}}_{2-x}{\mathrm{S}}_x$ mixed crystals for $x=0, 0.5, 0.75, 1, \mathrm{and} 1.25$. A pronounced negative minimum is observed near 150 K in the samples which exhibit the superlattice state. In contrast, in the nontransforming crystals, the TEP varies monotonically with temperature. We interpret the 150-K TEP dip in the transforming samples as due to a phonon-drag effect, in which the strong interaction between electrons and holes and the softening TA-phonon mode $L_1^{-}$—the mode actually defining the $2a_0\times 2c_0$ superlattice condition—plays a primary role.