Thermoelectric power and its correlation with conductivity in NbS3 whiskers

We report studies of the Seebeck coefficient S of the quasi-one-dimensional compound NbS3, together with the temperature dependence of its specific conductivity ss and heat capacity c(p). The monoclinic phase (NbS3-II) is studied over the temperature range T = 80-400 K, which covers two charge density wave (CDW) transitions at T-P1 = 360 K (CDW-1) and T-P2 = 150 K (CDW-2). The S(T) curves show features in the vicinities of both CDW transitions and appear to be correlated with the value of sigma(s)(300 K): The increase of S below T-P1 in the high-Ohmic samples reveals a complete dielectrization of the electronic spectrum, while in the low-Ohmic samples S decreases below T-P1 and even becomes negative below T-P2. The magnitude of S in low-Ohmic samples at T < T-P2 is well below k(B)/e approximate to 86 mu V/K, k(B) being the Boltzmann constant and e the elementary charge, which is surprisingly low for a usual CDW semiconducting state. Our results suggest that at T-P1 the main electronic band with p-type carriers becomes gapped, while some n-type carriers can remain in a separate band with low density of states. These carriers, whose concentration is defined by the compositional doping, are gapped at T-P2. We also report S for the triclinic dielectric phase of NbS3 (NbS3-I). Its low absolute value, similar to k(B)/e, and the anomalous temperature dependence demonstrate that NbS3-I is neither a semiconductor nor a CDW conductor in the usual sense.