Thermoelectric power and its correlation with conductivity in NbS3 whiskers

Physical Review B: Condensed Matter and Materials Physics. 2019. Vol. 99. Article 235155.

Zybtsev S., Pokrovskii V., Nasretdinova V., Zaitsev-Zotov S., Pryadun V. V., Kozlyakova E. S., Volkova O. S., Vasiliev A. N., Pai W. W., Staresinic D.

We report studies of the Seebeck coefficient S of the quasi-one-dimensional compound NbS3, together with the temperature dependence of its specific conductivity σs and heat capacity cp. The monoclinic phase (NbS3-II) is studied over the temperature range T = 80–400 K, which covers two charge density wave (CDW) transitions at TP1 = 360 K (CDW-1) and TP2 = 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 σs(300 K): The increase of S below TP1 in the high-Ohmic samples reveals a complete dielectrization of the electronic spectrum, while in the low-Ohmic samples S decreases below TP1 and even becomes negative below TP2. The magnitude of S in low-Ohmic samples at T < TP2 is well below kB/e ≈ 86 μV/K, kB being the Boltzmann constant and e the elementary charge, which is surprisingly low for a usual CDW semiconducting state. Our results suggest that at TP1 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 TP2. We also report S for the triclinic dielectric phase of NbS3 (NbS3-I). Its low absolute value, ∼kB/e, and the anomalous tem- perature dependence demonstrate that NbS3 -I is neither a semiconductor nor a CDW conductor in the usual sense.

Keywords: Seebeck effect, charge density wave, Peierls transition, NbS3