Charge-Density Transport in Transition-Metal Trichalcogenides.
The electrical transport properties along the chain axis of three transition-metal trichalcogenides, NbSe(,3), orthorhombic TaS(,3), and NbS(,3) have been measured at ambient pressure and at temperatures between 350 K and 4.5 K. These linear-chain compounds undergo Peierls transitions to charge-density-wave (CDW) states at T(,P) = 144 K and 59 K (NbSe(,3)), T(,P) = 215 K (TaS(,3)), and T(,P) = 155 K (NbS(,3)). The dc electric field dependence of the dc conductivity (sigma)(,dc)(E(,dc)) indicates a well-defined threshold field E(,T) for the onset of non-linear dc conduction, for all three materials. The non-linearity is interpreted in terms of Frohlich conduction. The complex low-field ac conductivity (sigma)(,ac)((omega)) is also found to be strongly frequency dependent below T(,P), again for all three materials. For NbSe(,3) and TaS(,3) the frequency -dependent response resembles that of an overdamped oscillator, with cross-over frequencies in the 100 MHz region. Both (sigma)(,dc)(E(,dc)) and (sigma)(,ac)((omega)) are analyzed in terms of a classical model of CDW transport by Gruner, Zawadowski, and Chaikin, and a quantum-mechanical tunneling model by Bardeen. For both NbSe(,3) and TaS(,3), it is found that (sigma)(,dc)(E(,dc)/E(,T)) = (sigma)(,ac)((omega)/(omega)(,T)), where (omega)(,T) is an empirically determined pinning frequency, of the order of 100 MHz. This scaling relation is predicted by the CDW tunneling model. For applied fields E(,dc) > E(,T), it is found that the current response contains discrete ac frequency components in TaS(,3), as previously observed in NbSe(,3). These coherent current oscillations, and the associated narrow -band "noise", have been studied in NbSe(,3) and TaS(,3) as functions of applied electric field and temperature, and compared to the predictions of the classical model and a model of current oscillations proposed by Bardeen. For NbSe(,3) and TaS(,3) a variety of experiments are described for a joint application of dc and ac driving fields. It is observed that an ac field strongly influences the dc response, and vice versa. For NbSe(,3), strong interference effects are observed where the intrinsic narrow -band noise frequency equals the externally applied ac frequency (omega)/2(pi). These results are again interpreted in terms of the classical and quantum models. A variety of ac-dc coupling phenomena observed in NbSe(,3) and TaS(,3) are shown to be analogous to effects observed in Josephson junctions, and are discussed in light of this analogy.
- Pub Date:
- February 1983
- Physics: Condensed Matter