Pinning, Memory and Motion of Charge-Density Waves in NbSe_3 and o-TaS_3(EXPERIMENTS ON MX_3 COMPOUNDS, International Symposium on NONLINEAR TRANSPORT AND RELATED PHENOMENA IN INORGANIC QUASI ONE DIMENSIONAL CONDUCTORS)

Abstract

Experimental studies of electrical memory phenomena in the charge-density wave (CDW) conductors NbS_3 and orthorhombic (o-) TaS_3 are described, in an account devoted partly to review and partly to the presentation of new results. Four classes of memory phenomenon are listed: I, hysteresis effects in the weak-field resistance; II, the 'pulse-memory' or 'overshoot' phenomenon, where a transient increase in conductivity occurs when current is first applied in a direction opposite to that in which it last flowed; III, phase-memory in the periodic response to steady fields; and IV, a miscellaneous group of transient phenomena, arising from current-induced changes in the threshold field for nonlinear conduction. A study of electrical hysteresis in o-TaS_3 leads to the tentative conclusion that a change in threshold behaviour near 140K is the result of an increase in the resistance of the CDW to deformation, rather than to the onset of commensurability, which it is suggested only occurs at or below 55K. The occurrence of the pulse-memory phenomenon III has made it possible to show that, at least near the threshold for CDW motion, the periodic response arises from a modulation of the single-electron conductivity, presumably local in nature, and not from modulation of the Frohlich current. Experiments on short sections of NbSe_3, in which the CDW is expected to behave as a single domain, provide evidence that the transients in the pulsememory phenomenon II then correspond to Frohlich currents generated as the CDW makes transitions between metastable distorted states. A current associated with a partial relaxation of the distortion, observed after removal of the applied field, provides what is believed to be a direct demonstration of Frohlich conduction. In larger crystals the metastable distortion is concentrated near the current terminals, apparently because of the presence of intermediate domains at whose boundaries the CDW loses coherence. Transients of type IV, clearly not due to Frohlich currents generated as the distortion changes, are then also observed.