On the origin of quasi-periodic current noise in charge-density wave conductors: Experiments on orthorhombic tantalum trisulphide

The quasi-periodic components, which appear in the voltage V developed across a charge-density wave (CDW) conductor when the steady current I exceeds the threshold current I_T at which motion of the CDW begins, have been observed in orthorhombic TaS₃ at 77 K at frequencies v down to less than 1 Hz. Although its relation to the nonlinear part of the current when I?I_T indicates that v is, or is a low multiple of, the ‘washboard’ frequency v/λ, where v is the velocity and λ the wavelength of the CDW, the amplitude of the periodic variation of V when v<1 kHz is much too great to represent a modulation of the Frohlich current. The alternative is that the variation of V arises from a periodic modulation of the single-electron conductivity, resulting from distortion of the CDW as it moves through the crystal. Such a modulation of conductivity has been demonstrated experimentally, by interrupting a current I>I_T at different stages of the cyclic variation of V, and then using a current I<I_T to observe the resistance of the specimen when the CDW is at rest. Mechanisms whereby a periodic dependence of resistance on the position of the CDW may arise are briefly discussed.