Charge density wave depinning in TaS₃

Abstract

Abstract Two polytypes of TaS3 have been synthesized, one with a monoclinic unit cell, the other with an orthorhombic unit cell. Monoclinic TaS3 has a structure similar to that of NbSe3 and undergoes two independent Peierls transitions at T 1 = 240 K and T 2 = 160 K. A unique Peierls transition occurs in orthorhomic TaS3 at T 0 = 215 K, with a slightly incommensurate periodic lattice distortion which locks to a commensurability of four lattice parameters along the chain direction at T'0 ∼ 130 K. Both TaS3 varieties show nonlinear transport properties associated with the motion of the charge density wave produced by applying an electric field with an amplitude higher than a threshold value. Measurements are reported of the threshold field and of the a.c. frequency measured by Fourier anaysis of the noise as a function of the temperature. Monoclinic TaS3 behaves similarly to NbSe3, with a higher threshold field for the upper temperature Peierls transition than for the lower one. For orthorhombic TaS3 it is shown that the threshold field decreases when the wavelength of the charge density wave becomes commensurate down to a value of 0·2 V cm−1. For both forms of TaS3, from the measurement of the current carried by the charge density wave in the nonlinear state and the fundamental frequency in the Fourier noise spectrum, it is deduced that the pinning periodicity is half the wavelength of the charge density wave. It is also shown that at low temperature the relative amplitude of the harmonics of the a.c. voltage decreases exponentially. The measurements indicate that in the commensurate state in orthorhombic TaS3 the number of charges carried by the charge density wave is twice the number measured in the incommensurate temperature range. A model is presented in which the commensurate transition arises from the locking between two strongly coupled lattice distortions on two types of chain.