Deformations of charge-density wave crystals under electric field

We report the effects of electric field induced deformations of quasi one-dimensional conductors with charge-density wave (CDW). The most pronounced sort of deformation is torsional strain (TS). The TS is found to comprise two contributions. The features of the 1st—the larger one—are threshold hysteretic dependence on electric field and high relaxation time τ : For o-TaS 3 τ ∼10 −2 s at T =80 K and falls as exp(900 K/ T ) with increasing T . The 2nd contribution is linear in electric field and does not drop with frequency increase. The amplitude of this contribution falls abruptly with T approaching the Peierls transition temperature T P from below. Similar features of TS are demonstrated for other CDW compounds: (TaSe 4 ) 2 I, K 0.3 MoO 3 and NbS 3 type II, for which T P ≈360 K. We attribute the 1st and the 2nd contributions to large (hysteretic) and small (near-equilibrium) CDW deformations, respectively, likely—shear at the surface. The TS is observed also above T P : For TaS 3 and (TaSe 4 ) 2 I typical torsional amplitude is 10 −1 °/V in the resonance regimes, corresponding to the piezomodulus ∼10 −9 m/V. A separate study of TS was performed at room temperature with AFM technique. Apart from this (‘intrinsic’) effect, we observe electrostatic contribution to the TS. In contrast to the intrinsic response, the electrostatic one is proportional to the potential either over the sample, or over an additional electrode (‘gate’) placed nearby, but not to the difference of potentials between the sample ends. It is typically 2 orders of magnitude less. The intrinsic TS reveals a new electromechanical effect at room temperature, presumably associated with the excitations of the pinned mode of the CDW fluctuations. Its observation opens prospects for application of quasi one-dimensional conductors as micro- and nano-actuators. Basing on the electrostatic-induced TS we propose prototypes of electrometers and a technique for measurements of work function changes. Using a cryogenic transmission electronic microscope we observe also flexural deformation of TaS 3 in the double-clamped mode, visible both in resonant and static modes.