Voltage Induced Torsional Strain in Tantalum Trisulfide in its CDW State

In 2007, Pokrovskii et al [Phys. Rev. Lett. 98, 206404 (2007)] reported that crystals of tantalum trisulfide spontaneously twist when a voltage near the CDW depinning threshold is applied. The direction of twist reverses when the voltage is reversed, so that the twist angle describes a characteristic hysteresis loop as a function of applied voltage or current. We have studied this effect by placing the sample inside a resonant RF cavity so that the twisting sample modulates the resonant frequency of the cavity. A magnetic wire is attached to the sample so that magnetic torque can also be applied to the sample. We have reproduced the ``Pokrovskii effect'' and also observed the complement: application of torque induces an additional voltage when the sample is biased above threshold. We have also found that the onset of the torsional strain occurs at a voltage slightly below that at which the shear modulus of the sample softens.