Dynamics of the electroreflective response ofTaS3

Abstract

We have observed a large s,1%d change in infrared reflectance of the charge-density-wave (CDW) conductor, orthorhombic TaS3, when its CDW is depinned. The change is concentrated near one current contact. Assuming that the change in reflectance is proportional to the degree of CDW polarization, we have studied the dynamics of CDW repolarization through position-dependent measurements of the variation of the electroreflectance with the frequency of square-wave voltages applied to the sample, and have found that the response could be characterized as a damped harmonic oscillator with a distribution of relaxation (i.e., damping) times. The average relaxation time, which increases away from the contacts, varies with applied voltage as t0a 1/ V p with p, 3 / 2, but the distribution of times broadens as the voltage approaches the depinning threshold. Very low resonant frequencies s, 1k Hzd indicate a surprisingly large amount of inertia, which is observable in the time dependence of the change in reflectance as a polarity-dependent delay of ,100 ms. Interest in quasi-one-dimensional conductors with sliding charge-density waves (CDW’s) 1,2 has continued for almost three decades because of the variety of unusual properties they exhibit. In the CDW ground state, a periodic lattice distortion is accompanied by a modulated electron density: n= n0 + n1 cosfQz+ wsz , tdg, where z is the direction of the conducting chains and Q is the CDW wave vector, n1 its amplitude, and w its local phase. Because CDW pinning results from the competition between deformations of the CDW (i.e., variations of n1 and w) and its interactions with impurities, CDW’s are also model systems for studying the effects of quenched disorder on a deformable periodic medium. 3