Frequency-dependent conductivity in K0.30MoO3

This chapter is an experimental review of ac conductivity of the 'blue bronze' K0.3MoO3, which is taken to be a prototype material for a discussion of charge-density wave (CDW) motion. The bulk of the review concentrates on low-frequency measurements. Three frequency regimes are discussed. The first regime is termed the dielectric relaxation regime (dc-10 MHz) where the dynamics can be described as the decay of an induced polarization of the CDW resulting from local deformations of the CDW. In this frequency regime on can neglect the effects of inertia and take the CDW mass to be zero. The dominant energy loss mechanism results because of screening of the CDW deformations by the normal electrons. The second frequency regime (10 - 100 GHz) is dominated by the pinned phase mode of the CDW. In this regime one must include inertia in describing the CDW dynamics. The effective mass of the CDW causes an inertial roll-off in the ac response with a frequency dependence which is approximately described by a response of a rigid CDW. At frequencies near the pinning energy, electrostatic screening is less important as a damping mechanism and deformations of the CDW do not occur. Damping mechanisms in this frequency realm are temperature dependent and are less well-understood. The third frequency regime is the far-infrared frequency region between the pinned phase mode and the single electron energy gap. This regime has been studied less intensely than the two low frequency regimes, however large resonances in infrared measurements of (TaSe4)2I and K0.30MoO3 suggesting further modes may be present in CDW materials. (author). 87 refs.; 16 figs.; 2 tabs