Quasi One-Dimensional Conductors: The Far Infrared Problem

Abstract

In the metallic state the quasi one-dimensional conductors are characterized by very high values of dc conductivity equaling in some cases pure copper at room temperature. Thus one might expect their optical properties to follow the Drude free electron theory where the conductivity remains equal to the dc value up to a frequency of the order of the relaxation time. Instead the quasi one-dimensional conductors show a region of depressed optical conductivity in the far infrared. The dc conductivity¹ at low temperature of a material such as (TMTSF)₂ClO₄ is of the order of 5·10⁵ (Ω·cm)^-1 whereas the optical conductivity² at 50 cm^-1 is only a few hundred (Ω·cm)^-1. The change from high to low conductivity occurs in the experi mentally difficult microwave region and very few experiments have been done at these limiting frequencies. The phenomenon of suppressed far infrared conductivity is almost universal in the quasi one-dimensional materials^2–7. It was first observed in TTF-TCNQ but subsequently also reported in (TMTSF)₂X family of compounds in their conducting state. This discrepancy has been generally explained in terms of a narrow collective mode with a very large effective oscillator mass centered at zero frequency. Such a model implies that the electrical current at low temperature is carried by the collective mode.