Microwave conductivity measurements by the coaxial resonator technique. I. Description of the method and applications to TTF-TCNQ and related compounds

The coaxial-resonator technique, which has been used in the past to measure the surface impedance of metal wires, is also well suited to measuring the microwave conductivities of crystals of highly conducting organic salts such as tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ). In this technique the small needlelike crystal forms the center conductor of an open coaxial cavity which resonates whenever the crystal length is an integral multiple of microwave half-wavelengths. For reasonable choices of experimental parameters it is simple to show that losses in the crystal dominate the $Q$ of the cavity, and thus this method maintains good sensitivity at high conductivities where standard cavity perturbation methods are ineffective. In this paper we describe measurements made with three apparatuses based on the coaxial-resonator technique, spanning a frequency range from 10 to 40 GHz, on crystals of TTF-TCNQ, tetraselenafulvalene-TCNQ (TSeF-TCNQ), ${\mathrm{}\left(\mathrm{TTF}\right)\mathrm{}}_{0.5}$ ${\mathrm{}\left(\mathrm{TSeF}\right)\mathrm{}}_{0.5}$-TCNQ, and diselenadithiafulvalene-TCNQ. We also present an exact solution of the infinite coaxial line with lossy, anisotropic (but cylindrically symmetric) center conductor and comment on the problem of end effects in the resonator of finite length.