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Acoustic wave propagation in TTF-TCNQ Tiedje, J. Thomas

Abstract

Detailed measurements have been made of the temperature dependence of the velocity of three different modes of sound propagation in TTF-TCNQ crystals, in the range 7~300K. Values for the a and b axis Young's moduli and the shear modulus c₆₆ are inferred from the sound velocities. TTF-TCNQ. is found to be stiffer perpendicular to the conducting direction than parallel to it. The elastic anisotropy is typical of crystalline solids even though the anisotropy of the electrical conductivity is unusually large'. A small (1.5%) increase in the velocity of extensicnal waves below the meta1 -insulator transition is interpreted as being due to the disappearance of the conduction electrons. A quantitative theory of the low temperature velocity anomaly leads to an accurate estimate of the q → 0 electron-phonon coupling constant. The sound velocity measurements were made using an acoustic resonance technique. Resonant modes of vibration of single crystals of TTF-TCNQ were excited electrostatically and detected capacitively using a UHF carrier signal. The detection scheme is shown to be more sensitive than conventional d.c. biased capacitive pickups. A theoretical study of the electronic contribution to the attenuation of sound in one and two dimensional metals and semiconductors is presented. The attenuation in one dimensional metals is shown to be anomalously small. In both one and two dimensional metals, in the quantum limit the attenuation depends strongly on the direction of propagation of the wave. A transport equation solution to the problem of calculating the amplification of sound waves in a solid in the presence of a d.c. electric field is described. The treatment is much less complex than any that is currently available.

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