Far-infrared Studies of bis-tetramethyltetraselenafulvalene Salts [(TMTSF)₂X, X=(ClO₄, SbF₆ & ASF₆)]

Abstract

<p>Detailed far-infrared reflectivity measurements were done for three organic compounds, bis-tetramethyltetraselenafulvalene (TMTSF) perchlorate (ClO₄), hexafluoroantimonate (SbF₆) and hexafluoroarsenate (ASF₆). One of these compounds (TMTSF)₂ClO₄, was also studied by measuring the transmission through a grid of crystals. The grid technique was used to enhance the absorption of a material brought on by the application of a magnetic field. The (TMTSF)₂ClO₄ compound shows an increased absorption below ≃ 3.8 meV with a field of 0.2 Tesla.</p> <p>The reflectance data were Kramers-Kronig transformed to get the real part of the frequency dependent conductivity σ₁(ω), and the real part of the dielectric constant ε₁(ω). In the metallic state, σ₁(ω) can be described by two relaxation times. The first is an extremely long lifetime Tc (> 10ˉ¹¹ s) needed to account for the high dc conductivity (10⁴-10⁵ (Ω.cm)ˉ¹). We shall call this the 'zero frequency mode'. The high frequency 'tail' of the zero frequency mode can be seen in (TMTSF)₂SbF₆ and ASF₆. An effective mass M* of 250-650 m* was obtained for the zero frequency mode. The second is the frequency dependent scattering time tp due to phonons (Holstein process). In the Holstein process, the conductivity profile shows an initial increase in σ₁(ω) as a function of frequency before levelling off at the limiting value of Tp.</p> <p>None of the three compounds in the semiconducting (spin density waves (SDW)) state show the well developed SDW gap that is predicted theoretically. For (TMTSF)₂ClO₄, a gap of 20 cmˉ¹ was observed in the 'quenched state' while for (TMTSF)₂SbF₆ it is ≃ 180 cmˉ¹. In the latter compound, 'phase phonons' accompanied the SDW transition. This is the first observation of phase phonons in the SDW state. The third compound studied, (TMTSF)₂AsF₆, did not show any identifiable SDW gap.</p>