Pressure dependence of the Drude optical edge of tetrathiafulvalenium (TTF) and tetraselenafulvalenium (TSeF) tetracyanoquinodimethanide (TCNQ)

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Abstract
We have measured the pressure dependence of the Drude absorption edge in tetrathiafulvalenium-tetracyanoquinodimethanide (TTF-TCNQ) and tetraselenafulvalenium-tetracyanoquinodimethanide (TSeF-TCNQ) over the range of 0-60 kbar. The plasma frequency and optical conductivity are extracted from the data using a model which explicitly considers the two-stack nature of these materials. The pressure dependence of the plasma frequency is found to agree fairly well with that expected from a simple tight-binding model. The optical conductivity can be decomposed by a Mathiessen's rule technique in the manner typically used for the temperature dependence of the dc conductivity. To experimental accuracy, the temperature-independent term in the optical conductivity is also pressure independent. A calculation of the conductivity in terms of the electron-electron interaction yields a pressure dependence equal to that observed.