Tetramethyltetraselenafulvalene salts [(TMTSF)2X]: Anisotropy and effective dimensionality of the Fermi surface

With the use of an analytic approximation for the open Fermi surface of a system with highly anisotropic two-dimensional dispersion, it is shown that the plasma frequency anisotropy is linear in the bandwidth anisotropy. Comparison to the optical data on the Bechgaard salts of tetramethyltetraselenafulvalene [${\left(\mathrm{TMTSF}\right)}_{2}X$] implies a substantial transverse bandwidth of 140 meV, a full order of magnitude larger than previously supposed, and consistent with predominantly two-dimensional behavior. A critical discussion is presented of the model of, and supporting evidence for, large quasi-one-dimensional (1D) superconducting fluctuations. It is shown, using the above result, that there are several orders of magnitude discrepancy in the temperature obtained for crossover from 3D to 1D behavior. In addition the chief experimental results claimed as proof for fluctuations are susceptible to alternate interpretations.