Fermi-surface nesting, closed orbits, and the "threshold field" in an organic superconductor

Abstract

The Fermi surface of the organic superconductor ${\mathrm{}\left(\mathrm{TMTSF}\right)\mathrm{}}_2$P${\mathrm{F}}_6$ (bis-tetramethyltetraselena-fulvalenium hexafluorophosphate) should be open in a one-electron picture. Magnetotransport results, on the other hand, indicate the presence in the metallic state of small closed and compensated orbits. Such orbits can arise if there is an instability of commensurate wave vectors nesting the Fermi surface. The areas calculated here for these orbits are shown to strongly favor nonoptimal nesting over optimal nesting (i.e., the optimal areas are far too small). A model of instability commensuration change at the observed threshold associated with the Shubnikov-de Haas effect, driven by the magnetic field dependence of the Landau-level energies for the optimal orbits, is seen to be consistent with the present results and with many of the novel properties of this material.