Critical imperfect nesting in (TMTSF)2PF6.

Abstract

We have investigated the temperature, pressure, and field dependence of the spin density wave (SDW) transition in the organic superconductor (TMTSF${)}_{2}$${\mathrm{PF}}_{6}$ and derived parameters for this transition. The SDW can be characterized by the degree to which the two Fermi surfaces couple or ``nest'' as measured by the imperfect nesting bandwidth ${\mathit{t}}_{\mathit{b}}^{\ensuremath{'}}$. Perfect nesting (maximum ${\mathit{T}}_{\mathrm{SDW}}$) occurs for ${\mathit{t}}_{\mathit{b}}^{\ensuremath{'}}$=0, and increasing pressure increases ${\mathit{t}}_{\mathit{b}}^{\ensuremath{'}}$ and suppresses ${\mathit{T}}_{\mathrm{SDW}}$ to 0 K at a critical value ${\mathit{t}}_{\mathit{b}}^{\ensuremath{'}\mathrm{*}}$. The increase of ${\mathit{T}}_{\mathrm{SDW}}$ with field has been shown to be orbitally driven, and the field dependence is nearly quadratic for fields in the c direction. We have made quantitative observations of the field dependence of ${\mathit{T}}_{\mathrm{SDW}}$ at ambient pressure to 29 T and under hydrostatic pressure to 8 T. From our measurements at ambient pressure we are able to extract ${\mathit{t}}_{\mathit{b}}^{\ensuremath{'}}$ and ${\mathit{t}}_{\mathit{b}}^{\ensuremath{'}\mathrm{*}}$ using mean field theory and observe ${\mathit{T}}_{\mathrm{SDW}}$=12.1\ifmmode\pm\else\textpm\fi{}0.1 K, ${\mathit{t}}_{\mathit{b}}^{\ensuremath{'}}$=4.5\ifmmode\pm\else\textpm\fi{}0.3 K, and ${\mathit{t}}_{\mathit{b}}^{\ensuremath{'}\mathrm{*}}$=11.3\ifmmode\pm\else\textpm\fi{}0.2 K. Under a hydrostatic pressure of 5.2\ifmmode\pm\else\textpm\fi{}0.2 kbar, ${\mathit{t}}_{\mathit{b}}^{\ensuremath{'}}$ increases to 11.0\ifmmode\pm\else\textpm\fi{}0.3 K with ${\mathit{T}}_{\mathrm{SDW}}$ decreasing to 3.3\ifmmode\pm\else\textpm\fi{}0.1 K. The high pressure results are consistent with the value of ${\mathit{t}}_{\mathit{b}}^{\ensuremath{'}\mathrm{*}}$ from the ambient pressure measurements. We also observe no dependence of ${\mathit{T}}_{\mathrm{SDW}}$ with field in the a or b directions, in contrast to previous results. \textcopyright{} 1996 The American Physical Society.