CRITICAL IMPERFECT NESTING IN (TMTSF)(2)PF6

We have investigated the temperature, pressure, and field dependence o f the spin density wave (SDW) transition in the organic superconductor (TMTSF)(2)PF6 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 banawidth t'(b). Per fect nesting (maximum T-SDW) occurs for t'(b) = 0, and increasing pres sure increases t'(b) and suppresses T-SDW to 0 K at a critical value t '(b). The increase of T-SDW with field has been shown to be orbitally driven, and the field dependence is nearly quadratic for fields in th e c direction. We have made quantitative observations of the field dep endence of T-SDW at ambient pressure to 29 T and under hydrostatic pre ssure to 8 T. From our measurements at ambient pressure we are able to extract t'(b) and t'(b) using mean field theory and observe T-SDW = 12.1+/-0.1 K, t'(b) = 4.5+/-0.3 K, and t'(b) = 11.3+/-0.2 K. Under a hydrostatic pressure of 5.2+/-0.2 kbar, t'(b) increases to 11.0+/-0.3 K with T-SDW decreasing to 3.3+/-0.1 K. The high pressure results are consistent with the value of t'(b) from the ambient pressure measurem ents. We also observe no dependence of T-SDW with field in the a or b directions, in contrast to previous results.