Critical imperfect nesting in (TMTSF)(2)PF6

We have investigated the temperature, pressure, and field dependence of 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). Perfect nesting (maximum T-SDW) occurs for t'(b) = 0, and increasing pressure 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 the c direction. We have made quantitative observations of the field dependence of T-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 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 measurements. We also observe no dependence of T-SDW with field in the a or b directions, in contrast to previous results.