Spin-isospin structure and pion condensation in nucleon matter

We report variational calculations of symmetric nuclear matter and pure neutron matter, using the new Argonne $v_{18}$ two-nucleon and Urbana IX three-nucleon interactions. At the equilibrium density of 0.16 ${\mathrm{fm}}^{-3}$ the two-nucleon densities in symmetric nuclear matter exhibit a short-range spin-isospin structure similar to that found in light nuclei. We also find that both symmetric nuclear matter and pure neutron matter undergo transitions to phases with pion condensation at densities of 0.32 ${\mathrm{fm}}^{-3}$ and 0.2 ${\mathrm{fm}}^{-3}$, respectively. Neither transtion occurs with the Urbana $v_{14}$ two-nucleon interaction, while only the transition in neutron matter occurs with the Argonne $v_{14}$ two-nucleon interaction. The three-nucleon interaction is required for the transition to occur in symmetric nuclear matter, whereas the transition in pure neutron matter occurs even in its absence. The behavior of the isovector spin-longitudinal response and the pion excess in the vicinity of the transition, and the model dependence of the transition are discussed.