Characteristic Aspects of Pion-Condensed Phases

Characteristic aspects of pion-condensed phases are described in a simple model, for the system involving only nucleons and pions which interact through the π — N P-wave interaction. We consider one typical version in each of three kinds of pion condensation; the one of neutral pions (π0), the one of charged pions (πc) and the combined one in which both the π0 and πc condensations are coexistent. Emphasis is put on the description to clarify the novel structures of the nucleon system which are realized in the pion-condensed phases. At first, it is shown that the π0 condensation is equivalent to the particular nucleonic phase realized by a structure change of the nucleon system, where the attractive first-order effect of the one-pion-exchange (OPE) tensor force is brought about coherently. The aspects of this phase are characterized by the layered structure with a specific spin-isospin order with one-dimensional localization (named the ALS structure in short), which provides the source function for the condensed π0 field. We utilize both descriptions with use of fields and potentials for the π0 condensation. Next, the πc condensation realized in neutron-rich matter is described by adopting a version of the traveling condensed wave. In this phase, the nucleonic structure becomes the Fermi gas consisting of quasi-neutrons described by a superposition of neutron and proton. In this sense the structure change of the nucleon system for the πc condensation is moderate, and the field description is suitable. Finally, as the most favorable type of pion condensation in a sense that all the charge degrees of freedom are utilized, we describe a coexistent pion condensation, in which both the π0 and πc condensations coexist without interference in such a manner that the πc condensation develops in the ALS structure. Although the model adopted here is the simplest one with use of the driving interaction only, it provides us with the characteristic aspects of the pion-condensed phases persisting in the realistic situation, where other ingredients affecting the pion condensation are taken into account.