Solidification and Pion Condensation in Nuclear Medium: Alternating-Layer-Spin Structure with One-Dimensional Localization Accompanying 0 Condensate

Abstract

Localization of nucleon and π0 condensation in nuclear medium are studied in a unified way, by adopting two kinds of descroptions; namely in the potential description with the OPEP and in the description with explicit use of the pion field coupled to the nucleon field by the (σ·∇) interaction. The equivalence of these two descriptions shown in the mean field approximation provides an important starting point. In the former description, the strong tensor force of the OPEP brings about the one-dimensional solid-like structure in which nucleons localize to form the layers with their spin directions aligned perpendicular to the layers and changing in alternation layer by layer. We abbreviate this to [ALS] (Alternating-Layer-Spin) structure. In the latter description, the condensed π0 field is realized by the source function with the spin-density periodicity given by this nucleon [ALS] structure. Mechanism of energy gain in such a new phase is clarified by showing that the condensation energy overwhelms the increase of nucleon kinetic energy due to localization. Exchange effect, which is ignored in the usual mean field approximation, is not negligible. However, it becomes small if the δ-function part of the OPEP is removed. The nucleon system suddenly turns into a well-localized state as the density exceeds the critical value near the normal density ρ0. This means the growth of "solidification" accompanying the π0 condensation at the density higher than ρ0 and also indicates a wide range of validity of our model starting from the nucleon [ALS] structure.