A representation of SU(2) and isospin waves in a solid nuclear matter

A representation of SU(2) is given in terms of Fermi operators. Then, it is applied to construct physical states of quantized isospin-waves (which are called 'isomagnons' in the present paper) for the sake of variational evaluation of the ground state energy under pion condensation on a dense solid neutron matter. The mixing of protons is described in terms of the excitation of isomagnons. One pion exchange effect is also taken into account. The condensation of physical isomagnons is shown to bring about the energy gain of O(100 MeV) per nucleon. (author)