CALCULATION OF SPIN REDUCTION FOR MAGNONS IN HEXAGONAL COMPOUNDS IN MAGNETIC-FIELD BY THE NUMERICAL CONSTRUCTION OF BOGOLIUBOV TRANSFORMATION

The spin reduction Delta S-j by the quantum fluctuation of magnons is calculated for the triangular Heisenberg antiferromagnet in magnetic f ield for several types of Neel stares. Delta S-j depends on the sublat tice j except for the type of spiral symmetry. The behavior of Delta S -j is discussed in connection with Goldstone modes, one of which corre sponds to the deformation of spin structure within the non-trivial con tinuous degeneracy combining those types. Applying the analysis of Del ta Sj to CsCuCl3, we calculate the change of Beta S-j associated with the phase transition discussed by Nikuni and Shiba. The distinctive ro le of ferromagnetic ordering to Delta S-j is pointed out.