A theoretical study of two-magnon light scattering in triangular antiferromagnets on a plane

Two-magnon Raman scattering in triangular Heisenberg antiferromagnets on a two-dimensional plane is discussed on the basis of the 120 degrees structure, where spins are confined on the plane by the single-ion-type anisotropy, by the use of the exchange-scattering mechanism. Spectra obtained by calculation reflect the symmetric properties of the present non-collinear spin structure. In addition to the normal structure in spectra of the alpha mode which is symmetric with respect to space inversion, we can find anomalous structure of beta and gamma modes which lacks symmetry with respect to space inversion. The former appears at twice the excitation energy for the single mode, which is also found in collinear structures for two-sublattice Neel states. In contrast, the latter is caused by the following anomaly characterizing the present 120 degrees structure in the triangular lattice antiferromagnets: the magnon for the beta mode with wave-number vector +k is effectively coupled with that for the gamma mode with +k.