Nambu-Goldstone mode in K3Na(SeO4)2 crystal

Abstract

The Nambu-Goldstone optical mode has been observed in the ferroelastic crystals for the first time. The amplitudon and phason modes have been identified in the Raman spectra of K3Na(SeO4)2 (KNSe) crystal. We discuss the occurrence of such lattice vibration concerning to the possible presence of an incommensurate phase. The potential scenario of the dynamics of SO4 tetrahedron leading to the appearance of IC phase accompanied with critical temperature behavior of two external vibrations of Ag symmetry is given together with molecular mechanism of phase transitions in the material studied. The effect of spatial reorganization of the crystal lattice associated with the ferroelastic domains of the KNSe crystal of W and W' type is also discussed. We show that the emergence of such domain structure may also be a source of incommensurability. Finally, both amplitudon and phason have been classified as type A propagating and diffusion modes respectively.