Temperature behaviour of optical phonons near Tc in triglycine sulphate and triglycine selenate

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Abstract
The temperature dependences of optical phonon mode frequencies near Tc in TGS/TGSe were observed by using polarized Raman and infrared spectroscopy and were classified into four categories. Two of these categories are discussed in this article.About five out of 177 observed modes demonstrate a strong frequency shift below Tc while above Tc the frequency remains constant. The halfwidth γ of one of these five modes was studied and shows a maximum at Tc which is related with the pseudospin fluctuations near Tc. The internal SO42 7SeO42- -modes are characterized by a (rounded) steplike shift near Tc. These frequency shifts have a temperature dependence corresponding to a two-spin correlation function (σμMσv between pseudospins σμ and σv. All observations can be interpreted by a generalization of the usual pseudospin-phonon interaction theory. The experiments show strong evidence for the existence of an interaction quadratic in the phonon coordinates and linear as well as quadratic in the pseudospin coordinates.