Anomalous reduced sound velocity of multiferroic TbMn2O5

The anomalous reduced sound velocity of multiferroic TbMn2O5 (TMO) has been studied using Green's function technique. To achieve this aim, the anharmonic phonon-phonon interaction and the spin-phonon interaction were used. It was shown that the reduced velocity of sound of TMO exhibits a kink at the ferroelectric phase transition temperature TC. This can be explained as an effect of vanishing ferroelectric ordering above TC. It was found that the reduced sound velocity increases with increasing V(3) (the third-order atomic force constants of the anharmonic phonons) in the interval T < TC, whereas the reduced sound velocity remains unchanged in the interval TC < T < TN. It was also found that the reduced sound velocity increases with the increase of V(4) (the fourth-order atomic force constants of the anharmonic phonons) in the interval T < TN. In addition, the ferroelectric phase transition temperature TC decreases when V(4) increases in the interval T < TN. Those theoretical results are in agreement with the experimental data.