The effects of chemical disorder on the vibrational modes of relaxor ferroelectric PMN

We investigate the relaxor ferroelectric Pb(Mg_1/3Nb_2/3)O₃ (PMN), the disordered counterpart of PbTiO₃ (PTO), by employing a neural-network potential energy model [1]. PMN is recognized as a dielectric analog of magnetic spin glasses and also exhibits similarities with fragile structural glasses. Inherent structures are obtained by quenching to classical ground states for various disorder realizations, followed by diagonalization of the dynamical matrix to obtain the vibrational modes. Disorder-induced quasi-localized modes are identified in the low-frequency regime, in contrast to the extended phonon-like modes found in PTO. These quasi-localized modes result in a density of states (DOS) that follows a universal D(ω) ∼ ω⁴ scaling behavior, and is accompanied by an enhanced DOS of low-frequency extended modes. The calculated specific heat in the temperature range from 5 K to 100 K agrees well with experiments for both PMN and PTO [2]. Notably, the Boson peak observed around 10 K in PMN is quantitatively reproduced. Our analysis shows that the peak is due to the excess DOS arising from disorder.

[1] Linfeng Zhang et al. Physical review letters, 120(14):143001, 2018.

[2] Makoto Tachibana and Eiji Takayama-Muromachi. Physical Review B, 79(10):100104, 2009.