Phase competition in quantum chritical multiferroics Rb2Cu2Mo3O12 and Cs2Cu2Mo3O12

Rb_2Cu_2Mo_3O_12 and Cs_2Cu_2Mo_3O_12 are quasi-1D magnets recently demonstrated to host unconventional multiferroic phenomena [1,2]. Despite the close similarity of structure and magnetic phase diagram shapes, only Cs_2Cu_2Mo_3O_12 demonstrates dielectric relaxation by magnons [1]. Here, we explore the phase space of the spin ladder model associated with this family of materials using Density Matrix Renormalization Group. We find the model to be in the vicinity of a triple point in the phase space, with three closely competing phases. Comparing the computed spin correlations in different phases with experimental data, we conclude that the ordered phases realized in Rb_2Cu_2Mo_3O_12 and Cs_2Cu_2Mo_3O_12 are actually different and discuss the consequences for explaining the observed dielectric behavior.

[1] Flavian et al. Phys. Rev. Lett. 130, 216501 (2023)

[2] Hayashida et al., Phys. Rev. Research 3, 033053 (2021)