Spin-lattice coupling driven ferroelectric transition in one-dimensional organic quantum magnets

We propose a theoretical quantum spin model for a one-dimensional (1D) spin-Peierls (SP) system to describe its ferroelectricity driven by the spin-lattice coupling, and further investigate the ferroelectric (FE) SP transition for a known 1D organic donor-acceptor charge transfer compound, which was experimentally proved to show ferroelectricity, by means of many-body Green's function theory. It is found that the transition temperature (T-c), polarization (P) and dielectric constant are in agreement with experimental results. Meanwhile, it is shown that the two-site thermal entanglement entropy is a good indicator of a FE transition. In addition, the potential magnetoelectric behavior is taken into account. On the one hand, when the magnetic field is turned on, it makes P and T-c decrease, and drives the FE transition from the second order to the first order. Nevertheless, the FE dimerized-singlet state may collapse for high enough magnetic fields, leading to the restoration of paraelectric uniform stack of donor acceptor. On the other hand, as the electric field is applied along the chain, the FE phase is evidenced by the electric polarization (P)-field (E) hysteresis loops. It is also found that the magnetization M decreases but the polarization P can be enhanced with increasing electric field, which makes the FE transition exhibit crossover behavior, since the electrostatic energy predominates over the spin-lattice coupling.