Quantum Materials - 14

Abstract

The magnetoelectric effect, where electric polarization is induced by an external magnetic field, is typically explored in classical spins systems. In quantum dimer systems, the exact ground state is a product of spin-singlet states on each dimer. When a magnetic field is applied, magnon excitations of a triplet (triplon) can be induced, propagating through inter-dimer exchange interactions (Fig. 1). It has been reported that such triplon states can induce enhanced finite electric polarization due to quantum effects [1].
In this study, we qualitatively analyze the magnetoelectric effect in the quantum dimer systems TlCuCl₃, BaCuSi₂O₆, and BaVSi₂O₇. We construct a realistic multi-orbital Hubbard model based on DFT electronic structure calculations, followed by the application of superexchange theory to derive a general spin Hamiltonian and a spin model for polarization [2]. According to our theory, the induced polarization is characterized by a 3×3 tensor of intra-dimer interactions, which becomes finite in the triplon phase. While in BaCuSi₂O₆, the tensor is constrained by symmetry to the well-known Katsura-Nagaosa-Balatsky theory [3], leading to only antisymmetric elements being finite, other materials exhibit more complex symmetry constraints. We investigate the constructed spin models using the bond-operator formalism and discuss the microscopic origin of the quantum enhancement of the magnetoelectric effect due to the induced triplon states. Additionally, we find that the weaker inter-dimer exchange in BaVSi₂O₇ leads to a higher critical field compared to other systems.