Quantum-Phase-Transition-Induced Multiferroics and Higgs Mode in Integer Spin Systems in Noncentrosymmetric Lattice with Strong Single-Ion Anisotropy

In integer spin systems with a strong easy-plane type single-ion anisotropy, a paramagnetic ground state is stabilized with a finite excitation gap. Applying a magnetic field or pressure can collapse the excitation gap and induce quantum phase transition into a magnetically ordered state. In a lattice without the inversion symmetry at the magnetic ion site, electric polarization emerges spontaneously by the quantum phase transition. We study how excitation modes evolve with field or pressure and analyze the dynamical correlation functions of the electric polarization. In the case of the pressure-induced order, a longitudinal mode exists in the low-energy region in the vicinity of the quantum critical point. It corresponds to a Higgs mode and is observable as an electromagnon that is excited by the electric field component of light absorption.