Multiferroic metal with huge polar distortion driven by spin ordering: monolayer Fe₃GeTe₂

Classical and quantum electrodynamics show that the simultaneous existence of both ferroelectric instability and free electrons in a single-phase material is contradictory. However, it has been suggested that decoupling between softening transverse optical phonon mode and the free electrons might allow the coexistence of electric polarization and free electrons a few decades ago, thus they are called polar metal. In exploring numerous polar metal candidates, a rule of thumb is to look for a material where the atom for polar displacement and the atom for free electrons are distinct. In this work, we study a polar metallic behavior in a two-dimensional material composed of an odd number of layers. In this system, by using the first-principle calculations, the mirror symmetry breaking which originates from the spin configuration is possible without destroying the metallicity. Intriguingly, the single atom is responsible for both polar distortion and free electrons, thus a new class of polar metallicity. We demonstrate that the Fe₃GeTe₂ monolayer with three layers of Fe with broken mirror symmetry due to the spin configurations could show polar metallic behavior. The formation of ferroelectrics by magnetism in low-dimensional systems will open a new chapter in the discovery and application of new states of low-dimensional materials as a general method applied to all 2-dimensional materials and even several layers of metals.