Unexpected Fractional Ferroelectricity in Monolayer InSe: Breaking Symmetry Constraints

In their April 16, 2025, article titled Generalized Neumann’s Principle as a Unified Framework for Fractional Quantum and Conventional Ferroelectricity, researchers Hongsheng Pang and Lixin He present a novel theoretical framework that extends Neumann’s principle to unify fractional quantum ferroelectricity with conventional ferroelectricity. Their work offers a systematic method for identifying such phenomena across crystallographic groups, providing deeper insights into the control of polarization in materials.

Monolayer InSe exhibits in-plane polarization despite symmetry constraints, traditionally considered forbidden. Ji et al. proposed fractional ferroelectricity (FQFE), suggesting it violates Neumann’s principle. The authors introduced a generalized Neumann’s principle, unifying FQFE and conventional ferroelectricity under one framework. They developed a systematic method to identify all symmetry-allowed FQFE cases across 32 crystallographic point groups. Additionally, they demonstrated that FQFE can be switched via coupling with conventional polarization, using HfZnN as an example to reveal the underlying mechanism and propose strategies for identifying similar materials.

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