Type-II Antiferroelectricity

TL;DR

This paper introduces a new class of antiferroelectric materials called type-II AFEs, characterized by opposite polarizations in momentum space, which coexist with antiferromagnetism and exhibit unique physical phenomena.

Contribution

The study formulates the order parameter of type-II AFEs via Berry-phase theory, identifies symmetry constraints, and discovers their coexistence with antiferromagnetism in real materials.

Findings

Type-II AFEs have opposite polarizations in momentum space.

Type-II AFEs intrinsically coexist with antiferromagnetism.

Unique phenomena include spin current generation and boundary spin polarization.

Abstract

Antiferroelectricity (AFE) is a fundamental concept in physics and materials science. Conventional AFEs have the picture of alternating local electric dipoles defined in real space. Here, we discover a new class of AFEs, termed type-II AFEs, which possess opposite polarizations defined in momentum space across a pair of symmetry decoupled subspaces. Unlike conventional AFEs, the order parameter of type-II AFEs is rigorously formulated through Berry-phase theory and can be quantitatively extracted from the electronic band structure. Focusing on a subclass of type-II AFEs that preserve spin-rotation symmetry, we establish the relevant symmetry constraints and identify all compatible spin point groups. Remarkably, we find that type-II AFE order intrinsically coexists with antiferromagnetism, revealing a robust form of magnetoelectric coupling. We construct an altermagnetic model and identify several concrete antiferromagnetic/altermagnetic materials, such as FeS, Cr2O3, MgMnO3, monolayer MoICl2 and bilayer CrI3, that exhibit this novel ordering. Furthermore, we uncover unique physical phenomena associated with type-II spin-AFE systems, including spin current generation upon AFE switching and localized spin polarization at boundaries and domain walls. Our findings reveal a previously hidden class of quantum materials with intertwined ferroic orders, offering exciting opportunities for both fundamental exploration and technological applications.