Quantum Geometric Origin of Strain-Induced Ferroelectric Phase Transitions

TL;DR

This paper reveals that the quantum geometric properties of electron-phonon coupling, specifically Berry curvature, are fundamental in understanding strain-induced ferroelectric phase transitions, linking microscopic electronic effects to macroscopic polarization changes.

Contribution

It introduces a novel quantum geometric framework based on Berry curvature of EPC to explain ferroelectric phase transitions under strain, supported by first-principles calculations.

Findings

Berry curvature of EPC influences interatomic forces

Strain can invert EPC Berry curvature, causing phonon softening

The theory accurately describes experimental DFT results for BiOCl

Abstract

Strain-regulated ferroelectric (FE) materials have long attracted significant attention due to their diverse applications. While soft-phonon theory and the (pseudo) Jahn-Teller effect have achieved considerable success in providing phenomenological descriptions and general understanding, the detailed connection between these perspectives and their microscopic dependence on strain regulation remains unclear. Here, under the framework of density-functional perturbation theory (DFPT), we demonstrate that the Berry curvature of electron-phonon coupling (EPC) plays a pivotal role in the interatomic force matrix (IFM). A subsequent model analysis shows that external strain can reverse the polarity of the EPC Berry curvature in (quasi)-degenerate electronic subsystems through band inversion, thereby directly leading to phonon softening. The general theory is then applied to the BiOCl monolayer as a benchmark, which offers an accurate description of the density functional theory (DFT) calculations. This mechanism is further observed across a broad range of materials through ab initio calculations, providing an insightful perspective on EPC quantum geometry in lattice dynamics and FE phase transitions.