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Sliding ferroelectrics are a type of two-dimensional (2D) material realized by stacking nonpolar monolayers (atom-thick layers that lack an electric dipole). When these individual layers are stacked, they produce ferroelectric materials with an intrinsic polarization (i.e., in which positive and negative charges are spontaneously separated), which can be switched using an external electric field that is perpendicular to them.

Understanding the mechanisms driving the switching of this polarization in sliding ferroelectrics has been a key goal of many studies rooted in physics and materials science. This could ultimately inform the development of new advanced nanoscale electronics and quantum technologies.

Researchers at Westlake University and the University of Electronic Science and Technology of China recently uncovered a new mechanism that could drive the switching of polarization in sliding ferroelectrics. Their paper, in Â鶹ÒùÔºical Review Letters (PRL), suggests that polarization switching in the materials is prompted by wave-like movements of domain walls (i.e., boundaries between regions with an opposite polarization), rather than by synchronized shifts affecting entire monolayers at once, as was assumed by some earlier works.

"In recent years, sliding ferroelectrics have garnered significant attention for their potential to expand the family of van der Waals ferroelectric materials in low-dimensional systems," Shi Liu, senior author of the paper, told PRL. "The core idea is to engineer out-of-plane polarization in two-dimensional structures by stacking nonpolar monolayers with carefully tuned interlayer shifts. This concept opens exciting possibilities for novel functionalities in nanoscale devices."

The main objective of this recent study by Liu and his colleagues was to answer a fundamental research question that had not yet been fully addressed in earlier research. Specifically, they wanted to shed light on how an out-of-plane electric field can switch the polarization in sliding ferroelectrics.

"Thermodynamically, the answer seems straightforward—an out-of-plane field should reverse the out-of-plane polarization," said Liu. "However, the situation is more subtle in sliding ferroelectrics because the polarization originates from in-plane atomic displacements between layers. This led us to a conceptual puzzle: how can an electric field applied perpendicular to the layers induce lateral atomic motion?"

When they reviewed previous papers, Liu and his colleagues realized that many of them merely assumed that an out-of-plane electric field could prompt in-plane shifts or that an entire monolayer in sliding ferroelectrics could move at once. Yet they found this to be highly improbable, particularly in macroscopic samples, where the collective movement of an entire monolayer is not likely to take place.

"Our study aimed to resolve this inconsistency by exploring the microscopic mechanisms that enable polarization switching in sliding ferroelectrics, and to clarify the role of the electric field in facilitating such atomic rearrangements," explained Liu.

"To investigate the switching mechanism behind out-of-plane polarization in 2D sliding ferroelectrics, we employed molecular dynamics (MD) simulations, a powerful computational tool that allows us to simulate how atoms move and interact over time and across relatively large length scales."

As part of their recent study, Liu and his colleagues first developed a type of deep neural network-based model known as the deep potential (DP) model. This computational model was trained on data derived from quantum mechanical calculations rooted in density functional theory (DFT).

"The DP model acts as a highly accurate classical force field, capable of capturing the complex atomic interactions essential for realistic MD simulations," said Liu. "We also incorporated atomic-environment-dependent Born effective charges (BECs) into our simulations. BECs are tensors that relate field-induced atomic forces to external electric fields, crucial for simulating how the structure responds dynamically to electric stimuli."

Ultimately, the researchers successfully used their methods to simulate electric-field-driven polarization switching in sliding ferroelectrics that entailed a dynamic charge transfer process at the atomic scale and with high fidelity. This allowed them to gather new insights that could improve the understanding of this class of materials.

"Firstly, we show that an out-of-plane electric field alone cannot reverse the polarization of a single domain in sliding ferroelectrics," said Liu. "Instead, polarization switching occurs via symmetry-breaking domain walls (DWs), enabled by the tensorial nature of Born effective charges. Secondly, we identify a new type of domain wall dynamics that displays an anomalous temperature dependence."

The researchers' simulations also suggest that the velocity of the motion of domain walls increases as the temperature decreases, which is the opposite of what typically happens in conventional ferroelectrics. Liu and his colleagues dubbed this phenomenon "superlubric domain wall motion," drawing an analogy to a state known as superlubricity that occurs in frictionless mechanical systems.

This recent paper could inform new research focusing on sliding ferroelectrics and their underlying physics. In the future, it could also contribute to the development of various technologies based on these 2D materials, including nanoscale devices in cryogenic environments.

"As part of our future studies, we plan to explore relativistic-like kinematics of domain walls at low temperatures, with a particular interest in understanding how quantum effects affect structural dynamics in sliding ferroelectrics under cryogenic conditions," added Liu. "This could reveal new physical regimes and switching behaviors not accessible at higher temperatures."

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