Sliding ferroelectrics are a new exciting family of two-dimensional (2D) materials, and this homeogenized mechanism has taken researchers on a wild ride to profound discoveries. Shi Liu and his colleagues at MIT developed an innovative new study. They found a singular effect in which wavelike domain walls exhibit peculiar temperature dependence. Their findings, published in Physical Review Letters, focus on the intricate dynamics of polarization switching within these materials, which could pave the way for advanced applications in nanoscale devices.
Sliding ferroelectrics are formed by stacking these nonpolar monolayers—atom-thin sheets lacking an electric dipole. By precisely controlling the interlayer displacements, scientists hope to design out-of-plane polarization in these low-D materials. Liu emphasized the exciting potential of sliding ferroelectrics. He thinks they’re just getting started and that they can widen the library of van der Waals ferroelectric materials in low-dimensional systems.
The Role of Deep Neural Networks
So, Liu and his team developed a new deep neural network-based model to improve their research. To denote their new approach, they termed this novel model the deep potential (DP) model. This innovative approach acts as a highly accurate classical force field, capturing complex atomic interactions essential for realistic molecular dynamics simulations.
“The DP model acts as a highly accurate classical force field, capable of capturing the complex atomic interactions essential for realistic MD simulations,” – Shi Liu
By modeling this process, the researchers found that polarization switching can be suppressed in sliding ferroelectrics. Liu recognized right away that longstanding assumptions about electric field-induced spectral shifts required a rethink.
“Thermodynamically, the answer seems straightforward—an out-of-plane field should reverse the out-of-plane polarization,” – Shi Liu
What these findings showed was the picture was actually more nuanced. Liu explained that the polarization arises from in-plane atomic displacements between layers, leading to a perplexing question: how can an electric field applied perpendicular to the layers induce lateral atomic motion?
Discovering Superlubric Domain Wall Motion
The study uncovered a novel dynamic domain wall phenomenon called “superlubric domain wall motion.” Liu and his colleagues noticed that this motion mirrors an analogy to superlubricity—a condition recognized in ideal mechanical systems free of friction.
“Instead, polarization switching occurs via symmetry-breaking domain walls (DWs), enabled by the tensorial nature of Born effective charges,” – Shi Liu
This finding provides important context. More importantly, it uncovers how polarization switching can occur not by physically shifting a whole monolayer, but rather through the displacement of domain walls in turn. Born effective charges (BECs) were individually identified as a key discovery by the researchers. These tensors relate the induced forces on atoms due to external electric fields.
That finding surfaced an unusual temperature dependence in polarization switching, fueled by this newly discovered superlubric domain wall motion. Liu, who led the research, commented on how important their findings are and what these results mean for future studies.
“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,” – Shi Liu
Implications for Nanoscale Devices
The long-term effects of this research may be significant for the design of nanoscale devices. The unprecedented capability to dynamically control polarization via disruptive mechanisms presents thrilling opportunities for multifunctionality in next generation materials.
Liu noted that by understanding and harnessing these phenomena, researchers can explore new functionalities within nanoscale systems, driving advancements in technology and materials science.
“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,” – Shi Liu