Van der Waals bilayers: Monolayer stiffness stabilizes ferroelectricity above RT

Certain two-dimensional (2D) van der Waals-bonded bilayers (e.g., WTe₂ and h-BN) exhibit “sliding ferroelectricity (SF),” where electric polarization arises from specific stacking configurations between monolayers and can be switched by interlayer sliding with an ultra-low energy barrier. This enables energy-efficient switching and enhances material flexibility, making it promising for ultra-thin, low-power ferroelectric devices, such as non-volatile memory and flexible nanoelectronics.

An open question in ferroelectricity research is why SF in WTe₂ and h-BN remains stable well above room temperature (RT) despite their ultra-low electric-polarization switching barriers. Conventional phase transition theories suggest these 2D materials should not exhibit stable ferroelectric order without a substantial barrier, and thus cannot explain the remarkable stability observed in sliding ferroelectrics, where an ultra-low switching barrier has been confirmed experimentally.

In a 2023 article, Ping Tang and Gerrit E.W. Bauer from AIMR developed a thermodynamic model to address this conundrum. They proposed that the thermal stability of SF originates from the high in-plane mechanical stiffness of the monolayers, which suppresses thermal fluctuations and reinforces ferroelectric order. Using a mean-field self-consistent phonon approximation, they calculated electric susceptibility and specific heat, quantitatively linking monolayer mechanical properties to ferroelectric stability.

A key finding was the identification of a soft “sliding phonon” mode that governs the phase transition in sliding ferroelectrics. Unlike conventional ferroelectric phase transitions, where dipole flipping occurs within individual unit cells, the ferroelectric-to-paraelectric transition here is driven by a thermally activated collective sliding of the bilayer, triggered by this soft phonon at the Curie temperature.

“While the ultralow switching barrier from weak interlayer van der Waals forces does not favor stable ferroelectric order, the intralayer stiffness of individual monolayers enhances ferroelectric stability by suppressing the thermally activated collective sliding motion,” says Tang. “This balance between barrier and rigidity explains the high Curie temperatures of WTe₂ and h-BN and distinguishes them from other low-dimensional ferroelectric materials.”

These results demonstrate that the high Curie temperatures of WTe₂ and h-BN bilayers stem from the interplay between their ultra-low switching barriers and significant intralayer rigidity, which prevents rapid thermal depolarization, unlike in other low-dimensional magnetic or ferroelectric materials.

“Our work establishes a unifying theoretical framework for the mechanism underlying the thermal stability of sliding ferroelectricity in van der Waals bilayers,” concludes Tang. “By identifying a soft sliding phonon as the phase transition driver and highlighting the role of monolayer stiffness, we not only elucidated the phase transition mechanism of sliding ferroelectrics but also extended it to structural transitions in non-ferroelectric bilayers. These insights have broad implications for next-generation low-power electronics, spintronics, and bilayer twistronics.”

A personal insight from Dr. Ping Tang

What aspect of this research was most rewarding for you, and why?

The most rewarding part was solving a long-standing problem with a simple analytical model. As a theoretical physicist, I aim to understand nature by distilling complexity into its essential components. This work resolved an apparent paradox in van der Waals bilayer ferroelectrics through an intuitive mean-field approach, revealing that a soft "sliding phonon" mode governs the phase transition. Demonstrating intricate physical phenomena with a minimal yet insightful model was deeply satisfying and reinforced my belief in the power of elegant theoretical frameworks.

This article was written by Patrick Han, Ph.D. (patrick@sayedit.com).

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Advanced Institute for Materials Research (AIMR)

Tohoku University

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AIMR aims to contribute to society through its actions as a world-leading research center for materials science and push the boundaries of research frontiers. To this end, the institute gathers excellent researchers in the fields of physics, chemistry, materials science, engineering, and mathematics and provides a world-class research environment.