🔬 materials science

Anomalous Pressure-Enhanced Polarization in Sliding Ferroelectrics

This study demonstrates that, contrary to conventional ferroelectrics, hydrostatic pressure enhances the polarization of sliding ferroelectric bilayer boron nitride by increasing interlayer potential and charge transfer, offering a new tuning mechanism for ultrathin memory devices.

Original authors: Lujin Min, Sahas Kamat, Ameer Mustafa, Nguyen The Duy, Kyle Nadel, Junhao Lin, Kenji Watanabe, Takashi Taniguchi, Daniel Bennett, Brad J. Ramshaw, Kenji Yasuda

Published 2026-09-28

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CC BY 4.0

Original authors: Lujin Min, Sahas Kamat, Ameer Mustafa, Nguyen The Duy, Kyle Nadel, Junhao Lin, Kenji Watanabe, Takashi Taniguchi, Daniel Bennett, Brad J. Ramshaw, Kenji Yasuda

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Ferroelectricity is a property of certain materials that act like tiny, permanent magnets, but for electric charge instead of magnetic fields. In these materials, positive and negative charges separate to create an internal electric field that can be flipped back and forth. This ability to switch states makes them the backbone of modern memory devices, allowing computers to store information as ones and zeros. For decades, scientists understood how these materials worked: the atoms inside them would shift slightly off-center, creating the electric field. A long-standing rule of thumb in physics was that if you squeezed such a material with immense pressure, you would crush those atomic shifts, weakening the electric field and eventually turning the material off.

However, a new class of materials has emerged that breaks this old rule. These are called sliding ferroelectrics, found in ultra-thin sheets of atoms stacked like layers of paper. In these materials, the electric field does not come from atoms moving inside a single layer, but from how the layers are stacked on top of one another. If you slide one layer slightly to the left or right relative to the one below it, the electric field flips. Because the layers are held together by weak forces rather than strong chemical bonds, scientists wondered if squeezing them would behave differently than in traditional materials. This question led researchers to a surprising discovery: in these sliding materials, pressure does not turn the electric field off; it turns it up.

A team of researchers at Cornell University and other institutions set out to test this idea using a specific material known as rhombohedral boron nitride. They created a tiny device by stacking a single layer of graphene, a material made of carbon atoms arranged in a honeycomb pattern, on top of two layers of this boron nitride. The graphene acted as a sensitive sensor, capable of detecting the invisible electric field generated by the sliding layers beneath it. The researchers placed this delicate stack inside a specialized chamber filled with oil, which allowed them to apply uniform pressure from all sides, simulating the crushing weight found deep within the Earth's crust.

As they gradually increased the pressure, they watched how the graphene responded. Instead of the electric signal fading away as predicted by the old rules, the signal grew stronger. The researchers measured the electric potential created by the sliding layers and found that as they squeezed the material, the voltage difference between the layers increased steadily. By the time they reached a pressure of 1.66 gigapascals, which is roughly 16,000 times the atmospheric pressure at sea level, the electric polarization had increased by about 80 percent compared to its strength at normal pressure. This was a direct, measurable enhancement of the material's ability to store electric charge, driven entirely by the compression.

The team also looked at how the atoms inside the material were behaving to understand why this was happening. In traditional ferroelectrics, pressure forces atoms closer together, which increases the repulsion between them and stops them from shifting to create a field. But in these sliding layers, the pressure simply pushes the two sheets of boron nitride closer to each other. This reduced gap allows the electrons to interact more strongly across the layers, effectively amplifying the charge transfer that creates the electric field. The researchers confirmed this by running computer simulations that matched their experimental results, showing that the effect was a natural consequence of the layers being squeezed together.

This finding challenges the conventional wisdom that pressure always suppresses ferroelectricity. While the researchers noted that the behavior of the material's internal domains—tiny regions where the electric field points in a specific direction—became more complex under pressure, the overall trend was clear: compression strengthened the effect. They also observed that the material's ability to switch its electric state remained robust, though the pressure did make it slightly harder for some parts of the material to flip their orientation. This suggests that while the fundamental electric field gets stronger, the physical movement of the layers might encounter more resistance from the surrounding environment.

The implications of this discovery reach beyond just understanding a new physical phenomenon. Because the electric field becomes stronger under pressure, it suggests a new way to tune these materials for use in future memory devices. If engineers can find ways to compress these layers locally, perhaps by using stress from surrounding materials, they could create memory bits that are easier to read and more distinct from one another. The researchers propose that this pressure-enhanced effect could be a universal feature of sliding ferroelectrics, opening a path to designing ultra-thin, high-performance electronic components that rely on the simple act of squeezing to improve their function. The work demonstrates that by looking at materials through the lens of how their layers slide and stack, rather than just how their atoms shift, scientists can find entirely new ways to control electricity.

Technical Summary: Anomalous Pressure-Enhanced Polarization in Sliding Ferroelectrics

Problem Statement
Conventional displacive ferroelectrics (e.g., BaTiO $_3$ ) rely on off-center ionic displacements within a unit cell to generate polarization. In these materials, hydrostatic pressure typically suppresses polarization by compressing the lattice, thereby enhancing short-range interatomic repulsions that favor a centrosymmetric, paraelectric phase. In contrast, "sliding ferroelectrics" found in van der Waals (vdW) layered materials derive their polarization from interlayer stacking registry rather than intra-unit-cell ionic distortion. While theoretical predictions suggest that reducing the interlayer spacing in sliding ferroelectrics (such as rhombohedral boron nitride, rBN) should strengthen interlayer orbital hybridization and charge transfer—potentially enhancing polarization—there has been no quantitative experimental verification of the relationship between hydrostatic pressure, interlayer charge transfer, and polarization in these systems.

Methodology
The authors investigated the pressure dependence of polarization in a prototypical sliding ferroelectric: parallel-stacked bilayer rBN.

Device Architecture: A heterostructure was fabricated using a dry-transfer method, consisting of a bilayer rBN ferroelectric layer adjacent to a monolayer graphene charge sensor, all encapsulated between hexagonal boron nitride (hBN) dielectrics. Graphite top and bottom gates provided electrostatic control.
Charge Sensing: The polarization state of the rBN bilayer creates a built-in interlayer potential ( $\Delta V_P$ ). This potential shifts the charge-neutrality point (CNP) of the adjacent graphene layer. By measuring the hysteretic shift of the graphene's CNP ( $\Delta V_{CNP}$ ) as a function of gate voltage, the authors could quantify the ferroelectric switching and the magnitude of the built-in potential.
Experimental Setup: Measurements were conducted in a hydrostatic pressure cell (CTF-HHPC60 piston-cylinder) using Daphne 7373 oil as the pressure medium. The device was cooled to cryogenic temperatures (1.5–1.7 K) to minimize thermal broadening of the resistance peaks. Pressures were applied up to 1.66 GPa.
Characterization: Longitudinal resistance ( $R_{xx}$ ) was measured as a function of bottom-gate voltage ( $V_B$ ) and top-gate voltage ( $V_T$ ) to map switching boundaries. Landau fan measurements were performed to extract the pressure-dependent capacitance of the bottom gate stack.
Theoretical Support: First-principles density functional theory (DFT) calculations were performed to model the pressure dependence of the interlayer potential and validate experimental trends.

Key Results

Pressure-Enhanced Polarization: Contrary to conventional ferroelectrics, hydrostatic pressure was found to enhance the polarization of bilayer rBN. The separation between the forward and backward charge-neutrality peaks ( $\Delta V_{CNP}$ ) increased monotonically from 217.4 mV at ambient pressure to approximately 318 mV at 1.66 GPa.
Quantitative Evolution: The built-in interlayer potential ( $\Delta V_P$ ) increased monotonically from 109 mV at ambient pressure to 159 mV at 1.66 GPa, representing a 46% increase over the measured pressure range. Consequently, the interlayer charge-transfer density ( $n_{CT}$ ) increased by approximately 80% (from $4.46 \times 10^{12}$ cm $^{-2}$ to $8.03 \times 10^{12}$ cm $^{-2}$ ).
Polarization and Piezoelectricity: The out-of-plane polarization ( $P$ ) increased linearly from 0.71 $\mu$ C cm $^{-2}$ to 1.29 $\mu$ C cm $^{-2}$ . This corresponds to a negative out-of-plane piezoelectric coefficient ( $d_{33} \approx -3.5$ pC N $^{-1}$ ), indicating that compressive stress increases polarization.
Mechanism: The enhancement is attributed to the compression of the vdW gap, which strengthens the wavefunction overlap between layers and amplifies the stacking-dependent charge redistribution. The geometric factor ( $d_B/t$ ) was treated as pressure-independent based on X-ray diffraction data, confirming that the observed changes are driven by electronic redistribution rather than purely geometric capacitance changes.
Domain Behavior: While the polarization magnitude increased, the switching behavior became more complex under pressure. At high pressures, the backward sweep exhibited two resistance peaks, suggesting the coexistence of up and down polarization domains and indicating that domain walls may become harder to switch or pinned under compression.

Significance and Claims
The paper establishes hydrostatic compression as a direct tuning parameter for interfacial polarization in sliding ferroelectrics, demonstrating a response fundamentally opposite to that of conventional displacive ferroelectrics.

Mechanistic Distinction: The authors claim this work confirms that the negative piezoelectric response in sliding ferroelectrics arises from the electronic, interlayer origin of polarization (interlayer charge transfer) rather than ionic displacement. This distinguishes sliding ferroelectrics from other pressure-enhanced systems like CuInP $_2$ S $_6$ , where the mechanism involves ionic redistribution.
Device Implications: The results suggest that increasing the readout contrast between binary polarization states is achievable through interlayer compression. The authors propose that this effect could benefit ultrathin "slidetronic" non-volatile memory devices by improving the discrimination between polarization states.
Scalability: While hydrostatic pressure was used as the experimental tool, the authors note that the underlying mechanism (reduction of the vdW gap) implies that similar polarization enhancement could be achieved through strain engineering or stressor layers in practical devices.

The study does not claim to have solved the challenge of domain wall pinning under pressure or to have demonstrated a fully functional memory device, but rather provides the fundamental electromechanical characterization necessary for such future developments.

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