Ferroelectric switching of interfacial dipoles in $α$-RuCl$_3$/graphene heterostructure

We demonstrate electrically switchable, non-volatile dipoles in graphene/thin hBN/$α$-RuCl$_3$ heterostructures, stabilized purely by interfacial charge transfer across an atomically thin dielectric barrier. This mechanism requires no sliding or twisting to explicitly break inversion symmetry and produces robust ferroelectric-like hysteresis loops that emerge prominently near 30~K. Systematic measurements under strong in-plane and out-of-plane magnetic fields reveal negligible effects on the hysteresis characteristics, confirming that the primary mechanism driving the dipole switching is electrostatic. Our findings establish a distinct and robust route to electrically tunable ferroelectric phenomena in van der Waals heterostructures, opening opportunities to explore the interplay between interfacial charge transfer and temperature-tuned barrier crossing of dipole states at the atomic scale.

💡 Research Summary

In this work the authors demonstrate a novel route to achieve electrically switchable, non‑volatile dipoles in a van‑der‑Waals heterostructure composed of graphene, an atomically thin hBN spacer, and the Mott‑insulating α‑RuCl₃. By inserting a few‑layer hBN sheet between graphene and α‑RuCl₃, the otherwise complete charge transfer that occurs in a direct graphene/α‑RuCl₃ contact is deliberately moderated. The thin dielectric still permits strong electrostatic coupling across the interface, allowing a modest amount of electrons to move from graphene into α‑RuCl₃ (or vice‑versa) and thereby generate opposite surface charges on the facing planes. This charge imbalance creates an out‑of‑plane electric dipole localized at the graphene/hBN/α‑RuCl₃ interface.

Transport measurements reveal that, unlike the direct‑contact device (no hysteresis at any temperature), devices with a 1–2‑layer hBN spacer exhibit a clear ferroelectric‑like hysteresis loop in the four‑terminal resistance when the top‑gate voltage is swept. The hysteresis is most pronounced around 30 K, grows in area upon further cooling, and persists down to the lowest measured temperature (≈1.5 K). Importantly, the loop is closed—i.e., the resistance returns to its original value after a full forward‑and‑reverse sweep—indicating a true bistable dipole state rather than a simple charge‑trapping artifact.

The top gate primarily reorients the interfacial dipoles rather than modulating the carrier density in graphene; this is evident from the modest resistance change under top‑gate bias compared with the strong response to back‑gate bias (which directly dopes graphene through SiO₂ and bottom hBN). Dual‑gate experiments confirm that the top gate controls dipole polarity while the back gate controls carrier concentration, providing a clean separation of the two functionalities.

The dipole state is remarkably stable. After aligning the dipoles by a series of bipolar top‑gate sweeps, the device was left with a floating top gate for five months. Subsequent measurements showed virtually unchanged hysteresis shape, coercive voltage, and a sharp resistance peak, with only a small horizontal shift (~+19 V) attributed to slow internal charge redistribution. This endurance far exceeds the typical lifetimes of charge‑trapping or ion‑migration mechanisms, supporting the interpretation of a collective, ordered dipole ensemble.

Systematic variation of the hBN thickness (from zero to ten layers) demonstrates that the hysteresis magnitude diminishes as the spacer becomes thicker, vanishing for ten‑layer hBN. Raman spectroscopy and density‑functional theory calculations corroborate that interfacial charge transfer decreases with increasing barrier thickness, confirming that a finite but limited charge flow is essential for dipole formation.

Magnetic‑field tests up to 9 T, both in‑plane and out‑of‑plane, show virtually no effect on the hysteresis onset, loop area, or coercive fields. This magnetic‑field independence indicates that the dipole switching is driven purely by electrostatic forces, despite α‑RuCl₃’s reputation as a Kitaev quantum magnet with strong spin‑orbit coupling.

The authors interpret the 30 K characteristic temperature as the thermal activation energy required to overcome the barrier between the two dipole orientations. Below this temperature the dipoles become pinned, leading to larger coercive voltages; above it thermal fluctuations allow the dipoles to switch more readily, giving rise to the observed hysteresis window.

Overall, the study establishes a robust, electrically controllable ferroelectric‑like phenomenon in a 2D heterostructure without relying on lattice distortions, layer sliding, or twist‑induced symmetry breaking. By exploiting interfacial charge redistribution across an atomically thin dielectric, the work opens a new design paradigm for non‑volatile memory elements, electrically tunable optoelectronic devices, and platforms to explore coupling between electronic ferroelectricity and exotic magnetic states in van‑der‑Waals materials.