[Paper Review] Electronic ratchet effect in a moiré system: signatures of excitonic ferroelectricity

Zhi-Ren Zheng, Xueqiao Wang+18 more|arXiv (Cornell University)|Jun 6, 2023

Advanced Memory and Neural Computing10 citations

TL;DR

Demonstrates an electronic ratchet effect in a layer-contrasting graphene–boron nitride moiré system, showing exciton-driven (excitonic) ferroelectricity with tunable remnant polarization and multiple memory states.

ABSTRACT

Electronic ferroelectricity represents a new paradigm where spontaneous symmetry breaking driven by electronic correlations, in contrast to traditional lattice-driven ferroelectricity, leads to the formation of electric dipoles. Despite the potential application advantages arising from its electronic nature, switchable electronic ferroelectricity remains exceedingly rare. Here, we report the discovery of an electronic ratchet effect that manifests itself as switchable electronic ferroelectricity in a layer-contrasting graphene-boron nitride moiré heterostructure. Our engineered layer-asymmetric moiré potential landscapes result in layer-polarized localized and itinerant electronic subsystems. At particular fillings of the localized subsystem, we find a ratcheting injection of itinerant carriers in a non-volatile manner, leading to a highly unusual ferroelectric response. Strikingly, the remnant polarization can be stabilized at multiple (quasi-continuous) states with behavior markedly distinct from known ferroelectrics. Our experimental observations, simulations, and theoretical analysis suggest that dipolar excitons are the driving force and elementary ferroelectric units in our system. This signifies a new type of electronic ferroelectricity where the formation of dipolar excitons with aligned moments generates a macroscopic polarization and leads to an electronically-driven ferroelectric response, which we term excitonic ferroelectricity. Such new ferroelectrics, driven by quantum objects like dipolar excitons, could pave the way to innovative quantum analog memory and synaptic devices.

Motivation & Objective

• Investigate electronic ferroelectricity driven by electronic correlations rather than lattice displacements.
• Explore how layer-asymmetric moiré potentials create coexisting itinerant and localized electronic subsystems.
• Determine whether dipolar excitons can act as the elementary ferroelectric units generating a switchable polarization.

Proposed method

• Fabricate BLG sandwiched between top and bottom BN with large mismatch to create layer-contrasting moiré potentials.
• Characterize transport via four-probe resistance maps as a function of top and bottom gate voltages to identify LSAS and ratcheting regimes.
• Extract itinerant and localized charge densities from Hall measurements and total charge densities to separate n_I and n_L.
• Analyze polarization dynamics using a phenomenological Preisach-like hysteron model to reproduce P-D loops.
• Propose and quantify exciton formation as the mechanism for dipolar ferroelectric units and compare with energy scales (gap Δ, binding energy E_B).
• Use simulations and ab initio insights to estimate interlayer exciton binding energies and localized state gaps.

Experimental results

Research questions

• RQ1 Do layer-contrasting moiré potentials in BLG/BN create coexisting itinerant and localized electronic subsystems that enable electronic ferroelectricity?
• RQ2 Can excitonic dipoles act as the elementary units driving switchable ferroelectric polarization in a moiré system?
• RQ3 What is the nature of the remnant polarization and its tunability via displacement field and gate voltages?
• RQ4 How does the coupling between itinerant and localized subsystems produce a ratchet-like, quasi-continuous memory behavior?

Key findings

• Persistence of remnant polarization manifested as non-volatile, switchable n_H at D = 0, indicating ferroelectric behavior.
• Observation of a cascade pattern where LSAS and ratcheting regimes alternate, revealing layer-polarized localized states coexisting with itinerant carriers.
• Evidence for exciton formation as the driving ferroelectric unit, producing a fixed-widith hysteresis and a quasi-continuous set of memory states.
• Demonstration of a continuously scalable ferroelectric loop whose area scales with the D-field range due to the ratchet effect.
• Estimate of sizable interlayer exciton binding energy (~100 meV) compatible with observed energy gaps and localization.
• A phenomenological model (Preisach-like) capturing the hysteresis and the three-stage dependence of remnant polarization on maximum applied displacement field.