Twist-Angle-Controlled Anomalous Gating in Bilayer Graphene/BN Heterostructures

Technical Summary: Twist-Angle-Controlled Anomalous Gating in Bilayer Graphene/BN Heterostructures

Problem Statement
Anomalous gating effects, specifically gate ineffectiveness and pronounced hysteresis, have been observed in graphene-based systems encapsulated in hexagonal boron nitride (BN). These phenomena have been linked to a potential ferroelectric state, with implications for ferroelectric field-effect transistors. However, the origin, stability, and reproducibility of these effects remain debated. Previous observations across various graphene structures (monolayer to multilayer, twisted and untwisted) lacked precise control over sample alignment, preventing a clear understanding of the underlying mechanisms. A critical open question is whether these effects depend on the formation of a moiré superlattice between graphene and BN or on the crystallographic alignment between the encapsulating BN layers.

Methodology
The authors fabricated dual-gated, dynamically rotatable van der Waals heterostructures consisting of Bernal-stacked bilayer graphene (BBG) semi-encapsulated in BN. The device architecture features a "reference rotator" (a bottom BN flake locked at a specific crystallographic alignment) and an "active rotator" (a top BN flake capable of dynamic rotation). This design allows for the continuous variation of the angular alignment ($\Theta_{BN}$) between the top and bottom BN layers within a single heterostructure, covering a range of approximately $140^\circ$.

Charge transport measurements were performed using four-probe resistance techniques. The study primarily utilized room temperature measurements on a primary sample (Sample 1) where the alignment between the bottom BN and graphene was $\approx 10^\circ$, with comparative measurements on additional samples and at low temperatures ($6$ K). The researchers systematically swept top and bottom gate voltages to map resistance, identifying the Charge Neutrality Point (CNP) and analyzing hysteresis loops and gate effectiveness.

Key Results
The study demonstrates that the anomalous gating response acts as an ON/OFF switch controlled by the relative twist angle between the two BN layers, rather than by the alignment between the graphene and the BN layers.

1. Angular Dependence and Periodicity: The anomalous gating effects are observed when the relative angle between the two BN layers ($\Theta_{BN}$) lies between $15^\circ$ and $45^\circ$. Crucially, the effect does not exhibit the expected $60^\circ$ periodicity associated with BN crystal symmetry, suggesting the mechanism is not tied to simple crystallographic commensurability or moiré superlattice formation with graphene.
2. Three Distinct Regimes: The authors classify the anomalous behavior into three distinct types based on the response to gate voltage sweeps:
   • Type I: Characterized by strong gate asymmetry, regions of top-gate ineffectiveness (where the CNP is pinned), and large hysteresis when the top gate is swept as the fast axis. This regime shows jumps in the CNP position, reminiscent of ferroelectric domain switching.
   • Type II: Exhibits both gate ineffectiveness and hysteresis, but with a different topology. Hysteresis is observed for both top and bottom gates, though with advanced (top gate) and delayed (bottom gate) characteristics. The hysteresis loop appears to be collapsing compared to Type I.
   • Type III: Displays extreme gate asymmetry where the bottom gate behaves normally (no hysteresis, standard gating), while the top gate shows minimal effectiveness and slight hysteresis. This regime has not been previously reported in the literature.
3. Role of Alignment: The alignment between the graphene and the BN layers ($\theta_{bBN-G}$) does not determine the presence of anomalous gating. The effect is governed solely by the relative angle between the top and bottom BN layers.
4. Mechanism Exclusion: The ability to toggle the effect on and off via rotation rules out disorder-induced effects (such as stack faults) as the primary cause. Furthermore, the lack of $60^\circ$ periodicity and the independence from graphene-BN alignment suggest the phenomenon is not driven by moiré-induced electronic band structure modifications or charge localization within a moiré potential.

Significance and Claims
The paper claims to clarify the necessary conditions for reproducing anomalous gating phenomena, identifying the inter-BN layer alignment as the governing parameter. By demonstrating that these effects are not dependent on the graphene-BN moiré superlattice, the authors rule out correlated electron mechanisms associated with such superlattices.

The authors modestly state that while their results exclude several existing hypotheses (disorder, moiré effects), the microscopic origin of the possible ferroelectric state in the BN layer and the specific angular dependence remain to be explained. They propose that the phenomenon likely arises at the interface where the two BN layers touch (potentially outside the graphene Hall bar in static devices) or involves a sliding mechanism, but definitive conclusions on the specific mechanism require further theoretical and experimental investigation. The work provides a controlled platform to distinguish between different theoretical models of ferroelectricity in 2D heterostructures.