Ferroelectric Quantum Point Contact in Twisted Transition Metal Dichalcogenides

In twisted transition metal dichalcogenides (tTMDs), atomic reconstruction gives rise to moiré domains with alternating ferroelectric polarization, whose domain size and overall electric dipole moment are tunable by an out-of-plane electric field. Previous transport measurements in Hall bar devices have successfully demonstrated the overall ferroelectric behavior of tTMDs from a collective ensemble of ferroelectric moiré domains. To locally probe a single ferroelectric moiré domain, we fabricate and study mesoscopic quantum transport via a gate-defined twisted molybdenum disulfide (tMoS2) quantum point contact (QPC). The local property of a single moiré domain is invulnerable to long-range disorder and twist-angle inhomogeneity, resulting in an unusually long conductance plateau with large electrical hysteresis. The comparison between local and global measurements confirms that antiferroelectricity can emerge from alternating polarization of individual ferroelectric domains. Using a QPC as a single charge sensor, we characterize the nature and time scale of different domain evolution mechanisms with single atomic dipole resolution. Our findings shed new light on the microscopic ferroelectric behavior and dynamics within a single tTMD moiré domain, paving the way toward more advanced ferroelectric quantum devices with tunable local Hamiltonian, such as ferroelectric tTMD quantum dots (QDs).

💡 Research Summary

In twisted transition‑metal dichalcogenides (tTMDs) atomic reconstruction creates a moiré superlattice composed of alternating AB and BA stacking domains, each possessing a spontaneous out‑of‑plane electric polarization of opposite sign. An external perpendicular electric field (E₀) expands domains whose polarization aligns with the field and shrinks the opposite ones, giving rise to a hysteretic ferroelectric response that has been demonstrated previously only in global Hall‑bar measurements. Because such macroscopic measurements average over many domains with varying size and twist‑angle disorder, they cannot reveal the intrinsic behavior of a single moiré domain.

The authors address this limitation by fabricating a gate‑defined quantum point contact (QPC) in twisted bilayer MoS₂ (tMoS₂). The heterostructure consists of hexagonal‑BN encapsulation, few‑layer graphite contacts, two slightly twisted MoS₂ flakes (<1°), and a set of locally patterned bottom gates. By applying a negative voltage to a pair of bottom gates (V_QPC) they deplete carriers beneath the gates and create a one‑dimensional constriction whose width can be tuned from ~100 nm down to the moiré lattice constant. A global silicon back gate (V_g) simultaneously controls the out‑of‑plane electric field (hence the domain polarization) and the overall carrier density in the QPC channel.

Transport measurements at 4 K reveal several striking features. When V_QPC is fixed at –7 V and V_g is swept, the four‑probe conductance shows 2–3 plateaus, but the first plateau is unusually wide and the conductance‑versus‑gate curve exhibits a large hysteresis of ΔV_g ≈ 120 V. The authors explain this by recognizing that electrons in the QPC experience two electric fields: (i) the external field E₀ proportional to V_g, and (ii) an internal “molecular” field Ω proportional to the local polarization P of the moiré domain adjacent to the QPC. Ω is itself hysteretic because P(E₀) follows a ferroelectric loop. Consequently the effective field felt by the QPC, E_eff = E₀ + Ω, inherits the hysteresis of Ω. By defining χ = ∂Ω/∂E₀, they show that when χ≈1 the carrier density in the QPC responds normally to V_g, producing ordinary plateaus; when χ≈0 the response is suppressed, so the conductance remains on the first plateau over a broad V_g range. Larger V_g scan windows increase the hysteresis width, indicating that the accessible electric field does not fully saturate the domain polarization.

To probe the dynamics of the domain, the QPC is parked at the steep half‑conductance point of the first plateau and V_g is held constant. The conductance then decays exponentially over several hours, with a characteristic time τ ≈ 1.2 h, consistent with a relaxation of an ensemble of weakly interacting dipoles toward equilibrium. Superimposed on this slow decay are telegraph‑like switches of ~0.02 µS, an order of magnitude above the noise floor, which the authors attribute to single‑dipole flips at the domain boundary near the QPC. The switching rate of individual dipoles is on the order of one minute, demonstrating that the QPC functions as a single‑dipole charge sensor.

For comparison, the authors perform global transport on the same flakes by simultaneously sweeping V_g and V_QPC with matched capacitive coupling, thereby eliminating the constriction. The resulting hysteresis loops are much smaller (≈ 60 V) and are displaced entirely to either positive or negative V_g, never crossing zero. This behavior is interpreted as an emergent antiferroelectric response of the ensemble of alternating domains: at low carrier density each domain hosts a single electron dipole, leading to equal and opposite polarizations that cancel globally; at higher carrier density the unequal domain sizes produce an imbalance, restoring a conventional ferroelectric loop in one polarity.

In summary, the work provides (1) the first local probe of a single moiré ferroelectric domain using a QPC, (2) a quantitative model linking the hysteretic internal molecular field to the observed conductance plateaus, (3) direct observation of single‑dipole switching with minute‑scale dynamics, and (4) a clear distinction between single‑domain ferroelectric and multi‑domain antiferroelectric behavior. These insights open pathways toward tTMD‑based quantum devices such as ferroelectric quantum dots, electrically tunable qubits, and platforms where local Hamiltonians can be engineered by the controllable ferroelectric landscape.