Tunable correlation-driven symmetry breaking in twisted double bilayer graphene

Abstract: A variety of correlated phases have recently emerged in select twisted van der Waals (vdW) heterostructures owing to their flat electronic dispersions. In particular, heterostructures of twisted double bilayer graphene (tDBG) manifest electric field-tunable correlated insulating (CI) states at all quarter fillings of the conduction band, accompanied by nearby states featuring signatures suggestive of superconductivity. Here, we report electrical transport measurements of tDBG in which we elucidate the fundamental role of spontaneous symmetry breaking within its correlated phase diagram. We observe abrupt resistivity drops upon lowering the temperature in the correlated metallic phases neighboring the CI states, along with associated nonlinear $I$-$V$ characteristics. Despite qualitative similarities to superconductivity, concomitant reversals in the sign of the Hall coefficient instead point to spontaneous symmetry breaking as the origin of the abrupt resistivity drops, while Joule heating appears to underlie the nonlinear transport. Our results suggest that similar mechanisms are likely relevant across a broader class of semiconducting flat band vdW heterostructures.

Tunable Correlation-Driven Symmetry Breaking in Twisted Double Bilayer Graphene

The paper presents an in-depth exploration and experimental analysis of twisted double bilayer graphene (tDBG) devices, investigating the tunable correlation-driven symmetry breaking observed within their phase diagrams. The focus lies primarily on exploring the resistivity and Hall effect in tDBG's electronic transport properties, highlighting the importance of spontaneous symmetry breaking in correlated electron systems.

Overview

Twisted van der Waals (vdW) heterostructures, particularly those with flat electronic bands, have revealed a plethora of correlated phases resulting from their small bandwidths and enhanced electron interactions at low temperatures. tDBG, composed of two Bernal-stacked bilayer graphene sheets rotated by a small angle, serves as a model platform for studying correlated semiconducting bands within moiré superlattices. Crucially, tDBG reveals tunable correlated insulating states (CI states) at quarter fillings of the conduction band, with phase boundaries marked by sharp resistivity drops and notable nonlinear $I$-$V$ characteristics as temperature decreases, suggesting unique symmetry-breaking behaviors.

Key Findings

• Temperature-Dependent Resistivity: The paper identifies abrupt resistivity drops in tDBG’s metallic phases neighboring the CI states when lowering the temperature, alongside nonlinear electrical responses under applied bias. Therein, coupled reversals of the Hall coefficient suggest spontaneous symmetry breaking rather than superconductivity as the primary origin of these behavior patterns.
• Band Structure and Density of States (DOS): Calculations show a semimetal-to-semiconductor transition at the charge neutrality point (CNP) and band flattening with increased displacement fields. The distinct transport features observed correlate well with theoretical predictions, verifying a bifurcation in van Hove singularity in the valence band as the displacement field increases.
• Correlated Phase Diagram: Across devices with twist angles ranging from 1.17$^\circ$ to 1.53$^\circ$, consistent qualitative transport traits were noted. These included semimetal-transitions, CI states stabilizing over displacement field ranges, halo-like resistive features proximate to CI states, and symmetry-breaking behavior influencing the electronic landscape.
• Hall Effect: Measurements reveal rapid sign changes in Hall coefficients, coinciding closely with boundaries of resistive halos, potentially denoting symmetry-broken bands. Sign reversals within these regions are suggestive of ordering transitions and band-restructuring instigated by electron correlations.

Implications and Future Directions

The study bolsters understanding of correlated electron behavior in flat-band vdW heterostructures, positing that spontaneous symmetry breaking within tDBG may influence broader classes of semiconducting moiré systems, including twisted WSe$_2$ and ABC trilayer graphene. By enhancing control over electron interactions through electric fields and twist angles, similar symmetry-breaking phenomena can be elicited and studied to unravel potential paths for novel electronic states and device functionalities.

The results underscore the need for further theoretical analysis to characterize potential electron-phonon coupling changes, inelastic scattering moderation, and magnetic ordering to elucidate the precise nature of resistivity deviations and band modifications. Additionally, a deeper exploration of the subtleties of magnetically-ordered ground states sans anomalous Hall effects may provide vital insights for both fundamental physics and emerging technologies in quantum electronics.

Conclusion

This paper contributes significantly to the detailed understanding of the correlated phase behavior in tDBG, opening gateways to manipulating electronic properties in tunable vdW heterostructures with potential applications in quantum computing and advanced material sciences. The implications of symmetry-breaking induce interesting possibilities for reconciling superconductivity-like behavior without reaching zero resistivity, posing critical questions around the dynamics of reduced scattering and electronic orderings at quantum phase transitions.