Correlated states in twisted double bilayer graphene (1903.06952v2)

Abstract: Electron-electron interactions play an important role in graphene and related systems and can induce exotic quantum states, especially in a stacked bilayer with a small twist angle. For bilayer graphene where the two layers are twisted by a "magic angle", flat band and strong many-body effects lead to correlated insulating states and superconductivity. In contrast to monolayer graphene, the band structure of untwisted bilayer graphene can be further tuned by a displacement field, providing an extra degree of freedom to control the flat band that should appear when two bilayers are stacked on top of each other. Here, we report the discovery and characterization of such displacement-field tunable electronic phases in twisted double bilayer graphene. We observe insulating states at a half-filled conduction band in an intermediate range of displacement fields. Furthermore, the resistance gap in the correlated insulator increases with respect to the in-plane magnetic fields and we find that the g factor according to spin Zeeman effect is ~2, indicating spin polarization at half filling. These results establish the twisted double bilayer graphene as an easily tunable platform for exploring quantum many-body states.

• The paper reveals that displacement fields can tune correlated insulating states in TDBG, showing a non-monotonic response near half-filling.
• The paper employs dual-gated TDBG devices and spin Zeeman effect measurements to identify a maximum gap of ~3.2 meV at a 1.33° twist angle.
• The paper’s findings pave the way for probing quantum many-body phases, including potential unconventional superconductivity and spin-polarized ground states.

Correlated Insulating States in Twisted Double Bilayer Graphene

The paper explores the electronic properties of twisted double bilayer graphene (TDBG), focusing on displacement-field-tunable correlated insulating states. This paper elaborates on the interaction-induced phenomena in TDBG, which resemble those seen in magic-angle twisted bilayer graphene (TBG), with a particular emphasis on the tunability offered by an external displacement field. The work presents significant experimental findings, corroborated by theoretical models, and highlights new avenues for investigating quantum many-body phenomena.

In TDBG, when two bilayer graphene systems are stacked with a small twist angle, their electronic structure displays flat bands crucial for enhancing electronic correlations. This phenomenon primarily occurs around the magic angle (θ≈1.1°). However, the precise control of θ presents challenges for device fabrication. Here, the authors overcome these limitations by utilizing displacement fields as an alternative means of modulating electronic phases, resulting in the observation of correlated insulating states at half-filling of the conduction band. These states manifest within a specific range of displacement fields |D|/ε₀ = 0.2V/nm to 0.6V/nm, indicating a non-monotonic relationship with D.

Key experimental tools included dual-gated TDBG devices with twist angles varying from 0.98° to 1.33°, allowing independent tuning of the carrier density and displacement field. The insulating states observed herein exhibited increased resistance when subjected to parallel magnetic fields. Notably, the g-factor inferred from spin Zeeman effect experiments was approximately 2, suggesting spin polarization at half filling. The thermally activated transport gap, typically indicative of insulating behavior, was highly dependent on both the displacement field and the twist angle. In particular, a twist angle of 1.33° yielded a maximum thermally activated gap of ~3.2 meV, observable up to temperatures above liquid helium temperature.

Among the specialized findings, the paper identified an experimental signature of a spin-polarized ground state at half filling, distinct from prior observations in TBG, facilitated by the tunability of electronic states via layer number in this twisted system. Moreover, Landau levels emanating from the half-filling correlated state showed unusual odd-number sequences and intriguing Hofstadter butterfly patterns, implying complex underlying electronic interactions influenced by displacement fields.

The implications of these discoveries suggest that TDBG could serve as a versatile platform for probing unconventional superconductivity, ferromagnetic Mott insulating behavior, Chern bands, and potentially spin-triplet topological superconductivity. As a result, TDBG provides a fertile ground for exploring new quantum phases and paves the way for designing novel materials with tunable electronic properties through simple electrical control mechanisms.

Future research could investigate the precise nature of the spin-polarized ground state and explore the impact of different displacement fields on the band structure in TDBG. Additionally, there is potential for exploring the role of disorder and inhomogeneity in both the twist angle and electronic behavior. This paper forms an essential foundation for ongoing theoretical and experimental endeavors striving to unravel the complexities of correlated states in two-dimensional materials.