[論文レビュー] Spin-polarized Correlated Insulator and Superconductor in Twisted Double Bilayer Graphene

Xiaomeng Liu, Zeyu Hao+6 more|arXiv (Cornell University)|Mar 19, 2019

Graphene research and applications被引用数 39

ひとこと要約

本論文は twist ed double bilayer graphene (TDBG) における displacement field および密度で調整可能な spin-polarized correlated insulating states および spin-polarized superconductivity を示し、平面内磁場による超導性の増強を含む。結果は moiré flat-band 系における spin-polarized pairing および ferromagnetic correlations の可能性を示唆する。

ABSTRACT

Ferromagnetism and superconductivity typically compete with each other since the internal magnetic field generated in a magnet suppresses the formation of spin-singlet Cooper pairs in conventional superconductors. Only a handful of ferromagnetic superconductors are known in heavy fermion systems, where many-body electron interactions promoted by the narrow energy bands play a key role in stabilizing these emergent states. Recently, interaction-driven superconductivity and ferromagnetism have been demonstrated as separate phenomena in different density regimes of flat bands enabled by graphene moire superlattices. Combining superconductivity and magnetism in a single ground state may lead to more exotic quantum phases. Here, employing van der Waals heterostructures of twisted double bilayer graphene (TDBG), we realize a flat electron band that is tunable by perpendicular electric fields. Similar to the magic angle twisted bilayer graphene, TDBG exhibits energy gaps at the half and quarter filled flat bands, indicating the emergence of correlated insulating states. We find that the gaps of these insulating states increase with in-plane magnetic field, suggesting a ferromagnetic order. Upon doping the ferromagnetic half-filled insulator, superconductivity emerges with a critical temperature controlled by both density and electric fields. We observe that the in-plane magnetic field enhances the superconductivity in the low field regime, suggesting spin-polarized electron pairing. Spin-polarized superconducting states discovered in TDBG provide a new route to engineering interaction-driven topological superconductivity.

研究の動機と目的

• Demonstrate electric-field tunable flat bands in twisted double bilayer graphene (TDBG) and identify correlated insulating states at half- and quarter-filling.
• Show that half-filled insulating gaps grow with in-plane magnetic field, indicating ferromagnetic order.
• Discover superconductivity upon doping the ferromagnetic insulator and map its dependence on density and displacement field.
• Characterize spin-polarized nature of the superconducting state and its response to parallel magnetic fields.
• Discuss implications for interaction-driven topological superconductivity in moiré systems.

提案手法

• Fabricate TDBG devices with twist angles around 1.24°–1.33° using a dry transfer method.
• Control electron density n and displacement field D with top and bottom graphite gates.
• Measure four-terminal resistivity ρ as a function of V_TG, V_BG, T, and magnetic fields (B_⊥, B_∥).
• Identify insulating gaps Δ_n_s/2 and Δ_n_s/(4) via Arrhenius activation behavior of ρ(T).
• Determine superconducting regions by observing a dome of low ρ around half-filled insulators and extract BKT characteristics from I–V curves.
• Analyze Zeeman-field dependence (g ≈ 2) of gap sizes and infer spin polarization of bands.

実験結果

リサーチクエスチョン

• RQ1 Can electric-field tuning of TDBG induce isolated flat bands with correlated insulating states at specific fillings?
• RQ2 Do half-filled insulating states in TDBG exhibit ferromagnetic spin polarization signaled by their response to in-plane magnetic fields?
• RQ3 Is superconductivity achievable by doping a ferromagnetic insulator in TDBG, and is it spin-polarized?
• RQ4 How does the in-plane magnetic field influence the superconducting transition temperature and critical behavior (e.g., BKT) in TDBG?

主な発見

• A flat, tunable conduction band arises in TDBG under displacement field, yielding correlated insulating states at half- and quarter-fillings.
• The half-filled insulator gap Δ_n_s/2 increases with in-plane magnetic field, consistent with spin-polarized (ferromagnetic) order.
• Doping the half-filled ferromagnetic insulator leads to superconductivity, with a dome-shaped dependence on density and displacement field.
• Superconductivity shows enhancement at low in-plane fields and exhibits a BKT transition, suggesting spin-polarized (likely triplet) pairing.
• The superconducting region is strongest near a density n_m and displacement field D_m, with T_BKT and critical field B_∥^c indicating unconventional spin-polarized pairing behavior.