Correlated Insulator Behaviour at Half-Filling in Magic Angle Graphene Superlattices (1802.00553v1)

Abstract: Van der Waals (vdW) heterostructures are an emergent class of metamaterials comprised of vertically stacked two-dimensional (2D) building blocks, which provide us with a vast tool set to engineer their properties on top of the already rich tunability of 2D materials. One of the knobs, the twist angle between different layers, plays a crucial role in the ultimate electronic properties of a vdW heterostructure and does not have a direct analog in other systems such as MBE-grown semiconductor heterostructures. For small twist angles, the moir\'e pattern produced by the lattice misorientation creates a long-range modulation. So far, the study of the effect of twist angles in vdW heterostructures has been mostly concentrated in graphene/hexagonal boron nitride (h-BN) twisted structures, which exhibit relatively weak interlayer interaction due to the presence of a large bandgap in h-BN. Here we show that when two graphene sheets are twisted by an angle close to the theoretically predicted 'magic angle', the resulting flat band structure near charge neutrality gives rise to a strongly-correlated electronic system. These flat bands exhibit half-filling insulating phases at zero magnetic field, which we show to be a Mott-like insulator arising from electrons localized in the moir\'e superlattice. These unique properties of magic-angle twisted bilayer graphene (TwBLG) open up a new playground for exotic many-body quantum phases in a 2D platform made of pure carbon and without magnetic field. The easy accessibility of the flat bands, the electrical tunability, and the bandwidth tunability though twist angle may pave the way towards more exotic correlated systems, such as unconventional superconductors or quantum spin liquids.

• The paper demonstrates how magic angle graphene superlattices exhibit half-filling insulating behavior driven by electron correlations.
• It employs advanced fabrication techniques such as tear and stack and careful h-BN encapsulation to achieve precise twist angles enabling reliable transport measurements.
• Transport analysis via quantum oscillations and Arrhenius fits reveals temperature-dependent activation gaps and magnetic suppression of insulator phases.

An Expert Review of Techniques and Transport Phenomena in Twisted Bilayer Graphene

Twisted bilayer graphene (TwBLG) has gained significant interest due to its unique electronic properties arising from the relative twisting of its two layers. This paper explores the manufacturing and transport measurement processes necessary to explore these properties in devices with various twist angles, including angles below the so-called "magic" angle of approximately 1.1 degrees. The research leverages extensive fabrication techniques and measurements to provide new insights into the electronic behavior of TwBLG.

Fabrication Techniques and Measurement Setup

The fabrication process involves exfoliating monolayer graphene and hexagonal boron nitride (h-BN) onto SiO\textsubscript{2}/Si chips. These materials are manipulated using a technique referred to as 'tear and stack' to achieve different twist angles. Various devices named D1, D2, D3, and D4 are constructed with this method, allowing for the examination of how slight differences in angle affect electronic properties. An important aspect involves using h-BN as encapsulation, creating a clean interface crucial for reliable electronic characteristics. Transport measurements, essential for understanding the electronic behavior of TwBLG, are conducted using lock-in techniques across a broad temperature range.

Analysis of Transport Phenomena

In the paper of device D1, variances in conductance through different temperature regimes reveal metal-like behavior, interrupted by insulating superlattice gaps. These behaviors align with theoretical expectations, cementing the role of the superlattice gaps and narrow bandwidth in TwBLG. Thermal activation gaps of insulating states are fitted using the Arrhenius formula, providing precise activation energies for states at different carrier densities.

Magnetotransport measurements on devices D1 and D3 unveil the manifestation of quantum oscillations typically associated with high-mobility systems. Landau level fan diagrams illustrate a distinctive sequence of filling factors in TwBLG, distinct from monolayer graphene, suggesting underlying factors associated with TwBLG’s unique band structure. The paper also identifies half-filling insulating phases that dissipate under increased magnetic fields, further reinforcing the significant impact of correlation effects in these materials.

Speculative Insights and Future Directions

Encounters with half-filling insulating phases (HFIPs) in device D3 highlight the possible influence of electron-electron interactions paired with reduced bandwidth. Observations further suggest that the suppression of HFIPs in a magnetic field arises from the Zeeman interaction on electron spins, rather than orbital effects, opening discussions around spin-related transport phenomena in TwBLG.

Measurements conducted in a Hall configuration reveal a reset of the Hall density at half-filling states, verifying new quasiparticles' behavior in the material. This discovery prompts further exploration into the particle-hole symmetry-breaking effects in TwBLG and their role in exotic electronic states.

Practical Implications and Theoretical Considerations

From a practical standpoint, the careful determination of twist angles remains crucial for tailoring the physical properties of TwBLG devices, thus impacting electronic, photonic, and possibly quantum computing applications. The capacitance and transport data yield a reduced Fermi velocity in these structures, elucidating the profound geometric influences on electronic characteristics within the flat-band regime.

Theoretically, the paper highlights how data, particularly regarding quantum oscillations and Hall effects, could continue to shed light on the behavior of composite fermionic states in these heterostructures. Precise Fermi velocity and twist angle data can refine models depicting advanced correlated phenomena, such as superconductivity and Hubbard-like behavior in low-angle TwBLG.

Conclusion

This examination of TwBLG devices from fabrication to transport phenomena provides substantial contributions to our understanding of materials governed by complex electron correlations and topologically nontrivial electronic bands. Future inquiries might navigate the potential applicability of these findings in quantum computing, or explore the untapped technological potentials arising from manipulating the interlayer physics and strain within TwBLG systems.