Strongly Correlated Chern Insulators in Magic-Angle Twisted Bilayer Graphene (2007.03810v1)

Abstract: Interactions among electrons and the topology of their energy bands can create novel quantum phases of matter. Most topological electronic phases appear in systems with weak electron-electron interactions. The instances where topological phases emerge only as a result of strong interactions are rare, and mostly limited to those realized in the presence of intense magnetic fields. The discovery of flat electronic bands with topological character in magic-angle twisted bilayer graphene (MATBG) has created a unique opportunity to search for new strongly correlated topological phases. Here we introduce a novel local spectroscopic technique using a scanning tunneling microscope (STM) to detect a sequence of topological insulators in MATBG with Chern numbers C = $\pm$ 1, $\pm$ 2, $\pm$ 3, which form near $\nu$ = $\pm$ 3, $\pm$ 2, $\pm$ 1 electrons per moir\'e unit cell respectively, and are stabilized by the application of modest magnetic fields. One of the phases detected here (C = +1) has been previously observed when the sublattice symmetry of MATBG was intentionally broken by hexagonal boron nitride (hBN) substrates, with interactions playing a secondary role. We demonstrate that strong electron-electron interactions alone can produce not only the previously observed phase, but also new and unexpected Chern insulating phases in MATBG. The full sequence of phases we observed can be understood by postulating that strong correlations favor breaking time-reversal symmetry to form Chern insulators that are stabilized by weak magnetic fields. Our findings illustrate that many-body correlations can create topological phases in moir\'e systems beyond those anticipated from weakly interacting models.

• The paper reveals that strong electron interactions in MATBG induce topological Chern insulating phases characterized by multiple high Chern numbers.
• The methodology combines DT-STS with low-temperature magnetic field experiments to precisely map quantum phase transitions and spectral gaps.
• The findings challenge weak interaction models and suggest new avenues for engineering correlated quantum states in two-dimensional materials.

Analyzing Strongly Correlated Chern Insulators in Magic-Angle Twisted Bilayer Graphene

This paper presents an intricate experimental and theoretical investigation into the emergence of strongly correlated Chern insulators in magic-angle twisted bilayer graphene (MATBG). By employing a novel density-tuned scanning tunneling spectroscopy (DT-STS) technique, the authors identify and characterize topological insulating phases in MATBG. This research underscores the significant influence of strong electron-electron interactions in producing these novel quantum phases, particularly in the absence of significant symmetry-breaking substrates.

Overview and Key Findings

The authors utilize DT-STS in conjunction with a perpendicular magnetic field to investigate MATBG at millikelvin temperatures. Their methodology allows them to detect a sequence of topological insulators with Chern numbers $C = \pm 1, \pm 2, \pm 3$ corresponding to different electron densities per moiré unit cell. This sequence is indicative of the presence of many-body correlations inducing these topological phases. The authors hypothesize that strong correlations lead to time-reversal symmetry breaking, forming Chern insulators that are relatively stable even under weak magnetic fields.

One significant conclusion from their findings is that strong interactions alone can produce not only previously observed phases, such as $C = +1$ when an external hBN substrate is not disrupting the sublattice symmetry, but also new Chern insulating phases. These observations challenge theories based solely on weakly interacting models and suggest that interactions can introduce a Haldane mass term that breaks time-reversal symmetry (T-symmetry), producing Chern bands necessary for explaining the observed phases.

Spectroscopic data shows a prominent set of gaps at the Fermi level, shifting systematically with the magnetic field, which underlines their topological nature. The spectroscopic measurements further indicate a deviation from the zero magnetic field condition leading to domain formation, complicating the identity of topologically trivial insulators.

Theoretical Implications

From a theoretical standpoint, the results deviate from traditional weakly interacting models and highlight the role of strong electron-electron interactions in moiré flat-band systems. The ability of interactions to induce a sign-switching Haldane mass term across the charge neutrality point is particularly intriguing. This suggests a new mechanism for generating high Chern numbers without relying on extrinsic symmetry-breaking fields.

This model provides a plausible explanation for the hierarchy of insulating phases observed in experimental data, where the sequence of states changes with carrier density and magnetic field. The theoretical formulation aligns with the observed spectroscopic asymmetry in the Zeroth Landau Levels (ZLLs), which are displaced depending on the charge state of the graphene system.

Practical Implications and Future Work

Practically, the work suggests that MATBG, even in the absence of symmetry-breaking substrates, can serve as a rich platform for exploring correlated topological phases. The research indicates potential engineering of new materials and devices capable of harnessing these phases for technological applications. The paper also prompts the exploration of fractional Chern insulators in systems with similar characteristics.

Future directions for research may include investigating alternative two-dimensional materials using the DT-STS technique. The experimental setup's adaptability for detecting topological responses at differing densities and magnetic fields makes it a promising tool for further advancements in the paper of moiré systems and beyond.

In conclusion, the paper presents vital experimental and theoretical insights into the emergence of strongly correlated Chern insulators in MATBG, emphasizing the critical role of electron-electron interactions in these processes. The findings demonstrate the potential for discovering new quantum materials with tailored properties, expanding the exploration frontier of topological and correlated phases in condensed matter physics.