Superconductors, Orbital Magnets, and Correlated States in Magic Angle Bilayer Graphene (1903.06513v2)

Abstract: Superconductivity often occurs close to broken-symmetry parent states and is especially common in doped magnetic insulators. When twisted close to a magic relative orientation angle near 1 degree, bilayer graphene has flat moire superlattice minibands that have emerged as a rich and highly tunable source of strong correlation physics, notably the appearance of superconductivity close to interaction-induced insulating states. Here we report on the fabrication of bilayer graphene devices with exceptionally uniform twist angles. We show that the reduction in twist angle disorder reveals insulating states at all integer occupancies of the four-fold spin/valley degenerate flat conduction and valence bands, i.e. at moire band filling factors nu = 0, +(-) 1, +(-) 2, +(-) 3, and superconductivity below critical temperatures as high as 3 K close to - 2 filling. We also observe three new superconducting domes at much lower temperatures close to the nu = 0 and nu = +(-) 1 insulating states. Interestingly, at nu = +(-) 1 we find states with non-zero Chern numbers. For nu = - 1 the insulating state exhibits a sharp hysteretic resistance enhancement when a perpendicular magnetic field above 3.6 tesla is applied, consistent with a field driven phase transition. Our study shows that symmetry-broken states, interaction driven insulators, and superconducting domes are common across the entire moire flat bands, including near charge neutrality.

• The paper demonstrates multiple superconducting domes up to 3 K and correlated insulating states at all integer fillings through precise twist angle control.
• The paper reveals orbital magnetism linked to nonzero Chern numbers, with magnetic field–induced hysteretic transitions confirming Chern insulating behavior.
• The paper uncovers complex Landau level patterns and quasiparticle dynamics that indicate exotic symmetry-breaking states beyond conventional band theory.

Analysis of Superconductors, Orbital Magnets, and Correlated States in Magic Angle Bilayer Graphene

This paper presents a detailed investigation into the complex interactions and emergent states exhibited by Magic Angle Bilayer Graphene (MAG). The research explores superconducting behavior, orbital magnetism, and correlated insulating states, elucidating the role of electronic correlations within the flat bands of MAG. The authors have constructed devices with a high degree of twist angle precision, thereby reducing disorder and allowing for the observation of correlated states at all integer fillings, including charge neutrality, and the discovery of several superconducting domes.

Overview

The paper explores the electronic properties of bilayer graphene twisted to specific angles, often referred to as "magic angles." These configurations give rise to flat moiré minibands characterized by low-energy electron states that are conducive to strong correlation phenomena. By minimizing the deviation in twist angle—reported to be less than 0.02º across a span of approximately 10µm—these devices exhibit uniform behavior that surpasses previously reported devices regarding angle homogeneity.

Key Findings

1. Superconductivity and Insulating States:
   • Superconductivity is observed at critical temperatures up to approximately 3 K, with multiple superconducting domes detailed in response to varying electron density, particularly at moiré band filling factors of ν = 0, ±1, ±2, and ±3.
   • The appearance of resistance peaks at all integer fillings suggests the presence of interactions-driven insulating states not explained by single-particle models.
   • A notable observation is the superconducting dome on the lower carrier density side of ν = −2, a dome that had previously been established, extends up to much higher critical temperatures with this improved device.
2. Chern Insulators and Orbital Magnetism:
   • The paper reports orbital magnetism linked to Chern numbers in the insulating states located at ν = ±1, wherein non-zero Chern numbers and associated magneto-resistance change upon the application of a perpendicular magnetic field are documented.
   • Above certain magnetic field thresholds, the resistance demonstrates hysteretic transitions, indicating field-induced phase changes and supporting the presence of Chern insulating behavior.
3. Landau Levels and Quasiparticle Dynamics:
   • The observations include Landau fan patterns signaling broken spin/valley degeneracies over large filling factor intervals. Interesting behavior arises at transitions, such as the crisscross pattern of Landau levels, associated with changes in band degeneracy.
   • The absence of quantum oscillations suggests new quasiparticle formations that potentially indicate exotic symmetry-breaking states not commonly associated with conventional band theory.

Implications and Future Directions

This research provides a deeper understanding of the role of electron-electron interactions in novel quantum states, with implications for technological applications such as quantum computing and advanced materials. The strong correlation effects seen at integer fillings and the associated multiplicity of superconducting states underscore the necessity of understanding these phenomena in the broader context of two-dimensional material systems.

Further exploration is suggested to clarify the precise nature of the superconducting pairing mechanisms and the interactions leading to Chern insulating phases, possibly through focusing on how local strain and external fields modulate the interplay of superconductivity and insulating phases. These insights may prompt strategic enhancements in methodologies aimed at exploring other twist-angle materials with similar strongly-correlated electron phenomena.

In summary, this paper significantly advances our comprehension of MAG's electronic behavior, highlighting the complex interplay of twist angle precision, strong electron correlations, and novel superconducting phases.