Magic-angle graphene superlattices: a new platform for unconventional superconductivity (1803.02342v2)

Abstract: The understanding of strongly-correlated materials, and in particular unconventional superconductors, has puzzled physicists for decades. Such difficulties have stimulated new research paradigms, such as ultra-cold atom lattices for simulating quantum materials. Here we report on the realization of intrinsic unconventional superconductivity in a 2D superlattice created by stacking two graphene sheets with a small twist angle. For angles near $1.1^\circ$, the first `magic' angle, twisted bilayer graphene (TBG) exhibits ultra-flat bands near charge neutrality, which lead to correlated insulating states at half-filling. Upon electrostatic doping away from these correlated insulating states, we observe tunable zero-resistance states with a critical temperature $T_c$ up to 1.7 K. The temperature-density phase diagram shows similarities with that of the cuprates, including superconducting domes. Moreover, quantum oscillations indicate small Fermi surfaces near the correlated insulating phase, in analogy with under-doped cuprates. The relative high $T_c$, given such small Fermi surface (corresponding to a record-low 2D carrier density of $10^{11} \textrm{cm}^{-2}$ , renders TBG among the strongest coupling superconductors, in a regime close to the BCS-BEC crossover. These novel results establish TBG as the first purely carbon-based 2D superconductor and as a highly tunable platform to investigate strongly-correlated phenomena, which could lead to insights into the physics of high-$T_c$ superconductors and quantum spin liquids.

• The paper reveals intrinsic unconventional superconductivity in twisted bilayer graphene when rotated to a magic angle of ~1.1°.
• The research employs precise stacking and electrostatic doping to produce superconducting domes with a critical temperature up to 1.7 K.
• The study highlights low carrier densities and quantum oscillations that mimic Fermi surface features observed in under-doped cuprates.

Magic-angle Graphene Superlattices: A Platform for Unconventional Superconductivity

The potential of twisted bilayer graphene (TBG) to serve as a groundbreaking platform in condensed matter physics has been detailed in the paper "Magic-angle graphene superlattices: a new platform for unconventional superconductivity." This paper investigates the occurrence of intrinsic unconventional superconductivity within a 2D lattice generated by the rotation of two graphene sheets to specific angles, notably the 'magic' angle of approximately 1.1 degrees. Within this unique configuration, known as Magic-Angle Twisted Bilayer Graphene (MA-TBG), ultra-flat conductive bands emerge, facilitating correlated electronic states with intriguing properties similar to those found in high-temperature superconductors.

Experimental Findings

Through precise rotation and stacking of graphene layers, the authors observe superconductivity associated with zero-resistance states that can be modulated by electrostatic doping. The material shows a critical superconducting temperature ($T_c$) of up to 1.7 Kelvin. The experimental paper highlights:

1. Phase Diagram Features: The temperature-density phase diagram of MA-TBG reveals the presence of superconducting domes, akin to those observed in cuprates, complemented by correlated insulating states. Notably, these states are prominent at charge densities as low as $1\times10^{11} \text{ cm}^{-2}$, marking a significant departure from conventional 2D superconductors.
2. Quantum Oscillations: Quantum oscillations near the correlated insulating phase reveal small Fermi surfaces reminiscent of those in under-doped cuprates, suggesting unique electronic properties.
3. Carrier Density: The research documents an unprecedentedly low carrier density that supports superconductivity, emphasizing the strong-coupling nature of the regime, which aligns closely with the BCS-BEC crossover.

Implications and Speculations

This research positions MA-TBG as a highly tunable and versatile platform for exploring phenomena pertinent to strongly correlated electron systems. The findings have substantial implications for the paper of superconductivity in two-dimensional materials, offering pathways to simulate and understand high-$T_c$ superconductors and quantum spin liquids.

The results demonstrate that MA-TBG can potentially be manipulated through varied parameters such as twist angle and gate-induced electric fields, paving the way for enhanced understanding and possibly optimized superconducting properties. Additionally, the unique triangular symmetry inherent within the graphene superlattice prompts discussions on the emergent electronic behavior and supports diverse and novel pairing symmetries, which could differ fundamentally from known systems like cuprates.

Future Developments

Future investigations might delve further into the nature of superconducting pairing within MA-TBG, utilizing advanced experimental setups like tunneling and Josephson junctions to explore unconventional superconductivity traits. Considerations of the lattice's triangular symmetry and its impact on quantum states could drive theoretical insights and highlight critical mechanisms underlying unconventional superconductivity.

In conclusion, this paper not only marks a significant step in understanding graphene's potential in condensed matter physics but also opens a corridor for further research into strongly correlated electron phenomena in adjustable two-dimensional platforms, strengthening the bridge to novel quantum technologies.