Tuning superconductivity in twisted bilayer graphene (1808.07865v2)

Abstract: Materials with flat electronic bands often exhibit exotic quantum phenomena owing to strong correlations. Remarkably, an isolated low-energy flat band can be induced in bilayer graphene by simply rotating the layers to 1.1$^{\circ}$, resulting in the appearance of gate-tunable superconducting and correlated insulating phases. Here, we demonstrate that in addition to the twist angle, the interlayer coupling can also be modified to precisely tune these phases. We establish the capability to induce superconductivity at a twist angle larger than 1.1$^{\circ}$ $-$ in which correlated phases are otherwise absent $-$ by varying the interlayer spacing with hydrostatic pressure. Realizing devices with low disorder additionally reveals new details about the superconducting phase diagram and its relationship to the nearby insulator. Our results demonstrate twisted bilayer graphene to be a uniquely tunable platform for exploring novel correlated states.

• The paper reveals that precise control of hydrostatic pressure and twist angle induces novel superconducting and correlated insulating phases in tBLG.
• The paper employs high-mobility devices and detailed quantum oscillation measurements to show that pressure narrows the band, achieving T₍c₎ above 3 K at ~1.3 GPa.
• The paper highlights that tuning interlayer coupling and minimizing disorder provides practical pathways for engineering quantum materials with tailored electronic properties.

Tuning Superconductivity in Twisted Bilayer Graphene

The paper "Tuning superconductivity in twisted bilayer graphene" explores the phenomena of superconductivity and correlated phases in twisted bilayer graphene (tBLG). The central focus is on the conditions that lead to these states, particularly examining the effects of twist angle and interlayer coupling, with an innovative approach using hydrostatic pressure to modulate these characteristics.

Twisted bilayer graphene (tBLG) has garnered attention due to its tunable electronic properties. By rotating the graphene layers to a specific "magic angle" of approximately 1.1°, the system exhibits strongly correlated electronic phases such as superconductivity and insulating states. In this paper, the authors extend the understanding of these phenomena by demonstrating the impact of interlayer interaction and pressure on the electronic properties of tBLG.

Key Findings

• Superconducting and Correlated Insulating States: The paper presents results from measuring the superconducting and insulating phases in tBLG devices with twist angles near the magic angle. Novel superconducting pockets are observed near half-filling of the electron-doped band, a deviation from prior findings focused on hole-doped bands.
• Pressure as a Tuning Parameter: The paper details experiments on a device with a 1.27° twist angle, showing the ability to induce correlated insulating phases and superconductivity through the application of hydrostatic pressure. This expands the ability to tune electronic correlations in tBLG beyond precise control of the twist angle alone.
• Pressure-Induced Bandwidth Tuning: It is demonstrated that the flat band condition can be achieved at twist angles larger than the conventional magic angle by reducing interlayer spacing via pressure. This insight offers a practical method to enhance the energy scale of correlated states, potentially increasing the superconducting transition temperature ($T_c$) significantly, with a reported $T_c$ reaching above 3 K at ~1.3 GPa.
• Structure and Disorder Considerations: Careful fabrication of high-mobility devices reveals a complex phase diagram where the interplay of superconductivity and correlated insulating states can be finely studied, emphasizing the influence of disorder and mechanical stability on the electronic properties.
• Quantum Oscillations and Fermi Surface Analysis: High homogeneity samples allowed detailed investigation of quantum oscillations, providing insights into the electronic band structure and the intricate role of interactions at various commensurate fillings.

Implications and Future Directions

The results presented hold significant implications for the paper of unconventional superconductors. The ability to modify the superconducting and insulating phases through controllable parameters like pressure and interlayer spacing opens pathways for designing and fabricating devices with desired electronic properties, enhancing the potential for practical applications.

In terms of future research directions, the findings suggest several avenues:

1. Higher Pressure Studies: By continuing to explore the effects of pressure on tBLG, further enhancements in superconducting properties might be realized, providing comprehensive insights into the pressure-induced tuning mechanisms.
2. Homogeneity and Disorder: Refining fabrication methods to minimize inhomogeneity and disorder could provide clearer insights into the intrinsic properties of tBLG, essential for theoretical modeling and understanding of correlated electron systems.
3. Theoretical Modeling: The paper reinforces the need for accurate theoretical models that can account for the experimental phenomena observed, particularly tackling the role of electron-electron interactions and symmetry-breaking mechanisms in the emergence of superconductivity.

In conclusion, this work significantly contributes to the understanding of highly tunable superconducting systems, exemplifying how physical parameters can be leveraged to explore new quantum phenomena in two-dimensional materials. This research not only highlights the versatility of tBLG as a model system for strongly correlated electron systems but also paves the way for innovations in quantum materials engineering.