Signatures of Gate-Tunable Superconductivity in Trilayer Graphene/Boron Nitride Moiré Superlattice

Abstract: Understanding the mechanism of high temperature (high Tc) superconductivity is a central problem in condensed matter physics. It is often speculated that high Tc superconductivity arises from a doped Mott insulator as described by the Hubbard model. An exact solution of the Hubbard model, however, is extremely challenging due to the strong electron-electron correlation. Therefore, it is highly desirable to experimentally study a model Hubbard system in which the unconventional superconductivity can be continuously tuned by varying the Hubbard parameters. Here we report signatures of tunable superconductivity in ABC-trilayer graphene (TLG) / boron nitride (hBN) moir\'e superlattice. Unlike "magic angle" twisted bilayer graphene, theoretical calculations show that under a vertical displacement field the ABC-TLG/hBN heterostructure features an isolated flat valence miniband associated with a Hubbard model on a triangular superlattice. Upon applying such a displacement field we find experimentally that the ABC-TLG/hBN superlattice displays Mott insulating states below 20 Kelvin at 1/4 and 1/2 fillings, corresponding to 1 and 2 holes per unit cell, respectively. Upon further cooling, signatures of superconducting domes emerge below 1 kelvin for the electron- and hole-doped sides of the 1/4 filling Mott state. The electronic behavior in the TLG/hBN superlattice is expected to depend sensitively on the interplay between the electron-electron interaction and the miniband bandwidth, which can be tuned continuously with the displacement field D. By simply varying the D field, we demonstrate transitions from the candidate superconductor to Mott insulator and metallic phases. Our study shows that TLG/hBN heterostructures offer an attractive model system to explore rich correlated behavior emerging in the tunable triangular Hubbard model.

• The paper reveals gate-tunable superconductivity in a TLG/hBN moiré system by observing superconducting domes emerging near a 1/4 filling Mott state.
• It employs a dual-gated heterostructure to control carrier concentration and miniband bandwidth, achieving superconducting phases below 1 K with a critical current of ~10 nA.
• The findings highlight the potential of graphene-based moiré superlattices for exploring correlated electron phenomena and validating theoretical models like the Hubbard model.

Superconductivity and Correlated States in Trilayer Graphene/HBN Moiré Superlattice

In their study, Guorui Chen et al. explore the capacity of the ABC-stacked trilayer graphene (TLG) and hexagonal boron nitride (hBN) moiré superlattice as a tunable model system for investigating unconventional superconductivity and correlated electron behavior. This research is situated within the broader inquiry concerning high-Tc superconductivity whose mechanics often involve strongly correlated electron systems, as theories like the Hubbard model suggest. However, empirical investigations are essential due to the difficulty in solving these models exactly, and the TLG/hBN setup serves as an attractive experimental analog with tunable parameters.

Experimental Framework and Findings

The researchers utilized a dual-gated TLG/hBN heterostructure to manipulate the electronic properties through an externally applied vertical displacement field (D field). This setup allows precise control over carrier concentration and miniband bandwidth, effectively tuning transitions between different electronic phases.

Their investigations identify Mott insulating states below 20 K at 1/4 and 1/2 fillings of the moiré miniband, displaying substantial electron-hole asymmetry. Beyond critical cooling to below 1 K, the authors report the emergence of superconductivity near the 1/4 filling Mott state, marked by superconducting domes on both electron- and hole-doped sides. Their analysis was substantiated through resistance measurements under varying conditions of temperature, gate voltage, and magnetic fields. Notably, strong superconducting signatures were evident from substantial reductions in resistance and from measurements of critical current.

The study's results are quantitatively robust; for example, the critical current was measured at approximately 10 nA, and the superconducting phase was found to disappear under a perpendicular magnetic field of about 0.7 T as well as an in-plane magnetic field approaching 1 T. These findings emphasize a complex but coherent pattern of phase transitions driven by the displacement field, transitioning from metallic to insulating and superconducting phases as the field is varied.

Implications and Future Directions

This study contributes significant insights into the tunability and versatility of TLG/hBN moiré superlattices as platforms for exploring correlated quantum phenomena. The clear observation of Mott insulating and superconducting transitions facilitated by an adjustable D field highlights the potential of these materials in the field of condensed matter physics, particularly for testing the validity of theoretical models like the Hubbard model on triangular lattices. The capability of inducing different states—metallic, superconducting, and insulating—alongside their sensitivity to external field tuning, opens up avenues for further investigation into exotic phases such as spin liquids, tunable Chern bands, and topological superconductivity.

The conclusive establishment of these phenomena provides a promising trajectory for future research in quantum materials. Of particular interest would be the detailed exploration into the nature of the superconducting phase at different dopings and field conditions, and the examination of potential topological characteristics of the resulting electronic states in varying environmental parameters. This work not only underscores the utility of graphene-based systems in fundamental research but also directs attention towards practical applications where tunable superconductivity could be leveraged for new technologies.