Superconductivity in 5.0° Twisted Bilayer WSe2
Background Introduction
In recent years, the discovery of superconductivity in twisted bilayer and trilayer graphene has sparked widespread interest. The key feature of these systems lies in the interplay between interlayer coupling and moiré superlattices, which gives rise to low-energy flat bands with strong correlations. Similar flat bands can also be induced by moiré patterns in lattice-mismatched or twisted heterostructures of other two-dimensional materials, such as transition metal dichalcogenides (TMDs). Although a variety of correlated phenomena have been observed in moiré TMDs, robust experimental evidence of superconductivity has remained elusive. This paper reports the observation of superconductivity in 5.0° twisted bilayer WSe₂, with a maximum critical temperature of 426 mK. This discovery demonstrates that moiré flat-band superconductivity is not limited to graphene structures, and the intrinsic properties of TMDs (such as bandgap, strong spin-orbit coupling, spin-valley locking, and magnetism) provide the possibility of exploring a broader superconducting parameter space.
Source of the Paper
This paper was co-authored by Yinjie Guo, Jordan Pack, Joshua Swann, and others, with contributors from multiple institutions including Columbia University, the University of Tennessee, and the National Institute for Materials Science in Japan. The paper was published in Nature on January 23, 2025, under the title “Superconductivity in 5.0° twisted bilayer WSe₂.”
Research Process and Results
1. Device Fabrication and Experimental Design
The research team first fabricated a 5.0° twisted bilayer WSe₂ (tWSe₂) device. The device adopted an AA-stacked structure, with exfoliated WSe₂ layers stacked on a SiO₂ substrate using the dry-transfer method. To increase the range of displacement fields and densities, the team used thin hexagonal boron nitride (BN) as a dielectric spacer. The top and bottom gates were made of few-layer graphite, and the contact regions utilized graphite and RuCl₃ as charge-transfer dopants. The device design included three gates for tuning channel density and displacement fields.
2. Electronic Transport Measurements
The research team conducted electronic transport measurements in a low-temperature environment, including four-terminal and two-terminal resistance measurements. Using low-frequency lock-in techniques, the team measured resistance changes at different temperatures and magnetic fields. The experimental results showed that no correlated behavior was observed at zero displacement field. However, when a displacement field was applied to bring the van Hove singularity (VHS) of the moiré band close to half-filling, a superconducting region emerged. The superconducting state appeared in a limited region adjacent to a metallic state, which was believed to arise from antiferromagnetic (AFM) order-induced Fermi surface reconstruction.
3. Interplay Between Superconductivity and Magnetic Phases
By applying a perpendicular magnetic field to suppress superconductivity, the research team further investigated the interplay between superconductivity and magnetic phases. The experimental data revealed a sharp boundary between superconductivity and magnetic phases, reminiscent of spin-fluctuation-mediated superconductivity. The team also analyzed the effects of displacement fields on the density of states and wavevector-dependent susceptibility through theoretical calculations. They found that as the displacement field increased, the susceptibility increased, but the nesting of the Fermi surface decreased. This suggests that above a critical displacement field, the Stoner criterion is satisfied, leading to the emergence of magnetic order.
4. Temperature-Dependent Studies
The research team also studied the temperature dependence of superconductivity. The experimental results showed that as the temperature increased, the superconducting region gradually shrank, eventually leaving only a region of increased resistance associated with the magnetic phase. This phenomenon further confirmed the close relationship between superconductivity and magnetic phases.
Conclusions and Significance
This paper reports the first observation of superconductivity in 5.0° twisted bilayer WSe₂, with a maximum critical temperature of 426 mK. This discovery demonstrates that moiré flat-band superconductivity is not limited to graphene structures, and the intrinsic properties of TMDs provide the possibility of exploring a broader superconducting parameter space. The study also revealed the interplay between superconductivity and magnetic phases, suggesting that spin fluctuations may play a key role in superconducting pairing. This finding offers new perspectives for understanding strongly correlated phenomena in two-dimensional materials and provides important references for the future design of novel superconducting materials.
Research Highlights
- First Observation of Superconductivity in Twisted Bilayer WSe₂: This discovery expands the scope of research on moiré flat-band superconductivity.
- Interplay Between Superconductivity and Magnetic Phases: The study reveals the close relationship between superconductivity and magnetic phases, indicating that spin fluctuations may play a key role in superconducting pairing.
- Temperature-Dependent Studies: By studying the temperature dependence of superconductivity, the relationship between superconductivity and magnetic phases is further confirmed.
Other Valuable Information
The research team also developed a novel device fabrication method, achieving high-quality contacts through the dry-transfer method and RuCl₃ doping techniques. This method provides an important reference for future studies of correlated phenomena in other two-dimensional materials.
The research in this paper not only offers new perspectives for understanding superconductivity in two-dimensional materials but also provides important references for the future design of novel superconducting materials.