Rhombohedral Graphene Exhibits Stabilized Spin-Triplet Superconductivity Beyond Pauli Limit with Gate-Tunable Critical Fields

Superconductivity, the ability of a material to conduct electricity with zero resistance, typically breaks down in the presence of magnetic fields, but researchers continually seek materials that defy this limitation. Manish Kumar, Derek Waleffe, and Anna Okounkova, at the University of Washington, alongside colleagues including Raveel Tejani and Kenji Watanabe, now report the surprising discovery of robust superconductivity in a specific form of graphene, a single-layer sheet of carbon atoms, stacked into a rhombohedral heptalayer structure. This material not only exhibits superconductivity despite an applied magnetic field, but actually requires it, with critical field strengths exceeding established theoretical limits, and the team observes this effect across a wide range of electrical conditions. The discovery establishes intriguing new properties of spin-triplet superconductivity in this thick graphene stack, and the observation of a weak superconducting diode effect suggests a potential link between superconductivity and valley imbalance within the material’s unique electronic structure.

Magnetic Field Suppression in Ultra-Thin NbSe2

Researchers are investigating how magnetic fields disrupt superconductivity, a phenomenon where materials conduct electricity with no resistance. This suppression occurs when the energy associated with electron spin exceeds the energy holding the superconducting state together. The team focuses on unconventional superconductors, materials where superconductivity arises through mechanisms different from those in conventional materials. They achieve this by creating and studying ultra-thin films of niobium diselenide, carefully engineered down to just a few atomic layers. This precise control allows them to examine how the material’s electronic structure responds to magnetic fields.

The research centres on observing how superconductivity evolves as the magnetic field strength increases, specifically searching for a fundamental change in the superconducting state. The study reveals that superconductivity persists to surprisingly high magnetic fields, exceeding previous expectations for this material. This work demonstrates a robust topological superconducting phase, characterised by the presence of protected edge states, even in strong magnetic fields. These edge states, confined to the boundaries of the ultra-thin film, possess unique properties and could be useful in quantum computing. The researchers establish a clear link between the material’s electronic structure, the applied magnetic field, and the emergence of this topological phase. This provides crucial insights into the interplay between superconductivity, magnetism, and topology in two-dimensional materials, paving the way for the development of new quantum technologies.

Graphene Superconductivity, Transition and Resistivity Measurements

Detailed measurements of electrical resistance characterize superconductivity in a graphene-based device. The analysis focuses on the superconducting transition, precisely determining the critical temperature and sharpness of the transition at various gate voltages and magnetic fields. Researchers also investigated the residual resistance observed in the superconducting state, a crucial parameter for understanding the nature of superconductivity. Theoretical analysis explores how spin polarization and valley imbalance affect the superconducting state, particularly in the presence of an in-plane magnetic field, supported by detailed calculations exploring the interplay between these factors and superconductivity.

The theoretical section delves into the physics of the graphene device, exploring how spin polarization and valley imbalance can influence the superconducting state. It considers the effects of interactions between electrons, which can favour specific spin or valley configurations. The analysis examines how an in-plane magnetic field affects the spin polarization and valley imbalance, predicting transitions between different spin/valley configurations. The experimental section presents data on the temperature dependence of the resistivity of the graphene device at a wide range of gate voltages and magnetic fields. The resistivity curves are carefully analysed to determine the critical temperature and the sharpness of the superconducting transition. The key takeaway is that the graphene device exhibits complex superconducting behaviour influenced by spin polarization, valley imbalance, and the in-plane magnetic field, with superconducting properties tunable by varying the gate voltage and magnetic field.

Magnetic Field Stabilizes Superconductivity in Graphene

Scientists have discovered a novel superconducting state in rhombohedral heptalayer graphene, aligned with hexagonal boron nitride, that is both induced and stabilized by an in-plane magnetic field. Measurements of electrical resistance reveal a sharp peak at zero magnetic field, indicating two small superconducting regions, which transforms into an extended superconducting region when a modest magnetic field of 1 Tesla is applied. This behaviour occurs along a feature that tracks approximately constant electron density, indicating a consistent number of charge carriers as the superconducting state emerges. The team observed that the superconducting state arises from a region where the density of states is large, and is consistent with spin-triplet pairing. These results establish a new pathway for inducing and stabilizing superconductivity in graphene-based materials, potentially opening avenues for novel electronic devices and applications.

Magnetic Field Stabilizes Unexpected Superconductivity

Scientists have discovered a novel superconducting state in rhombohedral heptalayer graphene, aligned with hexagonal boron nitride, that is both induced and stabilized by an in-plane magnetic field. Measurements of electrical resistance reveal a sharp peak at zero magnetic field, indicating two small superconducting regions, which transforms into an extended superconducting region when a modest magnetic field of 1 Tesla is applied. This behaviour occurs along a feature that tracks approximately constant electron density, indicating a consistent number of charge carriers as the superconducting state emerges. The team observed that the superconducting state arises from a region where the density of states is large, and is consistent with spin-triplet pairing. These results establish a new pathway for inducing and stabilizing superconductivity in graphene-based materials, potentially opening avenues for novel electronic devices and applications.