Researchers from Brown University, Harvard University and National Institute for Materials Science (Tsukuba) have reported new insights into how superconductivity, electronic nematicity and strange metallicity are connected in 'magic-angle' twisted trilayer graphene, using angle-resolved transport measurements to track how these phases evolve.
Superconductivity - where electrical resistance drops to zero - is often linked to a breaking of rotational symmetry in the electronic system, known as nematicity. At the same time, many materials already show directional differences in electrical transport (anisotropy) before becoming superconducting. This has made it difficult to determine whether the symmetry breaking in the superconducting state is intrinsic, or simply inherited from the normal metallic phase. To address this, the researchers developed a method that measures electrical resistance as a function of direction. Instead of probing transport along a single axis, they continuously rotated the direction of current flow and mapped how resistance changes with angle. This allowed them to directly compare the angular behavior of three closely related states: the normal metallic phase, the superconducting phase, and the so-called strange metal phase, which exhibits unconventional temperature-dependent resistance.
The experiments were carried out in magic-angle twisted trilayer graphene, where three graphene layers are stacked with a small rotational offset. This structure enhances electron-electron interactions and gives rise to a range of correlated electronic phases.
A key finding is a clear directional relationship between these phases. The strongest superconducting response occurs along the direction where the metallic state shows the highest resistance. In contrast, strange metallic behavior aligns with the direction of lowest resistance in the metallic phase. In other words, the direction that appears least favorable for current flow in the normal state becomes the most favorable for superconductivity.
This result points to nematicity - driven by electron interactions that break rotational symmetry - as a central link between these phenomena. Rather than being separate effects, superconductivity, strange metallicity and nematic order appear to be closely intertwined.
This directional behavior places new constraints on the symmetry of the superconducting state, helping to narrow down possible theoretical explanations for how superconductivity emerges in strongly interacting systems.
The angle-resolved transport approach introduced here provides a practical way to probe the symmetry of electronic states in complex materials. The researchers suggest that applying this method to other graphene-based systems, moiré materials and high-temperature superconductors could reveal similar connections and help clarify the mechanisms behind unconventional superconductivity.
