The strong Ising spin–orbit coupling in certain two-dimensional transition metal dichalcogenides can profoundly affect the superconducting state in few-layer samples. For example, in NbSe₂, this effect combines with the reduced dimensionality to stabilize the superconducting state against magnetic fields up to ~35 T, and could lead to topological superconductivity. Here we report a two-fold rotational symmetry of the superconducting state in few-layer NbSe₂ under in-plane external magnetic fields, in contrast to the three-fold symmetry of the lattice. Both the magnetoresistance and critical field exhibit this two-fold symmetry, and it also manifests deep inside the superconducting state in NbSe₂/CrBr₃ superconductor-magnet tunnel junctions. In both cases, the anisotropy vanishes in the normal state, demonstrating that it is an intrinsic property of the superconducting phase. We attribute the behaviour to the mixing between two closely competing pairing instabilities, namely the conventional s-wave instability typical of bulk NbSe₂ and an unconventional d- or p-wave channel that emerges in few-layer NbSe₂. Our results demonstrate the unconventional character of the pairing interaction in few-layer transition metal dichalcogenides and highlight the exotic superconductivity in this family of two-dimensional materials.

某些二维过渡金属二硫属化物中的强伊辛自旋轨道耦合可以深刻影响几层样品中的超导状态。例如，在 NbSe ₂中，这种效应与降低的维度相结合，可在高达 ~35 T 的磁场中稳定超导状态，并可能导致拓扑超导。_{在这里，我们报告了在面内外部磁场下少层 NbSe 2}中超导态的两倍旋转对称性，与晶格的三重对称性形成对比。磁阻和临界场都表现出这种双重对称性，它也表现在 NbSe ₂ /CrBr _{3超导状态的深处}超导体-磁体隧道结。在这两种情况下，各向异性在正常状态下都消失了，这表明它是超导相的固有特性。我们将这种行为归因于两个紧密竞争的配对不稳定性之间的混合，即块状 NbSe _{2典型的常规}s波不稳定性和出现在少层 NbSe ₂中的非常规d或p波通道。我们的结果证明了少层过渡金属二硫属化物中配对相互作用的非常规特征，并突出了该二维材料家族中的奇异超导性。