Observation of a Two-fold Anisotropy in Ultrathin Mo₂C Superconducting Crystals

The structural phase and polymorphs play important roles in determining the symmetry and properties of materials, providing an important route to engineering exotic electronic phases and expanding the functionalities for applications. Here, we report the observation of a two-fold symmetry of transport anisotropy associated with the crystallographic directions in 2D ultrathin Mo₂C superconducting crystals. Through angle-resolved transport measurements, we demonstrate the modulation of the longitudinal and transverse resistance on an in-plane magnetic field. The onset of anisotropy coincides with an enhanced superconducting fluctuation near the superconducting transition. Moreover, large nonreciprocal charge transport is observed, indicating spatial inversion symmetry breaking. These results can be understood from the spontaneous symmetry breaking due to the emergence of mixed polymorphs and metastable phases in ultrathin Mo₂C crystals, which is further supported by atomic-scale characterizations. Our work reveals the potential of engineering phases and polymorphs to manipulate superconducting properties and functionalities in ultrathin transition metal carbides.