Encapsulation epitaxy of air-stable 2D superconductors for quantum circuits

Two-dimensional (2D) superconductors are emerging platforms supporting both strongly correlated physics and quantum information science^1,2. Their reduced dimensionality, atomically flat interfaces and high crystallinity are particularly attractive for realizing compact lumped-element devices in superconducting circuits^3-5. However, large-scale synthesis of monolayer 2D superconductors remains challenging as they are easily oxidized in air⁶. Here we report an 'encapsulation epitaxy' mechanism that enables the growth of large-area (more than 1 inch), air-stable, monolayer niobium diselenide (NbSe₂) films (1L-NbSe₂) and explore their potential for superconducting quantum circuits. This work represents a distinct growth phenomenon in which a 2D encapsulation layer, such as graphene or hexagonal boron nitride, pre-deposited on a 3D substrate (for example, SiO₂ or Si₃N₄) simultaneously serves as a template for the epitaxial growth of 1L-NbSe₂ underneath it at the encapsulation-substrate interface and as a protective capping layer against ambient degradation. The as-grown 1L-graphene/NbSe₂ heterostructures exhibit robust superconductivity (superconducting transition temperature T_c ≈ 1 K) and enhanced charge density waves (CDWs; CDW transition temperature T_CDW ≈ 177 K). We further demonstrate the integration of 1L-NbSe₂ into superconducting circuits by developing oxidation-free transfer and superconducting edge-contact techniques. The 1L-NbSe₂ in these circuits feature a measured kinetic inductance L_K ≈ 0.7 nH □^-1, making it suitable for quantum circuits requiring elements with high kinetic inductance. This encapsulation-epitaxy methodology enables the production of air-stable 2D superconductors and van der Waals heterostructures, holding promise for wafer-scale, monolithic fabrication of superconducting quantum circuitry.