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 science1,2. Their reduced dimensionality, atomically flat interfaces and high crystallinity are particularly attractive for realizing compact lumped-element devices in superconducting circuits3-5. 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 Tc ≈ 1 K) and enhanced charge density waves (CDWs; CDW transition temperature TCDW ≈ 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 LK ≈ 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.