Measuring superfluid stiffness in 2D superconductors via capacitive coupling to a high-impedance superconducting microwave resonator

A variety of unconventional superconducting (SC) phases have been observed in two-dimensional van der Waals (vdW) materials. However, the small volume and inherently low superfluid stiffness of these materials make them difficult to probe using many of the conventional probes of superconductivity. Recently, hybrid superconducting microwave resonators have emerged as a promising tool to directly probe the superfluid stiffness in 2D superconductors. However, such devices face limitations due to undesired proximity effects from SC electrodes as well as difficulties in achieving high-quality contacts. Here, we present an approach that circumvents these issues by capacitively, rather than galvanically, coupling the vdW materials to the SC microwave circuit. By employing high-impedance resonators of niobium nitride, we enhance the measurement sensitivity and demonstrate this method by probing the superfluid stiffness of thin NbSe₂. Our results establish a general route for contact-less coupling to a wide range of vdW materials and enable the study of intrinsic unconventional superconductivity that avoids the spurious proximity effect.