Title: Listening to the sound of superfluid
Abstract: The exploration of unconventional superconductivity has entered a new frontier with the emergence of exotic phases in quantum materials—from moiré superlattices to topological semimetals. Probing the superfluid properties and pairing symmetry in these systems is essential to understanding their unconventional behavior, yet traditional techniques often falter when applied to atomically thin materials or those with extremely low critical temperatures. Here, we present a novel approach that “listens to the sound of superfluid” by probing the kinetic inductance of superconductors through microwave resonant cavities. Variations in superfluid stiffness perturb the cavity resonance frequency, enabling precise measurements of the London penetration depth with parts-per-million sensitivity. This technique provides unprecedented access to the superfluid response in fragile and low-temperature superconductors. Applied to magic-angle-twisted trilayer graphene and the Weyl semimetal MoTe₂, our measurements uncover compelling signatures of nodal superconductivity. In twisted trilayer graphene, we observe a linear temperature dependence of the superfluid stiffness and a zero-temperature stiffness that scales linearly with the critical temperature—echoing Uemura’s relation in cuprates. In MoTe₂, the penetration depth exhibits a T2 dependence down to millikelvin temperatures. Most strikingly, both systems display the anomalous nonlinearMeissner effect, where the superfluid response becomes current-dependent—a hallmark of nodal quasiparticles. These results offer evidence for unconventional pairing in both moiré and topological superconductors, demonstrating how “listening” to the subtle resonances of quantum fluids can illuminate the hidden symmetries of correlated electron matter.
Ref.: A. Banerjee, et. al., Nature 638, 93 (2025).