Circuit QED detection of induced two-fold anisotropic pairing in a hybrid superconductor-ferromagnet bilayer

TL;DR

This paper introduces a microwave cQED-based probe to measure superfluid density in hybrid superconductor-ferromagnet systems, revealing anisotropic, nodal triplet pairing and coupling effects near ferromagnetic resonance.

Contribution

Developed a novel microwave cQED technique to measure superfluid density in micron-scale superconductors, applied to superconductor/ferromagnet bilayers, revealing anisotropic pairing and spin dynamics coupling.

Findings

Proximity-induced superfluid density is two-fold anisotropic.

Superfluid density exhibits power-law temperature scaling.

Microwave response is strongly affected near ferromagnetic resonance.

Abstract

Hybrid systems represent one of the frontiers in the study of unconventional superconductivity and are a promising platform to realize topological superconducting states. Owing to their mesoscopic dimensions, these materials are challenging to probe using many conventional measurement techniques, and require new experimental probes to successfully characterize. In this work, we develop a probe that enables us to measure the superfluid density of micron-size superconductors using microwave techniques drawn from circuit quantum electrodynamics (cQED). We apply this technique to a paradigmatic hybrid system, the superconductor/ferromagnet bilayer, and find that the proximity-induced superfluid density is two-fold anisotropic within the plane of the sample and exhibits power law temperature-scaling which is indicative of a nodal superconducting state. These experimental results are consistent with the theoretically predicted signatures of induced triplet pairing with a nodal $p$-wave order parameter. Moreover, we unexpectedly observe drastic modifications to the microwave response at frequencies near the ferromagnetic resonance, suggesting a coupling between the spin dynamics and induced superconducting order in the ferromagnetic layer. Our results offer new insights into the unconventional superconducting states induced in superconductor/ferromagnet heterostructures and simultaneously establish a new avenue for the study of fragile unconventional superconductivity in low-dimensional materials such as van der Waals heterostructures.