AC-flux-driven SQUID diode spectroscopy as a probe of current-phase relations

Abstract

The current-phase relation (CPR) of a Josephson junction encodes microscopic information on superconducting states through higher-order and fractional harmonics. However, their unambiguous extraction is challenging, as different CPR components produce nearly identical static interference patterns that are further obscured by device asymmetries, damping, and dynamical effects. Here, we propose probing individual CPR harmonics via the ac magnetic-flux-driven diode effect in asymmetric dc SQUIDs with unequal junction critical currents. Using two complementary reductions of the fast-driven dynamics -- a Kapitza-type perturbation theory for the conventional junction and a Jacobi--Anger averaging for a general CPR -- we show that ac flux modulation dresses each harmonic with a distinct Bessel function, yielding characteristic signatures in the diode efficiency $\eta(\phi_{\rm ac},\omega)$ as a function of ac flux amplitude $\phi_{\rm ac}$ and frequency $\omega$. We verify and extend these predictions by numerical solutions of the coupled dynamical equations for CPRs containing $\sin\varphi$, $\sin(\varphi/2)$, and $\sin 2\varphi$ terms ($\varphi$: superconducting phase difference), and construct phase diagrams of $\eta(\phi_{\rm ac},\omega)$. Distinct CPR components are revealed to produce characteristic weak, sparse, dense, or intermodulated arc patterns that remain robust in both overdamped and underdamped regimes. This suggests ac-flux-driven SQUID diode spectroscopy as a probe of current-phase relations in topological materials, multiband systems, and other unconventional superconductors.