2023/07/11 by Julian Linek, Linek, J., Michał J. Wyszyński +11
Engineering · Physics and Astronomy · #FOS: Physical sciences #Physics of Superconductivity and Magnetism #Superconducting Materials and Applications #Superconductivity (cond-mat.supr-con) #Superconductivity in MgB2 and Alloys
paper · pdf · doi:10.48550/arxiv.2307.05724
openalex publication_date 2023/07/11 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
We investigate the coupling factor ϕμ that quantifies the magnetic flux Φ per magnetic moment μ of a point-like magnetic dipole that couples to a superconducting quantum interference device (SQUID). Representing the dipole by a current-carrying loop, the reciprocity of mutual inductances of SQUID and loop provides a way of calculating ϕμ(r, eμ) vs.~position r and orientation eμ of the dipole anywhere in space from the magnetic field B(r) produced by a supercurrent circulating in the SQUID loop. We use numerical simulations based on London and Ginzburg-Landau theory to calculate ϕμ from the supercurrent density distributions in various SQUID geometries. We treat the far-field regime (r\gtrsim a= inner size of the SQUID loop) with the dipole placed on the symmetry axis of circular or square shaped loops. We compare expressions for ϕμ from filamentary loop models with simulation results for loops with finite width w (outer size A>a), thickness d and London penetration depth λL and show that for thin (d≪ a) and narrow (w < a) loops the introduction of an effective loop size a\rm eff in the filamentary loop-model expressions results in agreement with simulations. For a dipole placed in the center of the loop, simulations provide an expression ϕμ(a,A,d,λL) that covers a wide parameter range. In the near-field regime (dipole centered at small distance z above one SQUID arm) only coupling to a single strip representing the SQUID arm has to be considered. Here, we compare simulations with an analytical expression derived for a homogeneous current density distribution, which yields excellent agreement for λL>w,d. Moreover, we analyze ϕμ provided by the introduction of a constriction in the SQUID arm below the magnetic dipole.