Direct observation of a superconducting vortex diode

Abstract

The interplay between magnetism and superconductivity can lead to unconventional proximity and Josephson effects. A related phenomenon that has recently attracted considerable attention is the superconducting diode effect, in which a nonreciprocal critical current emerges. Although superconducting diodes based on superconductor/ferromagnet (S/F) bilayers were demonstrated more than a decade ago, the precise underlying mechanism remains unclear. While not formally linked to this effect, the Fulde–Ferrell–Larkin–Ovchinikov (FFLO) state is a plausible mechanism due to the twofold rotational symmetry breaking caused by the finite center-of-mass-momentum of the Cooper pairs. Here, we directly observe asymmetric vortex dynamics that uncover the mechanism behind the superconducting vortex diode effect in Nb/EuS (S/F) bilayers. Based on our nanoscale SQUID-on-tip (SOT) microscope and supported by in-situ transport measurements, we propose a theoretical model that captures our key results. The key conclusion of our model is that screening currents induced by the stray fields from the F layer are responsible for the measured nonreciprocal critical current. Thus, we determine the origin of the vortex diode effect, which builds a foundation for new device concepts.

Flux-pinning mediated superconducting diode effect in NbSe2/CrGeTe3 heterostructure

“…This simple explanation captures the observed asymmetry in an asymmetric magnetic environment B x (0) ̸ = 0 even if pinning centers are homogeneously distributed. To obtain a deeper phenomenological understanding of the SDE, combining the transport measurements (as in this work) with direct microscopic imaging of vortex dynamics will be useful to reveal the actual operation mechanism at play [47]. We also anticipate that employing additional constrictions of varying dimensions to exploit extrinsic pinning, as in [2], will offer pathways to further increase the Q-factors.…”

Unconventional Superconducting Diode Effects via Antisymmetry and Antisymmetry Breaking

“…Consequently, a significant discrepancy between the I c curves for positive and negative currents is observed (Figure l), leading to a flux-quantum diode effect. This current-driven nonreciprocal effect arises directly from the breaking of space inversion symmetry in the pinning potential. − ,− In our case, the symmetry breaking occurs from the asymmetric distribution of positive and negative magnetic charges (Figure f). The observed flux-quantum diode demonstrates a substantial superconducting diode efficiency ([| I c (+) – I c (−)|]/[ I c (+) + I c (−)]) of up to 20% over a wide magnetic field range (Figure S4).…”

“…Unlike traditional superconducting diodes, which typically display simultaneous nonreciprocal transport in response to both I and B , − ,− ,, the superconducting diode shown in Figure l exhibits a symmetric response to magnetic field B , contrary to the principle of magnetochiral anisotropy. To unveil the underlining mechanism behind this unconventional superconducting diode effect, we analyze the symmetry of magnetic charges and their interaction with flux-quanta.…”

“…On the other hand, a special type of superconducting diodes, relying on ratchet motion of superconducting flux-quanta, has been extensively investigated for years. − In type-II superconductors, magnetic flux penetrates the material in the form of individual flux-quanta (also known as vortices). The motion of such flux-quanta directly impacts the electromagnetic properties of superconductors .…”

“…Although asymmetric magnetic field responses have been widely demonstrated in superconducting diodes, − ,− satisfying magnetochiral anisotropy, these diodes exhibit a simultaneous current-driven nonreciprocal effect known as the regular diode effect. However, our observed superconducting odd-parity magnetotransport effect displays symmetric responses regarding electric currents.…”