Direct observation of a superconducting vortex diode

Area of Science:

• Condensed Matter Physics
• Quantum Materials Science
• Spintronics

Background:

• The superconducting diode effect, characterized by a nonreciprocal critical current, arises from the interplay of magnetism and superconductivity.
• Previous studies on superconductor/ferromagnet (S/F) bilayers demonstrated this effect, but the underlying mechanism remained elusive.
• The Fulde-Ferrell-Larkin-Ovchinikov (FFLO) state was a proposed, though unconfirmed, mechanism due to symmetry breaking.

Purpose of the Study:

• To directly observe and elucidate the mechanism responsible for the superconducting vortex diode effect in S/F bilayers.
• To investigate the role of asymmetric vortex dynamics in generating a nonreciprocal critical current.
• To establish a foundational understanding for the development of novel superconducting devices.

Main Methods:

• Utilized a nanoscale SQUID-on-tip (SOT) microscope for direct observation of vortex dynamics.
• Performed in-situ transport measurements to complement microscopic observations.
• Developed a theoretical model to explain the observed phenomena and experimental results.

Main Results:

• Directly observed asymmetric vortex dynamics in Nb/EuS (S/F) bilayers.
• Identified screening currents induced by stray magnetic fields from the ferromagnet layer as the cause of nonreciprocal critical current.
• Validated the theoretical model with experimental data, confirming the mechanism.

Conclusions:

• The vortex diode effect in S/F bilayers is driven by screening currents from the ferromagnet layer.
• This finding clarifies the origin of the nonreciprocal critical current in these systems.
• Provides a basis for designing new superconducting electronic devices leveraging this effect.