Anomalous Josephson current through a topological noncoplanar ferromagnetic trilayer

Area of Science:

• Condensed Matter Physics
• Spintronics
• Materials Science

Background:

• Topological insulators (TIs) exhibit unique electronic properties.
• Josephson junctions are crucial for superconducting electronics.
• Ferromagnetic insulators (FIs) can introduce spin polarization into superconducting systems.

Purpose of the Study:

• Investigate the anomalous Josephson effect in a TI-based Josephson junction incorporating a ferromagnetic insulator (FI) trilayer with noncoplanar magnetizations.
• Explore the influence of magnetic properties on superconducting correlations and supercurrent behavior.
• Analyze the conditions for 0-π state transitions and the behavior of the $\phi_0$ supercurrent.

Main Methods:

• Fabrication and characterization of a TI-based Josephson junction with a specific FI trilayer structure.
• Theoretical modeling to analyze spin-singlet and spin-triplet correlations.
• Investigation of the chiral Dirac energy band structure influenced by magnetism.
• Experimental or simulation-based measurement of Josephson supercurrents and free energies.

Main Results:

• Observed equal fractional spin-singlet and -triplet correlations arising from the magnetism-tuned chiral Dirac energy band structure.
• Demonstrated an anomalous Josephson supercurrent with a 0-π state transition controlled by the exchange field strength of the first FI region.
• Showcased a gradual decrease in $\phi_0$ supercurrent and maximum supercurrent with increasing exchange field strengths in the second and third FI regions.
• Found that increasing FI region length also decreases $\phi_0$ supercurrent and maximum supercurrent, distinct from exchange field effects.

Conclusions:

• The noncoplanar magnetizations in the FI trilayer are key to inducing the anomalous Josephson effect and fractional spin correlations.
• The exchange field strength and length of FI regions offer tunable control over the Josephson supercurrent and phase transitions.
• This study provides a pathway for designing novel superconducting devices with tailored spintronic functionalities.