Robust field-free superconducting diode effect in FeTe0.55Se0.45

The superconducting diode effect (SDE), a nonreciprocal phenomenon where the critical supercurrent differs with the direction of current flow, is a promising foundation for superconducting logic and memory. Realizing its full potential requires a diode that operates without an applied magnetic field, demanding the breaking of both time reversal and inversion symmetries within the super-conductors. Zero-field SDE is often observed in heterostructures, while rare in single materials. Here, we report the observation of a robust, field-free SDE in thin flake FeTe 0.55 Se 0.45. A definitive signature of the effect is the emergence of a pronounced second harmonic response near the superconducting transition under zero applied field, with an amplitude comparable to the standard first harmonic signal. The SDE persists at zero field, and maintains its polarization robustly under both high positive and negative magnetic field—an even symmetry with respect to the magnetic field, a key distinction from mechanisms based on finite-momentum pairing or magnetochiral anisotropy. We systematically exclude alternative origins from device geometry, thermal gradient, chiral domains or extrinsic magnetic order. Instead, our analysis indicates that local strain/polarization-induced symmetry breaking is a primary factor in generating and enhancing the effect. These results establish FeTe 0.55 Se 0.45 as a compelling, structurally simple material platform for intrinsic field-free superconducting diodes.