The superconducting diode effect (SDE), a nonreciprocal phenomenon where the critical supercurrent differs depending on 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, which generally requires the breaking of both time-reversal and inversion symmetries in the superconducting system. Zero-field SDEs are often observed in heterostructures but remain rare in single materials. Here, we report the observation of a robust, field-free SDE in thin flakes of FeTe 0.55 Se 0.45 . A sensitive probe of nonreciprocal transport 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 polarity under both large positive and negative magnetic fields, exhibiting an even-in-field symmetry that distinguishes it from mechanisms based on finite-momentum pairing or magnetochiral anisotropy. We systematically rule out alternative origins, including device geometry, thermal gradients, chiral domains, and 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 identify iron-based high-temperature superconducting platforms as highly promising candidates for the field-free superconducting diode effect, benefiting from their coexistence of strong intrinsic disorder and superconductivity in a structurally simple form.
Dong et al. (Sun,) studied this question.