Observation of field-odd and field-free superconducting diode effects in $\mathrm{Mo}_{2}\mathrm{C}$ nanoflakes

Abstract The superconducting diode effect (SDE) enables nonreciprocal supercurrent flow, holding immense potential for ultra-low-power quantum electronics. Intrinsic SDE typically requires materials with inherent symmetry breakings. Here, we report the discovery of SDE in chemical vapor deposition-grown molybdenum carbide ( $\mathrm{Mo}_{2}\mathrm{C}$ Mo 2 C ) nanoflakes, a material traditionally considered centrosymmetric. Strikingly, this system uniquely hosts both field-odd and field-free SDEs. Transport measurements reveal a field-odd SDE with tunable efficiency exceeding 40% at 4 K under a perpendicular in-plane magnetic field. In a separate sample, a robust field-free SDE persists under zero-field and field-coolings. Out-of-plane field sweeps confirm the intrinsic nature of these phenomena. We discuss three possible mechanisms including charge density wave, domain-boundary supercurrents, and mixed superconducting phases, and we find mixed superconducting phases to be the most plausible driver of this unexpected combination of symmetry breakings. Our findings establish air-stable $\mathrm{Mo}_{2}\mathrm{C}$ Mo 2 C as an ideal platform for nonreciprocal superconducting electronics operating at liquid-helium temperatures, expanding the search for SDE into nominally centrosymmetric superconductors.