Spontaneous Hall effect in superconducting materials

The spontaneous Hall effect (SHE), observed as a finite zero-field transverse signal, occurs in both conventional and unconventional superconductors, appearing as a peak near the superconducting transition temperature. The origin of SHE is debated, with proposed explanations ranging from intrinsic mechanisms, such as spontaneous symmetry breaking and time-reversal symmetry breaking (BTRS), to extrinsic factors like material inhomogeneities. Previous studies have linked the SHE to Abrikosov vortex motion or BTRS, while others suggest that spatial inhomogeneities, such as non-uniform critical temperature distributions or structural asymmetries, could induce the observed transverse voltage. We conducted a comparative study of the SHE in various superconducting materials, including conventional superconductors (e.g., Nb) and unconventional ones (e.g., Fe(Se,Te)). Our findings show distinct SHE peaks around the superconducting transition, with variations in height, sign, and shape, indicating a possible common mechanism independent of the specific material. We propose that spatial inhomogeneities in the critical temperature, caused by local chemical composition variations or disorder, could explain the appearing of the SHE. This hypothesis is supported by our experimental data and finite element simulations, suggesting a unified extrinsic origin for the SHE in different superconductors.