Concurrent Ferromagnetism and Superconductivity in Fe(Te,Se) van der Waals Josephson Junctions

Iron-based 2D layered material FeTe_xSe_1-x (FTS) has been identified as a high-Tc intrinsic topological superconductor (TSC) with s-wave bulk superconductivity proximitizing Dirac surface states. So far, the evidence of TSC and trace of Majorana bound states in FTS have only been reported using local scanning spectroscopy methods or ARPES experiments. Here we report the first device transport measurements with a convincing smoking gun of topological superconductivity in Fe(Te_0.59Se_0.41) van der Waals Josephson junctions (vJJ). We discovered an unconventional ferromagnetic ordering coexisting with superconductivity by observing magnetic hysteresis loops of junction resistance. Such hysteresis behavior only exists below superconducting temperature Tc, suggesting that the ferromagnetic ordering is strongly correlated with superconductivity. Fraunhofer patterns in vJJ exhibit features of mixing 0-π junctions confirming the non-trivial pairing mechanism in this system. The spontaneous time-reversal-symmetry breaking is also manifested through the field-free stochastic superconducting diode effect, which provides insights into the ferromagnetic domain behavior through thermal cycling above Tc.