Giant Negative Spin Polarization Induced by Strong Coulomb Interaction in Superconducting High-Entropy Alloy NbTaTiZrFe

Diamagnetism expels external magnetic fields due to the orbital motion of electrons (orbital diamagnetism) or the Meissner effect in superconductors. The magnetic susceptibility of orbital diamagnetism is generally very small within the order of 10⁻⁶ to 10⁻⁴ emu cm⁻³ Oe⁻¹ because the orbital diamagnetism originates from the 2^nd-order perturbation of atomic Hamiltonian in fully occupied orbitals. Here, we report the giant negative spin-polarization (GNSP) in the Fe-based high entropy alloy NbTaTiZrFe superconductor, synthesized by a powder metallurgical method. This compound exhibits a pronounced diamagnetic response below 42 K, and it reaches up to −60% of diamagnetic external field shielding at the above superconducting transition temperature (T_c) of 6 K in zero-field-cooled conditions, transitioning to ferromagnetic behavior under field-cooled conditions. The itinerant negative spin polarization is supported by substantial diamagnetic signal at low magnetic fields paired with a strong ferromagnetic coercive force (H_coer = 1800 Oe), a ferromagnetic spin flip signal exclusive to the diamagnetic state, and metallic diamagnetism verified by scanning magnetic force microscopy, and spin-resolved density functional theory calculation. The GNSP originated from the strong antiparallel correlation between Fe spins and the other spins of diamagnetic elements by strong Coulomb interaction. The coexistence of GNSP and stable superconductivity strongly implies the possibility of S_z = -1 type spin-triplet superconductivity, which can be a promising platform for exploring Majorana fermions and their potential applications in quantum computation.