Anomalous hysteresis as evidence for a magnetic-field-induced chiral superconducting state in LiFeAs

Magnetometry measurements in high-quality LiFeAs single crystals reveal a change in the sign of the magnetic hysteresis in the vicinity of the upper critical field H_c2, from a clear diamagnetic response dominated by the pinning of vortices to a considerably smaller net hysteretic response of opposite sign, which disappears at H_c2. If the diamagnetic response at high fields results from pinned vortices and associated screening supercurrents, this sign change must result from currents circulating in the opposite sense, which give rise to a small field-dependent magnetic moment below H_c2. This behavior seems to be extremely sensitive to the sample quality or stoichiometry, as we have observed it only in a few fresh crystals, which also display the de Haas van Alphen effect. We provide arguments against the surface superconductivity, the flux compression, and the random π junction scenarios, which have been previously put forward to explain a paramagnetic Meissner effect, below the lower critical field H_c1. The observed anomalous hysteresis at high fields will be compatible with the existence of chiral gap wave functions, which possess a field-dependent magnetic moment. Within a Landau-Ginzburg framework, we demonstrate how a (d _x2-y2+id_xy) or a (p_x+ip_y) chiral superconducting component can be stabilized in the mixed state of s _± superconductor, due to the combined effects of the magnetic field and the presence of competing pairing channels. The realization of a particular chiral pairing depends on the microscopic details of the strengths of the competing pairing channels.