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 pi 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. DOI: 10.1103/PhysRevB.87.024512