Superconducting vortices carrying a temperature-dependent fraction of the flux quantum*

Vortices in superconductors typically carry exactly one quantum of magnetic flux. Here we use SQUID microscopy to investigate isolated vortices in the hole-overdoped Ba_1−xK_xFe₂As₂ (x = 0.77). In many locations, we observed vortices that carried only part of a flux quantum, with a magnitude that varied continuously with temperature. These features may be quantum vortices with non-universally quantized (fractional) magnetic flux whose magnitude is determined by the temperature-dependent parameters of a multiband superconductor.