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

This work investigates isolated vortices in the hole-overdoped Ba_1-xK_xFe₂As₂ (x=0.77) by using scanning superconducting quantum interference device (SQUID) magnetometry.  At low field, we observed vortices that were well separated from their neighbors. In many locations, we observed vortices that carried only part of a flux quantum, with a magnitude that varied continuously with temperature. We interpret these features as vortices with non-quantized magnetic flux, sometimes called fractional or partial vortices whose magnitude is determined by the temperature-dependent parameters of a multiband superconductor. We further characterize the mobility and manipulability of the fractional vortices.