  Magnetic field is expelled from a superconductor, unless it forms quantum
vortices, consisting of a core singularity with current circulating around it.
The London quantization condition implies that there is one core singularity
per quantum of magnetic flux in single-component superconductors, while in
multiband materials fractional vortices are possible. Here, we report the first
observation of quantum vortex core fractionalization on the potassium
terminated surface of multiband superconductor KFe2As2 by scanning tunneling
microscopy. We observe splitting of an integer-flux vortex into several
fractional vortices, leading to disparity between numbers of flux quanta and
vortex cores. Our findings demonstrate that fractionalized core singularities
are possible in a multiband superconductor, opening avenue for new experimental
platforms with quasiparticles with fractional statistics.
