  Vortex is a topological defect in the superconducting condensate when a
magnetic field is applied to a type-II superconductor, as elucidated by the
Ginzburg-Landau theory. Due to the confinement of the quasiparticles by a
vortex, it exhibits a circular shaped pattern of bound states with discrete
energy levels, as predicted by the Caroli-de Gennes-Matricon theory in 1964.
Here, however, we report a completely new type of vortex pattern which is
necklace-like in an iron-based superconductor KCa2Fe4As4F2. Our theoretical
analysis shows that this necklace-like vortex pattern arises from selective
off-shell interference between vortex bound states of opposite angular momenta
in the presence of rotational symmetry breaking due to disorders. This
fascinating effect can be observed in a system with a small Fermi energy and
wave vector, conditions fortuitously met in our samples. Our results not only
disclose a novel vortex structure but also provide insights into comprehending
the physics of the superconducting condensate.
