  Majorana zero modes (MZMs) that obey the non-Abelian statistics have been
intensively investigated for potential applications in topological quantum
computing. The prevailing signals in tunneling experiments "fingerprinting" the
existence of MZMs are the zero-energy bound states (ZEBSs). However, nearly all
of the previously reported ZEBSs showing signatures of the MZMs are observed in
difficult-to-fabricate heterostructures at very low temperatures and
additionally require applied magnetic field. Here, by using in-situ scanning
tunneling spectroscopy, we detect the ZEBSs upon the interstitial Fe adatoms
deposited on two different high-temperature superconducting one-unit-cell-thick
iron chalcogenides on SrTiO3(001). The spectroscopic results resemble the
phenomenological characteristics of the MZMs inside the vortex cores of
topological superconductors. Our experimental findings may extend the MZM
explorations in connate topological superconductors towards an applicable
temperature regime and down to the two-dimensional limit. While a concrete
understanding of the observations is lacking, possible explanations involving
novel 2D superconducting states with spin-orbit coupling, spontaneous
nucleation of anomalous vortices at the magnetic sites, and noncoplanar
magnetic ordering may further stimulate theoretical understandings of the
scarcely captured ZEBSs in strongly correlated systems with multiband Cooper
pairing.
