  The discovery of enhanced superconductivity (SC) in FeSe films grown on
SrTiO3 (FeSe/STO) has revitalized the field of Fe-based superconductors. In the
ultrathin limit, the superconducting transition temperature Tc is increased by
almost an order of magnitude, raising new questions on the pairing mechanism.
As in other unconventional superconductors, antiferromagnetic spin fluctuations
have been proposed as a candidate to mediate SC in this system. Thus, it is
essential to study the evolution of the spin dynamics of FeSe in the ultrathin
limit to elucidate their relationship with superconductivity. Here, we
investigate and compare the spin excitations in bulk and monolayer FeSe grown
on STO using high-resolution resonant inelastic x-ray scattering (RIXS) and
quantum Monte Carlo (QMC) calculations. Despite the absence of long-range
magnetic order, bulk FeSe displays dispersive magnetic excitations reminiscent
of other Fe-pnictides. Conversely, the spin excitations in FeSe/STO are gapped,
dispersionless, and significantly hardened relative to the bulk counterpart. By
comparing our RIXS results with simulations of a bilayer Hubbard model, we
connect the evolution of the spin excitations to the Fermiology of the two
systems. The present study reveals a remarkable reconfiguration of spin
excitations in FeSe/STO, which is essential to understand the role of spin
fluctuations in the pairing mechanism.
