Ultrathin films of FeSe grown on SrTiO3 (FeSe/STO) exhibit a dramatic increase in superconducting transition temperature (Tc), prompting investigations into the underlying pairing mechanism. This study uses resonant inelastic x-ray scattering (RIXS) and quantum Monte Carlo (QMC) calculations to analyze spin excitations in bulk and monolayer FeSe/STO. While bulk FeSe displays dispersive magnetic excitations, FeSe/STO shows gapped, dispersionless, and significantly hardened excitations. By comparing RIXS results with simulations of a bilayer Hubbard model, the researchers link the evolution of spin excitations to the Fermiology of the two systems, revealing a remarkable reconfiguration of spin excitations in FeSe/STO crucial for understanding the role of spin fluctuations in the pairing mechanism.

Jonathan Pelliciari, Seher Karakuzu, Qi Song, Riccardo Arpaia, Abhishek Nag, Matteo Rossi, Jiemin Li, Tianlun Yu, Xiaoyang Chen, Rui Peng, Mirian García-Fernández, Andrew C. Walters, Qisi Wang, Jun Zhao, Giacomo Ghiringhelli, Donglai Feng, Thomas A. Maier, Ke-Jin Zhou, Steven Johnston, Riccardo Comin