Probing emergent excitations with middle-energy electron energy loss spectroscopy

Probing the emergent excitations in a range of 1-50eV is fundamentally important to understand a variety of emergent phenomena in quantum materials, however, the characterization techniques of which are very limited at present. Here, we take high-temperature superconducting YBa₂Cu₃O_7-δ films (T_c∼90K) as an archetype to demonstrate the powerful capabilities of middle-energy electron energy loss spectroscopy (EELS) in determining the emergent excitations. Three dominative features (2.9, 12.9, and 25 eV) are observed in the EELS spectra, one of which is a newly observed peak near 2.9 eV resulting from excitations between the Zhang-Rice state and the upper Hubbard band. Moreover, incident energy-dependent EELS spectra indicate that the signals of these three features are strongly dependent on the energy of incident electrons. Our work provides a route to probe emergent excitations in the energy range 1-50eV with utilizing high-resolution two-dimensional energy and momentum mapping.