across the metal-insulator transition

Rare-earth nickelates exhibit a variety of intriguing phenomena, including a metal-insulator transition, long-range magnetic order, and ferroelectricity. Using a combination of reflection momentum-resolved electron energy loss spectroscopy (EELS), angle-resolved photoemission spectroscopy, and oxide molecular beam epitaxy, we study the collective excitations of NdNiO₃ thin films. We observe a multitude of collective excitations, including phonons, plasmons, and d-d excitations in the high-temperature metallic phase. As the insulating antiferromagnetic phase is reached, we observe a strong suppression of the plasmons and novel, dipole-active modes emerge in the EELS spectra. These modes are close in energy to magnon excitations and exhibit highly asymmetric line shapes. We discuss the possible origin of these modes, including the relationship between magnetic excitations and the electric dipole degrees of freedom.