Ultrafast dynamical charge-lattice coupling in rare-earth nickelate thin films studied by time-resolved terahertz spectroscopy

Abstract

Abstract Rare-earth nickelates exhibit a rich phase diagram formed by the complex interplay of intertwined and competing energetics of fundamental entities. To unwind the coupling and interaction mechanisms of fundamental entities underneath, time-resolved terahertz (THz) spectroscopy was implemented to understand non-equilibrium carrier and lattice dynamics of epitaxial thin films of La x Eu1−x NiO3 (x = 0, 0.25, 0.50, 1) systems, where x = 0 is insulating while remaining are metallic at room temperature. The THz transmittance of the insulating and metallic phases exhibit contrasting photo-induced phases associated with bi-exponential and mono-exponential relaxation mechanisms, respectively. A pronounced oscillatory feature superimposed on the mono-exponential relaxation manifests only in the metallic phase. As ascribed to the acoustic phonons, the ‘x’ dependent behavior of this feature reveals an inverse relation between the strength of electron–phonon coupling and the magnitude of conductivity. In the insulating state, in contrast, the relaxation time constants are associated with the recovery of charge-ordering and electron–phonon thermalization. This dynamical lattice–charge interaction study demonstrates use of this ultrafast phenomena in nickelate thin films in new generation ultrafast photo-acoustic devices as an alternate to conventional surface acoustic wave device.