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 LaxEu1-xNiO3 (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.

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The magnetic field control of quantum matter enables to study of various exotic phenomena which emerge in complex oxide materials. The thin film heterostructures of these systems, potentially relevant in oxide electronics, require a great deal of care in distinguishing the properties of the film from that of the substrate underneath. Here, we unravel a unique low energy excitation mode in NdGaO3 crystals which is an important constituent of oxide thin film heterostructures. Employing terahertz (THz) time-domain spectroscopy with control of the external magnetic field, an excitation mode resonating at a frequency of 0.2 THz was observed along two key [001] and [110] crystallographic orientations. The mode develops and strengthens with an application of increasing magnetic field and weakens with increasing temperature; all attributes conforming to the predictions for the energetics of Zeeman splitting and intra-level transition within Kramer’s doublet ground state. A dominance of the magnetic field control of this mode along [110] compared to that along [001] orientation also further unveils a structural control. NdGaO3 crystals are widely used as substrates in oxide thin film heterostructures; hence, while realizing ultrafast control of magnetic order, specifically in antiferromagnetic spintronics devices, the Kramer’s double mode should be carefully separated from ordered spin resonance mode.

Transparent conducting materials are inevitable in the fast-developing optoelectronic and photovoltaic industries. Correlated metals are emerging classes of materials that possess a charge density comparable to the metals in which the correlation effects provide transparency. So, understanding the fundamental physics of these materials is equally important to improve the performance of devices. We have investigated the low energy and non-equilibrium dynamics of the CaVO3 (CVO) thin film using terahertz time-domain and time-resolved terahertz spectroscopic measurements. Though the electrical resistivity of the CVO thin film shows a Fermi liquid-like signature, the terahertz conductivity dynamics unveil the presence of metal-insulator transition. Furthermore, the mass renormalization effects indicate the competition between electron correlations and phonon interactions in driving the ground state of this system. It is clear that the relaxation of photo-excited carriers is through electron–phonon thermalization, and comprehensive studies show the metallic nature of the system with electron correlations. Thus, the extracted optical and electrical parameters of CVO are comparable with the existing transparent conducting materials and, hence, make this system another potential candidate for transparent electronics.

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