Photoinduced structural dynamics across metal-insulator transition in nickelate thin films

Rare earth nickelates display metal-insulator transition (MIT) which is accompanied by a magnetic transition, charge ordering, and a crystal structure change from orthorhombic to monoclinic. The size of the rare-earth cation affects the onset of MIT and the magnetic transition. Laser-induced excitation drives the transition at ultrafast timescales and can be used in combination with time resolved x-ray diffraction to disentangle the contribution of competing degrees of freedom. In this study, we focused on measuring the laser fluence dependent photoinduced structural response of epitaxial NdNiO3/ SrTiO₃ (NNO/STO) and SmNiO3/SrTiO₃ (SNO/STO) thin films. We utilized time-resolved x-ray diffraction to observe the evolution of the out of plane (002)pc Bragg peak and the in-plane (1-13)/2_pc Bragg peak of NNO and SNO after laser excitation. The out of plane lattice parameter of NNO contracts for low fluences and expands for high fluences after laser excitation whereas an expansion in the SNO out of plane lattice parameter is observed for all fluences. The significantly faster structural recovery timescales of NNO compared to SNO indicate the role of the initial magnetic state on the photoinduced process.