Fluence dependent structural changes in photoexcited NdNiO₃ thin films

Rare earth nickelates display metal-insulator transition which is accompanied by a magnetic transition, charge ordering, and a crystal structure change from orthorhombic to monoclinic. 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 thin films(NNO/STO) of 9 nm and 19 nm thickness. 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 after laser excitation. Our measurement shows a contraction of the out of plane lattice parameter for low fluences and expansion for high fluences after laser excitation. The observed sign reversal of the peak shift requires a fluence threshold in the range of 1.8-2.4 mJ/cm² for both films. We observe a thermally inaccessible state for higher laser fluences which recovers with a time constant of 200 ps.