Ultrafast modification of coherent phonons during the photoinduced insulator-to-metal phase transition in neodymium nickelate

Abstract

The interplay between electronic and lattice degrees of freedom in the insulator-to-metal transition (IMT) in rare-earth nickelates is a long-standing question. In the present work, broadband ultrafast transient reflectivity (TR) spectroscopy is applied to study the photoinduced IMT in NdNiO3. Both coherent and incoherent terms of the TR signal show discontinuous behavior around the same pump fluence value. A drastic drop in the sample reflectivity appearing at ∼100 fs timescale in the high excitation density regime indicates the closing of the gap across the IMT. In this regime, coherent phonons associated with the low-temperature crystal phase are not observed even at early time delays, indicating an ultrafast transformation of the lattice potential. A detailed analysis of the coherent phonons indicates a strong coupling between some phonon modes, electronic excitations, and possibly the magnetic order. In this study, we provide insights into the ultrafast dynamics of rare-earth nickelates.