Structure and photoemission spectroscopy of strain-controlled metal-insulator transition in $NdNiO3$ thin films

We study epitaxial $NdNiO3$ thin films on $NdGaO3(001)$ single-crystal substrates grown using a pulsed-laser deposition method. The films show a clear first-order metal-insulator transition (MIT) at $TMI∼240K$⁠, which is significantly higher than $TMI∼190K$ in bulk $NdNiO3$⁠. The x-ray reciprocal space map shows in-plane tensile and out-of-plane compressive strain, stabilizing the more distorted $NdNiO3$ thin films with the higher transition temperature. Hard x-ray photoemission shows changes across the MIT and the bandwidth-controlled charge-transfer gap opens due to the reduced $p-d$ hybridization in the low-temperature insulating phase.