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Understanding and controlling phase transition in the transition-metal oxides is interesting from the fundamental physics point of view. The pd hybridization are of considerable issue to tune the phase transition temperature. Here, the transport properties of the NdNiO3 with artificial ultra-thin NiO insert layer have been investigated. The relationship between the essential parameters of the pd hybridization (T) and the metal-insulator transition (MIT) temperature has been clearly elucidated a monotone decreasing relationship. This work realized the combination of the engineering hybridization and the phase transition temperature, which might be significant for the development of multifunctional materials in the transition-metal oxides.

A high-spin state of SrRuO3 at the interface and large magnetic coupling were observed in SrRuO3/LaNiO3 superlattices. X-ray absorption spectroscopy and X-ray linear dichroism results show preferential occupation of the d3z2-r2 orbitals. The origin of the high-spin state in SrRuO3 and the large-scale magnetic coupling is closely related to the orbital reconstruction. The reduced energy of the d3z2−r2 orbitals and enhanced densities of the eg of SrRuO3 contribute to the interface magnetic moment. The orbital occupancy is strongly correlated with the interface magnetic properties, thereby extending the concept of orbital degrees of freedom modulation on magnetic coupling properties. Impact statement A high-spin state in SRO at the interface and large magnetic coupling is observed in SRO/LNO superlattices and the origin of these properties is closely related to orbital reconstruction.

Soft X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) have become essential experimental tools for the investigations the complex physics of transition metal oxide (TMO) heterostructures. XAS has been long used to determine the valence, the orbital and magnetic properties of transition metals. More recently, linear and circular dichroism in XAS have been widely applied to determine the crystal field splitting, the atomic orbital and spin moments, and the magnetic order of 3d-states, in bulk sample, in thin films and at atomically-sharp interfaces. Although less common, RIXS is also gaining popularity for its capability of accessing local and collective excitations at a time; the recent technical advances have been established RIXS as an important method for the determination of the electronic and magnetic properties of TMOs. This chapter is a brief review of the salient XAS and RIXS results on TMO and TMO heterostructures published in the last 15 years.