Manipulating the metal-to-insulator transition of NdNi O3 films by orbital polarization

We investigate the film thickness dependent metal-to-insulator transition temperature (TMIT) of NdNiO3 films under tensile and compressive strain states. For the films exceeding the critical thickness for strain relaxation, TMIT varies gradually with the film thickness caused by strain relaxation. The variation tendency differs dramatically for the films below the critical thickness: an increase (decrease) of TMIT with increasing the film thickness for the case of tensile (compressive) strain, which is attributed to the decaying of orbital polarization. As the overlap of O2px,y orbits with Ni3dx2-y2 orbits determines TMIT, a decrease of x2-y2 orbital occupation with increasing film thickness would reduce the orbital overlap and resultant enhanced TMIT for tensile strained films, while their compressive counterparts do the opposite. Our findings identify the importance of orbital polarization in regulating the metal-to-insulator transitions, opening up a new perspective for orbital physics in transition metal oxides.