We investigate the film thickness dependent metal-to-insulator transition temperature $\left(T_{\mathrm{MIT}}\right)$ of $\mathrm{NdNi}{\mathrm{O}}_3$ films under tensile and compressive strain states. For the films exceeding the critical thickness for strain relaxation, $T_{\mathrm{MIT}}$ 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 $T_{\mathrm{MIT}}$ 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 $\mathrm{O}2p_{x,y}$ orbits with $\mathrm{Ni}3d{x}^2-y^2$ orbits determines $T_{\mathrm{MIT}}$, a decrease of $x^2-y^2$ orbital occupation with increasing film thickness would reduce the orbital overlap and resultant enhanced $T_{\mathrm{MIT}}$ 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.

• Received 8 March 2016
• Revised 6 May 2016

DOI:https://doi.org/10.1103/PhysRevB.93.235102