Metal-Insulator Transition and Orbital Reconstruction in Mott-Type Quantum Wells Made of ${\mathrm{NdNiO}}_3$

The metal-insulator transition and the underlying electronic and orbital structure in $e_g^1$ quantum wells based on ${\mathrm{NdNiO}}_3$ were investigated by dc transport and resonant soft x-ray absorption spectroscopy. By comparing quantum wells of the same dimension but with two different confinement structures, we explicitly demonstrate that the quantum well boundary condition of correlated electrons is critical to selecting the many-body ground state. In particular, the long-range orderings and the metal-insulator transition are found to be strongly enhanced under quantum confinement by sandwiching ${\mathrm{NdNiO}}_3$ with the wide-gap dielectric ${\mathrm{LaAlO}}_3$, while they are suppressed when one of the interfaces is replaced by a surface (interface with vacuum). Resonant spectroscopy reveals that the reduced charge fluctuations in the sandwich structure are supported by the enhanced propensity to charge ordering due to the suppressed $e_g$ orbital splitting when interfaced with the confining ${\mathrm{LaAlO}}_3$ layer.

Figure 1

Cartoons of the two quantum well geometries with potential schematics. (a) Confinement by wide-gap dielectrics (${\mathrm{LaAlO}}_3$) on both sides (sandwiched slab). (b) Confinement by a dielectric (${\mathrm{LaAlO}}_3$) on one side and vacuum on the other.

Figure 2

Sheet resistance versus temperature in ${\mathrm{NdNiO}}_3$ quantum wells under OLS (a) and SWS (b). Solid curves are warming data. Cooling data are shown in dashed curves for samples that have hysteresis. Inset of (a): warming curves for 5-u.c. slabs in both geometries. The dashed line is a guide to eye for the extrapolation to lower temperatures in SWS. Inset of (b): comparison of the 10-u.c. OLS and SWS.

Figure 3

Polarization-dependent Ni $L_2$-edge x-ray absorption spectrum (upper) and linear dichroism spectra (lower) ($I_{E\parallel c}-I_{E\parallel a,b}$) of 5- and 10-u.c. slabs of ${\mathrm{NdNiO}}_3$ in OLS and SWS. Absorption spectrum of bulk ${\mathrm{NdNiO}}_3$ is included for comparison. Note, different samples are compared by normalization to the whiteline intensity at the $L_2$ edge.