Mott physics is characterized by an interaction-driven metal-to-insulator transition in a partially filled band. In the resulting insulating state, antiferromagnetic orders of the local moments typically develop, but in rare situations no long-range magnetic order appears, even at zero temperature, rendering the system a quantum spin liquid. A fundamental and technologically critical question is whether one can tune the underlying energetic landscape to control both metal-to-insulator and Néel transitions, and even stabilize latent metastable phases, ideally on a platform suitable for applications. Here we demonstrate how to achieve this in ultrathin films of NdNiO 3 with various degrees of lattice mismatch, and report on the quantum critical behaviours not reported in the bulk by transport measurements and resonant X-ray spectroscopy/scattering. In particular, on the decay of the antiferromagnetic Mott insulating state into a non-Fermi liquid, we find evidence of a quantum metal-to-insulator transition that spans a non-magnetic insulating phase.

Using resonant x-ray spectroscopies combined with density functional calculations, we find an asymmetric biaxial strain-induced d-orbital response in ultrathin films of the correlated metal LaNiO3 which are not accessible in the bulk. The sign of the misfit strain governs the stability of an octahedral "breathing" distortion, which, in turn, produces an emergent charge-ordered ground state with an altered ligand-hole density and bond covalency. Control of this new mechanism opens a pathway to rational orbital engineering, providing a platform for artificially designed Mott materials.

In pursuit of rational control of orbital polarization, we present a combined experimental and theoretical study of single-unit-cell superlattices of the correlated metal LaNiO3 and the band insulator LaAlO3. Polarized X-ray absorption spectra show a distinct asymmetry in the orbital response under strain. A splitting of orbital energies consistent with octahedral distortions is found for the case of compressive strain. In sharp contrast, for tensile strain, no splitting is found although a strong orbital polarization is present. Density functional theory calculations including a Hubbard U -term reveal that this asymmetry is a result of the interplay of strain and confinement that induces octahedral rotations and distortions and altered covalency in the bonding across the interfacial Ni-O-Al apical oxygen, leading to a charge disproportionation at the Ni sites for tensile strain.

We have synthesized epitaxial NdNiO3 ultrathin films in a layer-by-layer growth mode under tensile and compressive strain on SrTiO3 (001) and LaAlO3 (001), respectively. A combination of x-ray diffraction, temperature dependent resistivity, and soft x-ray absorption spectroscopy has been applied to elucidate electronic and structural properties of the samples. In contrast to the bulk NdNiO3, the metal-insulator transition under compressive strain is found to be completely quenched, while the transition remains under the tensile strain albeit modified from the bulk behavior.

We investigate the electronic reconstruction in (LaNiO3)n/(LaAlO3)3 (n =3, 5 and 10) superlattices due to the quantum confinement (QC) by d.c. transport and resonant soft x-ray absorption spectroscopy. In proximity to the QC limit, a Mott-type transition from an itinerant electron behavior to a localized state is observed. The system exhibits tendency towards charge-order during the transition. ab initio cluster calculations are in good agreement with the absorption spectra, indicating that the apical ligand hole density is highly suppressed resulting in a strong modification of the electronic structure. At the dimensional crossover cellular dynamicalmean-field calculations support the emergence of a Mott insulator ground state in the heterostructured ultra-thin slab of LaNiO3. PACS numbers:Prompted by the discovery of high-T c superconductivity in cuprate compounds there has been a surge of activity to discover materials with even higher transition temperature [1]. Recent remarkable advances in synthesis of artificial layers of complex oxides along with the progress in computational methods have re-energized the search for novel superconductors outside of the cuprate family [2,3].Towards the challenge, a recent proposal has been put forward to use heterostructuring and orbital engineering to turn hole-doped alternating unit-cell thin layers of a correlated metal LaNiO 3 (LNO) and band-gap dielectric LaAlO 3 (LAO) [4]. The proposal utilizes the Ni III 3d 7 low-spin state in the bulk with a single unpaired electron occupying the degenerate (d x 2 −y 2 /d 3z 2 −r 2 ) e g orbital whose nodes point to the planar and apical ligands of the octahedra, respectively. Both t-J and LDA+DMFT calculations [4,5] suggest that the quantum confinement together with the electronic correlations should make it possible to localize or empty the d 3z 2 −r 2 band leaving the conduction electron in the d x 2 −y 2 band. Epitaxial strain is also suggested as a mean of the orbital control to manipulate the d 3z 2 −r 2 orbital to appear above d x 2 −y 2 and play the analogous role of the axial orbital of the high-T c cuprates.Despite the attractive simplicity of the structure, the experimental realization of LNO/LAO superlattice (SL) presents a large degree of ambiguity in selecting a specific electronic ground state caused by (i) the intrinsic propensity of orthonickelates to charge-ordering [6,7], (ii) the orbital polarization due to chemical confinement imposed by interfacial bonding [8,9], (iii) polar discontinuity [10,11], (iv) epitaxial constraint on the crystal symmetry [12,13] and (v) quantum confinement [14,15]. Even for the undoped LNO/LAO SL (i.e. the proposed parent cuprate-like compound), the synergetic action of these phenomena will likely render the theoretical problem intractable to a priori predict whether the cuprate-like physics can be experimentally realized.Here we report on emergence of a Mott-type metalinsulator transition (MIT) at the dimensional crossover in the experimentally realized (LNO) n /(LAO) 3 SLs. By usin...