Disentangled Cooperative Orderings in Artificial Rare-Earth Nickelates

Coupled transitions between distinct ordered phases are important aspects behind the rich phase complexity of correlated oxides that hinder our understanding of the underlying phenomena. For this reason, fundamental control over complex transitions has become a leading motivation of the designer approach to materials. We have devised a series of new superlattices by combining a Mott insulator and a correlated metal to form ultrashort period superlattices, which allow one to disentangle the simultaneous orderings in ${\mathrm{RENiO}}_3$. Tailoring an incommensurate heterostructure period relative to the bulk charge ordering pattern suppresses the charge order transition while preserving metal-insulator and antiferromagnetic transitions. Such selective decoupling of the entangled phases resolves the long-standing puzzle about the driving force behind the metal-insulator transition and points to the site-selective Mott transition as the operative mechanism. This designer approach emphasizes the potential of heterointerfaces for selective control of simultaneous transitions in complex materials with entwined broken symmetries.

Figure 1

(a) Phase diagram of bulk ${\mathrm{RENiO}}_3$ [9, 10, 11]. Arrows denote individual members, which are used in this Letter to make superlattices. (b) Rock-salt-type charge ordering with exaggerated ${\mathrm{Ni}}^{3\pm \delta }$ radius variations and $E^{\prime }$-AFM spin ordering [25] for bulk ${\mathrm{RENiO}}_3$ (RE and O atoms omitted for clarity). Pseudocubic (pc) (1 1 1) planes are highlighted. Schematic crystal structure of (c) 1ENO/1LNO and (d) 2ENO/1LNO superlattices grown along the pseudocubic [0 0 1] direction. All Ni’s in 1ENO/1LNO SL have Eu in one side and La on the opposite side along $[001{]}_{\mathrm{pc}}$. However, 2ENO/1LNO has two types of Ni: Ni(I) with Eu and La on opposite sides and Ni(II) with Eu on both sides.

Figure 2

Upper panels of (a) and (b) display the temperature dependence of the dc resistivity of 1ENO/1LNO and 2ENO/1LNO, respectively. Lower panels show the temperature dependence of the ($(1/201/2{)}_{\mathrm{or}}$ Bragg peak intensity corresponding to an $E^{\prime }$-type antiferromagnetic state. Magnified view of the resistivity around $T_{\mathrm{MIT}}$ has been shown in Supplemental Material [39]. The inset in (b) shows the measured magnetic scattering for 100 and 230 K for 2ENO/1LNO.

Figure 3

(a) $(011{)}_{\mathrm{or}}$ resonance energy scan for the 1ENO/1LNO SL at various temperatures. Resonance does also persist in the metallic state (b). (c) Temperature dependence of CO parameter [45] for 1ENO/1LNO. (d) $(011{)}_{\mathrm{or}}$ resonance for the 2ENO/1LNO SL showing a strong resonance signal but no significant change across $T_{\mathrm{MIT}}\sim 245\mathrm{K}$ is observed.

Figure 4

(a) XAS and XMCD of the 2ENO/1LNO SL, measured with $H=5\mathrm{T}$ at 50 K and XAS of NiO. To rule out any artifacts, XMCD was measured with both $+5$ and $-5\mathrm{T}$, and their difference (divided by 2) has been plotted. The strong artifacts in XMCD at the very strong La $M_4$ peak position do not exactly cancel with this field flipping, resulting in the small artifacts indicated by *. (b) Real and imaginary part of the magnetic scattering factor. (c) Energy dependence of the magnetic scattering intensity was simulated using the data shown in (b) and is compared with the experimentally observed energy scan of the $E^{\prime }$ magnetic structure.