Control of hidden ground-state order in $\mathrm{NdNi}{\mathrm{O}}_3$ superlattices

The combination of charge and spin degrees of freedom with electronic correlations in condensed matter systems leads to a rich array of phenomena, such as magnetism, superconductivity, and novel conduction mechanisms. While such phenomena are observed in bulk materials, a richer array of behaviors becomes possible when these degrees of freedom are controlled in atomically layered heterostructures, where one can constrain dimensionality and impose interfacial boundary conditions. Here, we unlock a host of unique, hidden electronic and magnetic phase transitions in $\mathrm{NdNi}{\mathrm{O}}_3$ while approaching the two-dimensional (2D) limit, resulting from the differing influences of dimensional confinement and interfacial coupling. Most notably, we discover a phase in fully 2D, single-layer $\mathrm{NdNi}{\mathrm{O}}_3$, in which all signatures of the bulk magnetic and charge ordering are found to vanish. In addition, for quasi-two-dimensional layers down to a thickness of two unit cells, bulk-type ordering persists but separates from the onset of insulating behavior in a manner distinct from that found in the bulk or thin-film nickelates. Using resonant x-ray spectroscopies, first-principles theory, and model calculations, we propose that the single-layer phase suppression results from an alternative mechanism of interfacial electronic reconstruction based on ionicity differences across the interface, while the phase separation in multilayer $\mathrm{NdNi}{\mathrm{O}}_3$ emerges due to enhanced 2D fluctuations. These findings provide insights into the intertwined mechanisms of charge and spin ordering in strongly correlated systems in reduced dimensions and illustrate the ability to use atomic layering to access hidden phases.

Figure 1

(a) X-ray diffraction $\left(\theta \text{−}2\theta \right)$ scan around the (002) Bragg peak for $\mathrm{NdNi}{\mathrm{O}}_3$ film (NNO) and superlattices (SL) grown on $\mathrm{LaAl}{\mathrm{O}}_3$ (LAO). Inset shows x-ray reflectivity for $m=4$ and $m=1$ superlattices with simulated fits. (b)–(d) Postgrowth RHEED patterns for (b) $m=4$, (c) $m=2$, and (d) $m=1$ superlattices.

Figure 2

(a) Resistivity vs temperature for $\mathrm{NdNi}{\mathrm{O}}_3$ film and superlattices (solid and dashed line for 50-uc film shows resistance measured during heating and cooling, respectively). (b)–(d) Resistivity (ρ) vs temperature for $m=1$ superlattice (blue circles) and linear fit (red line) to (b) small polaron hopping [Eq. (1)], (c) activated conduction [Eq. (2)], and (d) variable range hopping [Eq. (3)] models. (e)–(h) Polaron energy, $W$, determined by Eq. (4), for the (e) 50-uc film and (f) $m=4$, (g) $m=2$, and (h) $m=1$ superlattices.

Figure 3

(a) $T=300\mathrm{K}$ and (b) $T=77\mathrm{K}$ Ni $L$ edge XAS data. Insets provide a zoom-in of $L_3$ multiplet structure for respective temperatures, showing splitting of peaks $A$ and $B$.

Figure 4

Spectral parameters extracted from fits of $\mathrm{Ni}{L}_3$ XAS data for $\mathrm{NdNi}{\mathrm{O}}_3$ film and superlattices as a function of temperature $\left(Q_I=\mathrm{peak}\mathrm{height}\mathrm{ratio},{\gamma }_A=\mathrm{FWHM}\mathrm{of}\mathrm{peak}A,\mathrm{\Delta }{E}_0=\mathrm{peak}\mathrm{splitting}\right)$. Error bars are determined from fit uncertainties. Dashed lines are guides to the eye, and green shading indicates the temperature region of the $\mathrm{\Delta }{E}_0$ transition.

Figure 5

(a) Schematic of RSXS measurement (fixed $\chi ={45}^{\circ }$, varied $\vartheta )$. $\chi$ and $\vartheta$ are the rotation angles of the sample about the incident beam direction. (b) Energy dependence of $q=(\frac{1}{4},\frac{1}{4},\frac{1}{4})$ diffraction intensity through the Ni $L$ edge for the 50-uc $\mathrm{NdNi}{\mathrm{O}}_3$ film at $T=30\mathrm{K}$. (c) Angular scan along the (111) direction through the magnetic Bragg reflection at the $\mathrm{Ni}{L}_3$ peak energy $\left(E=853\mathrm{eV}\right)$ and $T=30\mathrm{K}$ (symbols=normalized data and solid lines=Lorentzian fits). The left axis corresponds to 50 uc film and the right axis to the superlattices.

Figure 6

Temperature dependence of the integrated intensity normalized at $T=30\mathrm{K}$ for magnetically ordered $\mathrm{NdNi}{\mathrm{O}}_3$ film and superlattices. Dashed lines are fits to models of the magnetic-order parameter (see text for details) with $T_N=138$, 152, and 137 K for the 50-uc film, $m=4$, and $m=2$ superlattices, respectively.

Figure 7

(a) In-plane $\left(I_{xy},\vec{E}\perp \vec{c}\right)$ and (b) out-of-plane $\left(I_z,\vec{E}\parallel \vec{c}\right)$ absorption spectra for $m=4$ and $m=1$ superlattices at $T=270\mathrm{K}$ zoomed in around the $\mathrm{Ni}{L}_3$ peak. Measured data and two-peak fits are shown as open circles and solid lines, respectively. (c) Peak height ratio dichroism $\left(\mathrm{\Delta }{Q}_I=Q_{I_z}-Q_{I_{xy}}\right)$ as a function of temperature for $\mathrm{NdNi}{\mathrm{O}}_3$ film and superlattices. The right side of (c) shows schematic energy level diagrams corresponding to positive and zero $\mathrm{\Delta }{Q}_I$.

Figure 8

Schematic of the $\mathrm{NdNi}{\mathrm{O}}_3/\mathrm{NdAl}{\mathrm{O}}_3$ interface depicting the metal-oxygen bonding [single (100) plane displayed and Nd not shown for clarity]. Blue curves indicate hopping of mobile electrons between neighboring sites, which is suppressed between Al and ${\mathrm{O}}_{\mathrm{b}}$ due to the high energy of the unoccupied states on Al.

Figure 9

Phase diagram of $\mathrm{NdNi}{\mathrm{O}}_3$ superlattices as a function of the number of confined $\mathrm{NdNi}{\mathrm{O}}_3$ layers, $m$, along with data from a 50-uc-thick $\mathrm{NdNi}{\mathrm{O}}_3$ film $\left(\mathrm{PM}=\mathrm{paramagnetic},\mathrm{AFM}=\mathrm{antiferromagnetic},\mathrm{CO}=\mathrm{charge}\mathrm{ordered}\right)$. $T_{MI}$ (red squares) and $T_W$ (green triangles) are determined from transport, $T_{CO}$ is determined from XAS, and $T_N$ (blue circles) is determined from RSXS. Error bars denote uncertainty in fits to experimental data. Solid and dashed lines are guides to the eye.

Figure 10

Schematics of the atomic and electronic structure for (a) the high-temperature, bulk metallic phase; (b) the low-temperature bulk charge- and spin-ordered phase; (c) the $m=2$ superlattice; and (d) the $m=1$ superlattice. Ni and O are shown as gray and red circles, respectively. Orbital configurations ignore O $p$ states for clarity.

Figure 11

Examples of measured $\mathrm{Ni}{L}_3$ edge XAS spectra (circles), along with results of two-peak fitting (solid and dashed lines) using Eq. (A1). Spectra shown are from the (a) 50-uc $\mathrm{NdNi}{\mathrm{O}}_3$ film at $T=300\mathrm{K}$ and (b) 77 K, and from the (c) $m=1$ $\mathrm{NdNi}{\mathrm{O}}_3/\mathrm{NdAl}{\mathrm{O}}_3$ superlattice at $T=300\mathrm{K}$ and (d) 77 K.

Figure 12

(a), (b) Energy dependence of $q=(\frac{1}{4},\frac{1}{4},\frac{1}{4})$ scattering intensity through the Ni $L$ edge for the (a) $m=4$ and (b) $m=2$ superlattice taken at $T=30\mathrm{K}$ and $T=15\mathrm{K}$, respectively. (c), (d) Reciprocal space map of magnetic scattering around $q=\left(\frac{1}{4}\frac{1}{4}\frac{1}{4}\right)$ for the (c) $m=4$ superlattice and (d) 50-uc $\mathrm{NdNi}{\mathrm{O}}_3$ film taken at the $\mathrm{Ni}{L}_3$ peak energy $\left(E=853\mathrm{eV}\right)$ and $T=30\mathrm{K}$. Inset of (d) shows linear intensity scale for both maps is shown (max/min are different for each map).

Figure 13

XAS data measured at $T=77\mathrm{K}$ for $\mathrm{NdNi}{\mathrm{O}}_3$ and $m=1$ superlattice, along with calculated spectra (see text for details of calculations).