Emergent behavior of LaNiO3 in short-periodic nickelate superlattices

Heterostructure engineering provides an efficient way to obtain several emergent phases of LaNiO3, as demonstrated in recent studies. In this work, a new class of short-periodic superlattice, consisting of LaNiO3 and EuNiO3, has been grown by pulsed laser interval deposition to investigate the effect of structural symmetry mismatch on the electronic and magnetic behaviors. Using synchrotron-based soft and hard x-ray resonant scattering experiments, we have found that these heterostructures undergo simultaneous electronic and magnetic transitions. Most importantly, LaNiO3 within these artificial structures exhibits a new antiferromagnetic, charge ordered insulating phase, which may be a potential candidate to achieve high temperature superconductivity.

The prediction of achieving high Tc superconductivity in LaNiO 3 through interface and orbital engineering has led to remarkable progress over the last 10 years in the synthesis and characterization of RENiO 3 -based heterostructures, 1-4 culminated with the very recent discovery of superconductivity in Nd 0.8 Sr 0.2 NiO 2 . 5 Bulk NdNiO 3 (NNO) is metallic and paramagnetic without any charge ordering (CO) and has an orthorhombic structure at room temperature. 6,7 On lowering the temperature, NNO undergoes simultaneous transitions to an insulating, antiferromagnetic, charge ordered phase having monoclinic structural symmetry. On the other hand, LaNiO 3 (LNO) has a rhombohedral structure and remains paramagnetic as well as metallic down to the lowest temperature. 6,7 The absence of any superconductivity in a LNO-based analogous compound (La 0.8 Sr 0.2 NiO 2 ) 5 with Ni in square planar oxygen coordination points toward possible strong interconnections between the electronic and magnetic phases of the parent compound and the doping induced superconductivity. In this paper, we have investigated whether an emergent antiferromagnetic, charge ordered phase can be achieved in a LaNiO 3 based system, which can be used to achieve unconventional superconductivity.
The heterostructure route has been shown to enable several unconventional behaviors of LNO (see Refs. 3 and 4, and references therein). For example, unlike the negligible orbital polarization of bulk LNO, the LaNiO 3 /LaTiO 3 heterostructure exhibits strong orbital anisotropy. [8][9][10] Helical spin configurations were realized in LNO through the interfacial charge transfer from La 0.67 Sr 0. 33 MnO 3 layers. 11 Such charge transfer can be further tuned by epitaxial strain. 12 The reduction in LNO film thickness results in an insulating phase due to the quantum confinement effect. [13][14][15][16][17][18] A ferromagnetic phase in the epitaxial LNO film on the LaAlO 3 (1 1 1) substrate was also demonstrated. 19 Interestingly, magnetic ordering, analogous to the E ′ -AFM phase and without any charge ordering, was obtained in two UCs (unit cells) of LNO layers sandwiched between the insulating spacer LaAlO 3 . 16,20,21 Despite all these findings and somewhat surprisingly, long-range charge ordered (CO), antiferromagnetic phase of LNO has not been demonstrated so far.
Apart from the abovementioned RENiO 3 /ABO 3 (B ≠ Ni) heterostructures, the epitaxial growth of two different members of the RENiO 3 series in a layer-by-layer way offers another interesting route to achieve unconventional behaviors. [22][23][24] For example, bulk EuNiO 3 (ENO) undergoes simultaneous electronic and structural transition at 410 K and a magnetic transition around TN ∼ 200 K. However, the artificial structure in the form of [1 UC ENO/1 UC LNO] (UC = unit cell in pseudocubic setting) superlattice (SL) grown on single crystalline NdGaO 3 exhibits all four simultaneous transitions similar to bulk NNO. 22 As illustrated in Fig. 1(a), each Ni in the 1ENO/1LNO SL [marked as Ni(I)] has Eu on one side and La on the opposite side along [0 0 1]pc, implying that each layer is equivalent to an ordered analog of the Eu 0.5 La 0.5 NiO 3 composition. 24   and Ni(III) in Fig. 1(b)]. Since bulk ENO and LNO have different octahedral rotational patterns a − a − c + vs a − a − a − , 25,26 strong structural competition between ENO and LNO layers in these [mENO/mLNO] systems can lead to emergent electronic and magnetic phases.
Here, we report the development of a new LNO-based heterostructure and demonstrate the emergence of a charge ordered antiferromagnetic insulating (AFI) phase of LaNiO 3 . To avoid complications caused by the quantum confinement and/or interfacial charge transfer, we have judiciously built a series of [m EuNiO 3 /m LaNiO 3 ] × N superlattices (here, N denotes the number of repeats) using pulsed laser deposition. [22][23][24] While the long-periodic superlattice (m = 8) shows entirely metallic behavior, the short-periodic superlattices undergo metal to insulator transition. Soft x-ray scattering experiments not only have found a simultaneous paramagnetic to antiferromagnetic transition but also confirmed that the magnetism is developed throughout the ENO and LNO layers. Resonant hard x-ray scattering experiments have revealed the signature of monoclinic symmetry and Ni resonance features in the metallic phase of the m = 5 film. The Ni resonance further increases after entering into the insulating phase, signifying the development of charge ordering throughout the whole superlattice. Magnetotransport measurements show a sign change in the Hall coefficient RH on crossing the transition temperature in all cases, which can be explained as a consequence of Fermi surface reconstruction. The combination of advanced experimental probes has confirmed the successful creation of an antiferromagnetic, charge ordered insulating phase in LNO, which is otherwise a paramagnetic metal in the bulk form.
mENO/mLNO superlattices were grown on single crystalline NdGaO 3 (1 1 0)or [(0 0 1)pc] (here, or and pc denote orthorhombic and pseudocubic setting, respectively) substrates using a pulsed laser deposition system. [22][23][24] The layer-by-layer growth of each unit cell of both ENO and LNO has been confirmed by in situ reflection high energy electron diffraction (not shown). For each SL, the deposition on the NGO substrate starts with an ENO layer and terminates with a LNO surface layer. The sample thickness (2 mN) is 36 UC for SLs with m = 1, 3, and 30 UC (32 UC) for the m = 5 (m = 8) sample. The sample quality has been checked by xray diffraction using a laboratory based x-ray diffractometer and also by using a six-cycle diffractometer in 6-ID-B beamline of Advanced Photon Source (APS) in Argonne National Laboratory. Magneto-transport properties of these samples have been measured in the four probe Van Der Pauw geometry using a Quantum design PPMS (Physical Property Measurement System). The issue of charge ordering in these thin films has been investigated by resonant hard x-ray scattering experiments on the Ni-K edge in 6-ID-B beam line of APS. Resonant soft x-ray scattering experiments at the Ni-L 3 edge were performed at beam line 4.0.2 of the Advanced Light Source (ALS) to probe the E ′ -antiferromagnetic order.
The effect of epitaxial strain of these ENO/LNO SLs has been investigated using synchrotron x-ray diffraction. Surprisingly, the effect of tensile strain in LaNiO 3 and EuNiO 3 ultra-thin films is compensated by octahedral rotations and breathing-mode distortions, instead of the expected out-of-plane compression. [26][27][28] Figure 1(c) shows representative diffraction scans along the (0 0 l)pc truncation rod. As seen, the diffraction pattern of each SL consists of a sharp substrate peak, a broad film peak (indicated by #), and a set of Kiessig fringes. The presence of the satellite peaks further testifies for the intended superlattice structure of each film. Surprisingly, the out-ofplane lattice constants of all of these SLs (cpc = 3.793 Å, 3.793 Å, and 3.791 Å for m = 1, 3, and 5 sample, respectively) are very similar to the cpc (3.8 Å) of the EuNiO 3 thin film on the NGO substrate. 28 This observation clearly suggests that the LNO layers follow the structural response of the ENO layers within these short-periodic superlattices.
The cpc of 8ENO/8LNO is slightly larger (3.807 Å), pointing toward different structural effects in the LNO layers within a long-periodic superlattice.
The modification in the electronic behavior as a function of m has been investigated by dc-transport measurements. Using a parallel resistor model with bulk-like ENO, bulk-like LNO, and the interfacial ELNO layer, the expected resistance of the SL can be expressed as  Fig. 2(a)]. In sharp contrast, all shortperiodic SLs exhibit first order MIT as a function of temperature.
The transport data of m = 1 SL also show the first order MIT of the interfacial ELNO layer [marked as Ni(I) in Fig. 1]. Unexpectedly, a large change in the layer number (e.g., m = 1 → 5) within each period does not result in any drastic change in the transition temperature (Table I). The observation of a MIT in the m = 5 film implies that bulk-like LNO layers marked by Ni(III) in Fig. 1(b) also become insulating below T MIT ∼ 120 K. The observation of strong thermal hysteresis signifies the spatial coexistence of metallic and insulating phases. [29][30][31][32] Apart from the bulk LNO, the metallic phase of RENiO 3 shows a distinct non-Fermi liquid (NFL) behavior with the NFL exponent tunable by hydrostatic pressure or by epitaxial strain. 23,[33][34][35] To extract the value of the power exponent n, we plot log(dRs/dT) vs log(T) shown in Fig. 2(b), which yields n = 1.05 ± 0.05, which is very close to n = 1 for T > 175 K for the m = 5 sample. Similar linear Tdependent behavior is also observed for the 1ENO/1LNO SL. 23 Such NFL behavior implies that the electronic behavior of LNO in these artificial structures is very different compared to the bulk LNO. The insulating phase of all RENiO 3 based systems hosts an E ′ -type antiferromagnetic phase (E ′ -AFM), characterized by the (1/2 0 1/2)or [≡(1/4 1/4 1/4)pc] magnetic wave vector. 36 A reliable estimate of the magnetic transition temperature (TN) can be obtained from resistivity analysis. 37,38 The d(lnRs)/d(1/T) vs T plot [ Fig. 2(c)] yields TN of 145 K, 115 K, and 110 K for SLs with m = 1, 3, and 5 SL, respectively, illustrating simultaneous electronic and magnetic transitions of these superlattices.
Simultaneous MIT and paramagnetic to E ′ -AFM transition for m = 1 SL were also confirmed by our earlier resonant soft x-ray diffraction (RSXD) experiments with the photon energy tuned to 852 eV. 22 Such a RSXD technique has been demonstrated as a very effective way to investigate the E ′ -AFM phase of various RENiO 3 thin films and heterostructures. 20,22,24,34,[39][40][41][42][43] As shown in Fig. 3(a), a (1/4 1/4 1/4)pc diffraction peak is also observed at 50 K in RSXD of m = 5 SL, which vanishes gradually with increasing T. A plot of the area under this peak as a function of T gives TN ∼ 115 ± 5 K [see Fig. 3(b)], which is very close to the values of TN obtained from our transport data shown in Fig. 2(c). Thus, apart from the  interfacial unit cells, bulk-like LNO layers also undergo simultaneous metal-insulator and paramagnetic-antiferromagnetic transitions in such heterostructures. The temperature dependence of the Lorentzian half width at half maximum (HWHM) of the E ′ -AFM peak is plotted in the upper panel of Fig. 3 Fig. 3(c)], which is larger than the film thickness along the [1 1 1]pc direction. This finding establishes that the E ′ -antiferromagnetic spin arrangement is developed throughout the entire sample, including the bulk-like LNO layers.
The microscopic origin of the simultaneous electronic and magnetic transitions in RENiO 3 can be accounted by the hole-Fermi surface nesting driven spin-density wave (SDW) transition. 20,40,44,45 The consequence of nesting driven transition in multi-band nickelates can be captured by measuring the Hall coefficient (RH) as a function of temperature. 38 For example, the previous measurements of RH 38 reveal that RH, dominated by the contribution from large hole pockets, is positive in the paramagnetic metallic phase and becomes negative below the SDW transition as the remaining small electron pockets contribute to RH in the antiferromagnetic insulating phase. Figures 4(a) and 4(b) show the variation of transverse resistance (Rxy) as a function of the applied magnetic field (H) after antisymmetrization 38 for the m = 3 and m = 5 SLs, respectively. These measurements were performed in the cooling run. As shown in Fig. 4(a), the positive slope of the curves RH (=tdRxy/dH, t = film thickness) is hole-like above 100 K for the m = 3 sample and then becomes electron-like. For the m = 5 film, RH is also hole-like down to 120 K and is completely electron-like at 80 K.
In the intermediate temperature range around 100 K, Rxy shows a non-linear behavior with H and switches from the electron-to holelike behavior with increasing H. Such a small value of Rxy indicates that the difference in volume between electron and hole pockets is very small at that temperature. Similar switching of RH was also observed in the 1ENO/1LNO SL (not shown). The crossover of RH in the vicinity of T MIT ∼ TN strongly suggests a SDW driven origin of the antiferromagnetic insulating (AFI) phase of these ENO/LNO superlattices.
After confirming the simultaneous electronic and magnetic transitions in SLs, we investigated the issue of the highly debated charge ordering. Unlike the bulk samples, thin films can host an insulating phase without any CO. 22,40,43,46 The conventional picture of charge disproportionation (CD) on Ni ions (d 7 +d 7 → d 7+δ + d 7−δ ) 40,47-51 is also challenged by the alternative mechanism of the bond disproportion (BD) transition, specially proposed for the most distorted members of the series. 52-56 Such BD transition (d 8 L+d 8 L → d 8 + d 8 L 2 , L represents a hole on oxygen) results in a rocksalttype pattern with alternating NiO 6 octahedra with d 8 having longbond (LB) and d 8 L 2 having short-bond (SB). Most importantly, the results of x-ray absorption spectroscopy and x-ray scattering experiments, conventionally used to conclude CD on Ni sublattices [48][49][50]57,58 can be also accounted by the BD transition. 21,56,59,60 However, a larger amount of BD can generate a significant amount of Ni eg charge disproportionation. For example, double cluster calculation with a BD of δd ∼ 0.2 Å by Green et al. 59 has not only obtained unequal spin values (S ∼ 1.0 and 0.1 on LB and SB sites, respectively) but also a difference of ∼0.3e in the eg electronic occupancy between LB and SB sites. It is important to note that   44,45 In order to investigate BD/CD in the weakly insulating phase of the m = 5 film, we have measured resonant x-ray scattering at the Ni-K edge (1 s → 4p) around (0 1 1)or [≡(−1/2 1/2 1/2)pc] and (1 0 1)or [≡(1/2 1/2 1/2)pc] reflections. 21,22,[48][49][50] The details of the energy and momentum dependence of these diffraction peaks are given in Ref. 61. Here, we note that the (−1/2 1/2 1/2)pc peak is allowed for monoclinic and forbidden for orthorhombic symmetry. On the other hand, the (1/2 1/2 1/2)pc reflection is allowed in both monoclinic and orthorhombic phases, but both of these reflections are forbidden for rhombohedral symmetry. As shown in Fig. 5(a), the m = 5 sample exhibits a resonance feature for the (−1/2 1/2 1/2)pc diffraction peak around 8.345 keV photon energy at 50 K. Generally, both the resonant intensity (Ires) and the off-resonant intensity (I offres ) decrease with the increase of T as previously reported for the insulating phase of the thick NdNiO 3 film and for the 1ENO/1LNO SL. 21,22,48 However, I offres increases strongly up to 90 K for m = 5 SL [ Fig. 5(b)] suggesting the existence of additional structural effects below 90 K. Furthermore, both Ires and I offres decrease above 90 K, where the magnetic correlation length also decreases. This strongly implies strong interconnection between charge ordering and magnetic ordering. Interestingly, Ires is also non-zero in the metallic phase and the line shape is very similar to the resonant line shape observed in the insulating phase of the EuNiO 3 thin film. 42 The resonance and off-resonance intensities for the (1/2 1/2 1/2)pc diffraction peak increase up to 90 K and shows a marked jump across the insulator to metal transition [see From these experimental results, we suggest the following scenario for simultaneous metal-insulator, CO, and magnetic transitions in these SLs. As shown in Fig. 1(a), m = 1 SL consists of only interfacial Nis, having EuO on one side and LaO layers on the opposite side. The site-centered type SDW transition [ Fig. 5(f)] in all of these interfacial layers can give rise to the AFI phase with CO. Apart from the interfacial NiO 2 layers, m = 5 SL also contain bulk-like ENO and bulk-like LNO layers. The interfacial layers and the bulklike LNO layers remain metallic down to 120 K and the bulk-like ENO layers are bond ordered insulators with monoclinic symmetry, resulting in the overall metallic behavior of the sample. In addition to the interfacial layers, bulk-like LNO layers also undergo the SDW transition to develop the antiferromagnetic, charge ordered, insulating phase throughout the 5ENO/5LNO sample. Unlike the pure SDW phase without CO obtained in the LaNiO 3 /LaAlO 3 SL through dimensionality control, 16,20,21 LaNiO 3 layers in the present study undergo the SDW transition in spite of the presence of three dimensional NiO 6 octahedral networks, resulting in this emergent charge ordered, antiferromagnetic phase of LaNiO 3 , which is unattainable in the bulk.
To summarize, our detailed measurements, including magnetotransport, hard x-ray resonant scattering, and soft x-ray resonant magnetic scattering, have found simultaneous metalinsulator, charge ordering, and magnetic transitions in these (EuNiO 3 )m/(LaNiO 3 )m SLs. Moreover, these measurements demonstrated strong interrelation between the spin and charge orders. In addition, the Hall coefficient shows crossover from the hole-like to electron-like behavior across the transitions. All of these experimental observations can be successfully accounted by the notion of Fermi surface nesting emerging in the bulk-like LNO layers and the interfacial layer. The emergent antiferromagnetic, charge ordered phase of LaNiO 3 can be explored as a parent compound to achieve high Tc superconductivity.