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The role of quenched disorder, a key control to obtain novel phases and functionalities, has not yet been determined in the complex phase diagram of $\mathrm{RNi}{\mathrm{O}}_{3}$ (R = rare-earth ion) perovskites. Here we present such a study by investigating $(\mathrm{L}{\mathrm{a}}_{0.5}\mathrm{E}{\mathrm{u}}_{0.5})\mathrm{Ni}{\mathrm{O}}_{3}$ (LENO) having large R-site cation disorder. We show that in the presence of quenched disorder, (i) the resistivity drops by a few orders of magnitude across the metal-insulator transition (MIT) but the MIT shows only a subtle decrease, (ii) compressive films are completely metallic while largely tensile films are completely insulating sans a MIT, (iii) orthorhombic distortion promotes sharp MIT, and (iv) a Fermi liquid behavior even as high temperature resistivity exceeds the Mott-Ioffe-Regel limit with a bad metallic state. The low-energy terahertz conductivity dynamics obey Drude and Drude-Smith models for compressive and tensile films, respectively. All these features of disordered LENO, which are not typical of prototype ordered $\mathrm{NdNi}{\mathrm{O}}_{3}$, reveal an extraordinary sensitivity to slight structural perturbations. This study depicts the ease with which a variety of electronic phases can be tuned in disordered nickelates and emphasize the need to incorporate quenched disorder as a key control in the phase diagram of nickelates.

We have used first-principles methods to investigate the factors that control the ground state of rare-earth nickelates, studying in detail the case of NdNiO${}_{3}$. Our results suggest a complex phase diagram, with the bulk compounds standing on the edge of various instabilities that can be triggered by both electronic (e.g., changes in the Coulomb repulsion) and structural (e.g., epitaxial mismatch) means. In particular, we reveal that several phase transitions can be induced by epitaxial strain in thin films and predict that a continuous transformation between insulating spin-density-wave- and metallic spin-spiral-like solutions occurs at moderate values of the in-plane mismatch. Our results provide a coherent picture of structural and electronic effects in nickelates and have implications for current experimental and theoretical work on these compounds.

The physics of nickel perovskites is rich with various competing electronic phases that can be tuned by chemical or external degrees of freedom. As such, nickelates show strong potential for oxide electronics devices based on strongly correlated systems. However, their complexity has hitherto challenged a detailed understanding of classical material engineering effects using, e.g., epitaxial strain. Here we investigate this important pending issue by comparing experimental data with results from first-principles calculations using the Heyd-Scuseria-Ernzerhof hybrid exchange-correlation functional. The theory properly describes the magnetic ground state as well as the preferred orbital occupation observed by x-ray linear dichroism. It also shows that the strain-induced modulation of the metal-to-insulator transition temperature is likely driven by changes in the bandwidth, rather than by the charge-transfer energy.

Ab initio calculations predict a metal-insulator transition at zero temperature to occur in La$_4$Ni$_3$O$_8$ at moderate pressures as a result of a pressure-induced spin-state transition. The spin-state transition that is seen at 105 K at ambient pressure from a low-temperature high-spin state to a high-temperature low-spin state has been observed to be shifted to lower temperatures as pressure is applied. From our calculations we find that a smaller unit cell volume favors the metallic low-spin state, which becomes more stable at 5 GPa. Similar physics should take place in the related compound La$_3$Ni$_2$O$_6$, but on a different energy scale, which may account for why the transition has not been observed in this material., 6 pages, 6 figures

Among the transition-metal oxides, bismuth nickelate is particular in that, under normal conditions, it crystallizes in a triclinic structure with charge ordering in the Bi sites. Upon increasing pressure, a phase transition takes the material to the $\mathrm{Gd}\mathrm{Fe}{\mathrm{O}}_{3}$-type orthorhombic phase commonly found in perovskite oxides; recent attention to $\mathrm{Bi}\mathrm{Ni}{\mathrm{O}}_{3}$ arises, in part, because this transition can be engineered to be accompanied by a colossal negative thermal expansion. On the other hand, the closely related rare-earth nickelates also show the orthorhombic phase, but as temperature decreases they undergo a metal-insulator transition to a monoclinic phase where charge ordering appears instead in the Ni sites. Since bismuth is similar in valence and size to some of the rare-earth atoms, it has been conjectured that a $\mathrm{Bi}\mathrm{Ni}{\mathrm{O}}_{3}$ phase with charge ordering in the Ni sites might also exist---this would make this perovskite unique in that it can harbor charge ordering in either set of cation species, while the other set contains identical nominal ionic charges. In this paper we show that such a phase indeed corresponds to a special point of the energy surface of $\mathrm{Bi}\mathrm{Ni}{\mathrm{O}}_{3}$ when studied with methods based on density-functional theory. The existence of this phase could explain the somewhat conflicting experimental reports regarding the valence state of the Ni ions in the high-pressure phase of $\mathrm{Bi}\mathrm{Ni}{\mathrm{O}}_{3}$.

Here, we reinvestigate the underlying charge and magnetic correlations in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{4}$ within a combined study of muon spin relaxation ($\ensuremath{\mu}\mathrm{SR}$), x-ray photon correlation spectroscopy, synchrotron radiation single-crystal x-ray diffraction, and neutron-scattering measurements. For ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{4}$ around 1/3 hole-doping, the significant charge correlations that are responsible for the emergence of hour-glass magnetic spectra are found to be slowly fluctuating checkerboard charge ordering (CBCO) correlations with an onset temperature being independent of hole-doping. This reveals a local origin of the CBCO correlations in the rather ionic cobaltates, thus being distinct from the more covalent nickelates. Moreover, we report the observation of a very similar temperature dependence of the intensities of in-plane high- and low-energy excitations within the ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CoO}}_{4}$ hour-glass spectrum. This observation shows that nano phase separation is distinct from conventional phase separation and that there is a coupling between nanometer-sized undoped and hole-doped islands. Based on this, it is possible to propose a microscopic scenario for the suppression of the outwards-dispersing branches in hour-glass magnetic spectra.

Rare-earth nickelates exhibit several temperature-driven phase transitions that are tunable by the size of the rare-earth ions, pressure, epitaxial strain in ultrathin films, etc. We investigate the metal-insulator and Neel transitions in a series of NdNiO3 thin films with varying degrees of lattice mismatch using ultralow frequency electrical noise measurements. The noise magnitude follows a 1/f behavior and is Gaussian in the high temperature paramagnetic metallic phase of the films, however deviations are seen in the low temperature paramagnetic and antiferromagnetic insulating phases. The noise magnitude increases by orders of magnitude at temperatures below the metal-insulator transition and is indicative of inhomogeneous electrical conduction arising from phase separation. The inhomogeneous nature of conduction is corroborated by the presence of a large non-Gaussian noise signature in the low temperature phases. Well below the Neel temperature, the noise behavior evolves between Gaussian and non-Gaussian over several hours pointing to dynamically competing ground states with subtle variations within the antiferromagnetic insulating phase.

The interplay of charge, spin, and lattice correlations strongly influence the insulator-metal (I-M) transition and magnetic ordering in rare earth nickelates. In this context, we explored the low-energy charge dynamics in structurally modulated $\mathrm{PrNi}{\mathrm{O}}_{3}$ (PNO) thin films to unravel the complexity of ground state across I-M transition using terahertz (THz) spectroscopy. The THz optical constants of compressive film on $\mathrm{LaAl}{\mathrm{O}}_{3}$ (100) substrate and the tensile films on $\mathrm{NdGa}{\mathrm{O}}_{3}$ (100), (001), (110), and (111) substrates with varying orthorhombic distortion exhibit remarkably distinct features as a function of frequency and temperature. The THz conductivity of compressive film sans any I-M transition follows the Drude model. In contrast, the tensile strained films exhibit non-Drude THz conductivity, a giant positive dielectric permittivity, and negative imaginary conductivity, all of which can be explained by the Drude-Smith model. This rich variety of low-energy dynamics manifests as a function of temperature, strain, and crystal orientation. Such distinct THz spectral features, as induced by a subtle variation in strain while crossing over from tensile to compressive strain and with varying degree of orthorhombicity coupled with oxygen vacancies, reveal a novel facet of structure-property relationship of PNO.

We present low temperature resistivity and magnetotransport measurements conducted on pristine and electron doped ${\mathrm{SmNiO}}_{3}$ (SNO). The low temperature transport in both pristine and electron-doped SNO shows a Mott variable range hopping with a substantial decrease in localization length of carriers by one order in the case of doped samples. Undoped SNO films show a negative magnetoresistance (MR) at all temperatures characterized by spin fluctuations with the evolution of a positive cusp at low temperatures. In striking contrast, upon electron doping of the films via hydrogenation, we observe a crossover to a linear nonsaturating positive $\mathrm{MR}\ensuremath{\sim}0.2%$ at 50 K. The results signify the role of localization phenomena in tuning the magnetotransport response in doped nickelates. Ionic doping is therefore a promising approach to tune magnetotransport in correlated perovskites.

The rare-earth nickelates (RNiO3) exhibit interesting phenomena such as unusual antiferromagnetic order at wavevector q = (1/2, 0, 1/2) and a tunable insulator-metal transition that are subjects of active research. Here we present temperature-dependent transport measurements of the resistivity, magnetoresistance, Seebeck coefficient, and Hall coefficient (RH) of epitaxial SmNiO3 thin films with varying oxygen stoichiometry. We find that from room temperature through the high temperature insulator-metal transition, the Hall coefficient is hole-like and the Seebeck coefficient is electron-like. At low temperature the N\'eel transition induces a crossover in the sign of RH to electron-like, similar to the effects of spin density wave formation in metallic systems but here arising in an insulating phase ~200 K below the insulator-metal transition. We propose that antiferromagnetism can be stabilized by bandstructure even in insulating phases of correlated oxides, such as RNiO3, that fall between the limits of strong and weak electron correlation., Comment: *Equally contributing first authors

We revisit the electronic and magnetic structure of bilayer, La${}_{3}$Ni${}_{2}$O${}_{6}$ and trilayer, La${}_{4}$Ni${}_{3}$O${}_{8}$ nickelates, in terms of detailed first-principles calculations. Through construction of an axial orbital, we show that the crystal-field splitting obtained in a single layer case, is modified substantially in the multilayer case leading to a near degeneracy of several levels and a possible bistability between low spin and high spin state of Ni in specific cases. The issue needs to be settled by further experimental studies, followed by theoretical investigations.

To elucidate the hole doping mechanism, we have systematically studied the Ni and Cu [ital K]-edge x-ray-absorption spectra in doped Nd[sub 2]NiO[sub 4], La[sub 2]NiO[sub 4], and La[sub 2]CuO[sub 4]. We find a significant difference in the character of the doped hole between the nickelates and cuprate. We observe that both strontium substitution and introduction of excess oxygen lead to a shift of the Ni absorption edge to higher energies, by nearly 3 eV in NaSrNiO[sub 4]. This result demonstrates that the doped holes, in addition to the O 2[ital p] character, have a substantial amount of Ni 3[ital d] character (30--40 %). The intensity of the 1[ital s]-3[ital d] transition increases upon doping, consistent with increasing number of [ital d] holes. In contrast, the Cu absorption edge in the doped cuprate is shifted by much smaller amounts, indicative of a lower amount of Cu 3[ital d] character for the doped hole. The strongly mixed Ni 3[ital d] and O 2[ital p] character of the doped hole distinguishes the electronic structure of the nickelate from that of the cuprate.

The effects of strong covalency across a strongly correlated interface between two transition-metal compounds are studied. Since the charge transfer is directional, the lowest electron-removal and -addition states are often not involved in the formation of covalent bonds across the interface. This paper shows that this can lead to the formation of unusual ground states not found in the bulk. For cuprates, the formation of ``Zhang-Rice triplets'' is observed. For nickelates, we demonstrate the possibility of in-plane or out-of-plane orbital switching, whereas cobaltates are prone to spin switching. For Co and Fe compounds, a change between antiferromagnetic superexchange and ferromagnetic double exchange is found. Calculations of x-ray magnetic dichroism are presented, which could provide insight into the presence of these unusual ground states.

The electrical, magnetic, and structural properties of ${\mathrm{Sr}}_{3}{({\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x})}_{2}{\mathrm{O}}_{7}\phantom{\rule{0.3em}{0ex}}(0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.2)$ are investigated. The parent compound ${\mathrm{Sr}}_{3}{\mathrm{Ru}}_{2}{\mathrm{O}}_{7}$ is a paramagnetic metal, critically close to magnetic order. We have found that, with a Ru-site doping by only a few percent of Mn, the ground state is switched from a paramagnetic metal to an antiferromagnetic insulator. Optical conductivity measurements show the opening of a gap as large as 0.1 eV, indicating that the metal-to-insulator transition is driven by the electron correlation. The complex low-temperature antiferromagnetic spin arrangement, reminiscent of those observed in some nickelates and manganites, suggests a long-range orbital order.

The crystal and magnetic structure of ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Sr}}_{1+x}\mathrm{Mn}{\mathrm{O}}_{4}$ $(0\ensuremath{\leqslant}x\ensuremath{\leqslant}0.6)$ has been studied by diffraction techniques and high resolution capacitance dilatometry. There is no evidence for a structural phase transition related to octahedron tilting like those found in isostructural cuprates or nickelates, but there are significant structural changes induced by the variation of temperature and doping which we attribute to a rearrangement of the orbital occupation.

Inelastic pulsed neutron scattering measurements on YBa2Cu3O6.95 single crystals indicate that the sample has a distinct a-b plane anisotropy in the oxygen vibrations. The Cu-O bond-stretching-type phonons, which are suspected to interact strongly with charge, are simultaneously observed along the a and b directions due to a 7-meV splitting arising from the orthorhombicity, even though the sample is twinned. The bond-stretching LO branch with the polarization along a ~perpendicular to the chain! loses intensity beyond the middle of the zone, indicating branch splitting as seen in doped nickelates, with the second branch being located at 10 meV below. The mode along b has a continuous dispersion. These modes show temperature dependence, which parallels that of superconductive order parameter, suggesting significant involvement of phonons in the superconductivity of this compound.

We characterize the local structure and correlations of charge stripes in ${\mathrm{La}}_{1.725}{\mathrm{Sr}}_{0.275}{\mathrm{NiO}}_{4}$ using transmission-electron microscopy. We present direct evidence that the stripe modulation is indeed one-dimensional within each ${\mathrm{NiO}}_{2}$ plane. Furthermore, we show that individual stripes tend to be either site centered or bond centered, with a bias towards the former. The spacing between stripes often fluctuates about the mean separation, contributing to a certain degree of frustration of the approximate body-centered stacking along the c axis. These results confirm ideas inferred from previous neutron-diffraction measurements on doped nickelates, and demonstrate that charge-stripe order is quite different from the conventional concept of charge-density-wave order.

${\mathrm{NdNiO}}_{3}$ is studied by electron diffraction and by Raman scattering, and drastic changes are observed at the metal-insulator transition. These features are explained in the framework of a charge disproportionation of nickel, which is at variance from earlier results, but consistent with recent studies on smaller rare earth nickelates. The Raman spectra in the insulator phase are discussed with respect to the elpasolite arrangement. It is shown that several frequencies directly probe the charge transfer, which leads us to conclude that this parameter evolves over about 50 K below the transition.