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 Physics , 2015, DOI: 10.1016/j.jmmm.2015.06.056 Abstract: The perovskite nickelates RNiO$_3$ (R: rare-earth) have been studied as potential multiferroic compounds. A certain degree of charge disproportionation in the Ni ions has been confirmed by high resolution synchrotron power diffraction: instead of the nominal Ni$^{3+}$ valence, they can have the mixed-valence state Ni$^{(3-\delta)+}$ and Ni$^{(3+\delta)+}$, though agreement has not been reached on the precise value of $\delta$ (e.g. for NdNiO$_3$, $\delta=0.0$ and $\delta=0.29$ were reported). Also, the magnetic ground state is not yet clear: collinear and non-collinear Ni-O magnetic structures have been proposed to explain neutron diffraction and soft X-ray resonant sccattering results in these compounds, and more recently a canted antiferromagnetic spin arrangement was proposed on the basis of magnetic susceptibility measurements. This scenario is reminiscent of the situation in the half-doped manganites. In order to gain insight into the ground state of these compounds, we studied the magnetic excitations of some of the different phases proposed, using a localized spin model for a simplified spin chain which could describe these compounds. We first analize the stability of the collinear, orthogonal, and intermediate phases in the classical case. We then explore the quantum ground state indirectly, calculating the spin excitations obtained for each phase, using the Holstein-Primakoff transformation and the linear spin-wave approximation. For the collinear and orthogonal ($\theta=\pi/2$) phases, we predict differences in the magnon spectrum which would allow to distinguish between them in future inelastic neutron scattering experiments.
 Physics , 2013, DOI: 10.1038/nphys2907 Abstract: For most metals, increasing temperature (T) or disorder will quicken electron scattering. This hypothesis informs the Drude model of electronic conductivity. However, for so-called bad metals this predicts scattering times so short as to conflict with Heisenberg's uncertainty principle. Here we introduce the rare-earth nickelates (RNiO_3, R = rare earth) as a class of bad metals. We study SmNiO_3 thin films using infrared spectroscopy while varying T and disorder. We show that the interaction between lattice distortions and Ni-O bond covalence explains both the bad metal conduction and the insulator-metal transition in the nickelates by shifting spectral weight over the large energy scale established by the Ni-O orbital interaction, thus enabling very low \sigma while preserving the Drude model and without violating the uncertainty principle.
 Physics , 2008, DOI: 10.1103/PhysRevB.78.104101 Abstract: We report a temperature-dependent Raman scattering investigation of thin film rare earth nickelates SmNiO3, NdNiO3 and Sm0.60Nd0.40NiO3, which present a metal-to-insulator (MI) transition at TMI and an antiferromagnetic-paramagnetic Neel transition at TN. Our results provide evidence that all investigated samples present a structural phase transition at TMI but the Raman signature across TMI is significantly different for NdNiO3 (TMI = TN) compared to SmNiO3 and Sm0.60Nd0.40NiO3 (TMI =/ TN). It is namely observed that the paramagnetic-insulator phase (TN < T < TMI) in SmNiO3 and Sm0.60Nd0.40NiO3 is characterized by a pronounced softening of one particular phonon band around 420 cm-1. This signature is unusual and points to an important and continuous change in the distortion of NiO6 octahedra (thus the Ni-O bonding) which stabilizes upon cooling at the magnetic transition. The observed behaviour might well be a general feature for all rare earth nickelates with TMI =/ TN and illustrates intriguing coupling mechanism in the TMI > T > TN regime.
 Physics , 2004, DOI: 10.1103/PhysRevLett.93.156401 Abstract: A microscopic model for the diluted spin-mixed compounds (R_xY_(1-x))_2BaNiO_5 (R=magnetic rare-earth) is studied using Quantum Monte Carlo (QMC). The ordering temperature is shown to be a universal function of the impurity concentration x and the intrinsic Ni-chain correlation length. An effective model for the critical modes is derived. The possibility of a quantum critical point driven by the rare earth-concentration and the existence of a Griffiths phase in the high dilution limit is investigated. Several possible experimental approaches to verify the results are put forward.
 Physics , 2012, DOI: 10.1103/PhysRevLett.109.156402 Abstract: A combination of density functional and dynamical mean field theory calculations are used to show that the remarkable metal-insulator transition in the rare earth nickelate perovskites arise from a site-selective Mott phase, in which the $d$-electrons on a half of the Ni ions are localized to form a fluctuating moment while the $d$-electrons on other Ni ions form a singlet with holes on the surrounding oxygen ions. The calculation reproduces key features observed in the nickelate materials, including an insulating gap in the paramagnetic state, a strong variation of static magnetic moments among Ni sites and an absence of "charge order". A connection between structure and insulating behavior is documented. The site-selective Mott transition may be a more broadly applicable concept in the description of correlated materials.
 Physics , 2014, DOI: 10.1103/PhysRevB.91.075128 Abstract: We propose a simple theoretical description of the metal-insulator transition of rare-earth nickelates. The theory involves only two orbitals per nickel site, corresponding to the low-energy anti-bonding $e_g$ states. In the monoclinic insulating state, bond-length disproportionation splits the manifold of $e_g$ bands, corresponding to a modulation of the effective on-site energy. We show that, when subject to a local Coulomb repulsion $U$ and Hund's coupling $J$, the resulting bond-disproportionated state is a paramagnetic insulator for a wide range of interaction parameters. Furthermore, we find that when $U-3J$ is small or negative, a spontaneous instability to bond disproportionation takes place for large enough $J$. This minimal theory emphasizes that a small or negative charge-transfer energy, a large Hund's coupling, and a strong coupling to bond-disproportionation are the key factors underlying the transition. Experimental consequences of this theoretical picture are discussed.
 Physics , 2014, Abstract: We report on tunneling measurements that reveal for the first time the evolution of the quasi-particle state density across the bandwidth controlled Mott metal to insulator transition in the rare earth perovskite nickelates. In this, a canonical class of transition metal oxides, we study in particular two materials close to the T=0 metal-insulator transition: NdNiO3 , an antiferromagnetic insulator, and LaNiO3, a correlated metal. We measure a sharp gap in NdNiO3, which has an insulating ground state, of ~ 30 meV. Remarkably, metallic LaNiO3 exhibits a pseudogap of the same order that presages the metal insulator transition. The smallness of both the gap and pseudogap suggests they arise from a common origin: proximity to a quantum critical point at or near the T=0 metal-insulator transition. It also supports theoretical models of the quantum phase transition in terms of spin and charge instabilities of an itinerant Fermi surface.
 Physics , 2013, DOI: 10.1103/PhysRevLett.112.106404 Abstract: We study a model for the metal-insulator (MI) transition in the rare-earth nickelates RNiO$_3$, based upon a negative charge transfer energy and coupling to a rock-salt like lattice distortion of the NiO$_6$ octahedra. Using exact diagonalization and the Hartree-Fock approximation we demonstrate that electrons couple strongly to these distortions. For small distortions the system is metallic, with ground state of predominantly $d^8\ligand$ character, where $\ligand$ denotes a ligand hole. For sufficiently large distortions ($\delta d_{\rm Ni-O} \sim 0.05 - 0.10\AA$), however, a gap opens at the Fermi energy as the system enters a periodically distorted state alternating along the three crystallographic axes, with $(d^8\ligand^2)_{S=0}(d^8)_{S=1}$ character, where $S$ is the total spin. Thus the MI transition may be viewed as being driven by an internal volume "collapse" where the NiO$_6$ octahedra with two ligand holes shrink around their central Ni, while the remaining octahedra expand accordingly, resulting in the ($1/2,1/2,1/2$) superstructure observed in x-ray diffraction in the insulating phase. This insulating state is an example of a new type of charge ordering achieved without any actual movement of the charge.
 Physics , 2013, DOI: 10.1103/PhysRevB.89.245133 Abstract: A full implementation of the $ab$ $initio$ density functional plus dynamical mean field theory (DFT+DMFT) formalism to perform total energy calculations and structural relaxations is proposed and implemented. The method is applied to the structural and metal-insulator transitions of the rare earth nickelate perovskites as a function of rare earth ion, pressure, and temperature. In contrast to previous DFT and DFT+$U$ theories, the present method accounts for the experimentally observed structure of $La$NiO$_3$ and the insulating nature of the other perovskites, and quantitatively reproduces the metal-insulator and structural phase diagram in the plane of pressure and rare earth element. The temperature dependence of the energetics of the phase transformation indicates that the thermal transition is driven by phonon entropy effects.
 Physics , 2008, DOI: 10.1088/0953-8984/20/43/434217 Abstract: In this contribution to the special issue on multiferroics we focus on multiferroicity driven by different forms of charge ordering. We will present the generic mechanisms by which charge ordering can induce ferroelectricity in magnetic systems. There is a number of specific classes of materials for which this is relevant. We will discuss in some detail $(i)$ perovskite manganites of the type (PrCa)MnO$_3$, $(ii)$ the complex and interesting situation in magnetite Fe$_3$O$_4$, $(iii)$ strongly ferroelectric frustrated LuFe$_2$O$_4$, $(iv)$ an example of a quasi one-dimensional organic system. All these are "type-I" multiferroics, in which ferroelectricity and magnetism have different origin and occur at different temperatures. In the second part of this article we discuss "type-II" multiferroics, in which ferroelectricity is completely {\it due to} magnetism, but with charge ordering playing important role, such as $(v)$ the newly-discovered multiferroic Ca$_3$CoMnO$_6$, $(vi)$ possible ferroelectricity in rare earth perovskite nickelates of the type RNiO$_3$, $(vii)$ multiferroic properties of manganites of the type RMn$_2$O$_5$, $(viii)$ of perovskite manganites with magnetic E-type ordering and $(ix)$ of bilayer manganites.
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