The perovksite rare-earth nickelates, RNiO$_3$ (R=La ... Lu), are a class of materials displaying a rich phase-diagram of metallic and insulating phases associated with charge and magnetic order. Being in the charge transfer regime, Ni$^{3+}$ in octahedral coordination displays a strong hybridization with oxygen to form $3d$ - $2p$ mixed states, which results in a strong admixture of $3d^8\underline{L}$ into $3d^7$, where $\underline{L}$ denotes a hole on the oxygen. To understand the nature of this strongly hybridized ground state, we present a detailed study of the Ni and O electronic structure using high-resolution soft X-ray Absorption Spectroscopy (XAS). Through a comparison of the evolution of the XAS line-shape at Ni L- and O K-edges across the phase diagram, we explore the changes in the electronic signatures in connection with the insulating and metallic phases that support the idea of hybridization playing a fundamental role.
We show that charge ordered rare-earth nickelates of the type RNiO3 (R= Ho, Lu, Pr and Nd) are multiferroic with very large magnetically induced ferroelectric (FE) polarizations. This we determine from first principles electronic structure calculations. The emerging FE polarization is directly tied to the long-standing puzzle of which kind of magnetic ordering is present in this class of materials: its direction and size indicate the type of ground-state spin configuration that is realized. Vice versa, the small energy differences between the different magnetic orderings suggest that a chosen …
The discovery of unconventional superconductivity in hole doped NdNiO2, similar to CaCuO2, has received enormous attention. However, different from CaCuO2, RNiO2 (R = Nd, La) has itinerant electrons in the rare-earth spacer layer. Previous studies show that the hybridization between Ni-dx2-y2 and rare-earth-d orbitals is very weak and thus RNiO2 is still a promising analog of CaCuO2. Here, we perform first-principles calculations to show that the hybridization between Ni-dx2-y2 orbital and itinerant electrons in RNiO2 is substantially stronger than previously thought. The dominant hybridizatio…
A slave rotor-Hartree Fock formalism is presented for studying the properties of the p-d model describing perovskite transition metal oxides, and a flexible and efficient numerical formalism is developed for its solution. The methodology is shown to yield, within an unified formulation, the significant aspects of the rare earth nickelate phase diagram, including the paramagnetic metal state observed for the LaNiO3 and the correct ground-state magnetic order of insulating compounds. It is then used to elucidate ground state changes occurring as morphology is varied from bulk to strained and un-…
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$^{(3-δ)+}$ and Ni$^{(3+δ)+}$, though agreement has not been reached on the precise value of $δ$ (e.g. for NdNiO$_3$, $δ=0.0$ and $δ=0.29$ were reported). Also, the magnetic ground state is not yet clear: collinear and non-collinear Ni-O magnetic structures have been proposed to ex…
Using first-principles calculations, we analyze the evolution of the electronic structure and magnetic properties of infinite-layer nickelates RNiO$_2$ (R= rare-earth) as R changes across the lanthanide series from La to Lu. By correlating these changes with in-plane and out-of-plane lattice parameter reductions, we conclude that the in-plane Ni-O distance is the relevant control parameter in infinite-layer nickelates. An antiferromagnetic ground state is obtained for all RNiO$_2$ (R=La-Lu). This antiferromagnetic state remains metallic across the lanthanide series and is defined by a multiorb…
The discovery of superconductivity in hole-doped infinite layer nickelates, RNiO2 (R = Nd, Pr, La) has garnered sustained interest in the field. A definitive picture of low-energy many-body states has not yet emerged. We provide new insights into the low-energy physics, based on our embedded dynamical mean-field theory calculations, and propose a correlation (U)-temperature (T) phase diagram. The key features are a low-T Fermi liquid (FL) phase, a high-T Curie-Weiss regime, and an antiferromagnetic phase in a narrow U-T region. We associate the onset of the FL phase with partial screening of N…
The rare earth nickelates RNiO3 are metallic at high temperatures and insulating and magnetically ordered at low temperatures. The low temperature phase has been predicted to be type II multiferroic, i.e. ferroelectric and magnetic order are coupled and occur simultaneously. Confirmation of those ideas has been inhibited by the absence of experimental data on single crystals. Here we report on Raman spectroscopic data of RNiO3 single crystals (R = Y, Er, Ho, Dy, Sm, Nd) for temperatures between 10 K and 1000 K. Entering the magnetically ordered phase we observe the appearance of a large number…
Non-magnetic insulating phase induced by Jahn-Teller effect in RNiO$_3$
Lawrence Livermore National Laboratory（劳伦斯利弗莫尔国家实验室）；Molecular Foundry, Lawrence Berkeley National Laboratory（分子发现中心，劳伦斯伯克利国家实验室）；Institute for Theoretical Physics, Clausthal University of Technology（理论物理研究所，克劳塔尔工业大学）；Institute for Theoretical Physics, Georg-August-Universität Göttingen（理论物理研究所，哥廷根大学）
Rare-earth nickelates R$^{3+}$Ni$^{3+}$O$_3$ (R=Lu-Pr, Y) show a striking metal-insulator transition in their bulk phase whose temperature can be tuned by the rare-earth radius. These compounds are also the parent phases of the newly identified infinite layer RNiO2 superconductors. Although intensive theoretical works have been devoted to understand the origin of the metal-insulator transition in the bulk, there have only been a few studies on the role of hole and electron doping by rare-earth substitutions in RNiO$_3$ materials. Using first-principles calculations based on density functional …
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 ins…
We study the mechanism of orbital-order melting observed at temperature T_OO in the series of rare-earth manganites. We find that many-body super-exchange yields a transition-temperature T_KK that decreases with decreasing rare-earth radius, and increases with pressure, opposite to the experimental T_OO. We show that the tetragonal crystal-field splitting reduces T_KK further increasing the discrepancies with experiments. This proves that super-exchange effects, although very efficient, in the light of the experimentally observed trends, play a minor role for the melting of orbital ordering in…
The search for oxide materials with physical properties similar to the cuprate high Tc superconductors, but based on alternative transition metals such as nickel, has grown and evolved over time. The recent discovery of superconductivity in doped infinite-layer nickelates RNiO2 (R = rare-earth element) further strengthens these efforts.With a crystal structure similar to the infinite-layer cuprates - transition metal oxide layers separated by a rare-earth spacer layer - formal valence counting suggests that these materials have monovalent Ni1+ cations with the same 3d electron count as Cu2+ in…
The electronic structures of substitutional rare-earth (RE) impurities in GaAs and cubic GaN are calculated. The total energy is evaluated with the self-interaction corrected local spin density approximation, by which several configurations of the open 4f shell of the rare-earth ion may be investigated. The defects are modelled by supercells of type REGa$_{n-1}$As$_n$, for n=4, 8 and 16. The preferred defect is the rare-earth substituting Ga, for which case the rare-earth valency in intrinsic material is found to be trivalent in all cases except Ce and Pr in GaN. The 3+ --> 2+ f-level is found…
Charge-ordered rare earth manganites Nd0.5Ca0.5MnO3, La0.25Nd0.25Ca0.5MnO3, Pr0.7Ca0.3MnO3 and Pr0.6Ca0.4MnO3 are found to exhibit dielectric constant anomalies around the charge-ordering or the magnetic transition temperatures. Magnetic fields have a marked effect on the dielectric properties, indicating the presence of coupling between the magnetic and electrical order parameters. The observation of magnetoferroelectricity in these manganites is in accord with the recent theoretical predictions of Khomskii and coworkers.
Single crystal growth of several rare earth hexaborides has been carried out by the floating zone technique. A high power Xenon arc lamp image furnace was used for the crystal growth. Large high quality crystals of LaB$_{6}$, CeB$_{6}$, PrB$_{6}$ and NdB$_{6}$, about 1 cc in volume have been obtained. Crystals of all these compounds have also been grown using enriched $^{11}$B isotope for use in neutron scattering experiments.
Based on the electronic band structure obtained from first principles DFT calculations, the opticalspectra of yttrium and neodymium nickelates are computed. We show that the results are in fairagreement with available experimental data. We clarify the electronic transitions at the origin of thefirst two peaks, highlighting the important role of transitions from t2g states neglected in previousmodels. We discuss the evolution of the optical spectra from small to large rare-earth cations andrelate the changes to the electronic band structure.
Exchange bias effect is an important attribute in several device applications. Traditionally, it is discussed as a form of exchange anisotropy at the interface between the ferromagnetic/antiferromagnetic layers of two distinct systems. We report here on the magnetically ordered alloys possessing the feature of unidirectional anisotropy, which reverses its sign across a characteristic temperature. These are admixed rare earth intermetallics, comprising two dissimilar rare earth (RE) ions, belonging to the two different halves of the 4f-series and imbibing the notion of ferromagnetic coupling be…
The magnetic properties, the magnetostriction, and the magnetoelectric effect in the d-electron free rare-earth aluminum borate TmAl$_3$(BO$_3$)$_4$ are investigated between room temperature and 2 K. The magnetic susceptibility reveals a strong anisotropy with the hexagonal c-axis as the hard magnetic axis. Magnetostriction measurements show a large effect of an in-plane field reducing both, the a- and c-axis lattice parameters. The magnetoelectric polarization change in a- and c-directions reaches up to 300 $μ$C/m$^2$ at 70 kOe with the field applied along the a-axis. The magnetoelectric pola…
We have investigated the electronic structure of rare-earth tetraborides, $\textrm{RB}_{4}$, using first-principle electronic structure methods (DFT) implemented in Quantum Espresso (QE). In this article we have studied heather-to neglected strong spin-orbit coupling (SOC) effects present in these systems on the electronic structure of these system in the non-magnetic ground state. The calculations were done under GGA and GGA+SO approximations using ultrasoft pseudopotentials and fully relativistic ultrasoft pseudopotentials (for SOC case). Perdew-Burke-Ernzerhof generalized gradient approxima…
We review recent studies of spin dynamics in rare-earth orthorhombic perovskite oxides of the type $RM$O$_3$, where $R$ is a rare-earth ion and $M$ is a transition-metal ion, using single-crystal inelastic neutron scattering (INS). After a short introduction to the magnetic INS technique in general, the results of INS experiments on both transition-metal and rare-earth subsystems for four selected compounds (YbFeO$_3$, TmFeO$_3$, YFeO$_3$, YbAlO$_3$) are presented. We show that the spectrum of magnetic excitations consists of two types of collective modes that are well separated in energy: gap…
Strong permanent magnets mainly consist of rare earths ($R$) and transition metals ($T$). The main phase of the neodymium magnet, which is the strongest magnet, is Nd$_2$Fe$_{14}$B. Sm$_{2}$Fe$_{17}$N$_{3}$ is another magnet compound having excellent magnetic properties comparable to those of Nd$_{2}$Fe$_{14}$B. Their large saturation magnetization, strong magnetocrystalline anisotropy, and high Curie temperature originate from the interaction between the $T$-3d electrons and $R$-4f electrons. This article discusses the magnetism of rare-earth magnet compounds. The basic theory and first-princ…
We show that the charge density wave (CDW) ground state below the Peierls transition temperature, $T_{CDW}$, of rare-earth tritellurides is not at its equilibrium value, but depends on the time where the system was kept at a fixed temperature below $T_{CDW}$. This ergodicity breaking is revealed by the increase of the threshold electric field for CDW sliding which depends exponentially on time. We tentatively explain this behavior by the reorganization of the oligomeric (Te$_x$)$^{2-}$ sequence forming the CDW modulation.
We present a computational study of resonant inelastic x-ray scattering (RIXS) and x-ray absorption in a representative rare-earth nickelate LuNiO$_3$. We study the changes in the spectra across the metal-insulator/site-disproportionation transition. In particular, we address the question of site-selectivity of the two methods in the disproportionated insulating phase and the signature of metal-insulator transition in the fluorescence-like feature of the RIXS spectra. To this end we use the local density approximation + dynamical mean-field theory (LDA+DMFT) approach combined with configuratio…
We present ac susceptibility measurements with the frequency spanning three orders of magnitude on single grain, icosahedral R - Mg - Zn (R = rare earth) quasicrystals. The freezing temperature in Gd-based, Heisenberg spin glasses in this family increases by ~ 2% with a frequency increase from 10 Hz to 10 kHz, whereas the freezing temperature in the non-Heisenberg members of the family is significantly more responsive to the frequency change (by 16 - 22 %), suggesting that an additional magnetic anisotropy distribution in the non-Heisenberg spin glasses causes changes in the low frequency magn…
Spin reorientation and magnetisation reversal are two important features of the rare-earth orthorhombic provskites ($RM$O$_{3}$'s) that have attracted a lot of attention, though their exact microscopic origin has eluded researchers. Here, using density functional theory and classical atomistic spin dynamics we build a general Heisenberg magnetic model that allows to explore the whole phase diagram of the chromite and ferrite compounds and to scrutinize the microscopic mechanism responsible for spin reorientations and magnetisation reversals. We show that the occurrence of a magnetization rever…
We provide a set of computational experiments based on \textit{ab initio} calculations to elucidate whether a cuprate-like antiferromagnetic insulating state can be present in the phase diagram of the low-valence layered nickelate family (R$_{n+1}$Ni$_n$O$_{2n+2}$, R= rare-earth, $n=1-\infty$) in proximity to half-filling. It is well established that at $d^9$ filling the infinite-layer ($n=\infty$) nickelate is metallic, in contrast to cuprates wherein an antiferromagnetic insulator is expected. We show that for the Ruddlesden-Popper (RP) reduced phases of the series (finite $n$) an antiferrom…
The magnetic properties of iron pnictide superconductors with magnetic rare-earth ions under strong magnetic field are investigated based on the cluster self-consistent field method. Starting from an effective Heisenberg model, we present the evolution of magnetic structures on magnetic field in RFeAsO (R=Ce, Pr, Nd, Sm, Gd and Tb) and RFe2As2 (R=Eu) compounds. It is found that spin-flop transition occurs in both rare-earth and iron layers under magnetic field, in good agreement with the experimental results. The interplay between rare-earth and iron spins plays a key role in the magnetic-fiel…
We study the rare-earth magnets on a honeycomb lattice, and are particularly interested in the experimental consequences of the highly anisotropic spin interaction due to the spin-orbit entanglement. We perform a high-temperature series expansion using a generic nearest-neighbor Hamiltonian with anisotropic interactions, and obtain the heat capacity, the parallel and perpendicular spin susceptibilities, and the magnetic torque coefficients. We further examine the electron spin resonance linewidth as an important signature of the anisotropic spin interactions. Due to the small interaction energ…
We describe the magnetic phases of hexagonal rare earth manganites RMnO3 using Landau theory. A minimal model based on four one-dimensional magnetic order parameters is developed.
The rare-earth compounds R2O2Bi (R=Tb, Dy, Er, Lu, Y) are newly discovered superconductors in the vicinity of a rare-earth magnetic long-range order. In this work, we determine the magnetic order of the parent compound Dy2O2Te by neutron scattering as the A-type antiferromagnetic structure below the Néel temperature TN=9.7K. The large staggered magnetic moment 9.4(1) μB per Dy at T=3.5K lies in the basal ab plane. In a magnetic field, anomalous magnetic properties including the bifurcation between zero-field- and field-cooling magnetization, a butterfly-shaped magnetic hysteresis, and slow mag…
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 ($δd_{\rm Ni-O} \sim 0.05 - 0.10Å$), however, a gap opens at the Fermi …
The analysis of the properties of rare-earth based materials relies on the Crystalline Electric Field theory. This theory has to be reconsidered in case of cage-type compounds, where the rare-earth ion can substantially depart from its, high symmetry, average position. It is shown that, in case of an orbital degeneracy at the cage center, a specific Jahn-Teller effect develops in the paramagnetic range: at low temperature, the distribution of the magnetic ion spreads inside the cage, the magnetic entropy is reduced, whereas the cage multipolar susceptibilities are increased. These consequences…
We studied the electronic structure of rare-earth ditelluride (ReTe_2 : Re = La, Pr, Sm, and Gd) using photoemission spectroscopy. From the x-ray photoelectron spectroscopy (XPS) study of the 3d core levels of rare-earth elements, we found that all the rare earth elements are trivalent. We have also made theoretical calculations using the Gunnarsson and Schoenhammer approximation and multiplet calculations for the rare earth elements to find that the La and Gd~3d peaks are well explained using our calculations. There is no considerable change in the line-shape of the Te~3d peaks depending on d…
The electronic structure of the rare earth nitrides is studied systematically using the {\it ab-initio} self-interaction corrected local-spin-density approximation (SIC-LSD). This approach allows both a localised description of the rare earth $f-$electrons and an itinerant description of the valence electrons. Localising different numbers of $f$-electrons on the rare earth atom corresponds to different valencies, and the total energies can be compared, providing a first-principles description of valence. CeN is found to be tetravalent while the remaining rare earth nitrides are found to be tri…
We report the magnetic properties of compounds in the KBaRE(BO3)2 family (RE= Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb), materials with a planar triangular lattice composed of rare earth ions. The samples were analyzed by x-ray diffraction and crystallize in the space group R-3m. Physical property measurements indicate the compounds display predominantly antiferromagnetic interactions between spins without any signs of magnetic ordering above 1.8 K. The ideal 2D rare earth triangular layers in this structure type make it a potential model system for investigating magnetic frustration in rare-earth-b…
Magnetic exchange in Kondo lattice systems is of the Ruderman-Kittel-Kasuya-Yosida type, whose sign depends on the Fermi wave vector, $k_F$ . In the simplest setting, for small $k_F$ , the interaction is predominately ferromagnetic, whereas it turns more antiferromagnetic with growing $k_F$. It is remarkable that even though $k_F$ varies vastly among the rare-earth systems, an overwhelming majority of lanthanide magnets are in fact antiferromagnets. To address this puzzle, we investigate the effects of a p-wave form factor for the Kondo coupling pertinent to nearly all rare-earth intermetallic…
The Crystalline Electric Field (CEF) influence is usually described by considering an ideally symmetrical rare-earth site. In the case of cage compounds, ample excursions of the rare-earth inside the cage require an adapted CEF description. A corrective, position dependent, CEF term accounts for the deviation from the perfect symmetry. In the paramagnetic range, a CEF level with orbital degeneracy thus acquires a width reflecting the rare-earth spatial distribution. In the case, frequent in cubic systems, of an orbitally degenerate CEF ground state at the center, this width introduces an addit…
We calculated electron susceptibility of rare-earth tritelluride compounds RTe$_3$ as a function of temperature, wave vector and electron-dispersion parameters. Comparison of results obtained with the available experimental data on the transition temperature and on the wave vector of a charge-density wave in these compounds allowed us to predict values and the evolution of electron-dispersion parameters with the variation of atomic number of rare-earth element R.
We present a comparative density-functional theory plus dynamical mean-field theory (DFT+DMFT) study of the two known superconducting members of the rare-earth (R) layered nickelate family: hole-doped RNiO$_{2}$ ($n=\infty$) and R$_{6}$Ni$_{5}$O$_{12}$ ($n=5$). At the same nominal carrier concentration, these two materials exhibit nearly identical electronic structures and many-body correlations effects: mass enhancements, self-energies, and occupations. However, the fermiology of the quintuple-layer nickelate is more two-dimensional-like than its infinite-layer counterpart making this new sup…
Rare-earth iron garnets $R_{3}$Fe$_{5}$O$_{12}$ are fascinating insulators with very diverse magnetic phases. Their strong potential in spintronic devices has encouraged a renewal of interest in the study of their low temperature spin structures and spin wave dynamics. A striking feature of rare-earth garnets with $R$-ions exhibiting strong crystal-field effects like Tb-, Dy-, Ho-, and Er-ions is the observation of low-temperature non-collinear magnetic structures featuring "umbrella-like" arrangements of the rare-earth magnetic moments. In this study, we demonstrate that such umbrella magneti…
Using the recently developed method we calculate the crystal field parameters in yttrium and lutetium aluminum garnets doped with seven trivalent Kramers rare-earth ions. We then insert calculated parameters into the atomic-like Hamiltonian taking into account the electron-electron, spin-orbit and Zeeman interactions and determine the multiplet splitting by the crystal field as well as magnetic $\hat{g}$ tensors. We compare calculated results with available experimental data.
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 th…
In this paper we study the effect of substituting R (rare-earth ion) by non-magnetic ions in the spin-1 chain material $R_2BaNiO_5$. Using a strong-coupling expansion and numerical density matrix renormalization group calculations, we show that spin-wave bound states are formed at the impurity site. Experimental consequences of the bound states are pointed out.
Formation of charge-density-waves in ordered electronic phases (such as charge-order) is an emergent phenomenon in the perovskite class of correlated oxides. This scenario is visualized to prevail in exotic RNiO3 (R=rare-earth) nickelates in which the structure controls the incipient charge-order to form in weak localization limit. However, any consequent effect demonstrating these nickelates in rare category of charge-density-waves conductors with controlled charge-lattice interactions has been a fundamental challenge so-far. Here, we present first evidence of the charge-density-waves in a pr…
In rare-earth cage compounds, the guest 4f ion cannot be considered as fixed at the centre of its cage. As result of the electronic degeneracy of the 4f shell, single-ion or collective mechanisms can redistribute the ion inside the cage, which can be described in terms of multipolar components. These mechanisms and their influence are here discussed and illustrated in relation with the rare-earth hexaboride series. Warning: Following our oral presentation, this manuscript should have appeared in the Proceedings of SCES 2014 (SCES 2014, International Conference on Strongly Correlated Electron S…
We describe current experimental and theoretical understanding of the pressure-induced volume collapse transitions occurring in the early trivalent rare earth metals. General features of orbitally realistic mean-field based theories used to calculate these transitions are discussed. Potential deficiencies of these methods are assessed by comparing mean field and exact Quantum Monte Carlo solutions for the one-band Hubbard and two-band periodic Anderson lattice models. Relevant parameter regimes for these models are determined from local density constrained occupation calculations.
The crystal field parameters of 13 trivalent lanthanide ions substituted for La in LaF$_3$ were calculated using the combination of the band structure and Wannier function calculations. Performing an atomic exact diagonalization with thus obtained crystal-field parameters we compute the crystal-field splitting of atomic multiplets. The calculation is compared with the available experimental results and a good agreement is found.
Nonlinear transport properties are reported in the layered DyTe$_3$ compound at temperature below the charge-density-wave (CDW) transition, $T_P=302$ K. Conductivity is increasing sharply above the threshold electric field. Under application of a rf field Shapiro steps are clearly observed. These features demonstrate for the first time CDW sliding in two-dimensional compounds.
Three independent contributions to the magnetically induced spontaneous polarization of multiferroic TbMn2O5 are uniquely separated by optical second harmonic generation and an analysis in terms of Landau theory. Two of them are related to the magnetic Mn3+/4+ order and are independent of applied fields of up to 7 T. The third contribution is related to the long-range antiferromagnetic Tb3+ order. It shows a drastic decrease upon the application of a magnetic field and mediates the change of sign of the spontaneous electric polarization in TbMn2O5. The close relationship between the rare-earth…
We have demonstrated electron-electron and electron-nuclear spin manipulations of Gd3+ ion in CaWO4 crystal. The results suggest that the studied system is perspective for multiqubit implementation in quantum computing.
The understanding of magnetic and electronic properties of rare-earth intermetallics is summarized. The properties of NdNi5 or ErNi5 are understood by considering a few electronic subsystems like the f electronic subsystem and the conduction-electron subsystem. The localized f electrons can give a substantial contribution to the low temperature specific heat with the entropy of R ln2 in case of Kramers ions. In this view the heavy-fermion behaviour is related with the removal of the Kramers degeneracy of the ground state of the f electronic subsystem.
We report measured dipolar asymmetry ratios at the LIII edges of the heavy rare earth metals. The results are compared with a first principles calculation and excellent agreement is found. A simple model of the scattering is developed, enabling us to re-interpret the resonant x-ray scattering in these materials and to identify the peaks in the asymmetry ratios with features in the spin and orbital moment densities.
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.
The general form of the nearest neighbour exchange interaction for rare earth pyrochlores is derived based on symmetry. Generally, the rare earth angular momentum degeneracy is lifted by the crystal electric field (CEF) into singlets and doublets. When the CEF ground state is a doublet that is well-separated from the first excited state the CEF ground state doublet can be treated as a pseudo-spin of some kind. The general form of the nearest neighbour exchange interaction for pseudo-spins on the pyrochlore lattice is derived for three different types of pseudo-spins. The methodology presented …
