Localized 4f States Research Articles

ELECTRONIC STRUCTURE AND OPTICAL PROPERTIES OF A PHOTOLUMINESCENT TANTALITE: EuKNaTaO5

The electronic structure and linear optical properties of luminescent material EuKNaTaO5 are investigated by employing full-potential linear augmented plane wave method. Our results show highly localized Eu 4f states which pinned in the energy range below the unoccupied Ta 5d states and over the occupied O 2p states. The optical spectra are analyzed and interpreted in terms of the electronic structure. It is found that Eu ions absorb the major parts of the incident energy below 3.3 eV. This is in accordance with the experimental result that EuKNaTaO5 phosphor is efficient under the excitation of 535 nm (2.3 eV) visible part of the spectrum. The linear optical properties are found to be anisotropic.

Electron correlation beyond the local density approximation: self-interaction correction ingadolinium

We report on detailed first-principles calculations which focus on the magnetic andstructural properties of the (0001) surface of gadolinium. The electronic correlation withinthe localized 4f states is treated within the self-interaction correction (SIC), thus goingbeyond the local spin-density approximation. The ferromagnetic ground state is predictedcorrectly if the SIC is applied; the effect of surface relaxations on Heisenberg exchangeparameters and on the Curie temperature are addressed by Monte Carlo calculations. TheSIC also has a profound effect on the dispersion of the d surface states, due tohybridization of the 4f states with the 5d valence states. The best agreement withphotoemission experiments is obtained within the transition state approximation, whichtakes into account the orbital relaxation. The Rashba spin–orbit coupling in the dsurface states is fully captured by our relativistic multiple scattering approach.

Ab initio studies of half-metallic ferromagnetism in carbon-doped

Abstract The electronic structures and magnetic properties in Carbon-doped CeO2 have been investigated by means of first-principles calculations based on the LSDA+U scheme. The results demonstrate a magnetic moment of 2.00 μB per supercell with one Carbon dopant which mainly stems from Hund’s rule coupling based on rather localized 2p, 5d and 4f states. The hole-mediated long-range magnetic coupling between local magnetic moments can be attributed to the collective effects of the p–p, p–d, and p–f hybridizations between C and neighboring O or Ce atoms. Ferromagnetism and half-metallic characteristics of C-doped CeO2 make it possible to be an ideal material for spintronic devices.

Effects of Phonon Confinement on Anomalous Thermalization, Energy Transfer, and Upconversion in Ln3+‐Doped Gd2O3 Nanotubes

AbstractThere is a growing interest in understanding how size‐dependent quantum confinement affects the photoluminescence efficiency, excited‐state dynamics, energy‐transfer and thermalization phenomena in nanophosphors. For lanthanide (Ln3+)‐doped nanocrystals, despite the localized 4f states, confinement effects are induced mostly via electron–phonon interactions. In particular, the anomalous thermalization reported so far for a handful of Ln3+‐doped nanocrystals has been rationalized by the absence of low‐frequency phonon modes. This nanoconfinement may further impact on the Ln3+ luminescence dynamics, such as phonon‐assisted energy transfer or upconversion processes. Here, intriguing and unprecedented anomalous thermalization in Gd2O3:Eu3+ and Gd2O3:Yb3+,Er3+ nanotubes, exhibiting up to one order of magnitude larger than previously reported for similar materials, is reported. This anomalous thermalization induces unexpected energy transfer from Eu3+ C2 to S6 crystallographic sites, at 11 K, and 2H11/2 → 4I15/2 Er3+ upconversion emission; it is interpreted on the basis of the discretization of the phonon density of states, easily tuned by varying the annealing temperature (923–1123 K) in the synthesis procedure, and/or the Ln3+ concentration (0.16–6.60%).

Dispersive properties of 4f states in rare‐earth systems

AbstractAngle‐resolved photoemission spectra of Ce, Yb, and Eu systems reveal dispersions and momentum dependent energy splittings of the 4f emissions that are obviously caused by the interaction of localized 4f states with delocalized valenceband states. The spectra may be described satisfactorily in the light of a simplified approach of the periodic Anderson model by a momentum dependence of the electron hopping matrix elements. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultrasonic study of the Yb-based heavy fermion compound YbRh2Zn20

We report ultrasonic measurements on the high quality single crystal of the Yb-based heavy fermion compound YbRh2Zn20 over a temperature range from 200 K to 0.5 K. A shallow, but clear minimum was observed in the temperature dependent elastic constants C11, (C11 – C12)/2 and C44 around 15 K, probably attributed to the ground state and low-lying excited states of Yb3 in the cubic CEF. We discuss the low-temperature elastic properties and possible energy level scheme of localized 4f state of Yb3 ions in YbRh2Zn20.CEF ground state developed at the low temperatures and physical parameters relating to a quadrupolar moment in YbRh2Zn20

Correlation effect on effective model with realistic electronic structure for heavy fermion compound YbAl3

Based on the recently proposed effective Hamiltonian for typical heavy fermion compound YbAl3, the correlation effect is systematically investigated in the physical properties. The band part of the Hamiltonian consists of conduction bands described by nearly free electron method, hybridization between the conduction and 4f-electrons, and the localized 4f-states of Yb ions on the lattice site. The correlation effect is considered by using the self-consistent perturbation theory with local approximation. The temperature dependence of the specific heat coefficient is calculated, and the anomalous behaviors are found in the low temperature region in accord with the experiments. We show that the anomalies may originate from the correlation effect and the structure of the non-interacting density of states.

Spin-resolved photoelectron spectroscopy of rare-earth overlayers on rare-earth and d-metal substrates

Spin- and angle-resolved photoelectron spectroscopy was applied for studies of electronic and magnetic structures of Eu/Gd and Ce/Fe. Ferromagnetic coupling of 4f moments of Eu and Gd was found in the 1 ML Eu/Gd(0 0 0 1) system with high net Eu magnetization at low temperatures reflected by a spin polarization of ∼15% of the Eu 4f state. In case of the 1 ML Ce/Fe(1 1 0) system the antiparallel orientation of the Ce 4f spins with respect to the magnetization direction of the Fe substrate was observed. Very different shapes of the spin-up and spin-down Ce 4f spectral weights can be explained within periodic Anderson model by spin-dependent hybridization between Ce localized 4f and itinerant valence band states.

Metamagnetism of Ferromagnetic Kondo Compound SmFe4P12

SmFe 4 P 12 is a novel ferromagnetic heavy fermion compound with a Curie temperature T c of 1.6 K. The electronic specific heat coefficient is 370 mJ/mol·K 2 . The magnetic susceptibility of La 1- x Sm x Fe 4 P 12 (0.6 ≤ x ≤0.85) with a paramagnetic ground state shows a broad maximum around 15 K. This phenomenon reminds us the case of CeRu 2 Si 2 which is in the boundary of itinerant and localized 4f-state. We measured the magnetization up to 42 T and found metamagnetic crossover at 22 T. The metamagnetism disappears with increasing temperature. Possible mechanism of the metamagnetism is discussed.

D- and f-Orbital Correlations in the REFeAsO Compounds

We estimate theoretically the strength of the local Coulomb interaction for the Fe 3d and Ce 4f shells in the REFeAsO compunds. In LaFeAsO and CeFeAsO we obtain values of the local Coulomb interaction parameter U for both Fe and Ce which are larger than those of elemental Fe and Ce metals. The Fe 3d bandwidth of REFeAsO is found to increase slightly as one moves along the RE-series. Using a combined local density approximation and dynamical mean-field theory (LDA+DMFT) approach, we study the behaviour of the localized 4f states along the rare-earth oxyarsenides REFeAsO series (RE=Ce,Pr,Nd). In CeFeAsO the occupied Ce 4f band is located just below the Fe 3d band leading possibly to a Kondo screening of the 4f local moment under applied pressure, while the unscreened local moment behaviour is expected for the Pr and Nd compounds.

The actinides—a beautiful ending of the Periodic Table

The 5f elements, actinides, show many properties which have direct correspondence to the 4f transition metals, the lanthanides. The remarkable similarity between the solid state properties of compressed Ce and the actinide metals is pointed out in the present paper. The α–γ transition in Ce is considered as a Mott transition, namely, from delocalized to localized 4f states. An analogous behavior is also found for the actinide series, where the sudden volume increase from Pu to Am can be viewed upon as a Mott transition within the 5f shell as a function of the atomic number Z. On the itinerant side of the Mott transition, the earlier actinides (Pa–Pu) show low symmetry structures at ambient conditions; while across the border, the heavier elements (Am–Cf) present the dhcp structure, an atomic arrangement typical for the trivalent lanthanide elements with localized 4f magnetic moments. The reason for an isostructural Mott transition of the f electron in Ce, as opposed to the much more complicated cases in the actinides, is identified. The strange appearance of the δ-phase (fcc) in the phase diagram of Pu is another consequence of the border line behavior of the 5f electrons. The path leading from δ-Pu to α-Pu is identified.

Fermi surface changes due to localized–delocalized f-state transitions in Ce-115 and Pu-115 compounds

The current theoretical understanding of the electronic structures of the CeMIn 5 compounds (with M=Co, Rh, and Ir) and PuCoGa 5 and PuRhGa 5 is reviewed. Both the Ce-115 and Pu-115 compounds have in common that their f-electrons appear to be poised on the edge of a localization–delocalization boundary. We consider here the implications of this characteristic on the emergent electronic structure, addressing, in particular, the computed Fermi surfaces. In order to capture the electronic structures to the best as possible, we employ three different density-functional theory based computational approaches, viz. the local-spin density approximation (LSDA) for delocalized f-states, the open-core approach for localized f's, and the LSDA+ U approach for moderately localized f's. The Fermi surfaces of CeCoIn 5 and CeIrIn 5 are rather well described assuming delocalized 4f electrons. Conversely, the better description is obtained for CeRhIn 5 assuming a localized 4f state. For the Pu-115 compounds, LSDA calculations predict a rather two-dimensional Fermi surface. However, the Fermi surface of PuCoGa 5 computed with the around-mean field LSDA+ U scheme is also quite two-dimensional and, moreover, similar to the LSDA Fermi surface. Taking other available experimental data on PuCoGa 5 into account, it appears that the electronic structure of PuCoGa 5 is best described by the AMF-LSDA+ U approach with a Coulomb U of about 3 eV.

Magnetic Form Factor ofα-Ce: Towards Understanding the Magnetism of Cerium

A favored interpretation of the gamma <--> alpha phase transition in cerium postulates the transformation of the localized 4f state in gamma-Ce to a weakly correlated itinerant 4f band in alpha-Ce. However, results of high-energy neutron inelastic scattering measurements, presented here, show clearly that the magnetic susceptibility response from alpha-Ce follows the Ce3+ form factor despite the large, 30-fold, increase in its spectral width relative to that in gamma-Ce. This observation provides, for the first time, indisputable evidence for the localized character of the 4f state in the alpha phase. The present findings appear consistent with recent calculations combining dynamical mean-field theory with the local density approximation that indicate a strongly correlated 4f state in alpha-Ce. The localized 4f state is also fundamental to the Kondo volume collapse theories for the gamma <--> alpha phase transition in cerium.

Field-Induced Suppression of the Heavy-Fermion State inYbRh2Si2

We report dc-magnetization measurements on YbRh2Si2 at temperatures down to 0.04 K, magnetic fields B< or =11.5 T, and under hydrostatic pressure P< or =1.3 GPa. At ambient pressure a kink at B* =9.9 T indicates a new type of field-induced transition from an itinerant to a localized 4f state. This transition is different from the metamagnetic transition observed in other heavy-fermion compounds, as here ferromagnetic rather than antiferromagnetic correlations dominate below B*. Hydrostatic pressure experiments reveal a clear correspondence of B* to the characteristic spin fluctuation temperature determined from specific heat.

Photoemission study of U xM 100−x films (M=Au, Pd, Ag) prepared by sputter co-deposition

Thin films of U x M 100− x (M=Au, Pd, Ag; x = 0 – 100 at % ) have been prepared by sputter codeposition from uranium and M targets. They have been studied in situ by ultra violet and X-ray photoemission spectroscopy (UPS and XPS, respectively). For all three systems, we observed regimes of composition where the 5f spectra have two components, one at the Fermi-level and one at the higher binding energy, characteristic for strong electron correlation, whereas in Pd and Ag matrices, transition from itinerant to localized behaviour takes place with dilution (in U 25Pd 75 and U 5Ag 95) and there is no domain of purely localized 5f states for U diluted in the Au matrix.

Density functional theory studies of the structure and electronic structure of pure and defective low index surfaces of ceria

We present periodic density functional theory (DFT) calculations of bulk ceria and its low index surfaces (1 1 1), (1 1 0) and (1 0 0). We find that the surface energies increase in the order (1 1 1) > (1 1 0) > (1 0 0), while the magnitude of the surface relaxations follows the inverse order. The electronic properties of the bulk and surfaces are analysed by means of the electronic density of states and the electron density. We demonstrate that the bonding in pure ceria is partially covalent and analysis of the resulting electronic states confirms the presence of localised Ce 4f states above the Fermi level. The surface atoms show only a small change in the charge distribution in comparison to the bulk and from the DOS the main differences are due to the changes in the oxygen 2p and cerium 5 d states. Investigation of the atomic and electronic structure of an oxygen vacancy on the (1 0 0) surface shows the problems DFT can have with the description of strongly localised systems, wrongly predicting electron delocalisation over all of the cerium atoms in the simulation cell. We demonstrate an improvement in the description of the strongly correlated cerium 4f states in partially reduced ceria by applying the DFT+U methodology, which leads to the appearance of a new gap state between the valence band and the empty Ce 4f band. Analysis of the partial charge density shows that these states are localised on the Ce III ions neighbouring the oxygen vacancy. In terms of classical defect chemistry, the vacancy is bound by two neighbouring Ce III ions, which have been reduced from Ce IV, i.e. V O · · + 2 Ce Ce ′ . The remaining Ce ions are in the Ce IV oxidation state. The localisation of Ce 4f electrons modifies the predicted structure of the defective surface.

Specific-heat and Magnetization Studies in Ce(NixCu1-x)5 Heavy-fermion Compounds

We have prepared the Ce(NixCu1-x)5 compounds for a wide range of concentrations of x∈〈0;0.9〉 and studied them by X-ray powder diffraction at room temperature and measured magnetization and specific heat as functions of temperature and magnetic field. Over the entire studied concentration range, the CaCu5-type hexagonal structure (the space group P6/mmm) was observed. All prepared samples were found paramagnetic down to 2 K, some of them exhibiting pronounced heavy-fermion features. The evolution of temperature dependencies of the magnetic susceptibility may be conceived in terms of gradual change of the Ce 4f-electrons character from valence fluctuating with strong admixture of Ce4+ valence state in the case of the nickel-rich compounds, towards the localized 4f states in the Ce3+ ion in the case of the copper-rich compounds. The specific heat for x 0.5 a peak is observed on the c/T vs. T curves, which can be tentatively attributed to freezing of localized Ce magnetic moments rather than to magnetic ordering transition.

Photoemission and x-ray absorption spectroscopy study of electron-doped colossal magnetoresistive manganiteLa0.7Ce0.3MnO3films

The electronic structure of La0.7Ce0.3MnO3 (LCeMO) thin film has been investigated using photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). The Ce 3d core-level PES and XAS spectra of LCeMO are very similar to those of CeO2, indicating that Ce ions are far from being trivalent. A very weak 4f resonance is observed around the Ce 4d $\to$ 4f absorption edge, suggesting that the localized Ce 4f states are almost empty in the ground state. The Mn 2p XAS spectrum reveals the existence of the Mn(2+) multiplet feature, confirming the Mn(2+)-Mn(3+) mixed-valent states of Mn ions in LCeMO. The measured Mn 3d PES/XAS spectra for LCeMO agrees reasonably well with the calculated Mn 3d PDOS using the LSDA+U method. The LSDA+U calculation predicts a half-metallic ground state for LCeMO.

高圧下ＮＭＲ 希土類化合物ＹｂＩｎＣｕ４の高圧下ＮＭＲ／ＮＱＲ

The NQR technique was applied to investigate the valence phase transition of YbInCu4 under high pressures. The Cu NQR frequency νQ increases with increasing pressure in both of low and high temperature phases. The transition temperature Tv decreases with increasing pressure at a rate of -2.2 K/kbar. The nuclear spin-lattice relaxation rate T1-1 shows no temperature and pressure dependence in the high temperature phase, suggesting the localized 4f state of Yb3+ ion. In the low temperature phase, (T1T)-1 are constant at various pressures and show a Korringa like behavior.

Crystalline and electronic structure of Pu–Ce and Pu–Ce–Ga alloys stabilized in the δ phase

In the actinides series (which corresponds to the progressive filling of the 5f sub-shell), plutonium lies at the changeover for the behavior of the 5f electrons between the light actinides (up to Np) with delocalized 5f states, and the heavy actinides (from Am on) with localized 5f states. At this boundary, the expanded δ-phase exhibits an intermediate and thus controversial behavior of its 5f electrons. This high temperature δ-phase can be stabilized at and below room temperature by alloying with so-called deltagen solutes Ga, Al, Ce and Am. In this work, some Pu–Ce and Pu–Ce–Ga alloys were studied using several techniques (dilatometry, X-ray diffraction (XRD), extended X-ray absorption fine structure spectroscopy (EXAFS), electrical resistivity and magnetic susceptibility). It is found that the mechanism of δ-Pu stabilization is far from straightforward as both Pu 5f and Ce 4f electronic states are involved, inducing complex crystalline organization while no clear localization of the 5f states can be deduced from experimental results. Ternary Pu–Ce–Ga alloys show cooperative deltagen effects of Ce and Ga.