Localized 4f Electrons Research Articles

Evolution of atomic and electronic structure of magnetic Gd-doped gold clusters

The evolution of atomic and electronic structure of small Aun (n = 1–16, and 55) clusters doped with a Gd atom has been investigated using density functional theory within generalized gradient approximation for the exchange–correlation energy. Pure gold neutral clusters with n up to 15 are planar. However, with the doping of a Gd atom, the atomic structure of gold clusters changes, and there is a transition from planar-like structures to three dimensional structures at n = 10. The electronic structure of Gd-doped gold clusters shows a sharp increase in the highest occupied–lowest unoccupied molecular orbital (HOMO–LUMO) gap for certain sizes giving rise to their magic behavior. All clusters are magnetic with large magnetic moments ranging from 6 to 8 μB primarily due to the localized 4f electrons on Gd. This makes such clusters with large HOMO–LUMO gaps magnetic superatoms. The main interaction between gold and gadolinium atoms in the clusters is due to hybridization between Au-6s and Gd-5d6s orbitals. Our results indicate the emergence of a wheel structure for Gd@Au7, a symmetric cage structure at n = 15 for Gd@Au15 and n = 16 for Gd@Au16+ and Eu@Au16 corresponding to an electronic shell closing at 18 valence electrons leaving aside the f electrons on Gd while for Gd-doped Au55 corresponding to 58 valence electrons, a Au9Gd@Au46 core–shell structure is obtained in which the Gd atom connects the core of Au9 with the Au46 shell. The binding energy shows odd–even oscillations with enhancement due to Gd doping compared with pure gold clusters. Such magnetic clusters of gold could have multifunctional biological applications in drug delivery, sensor, imaging, and cancer treatment.

Crossover between BCS superconductor and BEC states in the attractive Anderson lattice model

Using the method of Green functions equation of motion we considered the three dimensional Anderson lattice model with attractive interaction between f-electrons. We studied the appearance of superconductivity as a function of this attractive interaction and a hybridization between localized f-electrons and conduction c-electrons. The zero temperature phase diagram as a function of the hybridization and the strength of the attractive interaction is obtained. In this phase diagram a quantum phase transition is found which is characterized by a break up of the conventional BCS superconductor state, as the hybridization increases after a certain critical value, Vc, and, at the same time, the beginning of a metallic-like state with preformed pairs. At sufficiently large attractive interactions and for V>Vc, a superfluid state of Bose condensed pairs appears. We investigate for a possible coexistence between these pairs that form the superfluid state, characterized by gapped dispersion relations, and the metallic-like state characterized by gapless electronic excitations.

Gd3+ electron spin resonance spectroscopy on LaO1 − x F x FeAs superconductors

We report an electron spin resonance (ESR) spectroscopy study on polycrystalline samples of the LaO1 − xFxFeAs (x = 0 and 0.1) compound with small levels of Gd doping (2% and 5%). The Gd ESR signal is found to be sensitive to the magnetic phase transition from the paramagnetic to the spin density wave (SDW) state occurring in the parent LaO1 − xFxFeAs compounds at TSDW ∼ 130 K. Interestingly, the analysis of the low-temperature ESR spectra of the c-axis oriented Gd1 − yLayOFeAs samples gives evidence for the magnetically nonequivalent Gd sites and also for sites having a different local charge environment. The analysis of the temperature dependence of the ESR linewidths gives evidence for a coupling of the localized 4f electrons of Gd to the conduction electrons in the FeAs layers. The ESR data reveal that the fluorine substitution, which provides electron doping, suppresses the SDW order and enhances the density of states in the electronic bands stemming from the xz and yz orbital states of Fe to which the 4f electrons are most strongly coupled.

A theoretical and experimental study of magnetism in Gd2In

We present detailed theoretical calculations and careful experiments to elucidate the magnetic properties of Gd2In. Our calculations suggest that Gd2In is a rather unique system where both localized 4f electrons as well as itinerant 5d electrons all originating from Gd are responsible for its novel magnetic properties with hardly any role of In s, p electrons. A careful analysis of the magnetization and magnetoresistance data as a function of field revealed that the magnetic behavior of Gd2In may be divided into three distinct regions. We argue with the aid of first principles electronic structure calculations that these three regions originate due to the different response of the localized 4f and itinerant 5d electrons of Gd upon application of the magnetic field. We show that in addition to the Gd 4f electrons, the Gd 5d electrons are also important for the metamagnetic behavior of Gd2In. These results establish the important role of Gd 5d electrons in the magnetic properties of Gd2In that so far only remained a speculation.

Change in magnetic properties induced by swift heavy ion irradiation in CeO2

We have studied the magnetic properties of pure CeO2 irradiated with swift heavy ions. Experimental results showed that the ferromagnetism was induced even at room temperature by 200MeV Xe ion irradiation. The value of saturation magnetization, Ms, systematically changes as a function of ion-fluence. The value of Ms increases with increasing the ion-fluence and reaches a maximum value at about the fluence of 2.0×1013/cm2, and then decreases. The X-ray diffraction (XRD) spectra show that the lattice constant of CeO2 increases with increasing ion-fluence. This result and our previous XPS result show that the ion-irradiation produced oxygen vacancies. The present study implies that the ferromagnetic behavior of the ion-irradiated CeO2 is attributed to the magnetic moments of localized 4f electrons on Ce3+ atoms, which are related to oxygen vacancies.

Metal-Insulator Transition of Plutonium Hydrides: DFT + U Calculations in the FPLAPW Basis

We report on the first-principles calculations of the electronic structure of face-centered cubic PuH2 and hexagonal PuH3 combining the full potential linearized augmented plane-wave basis with the density functional theory plus a Hubbard parameter U for considering the strong Coulomb correlation between localized Pu 5f electrons. Most importantly, the findings provide evidence for the first time that a spectacular metal-insulator transition occurs on the phase transformation from PuH2 to PuH3.

Oxygen vacancy formation in CeO2 and Ce1−xZrxO2 solid solutions: electron localization, electrostatic potential and structural relaxation

Ceria (CeO(2)) and ceria-based composite materials, especially Ce(1-x)Zr(x)O(2) solid solutions, possess a wide range of applications in many important catalytic processes, such as three-way catalysts, owing to their excellent oxygen storage capacity (OSC) through the oxygen vacancy formation and refilling. Much of this activity has focused on the understanding of the electronic and structural properties of defective CeO(2) with and without doping, and comprehending the determining factor for oxygen vacancy formation and the rule to tune the formation energy by doping has constituted a central issue in material chemistry related to ceria. However, the calculation on electronic structures and the corresponding relaxation patterns in defective CeO(2-x) oxides remains at present a challenge in the DFT framework. A pragmatic approach based on density functional theory with the inclusion of on-site Coulomb correction, i.e. the so-called DFT + U technique, has been extensively applied in the majority of recent theoretical investigations. Firstly, we review briefly the latest electronic structure calculations of defective CeO(2)(111), focusing on the phenomenon of multiple configurations of the localized 4f electrons, as well as the discussions of its formation mechanism and the catalytic role in activating the O(2) molecule. Secondly, aiming at shedding light on the doping effect on tuning the oxygen vacancy formation in ceria-based solid solutions, we summarize the recent theoretical results of Ce(1-x)Zr(x)O(2) solid solutions in terms of the effect of dopant concentrations and crystal phases. A general model on O vacancy formation is also discussed; it consists of electrostatic and structural relaxation terms, and the vital role of the later is emphasized. Particularly, we discuss the crucial role of the localized structural relaxation patterns in determining the superb oxygen storage capacity in kappa-phase Ce(1-x)Zr(1-x)O(2). Thirdly, we briefly discuss some interesting findings for the oxygen vacancy formation in pure ceria nanoparticles (NPs) uncovered by DFT calculations and compare those with the bulk or extended surfaces of ceria as well as different particle sizes, emphasizing the role of the electrostatic field in determining the O vacancy formation.

Lattice contraction of cerium hydrides from first-principles LDA + U calculations

Ce can be loaded with H forming complicated continuous solid solution and compounds, and causing remarkable electronic-structure changes. First-principles pseudopotential plane wave method with adding a Hubbard parameter U for considering the strong Coulomb correlation between localized 4f electron is employed to investigate the electronic and structural properties of stoichiometric and nonstoichiometric face-centered cubic (fcc) Ce hydrides (CeHx, x = 2, 2.25, 2.5, 2.75 and 3, respectively.). The most remarkable result is the decreasing trend of the calculated lattice parameters with increasing H composition, which is resulted from the associated effects of the enhanced chemical bonding owing to the participation of Ce 5d electron and, the size effects owing to the small H atomic radius and the large volume of octahedral interstice thus in favor of reducing the atomic distance for the formation of chemical bonding between Ce and octahedral H atoms.

Localized 5f electrons in superconducting PuCoIn5: consequences for superconductivity in PuCoGa5

The physical properties of the first In analog of the PuMGa5 (M = Co, Rh) family of superconductors, PuCoIn5, are reported. With its unit cell volume being 28% larger than that of PuCoGa5, the characteristic spin-fluctuation energy scale of PuCoIn5 is three to four times smaller than that of PuCoGa5, which suggests that the Pu 5f electrons are in a more localized state relative to PuCoGa5. This raises the possibility that the high superconducting transition temperature Tc = 18.5 K of PuCoGa5 stems from the proximity to a valence instability, while the superconductivity at Tc = 2.5 K of PuCoIn5 is mediated by antiferromagnetic spin fluctuations associated with a quantum critical point.

First principles description of the insulator–metal transition in europium monoxide

Europium monoxide, EuO, is a ferromagnetic insulator. Its electronic structure under pressure and doping is investigated by means of density functional theory. We employ spin polarized electronic structure calculations including onsite electron–electron interaction for the localized Eu 4f and 5d electrons. Our results show that under pressure the ferromagnetism is stable, both for hydrostatic and uniaxial pressure, while the compound undergoes an insulator–metal transition. The insulator–metal transition in O deficient and Gd doped EuO is reproduced for an impurity concentration of 6.25%. A 10 monolayer thick EuO(100) thin film is predicted to be an insulator with a narrow band gap of 0.08eV.

Synthesis, Structure, Chemical Bonding, and Magnetism of the Series RELiGe2 (RE = La–Nd, Sm, Eu)

This article focuses on the synthesis and the crystal chemistry of six members of a series of rare-earth metal based germanides with general formula RELiGe(2) (RE = La-Nd, Sm, and Eu). The structures of these compounds have been established by single-crystal X-ray diffraction (CaLiSi(2) structure type, space group Pnma, Z = 4, Pearson symbol oP16). The chemical bonding within this atomic arrangement can be rationalized in terms of anionic germanium zigzag chains, conjoined via chains of edge-shared LiGe(4) tetrahedra and separated by rare-earth metal cations. The structure can also be viewed as an intergrowth of AlB(2)-like and TiNiSi-like fragments, or as the result of the replacement of 50% of the rare-earth metal atoms by lithium in the parent structure of the REGe monogermanides. Except for LaLiGe(2) and SmLiGe(2), the remaining four RELiGe(2) phases exhibit Curie-Weiss paramagnetism above about 50 K. In the low temperature regime, the localized 4f electrons in CeLiGe(2), PrLiGe(2), and SmLiGe(2) order ferromagnetically, while antiferromagnetic ordering is observed for NdLiGe(2) and EuLiGe(2). The calculated effective magnetic moments confirm RE(3+) ground states in all cases excluding EuLiGe(2), in which the magnetic response is consistent with Eu(2+) configuration (J = S = 7/2). The experimental results have been complemented by tight-binding linear muffin-tin orbital (TB-LMTO) band structure calculations.

237Np Mössbauer studies on actinide superconductors and related materials

Actinide materials play a special role in condensed matter physics, spanning behaviours of itinerant d-electron and localized 4f-electron materials. This duality of the 5f electrons confer to actinide-based intermetallic compounds a broad variety of physical properties such as magnetic or multipolar ordering, heavy fermion behaviour, quantum criticality, unconventional superconductivity... 237Np Mossbauer spectroscopy is a unique microscopic tool for gaining information on the electronic and magnetic properties of Np systems.

First-principles study of electronic structure and metal–insulator transition of plutonium dihydride and trihydride

The electronic structures of cubic PuH2 and hexagonal PuH3 have been calculated by combining the full potential linearized augmented plane wave method (FLAPW) with the local spin density and generalized gradient approximation plus a Hubbard parameter U (LSDA+U and GGA+U) for considering the strong Coulomb correlation between localized Pu 5f electrons. Our study indicates that PuH2 is metallic, while PuH3 is a semiconductor with a small band gap about 0.26eV. The bonds in PuH2 system have some covalent character. For PuH3, besides the covalent bonds, particularly, the bonding between Pu and octahedral H atoms is of prominent ionicity. In addition, the conductivity and resistivity data is also worked out at 300K and low temperature of 4K. The conductivity decreases from a metallic behavior of PuH2 to the semiconducting region of PuH3. The experimentally undetermined metal–insulator transition has been firstly discovered in Pu–H system theoretically.

Emerging local Kondo screening and spatial coherence in the heavy-fermion metal YbRh2Si2

The entanglement of quantum states is both a central concept in fundamental physics and a potential tool for realizing advanced materials and applications. The quantum superpositions underlying entanglement are at the heart of the intricate interplay of localized spin states and itinerant electronic states that gives rise to the Kondo effect in certain dilute magnetic alloys. In systems where the density of localized spin states is sufficiently high, they can no longer be treated as non-interacting; if they form a dense periodic array, a Kondo lattice may be established. Such a Kondo lattice gives rise to the emergence of charge carriers with enhanced effective masses, but the precise nature of the coherent Kondo state responsible for the generation of these heavy fermions remains highly debated. Here we use atomic-resolution tunnelling spectroscopy to investigate the low-energy excitations of a generic Kondo lattice system, YbRh(2)Si(2). We find that the hybridization of the conduction electrons with the localized 4f electrons results in a decrease in the tunnelling conductance at the Fermi energy. In addition, we observe unambiguously the crystal-field excitations of the Yb(3+) ions. A strongly temperature-dependent peak in the tunnelling conductance is attributed to the Fano resonance resulting from tunnelling into the coherent heavy-fermion states that emerge at low temperature. Taken together, these features reveal how quantum coherence develops in heavy 4f-electron Kondo lattices. Our results demonstrate the efficiency of real-space electronic structure imaging for the investigation of strong electronic correlations, specifically with respect to coherence phenomena, phase coexistence and quantum criticality.

First-principles study on the crystal, electronic structure and mechanical properties of hexagonal Al 3RE (RE = La, Ce, Pr, Nd, Sm, Gd) intermetallic compounds

The structural properties, elastic properties and electronic structures of hexagonal Al 3RE intermetallic compounds are calculated by using first-principles calculations based on density functional theory. Since there exists strong on-site Coulomb repulsion between the highly localized 4f electrons of RE atoms, we present a combination of the GGA and the LSDA+ U approaches in order to obtain the appropriate results. The GGA calculated lattice constants for the hexagonal Al 3RE intermetallic compounds are in good agreement with available experimental values. The results of cohesive energy indicate that these compounds can be stable under absolute zero Kelvin and the stability of Al 3Gd is the strongest in all of the hexagonal Al 3RE compounds. The densities of states for GGA and LSDA+ U approaches are also obtained for the Al 3RE intermetallic compounds. The mechanical properties are calculated from the GGA method in this paper. According to the computed single crystal elastic constants, Al 3La, Al 3Sm and Al 3Gd are mechanically unstable, while Al 3Ce, Al 3Pr and Al 3Nd are stable. The polycrystalline elastic modulus and Poisson’s ratio have been deduced by using Voigt–Reuss–Hill (VRH) approximations, and the calculated ratio of bulk modulus to shear modulus indicates that Al 3La compound is ductile material, but Al 3Ce, Al 3Pr, Al 3Nd, Al 3Sm and Al 3Gd are brittle materials.

Spectroscopic characterization of actinide materials

Advanced spectroscopic techniques provide new and unique tools for unraveling the nature of the electronic structure of actinide materials. Inelastic neutron scattering experiments, which address temporal aspects of lattice and magnetic fluctuations, probe electromagnetic multipole interactions and the coupling between electronic and vibrational degrees of freedom. Nuclear magnetic resonance clearly demonstrates different magnetic ground states at low temperature. Photoemission spectroscopy provides information on the occupied part of the electronic density of states and has been used to investigate the momentum-resolved electronic structure and the topology of the Fermi surface in a variety of actinide compounds. Furthermore, x-ray absorption and electron energy-loss spectroscopy have been used to probe the relativistic nature, occupation number, and degree of localization of 5f electrons across the actinide series. More recently, element- and edge-specific resonant and non-resonant inelastic x-ray scattering experiments have provided the opportunity of measuring elementary electronic excitations with higher resolution than traditional absorption techniques. Here, we will discuss results from these spectroscopic techniques and what they tell us of the electronic and magnetic properties of selected actinide materials.

Thermodynamics of fission products in dispersion fuel designs – First-principles modeling of defect behavior in bulk and at interfaces

Density functional theory (DFT) calculations of fission product (Xe, Sr, and Cs) incorporation and segregation in alkaline earth metal oxides, HfO 2 and UO 2 oxides, and the MgO/(U, Hf, Ce)O 2 interfaces have been carried out. These calculations demonstrate that the fission product incorporation energies in MgO are higher than in HfO 2. However, this trend is reversed or reduced for alkaline earth oxides with larger cation sizes. In the case of UO 2, the calculations were performed using spin polarization and with a Hubbard U term characterizing the on-site Coulomb repulsion between the localized 5f electrons. The fission product solution energies in bulk UO 2 ± x have been calculated as a function of non-stoichiometry x, and were compared to that in MgO. The solution energies of fission products in MgO are substantially higher than in UO 2 ± x , except for the case of Sr in hypostoichiometric UO 2. Due to size effects, the thermodynamic driving force of segregation for Xe and Cs from bulk MgO to the MgO/fluorite interface is strong. However, this driving force is comparatively weak for Sr.

5f-electronic states of neptunium compounds: NpGe3, NpRhGa5 and NpCd11

We studied the electronic states of three typical neptunium compounds, NpGe3, NpRhGa5 and NpCd11. Among them, NpGe3 and NpCd11 with the cubic crystal structure are paramagnets without magnetic ordering, while NpRhGa5 with the tetragonal structure is an antiferromagnet. At high temperatures, the 5f electrons in NpGe3 are almost localized, and the magnetic susceptibility χ(T) approximately follows the Curie-Weiss law. With decreasing temperature, χ(T) shows a broad peak around 50 K, indicating a moderate heavy fermion state at lower temperatures. In fact, the results of the de Haas-van Alphen (dHvA) experiments for NpGe3 are well explained by the 5f-itinerant band model; namely, the 5f electrons contribute to the Fermi surface and the cyclotron effective mass. The cyclotron mass is thus enhanced, ranging from 2.6 to 16 times the rest mass of the electron m0. In the antiferromagnet NpRhGa5, it was clarified from the results of dHvA experiments and energy band calculations that the 5f electrons contribute to the Fermi surface. The cyclotron mass varies from 8.1 to 11.7 m0, and the magnetic moment is 0.89 μB/Np. In NpCd11, the 5f electrons are localized in the whole temperature range and do not contribute to the Fermi surface. The corresponding cyclotron mass is thus light, below 1 m0, revealing no hybridization between the localized 5f electrons and the conduction electrons. The Fermi surface consists of small closed Fermi surfaces, reflecting the small Brillouin zone associated to the large unit cell of the crystal structure in NpCd11. The long Np-Np nearest-neighbor distance (6.568 Å) results in the well localized 5f 4states. Magnetic susceptibility and magnetization curves are well explained in a crystalline electric field (CEF) scheme assuming a singlet ground state and an excited triplet state separated by a 120 K energy gap.

29Si-NMR study of magnetic anisotropy and hyperfine interactions in the uranium-based ferromagnet UNiSi2

UNiSi2 orders ferromagnetically below TCurie = 95 K. This material crystallizes in the orthorhombic CeNiSi2-type structure. The uranium atoms form double-layers, which are stacked along the crystallographic b axis (the longest axis). From magnetization measurements the easy (hard) magnetization axis is the c axis (b axis). 29Si-NMR measurements have been performed in the paramagnetic state. In UNiSi2, two crystallographic Si sites exist with orthorhombic local symmetry. The Knight shifts on each Si sites have been estimated from these spectra of random and oriented powders. The transferred hyperfine couplings have been also derived. It is found that the transferred hyperfine coupling constants on each Si sites are nearly isotropic, and that their Knight shift anisotropy comes from that of the bulk susceptibility. The nuclear-spin lattice relaxation rate 1/T1 shows temperature-independent behaviour, which indicates the existence of localized 5f electrons.

Origin of anomalous specific heat coefficient of YbAl3

By using recently proposed effective model for heavy fermion metallic systems, the temperature dependence of the specific heat coefficient of typical heavy fermion material YbAl3 is investigated. The Hamiltonian of the band term consists of the conduction electrons described with nearly free electron method, the localized 4f electrons of Yb ions and the hybridization term between the conduction and 4f electrons. In order to take account of the correlation effect, we reconstruct the low-energy effective Hamiltonian where the low-energy bands near the Fermi level are extracted. The self-consistent perturbation theory with local approximation is applied to this Hamiltonian, so that some physical quantities are calculated. The obtained temperature dependence of the specific heat divided by temperature shows two peak structures, which are in agreement with the experimental results of YbAl3. We show that one peak structure at the very low temperature region originates from the correlation effect and the other peak structure results from the band structure.