4f States Research Articles

Energetics and electronic structure of UAl4 with point defects

UAl4 ideal and defect structures were studied within the framework of the Density Functional Theory. The structural and magnetic ordering of UAl4 in paramagnetic, ferromagnetic, and antiferromagnetic states have been investigated, within the collinear and non-collinear spin approximation, using the GGA model, as embedded in the program package VASP. An antiferromagnetic (01¯1) layer structure with spins aligned to the [010] direction was found to be energetically preferred. The analysis of density of states and bonding charge density point out that the bonding mechanism consists primarily of band mixing between the U 5f and Al 3p states. Supercells were built from UAl4 unit cells with the established magnetic structure. For those supercells we calculated the energy of formation of vacancies and antisite defects taking into account the existence of three distinct aluminum sites. Point defect formation energies, local lattice relaxations, as well as the defect induced magnetic ordering and electronic density redistribution, are discussed. It is shown that antiferromagnetism is locally broken. Al antisites and U antisites in Al 4e Wyckoff positions are the constitutional point defects in Al-rich and U-rich oI20 UAl4, respectively. In this way we have presented here the first set of data which makes it possible to discuss and quantify the point defects concentrations in the experimental composition range for existence of this uranium aluminide.

Chemical bonding in equiatomic cerium intermetallics – The case of CeMgSn, CePdSn, and CeMgPb

The electronic and magnetic structures and the properties of chemical bonding in isopointal CeMgSn and CePdSn (both phases belong to the family of TiNiSi related intermetallics, space group Pnma) and CeMgPb belonging to the family of CeScSi intermetallics, space group I4/mmm, have been investigated within the density functional theory (DFT). The charge analyses indicate negatively charged tin and lead leading to assign the compounds as stannides and plumbides, as also illustrated by the mapping of the electron localization function ELF. Calculations within spin-degenerate non-magnetic spin-polarized ferro- (SP-F) and SP-antiferromagnetic configurations led to assign a major role of Ce 4f states in the onset of ordered moments within SP-AF ground states from energy differences. Chemical bonding analyses from crystal orbital overlap populations revealed the strongest interactions for Ce–Sn in CeMgSn, Ce–Pb in CeMgPb, and Ce–Pd in CePdSn.

NMR evidence of anisotropic Kondo liquid behavior inCeIrIn5

We report detailed Knight shift measurements of the two indium sites in the heavy fermion compound CeIrIn$_5$ as a function of temperature and field orientation. We find that the Knight shift anomaly is orientation-dependent, with a crossover temperature $T^*$ that varies by 50% as the field is rotated from (001) to (100). This result suggests that the hybridization between the Ce 4f states and the itinerant conduction electrons is anisotropic, a result that reflects its collective origin, and may lead to anisotropic Kondo liquid behavior and unconventional superconductivity.

Ultrafast two-photon Rabi oscillations in excited He driven by femtosecond intense laser fields

SynopsisWe demonstrate ultrafast multiphoton coherent excitation in near-infrared intense laser fields (≤10 TW/cm2) using excited helium in the 1s2p (1P) state as the target. Strong two-photon coupling between the 2p state and the 5f and 6f Rydberg states drives ultrafast Rabi oscillations in tens of femtosecond. Theoretical calculations solving the time-dependent Schrodinger equation were carried out to elucidate the strong field effects.

Tailoring of magnetic properties of ultrathin epitaxial Fe films by Dy doping

We report on the controlled modification of relaxation parameters and magnetic moments of epitaxial Fe thin films through Dy doping. Ferromagnetic resonance measurements show that an increase of Dy doping from 0.1% to 5% gives a tripling in Gilbert damping, and more importantly a strongly enhanced anisotropic damping that can be qualitatively understood through the slow-relaxing impurity model. X-ray magnetic circular dichroism measurements show a pronounced suppression of the orbital moment of the Fe with Dy doping, leading to an almost threefold drop in the orbital to spin moment ratio, ml/ms. Doping with Dy can therefore be used to control both dynamic and static properties of thin ferromagnetic films for improved performance in spintronics device applications, mediated through the antiferromagnetic interaction of the 4f and 3d states.

Electronic structure of the heavy fermion superconductor Ce2PdIn8: Experiment and calculations

The electronic structure of a heavy-fermion superconductor Ce2PdIn8 was investigated by means of X-ray photoelectron spectroscopy (XPS) and ab initio density functional band structure calculations. The Ce 3d core-level XPS spectra point to stable trivalent configuration of Ce atoms that is also reproduced in the band structure calculations within the generalized gradient approximation GGA+U approach. Analysis of the 3d9f2 weight in the 3d XPS spectra within the Gunnarsson-Schönhammer model suggests that the onsite hybridization energy between Ce 4f and the conduction band states, Δfs, is ∼120 meV, which is about 30 meV larger than Δfs in isostructural Ce2TIn8 compounds with T = Co, Rh, and Ir. Taking into account a Coulomb repulsion U on both the Ce 4f and Pd 4d states in electronic band structure calculations, a satisfactory agreement was found between the calculated density of states (DOS) and the measured valence band XPS spectra.

Interplay of electronic structure and unusual development in crystal structure of YbAuIn and Yb3AuGe2In3

First-principles calculations within the DFT are employed to investigate the relationship between the electronic structure and the unexpected features of the hexagonal cell parameters of YbAuIn and . Calculations indicate that YbAuIn is an intermediate valent system with one Yb 4f state pinned to the Fermi level, while is closer to integer valency with all Yb 4f states occupied. Structural relaxations performed on LaAuIn and LuAuIn analogues reveal that expansion of the c-parameter in is attributable to larger size of the divalent Yb compared with intermediate valent Yb.

Investigation of the transport properties of cation-substituted solid solutions Yb x Mn1 − x S

The electrical properties and thermopower of solid solutions YbxMn1 − xS with substitution concentrations 0 < x ≤ 0.2 have been studied in the temperature range of 77–1050 K. It has been found that the electrical conductivity has a semiconducting character, and the thermopower changes its sign from positive to negative as the substitution concentration in YbxMn1 − xS increases. It has been established that the activation energy decreases with increasing ytterbium concentration. The model of impurity donor 4f states satisfactorily agrees with the data on the electrical resistivity and the thermopower.

Electronic structure and oxygen ion conductivity of as-deposited Ce0.90Sm0.10O2−δ thin film prepared by RF magnetron sputtering

The physical properties and electronic structure of Sm-doped CeO2 (Ce0.90Sm0.10O2−δ) in the thin-film form have been studied. The as-deposited thin film exhibits (111) orientation on an Al2O3(0001) substrate. The lattice constant of the thin film is larger than that of the bulk crystal. The Ce0.90Sm0.10O2−δ thin film has the mixed valence states of Ce4+ and Ce3+. The electrical conductivity of Ce0.90Sm0.10O2−δ thin film in the temperature region from 200 to 700 °C is higher than that of the undoped CeO2 thin film and is independent of the oxygen partial pressure, indicating the occurrence of high oxygen-ion conduction. The valence band consists of the O 2p state hybridized with the Ce 4f state. The 40% Ce3+ (4f1L) state in Ce0.90Sm0.10O2−δ thin film, which was estimated from the resonant photoemission spectra, contributes to the high oxygen-ion conduction and low activation energy.

Electronic structure and energy level schemes of RE3+:LaSi3N5 and RE2+:LaSi3N5−xOx phosphors (RE=Ce, Pr, ND, Pm, Sm, Eu) from first principles

First principles calculations of rare earth (RE)-doped LaSi3N5 host lattice are performed to obtain the electronic structure, the band gap (BG), and the character of electronic transitions. Doping with both trivalent and bivalent RE cations is inspected. RE 4f states form two bands of occupied and unoccupied states separated by ~5eV. In RE3+-doped compounds 4f states are shifted by ~6eV to more negative energies compared with RE2+-compounds. This stabilization causes that RE3+ 4f bands are in a different position relative to the valence band and the conduction band than RE2+ 4f bands and therefore different electronic transitions apply. BG of RE3+-compounds decreases from ~4.6eV (Ce) to ~1.5eV (Eu). Except for Ce3+, exhibiting the 4f→5d transition, other RE3+-compounds show the charge transfer of the p→4f character. BG of RE2+-compounds increases from ~0.80eV (Ce, Pr) to ~0.95eV (Nd, Pm), ~1.43eV (Sm), and ~3.28eV (Eu) and the electronic transition is of the 4f→5d character. The energy level scheme constructed from ab initio calculated electronic structures agrees well with the experimental energy level diagram. The agreement demonstrates the reliability of the hybrid functional HSE06 to describe correctly bands of nonbonding RE 4f electrons.

A comparative study of the optical properties of TbRhGe and DyRhGe

In this paper, experimental and theoretical optical properties of TbRhGe and DyRhGe have been reported and compared. Our measurements by the ellipsometric method in the wavelength range 0.22–15 μm revealed significant differences in energy positions of the interband peaks in the spectral functions of these ternary compounds. The electronic structure of TbRhGe and DyRhGe calculated within the LSDA + U method was used to obtain theoretical optical conductivity, which is in good agreement with the experimental one. Further analysis allowed us to track changes in interband transitions related to the rare-earth 4f and rhodium 4d states, which are responsible for the observed differences in optical properties of TbRhGe and DyRhGe compounds.

First-principle calculations of the electronic and optical properties of Tm-doped anatase titanium dioxide

Doping titanium dioxide (TiO2) with impurities has been extensively investigated for the optimal use of solar energy in order to improve photocatalyst performance. Density functional theory calculations are performed to investigate the effects of Tm doping on the energy band structures and optical properties of anatase TiO2 with various doping concentrations. The results show that Tm doping can effectively reduce the band gap of pure anatase TiO2 by introducing Tm 4f states above the top of the valence band, and thus results in a red shift in the optical absorption edges. Furthermore, the calculations of optical properties indicate that Tm doping TiO2 at low levels presents the prominent optical absorbance in the low energy region. Our results have important implications for the understanding and further development of photocatalytic functionalities of anatase TiO2 by introducing a dopant for effective band gap engineering.

X-ray photoemission spectrum, electronic structure, and magnetism of UCuxSb2

The room temperature valence and core-level X-ray photoemission spectra of an UCu0.83Sb2 single crystal were measured using the Al Kα source. The related theoretical valence spectra were determined from densities of states for UCuxSb2 systems obtained from our band structure calculations using the FLAPW method in the LDA+U approximation, as implemented in the Wien2k code, and the supercell approach to simulate a deficit of the Cu atoms. The calculated spectrum of the Cu-deficit UCu0.75Sb2 is in good accord with the experimental one, revealing a complete localization of the Cu 3d electrons and a dual (both localized and itinerant) behavior as well as unusual spin-up polarization of the U 5f states near the Fermi level. Our calculated total magnetic moments on the uranium atom are in reasonable agreement with the experimental value of magnetization. Some localization and valence-mixing of the 5f-electrons are reflected by the triple-satellite (sats. 1-eV, 3-eV and 7-eV) structure, visible in the spectrum of the U 4f core-lines. Also the calculated Fermi surface of the stoichiometric system is complex, containing five spin-polarized sheets of different dimensionality with some nesting features.

Electronic spectra of DyF studied by four-component relativistic configuration interaction methods.

The electronic states of the DyF molecule below 3.0 eV are studied using 4-component relativistic CI methods. Spinors generated by the average-of-configuration Hartree-Fock method with the Dirac-Coulomb Hamiltonian were used in CI calculations by the KRCI (Kramers-restricted configuration interaction) program. The CI reference space was generated by distributing 11 electrons among the 11 Kramers pairs composed mainly of Dy [4f], [6s], [6p] atomic spinors, and double excitations are allowed from this space to the virtual molecular spinors. The CI calculations indicate that the ground state has the dominant configuration (4f(9))(6s(2))(Ω = 7.5). Above this ground state, 4 low-lying excited states (Ω = 8.5, 7.5, 7.5, 7.5) are found with dominant configurations (4f(10))(6s). These results are consistent with the experimental studies of McCarthy et al. Above these 5 states, 2 states were observed at T0 = 2.39 eV, 2.52 eV by McCarthy et al. and were named as [19.3]8.5 and [20.3]8.5. McCarthy et al. proposed that both states have dominant configurations (4f(9))(6s)(6p), but these configurations are not consistent with the large Re's (∼3.9 a.u.) estimated from the observed rotational constants. The present CI calculations provide near-degenerate states of (4f(10))(6p3/2,1/2), (4f(10))(6p3/2,3/2), and (4f(9))(6s)(6p3/2,1/2) at around 3 eV. The former two states have larger Re (3.88 a.u.) than the third, so that it is reasonable to assign (4f(10))(6p3/2,1/2) to [19.3]8.5 and (4f(10))(6p3/2,3/2) to [20.3]8.5.

Electronic structure of the PuCoIn5 compound

The electronic structure of the new superconductor PuCoIn5 has been examined within the LDA+U+SO method taking into account both strong electron correlations and spin-orbit coupling between 5f electrons of an actinide metal. As compared to the characteristics of the known superconductor PuCoGa5, Pu 5f states in PuCoIn5 are more localized but with an intermediate type of coupling between 5f electrons with a strong prevalence of the LS type. However, at a small volume corresponding to the volume of the unit cell of the PuCoGa5 superconductor, the electronic and spectral characteristics of PuCoIn5 are close to the corresponding parameters of PuCoGa5. It has been shown that the occupancy of Co 3d states in PuCoIn5 is close to the previously estimated limiting values providing the optimal doping regime in the electron subsystem of super-conductors of the Pu-115 series.

Ab-initio study of optoelectronic and magnetic properties of the orthorhombic NdMnO3 perovskite

Using the full potential linear augmented plane wave (FPLAPW) method based on density functional theory (DFT), we have studied the structural, electronic, optical and magnetic properties of the orthorhombic perovskite oxide NdMnO3, with both generalized gradient approximation (GGA) and GGA+U approaches, where U is on-site Coulomb interaction correction. We have analyzed the structural parameters, total and partial densities of states. The results show a half-metallic ferromagnetic ground state for NdMnO3 in GGA+U due to the strong hybridization effect between Mn 3d and Nd 4f states The integer value of the total magnetic moment is one of the significance of the half metallic nature of this material. Furthermore, we have computed optical properties of NdMnO3; we found pronounced peaks occurring in different parts of the spectrum. The results obtained, make the orthorhombic NdMnO3 a promising candidate for application in spintronics and opto-electronics.

Variations of magnetic properties ofUGa2under pressure

Electrical resistivity \rho(T) of the 5f ferromagnet UGa2 was investigated for single-crystal samples as a function of pressure and magnetic field. The Curie temperature monotonously increases from T$_{C}$ = 124 K under quasi-hydrostatic pressure up to 154 K at p = 14.2 GPa, after which it turns down steeply and reaches T$_{C}$ = 147 K at p = 15.2 GPa. At 20 GPa the compound is already non-magnetic. This dramatic variation is compatible with exchange interactions mediated by the 5f hybridization with the non-f states. The external pressure first enhances the exchange coupling of the 5f moments, but eventually suppresses the order by washing out the 5f moments. Such a two-band model is adequate for the weakly delocalized 5f states. The spin-disorder resistivity, which produces very high \rho-values (300 \mu\Omega.cm) is gradually suppressed by the pressure. In the paramagnetic state, this leads to a crossover from initial negative to positive d\rho/dT.

Electronic structure and transport properties of CeNi9In2

We investigated CeNi9In2 compound, which has been considered as a mixed valence (MV) system. Electrical resistivity vs. temperature variation was analyzed in terms of the model proposed by Freimuth for systems with unstable 4f shell. At low temperature the resistivity dependence is consistent with a Fermi liquid state with a contribution characteristic of electron–phonon interaction. Ultraviolet photoemission spectroscopy (UPS) studies of the valence band did not reveal a Kondo peak down to 14K. A difference of the spectra obtained with photon energies of low and high photoionization cross sections for Ce 4f electrons indicated that 4f states are located mainly close to the Fermi energy. The peaks related to f5/21 and f7/21 final states cannot be resolved but form a plateau between −0.3eV and the Fermi energy. X-ray photoemission spectroscopy (XPS) studies were realized for the cerium 3d level. The analysis of XPS spectra within the Gunnarsson-Schönhammer theory yielded a hybridization parameter of 104meV and non-integer f level occupation, being close to 3. Calculations of partial densities of states were realized by a full potential local orbital (FPLO) method. They confirm that the valence band is dominated by Ni 3d states and are in general agreement with the experiment except for the behavior of f-electrons.

Molecular orbital calculations of Eu-doped SrAl2O4 clusters

The electronic structures of undoped and Eu2+-doped SrAl2O4 clusters are evaluated from the relativistic molecular orbital (RMO) calculation. The calculated forbidden gaps between the highest and lowest unoccupied molecular orbitals of [SrAl6O21]22− and [Sr3Al12O38]34− clusters are calculated to be 8.3 and 7.3eV, respectively. When a Eu atom replaces a Sr atom, the Eu 4f states appear in the forbidden gap of the SrAl2O4 and the Eu 5d states are distributed widely in the conduction band. Combining the RMO calculation and the configuration interaction method for [EuO7]12− and [EuO8]14− clusters produces three or two theoretical peaks respectively for the transition 4f7→4f6 5d1 in Eu2+-doped SrAl2O4, respectively.

Two-component analysis of the 4f multiplet of samarium hexaboride

We present a detailed study on the temperature-dependence of the Sm 4f states in mixed-valent samarium hexaboride (SmB6) which, as a consequence of the hybridization with the conduction band states, largely determine the physical properties of the system. By comparing soft X-ray photoemission spectra of SmB6 with those of Sm0.35La0.65B6, we discuss the apparent differences and similarities of the two systems. Based on our analyses we show that the 4f spectra of SmB6 can be separated into two independent spectral contributions with the 4f peak maxima at different binding energies, representing two different electronic 4f configurations existing in the system.