Localized 4f States Research Articles

Antiferromagnetic-to-Ferromagnetic Transition in the GdNiSi1 –xAlx Compound

In this work, the electronic structure and magnetic properties of GdNiSi have been investigated within the framework of theoretical DFT + U method. The antiferromagnetic ordering of the Gd moments was obtained as the ground state, whereas Ni and Si have negligible magnetic moments. An antiferromagnetic-to-ferromagnetic transition was found when GdNiSi is doped with Al in our theoretical calculations. The antiferromagnetic-to-ferromagnetic transition takes place in the Gd sublattice, the magnetic moments at the Gd ions remain the same but the type of magnetic ordering changes. It was found that even at concentrations as low as \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$x = 0.1$$\\end{document}, the ferromagnetic ordering in the Gd sublattice is more favorable in total energy as the ground state which can be ascribed to the shortening of Gd–Gd distances and the oscillating behavior of the Ruderman–Kittel–Kasuya–Yosida interaction in the Al-doped compositions. The electronic structure demonstrates significant changes in the vicinity of the localized Gd 4f states that confirms the presence of the antiferromagnetic-to-ferromagnetic transition using our theoretical results for Al \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$x \\geqslant 0.1$$\\end{document} in good agreement with the previous experimental magnetic results.

Ab initio-based determination of lanthanoid-radical exchange as visualised by inelastic neutron scattering.

Magnetic exchange coupling can modulate the slow magnetic relaxation in single-molecule magnets. Despite this, elucidation of exchange coupling remains a significant challenge for the lanthanoid(iii) ions, both experimentally and computationally. In this work, the crystal field splitting and 4f-π exchange coupling in the erbium-semiquinonate complex [ErTp2dbsq] (Er-dbsq; Tp- = hydro-tris(1-pyrazolyl)borate, dbsqH2 = 3,5-di-tert-butyl-1,2-semiquinone) have been determined by inelastic neutron scattering (INS), magnetometry, and CASSCF-SO ab initio calculations. A related complex with a diamagnetic ligand, [ErTp2trop] (Er-trop; tropH = tropolone), has been used as a model for the crystal field splitting in the absence of coupling. Magnetic and INS data indicate antiferromagnetic exchange for Er-dbsq with a coupling constant of Jex = -0.23 meV (-1.8 cm-1) (-2Jex formalism) and good agreement is found between theory and experiment, with the low energy magnetic and spectroscopic properties well modelled. Most notable is the ability of the ab initio modelling to reproduce the signature of interference between localised 4f states and delocalised π-radical states that is evident in the Q-dependence of the exchange excitation. This work highlights the power of combining INS with EPR and magnetometry for determination of ground state properties, as well as the enhanced capability of CASSCF-SO ab initio calculations and purposely developed ab initio-based theoretical models. We deliver an unprecedentedly detailed representation of the entangled character of 4f-π exchange states, which is obtained via an accurate image of the spin-orbital transition density between the 4f-π exchange coupled wavefunctions.

Sm cluster on hexagonal boron nitride supported by Ir(111): Formation of magnetic cluster superlattice Sm13[formula omitted]h-BN[formula omitted]Ir(111)

The goal toward the ultimate density limit of magnetic information storage is now focused on the creations of identical magnetic clusters as well as their arrangements on a template surface for obtaining an equidistant, monodisperse and equally oriented pattern. To achieve this goal, the combination of rare-earth (RE) cluster and specific template is a viable route, e.g., h-BN monolayer on Ir(111) surface within a corrugated moiré structure (h-BN|Ir(111)) can act as a promising template to steer the intrinsically magnetic RE cluster in a well-defined orientation. Combining abinito molecular dynamic (AIMD) simulation and spin–orbital coupling density function theory (DFT) calculations, we report that magnetic Sm13 cluster can be stably deposited at the pore region of h-BN|Ir(111) template. The low-lying moiré region potential of h-BN|Ir(111) template, stable icosahedron structure of Sm13 cluster, and strong hybridization of Sm-5d6s orbitals with B/N-sp orbitals, play the synergetically roles to form Sm13@h-BN|Ir(111) cluster superlattice. The local magnetic moments of Sm atoms in Sm13 cluster are found to be robust against the structural configuration, localized site and inter-atom magnetic coupling, because their highly localized 4f states hybridize less with the spd states. Nevertheless, different deposition orientations and consequently the altered bonding interactions between Sm atoms and h-BN monolayer lead to different magnetic orders among Sm atoms. The appearances of large magnetic moment and giant magnetic anisotropy energy (MAE) of Sm13 cluster as well as its high deposition stability render Sm13@h-BN|Ir(111) for potential application of magnetic storage.

Localization for 4f electrons in a heavy fermion compound CeCu2Si2: Insights from dynamical mean-field theory

A first principles calculation is carried out on a typical heavy fermion system-CeCu2Si2 by using a many-body method combing density functional theory (DFT) and dynamical mean-field theory (DMFT) along with the on-site Coulomb repulsion (represented by the Hubbard U parameter) for capturing the electronic correlation due to the incompletely filled Ce 4f orbitals. The results establish that the average occupation of Ce 4f electrons nf is about 1.02 (mainly 4f1 atomic configuration), close to the nominal occupation in pure Ce metal and in good agreement with the spectrum function in this work and the available experimental observations. The imaginary part of the impurity Green function indicates that the Ce 4f j = 5/2 and j = 7/2 states have metallic and insulating behavior, respectively. The dehybridization between the Ce 4f orbitals and ligand valence orbitals in the vicinity of the Fermi level is responsible for the localized 4f state and heavy fermionic behavior in this system.

Experimental and ab Initio Studies of Deep-Bulk Traps in Doped Rare-Earth Oxide Thick Films

Lanthanum-doped CeO2 is a promising semiconductor for gas sensing. A combined study applying impedance spectroscopy and first-principle calculations was performed for pure and lanthanum-doped samples. The results showed a strong influence of the localized Ce 4f states on the electrical conduction processes, and an electrical resistance increase, as a function of the exposure to vacuum and air atmospheres. After its modification with a rare-earth element along with exposure to reducing and oxidizing atmospheres, the observed behavior suggested the presence of multi-traps, which depended on the described equilibrium between the oxygen vacancies (Vox ↔ VO. ↔VO..) in a disordered deep-bulk trap location. According to the DFT results, the multi-traps were formed with the creation of an oxygen vacancy far from the doping atom. They were considered to be responsible for the phenomena modifying the Debye-like response. The transfer of electrons from Ce(III) to the adsorbed oxygen species, decreasing the number of...

Electronic structure, Fermi surface and x-ray magnetic circular dichroism in the CeAgSb2

The electronic structure, Fermi surface, angle dependence of the cyclotron masses and extremal cross sections of the Fermi surface as well as x-ray magnetic circular dichroism (XMCD) in the CeAgSb2 compound were investigated from first principles using the fully relativistic Dirac linear muffin-tin orbital method. In our calculations Ce 4f states have been considered as: 1) itinerant using the generalized gradient approximation (GGA), 2) fully localized, treating them as core states, and 3) partly localized using the GGA + U approximation. The effect of the spin-orbit (SO) interaction and Coulomb repulsion U in a frame of the GGA + U method on the Fermi surface, orbital dependence of the cyclotron masses, and extremal cross sections of the Fermi surface are examined in details. We show that the conventional GGA band calculations fail to describe the Fermi surface of the CeAgSb2 due to wrong position of Ce 4f states (too close to the EF). On the other hand, fully localized (4f states in core) and the GGA + U approach produce similar Fermi surfaces and dHvA frequencies in the CeAgSb2. A good agreement with the experimental data of XMCD spectra at the Ce M4.5 edges was achieved using the GGA + U approximation. The origin of the XMCD spectra in the compound is examined. The core hole effect in the final states has been investigated using a supercell approximation. It improves the agreement be-tween the theory and the experiment of the XAS and the XMCD spectra at the Ce M4.5 edges.

Electronic Structure of Lanthanide-Doped Bismuth Vanadates: A Systematic Study by X-ray Photoelectron and Optical Spectroscopies

Monoclinic BiVO 4 has emerged in recent years as one of the most promising materials for photocatalytic evolution of oxygen under solar irradiation. However, it is in itself unable to phototcatalyze reduction of water to hydrogen due to the placement of the conduction band edge below the potential required for H 2 O/H 2 reduction. As a consequence, BiVO 4 only finds application in a hybrid system. Very recently, tetragonal lanthanide-doped BiVO 4 powders have been shown to be able to both reduce and to oxidize water under solar irradiation, but to date there has been no comprehensive study of the electronic properties of lanthanide-doped bismuth vanadates aimed at establishing the systematic trends in the electronic structure in traversing the lanthanide series. Here, the accessible family of lanthanide-doped BiVO 4 quaternary oxides of stoichiometry Bi 0.5 Ln 0.5 VO 4 (Ln = La to Lu, excluding Pm) has been studied by X-ray powder diffraction, X-ray photoemission spectroscopy, and diffuse reflectance optical spectroscopy. The compounds all adopt the tetragonal zircon structure, and lattice parameters decrease monotonically in traversing the lanthanide series. At the same time, there is an increased peak broadening in the diffraction patterns as the mismatch in ionic radius between Bi 3+ and the Ln 3+ ions increases across the series. Valence band X-ray photoemission spectra show that the final state 4f n-1 structure associated with ionization of lanthanide 4f n states is superimposed on the valence band structure of BiVO 4 in the quaternary materials: in the case of the Ce-, Pr- and Tb-doped BiVO 4 , 4f-related states appear above the top of the main valence band of BiVO 4 and account for the small bandgap in the Ce compound. In all cases, the 4f structure is characteristic of the lanthanide element in the Ln(III) oxidation state. Vanadium 2p and lanthanide 3d or 4d core level photoelectron spectra of those compounds where the lanthanide may in principle adopt a higher (Ln = Ce, Pr, Tb) or lower (Ln = Eu, Yb) oxidation state further confirm the prevalence of the Ln(III) valence state throughout. The visible region optical properties of all samples were studied by diffuse reflectance spectroscopy, with a particular focus on the optical bandgap and the details of transitions associated with localized 4f states. Taken together, the results demonstrate the remarkable tunability of optical and electronic properties for these quaternary materials.

Origin of Visible Light Photoactivity of the CeO2/ZnO Heterojunction

The synthesis of a mixed CeO2–ZnO oxide results in a photocatalyst active under visible light. The characterization of the new material shows that Ce does not enter as a dopant in ZnO but rather forms isolated CeO2 nanoparticles supported on the surface of larger particles of the more abundant zinc oxide phase. The as obtained material exhibits a band gap corresponding to UV light (∼3.3 eV), but nevertheless. it shows a relevant photoactivity under irradiation with photons with λ > 420 nm (visible light). The working hypothesis is that visible light irradiation leads to a charge separation and stabilization of a fraction of the carriers connected with the formation of the CeO2/ZnO interface. This phenomenon has been investigated by means of several methods. A specific EPR-based approach allowed to monitor and quantify the charge separation following the formation of holes in the valence band (VB) of the two materials. More complex is detecting the nature of the excited electrons, as this involves the formation of EPR invisible Ce3+ ions by trapping the electrons into localized 4f states of Ce ions at the interface between the two oxides. DFT calculations provide a rational for some of the observed phenomena and a basis for the discussion of the band alignment of the two systems as a consequence of the formation of a heterojunction. The theoretical results show that indeed electrons can be excited at the interface from the VB of the two oxides into the Ce 4f states with photons of 2.3 eV, thus justifying the occurrence of a visible-light activity despite the higher band gap of the two materials.

Surface- and pressure-induced bulk Kondo breakdown in SmB6

Motivated by the prediction of surface Kondo breakdown in topological Kondo insulators, we investigated the valence and magnetism of SmB6 with x-ray absorption and magnetic circular dichroism at the Sm M 4,5 and L 2 edges with surface and bulk sensitivity, respectively. A higher Sm valence state at the surface and in the bulk under pressure indicates the surface-and pressure-induced bulk Kondo breakdown. We confirmed the different magnetization origin from the surface (f electron) to the bulk (d electron) and ascertained the direct relationship between the disappearance of the Sm L 2-edge x-ray magnetic circular dichroism signal and the closure of the d-f hybridization gap in SmB6 bulk under pressure, paving the way for further investigations of the Kondo insulators. Above P c = 8.5 GPa, a more localized 4f state with higher valence and the disappearing of the 5d magnetism are observed, suggesting the closure of the hybridization gap in the bulk.

Strain Engineering of the CeNi5 System

The effect of strain on the CeNi5 system has been investigated using density functional theory (DFT). The studies have shown that localised Ce 4f and Ni 3d states carry the magnetic moment of the material. The Ce 4f moment remains relatively unchanged during strain whereas the Ni 3d moment increases as the strain becomes increasingly tensile in both the basal and non-basal directions. A significant strain-dependent interaction exists between delocalised, non-magnetic Ce 6s–Ni 4s states. A weaker group of Ni 3d–Ce 6s/Ni 4s and Ni 3d–Ni 3d interactions exist, indicating that competing localised and delocalised mechanisms act on the Ni 3d states during strain.

A density functional theory study of electronic and magnetic properties of rare earth doped monolayered molybdenum disulphide

The effects of Rare Earth (RE) atoms Sm, Eu, Gd, Tb, and Dy doping on structural, electronic, and magnetic properties of single layer MoS2 were investigated using first principles calculations. The analysis of electronic properties pointed out the appearance of host-impurity hybrid states on the edges of principal bands of the material, which appeared to cause narrowing of its bandgap. The values of total magnetic moment were calculated as 3.3 μB, 8.1 μB, 8.5 μB, 6.8 μB, and 6.4 μB for Sm-, Eu-, Gd-, Tb-, and Dy-doped MoS2, respectively. The underestimation of magnetic moment in the case of Sm doping is found which is assigned to shielding of 4f moments and quenching of angular momentum caused by Sm 4f–S 3p hybridization. On the other hand, the observed overestimation of magnetic moments for Eu and Gd cases is likely to be due to increase in angular momentum due to indirect exchange interaction of highly localized 4f states via 5d or 6s conduction electrons. The findings of this work point out the possibilities of tuning the band gap and magnetic properties of monolayer MoS2 upon RE doping for realization of p-type ferromagnetic semiconductors.

Suppression of ferromagnetism in solid solution CePdxGa4−x

In order to extend our knowledge on effects of substitution of gallium by transition metal in solid solutions of gallides with the tetragonal BaAl4-type crystal structure we studied physical properties of polycrystalline samples of CePdxGa4−x (x=0.2−0.8). X-ray diffraction confirmed that the unit-cell volume of the system changes in a non-monotonic way with a maximum around x∗ = 0.6. We performed magnetization, specific heat, electrical resistivity and thermoelectric power measurements in wide temperature and magnetic field ranges. Magnetic moments of Ce3+ ions are very close to the value corresponding to the localized 4f-state in the whole x-range covered, yet their variation with x coincides with that of lattice parameters, with a minimum close to x∗. The alloys order ferromagnetically at low temperatures, which causes distinct anomalies of the measured properties. The ordering temperature decreases with increasing the Pd content from 3.7 K for x=0.2 down to 1.3 K for x=0.8. The results suggest that no quantum critical transition can be approached within the solid solution existence range.

Coexistence of different magnetic moments in CeRuSn probed by polarized neutrons

We report on the spin densities in CeRuSn determined at elevated and at low temperatures using polarized neutron diffraction. At 285 K, where the CeRuSn crystal structure, commensurate with the CeCoAl type, contains two different crystallographic Ce sites, we observe that one Ce site is clearly more susceptible to the applied magnetic field whereas the other is hardly polarizable. This finding clearly documents that distnictly different local environment of the two Ce sites causes the Ce ions to split into magnetic Ce3+ and non-magnetic Ce(4-delta)+ valence states. With lowering the temperature, the crystal structure transforms to a structure incommensurately modulated along the c axis. This leads to new inequivalent crystallographic Ce sites resulting in a re-distribution of spin densities. Our analysis using the simplest structural approximant shows that in this metallic system Ce ions co-exist in different valence states. Localized 4f states that fulfill the third Hund's rule are found to be close to the ideal Ce3+ state (at sites with the largest Ce-Ru interatomic distances) whereas Ce(4-delta)+ valence states are found to be itinerant and situated at Ce sites with much shorter Ce-Ru distances. The similarity to the famous alpha-gamma transition in elemental cerium is discussed.

Valence States in CeVO4 and Ce0.5Bi0.5VO4 Probed by Density Functional Theory Calculations and X-ray Photoemission Spectroscopy

Ce is one of the few lanthanide elements to exhibit well-defined (III) and (IV) oxidation states in solid-state environments, and there is therefore ambiguity as to whether CeVO4 should be formulated as Ce(III)V(V)O4 or Ce(IV)V(IV)O4. To address this question, CeVO4 and Ce0.5Bi0.5VO4 have been studied by density functional theory calculations and X-ray photoemission spectroscopy. A peak above the main O 2p valence band in photoemission is attributed to localized Ce 4f states, in agreement with the calculations which show occupation of Ce 4f states. The Ce 3d core level spectrum is diagnostic of Ce(III) with no sign of a peak associated with 4f0 final states that are characteristic of Ce(IV) compounds. The experimental and theoretical results thus confirm that both compounds contain Ce(III) and V(V), rather than Ce(IV) and V(IV). In agreement with experiment, the calculations also show that the tetragonal zircon phase adopted by CeVO4 is more stable for Ce0.5Bi0.5VO4 than the monoclinic clinobisvanite phase adopted by BiVO4, so that the formation of the stereochemically active Bi(III) lone pairs is suppressed by Ce doping.

Electronic properties of CeRh1−xGexIn; evolution from an intermediate-valence to a localized 4f-state

We examined how electronic properties of the intermediate-valent CeRhIn are influenced by gradual substitution of rhodium with germanium. Results of specific-heat, magnetic susceptibility, electrical resistivity, thermopower and X-ray photoelectron spectroscopy (XPS) measurements performed in wide range of temperature and magnetic field on polycrystalline samples of CeRh1−xGexIn, for x = 0.1, 0.2 and 0.3, are presented and compared to corresponding data reported earlier for CeRhIn, its hydrides and CePdxRh1−x In solutions. A systematic shift from the intermediate-valence to a localized 4f-state with increasing content of germanium is evident from all obtained results. Non-Fermi liquid state is formed at low temperatures for the solution with the lowest Ge content, but it is destroyed by magnetic field and/or further doping with Ge.

Structural, electronic, and magnetic properties of the europium chalcogenides: A hybrid-functional DFT study

We analyze the structural, electronic, and magnetic properties of the europium chalcogenide series EuX (X = O, S, Se, and Te) using density-functional theory (DFT). To describe the localized 4f states of Eu, we utilize the parameter-free PBE0 and HSE hybrid functionals and found a systematic qualitative and quantitative improvement over the conventional local and semilocal functionals. Both hybrid functionals predict the lattice constant and the bulk modulus of all four compounds accurately. The semiconducting behavior, the opening of the band gap, as well as the nature of the band gap across the series is reproduced. By mapping the magnetic interactions to a Heisenberg model up to the next-nearest neighbors, we find the correct trend of the magnetic order of the Eu compounds (in magnetic coupling of the Eu 4f moments), from ferromagnetic in EuO to antiferromagnetic in EuTe, with critical temperatures that are in fair agreement with experiment.

Electronic structure of lanthanide oxide high K gate oxides

Graphical abstractDisplay Omitted Electronic structure of LnOx oxides.Explaining when 4f states enter band gap.Simplest first principles calculation of Ln oxides to date.How to choose Ln oxides for high K application. Lanthanide (rare earth) oxides are used in gate stacks as high K gate oxides and for threshold voltage control. It is important that such oxides do not have states in the main band gap due to localized 4f states. We present hybrid density functional calculations of the oxides which give an electronic structure in agreement with previous empirical models and that show which metal oxides have gap states.

Exciton condensation and its influence on the specific heat

Abstract In rare earth compounds with localized 4f states the observation of bound 4f-hole-5d-electron states, excitons, is questionable. On the other hand the same compounds exhibit p–d excitons, which are derived from itinerant bands. In rare earth compounds, which exhibit intermediate valence, 4f–5d hybridization produces a narrow, several 10 meV wide 4f band. Now 4f–5d excitons are possible and have been observed in TmSe 0.45 Te 0.55 and similar compositions. The special band structure of these materials permits an enormous amount of excitons (≈10 21 cm −3 ), which condense in a first order transition at low temperatures and high pressure. This static and immense concentration of excitons dominates the heat conductivity and the thermal diffusivity and even exhibits the phenomenon of superfluidity in a solid. The measured specific heat shows that phonons couple to these excitons forming exciton polarons.

Existence of Heavy Fermions in the Antiferromagnetic Phase of CeIn3

We report the pressure-dependent optical conductivity spectra of a heavy fermion (HF) compound CeIn3 below the Neel temperature of 10 K to investigate the existence of the HF state in the antiferromagnetic (AFM) phase. The peak due to the interband transition in the hybridization gap between the conduction band and nearly localized 4f states (c-f hybridization) appears at the photon energy of about 20 meV not only in the HF regime but also in the AFM regime. Both the energy and intensity of the c-f hybridization peak continuously increase with the application of pressure from the AFM to the HF regime. This result suggests that the c-f hybridization, as well as the heavy fermions, exists even in the AFM phase of CeIn3.

High pressure resistivity of UPd3

Electrical resistivity of UPd3, the only known binary U-based intermetallic with localized 5f states, was studied under high pressures up to 10 GPa, testing a possible incipient delocalization due to the lattice compression. The data obtained revealed no tendency to any delocalization. The shoulder related to crystal-electric field effects between 10 and 15 K remains visible, being only shifted slightly towards higher temperatures with the increase in pressure.