Localized 4f Electrons Research Articles

Norm-Conserving Pseudopotentials and Basis Sets To Explore Lanthanide Chemistry in Complex Environments.

A complete set of pseudopotentials and accompanying basis sets for all lanthanide elements are presented based on the relativistic, norm-conserving, separable, dual-space Gaussian-type pseudopotential protocol of Goedecker, Teter, and Hutter (GTH) within the generalized gradient approximation (GGA) and the exchange-correlation functional of Perdew, Burke, and Ernzerhof (PBE). The corresponding basis sets have been molecularly optimized (MOLOPT) using a contracted form with a single set of Gaussian exponents for the s, p, and d states. The f states are uncontracted explicitly with Gaussian exponents. Moreover, the Hubbard U values for each lanthanide element, for DFT+U calculations, are also tabulated, allowing for the proper treatment of the strong on-site Coulomb interactions of localized 4f electrons. The accuracy and reliability of our GTH pseudopotentials and companion basis sets optimized for lanthanides is illustrated by a series of test calculations on lanthanide-centered molecules, and solid-state systems, with the most common oxidation states. We anticipate that these pseudopotentials and basis sets will enable larger-scale density functional theory calculations and ab initio molecular dynamics simulations of lanthanide molecules in either gas or condensed phases, as well as of solid state lanthanide-containing materials, allowing further exploration of the chemical and physical properties of lanthanide systems.

Novel universality class for the ferromagnetic transition in the low carrier concentration systems UTeS and USeS exhibiting large negative magnetoresistance

We report the novel critical behavior of magnetization in low carrier concentration systems UTeS and USeS that exhibit the large negative magnetoresistance around the ferromagnetic transition temperatures T_C ~ 85 and 23 K, respectively. UTeS and USeS crystallize in the same orthorhombic TiNiSi-type crystal structure as those of uranium ferromagnetic superconductors URhGe and UCoGe. We determine the critical exponents, beta for the spontaneous magnetization M_s, gamma for the magnetic susceptibility chi, and delta for the magnetization isotherm at T_C with several methods. The ferromagnetic states in UTeS and USeS have strong uniaxial magnetic anisotropy. However, the critical exponents in the two compounds are different from those in the three-dimensional Ising model with short-range magnetic exchange interactions. Similar sets of the critical exponents have been reported for the uranium ferromagnetic superconductors UGe_2 and URhGe, and uranium intermetallic ferromagnets URhSi, UIr and U(Co_0.98Os_0.02)Al. The universality class of the ferromagnetic transitions in UTeS and USeS may belong to the same one for the uranium compounds. The novel critical phenomenon associated with the ferromagnetic transition is observed not only in the uranium intermetallic ferromagnets with the itinerant 5f electrons but also in the low carrier concentration systems UTeS and USeS with the localized 5f electrons. The large negative magnetoresistance in UTeS and USeS, and the superconductivity in UGe_2 and URhGe share the similarity of their closeness to the ferromagnetism characterized by the novel critical exponents.

Theoretical Study of Gilbert Damping in Rare-Earth Permanent Magnets

We investigate temperature dependence of the Gilbert damping of a magnetization of the Fe sublattice in rare-earth permanent magnets $R_{2}{\text {Fe}}_{14}{\text {B}}$ ( $R$ is a rare-earth element). Assuming that the spin angular moment of Fe dissipates to the phonon system via a localized 4f electron system, which interacts with the phonon system, we obtain the formulation of the Gilbert damping constant $\alpha $ on the basis of the ligand field theory and evaluate the temperature and material dependence of $\alpha $ . We show that the Gilbert damping of the magnetization arises only in certain temperature regions due to the discrete energy structure of the 4f electron system.

Magnetoelastic hybrid excitations in CeAuAl3

Nearly a century of research has established the Born-Oppenheimer approximation as a cornerstone of condensed-matter systems, stating that the motion of the atomic nuclei and electrons may be treated separately. Interactions beyond the Born-Oppenheimer approximation are at the heart of magneto-elastic functionalities and instabilities. We report comprehensive neutron spectroscopy and ab initio phonon calculations of the coupling between phonons, CEF-split localized 4f electron states, and conduction electrons in the paramagnetic regime of [Formula: see text], an archetypal Kondo lattice compound. We identify two distinct magneto-elastic hybrid excitations that form even though all coupling constants are small. First, we find a CEF-phonon bound state reminiscent of the vibronic bound state (VBS) observed in other materials. However, in contrast to an abundance of optical phonons, so far believed to be essential for a VBS, the VBS in [Formula: see text] arises from a comparatively low density of states of acoustic phonons. Second, we find a pronounced anticrossing of the CEF excitations with acoustic phonons at zero magnetic field not observed before. Remarkably, both magneto-elastic excitations are well developed despite considerable damping of the CEFs that arises dominantly by the conduction electrons. Taking together the weak coupling with the simultaneous existence of a distinct VBS and anticrossing in the same material in the presence of damping suggests strongly that similarly well-developed magneto-elastic hybrid excitations must be abundant in a wide range of materials. In turn, our study of the excitation spectra of [Formula: see text] identifies a tractable point of reference in the search for magneto-elastic functionalities and instabilities.

From Ceria Clusters to Nanoparticles: Superoxides and Supercharging

Several studies have reported a dramatically increased oxygen storage capacity (OSC) for small ceria nanoparticles (∼5 nm). Both experiments and theory have correlated this effect with superoxide ion formation. In previous studies, density functional theory (DFT) calculations with the PBE+U density functional have been used, and the obtained results were only in qualitative agreement with the experimental observations. One severe problem is the underbinding of the O2 molecule upon superoxide ion formation, which suggests that such species should not exist above room temperature. In this work, we use hybrid DFT functional to resolve this problem. We find that the discrepancy between theory and experiment originates from an incorrect estimate of the energy associated with the localized f-electrons with respect to the oxygen p-levels. By using average O2 adsorption energies from hybrid DFT calculations, extrapolated to large nanoparticles (3−10 nm), in conjunction with first-order desorption kinetics, we fin...

A high-throughput data analysis and materials discovery tool for strongly correlated materials

Modeling of f-electron systems is challenging due to the complex interplay of the effects of spin–orbit coupling, electron–electron interactions, and the hybridization of the localized f-electrons with itinerant conduction electrons. This complexity drives not only the richness of electronic properties but also makes these materials suitable for diverse technological applications. In this context, we propose and implement a data-driven approach to aid the materials discovery process. By deploying state-of-the-art algorithms and query tools, we train our learning models using a large, simulated dataset based on existing actinide and lanthanide compounds. The machine-learned models so obtained can then be used to search for new classes of stable materials with desired electronic and physical properties. We discuss the basic structure of our f-electron database, and our approach towards cleaning and correcting the structure data files. Illustrative examples of the applications of our database include successful prediction of stable superstructures of double perovskites and identification of a number of physically-relevant trends in strongly correlated features of f-electron based materials.

First-principles calculations of structural, electronic, magnetic, elastic and optical properties of tetragonal zircon-type CeVO4

We have performed comprehensive first-principle calculations to investigate the structural, electronic, magnetic, elastic and optical properties of the tetragonal zircon-type CeVO4. The local density approximation plus U (LDA+U) and generalized gradient approximation plus U (GGA+U) methods are used to interpret the strong Coulomb repulsion among the localized Ce 4f electrons. Without the effective U parameters, the obtained ground state of the ferromagnetic (FM) metal is not consistent with the experimental result. However, we can get the antiferromagnetic (AFM2: the magnetic ground state) insulating ground state for CeVO4 by LDA+U and GGA+U. It reveals that the result is in agreement with the experiment. For AFM2 CeVO4, the dependence of properties on the U parameters has been studied in detail. The effect of spin–orbit coupling (SOC) has also been investigated in detail. The demonstrated properties include volume, lattice parameters, bandgap, the density of states (DOS) and spin magnetic moment of Ce atom. With the Hubbard U parameter around 6 eV, the lattice parameters (a=7.396 Å, V=351.82 Å3, c/a=0.874), bulk modulus B0 (121 GPa), insulating gap (2.157 eV) and magnetic moment (0.97 [Formula: see text] per Ce atom) are consistent with the experiments and other theoretical results. Besides, the elasticity-related properties and optical properties have also been calculated for U=0,2,4 and 6 eV.

Activity and selectivity of propane oxidative dehydrogenation over VO3/CeO2(111) catalysts: A density functional theory study

The oxidative dehydrogenation (ODH) of propane on monomeric VO3 supported by CeO2(111) (VO3/CeO2(111)) is studied by periodic density functional theory calculations. Detailed energetic, structural, and electronic properties of these reactions are determined. The calculated activation energies of the breaking of the first and second C–H bonds of propane on the VO3/CeO2(111) catalyst are compared, and it is found that both the unique structural and electronic effects of the VO3/CeO2(111) catalyst contribute to the relatively easy rupture of the first C–H bond of the propane molecule during the ODH reaction. In particular, the so-called new empty localized states that are mainly constituted of O 2p orbitals of the ceria-supported VO3 species are determined to be crucial for assisting the cleavage of the first C–H bond of the propane molecule. Following this they become occupied and the remaining C–H bonds become increasingly difficult to break owing to the increasing repulsion between the localized 4f electrons at the Ce cations, resulting in the adsorption of more H and other moieties. This work illustrates that CeO2-supported monomeric vanadium oxides can exhibit unique activity and selectivity for the catalytic ODH of alkanes to alkenes.

Electronic structure and magnetic properties of the magnetically ordered intermediate valent Ce5RuGe2

We report here a comprehensive investigations of the band structure, magnetic properties, electrical transport, and specific-heat for ternary compound Ce5RuGe2 that crystallize in orthorhombic Y2HfS5-type structure. The results show a complex magnetic structure with the field-induced metamagnetic transitions in coexistence with spin-glass-like phase, displayed in the magnetic H−T phase diagram. Simultaneously, a combined experimental and theoretical study based on x-ray photoelectron spectroscopy (XPS) data together with ab initio band structure calculations indicate a mixed valence of Ce due to strong hybridization between Ce 4f states and conduction band. The obtained magnetic ground state of Ce5RuGe2, resulting from localized f-electron states and accompanying strong hybridization effect leading to valence instability of cerium ions are rarely observed for Ce-based intermetallic compounds.

The Smaller the Better: Hosting Trivalent Rare-Earth Guests in Cu–P Clathrate Cages

Summary The most striking difference between the trivalent rare-earth cations and their alkali and alkaline-earth peers is the presence of localized 4 f electrons. Placing trivalent rare-earth cations inside fullerene molecules or in between blocks of intermetallics has given rise to a plethora of fascinating properties and materials. A long-sought-after yet undiscovered material is a semiconducting compound with rare-earth cations situated inside the oversized polyhedral cages of a three-dimensional framework. In this work, we present a synthesis of unconventional clathrates, Ba 8−x R x Cu 16 P 30 , with the smallest pentagonal dodecahedral cages encapsulating La 3+ and Ce 3+ . Their composition and crystal structure were unambiguously determined by a combination of synchrotron powder diffraction, time-of-flight neutron powder diffraction, scanning transmission electron microscopy, and electron energy-loss spectroscopy. Our quantum-mechanical calculations and experimental characterizations show that the incorporation of the rare-earth cations significantly enhances hole mobility and reduces hole concentration, resulting in a drastic (700%) increase in the thermoelectric performance.

Disorder-induced f-electron localization in Nb and Y co-doped CeO2

We report the effect of the charge compensation on the electronic transport and optical properties of CeO2 co-doped with donor, Nb, and acceptor, Y, ions. As expected, the concentration of Ce3+ decreases with an increase in the Y content in Ce0.992−xNb0.008YxO2, where 0 ≤ x ≤ 0.008. More importantly, random electric fields generated by the Y ions bring additional disorder into the system. As a result, the high-temperature activation energy of conductivity increases significantly from 189 to 430 meV. A similar energy shift in the optical absorption peak centered at 1.3–1.5 eV is attributed to an increase in the energy gap separating the localized f-electrons from the empty Ce 4f band. The results underline the paramount importance of the disorder-induced Anderson localization of the f-electrons in ceria.

Many-body perturbation GW calculations for the electronic structures of both hexagonal and cubic REH3 (RE = Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu)

The electronic structures of REH3 (RE = Sm, Gd, Tb, Dy, Ho, Er, Tm, Lu) are extensively investigated using the parameter-free many-body perturbation GW calculations. The different symmetry hexagonal structures of REH3 almost present the same quasiparticle band structure both in the 4f-core and in the 4f-val cases. In the 4f-core case, the GGA + G0W0 calculations overestimate the direct band gap of the hexagonal structure; but the same G0W0 calculations in the cubic structure give a very suitable indirect band gap (around 1.9 eV). In the 4f-val case, for the hexagonal structure (GdH3), we find that the HSE03 calculations shift obviously the strongly localized 4f orbitals towards the high energy, and the shift is critical for the proper constructions and iterations of both G and W in the G0W/GW calculations of the electronic properties. In contrast to the cubic structure, the HSE03 calculations for the strong correlation of the localized 4f electrons in the hexagonal structure is not the critical factor for the acquirement of the appropriate band gap. The GGA + G0W0 calculations (GdH3) improve greatly the DFT band gap (0.2 eV) of the antiferromagnetic hexagonal structure and present a band gap with 1.0 eV; while the GGA + G0W0 calculations remove the small band overlap character (near Fermi level) of the ferromagnetic structure, and show a reasonable band gap with 1.5 eV at high symmetry Γ point. Therefore, to some extent, the GGA + G0W0 calculations can appropriately deal with the exchange-correlation interaction and the excited state in the ferromagnetic or antiferromagnetic hexagonal structure of GdH3.

First principle investigations of the Pbnm phase BiFeO3, BiFe0.875Mn0.125O3 and Bi0.875X0.125Fe0.875Mn0.125O3 (XBFM) (X = Ce, Gd, Lu)

The structural, magnetic, electronic and optical properties of Pbnm BiFeO[Formula: see text] (BFO), BiFe[Formula: see text]Mn[Formula: see text]O[Formula: see text] (BFM), Bi[Formula: see text]X[Formula: see text]Fe[Formula: see text]Mn[Formula: see text]O[Formula: see text] (XBFM) (X = Ce, Gd, Lu) have been investigated by the first principles within the PBE[Formula: see text]+[Formula: see text]U scheme. It is shown that the dopant Mn in the B sites and Ce (Gd, Lu) in the A sites improves the crystalline quality. The magnetic moments of Bi[Formula: see text]Ce[Formula: see text]Fe[Formula: see text]Mn[Formula: see text]O[Formula: see text] (CBFM) and Bi[Formula: see text]Gd[Formula: see text]Fe[Formula: see text]Mn[Formula: see text]O[Formula: see text] (GBFM) are 10 [Formula: see text] and 16 [Formula: see text] which mainly arises from the strongly localized unpaired 4f electrons of Ce and Gd, 3d electrons of Mn and Fe. While, as BFM, the magnetic moment of Bi[Formula: see text]Lu[Formula: see text]Fe[Formula: see text]Mn[Formula: see text]O[Formula: see text] (LBFM) is 9 [Formula: see text] indicating no effect on the magnetization is made by Lu for its full 4f states. The densities of states of the five systems hint that CBFM is metallic and BFM, GBFM, LBFM are still semiconductors. The metallic properties of CBFM arise from the hybridization between Ce-4f and Mn-3d states which leads the Mn [Formula: see text] states to be split into [Formula: see text] and [Formula: see text] lowering the bottom of the conduction band to cross the Fermi level. Furthermore, we also study the optical properties of Pbnm BiFeO[Formula: see text] (BFO), BiFe[Formula: see text]Mn[Formula: see text]O[Formula: see text] (BFM), Bi[Formula: see text]X[Formula: see text]Fe[Formula: see text]Mn[Formula: see text]O[Formula: see text] (XBFM) (X = Ce, Gd, Lu). In this work, the optical properties including the absorption spectrum, loss function, refractive index and reflectivity spectrum are discussed in detail.

Elastic moduli of the distorted Kagome-lattice ferromagnet Nd3Ru4Al12

The distorted kagome-lattice compound Nd3Ru4Al12 has the hexagonal structure. This compound is reported as a ferromagnet in which spins are aligned along the c-axis with the Curie temperature TC=39K. The nature of localized f-electrons is expected in Nd3Ru4Al12, and magnetic anisotropy can be attributed to a crystal electric field (CEF) effect. We performed ultrasonic measurements on a Nd3Ru4Al12 single-crystalline sample in order to investigate the phase transition at TC and the CEF effect. All longitudinal and transverse elastic moduli increase monotonically with decreasing temperature, and no clear elastic softening due to a quadrupole interaction is detected under the hexagonal CEF. This result is in contrast to an isomorphic compound Dy3Ru4Al12 with a remarkable elastic softening of the transverse modulus C44. At the ferromagnetic phase transition, the moduli show obvious elastic anomalies, suggesting characteristic couplings between a strain and a magnetic order parameter.

Low-temperature physical behavior in a novel compound CePtIn4

Single crystals of a novel ternary cerium indide CePtIn4 were grown by self-flux method and studied by means of X-ray diffraction, magnetic susceptibility, heat capacity and electrical resistivity measurements, carried out in wide ranges of temperature and magnetic field strength. The compound was found to crystallize with an orthorhombic crystal structure of the YNiAl4-type. It exhibits metallic conductivity and Curie-Weiss paramagnetism due to Ce3+ ions with well localized 4f-electrons. At TN = 1.65 K, CePtIn4 orders antiferromagnetically, possibly with a non-trivial magnetic structure. No superconductivity emerges down to 50 mK. Remarkably, the observed thermodynamic and electrical transport behavior in this compound does not give any hint at strong electronic correlations, typical for cerium intermetallics.

First-Principle Study of the Electronic and Magnetic Properties of Nd1−x La x FeSi Alloys (x = 0, 0.25, 0.50, 0.75, and 1)

The structural, electronic, and magnetic properties of Nd1−x La x FeSi (x = 0, 0.25, 0.50, 0.75, and 1) alloys are investigated based on the first-principle density functional theory by using the full-potential linear muffin-tin orbital (FP-LMTO) method. The exchange-correlation potential is treated with local density approximation (LDA). In addition, to treat localization of 4f-electrons properly, the LDA + U approach (where U is the Hubbard correlation term) is also employed. The calculated structural, electronic, and magnetic properties of these alloys within LDA and LDA + U approaches are compared. The calculated lattice constants of these alloys as a function of x within the LDA + U approach are in the best agreement with linear Vegard’s rule. Most importantly, we focus on the magnetic properties of these alloys and find that the magnetic interaction between Nd atoms is the Rudermen-Kittel-Kasuya-Yosida (RKKY) interaction type. Iron does not carry magnetic moment in these alloys and has just an induced small magnetic moment from Nd atoms.

Quantum spin fluctuations in the bulk insulating state of pure and Fe-doped SmB6

The intermediate-valence compound SmB6 is a well-known Kondo insulator, in which hybridization of itinerant 5d electrons with localized 4f electrons leads to a transition from metallic to insulating behavior at low temperatures. Recent studies suggest that SmB6 is a topological insulator, with topological metallic surface states emerging from a fully insulating hybridized bulk band structure. Here we locally probe the bulk magnetic properties of pure and 0.5 % Fe-doped SmB6 by muon spin rotation/relaxation methods. Below 6 K the Fe impurity induces simultaneous changes in the bulk local magnetism and the electrical conductivity. In the low-temperature insulating bulk state we observe a temperature-independent dynamic relaxation rate indicative of low-lying magnetic excitations driven primarily by quantum fluctuations.

Electronic states in the pressure-induced magnetically ordered phase in SmB6

We have carried out the high-pressure measurement of X-ray absorption spectroscopy on the intermediate valence compound SmB6 which shows magnetic ordering as well as an insulator-metal transition at critical pressure Pc ∼ 10 GPa. The valence of Sm atom at room temperature increases with increasing pressure, however it is far below a trivalent state at Pc. In contrast to cases of pressure-induced nonmagnetic-magnetic transition in Yb compounds, which mostly occurs in the scheme of well localized 4f electrons, the present observation suggests that electronic system in SmB6 still possesses strong delocalized characters at Pc.

Simulating heavy fermion physics in optical lattice: Periodic Anderson model with harmonic trapping potential

Periodic Anderson model (PAM), where local electron orbitals interplay with itinerant electronic carriers, plays an essential role in our understanding on heavy fermion materials. Motivated by recent proposal of simulating Kondo lattice model (KLM) in terms of alkaline-earth metal atoms, we make a further step toward simulation of PAM, which includes crucial charge/valence fluctuation of local f-electron beyond purely low-energy spin fluctuation in KLM. To realize PAM, transition induced by suitable laser between electronic excited and ground state of alkaline-earth metal atoms ($^{1}S_{0}$$\rightleftharpoons$$^{3}P_{0}$) is introduced, and it leads to effective hybridization between local electron and conduction electron in PAM. Generally, the $SU(N)$ version of PAM can be realized by our proposal, which gives a unique opportunity to detect large-$N$ physics without complexity in realistic materials. In the present work, high temperature physical feature of standard ($SU(2)$) PAM with harmonic trapping potential is detailed analyzed by quantum Monte Carlo and dynamic mean-field theory. Indications for near-future experiments are provided. We expect our theoretical proposal and (hopefully) forthcoming experiment will deepen our understanding on heavy fermion systems and at the same time triggers further studies on related Mott physics, quantum criticality and non-trivial topology in both inhomogeneous and nonequilibrium realm.

Ubiquity of unconventional phenomena associated with critical valence fluctuations in heavy fermion metals

Ubiquity of unconventional phenomena observed in a series of heavy fermion metals is discussed on the basis of an idea of critical valence fluctuations. After surveying experimental aspects of these unconventional behaviors in prototypical compounds, , under pressure, we propose that sharp valence crossover phenomena are realized in , , and by tuning the pressure and the magnetic field simultaneously, on the basis of previous results for an extended Anderson lattice model with the Coulomb repulsion between localized f-electron and itinerant conduction electrons.