Localized 4f Electrons Research Articles

Harnessing 4f Electron Itinerancy for Integrated Dual-Band Redox Systems Boosts Lithium-Oxygen Batteries Electrocatalysis.

In-depth comprehension and manipulation of band occupation at metal centers are crucial for facilitating effective adsorption and electron transfer in lithium-oxygen battery (LOB) reactions. Rare earth elements play a unique role in band hybridization due to their deep orbitals and strong localization of 4felectrons. Herein, we anchor single Ce atoms onto CoO, constructing a highly active and stable catalyst with d-fa dual-band redox center. It is discovered that the itinerant behavior of 4felectrons introduces an enhanced spin-orbit coupling effect, which facilitates ideal σ/π bonding and flexible adsorption between the Ce/Co active sites and *O. Simultaneously, the injection of localized Ce 4felectrons strengthens the orbital bonding capacity of Co-O, effectively inhibits the dissolution of Co sites and improves the structural stability of the cathode material. Bracingly, the Ce1/CoO-based LOB exhibits an ultra-low charge-discharge polarization (0.46 V) and stable cyclic performance (1088 hours). This work breaks through the traditional limitations in catalyst activity and stability, providing new strategies and theoretical insights for developing high-performance LOBs powered by rare-earth elements.

Investigation of optical properties of Ce and Eu-doped Gd2SiO5 insights from GGA + U calculations

Gadolinium silicate, (Gd2SiO5) co-doped with Ce and Eu has been found to exhibit enhanced luminescence efficiency, which makes it a promising material for use in scintillators and phosphors. It has excellent scintillation properties such as high density and high Zeff. In this study, we used density functional theory (DFT) calculations within the Wien2k software to investigate the effect of Ce and Eu concentration and native defects on the electronic structure and optical properties of Ce and Eu co-doped Gd2SiO5. We utilized the DFT + U method to treat the localized 4f electrons of Ce and Eu. Our results indicate that the electronic structure and optical properties of Ce and Eu co-doped Gd2SiO5 are significantly affected by the concentration of the dopants and presence of native defects. We found that increasing the concentration of Ce and Eu dopants leads to a shift in the bandgap to lower energies, resulting in enhanced absorption and emission spectra. Moreover, our calculations reveal that presence of oxygen vacancies and Gd interstitials can induce new defect levels in the bandgap, which may affect the luminescence properties of the material. Our study provides valuable insights into the atomic-level mechanisms that govern the luminescence properties of Ce and Eu co-doped Gd2SiO5 which can aid in the design and optimization of luminescent materials for various applications.

Strain-induced magnetic anisotropy of multi-domain epitaxial EuPd2 thin films

Europium intermetallic compounds show a variety of different ground states and anomalous physical properties due to the interactions between the localized 4f electrons and the delocalized electronic states. Europium is also the most reactive of the rare earth metals which might be the reason why very few works are concerned with the properties of Eu-based thin films. Here we address the low-temperature magnetic properties of ferromagnetic EuPd2 thin films prepared by molecular beam epitaxy. The epitaxial (111)-oriented thin films grow on MgO (100) with eight different domain orientations. We analyze the low-temperature magnetic hysteresis behavior by means of micromagnetic simulations taking the multi-domain morphology explicitly into account and quantify the magnetic crystal anisotropy contribution. By ab initio calculations we trace back the microscopic origin of the magnetic anisotropy to thin film-induced uniform biaxial strain.

Nature of the Unconventional Heavy-Fermion Kondo State in Monolayer CeSiI.

CeSiI has been recently isolated in the ultrathin limit, establishing CeSiI as the first intrinsic two-dimensional van der Waals heavy-fermion material up to 85 K. We show that, due to the strong spin-orbit coupling, the local moments develop a multipolar real-space magnetic texture, leading to local pseudospins with a nearly vanishing net moment. To elucidate its Kondo-screened regime, we extract from first-principles the parameters of the Kondo lattice model describing this material. We develop a pseudofermion methodology in combination with ab initio calculations to reveal the nature of the heavy-fermion state in CeSiI. We analyze the competing magnetic interactions leading to an unconventional heavy-fermion order as a function of the magnetic exchange between the localized f-electrons and the strength of the Kondo coupling. Our results show that the magnetic exchange interactions promote an unconventional momentum-dependent Kondo-screened phase, establishing the nature of the heavy-fermion state observed in CeSiI.

The reverse quantum limit and its implications for unconventional quantum oscillations in YbB12.

The quantum limit in a Fermi liquid, realized when a single Landau level is occupied in strong magnetic fields, gives rise to unconventional states, including the fractional quantum Hall effect and excitonic insulators. Stronger interactions in metals with nearly localized f-electron degrees of freedom increase the likelihood of these unconventional states. However, access to the quantum limit is typically impeded by the tendency of f-electrons to polarize in a strong magnetic field, consequently weakening the interactions. In this study, we propose that the quantum limit in such systems must be approached in reverse, starting from an insulating state at zero magnetic field. In this scenario, Landau levels fill in the reverse order compared to regular metals and are closely linked to a field-induced insulator-to-metal transition. We identify YbB12 as a prime candidate for observing this effect and propose the presence of an excitonic insulator state near this transition.

Two-dimensional heavy fermion in a monoatomic-layer Kondo lattice YbCu2

The Kondo effect between localized f-electrons and conductive carriers leads to exotic physical phenomena. Among them, heavy-fermion (HF) systems, in which massive effective carriers appear due to the Kondo effect, have fascinated many researchers. Dimensionality is also an important characteristic of the HF system, especially because it is strongly related to quantum criticality. However, the realization of the perfect two-dimensional (2D) HF materials is still a challenging topic. Here, we report the surface electronic structure of the monoatomic-layer Kondo lattice YbCu2 on a Cu(111) surface observed by synchrotron-based angle-resolved photoemission spectroscopy. The 2D conducting band and the Yb 4f state, located very close to the Fermi level, are observed. These bands are hybridized at low-temperature, forming the 2D HF state, with an evaluated coherence temperature of about 30 K. The effective mass of the 2D state is enhanced by a factor of 100 by the development of the HF state. Furthermore, clear evidence of the hybridization gap formation in the temperature dependence of the Kondo-resonance peak has been observed below the coherence temperature. Our study provides a new candidate as an ideal 2D HF material for understanding the Kondo effect at low dimensions.

Pressure modulated charge transfer and phonon interactions drive phase transitions in uranium–aluminum laves phases

Lanthanide AB2 intermetallic compounds known as Laves phases have itinerant 3d and localized 4f electrons, which lead to interesting physical properties such as magnetic anisotropy and high Curie temperatures. Actinide Laves phases can display physical properties that are similarly intriguing. However, at reduced A–A spacing the C14 and C15 polytypes may exhibit larger wavefunction overlap for their 5f electron states and distinct characteristics for phases with more delocalized chemical bonding. The C36 polytype, on the other hand, is extraordinarily rare (<5% of known Laves phases). UAl2 is the only known actinide Laves phase to show a pressure-controllable C15 → C36 transition. Here, we apply first principles calculations to determine the origin of the C15 → C36 phase transition and reveal the differences between the corresponding properties of each phase. Pressure increases lead to bond compression–induced electron transfer from Al to U, which drives dynamic instability in the C15 phonon modes because of the uniform U–U bonding environment. The opposite phenomena is observed in C36: varied U–U bonding environments are vibronically more stable after charge transfer. We find that the interplay between charge transfer, chemical bonding, and phononic stability are central to predicting phase transitions and corresponding changes in physical properties for both C15 and C36 UAl2.

Electronic Band Structure of Heavy Fermion Compound Cecoge2

The following article provides a thorough examination of the electronic band structure observed in heavy fermion compounds, which are a type of material that has received considerable interest within the realm of condensed matter physics. The compounds under consideration exhibit significantly high charge carrier masses, which give rise to intriguing electronic phenomena when subjected to low temperatures. Through the analysis of electronic band structures, valuable insights can be obtained regarding the distinctive characteristics displayed by these captivating materials. The research centers on the distinctive attributes and theoretical frameworks employed to elucidate the electronic properties of the subjects under investigation. In this study, we present an introduction to heavy fermions and their experimental manifestations, including the observation of enhanced specific heat and low-temperature resistivity. The present study delves into the theoretical examination of the Kondo effect, which involves the emergence of heavy quasi-particles resulting from the hybridization process between localized f-electrons and conduction electrons. This paper examines the utilization of band structure calculations and various spectroscopic techniques, including angle-resolved photoemission spectroscopy (ARPES), inelastic neutron scattering, and transport measurements. The experimental results demonstrate the presence of hybridization gaps, the characteristics of the Fermi surface topology, and the occurrence of spin fluctuations. This study investigates the effects of crystal symmetry, spin-orbit coupling, and external perturbations on the electronic band structure. Specifically, it explores how these factors influence hybridization strength, Fermi surface topology, and quantum phase transitions. The abstract provides a concise overview of the existing knowledge, acknowledges the obstacles encountered, and proposes potential avenues for further investigation. The significance of this research lies in its ability to elucidate the fundamental principles of heavy fermion compounds, as well as explore their potential practical implications.

Making Sense of the Growth Behavior of Ultra-High Magnetic Gd2-Doped Silicon Clusters

The growth behavior, stability, electronic and magnetic properties of the Gd2Sin− (n = 3–12) clusters are reported, which are investigated using density functional theory calculations combined with the Saunders ‘Kick’ and the Artificial Bee Colony algorithm. The lowest-lying structures of Gd2Sin− (n = 3–12) are all exohedral structures with two Gd atoms face-capping the Sin frameworks. Results show that the pentagonal bipyramid (PB) shape is the basic framework for the nascent growth process of the present clusters, and forming the PB structure begins with n = 5. The Gd2Si5− is the potential magic cluster due to significantly higher average binding energies and second order difference energies, which can also be further verified by localized orbital locator and adaptive natural density partitioning methods. Moreover, the localized f-electron can be observed by natural atomic orbital analysis, implying that these electrons are not affected by the pure silicon atoms and scarcely participate in bonding. Hence, the implantation of these elements into a silicon substrate could present a potential alternative strategy for designing and synthesizing rare earth magnetic silicon-based materials.

Site‐dependent correlation strength and occupation number of 5f electrons in alpha phase plutonium metal

AbstractIn order to quantify the site‐dependent correlation strengths in terms of the quasi‐particle weights and the occupation numbers of 5f electrons in alpha phase plutonium metal with eight crystallographically nonequivalent atomic sites Pun (n = 1–8), we perform a first principles calculation by using a many‐body method merging density functional theory with dynamical mean‐field theory plus the relativistic and correlation effects. The quasi‐particle weight, the electronic spectrum function, the hybridization function, as well as the occupation number of Pu 5f electrons all suggest that Pu1 and Pu8 atoms have the most itinerant and the most localized 5f electrons, respectively, while the other atoms Pun (n = 2–7) exhibit an intermediate correlation strength. The quasi‐particle weights demonstrate that only the jj or intermediate coupling scheme is more appropriate for Pu 5f electrons in comparison with the Russel–Saunders (LS) scheme. The electronic spectrum function and the occupation analysis establish that Pu 5f electrons simultaneously have dual itinerant and localized regimes with average 5f occupation numbers of n5f between 4.8994 and 4.9628, which are mainly composed of 5f4, 5f5, and 5f6 configurations, irrespective of different atomic sites. Finally, we also estimate the so‐called quasi‐particle band structure to directly compare with angle‐resolved photoemission spectrum.

Field-induced Lifshitz transition in the magnetic Weyl semimetal candidate PrAlSi

Lifshitz transition (LT) refers to an abrupt change in the electronic structure and Fermi surface and is associated to a variety of emergent quantum phenomena. Amongst the LTs observed in known materials, the field-induced LT has been rare and its origin remains elusive. To understand the origin of field-induced LT, it is important to extend the material basis beyond the usual setting of heavy fermion metals. Here, we report on a field-induced LT in PrAlSi, a magnetic Weyl semimetal candidate with localized 4f electrons, through a study of magnetotransport up to 55 T. The quantum oscillation analysis reveals that across a threshold field B* ≈ 14.5 T the oscillation frequency (F1 = 43 T) is replaced by two new frequencies (F2 = 62 T and F3 = 103 T). Strikingly, the LT occurs well below the quantum limit, with obvious temperature-dependent oscillation frequency and field-dependent cyclotron mass. Our work not only enriches the rare examples of field-induced LTs but also paves the way for further investigation of the interplay among topology, magnetism, and electronic correlation.

Multi-experimental determination of magnetic transition in Weyl semimetals RAlGe

The single crystals of Weyl semimetals or candicate RAlGe (R = La, Ce, Pr and Nd) have been synthesized by self-flux method and confirmed forming in noncentrosymmetric LaPtSi type. Compared with nonmagnetic LaAlGe, the configurations of localized 4f electrons strongly dominate the magnetic, thermal and electric behaviors of other RAlGe. The main critical temperatures for magnetic transition are robust around 5.1 K, 15.4 K and 5.2 K for CeAlGe, PrAlGe and NdAlGe. PrAlGe and NdAlGe have c easy axis with similar anisotropy, but the easy direction for CeAlGe lies in a axis below 12 K. Their λ-shaped specific heat reveal 2nd order phase transitions. The magnetic entropy for PrAlGe and NdAlGe approach to the values of Hund’s ground states at about 60 K and 50 K, while that for CeAlGe reaches 87 % of Rln2 at Tc stemming from a doublet ground state. It’s significant to combine multi-experiments and effective fittings to comparatively explore Weyl RAlGe family for the potential application in spintronics.

Excellent 5f-electron magnet of actinide atom decorated gh-C3N4 monolayer.

Atomic functionality of two-dimensional (2D) materials, typically with a controllable doping route for offering regular atomic arrangement as well as excellent magnetism, is crucial for both fundamental studies and spintronic applications. Here, the adsorptions of the 5f-electron actinide series (An = Ac-Am) on porous graphene-like carbon-nitride (gh-C3N4) layers are explored to determine their structural stabilities, electronic nature and magnetic properties using the combination of density functional theory (DFT) calculations, ab initio molecular dynamics (AIMD), Monte Carlo (MC) simulations and chemical bonding analyses. Our investigations reveal that each An atom can be individually adsorbed at the vacancy site of gh-C3N4 sheet with high energetic, thermal and dynamical stabilities, which are rooted in the major interactions of ionic An-N bonding as well as the minor interactions of covalent bonding of An-5f6d states with N-2s2p states. The delocalization of a very few 5f electrons is dependent on whether they occupy the suborbitals that are matching and conducive to hybridize with the ligand orbitals forming the 5f-2s2p covalent bonds. We propose that the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism plays a determining role for the inter-atomic 5f-5f magnetic exchange via the 6d electrons as the conduction electrons. Large magnetic moment and magnetic anisotropy energy (MAE) from the localized 5f electrons, together with the metallic characteristics owing to the delocalized 6d electrons, render these An-based 2D materials excellent metallic magnets, especially for the U@gh-C3N4 system with the modest magnetic moment of 0.6 μB, large MAE of 53 meV and high Curie temperature (TC) of 538 K.

Norm-Conserving 4f-in-Core Pseudopotentials and Basis Sets Optimized for Trivalent Lanthanides (Ln = Ce–Lu)

We present here a set of scalar-relativistic norm-conserving 4f-in-core pseudopotentials, together with complementary valence-shell Gaussian basis sets, for the lanthanide (Ln) series (Ce-Lu). The Goedecker, Teter, and Hutter (GTH) formalism is adopted with the generalized gradient approximation (GGA) for the exchange-correlation Perdew-Burke-Ernzerhof (PBE) functional. The 4f-in-core pseudopotentials are built through attributing 4f-subconfiguration 4fn (n = 1-14) for Ln (Ln = Ce-Lu) into the atomic core region, making it possible to circumvent the difficulty of the description of the open 4fn valence shell. A wide variety of computational benchmarks and tests have been carried out on lanthanide systems including Ln3+-containing molecular complexes, aqueous solutions, and bulk solids to validate the accuracy, reliability, and efficiency of the optimized 4f-in-core GTH pseudopotentials and basis sets. The 4f-in-core GTH pseudopotentials successfully replicate the main features of lanthanide structural chemistry and reaction energetics, particularly for nonredox reactions. The chemical bonding features and solvation shells, hydrolysis energetics, acidity constants, and solid-state properties of selected lanthanide systems are also discussed in detail by utilizing these new 4f-in-core GTH pseudopotentials. This work bridges the idea of keeping highly localized 4f electrons in the atomic core and efficient pseudopotential formalism of GTH, thus providing a highly efficient approach for studying lanthanide chemistry in multi-scale modeling of constituent-wise and structurally complicated systems, including electronic structures of the condensed phase and first-principles molecular dynamics simulations.

Inter-configuration fluctuation for 5f electrons in uranium hexafluoride: A many-body study

A quantum mechanics calculation is performed on uranium hexafluoride (UF6) by using density functional theory (DFT) combing with dynamical mean-field theory (DMFT) scheme, taking into account the spin–orbit coupling (SOC) interaction and the on-site 5f-5f Coulomb repulsion. Results demonstrate that SOC-splitting j = 5/2 and j = 7/2 manifolds are in the metallic and insulating regimes, respectively, albeit both are in weakly correlated states. Ratio of the quasi-particle weights R = Zj=7/2/Zj=5/2 is about 1, suggesting that UF6 is not in the so-called orbital-selective localized state. Russel-Saunders (LS) coupling scheme is feasible for U 5f electrons. U 5f-conduction electrons hybridization, 5f-5f correlation, SOC interaction and final state effects yield a complicated spectrum function. Occupation probability analysis shows that 5f electrons exhibit the so-called inter-configuration fluctuation (ICF) due to admixing of 5f1 and 5f3 atomic configurations into 5f2 configuration with an average occupation number of n5f ∼ 1.8585, mainly arising from the complicated quantum mechanics processes including the dual nature of localized and itinerant 5f electrons, a quantum mechanical mixture of the energetically degenerate 5fn configurations, as well as the flexible outer electronic configuration of U ions. Finally, the momentum-resolved electronic spectrum function (the so-called quasiparticle band structure) is also discussed.

Robust Magnetic Order Upon Ultrafast Excitation of an Antiferromagnet

AbstractThe ultrafast manipulation of magnetic order due to optical excitation is governed by the intricate flow of energy and momentum between the electron, lattice, and spin subsystems. While various models are commonly employed to describe these dynamics, a prominent example being the microscopic three temperature model (M3TM), systematic, quantitative comparisons to both the dynamics of energy flow and magnetic order are scarce. Here, an M3TM was applied to the ultrafast magnetic order dynamics of the layered antiferromagnet GdRh2Si2. The femtosecond dynamics of electronic temperature, surface ferromagnetic order, and bulk antiferromagnetic order were explored at various pump fluences employing time‐ and angle‐resolved photoemission spectroscopy and time‐resolved resonant magnetic soft X‐ray diffraction, respectively. After optical excitation, both the surface ferromagnetic order and the bulk antiferromagnetic order dynamics exhibit two‐step demagnetization behaviors with two similar timescales (&lt;1 ps, ∼10 ps), indicating a strong exchange coupling between localized 4f and itinerant conduction electrons. Despite a good qualitative agreement, the M3TM predicts larger demagnetization than the experimental observation, which can be phenomenologically described by a transient, fluence‐dependent increased Néel temperature. The results indicate that effects beyond a mean‐field description have to be considered for a quantitative description of ultrafast magnetic order dynamics.

Magnetic properties of NdFe[formula omitted]Ti and YFe[formula omitted]Ti, from experiment and theory

NdFe11Ti and YFe11Ti serve as prototypes for rare-earth (RE) lean or REfree magnets with the ThMn12-type structure. Although NdFe11Ti has been studied for a long time the origin of its complex magnetism at low temperature is so far not well-understood. We present a comprehensive theoretical and experimental study of the magnetic properties of NdFe11Ti and RE-free YFe11Ti to elucidate the influence of the 4f electrons. The partially localized 4f electrons of Nd are the driving force behind the complex behavior of the magnetocrystalline anisotropy which changes from cone to uniaxial above 170 dK. The spontaneous magnetization and the five leading anisotropy constants were determined from high-quality single crystal samples over a wide temperature range using field dependencies of magnetization measured along the principle crystallographic directions. The experimental data are compared with density functional theory combined with a Hartree-Fock correction (+U) and an approximate dynamical mean-field theory.

Exploring s-d, s-f, and d-f Electron Interactions in AgnCe+ and AgnSm+ by Chemical Reaction toward O2.

We investigate gas-phase reactions of free AgnCe+ and AgnSm+ clusters with oxygen molecules to explore s-d, s-f, and d-f electron interactions in the finite size regime; a Ce atom has a 5d electron as well as a 4f electron, whereas a Sm atom has six 4f electrons without 5d electrons. In the reaction of AgnCe+ (n = 3-20), the Ce atom located on the cluster surface provides an active site except for n = 15 and 16, as inferred from the composition of the reaction products with oxygen bound to the Ce atom as well as from their relatively high reactivity. The extremely low reactivity for n = 15 and 16 is due to encapsulation of the Ce atom by Ag atoms. The minimum reactivity observed at n = 16 suggests that a closed electronic shell with 18 valence electrons is formed with a delocalized Ce 5d electron, while the localized Ce 4f electron does not contribute to the shell closure. As for AgnSm+ (n = 1-18), encapsulation of the Sm atom was observed for n ≥ 15. The lower reactivity at n = 17 than at n = 16 and 18 implies that an 18-valence-electron shell closure is formed with s electrons from Ag and Sm atoms; Sm 4f electrons are not involved in the shell closure as in the case of AgnCe+. The present results suggest that the 4f electrons tend to localize on the lanthanoid atom, whereas the 5d electron delocalizes to contribute to the electron shell closure.

Novel Valence Transition in Elemental Metal Europium around 80GPa.

Valence transition could induce structural, insulator-metal, nonmagnetic-magnetic and superconducting transitions in rare-earth metals and compounds, while the underlying physics remains unclear due to the complex interaction of localized 4f electrons as well as their coupling with itinerant electrons. The valence transition in the elemental metal europium (Eu) still has remained as a matter of debate. Using resonant x-ray emission scattering and x-ray diffraction, we pressurize the states of 4f electrons in Eu and study its valence and structure transitions up to 160GPa. We provide compelling evidence for a valence transition around 80GPa, which coincides with a structural transition from a monoclinic (C2/c) to an orthorhombic phase (Pnma). We show that the valence transition occurs when the pressure-dependent energy gap between 4f and 5d electrons approaches the Coulomb interaction. Our discovery is critical for understanding the electrodynamics of Eu, including magnetism and high-pressure superconductivity.

Peculiarities of the electric resistivity behavior of R3(Ce,Nd,Sm)Cu4Sn4, R(Gd,Tb,Ho)NiSn2, DyNiSi, and DyNiSi3 compounds in magnetic fields

The resistance of ternary intermetallic compounds, namely RE(Ce, Nd, Sm)3Cu4Sn4 (space group Immm), RE(Gd, Tb, Ho)NiSn2 and DyNiSi (space group Pnma) and DyNiSi3 (space group Cmmm), has been studied in the temperature range up to 0.3 K and magnetic field up to 12 Tl. The temperature dependencies of electrical resistivity evidently show the anomalies at low temperatures. The anomalies of resistance temperature dependencies observed at the absence of an external magnetic field, agree well with magnetic phase transitions. In the majority of the investigated compounds, an increase of the external magnetic field leads to an erosion of the peculiarities on the temperature dependency of resistivity, right up to complete disappearance. Such effect is caused by the corresponding magnetic order phase transitions. Magnetic field influence on the temperature dependency of resistance is treated as an influence on the magnitude of hybridization between (s-d) conductivity electrons and localized f-electrons including influence on change on the charge caries mobility due to a possible spin-compensated interaction. Electron transport properties expose peculiarities in dependency of different chemical environment vicinity in elementary cell of crystal lattice and of magnetic state of the compound.