4f States Research Articles

Angle-resolved photoemission spectroscopy study of rare-earth tritelluride charge density wave compounds: RTe3 (R = Pr, Er)

The electronic structures of layered rare-earth tritelluride RTe3 (R = Pr, Er) charge density wave (CDW) compounds have been investigated by performing R 4d → 4f resonant photoemission spectroscopy (RPES) and angle-resolved photoemission spectroscopy (ARPES) measurements for high-quality single crystals. R 4d → 4f RPES measurements reveal that the R 4f states do not contribute directly to the CDW formation in RTe3 but that the R 4f-Te 5p hybridization in PrTe3 is significantly larger than that in ErTe3. In the photon-energy maps for the Fermi-edge (E F)-crossing states in RTe3, straight vertical dispersions are observed along k c , demonstrating the 2D character for the near-E F states in both R = Pr and Er. This finding implies the weak interlayer interaction between R-Te(1) and Te(2)–Te(3) layers, which is supported by the similar linear dichroism in ARPES for PrTe3 and ErTe3. The CDW-induced Fermi surface of PrTe3 exhibits two-fold symmetric features while that of ErTe3 exhibits four-fold symmetric features. This finding reveals different CDW distortions in PrTe3 and ErTe3, the origin of which is likely to be different ionic sizes of R ions and different R 4f-Te 5p hybridization.

Electronic structure and magnetism in UGa2 : DFT+DMFT approach

The debate whether uranium 5f electrons are closer to being localized or itinerant in the ferromagnetic compound UGa2 is not yet fully settled. The experimentally determined magnetic moments are large, approximately 3 Bohr magnetons, suggesting the localized character of the 5f electrons. In the same time, one can identify signs of itinerant as well as localized behavior in various spectroscopic observations. The band theory, employing local exchange-correlation functionals, is biased toward itinerant 5f states and severely underestimates the moments. Using material-specific dynamical mean-field theory (DMFT), we probe how a less approximate description of electron-electron correlations improves the picture. We present two variants of the theory: starting either from spin-restricted (LDA) or spin-polarized (LSDA) band structure. We show that the L(S)DA+DMFT method can accurately describe the magnetic moments in UGa2 as long as the exchange interaction between the uranium 6d and 5f electrons is preserved by a judicious choice of the spin-polarized double-counting correction. We discuss the computed electronic structure in relation to photoemission experiments and show how the correlations reduce the Sommerfeld coefficient of the electronic specific heat by shifting the 5f states slightly away from the Fermi level.

Electronic structure and coexistence of superconductivity with magnetism in RbEuFe4As4

In the novel stoichiometric iron-based material RbEuFe$_{4}$As$_{4}$ superconductivity coexists with a peculiar long-range magnetic order of Eu 4f states. Using angle-resolved photoemission spectroscopy, we reveal a complex three dimensional electronic structure and compare it with density functional theory calculations. Multiple superconducting gaps were measured on various sheets of the Fermi surface. High resolution resonant photoemission spectroscopy reveals magnetic order of the Eu 4f states deep into the superconducting phase. Both the absolute values and the anisotropy of the superconducting gaps are remarkably similar to the sibling compound without Eu, indicating that Eu magnetism does not affect the pairing of electrons. A complete decoupling between Fe- and Eu-derived states was established from their evolution with temperature, thus unambiguously demonstrating that superconducting and a long range magnetic orders exist independently from each other. The established electronic structure of RbEuFe$_{4}$As$_{4}$ opens opportunities for the future studies of the highly unorthodox electron pairing and phase competition in this family of iron-based superconductors with doping.

Spectroscopic properties of Tm:Bi4Ge3O12 crystals grown by the micro-pulling-down method

Tm3+-doped bismuth germanate crystals (Tm:BGO) with various doping concentrations of 0.5, 0.75, 1.0, 1.5, and 2.0 at.% were grown by the micro-pulling-down method (μ-PD). The effect of thulium concentration on the growth, structure, absorption spectra, emission spectra and fluorescence lifetime of 3H4 and 3F4 state was studied. Under excitation of 792 nm, Tm:BGO crystals exhibit two fluorescence bands corresponding to the 3H4 → 3F4 and the 3F4 → 3H6 transitions. The fluorescence lifetime of 3H4 state decreased strongly as the thulium concentration increased, while the lifetime of 3F4 state was relatively constant for all thulium concentrations.

Spectroscopy of Tm3+-doped CaF2 waveguiding thin films grown by Liquid Phase Epitaxy

Thulium-doped calcium fluoride (Tm:CaF2) single-crystalline films are grown on (100)-oriented undoped bulk CaF2 substrates by Liquid Phase Epitaxy (LPE) using LiF as a solvent. Their spectroscopic properties are studied in details. For ~2 at.% Tm-doped films, the active ions are predominantly isolated and are located in oxygen-assisted trigonal sites, C3v(T2). With increasing the Tm doping level, ion clustering is promoted but it is slowed down as compared to bulk crystals. For ~6 at.% Tm-doped films, the majority of active ions form clusters resulting in a “glassy-like” spectroscopic behavior (smooth and broad spectral bands). For such layers, the maximum stimulated-emission cross-section for the 3F4 → 3H6 transition is 0.14 × 10−20 cm2 at 1856 nm, the luminescence lifetime of the 3F4 state is 21.70 ms and the emission bandwidth exceeds 180 nm. The transition probabilities of Tm3+ ions are determined using the Judd-Ofelt theory. The waveguiding properties of the Tm:CaF2 films are confirmed. Highly-doped Tm:CaF2 epitaxial films are promising for waveguide lasers at ~2 μm.

Новый магнитный сверхпроводник стехиометрического состава EuRbFe<sub>4</sub>As<sub>4</sub>

In the novel stoichiometric iron-based material RbEuFe4As4 superconductivity coexists with a peculiar long-range magnetic order of Eu 4f states; their coexistance is puzzling and represents a challenge for both experiment and theory. Using angle-resolved photoemission spectroscopy, resonant photoemission spectroscopy, Andreev reflection spectroscopy and scanning tunneling spectroscopy we have addressed this puzzle and unambigously shown that Fe- and Eu-derived states are largely decoupled and that superconducting and a long range magnetic orders exist almost independently from each other.

The Limitations of 5f Delocalization and Dispersion

Delocalization in the 5f states of the actinides is an important phenomenon, but poorly quantified. Here, the fundamental limitations of 5f dispersion measurements using angle and momentum resolved variants of photoelectron spectroscopy will be discussed. A novel approach will be suggested, based on a theoretical projection, which should circumvent these limitations: M4,5 X-ray emission spectroscopy. This analysis will utilize the case study of U metal, which can be considered to be the paramount example of 5f dispersion.

New Cage-Like Cerium Trihydride Stabilized at Ambient Conditions

New Cage-Like Cerium Trihydride Stabilized at Ambient Conditions

State-selective charge exchange cross sections in Be^{4+} -hbox {H}(2textit{lm}) collision based on the classical trajectory Monte Carlo method

A three-body classical trajectory Monte Carlo method is used to calculate the nl state-selective charge exchange cross sections in hbox {Be}^{mathrm {4+}}+ H(2lm) collisions in the energy range between 10 and 200 keV/amu. We present partial cross sections for charge exchange into hbox {Be}^{mathrm {3+}}(nl) (textit{nl} = 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f) states as a function of impact energy. Our results are compared with the previous classical and quantum-mechanical results. We show that the classical treatment can able to describe reasonably well the charge exchange cross sections.Graphic abstract

Experimental and Theoretical Study of the Electronic Structures of Lanthanide Indium Perovskites LnInO3.

Ternary lanthanide indium oxides LnInO3 (Ln = La, Pr, Nd, Sm) were synthesized by high-temperature solid-state reaction and characterized by X-ray powder diffraction. Rietveld refinement of the powder patterns showed the LnInO3 materials to be orthorhombic perovskites belonging to the space group Pnma, based on almost-regular InO6 octahedra and highly distorted LnO12 polyhedra. Experimental structural data were compared with results from density functional theory (DFT) calculations employing a hybrid Hamiltonian. Valence region X-ray photoelectron and K-shell X-ray emission and absorption spectra of the LnInO3 compounds were simulated with the aid of the DFT calculations. Photoionization of lanthanide 4f orbitals gives rise to a complex final-state multiplet structure in the valence region for the 4fn compounds PrInO3, NdInO3, and SmInO3, and the overall photoemission spectral profiles were shown to be a superposition of final-state 4fn–1 terms onto the cross-section weighted partial densities of states from the other orbitals. The occupied 4f states are stabilized in moving across the series Pr–Nd–Sm. Band gaps were measured using diffuse reflectance spectroscopy. These results demonstrated that the band gap of LaInO3 is 4.32 eV, in agreement with DFT calculations. This is significantly larger than a band gap of 2.2 eV first proposed in 1967 and based on the idea that In 4d states lie above the top of the O 2p valence band. However, both DFT and X-ray spectroscopy show that In 4d is a shallow core level located well below the bottom of the valence band. Band gaps greater than 4 eV were observed for NdInO3 and SmInO3, but a lower gap of 3.6 eV for PrInO3 was shown to arise from the occupied Pr 4f states lying above the main O 2p valence band.

Electronic, magnetic, optical and transport properties of wurtzite-GaN doped with rare earth (RE= Pm, Sm, and Eu): First principles approach

A density functional theory (DFT) and semiclassical Boltzmann transport equations (BTE) has been used to explore the electronic, magnetic, optical and transport properties of Ga1−xRExN (RE=Pm, Sm, and Eu). We have studied the impact of doping of rare-earth (RE) elements on GaN wurtzite structure. In all the doped systems, the stability is ferromagnetic state with minimum ground state energy. Our theoretical calculations revealed that all the doped compounds exhibit half-metallic ferromagnet (HMF) behavior with 100% spin polarization at the Fermi level witin GGA, mBJ and Spin-orbit coupling (SOC). However, within GGA+U, Ga1−xRExN exhibits dilute magnetic semiconducting behaviour with exact integer value of total magnetic moment. A large gap was established with the concentrations of 6.25%, 12.5%. The shift of fermi energy towards the bottom of the conduction band, make these systems an n-type. The mechanism of ferromagnetism in this system has been proposed according to this positioning. The majority-spin density of states (DOS) mainly comprises of Eu-4f states appears in the energy gap. Yet, in the case of the Pm, and Sm-doped GaN, some part of the impurity states of 4f lies below Fermi energy (Ef). The optical absorption for RE-doped GaN is significantly enhanced, in the ultraviolet region. We have noticed the improvement of electrical conductivity as compared to the pristine one for both the concentrations. These features, make RE-doped GaN a transparent material for optoelectronic tools.

Enhanced optical absorption of rutile TiO2 through (Sm, C) codoping: a first-principles study

In this Paper, we employ the density functional theory to investigate the electronic and optical properties of rutile TiO $$_2$$ , Sm doped and (Sm, C) co-doped TiO $$_2$$ materials. We use the Tran-Blaha modified Beck Johnson (TB-mBJ) potential. This functional can estimate the band gap of these compounds with more accuracy compared to the generalized gradient approximation (GGA). The obtained electronic band gap of the pristine TiO $$_2$$ is $$E_g = 2.76$$ eV, which is close to the experimental value $$E_g = 3.0$$ eV. We report also that Sm doped and (Sm, C) co-doped TiO $$_2$$ are stable in ferromagnetic state. The magnetic moment of samarium is consistent with other literature works. Furthermore, adding samarium atom with 4f states and carbon atom with activated 2p states improve appreciably the optical properties in the visible light range. The corresponding obtained optical band gap value of the (Sm,C) co-doped TiO $$_2$$ is an optimal one which evidences that the materials is capable of showing high power conversion efficiency. Additionally, the enhancement of optical absorption under the visible-light of these materials, especially the co-doped one can find suitable applications in optoelectronic devices.

Locating impurity and defect levels in the host band gap by first-principles calculations: Pure and Ce3+-doped YAlO3

The structural and electronic properties of the YAlO3 crystal (neat, Ce3+-doped and with antisite defects) were calculated from the first-principles. Comparison of the calculated electronic band structures of the ideal YAlO3 host with the YAlO3:Ce3+ and YAlO3 (antisite) cases allowed to determine the Ce3+ ground 4f state to be at 3.5 eV above the valence band top, in very good agreement with the Dorenbos model prediction. The complete energy level scheme of the YAlO3:Ce3+ crystal was obtained, with the cerium crystal field split 4f and 5d levels superimposed onto the host's band structure. It was shown that the yttrium antisite ions produce localized defect levels 0.87 eV below the conduction band, which enables the antisite defects to serve as the trap centers for the electron excitation energy in this host.

Structural and electronic properties of amorphous bismuth calcium borate from first-principle calculations

We investigatethe electronic and structural properties of two new amorphous materials, Bi6Ca2O28B12 and Bi6Ca2 O28B11:Tm, using density functional theory with a short-range description similar to that for crystalline solids and by considering the Tm 4f states as core states. Our results show that the valence band is formed by broadband states with a strong presence of the O − 2p orbital states in both materials. The thulium-doped structure shows an increased band gap, indicating the strong presence of this rare earth in connection with the oxygen. Both materials exhibit a narrow band gap. Calculations related to the electronic transition of the orbitals between the highest occupied valence band and the lowest unoccupied conduction band and the density charge show a significant contribution of bismuth and oxygen in the formation of both materials. Our results present important information that can assist in the investigation of the technological applications of devices with a design focus for solid-state lasers.

Alkali Uranyl Borates: Bond Length, Equatorial Coordination and 5f States

Three uranyl borates, UO2B2O4, LiUO2BO3 and NaUO2BO3, have been prepared by solid state syntheses. The influence of the crystallographic structure on the splitting of the empty 5f and 6d states have been probed using High Energy Resolved Fluorescence Detected X-ray Absorption Spectroscopy (HERFD-XAS) at the uranium M4-edge and L3-edge respectively. We demonstrate that the 5f splitting is increased by the decrease of the uranyl U-Oax distance, which in turn correlates with an increased bond covalency. This is correlated to the equatorial coordination change of the uranium. The role of the alkalis as charge compensating the axial oxygen of the uranyl is discussed.

Eu–Mn Charge Transfer and the Strong Charge–Spin–Electronic Coupling Behavior in EuMnO3

Based on first-principles calculations with the DFT + U method, the couplings of lattice, charge, spin, and electronic behaviors underlying the Eu-Mn charge transfer in a strongly correlated system of EuMnO3 were investigated. The potential valence transition from Eu3+/Mn3+ to Eu2+/Mn4+ was observed in a compressed lattice with little distortions, which is achieved under hydrostatic pressure and external strain. The intraplane antiferromagnetism (AFM) of Mn is proved to be instrumental in the emergence of Eu2+. Furthermore, we calculated the magnetic exchange interactions within two equilibrium structures of Eu3+Mn3+O3 and Eu2+Mn4+O3. Mn-Mn ferromagnetic exchange in the ab-plane is enhanced strongly in the Eu2+Mn4+O3 structure, contributing to the existence of mixed states. The versatile electronic structures were obtained within the Eu2+Mn4+O3 phase by imposing different magnetic configurations on the Eu and Mn sublattice, attributed to the coupling of charge transfer and magnetic orderings. It is found that the intraplane ferromagnetic ordering of Mn leads to a metallic electronic structure with the coexistence of Eu2+ and Eu3+, while the intraplane AFM Mn spin ordering leads to insulating states only with Eu2+. Notably, a half-metallic characteristic emerges at the magnetic ground state of CF ordering (C-type AFM for the Eu sublattice and ferromagnetic for the Mn sublattice), which makes such a supposed phase more intriguing than the conventional experimental phase. Additionally, the mixture of delocalized 4f with 5d states of Eu in the background of Mn 3d and O 2p orbitals implies a pathway of Eu 4f 5d ↔ O 2p ↔ Mn 3d for charge transfer between Eu and Mn. Our calculation shows that the Eu-Mn charge transfer could be expected in compressed EuMnO3 and the introduction of Eu2+ 4f states near the Fermi level plays an important role in manipulating the interlinks of charge and spin together with electronic behaviors.

Photogenerated charge separation and recombination path modification in monocline Lu2WO6via lattice transition and Bi-O antibonding states.

Monoclinic Lu2WO6 undergoes diphase-to-perovskite BiLuWO6 transition via selective occupancy of Bi in three Lu sites. The transformation mechanism, process, and structure stabilities are revealed by variable cell nudged elastic band method, video, and phonon spectrum. Lattice transition brings about photogenerated charge separation in BiLuWO6. This is verified by indirect band gap transition, high electron migration rate, weak exciton binding energy, large photocurrent response, and small impedance. The electron-hole life time is elongated to produce abundant superoxide and hydroxyl radicals for the degradation of rhodamine B and phenol molecules. Bi-O antibonding states serve as immediate energy levels to change the recombination path, inducing 340 nm excitation band and 510 nm green light emission of Lu2WO6. Furthermore, multicolor emission of 1 at% Bi3+ + RE3+ (RE = Sm/Eu/Dy)-codoped Lu2WO6 is acquired via synergistic modification of the Bi-O antibonding state and RE3+ 4f states. Thus, the photogenerated charge motion in Lu2WO6 is tuned to expand application fields.

Ferromagnetic alloy for high-efficiency photovoltaic conversion in solar cells: first-principles insights when doping SnO2 rutile with coupled Eu-Gd.

From results of first-principles all-electron full-potential augmented spherical-wave calculations within a generalized gradient approximation, a materials design for half-metallic ferromagnetic semiconductors based on (Eu,Gd)-doped SnO2 rutile is proposed. Moreover, their half-metallic ferromagnetic properties are homogenous and energetically stable for different crystallographic directions. Therefore, the interatomic exchange interaction between the spins of double impurity ions is a long-range ferromagnetic interaction that is sharply weakened when the distance between Eu–Gd increases. The double impurities most likely substitute adjacent Sn sites and result in strong ferromagnetic interactions by p–f hybridization between rare earth 4f and Op states. There is great interest in the configuration that has the lowest energy difference, where the double impurity substitutes the nearest neighbor Sn sites along the z-axis of SnO2 rutile. Generalized gradient approximation GGA and GGA+U calculations were performed. According to our revPBE-GGA calculations, the ferromagnetic compound is capable of absorbing 96% from the visible light. Furthermore, the transport properties at room temperature ensure excellent electrical conductivity, low thermal conductivity, and the most optimal figure of merit (ZT), which leads to high thermoelectric performance. As the latter are closely related to free flow charge carriers, we can subsequently predict that the ferromagnetic alloy will be able to be a great power source for highly effective photovoltaic conversion in solar cells. Further experimentation will be necessary to obtain confirmation of our ab initio predictions.

Rare earth ions doped mixed crystals for fast quantum computers with optical frequency qubits

The possibility of using mixed crystals highly doped with rare earth ions (REIs) as physical systems for creating fast quantum computers with a sampling time of 10−9 s is discussed. The electronic 4f states of rare earth ions with small values of the diagonal elements of the Judd–Ofelt​ matrix U(2) are proposed as optical frequency qubit levels. CNOT and other conditional gate operations are performed by exciting the rare earth ion into the 4f state with a large diagonal element of U(2), causing a Stark blockade. It is found that the main interaction responsible for this blockade is the quadrupole–quadrupole interaction. The large inhomogeneous broadening of the frequencies of the electronic transitions in mixed crystals and the weak interaction of 4f electrons with phonons make it possible to achieve a high computation rate and a long decoherence time of the qubits. An ensemble of closest REIs is described that can act as an OQC instance; the frequencies of the corresponding qubits can be found using the spectral hole burning method.

Lidar observations of the upper atmospheric nickel layer at Beijing ([formula omitted],[formula omitted

Ni atoms, produced in the Earth's upper atmosphere by meteoric ablation, are challenging to measure by lidar because the Ni atom density is low, the resonance scattering cross section is relatively small, and the transitions occur in the near-UV around 340 nm where absorption by the stratospheric ozone layer starts to become significant. A new Ni lidar has been designed and deployed at Yanqing station (40∘N,116∘E) Beijing, China over the period from 3 April 2019 to 31 December 2019. The combination of a frequency-doubled near-IR laser and narrow bandwidth filters enables stable and continuous operation. From 25 nights of measurements, the Ni peak density ranges from 98 to 460 cm−3 with the peak altitude between 80 and 88 km; the average peak density is 258±115cm−3. The average column abundances of Nickel varied between 1.52×108 and 6.0×108 cm−2 between mid-summer and mid-winter. An experiment was also carried out using the Ni lidar to determine the spectroscopic branching ratios from the excited Ni(3F4) state into 3 lower states; very close agreement with the literature values were obtained.