Ground State Of Ce Research Articles

Ground state of Ce3Bi4Pd3 unraveled by hydrostatic pressure

Noncentrosymmetric Ce$_{3}$Bi$_{4}$Pd$_{3}$ has attracted a lot of attention as a candidate for strongly correlated topological material, yet its experimental ground state remains a matter of contention. Two conflicting scenarios have emerged from a comparison to prototypical Kondo insulator Ce$_{3}$Bi$_{4}$Pt$_{3}$: either Ce$_{3}$Bi$_{4}$Pd$_{3}$ is a spin-orbit-driven topological semimetal or a Kondo insulator with smaller Kondo coupling than its Pt counterpart. Here we determine the ground state of Ce$_{3}$Bi$_{4}$Pd$_{3}$ via electrical resistivity measurements under hydrostatic pressure, which is a clean symmetry-preserving tuning parameter that increases hybridization but virtually preserves spin-orbit coupling. Ce$_{3}$Bi$_{4}$Pd$_{3}$ becomes more insulating under pressure, which is a signature of Ce-based Kondo insulating materials. Its small zero-pressure gap increases quadratically with pressure, similar to the behavior observed in the series Ce$_{3}$Bi$_{4}$(Pt$_{1-x}$Pd$_{x}$)$_{3}$, which indicates that Pt substitution and applied pressure have a similar effect. Our result not only demonstrates that Kondo coupling, rather than spin-orbit coupling, is the main tuning parameter in this class of materials, but it also establishes that Ce$_{3}$Bi$_{4}$Pd$_{3}$ has a narrow-gap Kondo insulating ground state.

Metamagnetism and crystal-field splitting in pseudohexagonal CeRh3Si2

CeRh$_3$Si$_2$ has been reported to exhibit metamagnetic transitions below 5~K, a giant crystal field splitting, and anisotropic magnetic properties from single crystal magnetization and heat capacity measurements. Here we report results of neutron and x-ray scattering studies of the magnetic structure and crystal-field excitations to further understand the magnetism of this compound. Inelastic neutron scattering (INS) and resonant inelastic x-ray scattering (RIXS) reveal a $J_z$\,=\,1/2 groundstate for Ce when considering the crystallographic $a$ direction as quantization axis, thus explaining the anisotropy of the static susceptibility. Furthermore, we find a total splitting of 78\,meV for the $J$\,=\,5/2 multiplet. The neutron diffraction study in zero field reveals that on cooling from the paramagnetic state, the system first orders at $T_{\text{N}_1}=4.7$\,K in a longitudinal spin density wave with ordered Ce moments along the $b$-axis (i.e. the [0 1 0] crystal direction) and an incommensurate propagation vector $\textbf{k}=(0,0.43,0$). Below the lower-temperature transition $T_{\text{N}_2}=4.48$\,K, the propagation vector locks to the commensurate value $\textbf{k}=(0,0.5,0)$, with a so-called lock-in transition. Our neutron diffraction study in applied magnetic field $H\parallel b$-axis shows a change in the commensurate propagation vector and development of a ferromagnetic component at $H=3$\,kOe, followed by a series of transitions before the fully field-induced ferromagnetic phase is reached at $H = 7$\,kOe. This explains the nature of the steps previously reported in field-dependent magnetization measurements. A very similar behaviour is also observed for the $H\parallel$ [0 1 1] crystal direction.

Stimulated Resonant Spin Amplification Reveals Millisecond Electron Spin Coherence Time of Rare-Earth Ions in Solids.

The inhomogeneity of an electron spin ensemble as well as fluctuating environment acting upon individual spins drastically shorten the spin coherence time T_{2} and hinder coherent spin manipulation. We show that this problem can be solved by the simultaneous application of a radio frequency (rf) field, which stimulates coherent spin precession decoupled from an inhomogeneous environment, and periodic optical pulses, which amplify this precession. The resulting resonance, taking place when the rf field frequency approaches the laser pulse repetition frequency, has a width determined by the spin coherence time T_{2} that is free from the effects of inhomogeneity and slow nuclear spin fluctuations. We measure a 50-Hz-narrow electron spin resonance and milliseconds-long T_{2} for electrons in the ground state of Ce^{3+} ions in the yttrium aluminum garnet (YAG) lattice at low temperatures, while the inhomogeneous spin dephasing time T_{2}^{*} is only 25ns. This study paves the way to coherent optical manipulation in spin systems decoupled from their inhomogeneous environment.

Correlation effect on electronic and lattice properties of cerium oxides: Insights from density functional theory to dynamical mean-field theory

In order to reveal static and dynamic correlation effect in Ce 2 O 3 and CeO 2 with an unified theoretical framework, we perform a first principles calculation on electronic and lattice properties of these two typical correlated systems using density functional theory (DFT) schemes as well as a many-body method merging DFT with dynamical mean field theory (DMFT), taking into account the spin–orbit coupling (SOC) interaction and on-site Coulomb repulsion between Ce 4 f states. Results show that ground state of Ce 2 O 3 is in the localized 4 f 1 atomic configuration, while CeO 2 is a mixed-valence n f = 0.82 correlated system, which are in agreement with recent theoretical calculations and experimental observations. Lattice parameters, bulk modulus and band gap are also consistent with other theoretical calculations and experimental values. In order to compare with experimental angle-resolved photoemission spectrum (ARPES), momentum-resolved electronic spectrum function is also discussed.

Spin-Orbit Coupling Changes the Identity of the Hyper-Open-Shell Ground State of Ce+, and the Bond Dissociation Energy of CeH+ Proves to Be Challenging for Theory.

Cerium (Ce) plays important roles in catalysis. Its position in the sixth period of the periodic table leads to spin-orbit coupling (SOC) and other open-shell effects that make the quantum mechanical calculation of cerium compounds challenging. In this work, we investigated the low-lying spin states of Ce+ and the bond energy of CeH+, both by multiconfigurational methods, in particular, SA-CASSCF, MC-PDFT, CASPT2, XMS-PDFT, and XMS-CASPT2, and by single-configurational methods, namely, Hartree-Fock theory and unrestricted Kohn-Sham density functional theory with 34 choices of the exchange-correlation functional. We found that only CASPT2, XMS-CASPT2, and SA-CASSCF (among the five multiconfigurational methods) and GAM, HCTH, SOGGA11, and OreLYP (among the 35 single-configuration methods) successfully predict that the SOC-free ground spin state of Ce+ is a doublet state, and CASPT2 and GAM give the most accurate multireference and single-reference calculations, respectively, of the excitation energy of the first SOC-free excited state for Ce+. We calculated that the ground doublet state of Ce+ is an intra-atomic hyper-open-shell state. We calculated the spin-orbit energy (ESO) of Ce+ by the five multiconfigurational methods and found that ESO calculated by CASPT2 is the closest to the experimental value. Taking advantage of the availability of an experimental D0 for CeH+ as a way to provide a unique test of theory, we showed that all the multiconfigurational methods overestimate D0 by at least 246 meV (5.7 kcal/mol), and only three functionals, namely, SOGGA, MN15, and GAM, have an error of D0 that is less than 200 meV (5 kcal/mol).

Crystal fields and Kondo effect: Magnetic susceptibility of Cerium ions in axial crystal fields

The thermodynamic Bethe ansatz equations of the Coqblin-Schrieffer model have been solved numerically. The full N=6 (J=5/2) degeneracy of the Hund's rule ionic ground state of Ce is taken into account. Results for the temperature dependent magnetic susceptibility parallel and perpendicular to the crystal axis are presented. The deviations, due to the Kondo effect, to the non-interacting ion results are pointed out.

Understanding the magnetism in noncentrosymmetric CeIrGe3 : Muon spin relaxation and neutron scattering studies

The magnetic properties of a pressure induced noncentrosymmetric heavy-fermion superconductor CeIrGe$_3$ have been investigated by muon spin relaxation ($\mu$SR), powder neutron diffraction (ND) and inelastic neutron scattering (INS) techniques at ambient pressure. For completeness we have also measured the ac magnetic susceptibility $\chi_{\rm ac}(T)$, dc magnetic susceptibility $\chi(T)$, dc isothermal magnetization $M(H)$ and heat capacity $C_{\rm p}(T,H)$ down to 2 K. CeIrGe$_{3}$ is known to exhibit pressure induced superconductivity ($T_{\rm c}\approx 1.5$ K) at a pressure of 20 GPa and antiferromagnetic ordering at 8.7 K, 4.7 K and 0.7 K at ambient pressure. Our $\chi_{\rm ac}(T)$ and $\chi(T)$ data show an additional anomaly near 6.2 K which is also captured in $C_{\rm p}(T)$ data. From $\chi_{\rm ac}(T)$, $\chi(T)$ and $C_{\rm p}(T)$ measurements we infer three antiferromagnetic transitions above 2 K at $T_{\rm N1}= 8.5$ K, $T_{\rm N2}= 6.0$ K and $T_{\rm N3}= 4.6$ K. Our $\mu$SR study also confirms the presence of three transitions through the observation of one frequency for $T_{\rm N2} < T\leq T_{\rm N1}$, two frequencies for $T_{\rm N3} < T\leq T_{\rm N2}$ and three frequencies for $T\leq T_{\rm N3}$ in the oscillatory asymmetry. The ND data reveal an incommensurate nature of the magnetic ordering at $T=7$ K with the propagation vector k = (0,0,0.688(3)), and a commensurate magnetic structure at $T=1.5$ K with the propagation vector locked to the value k = (0,0,2/3) and magnetic moments oriented along the $c$ axis. The commensurate structure couples a macroscopic ferromagnetic component, resulting in a strong dependence of the lock-in transition temperature on external magnetic field. The INS data show two well defined crystal electric field (CEF) excitations arising from the CEF-split Kramers doublet ground state of Ce$^{3+}$.

Thermal quenching of Ce3+luminescence in the cuspidine-type oxide nitride compounds Y4Si2−xAlxO7+xN2−x

Y3.98Ce0.02AlxO7+xN2−x system with x equal to 0, 0.2, 0.4, 0.6, 0.8, 1.0 and 1.2 doped with Ce3+ were synthesized by gas pressure sintering (GPS) system. It was found that Ce3+ at different sites in Y3.98Ce0.02AlxO7+xN2−x system creates several types of emission centers which have the same emission energy with maximum near 510nm but differ in quenching temperature. When part of Si-N is replaced by Al-O in Y3.98Ce0.02Si2O7N2, the position of the Ce3+ emission maximum slightly shifts to higher energy and the emission intensity as well as decay time of luminescence strongly decreases. Both thermal ionization to conduction band and thermal excitation to CTE (Cerium Trapped Exciton) are responsible for non-radiative quenching and low efficiency of the system. It is shown that the luminescence quenching by the CTE state activates at lower temperature because of the much lower activation energy. The detailed analysis of the temperature dependence of the luminescence decay shown that the various Ce3+ sites differ mainly by location of the Ce3+ localized states in the bandgap, whereas difference between the first excited states 5d1 and the ground state of Ce3+ is approximately the same.

Crystal-field states of Kondo lattice heavy fermionsCeRuSn3andCeRhSn3

Inelastic neutron scattering experiments have been carried out to determine the crystal field states of the Kondo lattice heavy fermions CeRuSn3 and CeRhSn3. Both the compounds crystallize in LaRuSn3-type cubic structure (space group Pm-3n) in which the Ce atoms occupy two distinct crystallographic sites with cubic (m-3) and tetragonal (-4m.2) point symmetries. The INS data of CeRuSn3 reveal the presence of a broad excitation centered around 6-8 meV which is accounted by a model based on crystal electric field (CEF) excitations. On the other hand, the INS data of isostructural CeRhSn3 reveal three CEF excitations around 7.0, 12.2 and 37.2 meV. The neutron intensity sum rule indicates that the Ce ions at both cubic and tetragonal Ce sites are in Ce3+ state in both CeRuSn3 and CeRhSn3. The CEF level schemes for both the compounds are deduced. We estimate the Kondo temperature T_K = 3.1(2) K for CeRuSn3 from neutron quasielastic linewidth in excellent agreement with that determined from the scaling of magnetoresistance which gives T_K = 3.2(1) K. For CeRhSn3 the neutron quasielastic linewidth gives T_K = 4.6 K. For both CeRuSn3 and CeRhSn3, the ground state of Ce3+ turns out to be a quartet for the cubic site and a doublet for the tetragonal site.

Combined optical emission and resonant absorption diagnostics of an Ar-O2-Ce-reactive magnetron sputtering discharge

We report the results on combined optical characterization of Ar-O2-Ce magnetron sputtering discharges by optical emission and resonant absorption spectroscopy. In this study, a DC magnetron sputtering system equipped with a movable planar magnetron source with a Ce target is used. The intensities of Ar, O, and Ce emission lines, as well as the absolute densities of Ar metastable and Ce ground state atoms are analyzed as a function of the distance from the magnetron target, applied DC power, O2 content, etc. The absolute number density of the Arm is found to decrease exponentially as a function of the target-to-substrate distance. The rate of this decrease is dependent on the sputtering regime, which should be due to the different collisional quenching rates of Arm by O2 molecules at different oxygen contents. Quantitatively, the absolute number density of Arm is found to be equal to ≈3×108cm−3 in the metallic, and ≈5×107cm−3 in the oxidized regime of sputtering, whereas Ce ground state densities at the similar conditions are found to be few times lower. The absolute densities of species are consistent with the corresponding deposition rates, which decrease sharply during the transition from metallic to poisoned sputtering regime.

Spin–Orbit Splitting Difference and Stokes Shift in Cerium3+-Activated Aluminate Phosphors

ABSTRACT Here we discuss the spin–orbit splitting difference in the 2F7/2 and 2F5/2 ground state of Ce3+-activated XMg2Al16O27(X˭Ba, Sr) phosphors. Recently we reported the optical properties in these host materials for the SSL purpose; there was no more literature available to date on these phosphor compositions. By using Gaussian curve fit analysis, we have estimated the energy level difference in the ground state of Ce3+ and the corresponding Stokes shift, and we compare these with those observed in previously reported works in different families of Ce3+-activated oxide-based phosphors.

Combined effect of chemical pressure and valence electron concentration through the electron-deficient Li substitution on the RE4LiGe4 (RE=La, Ce, Pr, and Sm) system

Four members of the RE 4 LiGe 4 ( RE =La, Ce, Pr, and Sm) system have been prepared by high-temperature reaction method and characterized by X-ray diffractions. All compounds crystallize in the orthorhombic Gd 5 Si 4 -type structure (space group Pnma , Pearson code oP 16) with bonding interactions for interslab Ge 2 dimers. The Li substitution for rare-earth elements in the RE 4 LiGe 4 system leads to a combined effect of the increased chemical pressure and the decreased valance electron concentration (VEC), which eventually results in the structure transformation from the Sm 5 Ge 4 -type with all broken interslab Ge–Ge bond for the parental RE 5 Ge 4 to the Gd 5 Si 4 -type structure for the ternary RE 4 LiGe 4 ( RE =La, Ce, Pr, and Sm) system. Site-preference between rare-earth metals and Li is proven to generate energetically the most favorable atomic arrangements according to coloring-problem, and the rationale is provided using both the size-factor and the electronic-factor related, respectively, to site-volume and electronegativity as well as QVAL values. Tight-binding, linear-muffin-tin-orbital (TB-LMTO) calculations are performed to investigate electronic densities of states (DOS) and crystal orbital Hamilton population (COHP) curves. The influence of reduced VEC for chemical bonding including the formation of interslab Ge 2 dimers is also discussed. The magnetic property measurements prove that the non-magnetic Li substitution leads to the ferromagnetic (FM)-like ground state for Ce 4 LiGe 4 and the co-existence of antiferromagntic (AFM) and FM ground states for Sm 4 LiGe 4 . Reported is a combined effect of the chemical pressure and the reduced VEC caused by the smaller monovalent non-magnetic Li substitution for the larger trivalent magnetic rare-earth metals in the RE 4 LiGe 4 ( RE =La, Ce, Pr, and Sm) system. This results in the structure transformation from the Sm 5 Ge 4 -type to the Gd 5 Si 4 -type structure and the changes in magnetic properties. • Four Li-containing intermetallic compounds RE 4 LiGe 4 ( RE =La, Ce, Pr and Sm) were synthesized. • The combined effect of the chemical pressure and the reduced VEC caused by Li substitution for a rare-earth metal results in the structure transformation. • Electronic structures and magnetic properties of title compounds were thoroughly investigated.

ESR study of Ce3+ ions in a CaGa2S4 single crystal

AbstractESR measurements have been carried out to determine the local environment and the ground state of Ce3+ ions in the orthorhombic CaGa2S4 at 4.2 K. More than 30 ESR lines are observed with almost the same angular dependence. The local symmetry and the anisotropic g ‐tensor for the four strongest ESR signals indicate that Ce3+ ions substitute the three independent Ca sites. A part of Ce3+ ions at the Ca2+ sites and a neighbouring sulphur vacancy are expected to form complex centers in view of the other weak ESR lines observed. The ground state of Ce3+ in the host lattice is shown to consist of mostly ϕ±1/2 by fitting the ESR spectra using the spin Hamiltonian. The upper 2F7/2 state is thought to be slightly mixed into the ground state 2F5/2 judging from a small difference in the fitting. (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Investigation of the photoionization of Ce3+ ions in a YAG crystal by microwave resonance technique

The kinetic and spectral characteristics of the complex dielectric constant of a Ce: YAG crystal under laser irradiation in 250–275 nm spectral range are investigated. The lifetimes of free charge carriers and charge carriers, localized at the lattice defects (color centers), are estimated. It was established that photoconductivity signal of the sample is essentially caused by one-photon ionization processes from the 2 F 5/2 ground state of Ce3+ ions.

Anomalous luminescence of impurity-bound excitons in lithium borate crystals doped with cerium ions

The spectra and relaxation kinetics of the anomalous (τ < 10 ns) luminescence of Li6GdB3O9:Ce3+ crystals have been experimentally detected. The time-resolved vacuum ultraviolet spectroscopy study has shown that optical transitions at 6.2 eV, caused by the transfer of an electron from the 4f1 ground state of Ce3+ to autoionizing states near the conduction band bottom of a crystal, lead to the formation of an impurity-bound exciton with the hole component localized on the 4f state of Ce3+ and the electron localized on states of the conduction band bottom. It has been found that the decay of such an exciton in Li6GdB3O9:Ce3+ occurs through radiative recombination, leading to fast luminescence at 4.25 eV. The energy threshold for the formation of the impurity-bound exciton has been determined. The distribution functions of elementary relaxations over the reaction rate constants H(k), which determine the relaxation kinetics and luminescence quenching processes, have been calculated.

Physical properties of Ce3−xTe4below room temperature

The physical properties of polycrystalline Ce3-xTe4 were investigated by measurements of the thermoelectric properties, Hall coefficient, heat capacity, and magnetization. The fully-filled, metallic x=0 compound displays a soft ferromagnetic transition near 4K, and analysis of the corresponding heat capacity anomaly suggests a doublet ground state for Ce^{3+}. The transition is suppressed to below 2K in the insulating x=0.33 composition, revealing that magnetic order in Ce3-xTe4 is driven by an RKKY-type interaction. The thermoelectric properties trend with composition as expected from simple electron counting, and the transport properties in Ce3Te4 are observed to be similar to those in La3Te4. Trends in the low temperature thermal conductivity data reveal that the phonons are efficiently scattered by electrons, while all compositions examined have a lattice thermal conductivity near 1.2W/m/K at 200K.

Luminescent Properties of Ce Doped LuAG Thin Films Prepared by Pechini Sol–Gel Method

Compared to phosphors, scintillating films are shown as a better feature for high–resolution display devices with low loss diffusion. Ce 3+ doped lutetium aluminum garnet (Lu3Al5O12, LuAG) scintillating films with different Ce 3+ concentration have been fabricated by Pechini sol–gel processing combined with the spin–coating technique and characterized by X–ray diffraction (XRD) and photoluminescence (PL) under excitation of UV–VIS light. The results of XRD patterns indicated that the specimen under study is a well–crystallized single–phase of cubic structure. Under an excitation of 442nm , it was found that the PL spectra was the typical Ce 3+ emissions located in the 460–650 nm consisting of two emission bands due to the transitions from the lowest 5d excited state (2D) to the 4f ground state of Ce 3+ , which matches well with the sensitivity curve of the Si–photodiode . The luminescent intensity of LuAG: Ce 3+ films varies with the Ce 3+ contents and reaches the maximum at 1.0 mol% doped. With the increase of calcining temperature, the luminescence intensity in the PL spectra of LuAG:Ce 3+ films was found to increased accordingly due to the improved crystallization.

Luminescence studies and effect of etching on cerium-doped CaS nanoparticles

Cerium-doped calcium sulphide nanoparticles were synthesized using the solid state diffusion method. The formed nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-visible absorption spectroscopy and photoluminescence (PL) spectroscopy. The XRD pattern confirmed a cubic CaS phase with an average grain size of 53 nm of the formed samples. The TEM image showed non-agglomerated particles with an average size of 60 nm, which is in close agreement with the XRD result. The PL-emission spectrum showed peaks at 506 and 565 nm due to the transition from the excited state to the ground state of Ce3+. The effect of etching has been studied on the luminescent properties of CaS:Ce phosphors. With an increase in the etching time there is decrease in the size of the particles, as a result of which the PL spectrum showed a slight blue shift. The UV-visible absorption spectrum also showed a blue shift with an increase in etching time, which is in agreement with the nanosize effect.

Simultaneous Occurrence of an Antiferroquadrupolar and a Ferromagnetic Transitions in Rare-Earth Palladium Bronze CePd3S4

Neutron powder diffraction experiments have been performed on CePd 3 S 4 as well as measurements of magnetic and transport properties on a single-crystalline sample. Magnetic susceptibility increases markedly below T C = 6.3 K and a spontaneous magnetization appears below T C . The logarithmic temperature dependence of electrical resistivity implies that CePd 3 S 4 is classified into a Kondo-lattice compound whose Kondo temperature is comparable to T C . Magnetic susceptibility is analyzed satisfactorily with a crystalline electric field model that the 4 f ground state of Ce 3+ is the Γ 8 quartet with the excited Γ 7 doublet located at 630 K. The magnetic structure of CePd 3 S 4 was determined to be a canted structure with a canting angle of 51°. Such a canted structure is a result of the ferromagnetic (FM) ordering and simultaneous [100]-type antiferroquadrupolar (AFQ) ordering at T C . The smaller magnetization values and an unexpectedly high T C can be explained by the simultaneous occurrence of AFQ an...

Magnetic behavior of cerium palladium bronze, CePd 3S 4

The magnetic susceptibility and magnetization measurements were performed on the single crystalline CePd 3S 4. A steep increase of the magnetic susceptibility below T C=6.3 K is due to a ferromagnetic (FM) ordering. From the analysis of the magnetic susceptibility based on a crystalline electric field (CEF) model, the 4 f ground state of Ce 3+ was deduced to be a quartet with an excited doublet situated at 1001 K. The experimental magnetization values were considerably smaller than the calculated ones that were computed by introducing a FM mean field as well as a CEF and the Zeeman effects. We proposed that not only the Kondo effect but also the suppression of magnetic moments by quadrupolar interactions is responsible for the unexpected small magnetizations.