Ce 4f Electrons Research Articles

Hydrogen-induced localization of Ce-4f electrons in mixed-valent CeNiIn: Observation of heavy-fermion and magnetically ordered states

Hydriding studies have been carried out on the mixed-valent compound CeNiIn having a Kondo temperature T K ∼ 100 K. Two distinct hydride phases, CeNiInH y, with y = 0.3–0.6 and 1.6 are obtained which retain the hexagonal structure of the parent compound. The higher hydride phase with a composition of y = 1.6 is relatively unstable and it transforms into the lower hydride phase over a short period of time. Magnetization measurements indicate valence transformation of cerium from mixed-valent for the parent compound to Ce 3+ for the hydrides. While the higher hydride composition CeNiInH 1.6 shows ferromagnetic ordering below ≅7 K, the lower hydride phase with a composition CeNiInH 0.6 shows heavy fermion behaviour. Thus, Ce valence changes from mixed-valent to normal trivalence via a heavy-fermion like state as a function of hydrogen concentration.

Angle-resolved photoemission study on CeTe 2

We have used angle-resolved photoemission spectroscopy as a function of excitation energy to elucidate the three dimensionality at the electronic structure of quasi-two-dimensional strongly correlated Ce 4f electron compound CeTe 2. We observed that there are two types of Te 5p bands along Γ Z symmetry line at the valence band, one is non-dispersive bands and the other is dispersive one. While the former is ascribed to the Te 5p bands at two-dimensional Te layer relating to the CDW formation, the latter to one at CeTe double layers.

Variation of electronic structures of CeAl 2 thin films with thickness studied by X-ray absorption near-edge structure spectroscopy

We report the X-ray absorption near-edge structures (XANES) of CeAl 2 thin films of various thickness, 40–120 nm, at Al K-, Ce L 3-, and Ce M 4,5-edges. It is found that the threshold of near-edge absorption features at the Al K-edge shifts to the higher photon energy as film thickness decreases, implying that Al loses p-orbital charge and the valence of Ce increases slightly as revealed from the XANES features at Ce L 3- and M 4,5-edges. Above observations suggest that there is charge transfer from Al to Ce as the surface to bulk ratio is varied. This induces change in the electronic structures of CeAl 2 thin films. The Ce 4f electrons are surface sensitive in nanoparticles compared to thin films. This work also shows that 4f electronic states of Ce ions are sensitive to the reduction of the coordination number induced by surface effects that would change their hybridization.

Magnetic properties of [formula omitted] Kondo-lattice system

We have carried out measurements of specific heat and magnetic susceptibility of Ce 1 - x Y x PdAl ( 0 ⩽ x ⩽ 0.4 ) polycrystalline samples to study the dilution effect on magnetic properties of Kondo-lattice system CePdAl. A substitution of nonmagnetic ions of Y for magnetic ions of Ce reduces the Néel temperature T N rapidly and enhances the paramagnetic Curie–Weiss temperature θ p . These results suggest that the increase of x predominantly leads to an increase of the exchange parameter J arising out of the interaction between the Ce 4f electron and the conduction electrons.

Anisotropic behavior in the CeRhSn single crystal

We report on magnetization, magnetic susceptibility, specific heat, electrical resistivity and thermal expansion measurements performed on a CeRhSn single crystal at temperatures between 2 and 300 K and in magnetic fields up to 14 T. No indication of magnetic ordering in low-temperature susceptibility, specific heat and resistivity was observed. Besides the maximum in the temperature dependence of resistivity for current along the a-axis no indication for the previously reported Ce-ion valence fluctuations was found in experimental data. In particular, the temperature dependence of susceptibility is monotonous and the magnetic specific heat above 20 K is negligible. The linear thermal expansion is isotropic and has the character usual for normal metal. Based on these results we propose for CeRhSn an alternative scenario considering that the Ce 4f-electrons are itinerant.

NMR studies of Ce-based filled skutterudites [formula omitted] and [formula omitted

We have carried out the 31 P NMR measurements on Ce-based filled skutterudites CeFe 4 P 12 and CeRu 4 P 12 , which display semiconductive behaviors due to the strong hybridization between Ce 4f and conduction electrons. At higher temperatures, the nuclear spin-lattice relaxation rate 1 / T 1 obeys an activation behavior 1 / T 1 ∝ exp [ - E g / k B T ] with an energy gap of E g / k B = 1300 and 1000 K for CeFe 4 P 12 and CeRu 4 P 12 , respectively. These are in good agreement with the values measured by various measurements.

Magnetic-Field-Induced Band-Structure Change in CeBiPt

We report on a field-induced change of the electronic band structure of CeBiPt as evidenced by electrical-transport measurements in pulsed magnetic fields. Above approximately 25 T, the charge-carrier concentration increases nearly 30% with a concomitant disappearance of the Shubnikov-de Haas signal. These features are intimately related to the Ce 4f electrons since for the non-4f compound LaBiPt the Fermi surface remains unaffected. Electronic band-structure calculations point to a 4f-polarization-induced change of the Fermi-surface topology.

Density-functional calculation of CeO2 surfaces and prediction of effects of oxygen partial pressure and temperature on stabilities

We have used density-functional theory to investigate (111), (110), (210), (211), (100), and (310) surfaces of ceria (CeO2). Compared with previous interatomic-potential-based studies, our calculations reported a slightly different relative stability ordering and significantly lower surface energies for the stoichiometric surfaces. Using a defect model, the surface stabilities were evaluated as functions of oxygen partial pressure and temperature. Our investigations were restricted to ideal surface terminations, without considering defect formation on those surfaces. We found that at 300 K, the stoichiometric (111) has the lowest free energy for a wide range of oxygen partial pressures up to 1 atm, and only at ultrahigh vacuum does the Ce-terminated (111) becomes the most stable one. The transition point for the Ce-terminated (111) surfaces moves to higher oxygen partial pressures when temperature increases. To improve the prediction of electron density of states, we used the local-density approximation plus U(J) correction method to correct the on-site Coulomb correlation and exchange interaction due to the strongly localized Ce-4f electrons. The optimal parameter combination of U = 7 eV and J = 0.7 eV was found to improve the O 2p-Ce 4f gap without much degradation of ground-state bulk properties or the O 2p-Ce 5d gap. The bulk and surface electronic structures were then analyzed based on the improved density of states.

Electronic and magnetic structures of CeTe2

We have investigated electronic structures of CeTe2 to explore the correlation between the magnetic, charge density wave, and superconducting phases by using the local spin density approximation (LSDA) and LSDA+U (U=on-site Coulomb interaction) methods. We have found that the contribution near EF from Ce 4f states is negligible, suggesting that Ce 4f electrons are not the superconducting carriers. The total energy calculation indicates that the ABBA-type antiferromagnetic configuration is the most stable, while the stability is weakened by pressure. We have discussed the possible superconducting mechanism in the magnetic phase of CeTe1.82.

F-Electron Behavior in Rare Earth Based Systems: Localization or Itinerancy?

Measurement of the two-dimensional angular correlation of the electron{ positron annihilation radiation complemented with ab initio calculations can provide decisive information about the character of the f-electrons in rare earth compounds. We provide examples of f-electron localized and f-electron itinerant systems, respectively. (i) In the case of the antiferromagnetic heavy fermion and superconductor CeIn3 the multisheet Fermi surface, reconstructed from our measurements in the paramagnetic phase, agrees closely with the predictions of band structure calculations regarding the Ce 4f electrons as fully localized. (ii) On the other hand, our studies of the antiferromagnet actinide based UGa3 in the paramagnetic phase, compared with calculations which include the efiects due to the non-uniform positron density and the electron{positron correlations, produce a substantial evidence that an unconstrained 5f-electron itinerant description applies.

Dense Kondo behavior in the electric resistivity and the specific heat of amorphous [formula omitted] alloy

Binary amorphous alloy system Ce x Mn 100 - x shows the dense Kondo behavior in the Ce-rich side. This fact is confirmed by the large T 2 coefficient A of electrical resistivity at low temperature and also the large linear specific heat coefficient γ . The result of log A versus log γ 2 plot for Ce-rich samples shows the systematic change, but deviates from the Kadowaki–Woods (KW) relation. If we take the orbital degeneracy of Ce 4f-electron into account, according to Kontani's recent analysis, the result satisfies the generalized KW-relation. Therefore, present amorphous Ce–Mn alloy seems to form the dense Kondo state or the heavy-fermion state at low temperature.

Hybridization and Magnetic Ground States in Heavy FermionCompound CeRhIn5

The magnetic properties of CeRhIn5 are studied by first-principlescalculation. Spin-orbit coupling is considered as well as differentcorrections including local density approximation plus on-site Coulombinteraction U (U = 1, 1.5, 3 eV) and orbital polarization. Theresults show the existence of only moderate correlation in CeRhIn5and Ce-4f electrons are on the border of localization and itinerancy.The effect of pressure by changing the volume of the unit cellis also studied.

Density functional theory studies of the structure and electronic structure of pure and defective low index surfaces of ceria

We present periodic density functional theory (DFT) calculations of bulk ceria and its low index surfaces (1 1 1), (1 1 0) and (1 0 0). We find that the surface energies increase in the order (1 1 1) > (1 1 0) > (1 0 0), while the magnitude of the surface relaxations follows the inverse order. The electronic properties of the bulk and surfaces are analysed by means of the electronic density of states and the electron density. We demonstrate that the bonding in pure ceria is partially covalent and analysis of the resulting electronic states confirms the presence of localised Ce 4f states above the Fermi level. The surface atoms show only a small change in the charge distribution in comparison to the bulk and from the DOS the main differences are due to the changes in the oxygen 2p and cerium 5 d states. Investigation of the atomic and electronic structure of an oxygen vacancy on the (1 0 0) surface shows the problems DFT can have with the description of strongly localised systems, wrongly predicting electron delocalisation over all of the cerium atoms in the simulation cell. We demonstrate an improvement in the description of the strongly correlated cerium 4f states in partially reduced ceria by applying the DFT+U methodology, which leads to the appearance of a new gap state between the valence band and the empty Ce 4f band. Analysis of the partial charge density shows that these states are localised on the Ce III ions neighbouring the oxygen vacancy. In terms of classical defect chemistry, the vacancy is bound by two neighbouring Ce III ions, which have been reduced from Ce IV, i.e. V O · · + 2 Ce Ce ′ . The remaining Ce ions are in the Ce IV oxidation state. The localisation of Ce 4f electrons modifies the predicted structure of the defective surface.

Electronic structure and de Haas-van Alphen quantities of [formula omitted] (M=Co, Rh, and Ir) compounds

We report a critical analysis of the electronic structures and de Haas-van Alphen (dHvA) quantities of the heavy-fermion superconductors CeCoIn5, CeRhIn5, and CeIrIn5. The ab initio calculated dHvA quantities are in good agreement with experimental data for CeCoIn5 and CeIrIn5, when we adopt the delocalized LSDA description for the Ce 4f states. For CeRhIn5, however, a better agreement with experiment is obtained when the Ce 4f electron is treated as a localized core electron. The implications for an emerging picture of the localization behavior of the 4f electron in these materials are discussed.

Specific-heat and Magnetization Studies in Ce(NixCu1-x)5 Heavy-fermion Compounds

We have prepared the Ce(NixCu1-x)5 compounds for a wide range of concentrations of x∈〈0;0.9〉 and studied them by X-ray powder diffraction at room temperature and measured magnetization and specific heat as functions of temperature and magnetic field. Over the entire studied concentration range, the CaCu5-type hexagonal structure (the space group P6/mmm) was observed. All prepared samples were found paramagnetic down to 2 K, some of them exhibiting pronounced heavy-fermion features. The evolution of temperature dependencies of the magnetic susceptibility may be conceived in terms of gradual change of the Ce 4f-electrons character from valence fluctuating with strong admixture of Ce4+ valence state in the case of the nickel-rich compounds, towards the localized 4f states in the Ce3+ ion in the case of the copper-rich compounds. The specific heat for x 0.5 a peak is observed on the c/T vs. T curves, which can be tentatively attributed to freezing of localized Ce magnetic moments rather than to magnetic ordering transition.

Magnetism in single crystal Ce 2Fe 17 with two types of magnetic ground states

Magnetization and neutron diffraction measurements have been performed on two Ce 2Fe 17 single crystals that have different magnetic ground states. Two types of magnetic structure in the ground states for Ce 2Fe 17 are confirmed. From the above results, it is deduced that the hybridization between the Fe 3d- and Ce 4f-electrons plays an important role on the magnetism of Ce 2Fe 17 in the ground states.

Transport properties of CeBiPt in magnetic fields up to [formula omitted

Electrical-transport measurements of the semimetal CeBiPt in magnetic fields up to 60 T reveal a drastic change of the electronic band structure. The oscillating Shubnikov–de Haas signal vanishes above about 25 T although the quantum limit is not yet reached. Above this field the magnetoresistance rises strongly independent of angle and temperature. These unique features are caused by the Ce 4f electrons as evidenced by the absence of any unconventional behavior in the sister compound LaBiPt.

Heat capacity studies of Ce and Rh site substitution in the heavy-fermion antiferromagnetCeRhIn5:Short-range magnetic interactions and non-Fermi-liquid behavior

In heavy fermion materials superconductivity tends to appear when long range magnetic order is suppressed by chemical doping or applying pressure. Here we report heat capacity measurements on diluted alloyes of the heavy fermion superconductor CeRhIn_5;. Heat capacity measurements have been performed on CeRh_{1-y}Ir_{y}In_5; (y <= 0.10) and Ce_{1-x}La_{x}Rh_{1-y}Ir_{y}In_5; (x <= 0.50) in applied fields up to 90 kOe to study the affect of doping and magnetic field on the magnetic ground state. The magnetic phase diagram of CeRh_{0.9}Ir_{0.1}In_5; is consistent with the magnetic structure of CeRhIn_5; being unchanged by Ir doping. Doping of Ir in small concentrations is shown to slightly increase the antiferromagnetic transition temperature T_{N} (T_{N}=3.8 K in the undoped sample). La doping which causes disorder on the Ce sublattice is shown to lower T_{N} with no long range order observed above 0.34 K for Ce_{0.50}La_{0.50}RhIn_5;. Measurements on Ce_{0.50}La_{0.50}RhIn_5; show a coexistence of short range magnetic order and non-Fermi-liquid behavior. This dual nature of the Ce 4f-electrons is very similar to the observed results on CeRhIn_5; when long range magnetic order is suppressed at high pressure.

Magnetic hyperfine fields on 140Ce probes substituting for the rare earth in RCo 2 laves phases

The magnetic hyperfine fields (MHF) acting on 140Ce probes substituting for the rare-earth sites in RCo 2 Laves phases (R=Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm) were determined by perturbed angular correlation spectroscopy. The MHF scales very well with the Co magnetic moment throughout the series as already observed with 181Ta, and to a lesser extent, with 111Cd probes. It is shown that the orbital contribution to the MHF is about one order of magnitude smaller than in the free Ce +3 ion. The decrease was attributed to the de-localization of the Ce 4f electrons.

Localized character of 4f electrons in CeRhx (x=2,3) and CeNix (x=2,5).

We have measured Ce 4f spectral weights of extremely alpha-like Ce transition metal intermetallic compounds CeRhx (x=2,3) and CeNix (x=2,5) by using the bulk-sensitive resonant photoemission technique at the Ce M5(3d(5/2)-->4f) edge. High energy resolution and longer escape depth of photoemitted electron at this photon energy enabled us to distinguish the sharp Kondo resonance tails at the Fermi level, which can be well described by the Gunnarsson-Schönhammer calculation based on the Anderson impurity Hamiltonian. On the other hand, the itinerant 4f band description shows big discrepancies, which implies that Ce 4f electrons retain localized characters even in extremely alpha-like compounds.