Intermediate Valence Systems Research Articles

On the edge of tetravalent Ce in CePd7

Data on the magnetic susceptibility, low-temperature specific heat and X-ray absorption spectroscopy (Ce LIII and MIV.V edges) for CePd7 and its reference compound YPd7 are presented. The respective Pauli susceptibilities chi 0 of 0.11*10-3 emu mol-1 and 0.50*10-3 emu mol-1 and the respective Sommerfeld coefficients gamma of 9.8 mJ K-2 mol-1 and 35.8 mJ K-2 mol-1 are much smaller (per atom) than those of the corresponding components (Ce, Y and Pd). The valence extracted from the Ce LIII edge is 3.5. The chi 0/ gamma -ratios are found to be 0.012 emu K-2 J-1 and 0.014 emu K2 J-1 for CePd7 and YPd7, respectively. Such a value for CePd7 corresponds to that of a free-electron system instead of an intermediate-valence system (0.035 emu K2 J-1).

Advances in the study of intermediate-valence and heavy-fermion effects in rare-earth and uranium systems

Three intermetallic systems, Yb−Al, U−Au and Ce−Co, are studied as most typical examples of intermediate-valence, heavy-fermion and coexistence of both intermediate-valence and heavy-fermion effects, respectively. YbAl2 and YbAl3, the two phases stable at low temperatures in the equilibrium phase diagram of Yb−Al, are both intermediate-valence systems with Yb valence values of 2.33 and 2.79 at room temperature. The modulation of these values by variations of external parameters like temperature, pressure and chemical substitutions with proper atoms was studied by structural, magnetic and spectroscopical techniques. Two phases are given also by U with Au at low temperatures with compositions UAu2 and U14Au51: both systems exhibit heavy-fermion properties although with different ordering mechanisms. U14Au51 is an antiferromagnetic heavy-fermion system with a Neel temperature around 23 K, UAu2 shows spin-fluctuating behaviour with a saturation in theT→0 K magnetic susceptibility. Effects of substitutions with thorium were investigated only in U14Au51: competition between strong antiferromagnetic coupling and Kondo-like behaviours is observed. The Ce-rich side of the Ce−Co phase diagram shows a single Ce24Co11 phase stable at room temperature interest in the present work but in CeCo2, which was demonstrated to be intermediate valent by resonant photoemission and XAS experiments and to exibit low magnetic moment and superconducting transition. The coexistence of intermediate-valence and heavy-fermion behaviours observed in Ce24Co11 and of intermediate valence and superconductivity in CeCo2 is shown by the analyses of XPS, XAS and magnetic experiments with suitable phenomenological models.

CORRELATION EFFECTS IN NiO: COMPARISON OF NEAR THRESHOLD EXCITATION SPECTROSCOPIES

At UHV-cleaved single-crystal NiO(100) surfaces core electron energy loss spectra (CEELS) at low primary energy and soft x-ray appearance potential spectra (SXAPS) have been measured at the oxygen K-threshold: the near-edge fine-structures have been investigated up to 40 eV above the threshold. Both spectroscopic methods probe in different ways the structure of the lowest empty states. CEELS is closely related to x-ray absorption spectroscopy (XAS), in both cases the excited final configuration consists of one core hole and one additional electron near the Fermi level E F. In APS we can study correlation effects very directly because the excited system consists of one core hole and two additional interacting electrons in localized resp. delocalized states near E F. The interpretation of the CEELS and SXAPS spectra at the oxygen K-edge is in agreement with a description of NiO as an intermediate valence system, where the charge-transfer gap is smaller than the Hubbard correlation energy. The comparison of our measured spectra with the experimental XAS of NiO shows the dominance of optically allowed channels in both spectroscopies.

Magnetic ordering in the heavy-fermion compounds CePtSi 2 and CeNiGe 2

The ground state of three CeTMX 2 compounds with CeNiSi 2 structure type was investigated by electrical-resistivity, magnetic-susceptibility and specific-heat measurements. Whereas CeNiSi 2 is an intermediate-valent system, we observe in CePtSi 2 and CeNiGe 2 magnetic ordering in the presence of heavy fermions. The trends in heavy-fermion formation on going from either the 111 or 122 to the 112 compounds are discussed.

EXTREMELY LOW DENSITY OF STATES IN CePd7

The low temperature specific heat and magnetic susceptibility measurements, up to room temperature, on CePd 7 are reported. The Sommefeld coefficient γ and the Pauli susceptibility χo were found to have the lowest values within the Ce compounds: γ = 9.8 mJ/mole K 2 and χo = 0.14 × 10−3 emu/mole . The ratio χo/γ = 0.014 emu K 2/ J is that of a free electron, instead of an intermediate valence system (0.036 emu K 2/ J ). Such a value suggests complete delocalization of the Ce 4f electrons.

An analysis of the ESR-linewidth results in intermediate valence systems Ce(Pd 3-xT x) ( T = Rh, Ag)

We study the deviation from the linear Korringa relaxation behavior using a previously suggested model. An analytical expression for the ESR-linewidth dependence with the temperature is applied to the intermediate valence pseudo-binary compounds Ce(Pd 3- x T x ) ( T = Rh, Ag). We discuss our results and compare them to those obtained by other models.

Functional expansion of the Anderson and SU(N) models

We study the Anderson and SU(N) lattice models that describe the Kondo and intermediate-valence systems, in the infinite-correlation limit (U\ensuremath{\rightarrow}\ensuremath{\infty}), employing the functional expansion. We use Hubbard operators that describe real electrons. In the lowest, nontrivial approximation, our expressions are similar to, but different from, those derived by several effective-Hamiltonian techniques, like the mean-field slave-boson (MFSB) technique. In the usual large-N limit, our results coincide with those of the equations of motion and Brillouin-Wigner expansions, which are exact in that limit. Our results at T=0 K are compared to those of the MFSB quasiparticle description, and we discuss the two approximations in the region in which they are different. We conclude that although the quasiparticle description should give better results for the thermodynamic properties, our treatment describes in a more physical way the overall behavior of the spectral density of the localized electrons. The structure near the chemical potential that is predicted in other methods is not obtained in our treatment, but I believe that it would appear if a higher-order approximation were employed.

Re-entrant and spin-glass-like behaviour of the Anderson lattice

The magnetic properties of intermediate valence systems described by the periodic Anderson model are studied within a mean-field approximation for a wide range of localized 4f-level occupation nf. It is found that the linear susceptibility chi 0 shows divergence at two critical temperatures Tc1 and Tc2, indicating instability in magnetic ordering. Ferromagnetic order exists for Tc1<or=T<or=Tc2, indicating a re-entrant behaviour. For another range of nf, the system exhibits spin-glass-like magnetic response with almost divergent non-linear susceptibility as the external magnetic field goes to zero, at a temperature T0 where the linear susceptibility has a cusp-like behaviour. As nf is scanned, it is found that Tc1 and Tc2 merge for a particular value of 4f-level position. This temperature Tg (=Tc1=Tc2) is found to increase with the increase in hybridization between the localized and band states. It is observed that the linear response for T>or=T0 can be represented by a scaling relation.

A microscopic model for the coupling of spin fluctuations and charge fluctuation in intermediate valency

A microscopic model is presented to show explicitly how dynamical charge and lattice fluctuations (at zero temperature the zero point motion) will-through spin orbit coupling-induce spin-flip terms off-electrons in intermediate valent and heavy fermion systems. A simple model Hamiltonian coupling charge, lattice, and spin degrees of freedom is derived and diagonalized exactly.

Magnetic relaxation spectra of YbPd2Si2

We present the results of inelastic neutron scattering experiments on the intermediate-valent system YbPd2Si2 to investigate the magnetic relaxation behaviour. We have performed measurements on polycrystalline samples with neutrons of incident energyE0=3.1 meV at temperatures between 1.5 K and 250 K, and withE0=12.7 meV andE0=50.8 meV at temperatures between 5 K and 50 K using time-of-flight spectrometers. At temperaturesT>50 K we find a pure quasielastic magnetic response with a rather broad linewidth typical for intermediate-valent systems. AtT≈50 K an inelastic excitation line appears at about 21 meV; its intensity increases with decreasing temperature. In the same temperature range (T 5 K.

Method for analyzing the ground state of intermediate-valence systems: Application to metallic SmS.

We give a method of obtaining the electronic structure of the ground state of intermediate-valence systems by means of a nonlocal potential deduced by the variations of the total energy with the number of f electrons. In addition, we deduce this potential from the Green's function for interacting f systems and we calculate the band structure and the density of states of golden-phase SmS for several values of the energy U and lattice parameters.

Resistivity anomalies due to valence fluctuations

It is shown that slow valence (= charge) fluctuation occuring in intermediate valent system lead to logarithmic divergencies in perturbation theory for the electrical resistivity, equivalent to those of two level systems. Charge fluctuations thus provide a consistent explanation for the observed resistivity anomalies.

151Eu-Mossbauer investigations in a new Eu-based mixed valent material EuIr2Si2

151 Eu-Mossbauer and magnetic susceptibility measurements on EuIr 2 Si 2 suggest that this material is an intermediate valent system. Variation of average valence V is very much pronounced compared to EuCu 2 Si 2 ; V = 2.3 and 2.8 at 300K and 4.2K respectively. Excitation energy, E ex = ( E 2+ g − E 3+ g ) and the spin fluctuation temperature, T sf , are strongly temperature dependent.

Specific intermediate-valence state of insulating 4fcompounds detected byL3x-ray absorption

The intermediate valence of formally tetravalent compounds has been detected by ${L}_{3}$ x-ray-absorption near-edge structure (XANES) in ${\mathrm{CeO}}_{2}$ and in ${\mathrm{PrO}}_{2}$ but not in ${\mathrm{UO}}_{2}$, which have the same ${\mathrm{CaF}}_{2}$ structure and large f and ligand mixing. The intermediate valence has been found both in ${\mathrm{CeO}}_{2}$ and in Ce(${\mathrm{SO}}_{4}$${)}_{2}$\ensuremath{\cdot}${4\mathrm{H}}_{2}$O, which have similar local structure but different crystal structure. We show that ${L}_{3}$ XANES final states are a direct probe of configuration interaction between 4${f}^{n}$ and 4${f}^{n+1}$L configurations in the ground state and that the weight of the 4${f}^{n+1}$L in the ground state can be deduced. The many-body final states arise from the characteristic properties of these materials: (i) the presence of localized 4f level above the oxygen 2p band separated by a gap \ensuremath{\delta}\ensuremath{\varepsilon} with relevant correlation energy ${U}_{\mathrm{ff}}$ (${U}_{\mathrm{ff}}$\ensuremath{\ge}\ensuremath{\delta}\ensuremath{\varepsilon}) and (ii) mixing of 4f localized states with ligand valence orbitals such that the hybridization energy V is of the same order of magnitude as the energy separation \ensuremath{\Delta}E between the many-body configuration 4${f}^{n}$ and 4${f}^{n+1}$L (V\ensuremath{\ge}\ensuremath{\Delta}E). These insulating materials, which cannot be classified as standard mixed-valence systems, are called here interatomic intermediate-valence systems.

Magnetic Order in Intermediate Valence Systems

AbstractA „hybridized”︁ s—f model is used in order to investigate the possibility of a coexistence of ferromagnetism and intermediate valence in systems which fluctuate between a magnetic and a nonmagnetic 4f configuration. The Curie temperature Tc, is determined, the local magnetic f‐moment mf, the valence nf and the static susceptibility χ as functions of the f‐level position Ef, the hybridization V, the s—f exchange g, and the Bloch bandwidth W for semiconducting as well as metallic 4f systems. It is found that electronic fluctuations destablize the local moments, but enhance the coupling between them. This leads to a Tc‐maximum just in the intermediate valence phase. Many results remarkably depend on, how the gap between the 4f‐level and the lower band edge (or Fermi‐edge) is closed, by a shift of the f‐level towards the conduction band (alloying!) or by a respective broadening of the band (external pressure!). Striking differences between semiconducting and metallic 4f‐systems are observed, while qualitatively similar behaviour is found for systems with ferromagnetic and systems with antiferro magnetic s—f exchange coupling. Magnetic phase diagrams in terms of Ef, W, V, and conduction band occupation n are derived and discussed. Qualitative agreement is found with recently performed experiments on systems like EuO, EuRh3B2, CeNixPt1−x, Ce(Rh1−xOsx)3B2,‥.

Group theoretical selection rules for f-c hybridization

The physical conditions under which the intersite hybridization is the same in equivalent crystallographic directions are discussed. The above symmetry of hybridization leads to a selection rule that only multiplets which belong to the same irreducible representations of the double point group can mix. This seems to confirm the empirical selection rules found recently in e.g. heavy fermions (HF) and in magnetically ordering intermediate valent systems.

Ultrasonic attenuation in superconducting intermediate valence systems

We calculate the longitudinal sound attenuation of a superconducting mixed-valent compound. We consider the periodic Anderson model with electron-phonon interaction. Assuming that the volume strain induces f-d transitions we obtain a peak immediately below Tc. With a modification to allow for two Tc a λ-shaped peak appears.

Thermodynamic Perturbation Theory for Intermediate Valence Systems with Two Magnetic Configurations: General Formalism††Project supported by the Science fund of the Chinese Academy of Sciences.

We present a generalization of thermodynamic perturbation theory to the intermediate valence systems with two magnetic configurations.

Self-trapping of a dipole coupled to a lattice: Application of a new theoretical method to an old problem

The static and dynamic properties of a spin-1/2 dipole with a transverse coupling to phonons are calculated using a generalization of a perturbation theory developed originally by Keiter, Kimball, Grewe, and Kuramoto for intermediate valence systems.

Calculation of fluctuations and photoemission properties in a tetrahedral-cluster model for an intermediate-valence system.

An exact solution of a four-site tetrahedral-crystal model, the smallest face-centered-cubic crystal, is presented in the case of an intermediate-valence system. The model consists of the following: (a) one extended orbital and one localized orbital per atom, (b) an interatomic transfer term between extended orbitals, (c) an interatomic hybridization between the localized and extended orbitals, (d) strong intra-atomic Coulomb repulsion between opposite-spin localized states, and (e) intermediate-strength intra-atomic Coulomb repulsion between the localized and extended states. These competing effects are examined as they manifest themselves in the intermediate-valence, photoemission, inverse-photoemission, and thermodynamic properties.