Valence Fluctuation Phenomena Research Articles

Pressure-induced changes of valence fluctuation in β−YbAlB4 probed by x-ray absorption spectroscopy

We report detailed valence fluctuation phenomena in the unique quantum-critical compound $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{YbAlB}}_{4}$ under pressures and at low temperatures. We directly observed the drastic change in the pressure dependence of the mean Yb valence at about 3 GPa with no pressure-induced structural transition. Below 3 GPa, the anomaly in the temperature dependences of the mean Yb valence was observed at about 55 K under ambient pressure, corresponding to a previously observed reduced Kondo coherence temperature, and this temperature rapidly decreases to 22 K at 2.2 GPa, in contrast to the monotonous decrease of the high Kondo temperature ($\ensuremath{\sim}250\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ at ambient pressure). The present results reveal that this reduced Kondo coherence temperature is responsible for the unconventional quantum critical behaviors in $\ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{YbAlB}}_{4}$. Above 3 GPa, it was found that the temperature dependence of the mean Yb valence exhibits a discontinuous change at low temperature (10.5 and 9 K under 4.8 and 5.5 GPa, respectively).

Synchrotron X-ray spectroscopy study on the valence state and magnetization in α-YbAl1-xFexB4 (x = 0.115) at low temperatures and high magnetic fields

Valence fluctuation phenomena in rare-earth intermetallic compounds have attracted attention because the quantum criticality of the valence transition has been proposed theoretically. Recently, it was found that β-YbAlB4 shows quantum criticality without tuning and has a strong mixed-valence state. In this study, we measured the magnetization curve and X-ray magnetoabsorption in α-YbAl1-xFexB4 (x = 0.115), which is a locally isostructural polymorph of β-YbAlB4. The magnetization and X-ray experiments were performed in fields up to 55 and 40 T, respectively. A small increase in the Yb valence was observed at fields where the magnetization curve exhibited a change in slope.

High-Magnetic-Field XMCD as a Novel Tool for the Study of Valence Fluctuation Phenomena—Application to Eu-based Intermetallic Compounds

X-ray magnetic circular dichroism (XMCD) in Eu-based valence fluctuating compounds, EuNi2(Si1−x Ge x )2 (x=0.82, 0.85) and EuNi2P2 are investigated at high magnetic fields up to 40 T. Distinct two XMCD peaks corresponding to different valence states, i.e., Eu2+ and Eu3+ states are observed in EuNi2(Si1−x Ge x )2 (x=0.82) and EuNi2P2 at the L absorption edges (2p→5d). This suggests that the Eu 5d electrons are magnetically polarized in the both valence states. Since Eu3+ state has nonmagnetic ground state J=0, where J is total angular momentum, finite XMCD of Eu3+ state can be closely related to the magnetic polarization of the conduction electrons that is induced by the local magnetic moments of Eu2+ (J=7/2) state through the strong hybridization.

Strongly correlated electron phenomena in filled skutterudite compounds

A rich variety of strongly correlated electron phenomena are found in filled skutterudite compounds. Selected examples of these phenomena and the skutterudite compounds in which they occur are briefly described in this paper. Examples considered include “hybridization gap” semiconductivity (“Kondo insulator” behavior), ferromagnetism, valence fluctuation phenomena, heavy fermion behavior, non-Fermi liquid behavior, and heavy fermion superconductivity.

Pseudogap Formation in Rare-Earth Compounds

The generalized periodic Anderson model with plane-wave conduction-band states is considered as a model to describe the properties of Ce, Sm, Eu, Tm and Yb compounds showing a variety of valence fluctuation phenomena. The mixing matrix elements between localized 4 f -electron states and the conduction-electron states, which are anisotropic in k space, are derived, and it is shown how pseudogap or real-but-anisotropic gap is formed in this model. The structures of gap or pseudogap to be derived here are considered to bear some of the characteristics of those of rare-earth compounds in such as heavy-fermion, Kondo-insulator and intermediate-valence states.

Coupling of the ionic motions with the electronic structure in the YBa2Cu3O7 high Tc superconductor

A theoretical analysis of the consequences of the experimentally measured double-well oscillation of the apical oxygen in YBa 2 Cu 3 O 7 is made. The electronic structure was calculated in the frozen phonon approximation with different apical oxygen positions of a cell consisting 4-unit cells. Chemical bond changes are related to chain-planes electronic interactions and with valence fluctuations of the Cu(1) atoms of the chains. A vibronic mechanism is substantiated as a candidate for strong phonon-electron interaction. The coupling of the ionic motions with electronic structure, in turn, explains valence fluctuation, bipolaron and magnetic-like phenomena in these type of compounds.

Experimental summary - valence fluctuations, heavy fermions, and high-T c superconductivity

Selected experiments on valence fluctuation, Kondo lattice, and heavy-fermion phenomena in rare earth and actinide materials, and high- T c superconductivity in copper oxides reported at this conference are summarized.

Observation of valence fluctuation phenomena in YbSi 2

Electrical and magnetic properties of YbSi 2 have been investigated experimentally by electrical resistivity and a.c. susceptibility measurements. The resistivity data may be summarized as following: 1. (i) at high temperatures the resistivity shows a tendency to saturate; 2. (ii) there is a negative curvature above T ≅ 65 K ( d 2ρ dT 2 < 0) and ρ exhibits an inflection point at 65 K; 3. (iii) below 37 K the resistivity shows a T 2 dependence. The a.c. susceptibility shows a broad peak at about 90 K. The low temperature T 2 resistivity and the broad peak in a.c. susceptibility suggest that YbSi 2 is a valence fluctuating system. The results will be discussed in terms of valence fluctuating phenomena. Measurements of the magnetic moment in YbSi 2 will be carried out in a SQUID magnetometer.

Variational ground state for the periodic Anderson model with an indirect hybridization.

As the direct on-site hybridization is forbidden by inversion symmetry in most of the mixed-valence compounds, an indirect on-site hybridization mediated by phonons has been introduced into the periodic Anderson model to constitute our model system. Then we try to construct a variational ground state for the model Hamiltonian by the following steps. First, we develop a new procedure to transform the model Hamiltonian by a unitary transformation of the displacement-operator type. Second, a two-phonon coherent state is taken as the trial-state vector for the ground state of the phonon subsystem and the parameters of the two-phonon coherent state, which are regarded as the adjustable parameters of the variational treatment, remain to be determined. Third, a Bogoliubov transformation is introduced to deal with the electron subsystem; the ground state and low-lying excited states are constructed directly. Finally, the parameters of the two-phonon coherent state are adjusted to ensure that the energy functional of our variational ground state is a stable minimum. Numerical calculations have been done and a nonzero energy gap and fluctuating valence have been obtained in various cases. Our results could be used to explain the small energy gap and valence-fluctuation phenomena observed in some Sm-based compounds.

A NEW APPROACH TO INVESTIGATING THE VALENCE FLUCTUATION COMPOUNDS

The ground state properties of the Sm-based valence fluctuation compounds are discussed by introducing an indirect on-site hybridization mediated by phonons, as the direct on-site hybridization is forbidden by inversion symmetry. The Hamihonian including this indirect hybridization is transformed into an effective Hamihonian of a "small polaron" type by means of a. canonical transformation. A self-consistent field approximation is used, a non-zero energy gap and the noninteger occupation of the f level have been given. The results obtained can be used to explain the small energy gap and the valence fluctuation phenomena observed in SmB6 and SmS.

A new model for the mixed valence compounds

The ground-state properties of the Sm-based mixed valence compounds are discussed by means of introducing an indirect on-site hybridization mediated by phonons, for the direct on-site hybridization is forbidden by inversion symmetry. A self-consistent field approximation is used, and a nonzero energy gap and a fluctuating valence state are given. The results obtained can be used to explain the small energy gap and the valence fluctuation phenomena observed in SmB6 and SmS.

Electron-phonon interaction in mixed-valence systems

The importance of electron-phonon coupling in valence-fluctuation phenomena is discussed. It is shown that the electron-phonon renormalized $d\ensuremath{-}f$ hybridization can account for the experimentally observed variation on the $4f$ width in some mixed-valence systems. A comparison is also made with previously obtained results.

On the valence fluctuation phenomena in SmS compounds

On the valence fluctuation phenomena in SmS compounds