Valence Fluctuation Compounds Research Articles

Superconducting and Fermi Surface Properties of a Valence Fluctuation Compound CeIr2

Superconducting and Fermi Surface Properties of a Valence Fluctuation Compound CeIr₂

Lattice instability coupled with valence degrees of freedom in valence fluctuation compound YbPd

Inelastic x-ray scattering (IXS) was used to examine the valence fluctuation compound YbPd at 300 K in the $\ensuremath{\Gamma}\ensuremath{-}X$, $\ensuremath{\Gamma}\ensuremath{-}M, X\ensuremath{-}M$, and X-R directions. The obtained spectra and determined phonon dispersion relations demonstrate lattice instabilities related to the successive transitions and correlations between phonon spectra and valence degrees of freedom in YbPd. The low-lying branches in the phonon dispersion relations as well as the broadened linewidths of the IXS spectra observed around the longitudinal $X$ point provide supporting evidence of correlation between a valence-lattice interaction and the successive transitions in YbPd. X-ray absorption spectroscopy was also performed in order to discuss correlations between lattice instabilities and Yb valence degrees of freedom in YbPd.

Charge, lattice and magnetism across the valence crossover in EuIr2Si2 single crystals

We present a detailed study of the temperature evolution of the crystal structure, specific heat, magnetic susceptibility and resistivity of single crystals of the paradigmatic valence-fluctuating compound . A comparison to stable-valent isostructural compounds (with Eu3+), and , (with Eu2+) reveals an anomalously large thermal expansion indicative of the lattice softening associated to valence fluctuations. A marked broad peak at temperatures around 65–75 K is observed in specific heat, susceptibility and the derivative of resistivity, as thermal energy becomes large enough to excite Eu into a divalent state, which localizes one f electron and increases scattering of conduction electrons. In addition, the intermediate valence at low temperatures manifests in a moderately renormalized electron mass, with enhanced values of the Sommerfeld coefficient in the specific heat and a Fermi-liquid-like dependence of resistivity at low temperatures. The high residual magnetic susceptibility is mainly ascribed to a Van Vleck contribution. Although the intermediate/fluctuating valence duality is to some extent represented in the interconfiguration fluctuation model commonly used to analyze data on valence-fluctuating systems, we show that this model cannot describe the different physical properties of with a single set of parameters.

Large Cyclotron Effective Mass Detected by de Haas–van Alphen Effect in YbCu2Si2

De Haas–van Alphen experiments were performed on the valence fluctuation compound YbCu2Si2. For the field direction close to the c-axis in the tetragonal structure, a very large cyclotron effective mass of 43 m0 was detected on branch β, which corresponds to the nearly cylindrical Fermi surface. This mass is one of the largest cyclotron masses detected experimentally in Yb compounds.

Temperature and Magnetic Field Dependent Yb Valence in YbRh2Si2 Observed by X-ray Absorption Spectroscopy

The temperature and magnetic field dependences of Yb valence were observed in the heavy fermion compound YbRh2Si2 by X-ray absorption spectroscopy. The measurements revealed that the Yb valence decreases with decreasing temperature in the range from 200 to 2 K and increases with increasing magnetic field in the range from 0 to 33 T without showing an abrupt change in the Yb valence. The Yb valence is in the range from 2.92 to 2.96 depending on temperature and magnetic field. With respect to the valence being 2.92 at 0 T and 2.93 at 33 T in 2 K, YbRh2Si2 is a valence fluctuation compound and does not reach the integer trivalent state at high magnetic field. These results endorse the conventional knowledge that the valence of Yb is very close to the integer value of 3+, decreases with decreasing temperature, and becomes closer to 3+ with increasing magnetic field.

Sm Atomic and Charge Dynamics in a Heavy Fermion Compound SmOs4Sb12

We carried out X-ray absorption spectroscopy, powder diffraction experiment using synchrotron radiation and inelastic X-ray scattering to elucidate Sm atomic and charge dynamics in SmOs 4 Sb 12 . The X-ray absorption spectra demonstrate that SmOs 4 Sb 12 is a Sm valence fluctuation compound. The inelastic X-ray scattering and powder diffraction experiments demonstrate the presence of the anharmonic Sm modes with low energy. We discussed here the correlation between the low-lying Sm modes and Sm valence fluctuation in SmOs 4 Sb 12 .

Effects of Magnetic Field, Pressure and Dilution of Yb on Phase Transitions in Valence Fluctuating Compound YbPd

High field magnetization, pressure effect on electrical resistivity and dilution of Yb with Lu were examined for YbPd. The high field magnetization suggests that half of Yb ions have magnetic moments between 50 K and 115 K up to 55 T. The phase transition temperatures, T 1 and T 2 , which are observed under ambient pressure at 125 and 105 K, respectively, decrease with increasing pressure and the transitions disappear at around 4 GPa. This indicates that the valence fluctuating state above T 1 and T 2 becomes stable down to zero temperature above 4 GPa. Substitution of Lu for Yb also leads to depression of T 1 and T 2 , which is possibly caused by disturbing the periodicity of Yb ions. These results suggest that the valence ordered state of Yb ions is realized in YbPd below T 1 or T 2 .

Soft X-ray angle-resolved photoemission study of YbCu2Ge2

Soft X-ray photoemission spectroscopies have been performed on YbCu2Ge2, which is thought to be a Yb divalent system, to investigate its bulk electronic structure including Yb 4f electrons. Small but finite Yb3+ multiplet peaks were found at 6-12 eV in the angle-integrated spectrum, suggesting that it is a valence fluctuation compound. An "anti-crossing" Yb 4f bands with conduction bands were observed in the angle-resolved spectrum. This is the evidence that the Yb 4f electrons form a band state through the hybridization with Ge 4p and Cu 3d electrons.

Observation of two charge ordering transitions in the valence-fluctuating EuPtP by resonant x-ray diffraction

Two first-order phase transitions of the valence-fluctuating compound EuPtP at ${T}_{1}=246\text{ }\text{K}$ and ${T}_{2}=200\text{ }\text{K}$ were examined by means of resonant x-ray diffraction at the $\text{Eu}\text{ }{L}_{3}$ absorption edge. Although previous M\"ossbauer spectroscopy and preliminary x-ray diffraction experiments have suggested charge-ordered states, characteristic energy spectra of the 111 and $11\frac{2}{3}$ reflections observed in the present experiment unambiguously indicated that twofold and threefold superstructures of Eu valence exist below ${T}_{2}$ and between ${T}_{1}$ and ${T}_{2}$, respectively. We also propose a mechanism to explain the charge ordering in EuPtP based on a charge-lattice coupling that alters a Coulomb repulsion.

Valence-selective XAFS spectroscopy using EuLγ4 emission

EuLγ4 emission spectra show a large chemical shift (∼5 eV), depending on the valence state of a compound. The applicability of this emission to valence-selective X-ray absorption fine structure spectroscopy was demonstrated by performing partial fluorescence yield measurements using a 1:1 mixture of EuS and Eu2O3 and using valence-fluctuating compounds, such as Eu3Pd20Ge6 and BaMgAl10O17:Eu.

ChemInform Abstract: Magnetic and Electronic Properties of Heavy Fermion Compound CeCu4In and Valence Fluctuating Compound CeNi4In.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Formation of a Hybridization Gap in a Cage-Like Compound CeFe2Al10

We report on the magnetic, transport, and thermal properties of a cage-like compound CeFe 2 Al 10 that crystallizes in the orthorhombic YbFe 2 Al 10 -type structure. A broad peak in the magnetic susceptibility at 70 K indicates that CeFe 2 Al 10 is a valence fluctuation compound. The electrical resistivity and the Hall coefficient exhibit sharp upturns below 20 K, where the thermopower shows a rapid decrease. These low-temperature anomalies in the transport properties resemble those of a typical Kondo semiconductor CeRhSb. These features indicate the formation of a hybridization gap in CeFe 2 Al 10 on cooling below 20 K. The energy gap is estimated as 15 K from the thermal activation energy of the resistivity. The magnetic contribution of the specific heat shows a Schottky-type maximum at 30 K that provides another evidence for the gap formation in CeFe 2 Al 10 .

Ground state and elementary excitations of a model valence-fluctuation system

The nature of the valence-fluctuation problem is described, and motivations are given for an Anderson lattice model Hamiltonian. A simple trial wavefunction is posed for the ground state, and the variational problem is solved. This demonstrates clearly that there is no Kondo-like divergence; the present “concentrated Kondo” problem is thus more simple mathematically than the single-impurity problem. Elementary excitations are studied by the Green's function techniques of Zubarev and Hubbard. Quenching of local moments and a large specific heat are found at low temperatures. The quasiparticle spectrum exhibits a gap, but ϵf does not lie in this gap. The insulationlike feature of SmB6, SmS, and TmSe at very low temperatures is explained in terms of a strongly reduced mobility for states near the gap, and reasons are given why this feature is not observed in other valence-fluctuation compounds.

Probe for Spin- and Valence-Selective X-ray Absorption Fine Structure Spectroscopy: EuLγ4 Emission

EuLgamma(4) emission spectra of EuS, EuF(3), EuCl(3), and Eu(2)O(3) were measured using a multicrystal, multidetector spectrometer. Splitting of the EuLgamma(4) band was observed in the EuS spectra and attributed to exchange interactions between Eu5p and Eu4f. It was shown that because of this splitting, the EuLgamma(4) emission could be used for performing spin-selective measurements. This emission exhibited a large chemical shift (approximately 5 eV), depending on the valence state of a compound. The applicability of this emission to valence-selective X-ray absorption fine structure spectroscopy was demonstrated through partial fluorescence yield measurements performed on a 1:1 mixture of EuS and Eu(2)O(3) and on a valence-fluctuating compound Eu(3)Pd(20)Ge(6).

Single crystal growth and electrical and magnetic measurements on [formula omitted

We have succeeded in growing single crystals of CeFe 4 Sb 12 by a Sb-self-flux method and measured the electrical resistivity and magnetization. The residual resistivity is about two times smaller than that of polycrystalline ones, suggesting that the sample quality is highly improved. The magnetization measurements at low temperature revealed that the sample contains almost no magnetic impurities which are always observed in polycrystalline samples. The weak temperature dependence and a broad maximum around 115 K in magnetic susceptibility, which is a typical feature of valence fluctuation compounds, have been observed.

Magnetoelastic effects and magnetic anisotropy in the intermediate-valence compound YbInCu 4

YbInCu 4 compound exhibits a first-order valence transition of Yb from a trivalent state to a mixed-valence state with decreasing temperature. Here we provide an overview of our recent high-field and high-pressure measurements of magnetization, thermal expansion and magnetostriction of the valence-fluctuating compound. The results are analyzed using the Kondo single-impurity model. It is shown that the change of the 4 f–conduction band electron hybridization is responsible for a large modification of the physical properties of YbInCu 4 at the valence transition. The volume magnetostriction is rather large for the low-temperature mixed-valence state, but strongly suppressed in the high-temperature local-moment state. From the magnetization measurements of the oriented YbInCu 4 single crystal, an appreciable anisotropy was found in the susceptibility, high-field magnetization, effective moment, paramagnetic Curie temperature and critical field of the valence transition. The type of the magnetic anisotropy and the value of anisotropic magnetoelastic coupling constant do not change significantly on going from the mixed-valence to the local-moment state. The anisotropic magnetostriction and magnetic anisotropy are described well in terms of a single-ion model of the interaction of anisotropic 4 f-shell of Yb ion with the crystal electric field.

Homoatomic Clustering in the Intermediate Valence Compounds Yb1−yAl3−xSix and Yb1−yAl3−xGex

Two new phases, Yb1−xAl3−xSix and Yb1−yAl3−xGex, were found by systematic investigations of the according ternary systems. The crystal structures of Yb1−yAl2.8Si0.2 and Yb1−yAl2.8Ge0.2 (defect HT-PuAl3 type) were studied by X-ray powder methods (CuKα1 radiation, λ=1.54056 Å, hexagonal system, space group P63/mmc (No. 194), a=6.009(1) and 6.015(1) Å, c=14.199(2) and 14.241(5) Å, V=444.0(2) and 446.2(3) Å3, 93 and 92 reflections, and 8200 and 8000 profile points for silicide and germanide, respectively). Full profile refinements with 11 and 13 structural parameters resulted in RI=0.049 and 0.054, and Rp=0.088 and 0.104, respectively. The ternary structures are distorted closest packings in comparison with the binary YbAl3 compound with AuCu3-type structure. They are characterized by the formation of Al3-, Si3-, and Ge3-homoatomic clusters and aluminum networks. Magnetization measurements show that both the silicide and germanide are valence fluctuation compounds with enhanced electronic density of states at the Fermi level similar to the binary YbAl3. The characteristic maximum of the magnetic susceptibility increases from ≈120 K for YbAl3 to ≈140 K for Yb1−yAl2.8Si0.2or Yb1−yAl2.8Ge0.2 and further to ≈150 K for Yb1−yAl2.75Si0.25. The S-shape of the electrical resistivity curves is also characteristic of valence fluctuations.

Charge fluctuation in Sm 3Te 4

Ultrasonic measurements for a valence-fluctuation compound Sm 3Te 4 have been performed. Considerable dispersion around 120 K due to a thermal hopping motion of 4f-electron among Sm 2+ and Sm 3+ has been observed in the ultrasonic attenuation. The relaxation time of the charge fluctuation 2π τ becomes infinite at low temperatures following as τ= τ 0exp( E/ k B T), here 2 πτ 0=2.5×10 −13 s and E=0.136 eV. The ln T dependence of the elastic constants similar to a two-level system of amorphous glass compounds is observed at low temperatures below 15 K. These results indicate a two-level system of the 4f-electron tunneling in the random potential of Sm 2+ and Sm 3+ ions due to the charge glass state.

Low-temperature magnetization study of the heavy-electron systems

The ground-state magnetic properties of Ce 0.75La 0.25B 6 and Sm 3Te 4 have been studied by means of static magnetization measurements at very low temperatures T ⩾ 50 mK. In Ce 0.75La 0.25B 6, the H − T phase diagram reveals three different ordered phases: in addition to the two phases (phase II and III) known for CeB 6, a new antiferromagnetic state appears before the antiferro-quadrupolar order sets in. In the valence fluctuation compound Sm 3Te 4, a spin-glass ordering is evidenced at T g ∼ 1.3 K. It is likely that the two valence states (Sm 2+, Sm 3) are randomly frozen at low temperature to form effectively a randomly diluted magnet.

The evolution of the crystal-field states upon increasing hybridization in

Inelastic neutron scattering (INS) experiments have been performed on polycrystalline for the concentrations x = 0.1, 0.28, 0.5, 0.65 to study the effect of the increasing hybridization between the f electrons and the band states on the crystal-field (CF) states. For both x = 0.1 and x = 0.28, CF level schemes very similar to that of pure have been found. In contrast, no well defined CF excitations could be detected for x = 0.5 and x = 0.65. The absence of properly developed CF states in high- Kondo and/or valence fluctuation compounds is a well known fact. Remarkably, in for x = 0.5 and x = 0.65, the Kondo temperatures as determined by quasielastic neutron scattering are at least one order of magnitude below the typical CF splitting energies observed for x = 0.1 and x = 0.28. The evolution of the CF splitting in and its distinctive behaviour is discussed in comparison with other heavy-fermion compounds.