Valence Fluctuation Systems Research Articles

A phenomenological study in CeCoSi system

We present phenomenological models calculations for the magnetic susceptibility, valence of Ce ions and the effective Bhor magneton number of Ce ion in CeCoSi system. We found that the susceptibility of this compound explained by the interconfiguration fluctuation (ICF) model. We obtain the valence of Ce ions 3.42 at the room temperature. This result suggests that CeCoSi system is a valence fluctuation system. We found the effective Bhor magneton number per atom 2.35 μB by the 2-state model which is close to the expected for a free Ce+3 ion.

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.

Novel lanthanide borohydrides: magnetism of all flavours

The rare-earth metals have high magnetic moments and a diverse range of magnetic structures. However, due to the inner-transition nature of lanthanide elements, the valence f orbitals of their trivalent cations usually do not mix substantially with the ligands' orbitals in the chemical compounds. The majority of lanthanide compounds is thus characterized by a rather ionic metal–ligand bonding and is hosting only weak crystal field effects. Several exceptions known encompass the valence fluctuation systems consisting of Sm, Ey, Tm or Yb combined with less electronegative nonmetal ligands (Si, S, Se, B etc.) or metals (Murani 2003 and references therein). This important class of lanthanide compounds for which crystal field effects are strong includes the classical systems: Yb3Si5 (Iandelli et al., 1979), YbB12 (Altshuler et al., 1998), and Yb3H8 (Drulis et al., 1999) . Even elemental Yb and Eu metals show valence transition at elevated pressure from di- to trivalent (Takemura & Syassen, 1985). These valence fluctuations are typically accompanied by electric resistivity changes: Ln(2+) → Ln(3+) + e–. Lanthanide borohydrides, Ln(BH4)3, constitute a rather poorly explored and novel group of compounds (Olsen et al., 2014). They are conveniently prepared via mechanochemical synthesis approach (high-energy milling). Quasi-ternary alkali metal-lanthanide borohydrides, MLn(BH4)4, are also available using this synthetic procedure (Wegner et al., 2013 [1] & Wegner et al., 2014 [2]). Here we explore for the first time the magnetic properties of Ln(BH4)3 and MLn(BH4)4 compounds, with particular emphasis on the thermally unstable systems (Ln= Sm, Yb and Eu) as contrasted with the reference case of much more thermally stable derivatives of ordinary lanthanides (Ln = Ho). We show that remarkably strong mixing of Ln(4f) and H(1s) states which causes thermal instability: Ln(3+) + BH4–→ Ln(2+) + BH4· leads in some cases to strong magnetic superexchange interactions between Ln(3+) centers [3].

Strongly hybridized electronic structure of YbAl2: An angle-resolved photoemission study

We report the electronic structure of a prototypical valence fluctuation system, YbAl2, using angle-resolved photoemission spectroscopy. The observed band dispersions and Fermi surfaces are well described in terms of band structure calculations based on local density approximation. Strong hybridization between the conduction and 4f bands is identified on the basis of the periodic Anderson model. The evaluated small mass enhancement factor and the high Kondo temperature qualitatively agree with those obtained from thermodynamic measurements. Such findings suggest that the strong hybridization suppresses band renormalization and is responsible for the valence fluctuations in YbAl2.

Kondo Lattice Behavior in the Valence Fluctuating Systems α- and β-YbAlB4

We have measured specific heat and magnetization of the Yb-based heavy Fermion systems α-YbAlB 4 and β-YbAlB 4 . α-YbAlB 4 is a locally isostructural polymorph of the first example of an Yb-based heavy Fermion superconductor β-YbAlB 4 , which exhibits pronounced non-Fermi-liquid behavior above the superconducting transition temperature T c = 80 mK. The results reveal a Fermi liquid ground state for α-YbAlB 4 in contrast to the quantum criticality found for β-YbAlB 4 . Interestingly, both systems exhibit Kondo lattice behavior with its characteristic temperature scale T * ∼8 K, despite of their strongly intermediate valence with the temperature scale of ∼ 200 K. We discuss the origin of this unusual behavior, making a comparison with the typical mixed valent compound YbAl 3 . Large Wilson ratios indicate the existence of ferromagnetic correlation as a possible origin for the Kondo lattice behavior.

Electronic structure, crystallographic, magnetic, and transport characterization of EuMn2 films

We report the formation of EuMn2, in the form of a thin film, grown by the reaction of Eu/Mn multilayers in the solid state phase at 480 K. X-ray diffraction (XRD) studies revealed that the EuMn2 crystallizes in a hexagonal structure, with the P6/mmc space group, and lattice parameters a=0.72 nm and c=1.07 nm. X-ray photoelectron spectroscopy measurements showed divalent europium forming the compound. Transition to a magnetically ordered phase was observed at 15 K. The value of 4πMs was estimated to be 1.5 kG at 2 K. An unusual behavior of the temperature dependence of resistivity was observed in the film, suggesting the presence of valence fluctuation systems.

Semiconducting behaviour in CeT2Al10 (T=Fe and Ru)

We present results of the magnetic susceptibility, electrical resistivity and thermopower for CeT2Al10 (T=Fe and Ru) crystallizing in the orthorhombic YbFe2Al10-type structure. The magnetic susceptibility for CeFe2Al10 shows a maximum at 70 K which is a characteristic of a valence fluctuation system. The electrical resistivity below 20 K exhibits semiconducting behaviour with an energy gap of 15 K. On the other hand, CeRu2Al10 displays Curie-Weiss behaviour of Ce3+ in the susceptibility down to 30 K, and undergoes an antiferromagnetic transition at TN=27 K. The semiconducting behaviour of the resistivity between 70 K and 30 K is described by a gap of 50 K. On cooling below TN, however, the resistivity exhibits a peak at 23 K and falls down. The thermopower shows two maxima at 22 K and 6 K. These findings suggest that the magnetic order induces a semiconductor-to-metal transition in CeRu2Al10.

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.

Neutron Scattering from α-Ce Systems at Thermal and Epithermal Neutron Energies

The γ-α phase transition in Ce has remained enigmatic ever since its discovery in 1927 by Bridgman [1]. With increasing pressure at ambient temperature an abrupt, ∼6 fold, drop in the susceptibility of Ce occurs above ∼9 kbar as the metal transforms from the γ to the α-phase, accompanied by a 14% volume reduction without any change in the crystalline structure which is fcc in both phases. The dramatic change in magnetic susceptibility of Ce metal (alloyed with a small concentration of Sc or Th to prevent β-phase formation) also occurs as a function of temperature. On cooling the Curie-Weiss susceptibility in the γ-Ce phase drops abruptly below the transition temperature of ∼110 K by roughly the same amount as with pressure, becoming practically temperature independent at lower temperatures. Such constant low-temperature susceptibilities, resembling Pauli paramagnetism, have been observed in a number of Ce-based alloys and compounds which are, therefore, considered α-Ce-like and also referred to as valence fluctuation systems.

Electrical, Magnetic and NMR Studies of Ge-Based Filled Skutterudites RPt4Ge12 (R=La, Ce, Pr, Nd)

We have synthesized the polycrystalline samples of Ge-based filled skutterudites R Pt 4 Ge 12 ( R =La, Ce, Pr, Nd) and investigated the lattice constant, electrical resistivity, magnetic susceptibility and nuclear magnetic resonance (NMR) systematically. LaPt 4 Ge 12 is confirmed to be a conventional BCS-type superconductor by the NMR measurements. Additional contribution ascribed to the rattling motion of the La ions has been observed in the nuclear relaxation at the La site at high temperatures. Temperature dependence of the magnetic susceptibility of CePt 4 Ge 12 shows a broad peak around 80 K, which is a typical feature of valence fluctuation systems, whereas PrPt 4 Ge 12 and NdPt 4 Ge 12 show the well-localized nature of 4 f -electrons. The NMR measurements in CePt 4 Ge 12 revealed that the nuclear spin–lattice relaxation rate 1/ T 1 shows an activation-type temperature dependence 1/ T 1 ∝exp (-Δ/ k B T ) with an energy gap Δ/ k B =150 K above 100 K, whereas it is proportional to the temperature belo...

Localization of magnetic moments of cerium in single crystallineCePt4In

Single crystals of $\mathrm{Ce}{\mathrm{Pt}}_{4}\mathrm{In}$ were studied by means of magnetic susceptibility and Seebeck coefficient measurements. Moreover, $\mathrm{Ce}\text{\ensuremath{-}}{\mathrm{L}}_{III}$ edge x-ray absorption near-edge structure experiments were performed on polycrystalline samples. The results show that the $4f$ electrons in this compound have fairly localized character, with rather stable $4{f}^{1}$ configuration. These findings supplement previous literature reports on $\mathrm{Ce}{\mathrm{Pt}}_{4}\mathrm{In}$ being a valence fluctuation system.

Ultrasonic Investigation on Charge Fluctuation of Valence Fluctuation and Rattling Systems

Ultrasonic Investigation on Charge Fluctuation of Valence Fluctuation and Rattling Systems

Impurity-induced transition and impurity-enhanced thermopower in the thermoelectric oxideNaCo2−xCuxO4

Various physical quantities were measured and analyzed for the Cu-substituted thermoelectric oxide ${\mathrm{NaCo}}_{2\ensuremath{-}x}{\mathrm{Cu}}_{x}{\mathrm{O}}_{4}.$ As was previously known, the substituted Cu enhances the thermoelectric power, while it does not increase the resistivity significantly. The susceptibility and the electron specific heat are substantially decreased with increasing x, which implies that the substituted Cu decreases the effective-mass enhancement. Through a quantitative comparison with the heavy-fermion compounds and the valence-fluctuation systems, we have found that the Cu substitution effectively increases the coupling between the conduction-electron and the magnetic fluctuations. The Cu substitution induces a phase transition at 22 K that is very similar to a spin-density-wave transition.

Transport properties ofCexY1−xCu2.05Si2:A heavy-fermion alloy system on the border of valence fluctuation

We report results of electrical resistivity and thermopower measurements from 2 to 300 K of the ${\mathrm{Ce}}_{x}{\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{2.05}{\mathrm{Si}}_{2}$ alloy system. The data show that the overall crystal field splitting does not depend on concentration and is the same as in the ${\mathrm{Ce}}_{x}{\mathrm{LaY}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{2.05}{\mathrm{Si}}_{2}$ alloy system. The results are compared with pressure experiments on the ${\mathrm{CeCu}}_{2}{\mathrm{Si}}_{2}$ compound, with investigations of ${\mathrm{U}}_{x}{\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Pd}}_{3}$ alloys and a recent analysis of the ${\mathrm{Ce}}_{x}{\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{2.05}{\mathrm{Si}}_{2}$ alloy system. We show that ${\mathrm{Ce}}_{x}{\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Cu}}_{2.05}{\mathrm{Si}}_{2},$ a heavy-fermion alloy system, evolves towards a valence fluctuation system.

Crossover from First-Order Valence Transition to Kondo Behavior in C15b-Type YbCu5-xInxSystem

YbCu 5 and YbCu 4 In forms a solid-solution (YbCu 5- x In x ) with cubic AuBe 5 -type (C15b) structure in the In-concentration range 0.1≦ x ≦1; and below x =0.1, it coexists with a hexagonal CaCu 5 -type YbCu 5 phase under ambient pressure. The magnetic susceptibility and high-field magnetization measurements reveal that the first-order valence transition in YbCu 4 In shifts to higher temperature and becomes broadened significantly with decreasing In-content, and simultaneously the delocalized mixed-valent state of Yb ion ( x =1) shows a continuous change toward the localized trivalent state ( x =0). The electrical resistivities of YbCu 5- x In x display various temperature-dependencies: ranging from a pronounced drop at x =1, via a minimum behavior for 0.4≤ x ≤0.7, to a negative logarithmic increasing behavior (-ln T ) for x ≤0.3. Above results suggest that YbCu 5- x In x goes through a crossover from a valence fluctuation system to a dence Kondo one with decreasing x . At x =0, the formation of Kondo la...

Transport, magnetic, and 119Sn Mössbauer studies on magnetically ordered valence fluctuating compound SmRuSn3

SmRuSn3 is a unique compound among the known Sm-based valence fluctuation (VF) systems. Its crystallographic structure has two inequivalent Sm sites and Sm ions at only one of them are in VF state while the Sm ions in the other site orders magnetically. Our 119Mössbauer studies show a quadrupolar splitting at the Sn site, consistent with the noncubic symmetry of the Sn site. A broadening of the Mössbauer spectrum is seen due to magnetic ordering of the material. The transferred hyperfine field at Sn site at 4.2 K is small.

Resistivity and LIII-edge absorption studies in valence fluctuation system Ce2Ni3Si5

From our x-ray (LIII-edge) absorption (XAS) investigations of Ce2Ni3Si5, we show that Ce-valence is temperature dependent; it is 3.07 and 3.11 at 280 and 8 K, respectively. We also report on our resistivity measurements of two related materials Ce2−xRxNi3Si5 (R=Y, Gd and x=0.1). Absence of any qualitative difference in the resistivities of these two samples suggests that the enhancement of resistivity at low temperature on introduction of impurity atoms is due to Kondo hole scattering implying that Ce2Ni3Si5 is a concentrated Kondo system.

Ce2Ni3Si5: A mixed-valence cerium compound.

The results of electrical resistivity (4.2--300 K), magnetic susceptibility (5--300 K) and specific heat (2--20 K) are reported on the ternary orthorhombic rare-earth compound ${\mathrm{Ce}}_{2}$${\mathrm{Ni}}_{3}$${\mathrm{Si}}_{5}$. The resistivity of the material is nearly temperature independent above 120 K unlike that of a normal metallic material. The magnetic contribution to resistivity shows a broad maximum around 200 K. At low temperatures, the resistivity follows a ${\mathit{T}}^{2}$ behavior in the temperature range 4--13 K. The inverse magnetic susceptibility deviates from Curie-Weiss behavior below 80 K where it exhibits a broad maximum in the \ensuremath{\chi} vs T curve. Specific heat measurements (2--20 K) give a value of \ensuremath{\gamma}\ensuremath{\approxeq}62 mJ/Ce mol ${\mathrm{K}}^{2}$ which is similar in magnitude to those encountered in valence-fluctuating systems. From these observations we conclude that ${\mathrm{Ce}}_{2}$${\mathrm{Ni}}_{3}$${\mathrm{Si}}_{5}$ is a Ce-based valence-fluctuation compound.

Band Calculations on YbB12, SmB6and CeNiSn

Many rare earth and actinide compounds are well known as heavy fermion systems with a large specific heat coefficient or as valence fluctuation systems with an energy-gap of activation type. Band calculation is a powerful method for investigat­ ing the electronic structures of these materials, although many-body effects are not sufficiently contained. If hybridization between /-electrons and conduction electrons is large, the Fermi surfaces are .explained as a result of a conventional band calcula­ tion using the local density approximation (LDA). If /-electrons are well localized and have a large magnetic moment, the Fermi surfaces are expected to be similar to those of non-I reference materials. Therefore investigations for the electronic struc­ tures of /-electron systems, provided the heavy masses cannot be expected, are important to know the role of /-electrons in their ground states. For the compounds with an energy-gap, results of band calculations using LDA are needed to answer the question whether hybridization causes a gap or not. Both of SmB6 1 > and YbB1z 2 >' 3 > are known as valence fluctuation compounds with clear gap. CeNiSn is known as a dense Kondo compound with a formation of an energy-gap. 4 >' 5 > The existence of the gap is suggested in the experiments. The resistivity of YbB12 and SmB6 rise in low temperatures in order of magnitude, while the resistivity in the c-direction of CeNiSn becomes about only 4 times larger below 5 K at most in three directiQns of the orthorhombic axes. Therefore, the energy gap in CeNiSn may have different charac­ ter. The compound SmB6 has a CaB6 type crystal structure with the space 'group of Pm3m. Trivalent compounds with this structure such as LaB6, PrB6, NdB6 and YB6 are good metals with one conduction electron per metallic ion, while divalent com­ pounds such as CaB6 and SrB6 are insulator. The band calculation for CaB6 calcu­ lated with LDA gives a small direct gap between the valence and conduction bands at the X points in the simple cubic Brillouin zone. 6 > The magnitude of the energy gap is 0.3 eV, in reasonable agreement with the experimental value, 0.4 eV suggested by

Variational ground state for the periodic Anderson model with an indirect hybridization. II. The Kondo-type singlet state compared with the magnetic state.

In this paper we extend our previously developed theory to consider the problems of phase stabilization and phase transition in the variational ground state of our model system. We have constructed a complex Bogoliubov transformation for the electron subsystem and found that through the electron-phonon interaction the f-f Hubbard-U term could play an important role in stabilizing the Kondo-type singlet state. We consider that this might explain why most valence-fluctuation systems have a strongly correlated f band but are not in any magnetic state.