Interconfiguration Fluctuation Research Articles

Electronic structure of the strongly correlated electron system plutonium hexaboride: A study from single-particle approximations and many-body calculations.

The electronic structure of the strongly correlated electron system plutonium hexaboride is studied by using single-particle approximations and a many-body approach. Imaginary components of impurity Green's functions show that 5fj=5/2 and 5fj=7/2 manifolds are in conducting and insulating regimes, respectively. Quasi-particle weights and their ratio suggest that the intermediate coupling mechanism is applicable for Pu 5f electrons, and PuB6 might be in the orbital-selective localized state. The weighted summation of occupation probabilities yields the interconfiguration fluctuation and average occupation number of 5f electrons n5f ~ 5.101. The interplay of 5f-5f correlation, spin-orbit coupling, Hund's exchange interaction, many-body transition of 5f configurations, and final state effects might be responsible for the quasiparticle multiplets in electronic spectrum functions. Prominent characters in the density of state, such as the coexistence of atomic multiplet peaks in the vicinity of the Fermi level and broad Hubbard bands in the high-lying regime, suggest that PuB6 could be identified as a Racah material. Finally, the quasiparticle band structure is also presented.

Valence Fluctuation of Uranium Ions in Uranium Sesquinitride Revealed by Dynamical Mean-field Theory Merged with Density Functional Theory.

The electronic properties, in particular, the occupation number of 5f electrons and the valence state of U ions in uranium sesquinitride (U2 N3 ) are studied by using density functional theory (DFT) calculations merged with dynamical mean-field theory (DMFT). The results demonstrate that j=5/2 and j=7/2 manifolds are in the weakly correlated metallic and weakly correlated insulating regimes, respectively. The quasi-particle weights indicate that LS coupling scheme is more feasible for 5f electrons, which are not in the orbital-selective localized state. The weighted summation of the occupation probabilities of 5fn (n=0,1,2,3,4) atomic configurations suggests that 5f electrons have the inter-configuration fluctuation, or the mixed-valence state for U ions, together with an average occupation number of 5f electrons n5f ∼2.234, which is in good agreement with the electron localization function (ELF) and occupation analysis based on other DFT-based calculations. The 5fn -mixing-driven inter-configuration fluctuation might originate from the dual nature of 5f electrons, and the flexible electronic configuration of U ions. Finally, the so-called quasiparticle band structure is also discussed.

Magnetic-field-induced valence change in Eu(Co1−xNix)2P2 up to 60 T

The solid solution 122 compounds, $\mathrm{Eu}{({\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{2}{\mathrm{P}}_{2}$, show a valence transition between divalent state and intermediate valence states at Eu, which is firmly correlated to multiple degrees of freedom in the solid such as the isostructural transition between the collapsed tetragonal (cT) and uncollapsed tetragonal (ucT) structures, $3d$ magnetism, and the formation of P-P dimers. To gain insights into the correlated behavior, we investigate the effect of high magnetic fields on the samples of $x=0.4$ and 0.5 using magnetostriction and magnetization measurements up to 60 T. The samples are in the Eu valence-fluctuating regime, where the possible structural transition from cT to ucT may be induced by the Eu valence change under the magnetic fields. For both samples, magnetostriction smoothly increases with increasing magnetic fields. The behavior is in good agreement with the calculated results using the interconfigurational fluctuation (ICF) model that describes the Eu valence change. This indicates that $\mathrm{\ensuremath{\Delta}}L$ represents the change of the Eu valence state in these compounds. Magnetization curves for both compounds show good agreement with the ICF model at high magnetic fields. In contrast, in the low-magnetic-field region, magnetization curves do not agree with the ICF model. These results indicate that the Eu valence changes manifest themselves in the magnetization curves at high magnetic fields and that the magnetism of the $3d$ electrons manifests itself in the magnetization at low magnetic fields. Hence, we conclude that the valence change occurs within the Eu valence fluctuation regime coupled with the cT structure. Thereby, we believe that the transition to ucT structure, which is firmly coupled with the divalent Eu state, does not occur within the magnetic field range of the present study. Even higher magnetic fields or pulsed magnetic fields with slower pulse durations may induce such large state changes in the present material, which is triggered by the Eu valence change under high magnetic fields.

Inter-configuration fluctuation for 5f electrons in uranium hexafluoride: A many-body study

A quantum mechanics calculation is performed on uranium hexafluoride (UF6) by using density functional theory (DFT) combing with dynamical mean-field theory (DMFT) scheme, taking into account the spin–orbit coupling (SOC) interaction and the on-site 5f-5f Coulomb repulsion. Results demonstrate that SOC-splitting j = 5/2 and j = 7/2 manifolds are in the metallic and insulating regimes, respectively, albeit both are in weakly correlated states. Ratio of the quasi-particle weights R = Zj=7/2/Zj=5/2 is about 1, suggesting that UF6 is not in the so-called orbital-selective localized state. Russel-Saunders (LS) coupling scheme is feasible for U 5f electrons. U 5f-conduction electrons hybridization, 5f-5f correlation, SOC interaction and final state effects yield a complicated spectrum function. Occupation probability analysis shows that 5f electrons exhibit the so-called inter-configuration fluctuation (ICF) due to admixing of 5f1 and 5f3 atomic configurations into 5f2 configuration with an average occupation number of n5f ∼ 1.8585, mainly arising from the complicated quantum mechanics processes including the dual nature of localized and itinerant 5f electrons, a quantum mechanical mixture of the energetically degenerate 5fn configurations, as well as the flexible outer electronic configuration of U ions. Finally, the momentum-resolved electronic spectrum function (the so-called quasiparticle band structure) is also discussed.

Slow crystalline electric field fluctuations in the Kondo lattice SmB6

This work reports on the temperature dependence of the electron spin resonance (ESR) of Gd$^{3+}$-doped SmB$_{6}$ single crystals at X- and Q-band microwave frequencies in different crystallographic directions. We found an anomalous inhomogeneous broadening of the Gd$^{3+}$ ESR linewidth ($\Delta H$) within 5.3 K $\leq T \leq$ 12.0 K which is attributed to slow crystalline electric field (CEF) fluctuations, slower than the timescale of the ESR microwave frequencies used ($\sim$10 GHz). This linewidth inhomogeneity may be associated to the coupling of the Gd$^{3+}$ $S$-states to the breathing mode of the SmB$_{6}$ cage, and can be simulated by a random distribution of the 4$^{th}$ CEF parameter, $b_4$, that strikingly takes negative and positive values. The temperature at which this inhomogeneity sets in, is related to the onset of a continuous insulator-to-metal phase transition. In addition, based on the interconfigurational fluctuation relaxation model, the observed exponential $T$-dependence of $\Delta H$ above $T\simeq$ 10 K gives rise to an excitation energy notably close to the hybridization gap of SmB$_{6}$ ($\Delta\simeq$ 60 K). This charge fluctuation scenario provides important ingredients to the physical properties of SmB$_{6}$. We finally discuss the interplay between charge and valence fluctuations under the view of slow CEF fluctuations in SmB$_{6}$ by coupling the Gd$^{3+}$ ions to the breathing phonon mode via a dynamic Jahn-Teller-like mechanism.

Inter-Configuration Fluctuation For 5f Electrons In Americium Trichloride Studied Via Density Functional Theory Combined with Dynamical Mean-Field Theory

Inter-Configuration Fluctuation For 5f Electrons In Americium Trichloride Studied Via Density Functional Theory Combined with Dynamical Mean-Field Theory

Thermoelectric Power in Ce Systems with Unstable Valence

In this paper, we report on a few exemplary tests of the applicability of analysis based on the interconfiguration fluctuation model (ICF) for a description of the temperature dependence of the thermoelectric power, S(T). The examples include a series of alloys: CeNi2(Si1−yGey)2, Ce(Ni1−xCux)2Si2, and the fluctuating valence (FV) compound CeNi4Ga. The two series develop from CeNi2Si2 being the FV system, where the f states occupation increases progressively with the Ge or Cu substitution. We find here that the ICF model parameters are of similar magnitude both for the analysis of the temperature dependence of the magnetic susceptibility and thermoelectric power. The ICF-type model appears to be a powerful tool for the analysis of S(T) dependences in Ce-based FV compounds and alloys.

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.

CeIr3: superconductivity in a phase based on tetragonally close packed clusters

We present the crystallographic analysis, superconducting and spectroscopic characterization, and theoretical modeling of CeIr3. Lattice parameters a = 5.2945(1) Å and c = 26.219(1) Å are found for the R-3m symmetry crystal structure, which are close to the literature values. CeIr3 is a moderate type-II superconductor (κGL = 17, λe–p = 0.65) below 2.5 K. Ce ions exhibit a strongly intermediate valence character as evidenced by x-ray photoelectron spectroscopy. The normal state magnetic susceptibility is weakly temperature dependent and follows the inter-configuration fluctuation model with a singlet Ce−4 f 0 ground state. Theoretical calculations support a non-magnetic ground state of the system and reveal that Ir−5d states are dominant at the Fermi level.

Influence of chemical composition on the X-ray photoemission, thermopower, specific heat, and magnetic properties of CeNi2(Si1-yGey)2

We report our studies of the intermediate compositions between CeNi2Si2 and CeNi2Ge2, i.e., the alloys CeNi2(Si1-yGey)2 by means of the thermopower, electrical resistivity, specific heat, magnetic susceptibility, and X-ray photoemission measurements. CeNi2Si2 is a fluctuating valence system and CeNi2Ge2 is known to show the heavy fermion behaviour. The change of the temperature dependence of the resistivity towards the typical metallic behaviour occurs below y ~ 0.25. The transition between CeNi2Si2 and CeNi2Ge2 is discussed in the frames of competition between the crystal electric field and Kondo interactions. It is found that valence stabilisation occurs for Ge content y > 0.25. The hybridization energy Δ determined from the XPS Ce 3d spectrum reflects well the behaviour of the parameter Eex obtained from the analysis of the magnetic susceptibility by the interconfiguration fluctuation model. It has been also shown that thermopower data can be successfully described employing the single ion model for 0.6 < y < 1.0 and two-band model including the crystal electric field splitting for y ≤ 0.25.

Valence Fluctuations in CeCo2 and Ti-Doped CeCo2

We report on the magnetic measurements of polycrystalline samples of CeCo2 and CeCo(2−x)Tix (x = 0.01, 0.02, 0.03, 0.04, and 0.05) which have been synthesized by an arc melting technique. All these compounds crystallize into the face-centered cubic (FCC) structure with the Fd\( \bar{3} \)m space group. The lattice parameter decreases linearly with increasing Ti content from 7.15808(5) A for x = 0 (CeCo2) to 7.15231(7) A for x = 0.05. The magnetic behavior of these compounds has been investigated in the temperature range 5–400 K. The zero field-cooled (ZFC) and field-cooled magnetization (FC) curves show irreversibility below T = 400 K. This result indicates that an inhomogeneous, dynamic magnetic state exists over a wide temperature range. The magnetic susceptibility for both ZFC and FC cases initially decreases with Ti content and then increases with further Ti addition. This behavior is interpreted in terms of band magnetism in the presence of magnetic clusters. This result indicates that the magnetic inhomogeneity of these alloys becomes dominant over a wide temperature range. The observed temperature dependence of the magnetic susceptibility leads us to suggest that these compounds are in a mixed-valence state of the magnetic Ce3+ ions and non-magnetic Ce4+ ions. This fact allows us to successfully interpret the ZFC magnetic susceptibility data with the two-level ionic inter-configuration fluctuations model. We also observe that the magnetic susceptibility increases by the addition of Ti, as evidenced by the enhancement of the formation of magnetic Co clusters due to local disorder. Finally, the magnetic state below the Curie temperatures are discussed based on Griffiths-like behavior.

Valence fluctuations of europium in the boride Eu4Pd29+xB8

We synthesized a high-quality sample of the boride Eu4Pd29+xB8 (x = 0.76) and studied its structural and physical properties. Its tetragonal structure was solved by direct methods and confirmed to belong to the Eu4Pd29B8 type. All studied physical properties indicate a valence fluctuating Eu state, with a valence decreasing continuously from about 2.9 at 5 K to 2.7 at 300 K. Maxima in the T dependence of the susceptibility and thermopower at around 135 K and 120 K, respectively, indicate a valence fluctuation energy scale on the order of 300 K. Analysis of the magnetic susceptibility evidences some inconsistencies when using the ionic interconfigurational fluctuation (ICF) model, thus suggesting a stronger relevance of hybridization between 4f and valence electrons compared to standard valence-fluctuating Eu systems.

Gradual Cerium Valence Change in the Solid Solution CeRu1–xPdxAl (x = 0.1–0.9)

AbstractThe solid solution CeRu1–xPdxAl was synthesized for x = 0.1–0.9 from the elements by arc‐melting and subsequent annealing and characterized by powder X‐ray diffraction. All members crystallize in the orthorhombic LaNiAl type structure, space group Pnma. The lattice parameters range from a = 718–722, b = 412–426, and c = 1588–1620 pm, but no linear change of the lattice parameters was found. Magnetic measurements reveal intermediate cerium valences, which change to more trivalent cerium with increasing Pd content. The susceptibility data was interpreted by either the Inter‐Configuration Fluctuation (ICF) model or the Curie‐Weiss law.

The solid solutions CeRu1–x Pd x Sn and CeRh1–x Pd x Sn – Applicability of the ICF model to determine intermediate cerium valencies by comparison with XANES data

Abstract Several samples of the solid solutions CeRu1–x Pd x Sn and CeRh1–x Pd x Sn have been synthesized by arc-melting and characterized by X-ray powder diffraction. Guinier powder patterns prove that the ZrNiAl-type structure is the dominating one, besides the CeRuSn and TiNiSi type structures. The structures of CeRu0.28Pd0.72Sn (ZrNiAl type, P6̅2m, a = 751.95(3), c = 418.70(2) pm, wR2 = 0.0274, 332 F 2 values, 14 variables) and CeRh0.66Pd0.34Sn (ZrNiAl type, P6̅2m, a = 750.26(3), c = 411.59(2) pm, wR2 = 0.0533, 358 F 2 values, 14 variables) were refined from single crystal diffractometer data. Magnetic measurements in combination with XANES (X-Ray Absorption Near Edge Structure) clearly proved intermediate cerium valencies for most compounds and revealed the best fitting parameters for those with the ICF model (Interconfiguration fluctuation). The electrical resistivity is also influenced by the substitutions. At low and high valence electron counts (VECs) metallic character is present, while around the VEC of CeRhSn the typical resistivity behavior for valence fluctuating compounds is observed.

Cerium Valence Change in the Solid Solutions Ce(Rh1-xRux)Sn

The solid solutions Ce(Rh1-xRux)Sn were investigated by means of susceptibility measurements, specific heat, electrical resistivity, X-ray absorption spectroscopy (XAS), and 119Sn Mössbauer spectroscopy. Magnetic measurements as well as XAS data show a cerium valence change in dependence on the ruthenium content. Higher ruthenium content causes an increase from 3.22 to 3.45 at 300 K. Furthermore χ and χ-1 data indicate valence fluctuation for cerium as a function of temperature. For example, Ce(Rh0:8Ru0:2)Sn exhibits valence fluctuations between 3.42 and 3.32 in the temperature range of 10 to 300 K. This could be proven by using the interconfiguration fluctuation (ICF) model introduced by Sales and Wohlleben. Cerium valence change does not influence the tin atoms as proven by 119Sn Mössbauer spectroscopy, but it influences the electrical properties. Ce(Rh0:9Ru0:1)Sn behaves like a typical valence fluctuating compound, and higher ruthenium content causes an increase of the metallic behavior

ESR of a magnetic impurity in a host with interconfiguration fluctuations

The electron spin resonance of a magnetic probe is considered in the interconfiguration-fluctuation host (ICF). The spin relaxation rate of a well-defined magnetic probe in the presence of the Coulomb interaction although satisfies the Korringa relation at low temperatures presents a minimum with a rise in temperature. The transition from the magnetic state to the non-magnetic state is also characterized by a minimum in the relaxation rate. The low temperature behavior of the relaxation rate with the distance between the magnetic probe and ICF ions is substantially lower than that of a pure metal.

ChemInform Abstract: Magnetism and Crystal Chemistry of Cerium Stannides.

Abstract The system Ce-Sn has been reinvestigated in the temperature range from 400 to 800 °C using X-ray powder diffraction techniques. From a characterization of its crystal structures the existence of nine binary compounds has been confirmed. Magnetic investigation of the binary cerium stannides in the temperature range from 4.2 to 600 K revealed a systematic tendency for ferromagnetic ordering of the ceriumrich compounds up to a ratio Ce:Sn ≈ 1, followed by antiferromagnetism up to a ratio of Ce:Sn ≈ 0.4, towards interconfigurational fluctuation in the compound richest in tin, CeSn 3 . Candidates for heavy fermion behaviour found in the transition regions, besides CeSn 3 , are α-Ce 5 Sn 3 and Ce 11 Sn 10 .

Valence fluctuations in YbNiAl4 compound

Measurements of the magnetic susceptibility, electrical resistivity, and heat capacity are reported for the polycrystalline YbNiAl4 intermetallic compound. χ(T) has been measured up to 1000 K and its behavior is typical of a compound with the valence fluctuation between Yb3+ and Yb2+. This is especially evident from a broad maximum observed at 400 K and interpreted in frames of the interconfiguration fluctuation model. The Sommerfeld coefficient derived from the heat capacity measurements takes a small value of γ=16 mJ mol−1 K−2. The low temperature resistivity follows the AT2 dependence characteristic of a Fermi liquid.

Valence fluctuation in Ce 2Co 3Ge 5 and crystal field effect in Pr 2Co 3Ge 5

Polycrystalline samples of ternary rare-earth germanides R 2Co 3Ge 5 (R=La, Ce and Pr) have been prepared and investigated by means of magnetic susceptibility, isothermal magnetization, electrical resistivity and specific heat measurements. All these compounds crystallize in orthorhombic U 2Co 3Si 5 structure (space group Ibam). No evidence of magnetic or superconducting transition is observed in any of these compounds down to 2 K. The unit cell volume of Ce 2Co 3Ge 5 deviates from the expected lanthanide contraction, indicating non trivalent state of Ce ions in this compound. The reduced value of effective moment ( μ eff ≈0.95 μ B ) compared to that expected for trivalent Ce ions further supports valence-fluctuating nature of Ce in Ce 2Co 3Ge 5. The observed temperature dependence of magnetic susceptibility is consistent with the ionic interconfiguration fluctuation (ICF) model. Although no sharp anomaly due to a phase transition is seen, a broad Schottky-type anomaly is observed in the magnetic part of specific heat of Pr 2Co 3Ge 5. An analysis of C mag data suggests a singlet ground state in Pr 2Co 3Ge 5 separated from the singlet first excited state by 22 K and a doublet second excited state at 73 K.