Valence Fluctuations Research Articles

Synthesis and structure determination of Ce6Cd23Te: a new chalcogen-containing member of the RE6Cd23T family (RE is a rare-earth metal and T is a late group 14, 15 and 16 element).

The ternary phase hexacerium tricosacadmium telluride, Ce6Cd23Te, was synthesized by a high-temperature reaction of the elements in sealed Nb ampoules and was structurally characterized by powder and single-crystal X-ray diffraction. The structure, established from single-crystal X-ray diffraction methods, is isopointal with the Zr6Zn23Si structure type (Pearson symbol cF120, cubic space group Fm-3m), a filled version of the Th6Mn23 structure with the same space group and Pearson symbol cF116. Though no Cd-containing rare-earth metal binaries are known to form with this structure, it appears that the addition of small amounts of a p-block element allows the formation of such interstitially stabilized ternary compounds. Temperature-dependent direct current (dc) magnetization measurements suggest local-moment magnetism arising from the Ce3+ ground state, with possible valence fluctuations at low temperature, inferred from the deviations from the Curie-Weiss law.

X線分光法による価数揺動現象およびそれを起源とする量子臨界現象の機構解明

X線分光法による価数揺動現象およびそれを起源とする量子臨界現象の機構解明

Switchable photovoltaic properties of multiferroic KBiFe2O5

We report here the preparation, ferroelectric, ferromagnetic and photovoltaic properties of polycrystalline KBiFe2O5 material. The formation of polycrystalline, orthorhombic KBiFe2O5 is confirmed through x-ray diffraction and HRTEM. The unsaturated ferroelectric hysteresis loop after poling indicates the leaky nature of the material that might be associated with the anion (oxygen) vacancies, not with the valence fluctuations of the Fe as confirmed by XPS. A weak ferromagnetism (remanent magnetization 0.005emu/g) in KBiFe2O5 as observed from magnetisation study is resulted from high spin state of Fe3+ in Fe2O3 tetrahedron. The optically active KBFO shows switchable photoresponse with positive and negative polarities which gives photo current (JSC)=20.7μA/cm2 and photo voltage (VOC)=2.04V under visible light illumination after poling. The observed switchable photo responses are explained on the basis of dipolar flipping in KBiFe2O5 due to the applied poling field.

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 in Sn(Pb)2P2S6 ferroelectrics

The valence fluctuations which are related to the charge disproportionation of phosphorous ions P4++P4+→P3++P+5 are the origin of ferroelectric and quantum paraelectric states in Sn(Pb)2P2S6 semiconductors. They involve recharging of SnPS3 or PbPS3 structural groups which can be represented as half-filled sites in the crystal lattice. Temperature–pressure phase diagram for Sn2P2S6 compound and temperature-composition phase diagram for (PbySn1–y)2P2S6 mixed crystals, which include tricritical points and where a temperature of phase transitions decrease to 0 K, together with the data about some softening of low energy optic phonons and rise of dielectric susceptibility at cooling in quantum paraelectric state of Pb2P2S6 are analyzed by GGA electron and phonon calculations and compared with electronic correlations models. The anharmonic quantum oscillators model is developed for description of phase diagrams and temperature dependence of dielectric susceptibility.

Magnetic Response and Valence Fluctuations in the Extended Anderson Lattice Model with Quasiperiodicity

We study valence fluctuations in the extended Anderson model on two-dimensional Penrose lattice, using the real-space dynamical mean-field theory combined with the continuous-time Monte Carlo method. Calculating $f$-electron number, $c$-$f$ spin correlations, and magnetic susceptibility at each site, we find site-dependent formations of the singlet state and valence distribution at low temperatures, which are reflected by the quasiperiodic lattice structure. The bulk magnetic susceptibility is also addressed.

Europium mixed-valence, long-range magnetic order, and dynamic magnetic response in EuCu2(SixGe1−x)2

In mixed-valence or heavy-fermion systems, the hybridization between local $f$ orbitals and conduction band states can cause the suppression of long-range magnetic order, which competes with strong spin fluctuations. Ce- and Yb-based systems have been found to exhibit fascinating physical properties (heavy-fermion superconductivity, non-Fermi-liquid states, etc.) when tuned to the vicinity of magnetic quantum critical points by use of various external control parameters (temperature, magnetic field, chemical composition). Recently, similar effects (mixed-valence, Kondo fluctuations, heavy Fermi liquid) have been reported to exist in some Eu-based compounds. Unlike Ce (Yb), Eu has a multiple electron (hole) occupancy of its $4f$ shell, and the magnetic Eu$^{2+}$ state ($4f^7$) has no orbital component in the usual $LS$ coupling scheme, which can lead to a quite different and interesting physics. In the EuCu$_{2}$(Si$_{x}$Ge$_{1-x}$)$_{2}$ series, where the valence can be tuned by varying the Si/Ge ratio, it has been reported that a significant valence fluctuation can exist even in the magnetic order regime. This paper presents a detailed study of the latter material using different microscopic probes (XANES, M\"ossbauer spectroscopy, elastic and inelastic neutron scattering), in which the composition dependence of the magnetic order and dynamics across the series is traced back to the change in the Eu valence state. In particular, the results support the persistence of valence fluctuations into the antiferromagnetic state over a sizable composition range below the critical Si concentration $x_c \approx 0.65$. The sequence of magnetic ground states in the series is shown to reflect the evolution of the magnetic spectral response.

Analysis of properties of modified kaolins and water-dispersion paints developed on their base

Analysis of the effect of modified kaolins on the formation of properties of water­dispersed paints was conducted. It was determined that, as a result of the kaolin modification by 3­aminopropyltriethoxysilane, their adsorption and physical and chemical properties (wettability and specific effective surface by water and benzene) were changed. Specific effective surface of kaolin from Glukhovetskiy deposit was increased by benzene by 26.47 %, from Prosianivskiy deposit – by 5.9 %. The same indicator by water was decreased by 26.1 % for the kaolin from Glukhovetskiy deposits and 3.4 % for the kaolin from Prosianivskiy deposit. The adsorption of acrylic­styrene polymer by respective kaolins was increased by 32.1 % and 23.8 %. The processes of interaction of kaolins with a modifier and styrene­acrylic film former were studied; the parameters of the main characteristic bands responsible for valence fluctuations of reactive capable groups of kaolins and film former were analyzed. The results of the study demonstrated that, due to the modification of kaolins by 3­aminopropyltriethoxysilane, their polymeric philicity was increased. The pre­modified kaolins, characterized by the most complete interaction with styrene­acrylic film former, are proposed to use as fillers for water­dispersion paints. This will ensure the higher performance properties of paints and coatings: elasticity at the level of 2–4 mm, the edge angle of wetting at the level of 89–95 degrees, tensile strength in the range of 5.53–6.81 MPa, which is by 18–51 % larger than when using non­modified fillers. It was determined that improving the compatibility of kaolins with the film former of water­dispersion paints allow forming coatings more resistant to corrosive liquids, with low water absorption at the level of 8.1–9.7 % mass %, i. e., by 17.9 % – 10.4 lower than when adding non­modified kaolins.

Magnetic and nonmagnetic doping dependence of the conducting surface states inSmB6

Kondo insulator $\mathrm{Sm}{\mathrm{B}}_{6}$ has attracted attention because it can realize new topological phenomena driven by the interplay between strong correlation effect and topology. However, its topological nature is still under debate. To examine the topological aspect, we demonstrate the nonmagnetic La and magnetic Ce doping dependence of the resistance of $\mathrm{Sm}{\mathrm{B}}_{6}$. Moreover, the resistance ratios of different thicknesses are analyzed to confirm the surface contribution. Lightly doped La samples show a purely conducting surface region at low temperature, whereas the lightly doped Ce samples do not have any conducting region at low temperature. Furthermore, based on the analysis of the electrical transport data of $\mathrm{S}{\mathrm{m}}_{1\ensuremath{-}x}\mathrm{L}{\mathrm{a}}_{x}{\mathrm{B}}_{6}$ $(0.0\ensuremath{\le}x\ensuremath{\le}1.0)$, an electronic phase diagram was found, composed of four regions: region I $(0.0\ensuremath{\le}x\ensuremath{\le}0.06)$, II $(0.1\ensuremath{\le}x\ensuremath{\le}0.15)$, III $(x\ensuremath{\approx}0.2)$, and IV $(0.25\ensuremath{\le}x\ensuremath{\le}1.0)$. Region I is characterized by the presence of conducting surface states, region II is characterized by the insulating phase due to the $d\ensuremath{-}f$ hybridization gap without the conducting surface state, region III is characterized by the disappearance of the $d\ensuremath{-}f$ hybridization gap and the existence of valence fluctuation, and region IV is a typical metallic state.

High-pressure synchrotron Mössbauer and X-ray diffraction studies: Exploring the structure-related valence fluctuation in EuNi2P2

The high-pressure effect on valence fluctuation of the ThCr2Si2-type intermetallic compound EuNi2P2 has been investigated using in situ synchrotron Mössbauer spectroscopy (SMS). The isomer shift of 151Eu in EuNi2P2 increases monotonically with increasing pressure up to 50GPa, suggesting a valence transition of the Eu from mixed toward trivalent. The synchrotron angle-dispersive X-ray diffraction (AD-XRD) experiment shows that EuNi2P2 remains in the tetragonal structure up to 32.5GPa at room temperature. We propose that the evolutions of bonding distance with pressure have an obvious effect on the valence fluctuation.

Localized and mixed valence state of Ce4fin superconducting and ferromagneticCeO1−xFxBiS2revealed by x-ray absorption and photoemission spectroscopy

We have performed Ce $L_3$-edge x-ray absorption spectroscopy (XAS) and Ce $4d$-$4f$ resonant photoemission spectroscopy (PES) on single crystals of CeO$_{1-x}$F$_x$BiS$_2$ for $x=0.0$ and 0.5 in order to investigate the Ce $4f$ electronic states. In the Ce $L_3$-edge XAS, mixed valence of Ce was found in the $x=0.0$ sample and the F-doping suppresses it, which is consistent with the results on polycrystalline samples. As for the resonant PES, we found that the Ce $4f$ electrons in both $x=0.0$ and $0.5$ systems respectively form a flat band at 1.0 eV and 1.4 eV below the Fermi level and there is no contribution to the Fermi surfaces. Interestingly, Ce valence in CeOBiS$_2$ deviates from Ce$^{3+}$ even though Ce $4f$ electrons are localized, indicating the Ce valence is not in a typical valence fluctuation regime. We assume that localized Ce $4f$ in CeOBiS$_2$ is mixed with the unoccupied Bi $6p_z$, which is consistent with the previous local structural study. Based on the analysis of the Ce $L_3$-edge XAS spectra using Anderson's impurity model calculation, we found that the transfer integral becomes smaller increasing the number of Ce $4f$ electrons upon the F substitution for O.

Synchrotron Radiation Mössbauer Spectroscopy Using 149Sm Nuclei

Synchrotron radiation 149Sm Mossbauer spectroscopy in the energy domain has successfully been carried out on Sm2O3, SmBe13, and SmB6. The isomer shift values were determined in these compounds to discuss their Sm valence state. We also investigated the time window effect on spectra, which is useful for measuring spectra with “high-energy-resolution”. This effect makes it possible to discuss the frequency of valence fluctuation in Sm compounds as well as determine Sm valence state precisely using the 149Sm Mossbauer effect. We discussed the fluctuation of the Sm valence in SmB6.

Superconductivity in CeOBiS2 with cerium valence fluctuation

Resistivities of single-crystalline as well as poly-crystalline samples of CeOBiS2 without fluorine doping were measured at temperatures down to 0.13 K, and were compared with those of poly-crystalline LaOBiS2 and PrOBiS2. Both poly-crystalline and single-crystalline CeOBiS2 exhibited zero resistivity below 1.2 K while poly-crystalline LaOBiS2 and PrOBiS2 did not show zero resistivity down to 0.13 K. Superconducting transition temperature of CeOBiS2 was reduced by increasing the applied current density. The superconductivity of CeOBiS2 without chemical doping is likely triggered by the carriers induced by the valence fluctuation between Ce3+ and Ce4+.

Pressure-Driven Quantum Criticality and T/H Scaling in the Icosahedral Au–Al–Yb Approximant

We report on ac magnetic susceptibility measurements under pressure of the Au-Al-Yb alloy, a crystalline approximant to the icosahedral quasicrystal that shows unconventional quantum criticality. In describing the susceptibility as $\chi(T)^{-1} - \chi(0)^{-1} \propto T^{\gamma}$, we find that $\chi(0)^{-1}$ decreases with increasing pressure and vanishes to zero at the critical pressure $P_{\rm c} \simeq 2$ GPa, with $\gamma~ (\simeq 0.5)$ unchanged. We suggest that this quantum criticality emerges owing to critical valence fluctuations. Above $P_{\rm c}$, the approximant undergoes a magnetic transition at $T \simeq 100$ mK. These results are contrasted with the fact that, in the quasicrystal, the quantum criticality is robust against the application of pressure. The applicability of the so-called $T/H$ scaling to the approximant is also discussed.

Valence of praseodymium in superconducting Pr(O,F)BiS2 single crystals

The praseodymium (Pr) valence in superconducting PrO1−xFxBiS2 single crystals was evaluated by X-ray photoelectron spectroscopy. The Pr 3d core level is split into 3d3/2 and 3d5/2 levels. Both PrO1−xFxBiS2 and Pr2S3, which contain only Pr3+, showed 3d spectra at a lower binding energy than Pr6O11, which contains both Pr3+ and Pr4+. Neither PrO1−xFxBiS2 nor Pr2S3 showed the characteristic satellites of Pr4+ in 3d5/2 spectra. The valence fluctuation between the Pr3+ and Pr4+ states was not confirmed, suggesting that Pr3+ is dominant in PrO1−xFxBiS2 single crystals.

Valence fluctuation and magnetic ordering in EuNi2(P1−xGex)2 single crystals

Unusual phases and phase transitions are seen at the magnetic–nonmagnetic boundary in Ce-, Eu- and Yb-based compounds. EuNi2P2 is a very unusual valence fluctuating Eu system, because at low temperatures the Eu valence stays close to 2.5 instead of approaching an integer value. The Eu valence, and thus the magnetic property in this system, can be tuned by Ge substitution in the P site as EuNi2Ge2 is known to exhibit the antiferromagnetc (AFM) ordering of divalent Eu moments with K. We have grown EuNi2(P1−xGex)2 () single crystals and studied their magnetic, thermodynamic and transport properties. Increasing Ge doping to x > 0.4 results in a well-defined AFM ordered state with K for x = 0.5. Moreover, the reduced value of magnetic entropy for x = 0.5 at TN suggests the presence of valance fluctuation/the Kondo effect in this compound. Interestingly, the specific heat exhibits an enhanced Sommerfeld coefficient upon Ge doping. Subsequently, electronic structure calculations lead to a non-integral valence in EuNi2P2 but a stable divalent Eu state in EuNi2Ge2, which is in good agreement with the experimental results.

ARPES view on surface and bulk hybridization phenomena in the antiferromagnetic Kondo lattice CeRh2Si2.

The hybridization between localized 4f electrons and itinerant electrons in rare-earth-based materials gives rise to their exotic properties like valence fluctuations, Kondo behaviour, heavy-fermions, or unconventional superconductivity. Here we present an angle-resolved photoemission spectroscopy (ARPES) study of the Kondo lattice antiferromagnet CeRh2Si2, where the surface and bulk Ce-4f spectral responses were clearly resolved. The pronounced 4f 0 peak seen for the Ce terminated surface gets strongly suppressed in the bulk Ce-4f spectra taken from a Si-terminated crystal due to much larger f-d hybridization. Most interestingly, the bulk Ce-4f spectra reveal a fine structure near the Fermi edge reflecting the crystal electric field splitting of the bulk magnetic 4f 15/2 state. This structure presents a clear dispersion upon crossing valence states, providing direct evidence of f-d hybridization. Our findings give precise insight into f-d hybridization penomena and highlight their importance in the antiferromagnetic phases of Kondo lattices.

Self-consistent modeling of thermal and elastic properties of unconventional superconductor PuCoGa5

A self-consistent thermodynamic model of PuCoGa5 is developed, which for the first time takes into account the anharmonicity of both acoustic phonons, described within a Debye model, and optical phonons, considered in an Einstein approximation. Within the framework of this model, we have calculated the temperature dependencies of lattice contributions to heat capacity, bulk modulus, volumetric coefficient of thermal expansion, Debye and Einstein temperatures and their Grüneisen parameters. The electronic heat capacity of PuCoGa5 is obtained, which demonstrates an unusual temperature dependence with two maxima. In addition, it is shown that an abnormal low temperature behavior of the bulk modulus of PuCoGa5 is not caused by the effects of lattice anharmonicity and is most likely due to the valence fluctuations, which is in agreement with previous studies.

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.

Unfolding the physics of URu2Si2 through silicon to phosphorus substitution.

The heavy fermion intermetallic compound URu2Si2 exhibits a hidden-order phase below the temperature of 17.5 K, which supports both anomalous metallic behavior and unconventional superconductivity. While these individual phenomena have been investigated in detail, it remains unclear how they are related to each other and to what extent uranium f-electron valence fluctuations influence each one. Here we use ligand site substituted URu2Si2-xPx to establish their evolution under electronic tuning. We find that while hidden order is monotonically suppressed and destroyed for x≤0.035, the superconducting strength evolves non-monotonically with a maximum near x≈0.01 and that superconductivity is destroyed near x≈0.028. This behavior reveals that hidden order depends strongly on tuning outside of the U f-electron shells. It also suggests that while hidden order provides an environment for superconductivity and anomalous metallic behavior, it's fluctuations may not be solely responsible for their progression.