Valence Instability Research Articles

The dominant role of critical valence fluctuations on high Tc superconductivity in heavy fermions

Despite almost 40 years of research, the origin of heavy-fermion superconductivity is still strongly debated. Especially, the pressure-induced enhancement of superconductivity in CeCu2Si2 away from the magnetic breakdown is not sufficiently taken into consideration. As recently reported in CeCu2Si2 and several related compounds, optimal superconductivity occurs at the pressure of a valence crossover, which arises from a virtual critical end point at negative temperature Tcr. In this context, we did a meticulous analysis of a vast set of top-quality high-pressure electrical resistivity data of several Ce-based heavy fermion compounds. The key novelty is the salient correlation between the superconducting transition temperature Tc and the valence instability parameter Tcr, which is in line with theory of enhanced valence fluctuations. Moreover, it is found that, in the pressure region of superconductivity, electrical resistivity is governed by the valence crossover, which most often manifests in scaling behavior. We develop the new idea that the optimum superconducting Tc of a given sample is mainly controlled by the compound’s Tcr and limited by non-magnetic disorder. In this regard, the present study provides compelling evidence for the crucial role of critical valence fluctuations in the formation of Cooper pairs in Ce-based heavy fermion superconductors besides the contribution of spin fluctuations near magnetic quantum critical points, and corroborates a plausible superconducting mechanism in strongly correlated electron systems in general.

Electronic structure and magnetic properties of the magnetically ordered intermediate valent Ce5RuGe2

We report here a comprehensive investigations of the band structure, magnetic properties, electrical transport, and specific-heat for ternary compound Ce5RuGe2 that crystallize in orthorhombic Y2HfS5-type structure. The results show a complex magnetic structure with the field-induced metamagnetic transitions in coexistence with spin-glass-like phase, displayed in the magnetic H−T phase diagram. Simultaneously, a combined experimental and theoretical study based on x-ray photoelectron spectroscopy (XPS) data together with ab initio band structure calculations indicate a mixed valence of Ce due to strong hybridization between Ce 4f states and conduction band. The obtained magnetic ground state of Ce5RuGe2, resulting from localized f-electron states and accompanying strong hybridization effect leading to valence instability of cerium ions are rarely observed for Ce-based intermetallic compounds.

Hill Plot Focusing on Ce Compounds with High Magnetic Ordering Temperatures and Consequent Study of Ce2AuP3

The Hill plot is a well-known criterion of the f-electron element interatomic threshold distance separating the nonmagnetic state from the magnetic state in actinides and lanthanides. We have reinvestigated the Hill plot of Ce compounds using the commercial crystallographic database CRYSTMET, focusing on a relationship between the Ce-Ce distance and the magnetic ordering temperature because a Ce compound with no other magnetic elements rarely has a magnetic ordering temperature that is higher than 20 K. The Hill plot of approximately 730 compounds has revealed that a Ce compound, particularly a ferromagnet, showing a high magnetic ordering temperature would require a short Ce-Ce distance with a suppression of the valence instability of the Ce ion. From the study, we developed interest in Ce2AuP3 with a Curie temperature of 31 K. The ferromagnetic nature of this material has been examined in terms of a doping effect, which suggests a possible increase in magnetic anisotropy energy.

The Pressure Dependence of Structural, Electronic, Mechanical, Vibrational, and Thermodynamic Properties of Palladium-Based Heusler Alloys

AbstractThe pressure dependent behaviour of the structural, electronic, mechanical, vibrational, and thermodynamic properties of Pd2TiX (X=Ga, In) Heusler alloys was investigated by ab initio calculations. The lattice constant, the bulk modulus and its first pressure derivative, the electronic band structure and the density of states (DOS), mechanical properties such as elastic constants, anisotropy factor, Young’s modulus, etc., the phonon dispersion curves and phonon DOS, entropy, heat capacity, and free energy were obtained under pressure. It was determined that the calculated lattice parameters are in good agreement with the literature, the elastic constants obey the stability criterion, and the phonon dispersion curves have no negative frequency which shows that the compounds are stable. The band structures at 0, 50, and 70 GPa showed valence instability at the L point which explains the superconductivity in Pd2TiX (X=Ga, In).

Towards first-principles prediction of valence instabilities in mixed stack charge-transfer crystals

Strongly correlated electrons delocalized on one-dimensional (1D) soft stacks govern the complex physics of mixed stack charge-transfer crystals, a well-known family of materials composed of electron-donor (D) and acceptor (A) molecules alternating along the 1D chain. The complex physics of these systems is well captured by a modified Hubbard model that also accounts for the coupling of electrons to molecular and lattice vibrational modes and for three-dimensional electrostatic interactions. Here we study several experimental systems to estimate relevant model parameters via density-functional theory calculations on DA units and isolated molecules and ions. Electrostatic intermolecular interactions, an important quantity not just to define the degree of charge transfer of the ground state but also to predict the propensity of the system towards multistability and hence towards discontinuous phase transitions, are also addressed. Results compare favorably with experimental data.

Phenomenology of the Neutral-Ionic Valence Instability in Mixed Stack Charge-Transfer Crystals

Organic charge-transfer (CT) crystals constitute an important class of functional materials, characterized by the directional charge-transfer interaction between π -electron Donor (D) and Acceptor (A) molecules, with the formation of one-dimensional ...DADAD... stacks. Among the many different and often unique phenomena displayed by this class of crystals, Neutral-Ionic phase transition (NIT) occupies a special place, as it implies a collective electron transfer along the stack. The analysis of such a complex yet fascinating phenomenon has required many years of investigation, and still presents some open questions and challenges. We present an updated and extensive summary of the phenomenology of the temperature induced NIT, with emphasis on the spectroscopic signatures of the transition. A much shorter summary is given for the NIT induced by pressure. Finally, we report on the exploration, by chemical substitution, of the phase space of ...DADAD... CT crystals, aimed at finding materials with important semiconducting or ferroelectric properties, and at understanding the subtle factors determining the crystal packing.

Thermal expansion of the heavy-fermion superconductor PuCoGa5

We have performed high-resolution powder x-ray diffraction measurements on a sample of $^{242}$PuCoGa$_{5}$, the heavy-fermion superconductor with the highest critical temperature $T_{c}$ = 18.7 K. The results show that the tetragonal symmetry of its crystallographic lattice is preserved down to 2 K. Marginal evidence is obtained for an anomalous behaviour below $T_{c}$ of the $a$ and $c$ lattice parameters. The observed thermal expansion is isotropic down to 150 K, and becomes anisotropic for lower temperatures. This gives a $c/a$ ratio that decreases with increasing temperature to become almost constant above $\sim$150 K. The volume thermal expansion coefficient $\alpha_{V}$ has a jump at $T_{c}$, a factor $\sim$20 larger than the change predicted by the Ehrenfest relation for a second order phase transition. The volume expansion deviates from the curve expected for the conventional anharmonic behaviour described by a simple Gr\"{u}neisen-Einstein model. The observed differences are about ten times larger than the statistical error bars but are too small to be taken as an indication for the proximity of the system to a valence instability that is avoided by the superconducting state.

Temperature-induced valence instability in the charge-transfer crystal TMB-TCNQ

The occurrence of so-called temperature-induced neutral-ionic transitions (TINIT) in mixed-stack charge-transfer crystals is quite rare. Here we reinvestigate one of the crystals which has been claimed to undergo such a transition, $3,{3}^{\ensuremath{'}},5,{5}^{\ensuremath{'}}$-tetramethylbenzidine-tetracyanoquinodimethane (TMB-TCNQ). Extensive optical data allow us to conclude that the transition should be classified as a valence instability, and not as a ``true'' TINIT, as the $\ensuremath{\sim}0.5$ neutral-ionic borderline is not crossed. The ionicity $\ensuremath{\varrho}$, or average charge at the molecular sites, indeed changes very little at the transition, from about 0.3 to about 0.4, and is accompanied by stack dimerization. The transition is first order with large hysteresis, and the crystal may crack or break. For this reason we have been unable to collect x-ray structural data on the low-temperature phase, but with the help of semiempirical calculations we are able to assess a plausible scenario for this peculiar phase transition and its mechanism.

Analysis of the crystal lattice instability for cage–cluster systems using the superatom model

We have investigated the lattice dynamics for a number of rare-earth hexaborides based on the superatom model within which the boron octahedron is substituted by one superatom with a mass equal to the mass of six boron atoms. Phenomenological models have been constructed for the acoustic and lowenergy optical phonon modes in RB6 (R = La, Gd, Tb, Dy) compounds. Using DyB6 as an example, we have studied the anomalous softening of longitudinal acoustic phonons in several crystallographic directions, an effect that is also typical of GdB6 and TbB6. The softening of the acoustic branches is shown to be achieved through the introduction of negative interatomic force constants between rare-earth ions. We discuss the structural instability of hexaborides based on 4f elements, the role of valence instability in the lattice dynamics, and the influence of the number of f electrons on the degree of softening of phonon modes.

Pressure-induced valence change toward the QCP in 4f-electron compounds determined by X-ray absorption spectroscopy

ABSTRACTIn strongly correlated electron systems, a fundamental understanding of the quantum critical behavior in the vicinity of a magnetic quantum critical point (QCP) remains one of the important issues for decades. In particular, the unconventional criticality, which cannot be explained by the spin fluctuation theory, has attracted much attention in 4f electron systems, and the role of the valence instability has been discussed both from theoretical and experimental points of view. Here, we present our systematic high pressure study of X-ray absorption spectra on YbNiGa and EuFeAs, revealing the characteristic pressure-induced valence crossover and possibly the relevance of the valence instability to the physical properties near the magnetic QCP.

Interplay of magnetism and valence instabilities in lanthanide systems

Abstract The valence instability in lanthanide systems is described within an extended periodic Anderson Hamiltonian (EPAM) which includes Coulomb repulsion between f- and conduction- electrons, allowing to describe both discontinuous and continuous valence variations. We investigate the connection between valence and magnetism in this model and show that it can be applied to several lanthanide compounds showing both magnetic and valence instabilities.

Flux growth of magnetoplumbite-type strontium ferrite single crystals with La–Co co-substitution

The Na2O flux method was successfully applied in the single crystal synthesis of the magnetoplumbite-type strontium ferrite with La–Co co-substitution (Sr1−xLaxFe12−yCoyO19), which is a familiar base material for commercial permanent magnets. The initial amount of Na2O flux was fixed in the molar ratio of Na/(Sr+La)=5.17. Composition analysis revealed discordance between La and Co concentrations (x>y) in the obtained crystals, indicating that the extra charge introduced by the Co2+ substitution for Fe3+ is not compensated by La3+ for Sr2+. In the present growth condition, the necessary amount of La is nearly twice that of Co (y/x≃0.55), suggesting valence instability of an appreciable amount of Fe3+ to Fe2+. Single crystalline magnetization measurements showed that the substitution markedly enhances the magnetic anisotropy and slightly raises the saturation magnetization. Remaining problems, which should be resolved before the experimental determination of the Co site occupation, are also discussed.

Spectral Properties and Energy Transfer of a Potential Solar Energy Converter

The energy transfer between Ce3+ and Eu2+ has been investigated in the host Ca3Sc2Si3O12 (CSS), prepared by a modified sol–gel method. Excitation and emission measurements from the near-infrared to the vacuum ultraviolet spectral regions have been performed upon CSS, Ce3+-doped CSS, Eu2+-doped CSS and Ce3+, Eu2+-co-doped CSS, at various concentrations, including experiments at temperatures range of 15–460 K. The energy transfer efficiency from Ce3+ to Eu2+ can approach 90%, and the Ce3+ donor decay curves for different Eu2+ acceptor concentrations in the codoped system were fitted by the Inokuti–Hirayama method, indicating that it is energy transfer induced by electric dipole interaction. The use of the Ce3+, Eu2+ couple in the CSS host as a wideband harvester with an emission profile tailored to the response of the silicon solar cell in solar energy conversion suffers from two main drawbacks relating to valence instability and emission quenching of Eu2+. Possible solutions are suggested.

Crystal structure and unstable valence in a novel intermetallic phase Ce2Ru2Al

New ternary intermetallic compound Ce2Ru2Al was prepared in polycrystalline form, and its crystal structure was determined from both powder and single crystal X-ray diffraction data. The aluminide was found to crystallize with an orthorhombic structure (space group Cmce) with the lattice parameters: a = 5.8914(4) Å, b = 9.9603(6) Å, and c = 6.9551(4) Å. The derived unit cell is an ordered variant of the binary La2Ni3 type, where one of two Ni sites is fully occupied with Ru atoms, while another one accommodates Al atoms. The structure of Ce2Ru2Al can be regarded as a sequence of atomic layers normal to b-axis. The remarkable structural feature is a short distance between Ce and Ru atoms equal to 2.7816(4) Å. Magnetic measurements revealed that Ce ions in Ce2Ru2Al possess unstable 4f shell that leads to intermediate valence behavior of the compound. Similar valence instability was observed before for other Ce intermetallics with short Ce–Ru bonds.

Control of the Valence Instability of Tetravalent Chromium in 1T-CrSe2 by Anion Substitution and Pressure

We report on the anion substitution and pressure effects on the layered compound 1T-CrSe2 with unusual-valent chromium. Owing to the competition between the orbital degrees of freedom and the valence instability of Cr4+, the systems of Cr(Se1−xSx)2 and Cr(Se1−yTey)2 exhibit several types of phase transition. As well as the parent compound CrSe2, Te-substituted samples and slightly S-substituted samples of x ≤ 0.025 show two phase transitions, which are likely to be related to both the valence change and orbital ordering of Cr4+. The highly S-substituted samples of x > 0.04 show one phase transition, which can be attributed to orbital ordering without a valence change. The intermediate S-substituted samples of \(0.025 < x \lesssim 0.04\) exhibit the above three types of phase transition and, in addition, another type of transition. In contrast, the pressure effect on CrSe2 is qualitatively different from these anion substitution effects, which indicates that these anion substitution effects are not simply explained by a chemical pressure effect but also by the change of the stability in the Cr4+ state.

Avoided valence transition in a plutonium superconductor

The d and f electrons in correlated metals are often neither fully localized around their host nuclei nor fully itinerant. This localized/itinerant duality underlies the correlated electronic states of the high-Tc cuprate superconductors and the heavy-fermion intermetallics and is nowhere more apparent than in the 5f valence electrons of plutonium. Here, we report the full set of symmetry-resolved elastic moduli of PuCoGa5--the highest Tc superconductor of the heavy fermions (Tc = 18.5 K)--and find that the bulk modulus softens anomalously over a wide range in temperature above Tc. The elastic symmetry channel in which this softening occurs is characteristic of a valence instability--therefore, we identify the elastic softening with fluctuations of the plutonium 5f mixed-valence state. These valence fluctuations disappear when the superconducting gap opens at Tc, suggesting that electrons near the Fermi surface play an essential role in the mixed-valence physics of this system and that PuCoGa5 avoids a valence transition by entering the superconducting state. The lack of magnetism in PuCoGa5 has made it difficult to reconcile with most other heavy-fermion superconductors, where superconductivity is generally believed to be mediated by magnetic fluctuations. Our observations suggest that valence fluctuations play a critical role in the unusually high Tc of PuCoGa5.

Electronic, magnetic, and transport properties of the isotypic aluminides SmT2Al10 (T = Fe, Ru)

We report the results of a comprehensive physical and magnetic property study of the new isotypic aluminides SmT2Al10 (T = Fe, Ru). These two compounds are members of a rare-earth based system which has become an exemplary case study of the interplay of magnetism and correlated electron phenomena. SmFe2Al10 and SmRu2Al10 are found to order in a putative antiferromagnetic spin arrangement at TN = 14.5 K and 12.5 K, respectively. Moreover, SmRu2Al10 shows a further phase transition at TSR = 5 K which is likely due to spin reorientation. The susceptibility of SmFe2Al10 points to a valence instability of the Sm ionic state at intermediate temperatures well above TN. Electronic and thermal transport confirm that SmFe2Al10 undergoes an antiferromagnetic superzone gap formation below TN, whereas SmRu2Al10 suffers a lattice anomaly driven magnetoelastic coupling at TN. Below TN, the physical properties of SmT2Al10 (T = Fe, Ru) are governed by magnons with an antiferromagnetic spin-wave spectrum that reveals spin-gap opening. Our findings in this work have exposed a new anomalous correlated compound in the RT2Al10 series. SmFe2Al10 has a magnetic ordered ground state in spite of an unstable valence at higher temperature. This is comparable with CeRu2Al10, which is a unique and controversial Kondo insulator that orders antiferromagnetic at TN = 27 K. Among the series of rare-earth RT2Al10 compounds, the presented Sm compounds are two new members with anomalously high magnetic ordering temperatures, and it is envisaged that together with the two very well studied compounds CeRu2Al10 and CeOs2Al10 our presented studies will enable a broader approach towards understanding the fascinating properties of this materials class.

Long Range Order and Spin-Fluctuations in Strongly Correlated Electron System with Valence Instability

Rare-earth based strongly correlated electron systems (SCES) exhibit a large variety of different ground states, ranging from the simple paramagnetism of crystal-field-split f-electron multiplets to highly unconventional Kondo-insulator states with a combination of charge gap, spin gap and valence instability, in which long-range magnetic order can eventually arise from an initially singlet state. The physical background for these properties of the electron subsystem may be clarified by performing detailed neutron scattering experiments, namely magnetic neutron scattering spectroscopy and diffraction. This report reviews the results of the previous and new experimental studies on a number of rare-earth intermetallic compounds, which shed light on peculiar features of those unusual ground states.

Fabrication and Scintillation Performance of Nonstoichiometric LuAG:Ce Ceramics

Nonstoichiometric LuAG:Ce Ceramics ([Lu(1–x)Cex]3Al5O12, x = 0.005) with different excess of Lu3+ were designed on the basis of Lu2O3‐Al2O3 phase diagram and fabricated by a solid‐state reaction method. Without using any traditional sintering aids, pure phase and good optical performance were obtained in such a Lu‐rich LuAG:Ce ceramics. In addition, scintillation efficiency and light yield of 1% excess of Lu3+ ceramic sample were found 16 times and 1.82 times higher than that of commercial Bi4Ge3O12 (BGO) single crystals, respectively. Such values are comparable or even better than those in most of LuAG:Ce single crystals. However, antisite defects were also induced by excess of Lu doping, whose luminescence was found at 350–410 nm in Lu‐rich LuAG:Ce ceramics. The relationship of excess content of Lu and the microstructure, optical quality, and scintillation performance were clarified and discussed. Furthermore, by utilizing X‐ray absorption near edge spectroscopy technique, the charge state stability of cerium in Lu‐rich LuAG:Ce ceramics was examined. It appears that the excess of isovalence Lu3+ doping has a negligible effect on the cerium valence instability and creation of stable Ce4+ center.

Possible charge-density wave, superconductivity, andf-electron valence instability inEuBiS2F

Superconductivity (SC) and charge-density wave (CDW) are two contrasting yet relevant collective electronic states which have received sustained interest for decades. Here we report that, in a layered europium bismuth sulfofluoride, EuBiS$_2$F, a CDW-like transition occurs at 280 K, below which SC emerges at 0.3 K, without any extrinsic doping. The Eu ions were found to exhibit an anomalously temperature-independent mixed valence of about +2.2, associated with the formation of CDW. The mixed valence of Eu gives rise to self electron doping into the conduction bands mainly consisting of the in-plane Bi-6$p$ states, which in turn brings about the CDW and SC. In particular, the electronic specific-heat coefficient is enhanced by ~ 50 times, owing to the significant hybridizations between Eu-4$f$ and Bi-6$p$ electrons, as verified by band-structure calculations. Thus, EuBiS$_2$F manifests itself as an unprecedented material that simultaneously accommodates SC, CDW and $f$-electron valence instability.