Intermediate Valence Compound Research Articles

Probing the bulk valence of Sm in SmB6 by studying the magnetic susceptibility under pressure

For one of the most famous and widely investigated intermediate-valence compound SmB6, we report the results of a precise experimental study of the pressure effect on magnetic susceptibility χ. The measurements of χ(P) were carried out under helium gas pressure P up to 2 kbar at fixed temperatures 78 and 300 K using a pendulum-type magnetometer. The observed pressure effect value, d lnχ/dP ∼ –6 Mbar–1, demonstrates a noticeable decrease in susceptibility under pressure, which is weakly dependent on temperature used. From model analysis of the experimental data, combined with the supplemented LSDA + U calculations of the electronic structure and van Vleck paramagnetism of the band states of SmB6, we have estimated the value of the initial pressure derivative for the intermediate valence of Sm. It is consistent with the literature data obtained by other methods and indicates an increase in valence with increasing pressure. It was shown that the valence of Sm and its pressure dependence are closely related to the detailed characteristics of the conduction band and 4f states.

Intermediate ytterbium valence in YbRhSn2

Abstract The stannide YbRhSn2 has been synthesized. An arc-melted Rh25Sn48 precursor sample was reacted with ytterbium in a sealed tantalum ampoule in an induction furnace. The structure of YbRhSn2 was refined from single-crystal X-ray diffractometer data: MgCuAl2 type, Cmcm, a = 433.10(3), b = 1076.63(8), c = 739.36(5) pm, wR2 = 0.0676, 444 F 2 values and 16 variables. The rhodium and tin atoms form a three-dimensional [RhSn2] δ− polyanionic network with short Rh–Sn (271–273 pm) and Sn–Sn (301–324 pm) distances. The tin substructure is an orthorhombically distorted lonsdaleite network. YbRhSn2 shows paramagnetic behavior with a reduced magnetic moment of 2.2(1) µB per ytterbium atom, classifying it as an intermediate-valent compound.

Anisotropy-driven quantum criticality in an intermediate valence system

Intermetallic compounds containing f-electron elements have been prototypical materials for investigating strong electron correlations and quantum criticality (QC). Their heavy fermion ground state evoked by the magnetic f-electrons is susceptible to the onset of quantum phases, such as magnetism or superconductivity, due to the enhanced effective mass (m*) and a corresponding decrease of the Fermi temperature. However, the presence of f-electron valence fluctuations to a non-magnetic state is regarded an anathema to QC, as it usually generates a paramagnetic Fermi-liquid state with quasiparticles of moderate m*. Such systems are typically isotropic, with a characteristic energy scale T0 of the order of hundreds of kelvins that require large magnetic fields or pressures to promote a valence or magnetic instability. Here we show the discovery of a quantum critical behaviour and a Lifshitz transition under low magnetic field in an intermediate valence compound α-YbAlB4. The QC origin is attributed to the anisotropic hybridization between the conduction and localized f-electrons. These findings suggest a new route to bypass the large valence energy scale in developing the QC.

Pressure effect on magnetic susceptibility of intermediate-valence compound YbInAu2

The pressure dependence of the magnetic susceptibility of the intermediate-valence compound YbInAu2 has been measured in the temperature range of 78−300 K, showing a strong and temperature-dependent increase in susceptibility under pressure. The initial analysis of the experimental data within the framework of the integer valence Coqblin–Schrieffer model has given certain estimates of the Kondo temperature and its pressure derivative, but did not allow to describe quantitatively the specific temperature dependence of the susceptibility itself. The Anderson impurity model with infinite Coulomb repulsion U, the main parameters of which are the position of the f level relative to the Fermi energy and its hybridization width, turned out to be more suitable to describe the obtained experimental data for YbInAu2, indicating a strong dependence of the f-level position on the atomic volume.

Electronic structure of the intermediate-valence compound EuNi2P2 studied by soft x-ray photoemission spectroscopy

We carried out angle-resolved photoemission (ARPES) experiments using soft x rays to investigate the electronic structure of the intermediate-valence compound ${\mathrm{EuNi}}_{2}{\mathrm{P}}_{2}$. Both the ${\mathrm{Eu}}^{2+}$ and ${\mathrm{Eu}}^{3+}$ components arising from the $4{f}^{6}$ and $4{f}^{5}$ final states were observed in the valence spectra, directly confirming an intermediate-valence character of Eu ions. The three-dimensional band structure was studied by ARPES measurements, and the ARPES results were compared with calculations based on the density-functional theory for the non-4$f$ reference compounds ${\mathrm{SrNi}}_{2}{\mathrm{P}}_{2}$ and ${\mathrm{YNi}}_{2}{\mathrm{P}}_{2}$. We found that the ARPES spectra up to just below the Fermi level are better reproduced by the calculation of ${\mathrm{SrNi}}_{2}{\mathrm{P}}_{2}$ rather than that of ${\mathrm{YNi}}_{2}{\mathrm{P}}_{2}$. The heavy-fermion bands in ${\mathrm{EuNi}}_{2}{\mathrm{P}}_{2}$ are thus considered to be formed through the hybridization between the dispersive valence bands, which resemble those for ${\mathrm{SrNi}}_{2}{\mathrm{P}}_{2}$, and the ${\mathrm{Eu}}^{2+}$ components located at the very vicinity of the Fermi level.

Nonmagnetic-magnetic transition and magnetically ordered structure in SmS

SmS, a prototypical intermediate valence compound, has been studied by performing high-pressure nuclear magnetic resonance measurements on a $^{33}$S-enriched sample. The observation of an additional signal below 15-20 K above a nonmagnetic-magnetic transition pressure $P_{\rm c2} \approx 2$ GPa gives evidence of a magnetic transition. The absence of a Curie-term in the Knight shift near $P_{\rm c2}$ indicates that the localized character of $4f$ electrons is entirely screened and the mechanism of the magnetic ordering is not described within a simple localized model. Simultaneously, the line shape in the magnetically ordered state is incompatible with a spin density wave order. These suggest that the magnetic order in SmS may require an understanding beyond the conventional framework for heavy fermions. The fact that hyperfine fields from the ordered moments cancel out at the S site leads us to a conclusion that the ordered phase has a type II antiferromagnetic structure.

Phonons, Q -dependent Kondo spin fluctuations, and 4f phonon resonance in YbAl3

The intermediate valence (IV) compound YbAl$_3$ exhibits nonintegral valence (Yb 4$f^{14-n_f}$ (5d6s)$^z$ where z = 2+n$_f$ = 2.75) in a moderately heavy (m* = 20-30me) ground state with a large Kondo temperature (T$_K$ ~ 500-600K). We have measured the magnetic fluctuations and the phonon spectra on single crystals of this material by time-of-flight inelastic neutron scattering (INS) and inelastic x-ray scattering (IXS). We find that at low temperature, the Kondo-scale spin fluctuations have a momentum (Q) dependence similar to that seen recently in the IV compound CePd$_3$ and which can be attributed to particle-hole excitations in a coherent itinerant 4$f$ correlated ground state. The Q-dependence disappears as the temperature is raised towards room temperature and the 4$f$ electron band states become increasingly incoherent. The measured phonons can be described adequately by a calculation based on standard DFT+$U$ density functional theory, without recourse to considering 4$f$ correlations dynamically. A low temperature magnetic peak observed in the neutron scattering at ~ 30meV shows dispersion identical to an optic phonon branch. This 4$f$/phonon resonance disappears for T > 150K. The phonons appear to remain unaffected by the resonance. We discuss several possibilities for the origin of this unusual excitation, including the idea that it arises from the large amplitude beating of the light Al atoms against the heavy Yb atoms, resulting in a dynamic 4$f$/3$p$ hybridization.

Insight into Kondo screening in the intermediate-valence compound SmOs4Sb12 uncovered by soft x-ray magnetic circular dichroism

Through a high-precision soft x-ray magnetic circular dichroism (XMCD) study of the intermediate-valence compound $\mathrm{Sm}{\mathrm{Os}}_{4}{\mathrm{Sb}}_{12}$, we show our successful approach of revealing unprecedented details of Kondo screening below a characteristic temperature of ${T}_{0}\ensuremath{\sim}20$ K in the paramagnetic phase. The multiplet XMCD structure at the Sm ${M}_{5}$ edge sensitive to the $4f$ configuration enabled us to observe a clear difference between the temperature evolution above and below ${T}_{0}$ with one- and two-component behavior, respectively. Our findings are in strong contrast to the conventional Kondo crossover that coincides with the valence transition, but are qualitatively accounted for by theoretical XMCD predictions combined with the two-fluid phenomenology recently proposed. This work contributes to the large context of Kondo physics that is closely related to quantum criticality in heavy fermion systems.

Electronic Structure of Yb(Ni1−xCox)3Ga9 Studied by Angle-resolved Photoelectron Spectroscopy

Chiral crystal YbNi3Ga9 is known as an intermediate valence compound in which a strong hybridization between the 4f orbitals and the conduction band is present. The Co-substitution to YbNi3Ga9 work...

Binary and Pseudobinary Invar Materials based on Intermediate Valence Compounds

A new type of binary and pseudobinary invar materials based on intermediate valence samarium compounds Sm0.8B6, Sm1 – xLaxB6 (x = 0, 0.1, 0.22, and 0.5), Sm0.67Y0.33S, Sm0.55Y0.45S, and Sm2.75C60 are developed. The Weiss two-level invar model is used to describe the anomalous contribution to the coefficient of thermal expansion (CTE) for Sm0.8B6 and Sm1 – xLaxB6 (x = 0, 0.1, 0.22, and 0.5). The minimum position of the CTE of samarium hexaboride as a function of the degree of lanthanum doping is determined. The obtained dependence allows one to predict the CTE minimum position for Sm1 – xLaxB6 compounds in the temperature range from 40 to 180 K and to optimize the developed composite invar materials, where the positive CTE of the functional material is compensated by the negative CTE of the valence-unstable system.

Pressure-induced first-order magnetic phase transition and Yb ordered moment in YbCu2Si2

We report on high-pressure x-ray absorption near edge structure and x-ray magnetic circular dichroism (XMCD) measurements at the Yb ${L}_{3}$ edge on ${\mathrm{YbCu}}_{2}{\mathrm{Si}}_{2}$. This intermetallic is a well-known intermediate valence compound which shows a pressure-induced paramagnetic-ferromagnetic phase transition at about 8 GPa. The pressure dependence of the Yb magnetization has been determined by following the evolution with pressure of the maximum of the dichroic signal at the energy corresponding to the electric quadrupole ($2{p}_{3/2}\ensuremath{-}4f$) transitions. The observed XMCD deduced magnetization curves confirm the ferromagnetic nature of the high-pressure phase as well as the first-order nature of the transition. The Yb ordered moment deduced from XMCD which amounts to about $1.25{\ensuremath{\mu}}_{B}$ at 8.7 GPa and 2.7 K is in very good agreement with the one estimated by M\"ossbauer spectroscopy for the magnetically ordered component of the spectra. This observation indicates that the paramagnetic and ferromagnetic phases coexist only in a narrow pressure range in our XMCD experiment. This is not the case for the M\"ossbauer measurements, which show that the paramagnetic component vanishes only at 20 GPa. Our results clarify the existing discrepancy with dc magnetization measurements that reported values of the Yb moment more than two times smaller. We propose that these apparent contradictory results are due to different experimental conditions which influence the range of coexistence of the paramagnetic and ferromagnetic phases, as found in many similar situations and frequently addressed in the literature.

Electronic states of CeT2X20 (T:transition metal, X=Zn and Cd)

The electronic state of single crystals of CeIr2(Zn1−xCdx)20 and CePt2Cd20 has been studied by measuring the electrical resistivity ρ and specific heat C. We found that by substituting Zn with Cd, the lattice parameter expands, and the electronic specific heat coefficient γ and A coefficient, which is T2-term in ρ, are enhanced. Our experimental results reveal that the electronic state of the intermediate valence compound CeIr2Zn20 changes to a moderately heavy fermion state due to the negative chemical pressure. On the other hand, C and ρ of CePt2Cd20 show a clear jump and an abrupt change of slope below 0.3 K, respectively, due to a magnetic transition. The relation between the hybridization strength and Ce valence state has also been investigated by X-ray absorption experiment.

Revisiting the physical properties of Ce2Ru3Ga9: Intermediate valence, or Kondo lattice system?

In this paper we discuss in details the experimental results regarding to the crystal structure, magnetic, electronic and thermal transport properties of the strongly correlated compound Ce2Ru3Ga9. Based on these data we report the observation of coexistence and competition between intermediate valence state on Ce ions and Kondo-like behaviour in a high-quality polycrystalline alloy. Intermediate valence behaviour manifests itself in the temperature dependence of the magnetic susceptibility (χ), which reveals a broad maximum at around 335 K as is frequently observed for Ce compounds with mixed-valent Ce3+/4+. The electrical resistivity (ρ) behaves in a manner characteristic of systems with strong electronic correlations. First varies in a metallic manner, typical for intermediate valence compounds and than demonstrating a behaviour pointing on the onset of coherence that develops above the room temperature. The thermoelectric power (S) shows a broad maximum with a relatively large value of Smax = 35.7 μV/K, which is typical for the Ce-based systems that exhibit a strong c−f hybridization. Both ρ(T) and S(T) show the −lnT behaviour at high temperatures thereby indicating on the presence of incoherent electron scattering on a local moment of Ce3+. Moreover, a particular attention has been paid to the low-T behaviour in the investigated physical quantities, as it is one of the focuses of this paper.

Microscopic Observation of Heavy Quasiparticle Formation in the Intermediate Valence Compound EuNi2P2: 31P NMR Study

We report $^{31}$P NMR measurements under various magnetic fields up to 7 T for the intermediate valence compound EuNi$_2$P$_2$, which shows heavy electronic states at low temperatures. In the high-temperature region above 40 K, the Knight shift followed the Curie--Weiss law reflecting localized $4f$ states. In addition, the behavior corresponding to the temperature variation of the average valence of Eu was observed in the nuclear spin-lattice relaxation rate $1/T_1$. With the occurrence of the Kondo effect, $1/T_1$ was clearly reduced below 40 K, and the Knight shift becomes almost constant at low temperatures. From these results, the formation of heavy quasiparticles by the hybridization of Eu $4f$ electrons and conduction electrons was clarified from microscopic viewpoints. Furthermore, a characteristic spin fluctuation was observed at low temperatures, which would be associated with valence fluctuations caused by the intermediate valence state of EuNi$_2$P$_2$.

Kondo-like behavior near the magnetic instability in SmB6 : Temperature and pressure dependences of the Sm valence

We report a systematic study of Sm valence in the prototypical intermediate valence compound SmB$_6$. Sm mean valence, $v_{\rm Sm}$, was measured by X-ray absorption spectroscopy as functions of pressure ($1<P<13$ GPa) and temperature ($3<T<300$ K). Pressure induced magnetic order (MO) was detected above $P_c = 10$ GPa by resistivity measurements. A shift toward localized $4f$ state with increasing $P$ and/or $T$ is evident from an increase in $v_{\rm Sm}$. However $v_{\rm Sm}$ at $P_c$ is anomalously far below 3, which differs from the general case of nonmagnetic-magnetic transition in Yb and Ce compounds. From the $T$ dependence of $v_{\rm Sm}(P,T)$, we found that $v_{\rm Sm}(P,T)$ consists of two different characteristic components: one is associated with low-energy electronic correlations involving Kondo like behavior, and the other with high-energy valence fluctuations.

Intermediate-valence state of the Sm and Eu in SmB6 and EuCu2Si2: neutron spectroscopy data and analysis

Magnetic neutron scattering data for Sm (SmB6, Sm(Y)S) and Eu (EuCu2Si2−xGex) intermediate-valence compounds have been analysed in terms of a generalized model of the intermediate-radius exciton. Special attention is paid to the correlation between the average ion’s valence and parameters of the low-energy excitation in the neutron spectra, such as the resonance mode, including its magnetic form factor. Along with specific features of the formation of the intermediate-valence state for Sm and Eu ions, common physical mechanisms have been revealed for systems based on these elements from the middle of the rare-earth series. A consistent description of the existing experimental data has been obtained by using the concept of a loosely bound hole for the Eu f-electron shell in the intermediate-valence state, in analogy with the previously established loosely bound electron model for the Sm ion.

Neutrons peek into f-electron bands

Solid-State Physics Neutron scattering can be used to tease out the details of collective magnetic excitations that yield well-defined peaks in the data. In principle, it could also be used to look into single-electron band excitations, but collecting enough data to capture broad distributions of intensity is tricky. Goremychkin et al. used neutron spectrometers that could efficiently capture a large amount of data by rotating the sample, a crystal of the intermediate-valence compound CePd3 (see the Perspective by Georges). The measured dynamical magnetic susceptibility, in combination with detailed ab initio calculations, showed the formation of coherent f-electron bands at low temperatures. Science , this issue p. [186][1]; see also p. [162][2] [1]: /lookup/doi/10.1126/science.aan0593 [2]: /lookup/doi/10.1126/science.aar2325

Coherent band excitations in CePd3: A comparison of neutron scattering and ab initio theory.

In common with many strongly correlated electron systems, intermediate valence compounds are believed to display a crossover from a high-temperature regime of incoherently fluctuating local moments to a low-temperature regime of coherent hybridized bands. We show that inelastic neutron scattering measurements of the dynamic magnetic susceptibility of CePd3 provides a benchmark for ab initio calculations based on dynamical mean field theory. The magnetic response is strongly momentum dependent thanks to the formation of coherent f-electron bands at low temperature, with an amplitude that is strongly enhanced by local particle-hole interactions. The agreement between experiment and theory shows that we have a robust first-principles understanding of the temperature dependence of f-electron coherence.

Optical study on intermediate-valence compounds Yb1−xLuxAl3**Project supported by the National Natural Science Foundation of China (Grant Nos. 11327806 and GZ1123) and the National Key Research and Development Program of China (Grant Nos. 2016YFA0300902 and 2017YFA0302904).

We report an optical spectroscopy study on intermediate valence system Yb1−xLuxAl3 with x = 0, 0.25, 0.5, 0.75, and 1. The Kondo temperature in the system is known to increase with increasing Lu concentration. Therefore, it is expected that the energy scale of the hybridization gap should increase with increasing Lu concentration based on the periodic Anderson model. On the contrary, we find that the spectral structure associated with the hybridization effect shifts monotonically to lower energy. Furthermore, the Lu substitution results in a substantial increase of the free carrier spectral weight and less pronounced plasma frequency reduction upon lowering temperature. We attribute the effect to the disruption of the Kondo lattice periodicity by the random substitution of Yb by Lu. The work highlights the importance of the lattice periodicity of the rare earth element for understanding the Kondo lattice phenomena.

Temperature and pressure dependences of Sm valence in intermediate valence compound SmB6

We report the results of the X-ray absorption spectroscopy (XAS) on the intermediate valence compound SmB6. The XAS measurements were performed near the nonmagnetic-magnetic phase boundary. Mean Sm valence vSm was estimated from absorption spectra, and we found that vSm near the boundary (P≥10GPa and T∼12K) is far below a trivalent state with magnetic characteristics. Although the result is markedly different from the cases of pressure induced magnetic orders in Yb and Ce compounds, it is likely that the large deviation from the trivalent state seems to be common in some Sm compounds which possess electronic configuration between 4f5 and 4f6 with multi 4f electrons.