Ce-based Compounds Research Articles

The thermodynamic properties and special features of spectra of elementary excitations of unstable valence Sm-and Ce-based compounds

The experimental data on the spectra of elementary excitations measured by inelastic neutron scattering and on the heat capacity and the coefficient of thermal expansion are used to analyze the correlation between the spectral characteristics of the electron and phonon subsystems and the special features of the temperature dependence of the thermodynamic properties of a number of unstable valence Sm-and Ce-based compounds. The anomalous behavior of the thermodynamic properties of these compounds is defined by the special features of their phonon and electron (4f and conduction electrons) spectra. The rearrangement of the 4f-electron spectrum as a result of temperature variation plays a decisive part in the formation of temperature dependences of the heat capacity and the coefficient of thermal expansion of unstable valence systems.

Effect of hydrogen absorption on the mixed-valent compound CeRhIn

The mixed valent Ce-based compound, CeRhIn, is found to absorb hydrogen and form two distinct hydride phases, CeRhInHx with x=0.55 and 1.58. The x=0.55 hydride retains the hexagonal structure of the parent compound. The x=1.58 hydride also has the hexagonal phase but contains an orthorhombic phase as well in nearly equal proportions. The higher hydride is found to be relatively unstable and transforms to the x=0.55 phase in a short time. Magnetization studies reveal that Ce ions continue to be in a mixed valent state in the x=0.55 hydride. The susceptibility of the x=1.58 sample exhibits Curie–Weiss behavior between 40 and 330 K with μeff=2.25 μB and θP=−92 K indicating the formation of nearly trivalent Ce ions in this hydride phase. It is suggested that hydrogenation results in increased d-band occupancy of Rh, due to partial transfer of electrons from hydrogen. This, in turn, results in decreased hybridization between Ce 4f electrons and those on the neighboring atoms, and a consequent shift of the valence of Ce towards 3+.

Thermoelectric properties of Ce-based Kondo semimetals and semiconductors

We report a systematic study of thermoelectric properties of a series of Ce-based compounds with the ε-TiNiSi-type structure; metallic CePtSn, semimetallic CeNiSn and CeRhSb, and semiconducting CeRhAs. In this sequence, both the Kondo temperature and hybridization-gap width increase, whereas the phonon thermal conductivity at high temperatures decreases. Opening of the pseudogap in CeRhSb and CeRhAs enlarges the thermopower, and leads to the maxima of the thermoelectric figure of merit, 1.7×10 −3 /K at 12 K and 1.0×10 −3/K at 115 K, respectively.

Anderson lattice inCeNiSi2intermediate valence compound

We present experimental results of electrical resistivity, Hall coefficient, magnetic susceptibility, and specific heat for single crystals of the Ce-based intervalent compound ${\mathrm{CeNiSi}}_{2}.$ The results show similar behaviors observed in Yb-based intervalent compounds and support a recent theory of the Anderson lattice, in which the Fermi-liquid coherence is gloval over the whole lattice. There is a low-temperature scale ${T}_{\mathrm{coh}}\ensuremath{\sim}50\mathrm{K}$ for the onset of Fermi-liquid coherence, in addition to a high-temperature scale ${T}_{K}^{*}\ensuremath{\sim}150\mathrm{K}$ for the Kondo lattice condensation. Therefore we conclude that two energy scales are generic in intermediate valence compounds based on Ce where the orbital degeneracy is smaller and where the size of the $4f$ orbital is larger than those based on Yb.

Effects of valence fluctuation and pseudogap formation on phonon thermal conductivity of Ce-based compounds withε-TiNiSi-type structure

We have measured the thermal conductivity of isostructural compounds CePtSn, CeNiSn, CeRhSb, and CeRhAs with the orthorhombic $\ensuremath{\epsilon}$-TiNiSi-type structure. It is found that the phonon thermal conductivity is reduced in a systematic way with increasing Kondo temperature ${T}_{K}.$ The scattering of phonons by valence fluctuations should play a dominant role in such a reduction. The gap formation in the electronic density of states enhances the phonon thermal conductivity significantly in CeRhAs with a gap width of 280 K, while it is weak in CeNiSn or CeRhSb with a pseudogap of 20--30 K. A phenomenological model is proposed for the unusual temperature dependence of phonon thermal conductivity by taking account of the strong dependence of the electron-phonon scattering rate on both ${T}_{K}$ and the energy gap.

Fermi Surface of Heavy Fermion Compounds CeTIn5(T = Rh, Ir, and Co): Band-Calculation and Tight-Binding Approach

Electronic properties of Ce-based heavy-fermion compounds are discussed by using the relativistic band-calculation method. To seek for a route from the band calculation to a many-body Hamiitonian, which is necessary to discuss the magnetic and superconducting properties, a minimal tight-binding model is proposed for f -electron systems.

Long-range magnetic order, spin-glass and evanescence of the magnetism in strongly correlated Ce-based compounds

The metallic rare-earth systems presenting strongly correlated electron properties are characterised by the existence of a strong 4f-conduction band hybridisation and, usually, by a non-negligible long-range magnetic interaction (RKKY type). In addition, other effects such as crystalline electric field or disorder contribute to the rich phenomenology found in these compounds. The actual interest is focussed on the compounds where the magnetic order is suppressed at a given critical pressure P C or, in the case of induced chemical pressure, at a critical concentration x C. In this region, corresponding to a zero temperature phase transition, non-Fermi liquid behaviour on the specific heat, resistivity or susceptibility has been found. The first part of this article will be devoted to analyse the mentioned magnetic interactions and to describe the role of the different implied parameters (cell volume, number of electrons, etc). In the second part, we present the CeNi 1− x Cu x system as an example of the interplay of the mentioned interactions leading to the disappearance of the long-range order. The spin-glass-like phase appearing in this system is discussed considering recent data, which confirm the existence of a cluster-glass behaviour. The effects of sample annealing are also discussed close to the magnetic non-magnetic crossover point.

Superconductivity inLa3Rh2Ge2and dense Kondo behavior inCe3Rh2Ge2

Magnetic and electrical transport measurements were carried out on two isostructural compounds ${\mathrm{La}}_{3}{\mathrm{Rh}}_{2}{\mathrm{Ge}}_{2}$ and ${\mathrm{Ce}}_{3}{\mathrm{Rh}}_{2}{\mathrm{Ge}}_{2}.$ The La-based phase is a weak Pauli paramagnet and shows typical metallic conductivity down to ${T}_{C}=3.5 \mathrm{K}$ where it becomes superconducting. The Ce-based compound exhibits localized magnetism due to the presence of trivalent Ce ions and undergoes an antiferromagnetic phase transition at ${T}_{N}=7.5 \mathrm{K}.$ Subsequently, at ${T}_{t}=5 \mathrm{K},$ some spin rearrangement takes place resulting in the development of a tiny ferromagnetic component. The electrical resistivity of ${\mathrm{Ce}}_{3}{\mathrm{Rh}}_{2}{\mathrm{Ge}}_{2}$ shows a temperature behavior characteristic of Kondo systems.

Hybridized nature of pseudogap in Kondo insulators CeRhSb and CeRhAs.

We studied the electronic structure of Kondo insulators CeRhSb and CeRhAs using high-resolution photoemission spectroscopy. We found that the 4f-derived density of states shows a depletion (pseudogap) at E(F) in contrast to metallic Kondo materials. It was found that the size of the f pseudogap is smaller than that of conduction electrons ( c pseudogap) while both scale well with the Kondo temperature. The present results indicate that the hybridization between 4f and conduction electrons near E(F) is essential for the Kondo gap in the Ce-based compounds.

Evidence for low-dimensional magnetic behaviour inCePt5Ge3

We present the results of magnetic susceptibility, magnetization, heatcapacity, resistivity and magnetoresistance measurements for thelow-dimensional Ce-based intermetallic compounds CePd5Ge3 andCePt5Ge3, which crystallize in the YNi5Si3-type structure.Magnetic order is manifested by a peak in both the magnetic susceptibilityand the specific heat at 1.9 K and 1.1 K for CePd5Ge3 andCePt5Ge3, respectively. For CePt5Ge3 a significant tail in thespecific heat from TN up to 5 K indicates pronounced antiferromagneticshort-range correlation of low-dimensional character above the 3Dlong-range order. Both compounds exhibit large negative magnetoresistanceat low temperature.

Superconductivity and magnetism in a new class of heavy-fermion materials

We report a new family of Ce-based heavy-fermion compounds whose electronic specific heat coefficients range from about 400 to over 700 mJ/mol-Ce K 2. Crystals in this family form as Ce n T m In 3 n+2 m , where T=Rh or Ir, n=1 or 2, and m=1, with a tetragonal structure that can be viewed as n-layers of CeIn 3 units stacked sequentially along the c-axis with intervening m-layers of TIn 2. Ambient and high-pressure studies show that the quasi-2D layers of CeIn 3 produce unconventional superconducting and magnetic ground states. This family should enable new understanding of the relationship between magnetism and superconductivity in heavy-fermion materials and more generally of why heavy-fermion superconductivity prefers to develop in one structure type and not another.

Magnetic and valence properties of the Ce-based quaternary borocarbide CeIr 2B 2C

We present here the magnetic and valence properties of the Ce-based quaternary compound, CeIr 2B 2C. From magnetic measurements, Ce-ions are found to be in a non-trivalent state in this material. Our Ce-L III edge results on CeIr 2B 2C as well as on the previously known intermediate valent CeRh 2B 2C confirm the presence of Ce ions both in 4+ and 3+ states. Both materials do not show superconductivity or magnetic ordering down to 2 K.

Formula omitted] and [formula omitted] NMR study in UCu 5Al

We have studied the microscopic properties of the tetragonal UCu 5Al Kondo compound by 27 Al and 63,65 Cu NMR in the paramagnetic state. NMR and susceptibility measurements performed on the powdered sample, but oriented along the applied field, showed χ ‖> χ ⊥. Plots of K ‖( T) against χ ‖( T) at temperatures T≥100 K yield the transferred hyperfine fields of +5.9 kOe/ μ B for 27 Al nuclei, and +5.3 and −7.0 kOe/ μ B for 65 Cu nuclei in crystallographically inequivalent Cu(2) and Cu(1) sites, respectively. The Knight shift vs. susceptibility plots for T<100 K exhibit a deviation from the linear behaviour (absolute values of shifts become smaller than expected). We attribute this finding to the crystalline electric field effect in similar way as it was reported for several Ce-based compounds. The random distribution of the Al and Cu(2) atoms in the crystal lattice we consider as a reason of an unusual broadening of the NMR spectra, particularly at low temperatures.

Effect of Pressure on Heavy-Fermion Metals**The project supported by National Natural Science Foundation of China

In this paper, we introduce a microscopic theory for the heavy-fermion metals under applied pressure in the framework of the slave-boson mean-field approximation of the U = ∞ Anderson lattice model. The density of states for f electrons and its variation with the pressure are calculated self-consistently The pressure-dependencies of the low-temperature specific-heat coefficient, magnetic susceptibility, and the resistivity in the electron-type and hole-type heavy-fermion metals are obtained. The results provide a unified explanation on the two opposite pressure-dependent effects observed in Ce-based and Yb-based compounds.

X-Ray Photoelectron Spectroscopy Study of U(Rh1-xPdx)3Alloys

The XPS valence-band and core-level spectra of pseudo-binary U(Rh 1- x Pd x ) 3 alloys for x =0, 0.83, 0.93, 0.97, and 1.0 were measured to investigate the systematics of their core-level spectra. We could consistently understand their U 4 f core-level spectra by assuming three final-state configurations, like the core-level spectra of mixed-valence Ce-based compounds. These final states are ascribed to predominance of 5 f 2 -, 5 f 3 -, and 5 f 4 -configurations, which are caused by the mixed valence with 5 f 2 (U 4+ ) and 5 f 3 (U 3+ ) character in the initial state. The results indicate the possibility of determining the valence states of other uranium compounds from the analysis of their core-level spectra.

Anomalous lattice dynamics in intermediate-valence CeNi

The low-energy phonon modes of the intermediate-valence system CeNi and the isostructural reference compound LaNi have been measured at room temperature by inelastic neutron scattering in the scattering planes (001) and (100). In general, the frequencies of the acoustic and lower optical-phonon branches are found to be appreciably lower in CeNi. This effect is especially dramatic for the acoustic modes propagating along the $a$ and $b$ directions. Maximum softening was observed near the zone boundary, where it can exceed 20% for some of the branches. These data represent the most pronounced effect of electron-lattice coupling in a Ce-based intermediate-valence compound that we are aware of.

Cerium Heavy-Fermion Compounds Near Their T=0 Magnetic-Non-Magnetic Boundary.

Measurements of the temperature-dependent specific heat and thermal expansion coefficient near a T=0 magnetic-nonmagnetic boundary, accessed in CeRh2Si2 by application of pressure and in CeRh2-xRuxSi2 at ambient pressure by chemical substitution, emphasize the role of disorder in producing non-Fermi-liquid behavior. Interestingly, superconductivity also develops near this boundary in some crystallographically-ordered Ce-based heavy-fermion compounds.

Configuration-dependent hybridization in electron spectroscopies of Ce-based compounds

In this paper, we analyze Ce $3d$ core-level photoemission and inverse-photoemission spectra in the framework of the single-impurity Anderson model. We show that the Gunnarsson-Sch\"onhammer model generally used to describe spectroscopic properties of Ce-based systems cannot account for the description of core-hole photoemission and inverse-photoemission spectra with the same set of parameters. By introducing $4f$ configuration-dependent hybridization terms, the situation is significantly improved, and a satisfactory agreement between experimental and calculated spectra is obtained. This result shows that, in contrast to what was previously claimed for highly hybridized compounds, the spectroscopic data of cerium compounds can be described in the framework of the single-impurity Anderson model.

Resonant photoemission study of the 4fspectral function of cerium in Ce/Fe(100) interfaces

In this paper, we present a resonant photoemission study of the cerium 4$f$ spectral function in Ce/Fe(100) interfaces. By covering cerium ultrathin films with lanthanum, we completely suppress the surface contribution of the spectra. Then we show that the cerium atoms at the interface are in an intermediate valent state, whereas the ${f}^{1}$ configuration is stabilized in the top layer. This method allows us to obtain the genuine 4$f$ spectral function of the interface, and could be extended to a study of Ce-based compounds.

Spin-glass freezing above the ordering temperature for the Kondo ferromagnetCeNi0.4Cu0.6

The low-temperature magnetic and transport properties of the orthorhombic CeNi{sub 0.4}Cu{sub 0.6} compound have been determined from the analysis of specific heat, ac magnetic susceptibility, electrical resistivity, elastic and inelastic neutron scattering. These measurements present intriguing experimental results that could not be explained within the usual phenomenology of Ce-based compounds. C{sub p} and {chi}{sub ac} present anomalies around 1 K corresponding to ferromagnetic order as confirmed by neutron diffraction. The magnetic structure is collinear with very reduced moments, 0.6{mu}{sub B}/Ce lying in the b direction. Additionally, a clear Kondo behavior is observed with a Kondo temperature T{sub K}=1.9K estimated from quasielastic neutron scattering. Above the ordering temperature, further anomalies are observed in C{sub p} and {chi}{sub ac} that could not be explained as originating from crystal electric field or Kondo effects. From the frequency and field dependence of the {chi}{sub ac}, above T{sub c}, a spin-glass state with a freezing temperature T{sub f}=2K is proposed for this compound. This unusual magnetic behavior is discussed in terms of mixed (positive and negative) Ruderman-Kittel-Kasuya-Yosida interactions, randomness (structural disorder), large hybridization (Kondo effect), and strong magnetocrystalline anisotropy (crystal electric field effects). {copyright} {ital 1997} {ital The American Physical Society}